GIFT OF 
 Professor W. S. Morley 
 
 Engineering Library 
 
 April 7, 1958 
 
DYNAMO ELECTRIC MACHINERY; 
 
 ITS CONSTRUCTION, DESIGN, 
 AND OPERATION 
 
 DIRECT CURRENT MACHINES 
 
 BY 
 
 SAMUEL SHELDON, A.M., PH.D. 
 
 PROFESSOR OF PHYSICS AND ELECTRICAL ENGINEERING AT THE POLYTECHNIC 
 
 INSTITUTE OF BROOKLYN, MEMBER OF THE AMERICAN INSTITUTE 
 
 OF ELECTRICAL ENGINEERS, AND FELLOW OF THE AMERICAN 
 
 ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE 
 
 ASSISTED BY 
 
 HOBART MASON, B.S. 
 
 NEW YORK: 
 D. VAN NOSTRAND COMPANY 
 
 23 MURRAY AND 27 WARREN STS. 
 1900 
 
COPYRIGHT, 1900, BY 
 D. VAN NOSTRAND COMPANY 
 
 \ - 
 
 
 ^v \JO 
 
 A 
 
PREFACE. 
 
 THIS book is intended to be used primarily in connec- 
 tion with instruction on courses of electrical engineering in 
 institutions for technical education. It is laid out on the 
 lines of the lectures and the instruction as given in the 
 Polytechnic Institute of Brooklyn. It is intended equally 
 as much for the general reader, who is seriously looking 
 for information concerning dynamo electrical machinery of 
 the types discussed, as well as a book of reference for 
 engineers. 
 
 The first two chapters are devoted to a brief but logical 
 discussion of the electrical and magnetic laws and facts 
 upon which the operation of this class of machinery 
 depends. Calculus methods have been employed in a few 
 places in these chapters, but the results arrived at by use 
 of them are in such a form that they can be utilized by the 
 reader who is unfamiliar with the processes of the calculus. 
 
 In the chapter on design it has seemed advisable to 
 express the flux density in lines per square centimeter. 
 Both the square centimeter and the square inch are used 
 in practice. The alteration of the formulas to square inch 
 units is obviously simple. 
 
 We wish to express our thanks to the various manufac- 
 turing companies who have so courteously given informa- 
 tion, and who have kindly loaned electrotypes of their 
 apparatus. 
 
 1)90644 
 
CONTENTS. 
 
 CHAPTER PAGE 
 
 I. ELECTRICAL LAWS AND FACTS i 
 
 II. MAGNETIC LAWS AND FACTS 12 
 
 III. ARMATURES 31 
 
 IV. FIELD MAGNETS 67 
 
 V. OPERATION OF ARMATURES 77 
 
 VI. EFFICIENCY OF OPERATION 92 
 
 VII. CONSTANT POTENTIAL DYNAMOS 103 
 
 VIII. CONSTANT CURRENT DYNAMOS 129 
 
 IX. MOTORS 161 
 
 X. SERIES MOTORS 185 
 
 XI. DYNAMOTORS, MOTOR-GENERATORS, AND BOOSTERS . . 208 
 
 XII. MANAGEMENT OF MACHINES 218 
 
 XIII. THE DESIGN OF MACHINES 232 
 
 XIV. TESTS 251 
 
DYNAMO ELECTRIC MACHINERY. 
 
 CHAPTER T. .V:..H' ''' ? ' 5 ' 
 
 ELECTRICAL LAWS AND FACTS. 
 
 i. Mechanical Units Force is that which tends to 
 
 produce, alter, or destroy motion. The units of force are 
 the pound and the dyne. The dyne is that force, which 
 acting on one gram for one second, will produce a velocity 
 of one centimeter per second. 
 
 Work is the production of motion against resistance. 
 The units of work are the foot-pound and the erg. The 
 foot-pound is the work done in lifting a body weighing one 
 pound one foot vertically. The erg is the work performed 
 by a force of one dyne in moving a body one centimeter 
 in the direction of its acting. The joule is a larger unit 
 much used, and is equal to io 7 ergs. 
 
 Energy is the capacity to do work. Energy is divided 
 into Kinetic energy and Potential energy. A body pos- 
 sesses kinetic energy in virtue of its motion, while poten- 
 tial energy is due to the separation or the disarrangement 
 of attracting particles or masses. A wound up spring has 
 potential energy because of the strained positions of the 
 molecules, while a weight raised to a height has potential 
 energy because of the separation of its mass from the 
 
2 DYNAMO ELECTRIC MACHINERY. 
 
 attracting mass of the earth. The potential energy of a 
 body is measured by the work required to put the body 
 into its strained condition. Kinetic energy is measured by 
 the product of the weight into the square of the velocity 
 divided by twice the acceleration due to gravity, or 
 
 Kinetic Energy = 
 
 '?.''; *e /,':.;' 2g 
 
 ' 'Power '{ the^ fcate of performance of work. Its units 
 
 lar^ tfta'hp'rEd-ptfwr and the watt. A horse-power is 33,000 
 foot-pounds per 'minlite. A watt is io 7 ergs per second. 
 One horse-power is equivalent to 746 watts. The number 
 of watts in an electrical circuit carrying a certain number of 
 amperes of current under a pressure of a certain number 
 of volts is expressed by the product of the amperes into the 
 volts. If we let T equal the torque or twisting moment 
 
 QTTH 
 
 and w equal the angular velocity (= - where n is the 
 
 60 
 
 number of revolutions per minute), then the horse-power 
 
 _ 6ow7 T _ zirnT 
 33000 33000 * 
 
 In a belt-driven machine the torque in the shaft is equal 
 to the difference in tension of the two sides of the belt 
 multiplied by the radius of the pulley in feet, hence 
 
 2. Absolute and Practical Units -- Since distinction 
 must continually be made between the absolute units and 
 the practical units, throughout this work the capital letters 
 /, E, and R will be used for the practical units, the am- 
 pere, the volt, and the ohm, respectively, and the lower- 
 case letters i, e, and r will stand for the absolute (C. G. S.) 
 units of current, pressure, and resistance respectively. 
 
ELECTRICAL LAWS AND FACTS. 3 
 
 The absolute unit of current is such that, when flowing 
 through a conductor of one centimeter length, which is 
 bent into an arc of one centimeter radius, it will exert a 
 force of one dyne on a unit magnet pole ( 10) placed at the 
 center. 
 
 The absolute unit of difference of potential exists between 
 two points when it requires the expenditure of one erg of 
 work to move a unit quantity of electricity from one point 
 to the other. This unit of quantity is the quantity which, 
 in a second, passes any cross-section of a conductor in 
 which a unit current is flowing. 
 
 The absolute unit of resistance is offered by a body when 
 it allows a unit current to flow along it between its two 
 terminals, when maintained at a unit difference of potential. 
 
 Current, f= /. 
 10 
 
 E.M.F., E = ioe. 
 Resistance, JR = loV. 
 
 It is convenient and rational to make a distinction be- 
 tween electromotive force and difference of potential. 
 Electromotive force is produced when a conductor cuts 
 magnetic lines of force, or when the electrodes of a pri- 
 mary battery are immersed in a solution. But a difference 
 of potential may exist merely because of an electric cur- 
 rent. Between any two points of a conductor carrying a 
 current there is that which would send a current through 
 an auxiliary wire connecting these points, and we call it 
 difference of potential. If the current in the original con- 
 ductor be doubled, the difference of potential between the 
 same two points will be doubled, showing that this differ- 
 ence of potential exists because of the current flowing in 
 
4 DYNAMO ELECTRIC MACHINERY. 
 
 the original conductor. The word pressure is used for 
 either difference of potential or for E.M.F. with obvious 
 relevancy. 
 
 3. Ohm's Law -- Ohm's law is expressed by the for- 
 mula r, 
 
 where 7 is the number of amperes flowing in an undivided 
 circuit, E the algebraic sum of all the electromotive forces 
 in that circuit, and R the sum of all the resistances in 
 series in that circuit. 
 
 The form of the equation E = IR, as applied to a por- 
 tion of a circuit, is much used under the name of Ohm's 
 law. In this case, however, E is not E.M.F., but differ- 
 ence of potential, as explained in the last article. 
 
 If, in a house lighted by electricity, the service maintains 
 a constant pressure of 100 volts at the mains where they 
 enter from the street, and no lights be turned on, then at 
 every lamp socket in the house there will be a pressure of 
 100 volts. If now a lamp be turned on, it will be working 
 on less than 100 volts, because of the drop or fall of po- 
 tential. If many lamps be turned on, a considerable drop 
 may occur. The drop is caused by the resistance of the 
 wires carrying the current from the place of constant po- 
 tential to the place where it is used, and the volts lost have 
 been consumed in doing useless work in heating the wires. 
 That the drop is proportional to the current flowing is 
 shown by a simple application of Ohm's law. 
 
 Let R be the resistance of the line, and E d the volts 
 drop caused thereby when a current / flows. Then 
 
 E* = IR* 
 
 from which it is evident that the drop varies as the cur- 
 rent when the resistance in the line is constant. 
 
ELECTRICAL LAWS AND FACTS. 5 
 
 4. Resistance of Conductors. The resistance R of a con- 
 ductor is expressed by the formula R = , where o- is a 
 
 A 
 
 constant called the resistivity, and depending upon the 
 material and the temperature of the conductor, / is the 
 length in centimeters, and A the cross-section in square 
 
 cms. The reciprocal of the resistivity, , is called the con- 
 ductivity of a substance. 
 
 The conductivity of copper depends on its purity, and on 
 its physical condition, soft copper having 1.0226 times the 
 conductivity of hard copper. Lake copper has a high con- 
 ductivity because of its pureness. The same is true of 
 electrolytic copper. This latter is now very largely used, 
 though for a while there was a prejudice against it because 
 it was said to be brittle. Temperature affects the resist- 
 ance of metals. In pure metals the increase of resistance 
 for a rise of i C. is about .004 times their resistance at 
 o C. Various alloys of iron, nickel, and manganese have 
 a high value for a-, and do not have so high a temperature 
 coefficient as given above. Iron heated in contact with 
 copper gives a large thermal E.M.F., which militates against 
 its alloys being used for resistances in measuring instru- 
 ments. 
 
 If in the foregoing expression for R the centimeter and 
 square centimeter be the units of length and cross-section 
 respectively, then the following list gives the value of o- for 
 various metals in microhms (i microhm = TTnJ V<yoff ohm). 
 
 Copper .... at oC, I -594 
 
 Iron .'...."" 9.5 
 
 Steel " " 13.0 
 
 18% German Silver " " 27. 
 
 30% " " 45- 
 
6 DYNAMO ELECTRIC MACHINERY. 
 
 A circular mil is a circle j-^^ inch in diameter, and a 
 wire one foot long and one circular mil cross-section is 
 called a mil-foot. The resistance of a mil-foot at o C. of 
 
 Copper = 9.59 ohms, 
 Iron =58. ohms, 
 Steel =82. ohms. 
 
 The American Institute of Electrical Engineers has 
 adopted as its standard resistivity for soft copper one given 
 by Matthiessen. A wire of standard soft copper, of uni- 
 form cross-section, of one meter length, and weighing one 
 gram, should have a resistance of 0.141729 international 
 ohms at o C. A commercial copper showing this resis- 
 tivity is said to have 100 per cent conductivity. Copper is 
 frequently found having a conductivity of 102 per cent. It 
 is in these cases almost invariably electrolytic copper. 
 
 5. Insulating Materials. Materials which are to be 
 used for insulating from each other the various electrical 
 circuits of dynamo electric machines should possess the 
 following properties : 
 
 They should have a high insulation resistance, and this 
 resistance should be maintained high over the range of 
 temperatures to be found in machines. They should 
 furthermore have a dielectric strength sufficient to pre- 
 clude any possibility of their being perforated by the 
 voltages liable to exist between the conductors which they 
 separate. This strength must also exist throughout all 
 probable ranges of temperature. They must possess such 
 physical properties as will permit of mechanical manipula- 
 tion, as they must be oftentimes bent and twisted. Of 
 course the chemical constitution should not be altered by 
 
ELECTRICAL LAWS AND FACTS. 7 
 
 any change of temperature to which they would be sub- 
 mitted. 
 
 Mica possesses the highest insuktion resistance and the 
 largest dielectric strength to be found. It requires 1000 
 volts to perforate a sheet I mil in thickness. Its chemical 
 constitution is unaffected by high temperatures. It is 
 not, however, mechanically strong. 
 
 Preparations of fibrous materials with linseed oil, which, 
 after being dried, have been thoroughly baked, are fairly 
 good insulators. As water is generally present in their 
 pores, their insulation resistance, upon heating, decreases 
 until the temperature has reached 100 C, and then it in- 
 creases. These preparations are mechanically flexible. 
 Preparations of fibrous material with shellac are good in- 
 sulators, but crack upon bending. 
 
 Vulcanized fibers are made by treating paper fiber chemi- 
 cally, and, when dried, they have a fairly high insulation 
 resistance, but they readily absorb moisture, and, upon dry- 
 ing, are liable to warp and twist. They furthermore be- 
 come brittle when heated. 
 
 Sheets of insulation made up from pieces of scrap mica 
 cemented together by linseed oil or preparations of shellac, 
 when carefully constructed with lapped joints, exhibit 
 nearly as good insulating and dielectric properties as sheet 
 mica. While not perfect mechanically, these sheets permit 
 of bending better than pure mica. 
 
 Vulcabeston, which is a preparation of asbestos and rub- 
 ber, exhibits fairly good insulating and mechanical quali- 
 ties, and is especially fitted for higher temperatures. Its 
 dielectric strength is about T J^ of that of mica. 
 
 6. Divided Circuits -- If a current I be flowing through 
 
8 DYNAMO ELECTRIC MACHINERY. 
 
 R y the undivided part of the circuit shown in Fig. I, and 
 if 7j and 7 2 be the currents flowing in the shunt resistances 
 RI and R y then / = 7 X + I v and, since the pressure E 
 upon each shunt is the same, by Ohm's law, 
 
 or Ii : 7 2 : : : 
 
 <1 ^2 
 
 currents in the branches of a divided circuit are in- 
 versely as the resistances of the branches. 
 
 If R e be a single resistance, that substituted for the 
 shunted resistances R l and R z will leave 7 unchanged, then, 
 by Ohm's law, 
 
 resistance equivalent to a number of shunted resistances 
 is equal to the reciprocal of the sum of the reciprocals of the 
 separate resistances. 
 
 RI 
 
 1 
 
 I 
 
 00 U O U U 
 
 Fig. x. 
 
 7. Power of Electric Current. A difference of poten- 
 tial e between two points requires e ergs of work to bring 
 
ELECTRICAL LAWS AND FACTS. 9 
 
 a unit quantity of electricity from one point to the other. 
 A unit of quantity is one absolute unit of current flowing 
 for one second. Hence a current i flowing for / seconds with 
 a difference of potential e does eit ergs of work. Likewise 
 a current / flowing / seconds gives It coulombs of quantity, 
 
 Eit 
 and with a difference of potential of E volts does r ergs 
 
 10 El 
 of work. Hence the work per second or the power is 7 
 
 absolute units. The practical unit of power is the watt, 
 and equals io 7 absolute units. Hence, remembering that 
 by Ohm's law E = IR, the power of a current in 
 
 For commercial currents and voltages the watt is a need- 
 lessly small expression, hence the kilowatt (= 1,000 watts) 
 is generally used as the unit of electrical power. It is repre- 
 sented by the abbreviation K. w. The horse-power is 
 equal to 746 watts, or approximately three-fourths of a K.W. 
 
 8. Heat Developed by a Current -- When a current / 
 does work in overcoming a resistance R, the work per- 
 formed is converted into heat. By the last article the 
 work thus done per second, or the power expended, will be 
 PR watts. Since this rate of production of heat is often 
 of no service, this expenditure of power is generally called 
 the PR loss. 
 
 This production of heat causes a rise of temperature in 
 the conductor, and the temperature will continue to rise till 
 the heat generated per second by the PR loss is exactly 
 counterbalanced by the rate of dissipation of heat by con- 
 duction, convection, and radiation. 
 
 The necessary resistances of electrical machines involve 
 
10 DYNAMO ELECTRIC MACHINERY. 
 
 the production of heat in their operation (as does also fric- 
 tion and reversal of magnetism), which causes a rise of tem- 
 perature. As insulating materials can survive only moder- 
 ately high temperatures, such machines must be designed 
 to operate without becoming too hot. This is accomplished 
 by decreasing the PR loss, by increasing the radiating sur- 
 face, and by supplying ventilation. 
 
 9. Fuses. These are devices intended to protect cir- 
 cuits from destruction or damage due to an excessive flow 
 of current through them. They protect them by being 
 themselves destroyed. They are generally made of lead 
 or alloys of lead. Lead is liable to become oxidized after 
 having been installed for some time. It is then liable to 
 form a tube of hard oxide, which is sufficiently strong to 
 hold molten lead in its interior, so as to maintain an elec- 
 trical contact in the circuit which should be broken. Some 
 alloys are not open to this objection. These alloys, in 
 the form of wires, strips, or ribbons, are fastened at each 
 end to copper terminals which are slotted to fit into fuse 
 receptacles. The wire with its terminals is called a fuse 
 link. Such a link is shown in Fig. 2. 
 
 Fig. 2. 
 
 Copper wires are sometimes used as fuses on trolley cars, 
 but the high melting point of copper prohibits its use as a 
 protective device on house circuits. 
 
 The current which will fuse a wire of lead alloy depends 
 in magnitude upon the length of the wire. Short lengths 
 
ELECTRICAL LAWS AND FACTS. II 
 
 of a wire of given cross-section and given material will 
 carry stronger currents than longer lengths. The heat 
 which is generated in the short ones escapes more rapidly, 
 owing to the larger masses of metal commonly forming the 
 terminals of the fuse. Fuses are rated to carry a given 
 amperage, and the rating is stamped upon the copper termi- 
 nals. According to the national code the fuses must, how- 
 ever, be able to carry indefinitely without melting such a 
 number of amperes that the rated capacity is but 80 per 
 cent of it. This permits the fuse to carry without melting 
 25 per cent above the rated capacity. 
 
 Fig. 3. 
 
 For high voltages, and for large amperages, inclosed 
 fuses are sometimes used, in which the fusible conductor is 
 surrounded by a packing of finely divided powder in which 
 borax is included as an element most desirable. Such a 
 fuse is shown in Fig. 3. 
 
12 DYNAMO ELECTRIC MACHINERY. 
 
 CHAPTER II. 
 
 MAGNETIC LAWS AND FACTS. 
 
 10. Strength of Magnet Pole A unit magnet pole is 
 
 one which will repel an equal like pole, when at a distance 
 of one centimeter, with a force of one dyne. 
 
 It follows from this definition that a pole m units strong 
 will repel a like unit pole with a force of m dynes. The 
 force exerted between two magnetic poles varies inversely 
 as the square of the distance between them. Hence the 
 force exerted between two magnetic poles of strengths m 
 and m' when d centimeters apart is defined by the equation 
 
 m m' 
 
 F " ~d^' 
 
 11. Intensity of Magnetic Field. A magnetic field is 
 of unit strength or intensity when a unit magnet pole placed 
 therein is acted upon by a force of one dyne, or when a 
 magnet pole m units strong is acted upon by a force of 
 m dynes. The strength of a field is usually represented 
 by 3C. 
 
 12. Magnetic Field and Lines of Force. The space 
 around a magnet where its action is felt is termed the field 
 of that magnet. This field may conveniently be consid- 
 ered as permeated by lines of force. These lines represent 
 the direction of the force exerted by the magnet, and by 
 their closeness to each other show the magnitude of this 
 force. 
 
MAGNETIC LAWS AND FACTS. 13 
 
 The student must not get the impression that, because 
 the lines spread apart, a point in the field could be chosen 
 where there would be no line. These lines may well be 
 considered as tubes or pencils of force, filling all the space 
 around the magnet. 
 
 The lines of force contained in any plane passed through 
 the magnet pole compose a magnetic spectrum, which can 
 be made visible by the familiar experiment of sprinkling 
 iron filings on a paper, which is laid over a magnet, and by 
 gently tapping it. 
 
 By convention one line of force per square centimeter is 
 considered to represent a field of unit strength, the square 
 centimeter being so taken that it is at all points perpendic- 
 ular to the lines cutting it. Hence the strength or inten- 
 sity 3C of any field can be expressed by the number of 
 lines of force per square centimeter. 
 
 Suppose a sphere of one centimeter radius to be circum- 
 scribed about a unit magnet pole. Another unit pole at 
 any point on the surface of this sphere will be acted upon 
 by a force of one dyne. Hence there exists a unit field at 
 any point on this surface. But there are 4 -n- square centi- 
 meters on this surface, and each square centimeter will 
 be cut by one line of force. Therefore, there emanate 
 from a tmit magnet pole 4 TT lines of force. Similarly a 
 magnet pole of strength m sends out 4 TT m lines of force. 
 
 A magnetic field is said to be uniform when it has the 
 same 3C at every point therein, or when the lines of force 
 are parallel. 
 
 13. Electro-Magnetic Induction. In 1831 Faraday and 
 Henry independently discovered that when a conductor was 
 moved in a magnetic field, an electromotive force was set 
 
DYNAMO ELECTRIC MACHINERY. 
 
 up in the conductor. This phenomenon is the foundation 
 of all modern electrical engineering. 
 
 An absolute unit of E.M.F. is produced when a conduc- 
 tor cuts one line of force per second. If the conductor 
 cuts two lines in the second, or one line in half a second, 
 then two units are produced. 
 
 If, in the short interval of time, dt seconds, dfy lines be 
 cut, then during that interval the value of the induced 
 E.M.F. will be 
 
 _ 
 ~ 
 
 dt 
 
 or, 
 
 = __ 8 vo its. 
 10 dt 
 
 The negative sign is used because the induced E.M.F. 
 tends to send a current in such a direction as to demag- 
 
 netize the field. When of no con- 
 sequence the negative sign will 
 hereafter be omitted. 
 
 If a conductor, Fig. 4, / centi- 
 meters long moves parallel to itself 
 with a uniform velocity of v cen- 
 timeters per second across a uni- 
 form magnetic field of strength 
 3C, its path making an angle a with 
 the direction of the lines of force, 
 then the number of lines cut per 
 second is 3lv sin a, and since the 
 rate of cutting is uniform, the E.M.F. at any instant is 
 
 e = 3lv sin a. 
 
 If there be a non-uniformity in the rate of cutting lines, 
 due either to an uneven field or an irregular motion, then 
 
 Fig. 4- 
 

 MAGNETIC LAWS AND FACTS. 15 
 
 the average value of the induced E.M.F. associated with 
 the cutting of < lines in the time, t seconds, will be e av = - 
 
 For suppose the time t to be divided into / equal and small 
 periods havirg a duration of A/ seconds. Furthermore, 
 suppose that during these successive periods A<', A<", A<'", 
 etc., lines be cut respectively. Then the induced E.M.F.'s 
 during these periods, which may be represented by /, /', e' 
 etc., respectively, will be as follows : 
 
 / = ^r 
 
 e " = ~KT ' 
 
 '" 
 
 A/ 
 
 Adding these / equations, and then dividing by /, gives the 
 equation above, viz., 
 
 '- = 7 or - = ^ volts ' 
 
 The average value of the induced E.M.F. is therefore inde- 
 pendent of the magnitude of the instantaneous values. 
 
 If a loop of wire revolve, uniformly or otherwise, in a 
 magnetic field which is uniform or otherwise, its sides cut 
 lines of force at various rates. The instantaneous E.M.F. 
 in the whole loop will be as before. 
 
 where < is the number of lines that links with, or that 
 passes through, the loop. If the loop be of n turns, then 
 
i6 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 the pressure will be n times as great, or during the inter- 
 val dt, 
 
 14. Direction of Induced E.M.F. The direction of 
 flow of a current induced in a closed circuit by mov- 
 ing it in a magnetic field is best represented by drawing 
 the conventional representation of the three dimensions 
 of space. If the flux be directed upwards, and the motion 
 of the conductor be to the right, then the E.M.F. will tend 
 to send a current toward the reader. If any one of these 
 conditions be changed it necessitates the change of one of 
 the others, and conversely the change of any two leaves 
 
 I 
 
 Moiior 
 
 Fig. 5. 
 
 Fig. 6. 
 
 the third unaltered. About the same idea is represented 
 in Fleming's Rule, which is as follows : 
 
 Let the index finger of the right hand point in the di- 
 rection of the flux, and the thumb in the direction of the 
 
MAGNETIC LAWS AND FACTS. I/ 
 
 motion. Bend the second finger at right angles with the 
 thumb and index finger, and it will point in the direction 
 of the E.M.F. 
 
 Another rule is : 
 
 Stand facing a north magnetic pole. Pass a conductor 
 downward. The current tends to flow to the left. 
 
 15. Inductance. Nearly every electrical circuit which 
 has a current flowing in it has lines of force linked with it, 
 due to that current. When the circuit is opened the dis- 
 appearance of the lines is accompanied by a cutting of the 
 circuit by those lines, and the cutting results in the pro- 
 duction of an E.M.F. This is called the E.M.F. of self- 
 induction. Its magnitude is dependent upon the rapidity 
 with which the field disappears, and upon a constant deter- 
 mined by the geometric shape of the circuit and the char- 
 acter of the medium in which it is placed. This constant 
 is called the self-inductance or the coefficient of self-induc- 
 tion of the circuit. It is generally represented by the 
 letter Z, and is that coefficient by which the time rate of 
 change of current in the circuit must be multiplied in order 
 to give the E.M.F. induced in the circuit. Its absolute 
 value is numerically represented by the number of lines of 
 force linked with the circuit per absolute unit of current in 
 that circuit. Its practical unit is io 9 times as large as the 
 absolute unit, and is called the henry. In a given circuit it 
 varies as the square of the number of turns of wire. Two 
 circuits may exercise a mutually inductive action upon each 
 other, and an E.M.F. may be induced in one by a change 
 of current in the other. This is called the E.M.F. of 
 mutual induction. In magnitude it depends upon the shape 
 and position of the two circuits, and upon the character 
 
18 DYNAMO ELECTRIC MACHINERY. 
 
 of medium in which they are placed. It is also dependent 
 upon a constant which is called the mutual inductance or 
 coefficient of muttial induction of the two circuits. It is 
 generally represented by the letter M. It is that coeffi- 
 cient by which the time rate of change of the current in one 
 of the circuits is multiplied in order to give the E.M.F. 
 induced in the other circuit. Its absolute value is numeri- 
 cally equal to the number of lines of force linked with one 
 of the circuits per absolute unit of current in the other cir- 
 cuit. Its practical unit is the same as the practical unit 
 of self -inductance, that is the henry, and is io 9 times as 
 large as the absolute unit. The coefficient of mutual in- 
 duction varies directly as the number of turns of wire in 
 either circuit. 
 
 16. Quantity of Electricity Traversing a Circuit Due 
 to a Change of Flux Linked with it. In many dynamo 
 tests, and in many magnetic investigations, it is necessary 
 to measure, generally by means of a ballistic galvanometer, 
 the quantity of electricity traversing a circuit due to a 
 change of flux linked with it. If the circuit have a resist- 
 ance of r and in dt time the flux linked with n turns 
 changes by d$, then the instantaneous current 
 
 dt 
 
 t = - 
 r 
 
 But the quantity, q = idt, hence 
 
 ndd 
 
 ?= 
 
 which is independent of time. So if the flux change from 
 <j> : to < 2 , then 
 
 9l <P2 
 
MAGNETIC LAWS AND FACTS. 19 
 
 or Q = _. microcoulombs. 
 
 100 J? 
 
 17. Work Performed by a Conductor Carrying a Current 
 and Moving in a Magnetic Field. Let a conductor carry- 
 ing a constant current i be moved in a direction perpen- 
 dicular to itself and to the lines of force of a magnetic field. 
 Suppose it to move for dt seconds, and in that time to cut 
 d$ lines of force. Then the induced E.M.F. e will be 
 
 -. The quantity of electricity dq that has to traverse 
 
 the circuit against this E.Af.F. during the time dt will 
 be idt. Since potential is a measure of work, the work 
 required to carry dq units of electricity against a difference 
 of potential e is edq ergs. Hence the work in ergs, 
 
 dw edq = idt X -p = *>/<. 
 
 Therefore the current /", in cutting < lines of force, per- 
 forms the work 
 
 w = i(j> ergs. 
 
 From this it is seen that the work done by a conductor 
 carrying a current and cutting lines of force is independent 
 of the time it takes to cut them. 
 
 In the above discussion, if the field be not uniform or 
 the motion be not uniform, the value of e will not be the 
 same for each instant of time. But since the result 
 obtained is independent of time, it is immaterial how the 
 lines are arranged, and how the rate of cutting varies. 
 
 18. Magnetic Potential. The magnetic potential at 
 any point is measured by the work required to bring a unit 
 magnet pole up to that point from an infinite distance. 
 
20 DYNAMO ELECTRIC MACHINERY. 
 
 The difference of magnetic potential between any two 
 points is measured by the work in ergs required to carry a 
 unit magnet pole from one to the other. The difference 
 of magnetic potential is a measure of the ability to send 
 out lines of force, or to set up a magnetic field. 
 
 19. Magnetomotive Force of a Circular Circuit Carry- 
 ing a Current. A thin circular conductor carrying a cur- 
 rent forms a magnetic shell. If a unit magnet pole be 
 taken from the top side of a shell, and carried around to 
 
 the bottom side, work must be 
 done, and this work is a measure 
 of the difference of potential be- 
 tween the two sides of the shell. 
 It is immaterial whether the 
 pole be carried from one side of 
 the shell to the other, or the 
 shell be turned bottom side up 
 
 around the pole. In the latter case it is clear that all the 
 lines emanating from the pole will be cut once, and once 
 only, by the conductor, wherefore 4^ lines will have been 
 cut. 
 
 If current i flows in the conductor, then, by 17, 
 
 Work in ergs /< = 4 TTZ. 
 
 If there be n turns of the conductor, each line of force 
 will be cut n times, and the work will be ^irni ergs. 
 
 Hence the difference of potential between the two sides 
 
 of a thin magnetic shell is 4 imi or H . 
 
 10 
 
 In this expression - - is a constant, and it is convenient 
 to regard nl as a single variable. In connection with it the 
 
MAGNETIC LAWS AND FACTS. 21 
 
 term ampere-turns is employed, and this is frequently 
 written ///. 
 
 Here the same argument holds as in 17, that the inten- 
 sity of field and the rate of cutting lines will vary as the 
 pole is in different parts of the path. But the total num- 
 ber of lines cut is the same in any case, so the expression 
 for work and potential is true, no matter what path the pole 
 takes. 
 
 20. Force Exerted on a Field by a Conductor Carrying a 
 Current When a conductor moves in a field perpendicu- 
 lar to itself and to the lines of force, then, from 1 7, 
 
 Work = /< ergs. 
 
 If the conductor be / centimeters long, and traverses a dis- 
 tance of s centimeters through a uniform field of strength 
 3C (3C lines per sq. cm.), then 
 
 < = /tfC, 
 and the 
 
 Work = Us 3C ergs. 
 But 
 
 Work = force X distance = Fs = Us 3C. 
 
 //TO 
 
 .-. ^=//X = = - dynes. 
 10 
 
 21. The Solenoid. A uniformly wound, long, straight 
 coil, carrying a current z, produces a uniform field 5C at its 
 center. This coil is called a solenoid, and may be consid- 
 ered as composed of magnetic shells arranged at equal dis- 
 tances from each other. It takes 471-2' ergs to move a unit 
 magnet pole from one side of a shell to the other ( 19), 
 and ^trin ergs to pass it through the n consecutive shells 
 
22 DYNAMO ELECTRIC MACHINERY. 
 
 of the solenoid. If these n shells occupy a length on the 
 solenoid of / centimeters, then 
 
 Work = force X distance = 3C/ = ^-rrm ergs, 
 
 and the magnetizing force, that is, the strength or intensity 
 of field, in the solenoid is 
 
 OC = 
 
 22. Permeability. The same difference of magnetic 
 potential between two points will produce more lines of 
 force in iron than in air. Iron is then said to be more per- 
 meable than air, or to have a greater permeability. If a 
 difference of magnetic potential could set up, at a certain 
 place, a field of strength 3C, with air as a medium, and one 
 of strength <&, with some other substance as a medium, 
 
 /n 
 
 then the ratio expresses the permeability of that sub- 
 stance. This ratio is usually represented by /x. As 3C 
 varies directly with the magnetic difference of potential, it 
 becomes a measure of it. Therefore 3C is called the mag- 
 netising force and <$> the flux density, the magnetic density, 
 or the induction per square centimeter. For air, vacuum, 
 and most substances /* = i. For iron, nickel, and cobalt //. 
 has a higher value, reaching, in the case of iron, as high 
 as 3000. Bismuth, phosphorus, water, and a few other sub- 
 stances, have a permeability very slightly less than unity. 
 
 A substance for which ^ = o would insulate magnetism. 
 There is no such substance. 
 
 The total magnetic flux, <, which passes through an 
 area of A square centimeters, in which the magnetic density 
 is , is represented by the equation 
 
MAGNETIC LAWS AND FACTS. 
 
 The permeability of air is constant for all magnetizing 
 forces. This is not the case with iron and other substances 
 which have a permeability greater than unity. The value 
 of /A, (&, and 3C, which are connected by the relation (& = /xX, 
 are given in the following table for average commercial 
 wrought iron, for cast iron, and for steel. The relations 
 which exist between (B and 3C are also shown in Figs. 8, 9, 
 and 10. These curves are technically known as &-5C 
 curves. 
 
 DATA FOR (B-OC CURVES. 
 
 AVERAGE FIRST QUALITY METAL. 
 
 
 AMPERE- 
 
 AMPERE- 
 
 WROUGHT AND 
 SHEET IRON. 
 
 CAST IRON. 
 
 CAST STEEL. 
 
 5C 
 
 PER CEN- 
 
 TURNS 
 PER INCH 
 
 
 KILO- 
 
 
 KILO- 
 
 
 KILO- 
 
 
 TIMETER 
 LENGTH. 
 
 LENGTH. 
 
 (B 
 
 LINES 
 
 PER 
 
 
 
 LINES 
 
 PER 
 
 & 
 
 LINES 
 
 PER 
 
 
 
 
 
 SQ. IN. 
 
 
 SQ. IN. 
 
 
 SQ.IN. 
 
 10 
 
 7-95 
 
 2O.2 
 
 11800 
 
 74 
 
 3900 
 
 25.2 
 
 I200O 
 
 77 
 
 20 
 
 15.90 
 
 40.4 
 
 14000 
 
 90 
 
 55 
 
 35-5 
 
 13800 
 
 8 9 
 
 3 
 
 
 60.6 
 
 15200 
 
 98 
 
 6500 
 
 42.0 
 
 I46OO 
 
 94 
 
 40 
 
 31.80 
 
 80.8 
 
 15800 
 
 102 
 
 7100 
 
 45-7 
 
 15400 
 
 99 
 
 
 39-75 
 
 IOI.O 
 
 16400 
 
 106 
 
 7700 
 
 49-5 
 
 16000 
 
 103 
 
 60 
 
 47.70 
 
 121. 2 
 
 16800 
 
 1 08 
 
 8200 
 
 53- 
 
 16400 
 
 106 
 
 80 
 
 100 
 
 63.65 
 79-5 
 
 161.6 
 
 2O2.O 
 
 17200 
 17600 
 
 III 
 
 114 
 
 8900 
 9300 
 
 57-2 
 60.0 
 
 l67OO 
 I76OO 
 
 1 08 
 "3 
 
 I2 5 
 
 99.70 
 
 252.5 
 
 17800 
 
 "5 
 
 9700 
 
 62.4 
 
 I82OO 
 
 117 
 
 150 
 
 119.25 
 
 33- 
 
 18000 
 
 116 
 
 IOIOO 
 
 65.8 
 
 I8600 
 
 1 20 
 
 3C = i -258 (nl per cm.) = .495 (nf per in.). <& = . 1 55 (0 per sq. in.). 
 
 23. Things Which Influence the Shape of the OHfC Curve. 
 In general all substances mixed with or alloyed with iron 
 lower its permeability. In steel and cast iron the per- 
 meability seems to be in inverse proportion to the amount 
 of carbon present. Carbon in the graphitic (not combined) 
 form lowers the permeability less than carbon when com- 
 bined. In cast iron and cast steel such substances as tend 
 to give softness and greater homogeneity to the metal 
 
DYNAMO ELECTRIC MACHINERY. 
 
 WROUGHT AND SHEET IRON 
 
 B.J per centimeter 16 
 
 10 20 30 40 50 60 70 80 90 100 110 120 
 
 30 
 
 a /per inch 40 
 
 Permeability^, 
 
 _2_40. 
 
 Fig. 8. 
 
 45.5 
 
 CAST IRON 
 
 MTpercentiint 
 
 60 70 80 9C 100 110 
 
 nl per incli 
 
 Permeability// 100 
 
 Fig. 9- 
 
MAGNETIC LAWS AND FACTS. 
 
 CAST STEEL 
 
 Fig. 10. 
 
 when present in limited amounts, say 2 per cent, increase 
 the value of /u. Aluminum and silicon act in this way. 
 
 The physical condition of the metal also affects its per- 
 meability. Chilling in the mold, when casting, lowers it, as 
 does tempering, or hardening the metal by working it. On 
 the other hand, annealing increases the permeability. 
 
 A piece of iron or steel, subjected to a small magnetizing 
 force, has its permeability increased by increasing the tem- 
 perature until a critical temperature is reached, when it falls 
 off rapidly to almost jmity. For stronger magnetization 
 the permeability does not rise so high at the critical tem- 
 perature, and does not fall off so sharply after it. The 
 value of this critical temperature lies between 650 C. and 
 900 C., depending on the test piece. 
 
 24. Reluctance and Permeance. In the flow of mag- 
 netic lines of force the reciprocal of the permeability, -, is 
 
26 DYNAMO ELECTRIC MACHINERY. 
 
 called the reluctivity. The total reluctance, tending to 
 oppose the passage of magnetic lines under the influence 
 of a magnetic difference of potential, is directly as the 
 length and the reluctivity of the medium and inversely as 
 its cross-section. Hence the total magnetic resistance or 
 
 Reluctance = - - : reluctivity. 
 cross-section 
 
 Reluctivity is usually represented by p(=_). Hence for 
 
 a medium of cross-section A square centimeters and length 
 / centimeters, the reluctance 
 
 Permeance is the reciprocal of the reluctance, hence the 
 permeance ^ ^ 
 
 * = */* 
 
 It must be remembered that p and p. are not constant for 
 any one substance, but depend for their values upon the 
 strength of the magnetizing force X which is acting upon 
 the substance. 
 
 25. Relation Between Magnetomotive Force, Magnetic 
 Flux, and Reluctance. These quantities are related to 
 each other the same as are E.M.F., current, and resistance, 
 
 viz., 
 
 _ Magnetomotive Force 
 Reluctance 
 
 In this respect electric current and magnetic lines are 
 similar. However, while electric circuits, in the main, ex- 
 ist in media of zero electric conductivity, and therefore 
 permit of accurate calculations, there being no appreciable 
 leakage, magnetic circuits must be situated in media which 
 
MAGNETIC LAWS AND FACTS. 
 
 have permeabilities of at least unity. In the latter case 
 much leakage is present, and precise calculations are out of 
 the question. In the designing of dynamo electric ma- 
 chinery, however, one or more paths of low reluctance are 
 presented to the magnetizing force, and these are pro- 
 tected by being so shaped that leakage paths offer a com- 
 paratively high reluctance. 
 
 26. Hysteresis. If a piece of iron become magnetized, 
 and the magnetizing force be then removed, the iron does 
 
 Magnetlztng 
 
 Fig. xi. 
 
 not become completely demagnetized. A certain magnet- 
 izing force in the opposite direction must be used to bring 
 it to a neutral state. This phenomenon has been termed 
 hysteresis. Because of hysteresis a (& 3C curve taken with 
 continuously increasing values of 3C to the maximum and 
 
28 DYNAMO ELECTRIC MACHINERY. 
 
 then with continuously decreasing values of 3C to a negative 
 maximum, and so on, will assume the shape shown in Fig. 
 1 1 . The distance O A represents the coercivity, that is, 
 the magnetizing force necessary to bring the iron from a 
 magnetic to a neutral state. The distance O C represents 
 the retentivity, that is, the amount of magnetic induction 
 left in the iron after the magetizing force has been removed. 
 The area inclosed by the curve represents the energy lost 
 in carrying the iron through one cycle, i.e., from a maximum 
 magnetization to a maximum in the opposite direction and 
 back to the orginal condition. 
 
 For suppose the magnetization to be due to a current 7 
 flowing in a solenoid of n turns. If, in a short interval of 
 time dt, a change of d$ be made in the flux which is linked 
 with the solenoid, then this change will induce an E.M.F. 
 in the solenoid which during the interval of time dt will be 
 equal to 
 
 nd<$> 
 
 E = - volts. 
 ioV/ 
 
 During this time work must be performed to maintain this 
 current /, and its magnitude is 
 
 io 
 
 for Idt represents the quantity of electricity which is trans- 
 ferred from one point to another, between which there ex- 
 ists a difference of potential E. Now $ = A($> (22) and 
 
 I O TP/ 
 
 hence d$ = Ad<$>. Furthermore, nl = - (21). Hence 
 the work during the time dt is 
 
 Eldt = 7 5C <i<$> joules. 
 
 I0 7 47T 
 
MAGNETIC LAWS AND FACTS. 29 
 
 Supposing the magnetizing force to vary cyclically, taking 
 T seconds to make one cycle, then the work per cycle is 
 
 Al r 
 EIT= _ / 3C</(B joules. 
 
 10 VJ ._ (R ' 
 UJ /Hax 
 
 If the number of cycles completed in one second be/, then 
 f = , and the work in joules per second, that is, the power 
 in watts, equals 
 
 f A J f* 1 ^IIHl K >7 A /* I ^/wax 
 
 ^/- ^- / 3C^/(B = 79 / volume / X./&. 
 io'47rj _(B ;))aj I0 J -(B,^ 
 
 The integral expression is evidently the area contained by 
 the hysteresis loop. 
 
 27. Steinmetz's Law The value of the integral ex- 
 pression is dependent upon (&, upon the retentivity of 
 the kind of iron, and upon its coercivity. Steinmetz has 
 shown that for all practical purposes the value of the inte- 
 gral may be expressed by the empirical formula 
 
 '+,. 
 
 where 77 is a constant depending upon the kind of iron. 
 Its value is given in the following table : 
 
 HYSTERETIC CONSTANTS. 
 
 Best soft iron or steel sheets o.ooi 
 
 Good soft iron sheets 0.002 
 
 Ordinary soft iron 0.003 
 
 Soft annealed cast steel 0.008 
 
 Cast steel 0.012 
 
 Cast iron 0.016 
 
 Hard cast steel 0.025 
 
30 DYNAMO ELECTRIC MACHINERY. 
 
 The hysteretic constant, if at first small, grows with age. 
 Its increase can be hastened by continued heating. The 
 increase may amount to 200 per cent. Annealing, while it 
 increases the permeability, also increases the hysteretic 
 constant, if it be originally very small. 
 
 The magnitude of the hysteretic constant is largely de- 
 pendent upon the mechanical structure of the iron. To 
 attain the smallest value the iron should not be of homoge- 
 neous structure, but should have a greater density in the 
 direction perpendicular to the direction of flux. 
 
ARMATURES. 31 
 
 CHAPTER III. 
 
 ARMATURES. 
 
 28. Dynamos Dynamos may be defined as machines 
 
 to convert mechanical energy into electrical energy by 
 means of the principle of electromagnetic induction. In 
 all commercial machines the mechanical energy is supplied 
 in the form of rotation, and the electrical energy is deliv- 
 ered either as " direct current" or "alternating current." 
 These machines are also frequently called generators. 
 
 29. Principle of the Action of a Dynamo. If a loop of 
 wire be revolved in a magnetic field about an axis perpen- 
 dicular to the lines of force, as in Fig. 1 2, then each side 
 (but not the ends) of the loop is a conductor moving across 
 the lines of a magnetic field, and as such will have an 
 E.M.F. induced in it. Since the motion of one conductor 
 is up while that of the other is down, the directions of the 
 induced E.M.F. 's in the two sides will be opposite to each 
 other, and since they are on opposite sides of a loop, the 
 pressure will be cumulative ; i. e., instead of neutralizing 
 each other, the two pressures will be added to each other. 
 If now the two ends of the wire from which the loop is 
 made be respectively connected with slip rings, and a cir- 
 cuit be completed through contacts sliding on them, a cur- 
 rent will flow. When the loop, in its revolution, reaches a 
 position (as illustrated in Fig. 1 2) such that the conductor 
 
DYNAMO ELECTRIC MACHINERY. 
 
 that was previously moving upward begins to move down- 
 ward, then the direction of the induced E.M.F. will be 
 changed in both sides of the loop, and the direction of the 
 
 Fig. 12. 
 
 current through the circuit will be changed. For each 
 complete revolution the current changes direction twice. 
 It is an alternating current, and the supposed machine is 
 an alternating current dynamo, or simply an alternator. 
 
 30. The Principle of the 
 Commutator A commu- 
 tator is used on the shaft of 
 a machine when it is de- 
 sired to get a direct or rec- 
 tified current. For the 
 single loop in the above 
 case, the commutator (Fig. 
 13) would consist of two similar cylindrical parts of metal, 
 insulated from each other, and affording sliding contact for 
 
 Fig. 13. 
 
ARMATURES. 
 
 33 
 
 two brushes. One end of the wire of the loop is attached 
 to one piece of the commutator, and the other to the other. 
 The brushes are so placed that at the instant the in- 
 duced E.M.F. in the loop changes its direction, the brushes 
 slide across from one segment of the commutator to the 
 other, and thus the current, while reversed in the loop, is 
 
 Fig. 14. 
 
 left flowing in the same direction in the outside circuit. 
 If the loop were wound double before the ends were at- 
 tached to the commutator segments, and if the speed of 
 revolution and the strength of the magnetic field were both 
 maintained constant, twice the E.M.F. would be produced, 
 but no more commutator segments would be necessary 
 (Fig. 14). 
 
 In the above cases at the instants of commutation there 
 would be no E.M.F. produced, and hence the current would 
 fall to zero twice every revolution. If two coils were placed 
 90 apart, one or the other would always be cutting lines 
 of force. Hence at no time could the pressure be zero. 
 
34 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 To satisfactorily collect current from this arrangement re- 
 quires four commutator segments and a system of connec- 
 tions similar to that shown 
 in Fig. 15. In this case 
 the E.M.F. would fluctu- 
 ate, but not so badly as in 
 the previous case. If we 
 increase the number of 
 loops, and correspondingly 
 increase the number of 
 commutator segments, we 
 decrease the fluctuation 
 of the E.M.F. until it be- 
 comes practically constant. In a bipolar machine with 12 
 commutator segments the fluctuation is 1.7 per cent of the 
 total E.M.F. 
 
 Fig, 15. 
 
 31. The Armature In a dynamo, the loops of wire in 
 
 which E.M.F. is induced by movement in a magnetic field, 
 together with the iron core that sustains them, with the 
 necessary insulation, and with the parts connected imme- 
 diately thereto, constitute the armature of a dynamo. The 
 conductors in which the 
 E.M.F. is generated are 
 called the inductors. An 
 armature in which both 
 sides of the loop of wire 
 cut lines of force, as in 
 the cases just described, 
 
 11 i T-L 
 
 is called a Drum Arma- 
 ture. A kind of armature less generally used is the Ring 
 Armature, illustrated diagrammatically in Fig. 16. Here 
 
ARMATURES. 
 
 35 
 
 the lines of force emanating from the N. pole of the field 
 magnets flow through the iron core of the ring, and very 
 few across the air space inside the ring. Hence when 
 wires are wound on the ring, and the whole is revolved 
 about an axis perpendicular to the plane of the ring, only 
 the wires on the outside face of the ring cut lines of force, 
 those on the inside serving only to complete the electrical 
 circuit. So a smaller portion of the wire on a ring arma- 
 ture is in action than on a drum armature. 
 
 A drum armature of large diameter and of short length 
 in the axial direction has more wire exposed on its ends 
 than on its periphery. The pole pieces are sometimes 
 placed at the ends, and the armature is then called a Disk 
 Armature. This type is seldom used in this country. 
 
 32. The Field Magnets. Almost all dynamos have 
 their magnetic fields produced by electro-magnets. These 1 
 
 Fig. 17. 
 
 are called the field magnets. In small machines these are 
 usually bipolar, i.e., having one N. and one S. pole, with 
 
36 DYNAMO ELECTRIC MACHINERY. 
 
 the armature revolving between. In large machines it is 
 usual to use multipolar field magnets, in which any even 
 number of poles alternately N. and S. are arranged in a 
 circle with their faces concentric with the armature. 
 
 Bipolar machines are made in many forms, a few of 
 which are shown in Fig. 17. 
 
 The magnetizing coils may be on both legs of the mag- 
 net, on one leg, or on the yoke which connects the two legs. 
 In the double horse-shoe type there are four windings, one 
 on each of the four legs. Such a field is sometimes said 
 to be of the consequent pole type. 
 
 33. Capacity of a Dynamo. By 13, in a bipolar 
 machine the average pressure between brushes equals the 
 product of the number of lines cut into the number of in- 
 ductors cutting them, divided by the time in seconds of 
 one revolution. Since each line is cut twice in one revolu- 
 tion by each conductor, the formula for the E.M.F. pro- 
 duced by the machine is 
 
 ^ -- 8 
 
 60 icr 
 
 where V is the number of revolutions per minute, < the 
 total flux through the loops, and ^ the number of inductors. 
 In drum armatures 5 = twice the number of loops ; in ring 
 armatures 5 = the number of loops. 
 
 The capacity of a machine is measured by the watts it 
 can send out, hence the capacity varies as EL It is seen 
 from the foregoing formula that the of. any machine may 
 be increased by increasing either V, <, or 5. 
 
 The value of Fis limited, (i) by considerations of me- 
 chanical safety and economy, and (2) by the desirability, in 
 the case of a dynamo, of directly connecting it to the steam 
 
ARMATURES. 37 
 
 engine or other prime mover, and in the case of a motor 
 the connection of it to the machine it operates. The speed 
 of small machines is greater than that of larger ones ; but 
 the peripheral velocity, that is, the velocity of a point on 
 the exterior of the armature, for all sizes, lies between 25 
 and 100 feet per second on belt-driven machines, and be- 
 tween 25 and 50 feet per second on direct connected 
 machines. On large (say 2,000 K.W.) multipolar machines, 
 having great diameter of armature, these values are often 
 exceeded. 
 
 The value of < depends upon the size of the machine, 
 and the permeability of the metal of its frame. To get a 
 large and economical (B the metal parts of the field magnets 
 are designed to have a very low magnetic reluctance. The 
 air-gap between the pole pieces and the armature, and the 
 space occupied by the revolving inductors, are each made 
 small. The armature inductors are wound upon an iron 
 core of low magnetic reluctance. These cores are fre- 
 quently slotted and the windings laid in the slots. Besides 
 reducing, to a certain extent, the magnetic reluctance by 
 this construction, a good mechanical means is furnished 
 for driving and protecting the inductors. Wires wound on 
 the exterior of a plain cylinder, or smooth core, under the 
 influence of high speeds and the "magnetic drag" which 
 they experience have a serious tendency to rub one an- 
 other, and chafe the insulation to its final destruction. 
 The armatures having slotted cores, which are also called 
 toothed core armatures, are to be recommended for gene- 
 rators that will be obliged to work under wide variations of 
 load. They cost more to build than smooth-core arma- 
 tures. 
 
 The numbers of inductors 5 on an armature can be in- 
 
38 DYNAMO ELECTRIC MACHINERY. 
 
 creased by decreasing the size of the wire. Sufficient 
 cross-section must, however, be left in the inductors to 
 carry the maximum current of the machine without causing 
 a heating of the armature to such a point as to endanger 
 the insulation. Good practice calls for from 400 to 800 
 circular mils cross-section of armature conductor per am- 
 pere. The smaller values are for intermittently acting 
 machines elevator motors for example. The larger 
 values are for machines that run continuously, such as 
 central-station generators. 
 
 34. Eddy or Foucault Currents in Armature Cores. 
 
 It is evident that an imaginary axial lamina of the iron core 
 of an armature is a conductor moving in a field, and there- 
 fore has in it an induced E.M.F. Since this lamina in it- 
 
 self forms a closed circuit, currents, called Foucault or eddy 
 currents, will flow in it, Fig. 18, and their energy will 
 appear in the form of heat, which will produce an undue 
 elevation of temperature of the armature. To avoid this 
 the iron of the core is laminated at right angles to the axis 
 of revolution, and the laminae are insulated from one 
 
ARMATURES. 39 
 
 another. The heating due to eddy currents is proportional 
 to the square of the thickness of the disks or laminae. 
 Commercial and mechanical reasons limit the decrease of 
 thickness. In good practice the thickness of armature 
 disks varies from .01" to .06." 
 
 For insulation between the disks reliance is usually 
 placed on the iron oxid that forms on them during their 
 manufacture. Generally every six disks or so a further 
 insulation is interposed by the use of shellac, japan, or 
 paper. Milling slots in laminated armature cores after set- 
 ting up causes burrs. These bridge the insulation between 
 the disks, and militate against the advantages sought after 
 by lamination. For small armatures the disks are punched 
 whole from sheet -iron, with the teeth and holes for the 
 shaft. These punchings are assembled on the shaft, and 
 held in place by brass collars set down on either side of the 
 pile by nuts on the shaft or by similar devices. In large 
 machines, parts or segments of the whole periphery are 
 punched separately, and these are assembled with joints 
 staggered. These large laminae are not directly attached 
 to the shaft, but are mounted upon a spider, which in turn 
 is connected with the shaft. A complete spider and core 
 is shown in Fig. 19. 
 
 In large armatures it is usual to make ducts or venti- 
 lating passages in the core by occasionally separating the 
 disks by the interposition of blocks of insulating material. 
 Such ventilation carries off the heat, and lessens the rise of 
 temperature of the armature when in operation. 
 
 35. Rating of Machines. The American Institute of 
 Electrical Engineers recommends that all electrical and 
 mechanical power be expressed, unless otherwise specified, 
 
DYNAMO ELECTRIC MACHINERY. 
 
 Fig. 19. 
 
ARMATURES. 41 
 
 in kilowatts ; that the full-load current of an electric gene- 
 rator be that current which, with the rated full-load volt- 
 age, gives the rated kilowatts ; that all guaranties on heat- 
 ing, regulation, and sparking should apply to the rated 
 load, except where expressly specified otherwise ; that 
 direct current generators should be able to stand an over- 
 load of 25 per cent for one-half hour without an increase 
 of temperature elevation exceeding 15 C. above that 
 specified for full load ; and that direct current motors 
 should, in addition, be able to stand an overload of 50 per 
 cent for one minute. 
 
 Concerning the normal permissible elevation of tempera- 
 ture the following statements are taken from articles 25 to 
 3 1 of the Institute's Standardization Report : 
 
 " Under regular service conditions, the temperature of 
 electrical machinery should never be allowed to remain at 
 a point at which permanent deterioration of its insulating 
 material takes place. 
 
 " The rise of temperature should be referred to the stan- 
 dard conditions of a room temperature of 25 C., a baro- 
 metric pressure of 760 mm. and normal conditions of 
 ventilation ; that is, the apparatus under test should neither 
 be exposed to draught nor inclosed, except where ex- 
 pressly specified. 
 
 " If the room temperature during the test differs from 
 25 C., the observed rise of temperature should be cor- 
 rected by per cent for each degree C. Thus, with a 
 room temperature of 35 C., the observed rise of tem- 
 perature has to be decreased by 5 per cent, and with 
 a room temperature of 15 C., the observed rise of tem- 
 perature has to the increased by 5 per cent. The 
 thermometer indicating the room temperature should 
 
42 DYNAMO ELECTRIC MACHINERY. 
 
 be screened from thermal radiation emitted by heated 
 bodies, or from draughts of air. When it is impracti- 
 cable to secure normal conditions of ventilation on ac- 
 count of an adjacent engine, or other sources of heat, the 
 thermometer for measuring the air temperature should be 
 placed so as fairly to indicate the temperature which the 
 machine would have if it were idle, in order that the rise of 
 temperature determined shall be that caused by the opera- 
 tion of the machine. 
 
 " The temperature should be measured after a run of 
 sufficient duration to reach practical constancy. This is 
 usually from 6 to 1 8 hours, according to the size and con- 
 struction of the apparatus. It is permissible, however, to 
 shorten the time of the test by running a lesser time on an 
 overload in current and voltage, then reducing the load to 
 normal, and maintaining it thus until the temperature has 
 become constant. 
 
 " In apparatus intended for intermittent service, as rail- 
 way motors, starting rheostats, etc., the rise of temperature 
 should be measured after a shorter time, depending upon 
 the nature of the service, and should be specified. 
 
 " In apparatus built for conditions of limited space, as 
 railway motors, a higher rise of temperature must be 
 allowed. 
 
 " In electrical conductors, the rise of temperature should 
 be determined by their increase of resistance. For this 
 purpose the resistance maybe measured either by galva- 
 nometer test or by drop-of -potential method. A temperature 
 coefficient of 0.4 per cent per degree C. may be assumed 
 for copper. Temperature elevations measured in this way 
 are usually in excess of temperature elevations measured 
 by thermometers. 
 
ARMATURES. 
 
 " It is recommended that the following maximum values 
 of temperature elevation should not be exceeded : 
 
 COMMUTATING MACHINES. 
 
 Field and armature by resistance, 50 C. 
 
 Commutator and brushes by thermometer, 55 C. 
 
 Bearings and other parts of machine, by thermometer, 40 C. 
 
 " Where a thermometer, applied to a coil or winding, in- 
 dicates a higher temperature elevation than that shown by 
 resistance measurement, the thermometer indication should 
 be accepted. In using the thermometer, care should be 
 taken so to protect its bulb as to prevent radiation from it, 
 and, at the same time, not to interfere seriously with the 
 normal radiation from the part to which it is applied. 
 
 " In the case of apparatus intended for intermittent ser- 
 vice, the temperature elevation, which is attained at the end 
 
 of the period corresponding to 
 the term of full load, should 
 not exceed 50 C. by resistance 
 in electric circuits. In the case 
 of railway, crane, and elevator 
 motors, the conditions of ser- 
 vice are necessarily so varied 
 that no specific period corre- 
 sponding to the full-load term 
 can be stated." 
 
 The manner in which temper- 
 ature elevation is affected by size of load and duration of 
 full load is shown in Figs. 20 and 21. The temperature 
 of stationary surfaces rises about 80 when radiating one 
 watt per square inch. The rise is but 15 to 20 when the 
 
 300 
 
 200 
 
 100 
 
 8 
 
 1 1 1 
 
 1 
 
 
 
 
 
 
 g RJSE IN TEMPERATURE CURV 
 "p7 800 K. W. SIZE 280 
 g- DIRECT DYNAMO 
 " RUN LONG ENOUGH AT EACK LOAD 
 - ATTAIN A CONSTANT TEMPERATMI 
 U- CROCKER-WHEELER ELECTRIC CC 
 AMPERE, N. J. 
 
 E 
 
 _]_ 
 
 TO 
 
 / 
 
 ., 
 
 -/- 
 
 o" 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 1 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 V ' 
 z 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 Si 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 [^ 
 
 
 
 
 
 
 
 
 Sj 
 
 - 
 
 
 
 
 
 
 
 
 
 
 
 i 
 
 
 
 
 
 
 
 
 
 
 
 
 5 
 
 j 
 
 OA 
 
 
 TE 
 
 RM! 
 
 U 
 
 fl 
 
 LL 
 
 LO* 
 
 n 
 
 422 
 
 1 
 
 Fig. 20. 
 
44 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 surface is rotating at 3,000 feet per minute in such a 
 manner as the surface of an armature rotates, and amounts 
 
 <u 
 110 
 50 
 .40 
 30 
 20 
 10 
 
 
 | 
 
 
 
 
 
 
 
 
 
 
 
 ,. ' 
 
 
 
 
 _ 
 
 
 
 
 
 
 S 
 
 
 
 
 
 
 
 
 tff 
 
 -^ 
 
 
 
 
 
 
 
 
 
 
 
 
 | 
 
 
 
 
 
 
 ^ 
 
 X 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 x 
 
 
 
 TEM 
 300 
 D 
 
 UNO 
 
 CROCKEF 
 
 PERATURE CL 
 K. W. SIZE 
 IRECT DYNAIV 
 
 JRVE 
 280 
 
 
 OAD 
 
 RICCO. , 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 S 
 
 
 
 / 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 -WHEELER ELECT 
 AMPERE, N. J. 
 
 
 
 | 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 I 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 P 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 5 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 TIME 
 
 N 
 
 HO 
 
 URS 
 
 
 
 
 
 
 
 421 
 
 .1 
 
 1567 
 Fig. 21. 
 
 8 9 10 
 
 to but 10 to 12 at a speed of 6,500 feet per minute. 
 Within limits, the ratio of rise of temperature to radiation 
 per unit surface is linear. 
 
 36. Definitions Concerning Armature Windings. In 
 
 some dynamos the inductors and commutator segments are 
 not all electrically connected with each other. In such 
 cases the winding is called an open-coil winding. This 
 definition must not be made to include the double or mul- 
 tiple windings to be described later, where two or more 
 closed-coil windings on the same core are not electrically 
 connected to one another. Fig. 22 shows a primitive open- 
 coil winding. In this type only those inductors on whose 
 commutator bars the brushes may for the moment be rest- 
 ing are in series with the external circuit. All the other 
 inductors are cut out and idle. 
 
 Open-coil windings are used chiefly on arc-lighting dyna- 
 mos, and will be further discussed in a following chap- 
 
ARMATURES. 
 
 45 
 
 ter devoted to such machines. Closed-coil windings are 
 much more generally used. In this case all the inductors 
 are engaged all the time, save when short circuited at com- 
 mutation, in adding E.M.F. to the circuit. Although there 
 are many kinds of closed-coil windings, they are all alike 
 in that the inductors form one or more endless circuits 
 completely around the armature core. 
 
 Before showing some of the many types of closed-coil 
 winding it will be well to define some of the terms used. 
 
 Fig. 22. 
 
 By inductor is meant that part of the winding conductor 
 which lies on the face of the armature that sweeps past 
 the pole pieces, and is that part of the conductor in which 
 E.M.F. is induced. In the following descriptions when 
 one inductor is mentioned there may be in reality a num- 
 ber of wires ; and, again, a loop said to be formed by two 
 inductors may be a loop of many turns, but the connec- 
 tions and placing would be the same as if actually there 
 
46 DYNAMO ELECTRIC MACHINERY. 
 
 were only one inductor. It simplifies the diagrams to 
 treat the subject in this manner. 
 
 That part of an armature winding which is electrically 
 connected directly between two consecutive commutator 
 segments is called a coil. 
 
 A two-circuit winding is one in which the current, on 
 entering the armature at one brush, finds two paths by 
 which it reaches the other brush. Since a closed-coil 
 winding is endless, there must invariably be two paths 
 when two brushes are used. 
 
 K four-circuit or multi-circuit winding is one in which 
 the current finds four or more paths through the arma- 
 ture. There are at least two circuits for every pair of 
 brushes used in collecting the current, unless the commu- 
 tator bars are cross-connected, as in Fig. 25. 
 
 If a winding is so arranged that one commutator bar 
 under a brush carries all the current from one side of the 
 armature to that brush, then the winding is said to be 
 single. If, however, the windings are so arranged that 
 two or more bars convey this current to the brush at once, 
 or if the current is commutated at two or more points on 
 the contact surface of the brush, then the winding is said 
 to be double or multiple. Triple and quadruple windings 
 are not infrequent on machines which carry very heavy 
 currents. 
 
 A singly-re-entrant winding is one in which, by successive 
 angular advances, all the coils have been laid when an 
 advance of 360 has been made. To be doubly-re-entrant 
 wound the angular advance between successive coils, in the 
 order of their winding, is doubled ; and the whole winding 
 is not complete until the armature has been gone around, 
 angularly, twice, i.e., through an advance of 720. On the 
 
ARMATURES. 47 
 
 second time around the coils fill up the interstices left by 
 the doubled pitch of the first round. Triply and quadruply 
 re-entrant windings are used. In these the circuit passes 
 around the armature three or four times. Any closed-coil 
 winding, single or multiple, may be singly or multiply re- 
 entrant, the re-entrancy being reckoned as great as that of 
 any single winding on the armature. 
 
 The two principal types of closed-coil armatures are 
 the gramme or ring armature, and the drum. 
 
 37. Ring-Armature Windings. As the name implies, 
 the ring-armature core consists of an annular ring around 
 which the armature conductors are wound in a continuous 
 spiral, or two or more separate but interleaved spirals in 
 multiple windings. These are tapped off at equal inter- 
 vals to the commutator bars. In ring armatures there is 
 but one inductor per loop of wire, the return being on the 
 inside of the ring where there is no magnetic flux. This 
 winding, though less generally used than the drum winding 
 is simpler and much more easily illustrated, and will be 
 treated first. 
 
 Fig. 24. 
 
 Fig. 23 shows the simplest of all dynamo armature 
 windings. It is a bipolar, singly-re-entrant, two-circuit, 
 single winding. 
 
 Fig. 24 shows a four-pole, four-circuit, singly-re-entrant, 
 
4 8 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 single winding. The number of coils should be a multiple 
 of the number of poles to electrically preserve a balance in 
 the four branches or circuits. 
 
 Fig. 25 is the same as Fig. 24 save that the commutator 
 bars are cross-connected. The current that would flow 
 out of two brushes in the previous case, now flows out of 
 one brush. This form is seldom used, since it reduces by 
 half the brush contact surface, and thus doubles the heat 
 
 Fig. 25. 
 
 loss in the transition of the current from the commutator 
 to the brush. 
 
 Fig. 26 shows a four-pole four-circuit singly-re-entrant, 
 single winding, where only half as many bars are used as 
 there are coils. A disadvantage is that coils of considerable 
 difference of pressure are adjacent, thus increasing the 
 difficulty of properly insulating them. Ordinarily, if it be 
 desired to halve the 'number of bars, it is better to unite 
 two adjacent coils in series, and treat them as one. But 
 if the magnetic distribution be uniform, this method of 
 connecting two coils that are in different parts of the field 
 
ARMATURES. 
 
 49 
 
 in series averages up the inequalities and facilitates spark- 
 less commutation. 
 
 Fig. 27 shows a bipolar, two-circuit, singly-re-entrant, 
 double winding. The advantages of the double winding 
 are : the current is commutated 
 at two points of the bearing-sur- 
 face of the brush, and therefore is 
 only half as heavy at any one point 
 as when only a single winding is 
 used ; and the successive bars of 
 one winding are separated by the 
 width of one bar plus two insula- 
 tions, thus making the short cir- 
 cuiting of a coil by dirt, arc, or 
 injury very unlikely. 
 
 In multipolar windings a distinc- 
 tion is made between the " short- 
 connection " and the " long-connection " types. In the 
 short-connection type coils in adjacent fields are connected 
 
 in series, while in the long-con- 
 nection type coils twice as far 
 apart are connected together. 
 
 Fig. 28 shows a long-connec- 
 tion, two-circuit, four-pole single 
 winding. Here only slight dif- 
 ferences of potential exist be- 
 tween contiguous coils. 
 
 Fig. 29 represents a ten-pole, 
 long-connection, two-circuit, sin- 
 gle winding. In these long-con- 
 nection types, which are all more or less highly re-entrant, 
 small mention is made of the re-entrancy. Strictly accord- 
 
 Fig. 27 . 
 
50 DYNAMO ELECTRIC MACHINERY. 
 
 ing to definition, the winding in Fig. 29 is re-entrant nine 
 
 times. 
 
 Fig. 30 is a four- 
 pole, short -connection, 
 two-circuit, single wind- 
 ing. Besides the com- 
 plication of the wind- 
 ings, this form as well 
 as all other short-con- 
 nection windings, is 
 open to the objection 
 that the contiguous 
 coils have, periodically, 
 the full E.M.F. of the 
 machine between them, 
 
 making heavy insulation necessary. 
 
 Fig. 31 gives a four-pole, two-circuit double-wound 
 
 Fig. 30. 
 
 Fig. 31. 
 
 armature, and Fig. 32 gives a similar winding for a six- 
 pole machine. 
 
ARMATURES. 51 
 
 38. Drum Armature Windings. Windings for drum 
 armatures are more varied and more complex than those 
 for ring armatures, and are much harder to portray dia- 
 grammatically. But few will be shown. 
 
 The most simple of these windings is shown in Fig. 33. 
 The diagram shows the drum and inductors in section, 
 with the connections of the commutator end in full lines 
 and those of the back (pulley) end in dotted lines. Those 
 inductors marked with a 4- are supposed to carry a current 
 in the direction from the observer into the paper, and 
 
 Fig. 32. Fig. 33. 
 
 those marked with a are supposed to carry a current from 
 the page to the observer. Those not marked are parts of 
 coils short-circuited by the brushes. This winding was 
 devised by von Hefner-Alteneck, and may be used on any 
 bipolar armature having half as many commutator bars as 
 slots, or, if it be smooth core, as many bars as coils. If n 
 be the number of bars and 2n the number of slots, then 
 the wire is started at bar I, passed back through slot I, 
 across the pulley end to slot n (or sometimes n 2, in the 
 
52 DYNAMO ELECTRIC MACHINERY. 
 
 figure n = 8). It is then brought forward through slot n, 
 and attached to bar 2. From bar 2 it passes back through 
 slot 3, across the back end and forward through slot n + 2 
 and connects to bar 3. Thus passing back through the 
 odd-numbered slots and forward through the even-num- 
 bered slots, n coils can be made to fill the 2;/ slots and 
 each can be attached to its own commutator bars. 
 
 Fig. 34 is very similar to the last, save that the wires 
 are laid two layers deep, thus allowing the conductor that 
 
 passed through slot i to return 
 through slot n + I which is 
 diametrically opposite. Both 
 this winding and the last are 
 classed as two-circuit single 
 windings. 
 
 In a bipolar machine a chord- 
 wound drum armature is one in 
 which the two inductors of one 
 loop are appreciably less than 
 1 80 apart, so that the wire at 
 Fig - 34 - the back end is a chord rather 
 
 than a diameter of the circle of the drum. The advan- 
 tages of this winding are that, on a given drum, it de- 
 creases the total length of wire necessary to give a definite 
 number of inductors, and that it reduces the bunching and 
 overlapping of the wires at the pulley end of the drum. 
 The disadvantage is that it is impossible to secure a per- 
 fectly electrically balanced winding by this method. This 
 objection does not hold in the case of multipolar gene- 
 rators, hence all multipolar drums are chord wound. 
 
 In the following figures the numbered radial lines will 
 represent armature inductors, the lines inside of them will 
 
ARMATURES. 
 
 53 
 
 represent their connections to the commutator segments, 
 and the lines outside of them will represent the cross 
 connections between inductors at the pulley end. The 
 brushes are placed in- 
 side the commutator 
 for convenience and 
 clearness. 
 
 Fig. 35 represents a 
 six-circuit, single wind- 
 ing with 80 inductors 
 and 40 segments. In 
 practice the inductors, 
 instead of all lying be- 
 side each other, would 
 probably be wound one 
 on top of another in 
 one slot. 
 
 Fig. 36 shows a rather simple single winding. Although 
 
 it is four pole it is but 
 two circuit, in which it 
 resembles Fig. 37, which 
 is, however, a triple wind- 
 ing. 
 
 Fig. 38 gives a six- 
 pole, two circuit, double 
 winding. 
 
 In winding armatures, 
 double or triple cotton in- 
 sulated copper wire is 
 generally used. Care 
 Fig ' 36 ' must be taken to well in- 
 
 sulate the wires, both from each other and from the core. 
 
54 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 Many of these styles of winding create very complex 
 
 masses of wire on the 
 ends of the drum, and 
 great care must be ex- 
 ercised both in regard to 
 insulating and fastening 
 at these points, so that 
 the movement of the 
 wires under the influence 
 of the "magnetic drag" 
 may not chafe the insu- 
 lation and short-circuit 
 the conductors. Mica is 
 the best insulator, and is 
 used where flat sheets are needed ; but its great cost, and 
 the difficulty of manipulating it, result in the extensive use 
 of canvas, oiled paper, rubber tape, vulcanized fiber, and 
 many patented manufac- 
 tured insulators. Much 
 reliance is placed upon 
 the liberal use of japan 
 and shellac, especially in 
 conjunction with canvas. 
 Where very large 
 wires are used on the 
 surface of an armature, 
 eddy currents are set up 
 in them by reason of one 
 side of the wire being 
 in a stronger field than Fig * 38 ' 
 
 the other. To avoid this a number of smaller insulated 
 wires are wound in parallel, to take the place of the larger 
 
ARMATURES. 55 
 
 one, or what is more economical of space, thin copper bars 
 set edgewise take the place of the round wire. 
 
 In the winding of multipolar armatures it is possible to 
 use formed coils, which are wound on a separate collapsible 
 forming block, and are afterward applied to the core. This 
 method is advantageous in that better insulation can be as- 
 sured, and damaged or burned-out coils can be replaced 
 without disturbing all of the windings. Fig. 39 shows a 
 General Electric Company's formed coil, and Fig. 40 some 
 of the Crocker Wheeler coils. 
 
 Fig. 39- 
 
 All armatures, whether wound with wire, or formed coils, 
 or shaped conductors, must be banded around to prevent 
 dislodgement of the conductors under influence of cen- 
 trifugal action. The wire used for this purpose is gener- 
 ally of hard-drawn brass or of phospher bronze, and on 
 railway motors of steel. It is wound over insulating strips 
 forming a band of several turns. The completed turns 
 are often sweated together with solder. 
 
 Many manufacturers punch a small recess in each side 
 of the teeth near the face. A strip of maple wood is fitted 
 
DYNAMO ELECTRIC MACHINERY, 
 
ARMATURES. 
 
 57 
 
 Fi2. 
 
 Fig. 42. 
 
 into the recesses, and acts like a cover to the slot, firmly 
 holding the windings in place, and presenting a neat ap- 
 pearance. 
 
 Figs. 41, 42, 43 show respectively a core, a partially 
 wound, and a completed General Electric Company's arma- 
 ture. Figs. 44 and 45 show small Westinghouse types. 
 
DYNAMO ELECTRIC MACHINERY. 
 
 Fig. 43. 
 
 Fig. 44. 
 
 Fig. 45- 
 
ARMATURES. 59 
 
 39. Commutators. The segments or bars of a commu- 
 tator are always of drop-forged, or hard-drawn copper. 
 The insulation between them is always of mica. There 
 are various grades of mica ; and for insulating purposes the 
 amber-colored mica, which must be free from iron, is to be 
 preferred. Besides being a good insulator, amber mica has 
 the additional advantage that it wears at the same rate as 
 copper ; thus after long use it leaves neither elevations nor 
 depressions on the commutator surface. 
 
 In fastening the bars considerable ingenuity is displayed ; 
 for they must not displace themselves with reference to 
 the windings, neither must one bar lift so as to be above 
 the level of its neighbors. If the latter occurs, then, when 
 the bar comes under a brush, it will lift it ; and as the 
 high spot moves out from under the brush the contact is 
 broken until the spring can reseat the brush. This causes 
 excessive wear and destructive sparking. 
 
 After a commutator has been for a time in use, it becomes 
 grooved and pitted, a condition which causes further spark- 
 ing and wear, and the commutator must be turned down 
 again to a true surface. The design of a commutator 
 should allow of good operation after it has been subjected 
 to this treatment. 
 
 Mechanical friction and the electrical losses that accom- 
 pany commutation will raise the temperature of the com- 
 mutator about 5 C. above that of the armature. To 
 secure successful operation a commutator must be de- 
 signed with a sufficient number of bars, so that the differ- 
 ence of potential between two adjacent bars shall not 
 exceed 10 volts. This would mean that a loo-volt bi- 
 polar machine should have at least 20 bars. The potential 
 between the brushes or around kalffae commutator is 100 
 
60 DYNAMO ELECTRIC MACHINERY. 
 
 volts, hence half the commutator must have 10 bars. 
 There is no general rule for the length of a commutator 
 bar, but one may roughly say that there should be at least 
 one inch per 100 amperes. 
 
 Commutators should be designed so as to expose a suffi- 
 cient area to radiate the heat which is communicated to 
 them. Except in the case of some special commutators, 
 which are supplied with cooling devices, at a peripheral 
 speed of 2,500 feet per minute, the radiation of one watt 
 per square inch of peripheral radiating surface will re- 
 sult in a rise of temperature of 20 C. The permissible 
 rise of 55 C., therefore, allows a radiation of 2.75 watts 
 per square inch. The heat to be radiated is due to the fol- 
 lowing causes : 
 
 a. Friction between the brushes and the commutator 
 
 /2 X 7T X 746\ 
 
 bars. This is equal to I - 1 times the product of 
 
 V 33' / 
 the following quantities : The radius of the commutator in 
 
 feet, the speed in revolutions per minute, the coefficient 
 of friction between the brushes and the commutator (0.3 
 for carbon brushes and 0.25 for copper brushes), and the 
 sum of the pressures of all the brushes upon the commuta- 
 tor. This latter should amount to 1.25 Ibs. per square inch 
 of rubbing surface. Copper brushes permit 200 amperes 
 per square inch of rubbing surface, and carbon brushes 
 40 amperes. 
 
 b. The contact resistance between the brushes and the 
 commutator. As there is always a drop of about one volt 
 at each point of contact, and as there is a drop at both the 
 positive and negative terminals, the watts represented by 
 these contact resistances are numerically equal to twice the 
 current of the machine. 
 
ARMATURES. 6l 
 
 c. The energy represented in the sparking at the brushes 
 and the heat due to waste currents in the short-circuited 
 segments. These two losses cannot be accurately calcu- 
 lated, but may be estimated as equal to about 6 per cent of 
 the total commutator loss. 
 
 ItTica collar- under- segmen 
 
 Fig. 46. 
 
 Fig. 46 gives a broken-away view of a General Electric 
 commutator, showing the methods of attachment and insu- 
 lation. 
 
 40. Collecting Devices. These consist of the brushes, 
 the brusJi holders, and the rockers. 
 
 Brushes for high potential machines are of carbon. Car- 
 bon against copper causes less wear than copper against cop- 
 per, and further, the greater resistance of a carbon brush 
 results in less sparking when it bridges two commutator bars 
 than would the lower resistance of a copper brush. Com- 
 bination brushes of carbon and copper are sometimes used. 
 Carbon brushes are set at an angle generally, though some 
 makers set them radially ; and in motors that must be re- 
 
62 DYNAMO ELECTRIC MACHINERY. 
 
 versed, as is the case with railroad and elevator motors, 
 they are invariably set radially. A surface contact of one 
 square inch per 40 amperes is usual for carbon brushes. 
 
 On low-potential machines copper brushes, set at an angle 
 of 45 with the tangent to the commutator surface at the 
 point of contact, are invariably used. This is because there 
 is less natural tendency to spark on low voltages, and be- 
 cause the resistance of carbon 
 brushes would be too great a 
 fraction of the whole resistance of 
 the circuit, and cause a wasteful 
 drop of potential. Copper brushes 
 must have their ends filed to give 
 sufficient surface contact, and this 
 
 is generally done with the aid of &jig, illustrated in Fig. 47. 
 The abrasion of carbon brushes is accomplished by means 
 of glasspaper. 
 
 Brush holders should permit of a low-resistance contact 
 between the brush and the leads, they should provide ad- 
 justment as to position and tension of the brushes, and 
 they should be arranged so that none of the springs shall 
 get hot and lose temper while in performance of its duties. 
 The tension on carbon brushes varies from i to 10 Ibs. per 
 square inch of contact surface. The lower limit is to be 
 found in large central station generators, and the higher 
 limit in small machines and in motors which are subjected 
 to frequent and sudden strains, as railway motors. The 
 coefficient of friction between brush carbon and copper 
 varies from 0.28 to 0.32. 
 
 Figs. 48 and 49 plainly show a Crocker Wheeler rigging 
 with parallel-motion brush holders. Fig. 50 shows a form 
 of General Electric holder. 
 
ARMATURES. 
 
 Fig. 48. 
 
 CLAMPING SCREW 
 
 BRUSH 
 PRESSURE SPRING 
 
 ADJUSTING 
 SCREW 
 
 HARD ROLLED COPPER LEAVES 
 
 Fig. 49- 
 
64 DYNAMO ELECTRIC MACHINERY. 
 
 Rockers are rings or attachments carrying the brush 
 holders, and they are mounted concentric with the com- 
 mutator. They are made to give all the brushes of the 
 machine, or sometimes all the positive brushes or all the 
 negative brushes at once, a motion around the axis, thus 
 adjusting all brushes by one movement. Fig. 5 1 shows 
 such a rocker. 
 
 41. Shafts, Bearings, and Oilers. Since armature 
 shafts generally have high speeds, and almost always are 
 subject to sudden large variations of load, the shafts, the 
 
 Fig. 50. 
 
 bearings, and the oiling facilities must be well designed. 
 Wiener gives the following approximate diameters of steel 
 shafts for drum armatures : 
 
 For ioo watts ........ i inch, 
 
 For 1,000 watts 2 inches, 
 
 For 10,000 watts 4! inches, 
 
 all to be turned down at the bearings. 
 
 It is necessary that the bearing-boxes be exactly in line, 
 and a form of self -alignment bearing is frequently used. If 
 undue wear in the bearings occur, the armature is apt to 
 
ARMATURES. 65 
 
 sag till it strikes a pole piece, which will damage the arma- 
 ture. Many machines use ordinary oil cups to secure 
 lubrication, while others make use of some device, as is 
 shown in Fig. 52. The shaft revolves in a cylindrical brass 
 with a spherical enlargement at its middle which rests upon 
 
 P~ .,/ ' r " j " <r-^*5 
 I * CT U '\.m*r A 
 
 Fig. 51. 
 
 a corresponding spherical bed of Babbit metal. This se- 
 cures self-alignment. Two slots are cut radially in the 
 brass, and allow two rings to rest upon the shaft. These 
 rings are also of brass, and have an inside diameter slightly 
 larger than the outside diameter of the brass cylinder. 
 
66 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 The pillow block is hollowed away under these rings, the 
 hollows serving as receptacles for the storage of oil. As the 
 
 Fig. 52. 
 
 shaft revolves, the rings also revolve at such a rate as to 
 carry a steady stream of oil up into the slots, thereby 
 lubricating the bearing. 
 
FIELD MAGNETS. 
 
 CHAPTER IV. 
 
 FIELD MAGNETS. 
 
 42. Parts of Field Magnets. The parts of a dynamo, 
 exclusive of the armature, which make up the magnetic 
 circuit, belong to the field magnets. Fig. 53 shows a con- 
 ventional bipolar horse-shoe type with the parts plainly 
 marked. The field cores are the iron centers in the mag- 
 netizing coils. The yoke connects the cores together at 
 one end while the other ends terminate in the pole pieces, 
 
 Fig. 54- 
 
 one being a north magnetic pole, the other a south. The 
 side of the pole piece embracing the armature is styled the 
 pole face, and the latter's projecting edges are fittingly 
 called the horns. Some dynamos have the magnetizing 
 coils on the yoke, thus making the latter serve also as 
 core. In different types different numbers of pieces pre- 
 vail, thus all the parts (save the coils) might be cast in 
 one piece or each might be made separately. 
 
68 DYNAMO ELECTRIC MACHINERY. 
 
 In multipolar machines the designation of the parts is 
 somewhat different than in the case of bipolar machines. 
 The particular designation often depends upon the manu- 
 facturer. Fig. 54 gives the designation used by the 
 Crocker Wheeler Company. 
 
 43. Magnetic Material. The materials used for field 
 magnetic circuits are three, cast iron, wrought iron, and 
 cast steel. The selection of material for a given machine 
 is governed by considerations of (a) weight, (b] first cost, 
 (c) economy and satisfactory regulation when in operation. 
 
 Cast iron has the great advantage of cheapness ; but it is 
 poor magnetically, hence more weight and bulk must be 
 employed to perform the same service as the magnetically 
 superior wrought iron. It costs more in copper to magne- 
 tize a cast-iron core, because more turns will be required, 
 and each turn will be longer than if the core were of better 
 material. 
 
 Wrought iron is the best magnetic material available. 
 It is used either in forgings, or in the form of plates 
 punched from the sheet. In either form it is expensive ; but 
 since less weight in a given machine is necessitated when 
 this metal is used, it is often chosen where portability 
 is required, as in the case of the marine dynamos, electric 
 railroad motors, and particularly motors for automobiles. 
 
 Cast steel is intermediate between cast iron and wrought 
 iron, both in cost and in magnetic properties, and is much 
 employed in good practice. The use of different metals in 
 different parts of the frame is very general. For instance, 
 a cast-iron yoke is used with cast-steel cores and pole 
 pieces, or a cast-iron or steel yoke is used with wrought- 
 iron cores and pole pieces. 
 
FIELD MAGNETS. 69 
 
 44. Shape of Field Magnets. There is a great vari- 
 ety of shapes of field magnets. Formerly each manufac- 
 turer had a type peculiarly his own, and this led to many 
 forms, some of little merit. These freak types are now 
 disappearing, and a few general types are adopted more or 
 less by all makers. In all forms, however, the polar span, 
 or part of the armature circle that is covered by pole faces, 
 is from 65 per cent to 75 per cent, or from 234 to 270. 
 In general a small number of poles in the field magnets re- 
 quires less copper in the exciting coil than does a larger 
 number, and also the fields can be excited more economi- 
 cally. But in large bipolar machines successful operation 
 under varying loads requires a large air gap between the 
 pole face and the armature. This increases the magnetic 
 reluctance and the energy necessary for excitation. Multi- 
 polar machines do not require so large an air gap. Further- 
 more, increasing the number of poles gives the mechanical 
 advantage of allowing a lower armature speed without low- 
 ering the potential of the output. Multipolar machines 
 will run cooler than bipolars of the same economy of 
 operation. 
 
 Speaking generally, though it is by no means a rule, 
 bipolar fields are used up to about 10 K.W., four-pole fields 
 from 10 K.W. to 100 K.W., six-pole fields from 100 K.W. to 
 300 K.W., and beyond that point eight or more poles are 
 generally used. 
 
 45. Methods of Excitation of Fields. Dynamos are 
 classified according to the five methods of exciting the 
 fields of the machine. They are : the Magneto, the 
 Separately Excited, the Shunt Wound, the Series Wound, 
 and the Compound Wound. 
 
DYNAMO ELECTRIC MACHINERY. 
 
 The magneto generator, Fig. 55, is one in which the 
 field is a permanent steel magnet, generally of horse-shoe 
 type. 
 
 The separately excited dynamo, Fig. 56, has, as its 
 
 MAGNETO DYNAMO 
 Fig. 55- 
 
 SEPARATELY EXCITED DYNAMO 
 
 Fig. 56. 
 
 name implies, its field coils traversed by a current other 
 than that produced by the machine. Alternating current 
 machines are nearly always of this type. 
 
 The shunt-wound machine, Fig. 57, has a large number 
 of turns of fine wire wound on its core, and the ends are 
 
 SHUNT WOUND 
 DYNAMO 
 
 Fig. 57- 
 
 SERIES WOUND 
 DYNAMO 
 
 Fig. 58. 
 
 connected to the terminals of the machine, thus being in 
 shunt with the outside circuit. The ampere turns requisite 
 for excitation are obtained by passing a small number of 
 amperes through a large number of turns. 
 
 The series-wound generator, Fig. 58, has all the cur- 
 
FIELD MAGNETS. 
 
 rent that is produced by the armature passed through 
 large conductors wound with fewer turns around the cores. 
 The exciting coils are then in series with the external cir- 
 cuit. The ampere turns required for excitation are ob- 
 tained by passing a large current through a small number 
 of turns. 
 
 The compound machine, Fig. 59, is one in which there 
 are both shunt and series coils on the field magnets. This 
 method of winding is used for purposes of regulation under 
 varying loads, as will be explained later. Compound wind- 
 ings are of two classes, the long shunt and the short sJiunt. 
 In the former, the current used in the shunt windings is 
 
 COMPOUND WOUND 
 DYNAMO LONG SHUNT 
 
 Fig. 59- 
 
 COMPOUND WOUND 
 DYNAMO SHORT SHUNT 
 
 Fig. 60. 
 
 also passed through the field windings along with the main 
 current. In the latter, the current from the shunt coils 
 passes directly back to the armature, avoiding the series 
 turns. Figs. 59 and 60 clearly show the two methods. 
 The short shunt is generally preferred. 
 
 46. Field Coils. The coils of a dynamo must, without 
 undue elevation of temperature, supply sufficient ampere 
 turns to give the required excitation. This temperature 
 rise will not be excessive when about o. 3 5 watts are radiated 
 per square inch of outer surface of the coil. If no account 
 be taken of the ends of the pole and coil, 0.6 watt may be 
 
72 DYNAMO ELECTRIC MACHINERY. 
 
 allowed per square inch. The field coils have no ventila- 
 tion due to their own motion as have armatures, hence 
 about 1000 circular mils per ampere must be allowed in 
 the wire which composes such coils. The cost of copper 
 is needlessly increased, if more than the necessary cross- 
 section be allowed. 
 
 Fig. 61. 
 
 Field coils are usually wound on brass or iron spools, 
 shaped to slip over the cores. Sometimes, especially in the 
 case of small machines, the coils are wound on frames, 
 which can be collapsed and removed. The coils of series 
 machines and the series coils of compound machines are 
 
FIELD MAGNETS. 
 
 73 
 
 often wound with copper ribbon instead of wire, or are even 
 made up of forged copper conductors, having a rectangular 
 cross-section. This is because the heavy currents require 
 such large cross-section of conductor that if made of wire 
 much space would be lost between the wires. The rear 
 coil in Fig. 61 is a series coil of shaped conductors. This 
 figure shows both the shunt and the series coil, as wound 
 by the Westinghouse Company, for a compound multipolar 
 railway generator. The binding which is seen on the shunt 
 coils in both illustrations should not be mistaken for the 
 wires of these coils. Field coils are wound with double 
 cotton-covered copper wire. Further insulation between 
 coil and core, and between series and shunt coils, is effected 
 by the use of fiber, fuller board, and mica. 
 
 47. Magnetic Leakage. Since air is not an insulator 
 of magnetism, but is simply much less permeable than 
 iron, it is evident that some of 
 the lines of force generated by 
 the field coils will not follow 
 around the desired path through 
 pole pieces and armature, but will 
 take a path through the air and 
 be of no utility in creating E.M.F. 
 in the revolving armature. Fig. 
 62 roughly represents some of 
 the paths such lines may take. 
 
 If $t be the total flux caused by the field coils and < rt be 
 the flux that passes through the armature, then the coeffi- 
 cient of magnetic leakage, 
 
 , *, 
 =*.' 
 
 and is always greater than unity. 
 
 Fig. 62. 
 
74 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 In practice X. varies from 1.25 to 1.4 in single horse- 
 shoe fields, and in the Edison type of inverted horse-shoe 
 and in double horse-shoe fields it varies from 1.5 to 1.75. 
 In multipolar machines X varies from i.i to 1.5. 
 
 To find the coefficient of magnetic leakage of small or 
 moderate sized machines proceed as follows : 
 
 Arrange the field-coils for separate excitation by a cur- 
 rent that can be conveniently commutated. Suppose the 
 machine to have a field of the double horse-shoe type, as in 
 Fig. 63. Take a few turns of fine insulated wire about 
 the middle of one coil, as c, d, and connect the ends to a 
 
 Fig. 63. 
 
 ballistic galvanometer of low sensioility. A low-reading 
 Weston voltmeter will answer. Suddenly commutate 
 the current in the field coils. The change in direction of the 
 flux in the core, from -f <j> to <f>, will induce E.M.F. in the 
 test coil, which will give a throw to the voltmeter needle. 
 The deflection is directly proportional to the flux in the 
 core. Repeat with the other coil, and the sum of the de- 
 flections obtained from cd and ef is directly proportional 
 to the total flux produced < r Now make a test coil of the 
 same number of turns and of the same resistance about 
 the armature, in such a position ab that it includes the area 
 
FIELD MAGNETS. 7$ 
 
 of the armature that is cut by the greatest number of lines 
 of force. Upon commutating tr e current a throw of the 
 needle will result, which is propcitional to the flux in the 
 armature < a . Hence the coefficient of magnetic leakage, 
 
 <#> t _ defl. at cd + defl. at ef 
 <f> a deflection at armature 
 
 The exciting current must remain constant during the 
 investigation. 
 
 The location of the different leakage paths may be found 
 by using test coils on different parts of the frame. The 
 difference between the throws observed at any two places 
 is a measure of the leakage between those two places. 
 
 Clearly the number of lines choosing paths through the 
 air will decrease as the permeability of the iron circuit 
 increases. An increase in the reluctance of the main 
 magnetic circuit will increase the leakage loss. 
 
 Armature cores vary in permeability under varying con- 
 ditions of load. As the load increases, this change pro- 
 duces an increase in the reluctance of the main magnetic 
 circuit. This results in an increase of the loss by leakage. 
 The coefficient of magnetic leakage is, therefore, different 
 with different loads. 
 
 48. Pole Pieces and Shoes. In general practice the 
 field cores and the frame of a generator are worked at a 
 flux density of at least 15,000 lines per sq. cm. 
 
 This is too high a value to use in the air gap. Therefore 
 pole shoes are put on the ends of the pole pieces to dis- 
 tribute this flux over a wider area where it has to pass 
 through the air, and to thus decrease the total reluctance 
 of the magnetic circuit. 
 
76 DYNAMO ELECTRIC MACHINERY. 
 
 49. Effect of Joints in the Magnetic Circuit. Since no 
 two pieces of metal can be put together with a perfect 
 joint, there is always an increase of reluctance in a mag- 
 netic circuit when a joint is introduced therein. Professor 
 Ewing found by experiment that at low magnetizations 
 (3C = 7.5) the increase of reluctance of a certain bar of 
 iron due to a joint was above 20 per cent, and that for 
 high magnetizations (3C = 70) the loss due to one joint was 
 less than 5 per cent. The difference is probably due to 
 the fact that the pieces under strong magnetizations attract 
 themselves so powerfully as to make a more perfect joint. 
 Ewing also found that a single cut in a bar acted upon the 
 reluctance of the bar as though the length of the bar had 
 been increased by amounts given in the following table : 
 
 For3C = 7.5 15 30 50 70 
 
 Equivalent length of i cut 
 
 in cms. of iron ... 4 2.53 i.io 0.43 0.22 
 
OPERATION OF ARMATURES. 77 
 
 CHAPTER V. 
 
 OPERATION OF ARMATURES 
 
 50. Process of Commutation. The simple process of 
 commutation as described in 30 is attended with some 
 difficulties in practice. Consider one coil of a plain ring 
 armature with the commutator bars attached thereto as in 
 Fig. 64. In position A, when the brush is on only one of 
 the bars in question, the action of the other coils of the 
 armature will be to force current in this one coil in the 
 direction indicated by the arrow. B is considered to be 
 
 the positive brush. In position D, when the brush has 
 passed over to the other bar entirely, the direction of the 
 current in this coil is in the other direction. Now this 
 change of direction must occur when the coil is in a weak 
 field, for it is observed that the coil is short circuited while 
 in position C, the circuit being completed through the coil, 
 the bars and the brush spanning the mica insulation at o. 
 If now at this moment the coil should be in a strong field, 
 and should be cutting many lines of force, too large an 
 
DYNAMO ELECTRIC MACHINERY. 
 
 E.M.F. would be produced, and as the resistance of the 
 circuit indicated is very low, an excessively strong current 
 might flow. When the brush slips past o the circuit is 
 broken, and a more or less serious sparking occurs accord- 
 ing to the strength of the current flowing at the instant of 
 break. Commutation must then be effected when the coil 
 is in such a position as not to cut many lines of force. It 
 follows that every commutating machine must have at 
 least two places where the effective field has a zero value. 
 Fig. 65 gives a rectified curve of the magnetic distribution 
 
 Fig. 65. 
 
 under the pole pieces and around the armature of a well- 
 designed bipolar machine, the ordinates of the curve giving 
 the flux density in the air gap. 
 
 The neutral plane is a plane passed through the axis of 
 the armature and a point in the field immediately surround- 
 ing the armature, where the inductively effective com- 
 ponent has a zero value. The coil in position C, Fig. 64 
 is supposed to be in the neutral plane. 
 
 The commutating plane is a plane passed through the axis 
 of the armature and through the points of contact of the 
 brushes. The segments are supposed to be connected with 
 parts of the armature windings lying on the same radius. 
 
OPERATION OF ARMATURES. 79 
 
 51. Influence of Self-induction of the Commutated 
 
 Coil. When the coil in Fig. 64 is in position A the cur- 
 rent flowing in it produces magnetic flux in the ring inde- 
 pendent of any inductive action of the field magnets of the 
 dynamo, and links the flux with itself. When the coil is 
 in position D, there is also a magnetic flux and linkage, but 
 its direction has been changed. Therefore, in passing 
 through the position C, the current in the coil and the 
 accompanying flux linked with the coil have decreased to 
 zero, and have afterwards risen in value in the opposite 
 direction. 
 
 This change of flux produces an E.M.F. in the coil inde- 
 pendent of any action of the field magnets (see 15). This 
 E.M.F. is called an electromotive force of self-induction and 
 tends to continue the flow of a current which has been 
 started, and tends to prevent any increase or decrease in 
 the strength of the current and to prevent the stopping or 
 starting of the current. The value of this self-induced 
 pressure with a given flow of current varies as the square 
 of the number of turns in the coil, as the cross-section of 
 the coil, and as the permeance of the magnetic circuit. 
 Because of self-induction it is evident that, if commutation 
 take place in the neutral plane, there is a liability that it 
 will be accompanied by excessive currents in the short-cir- 
 cuited coils and consequently by sparking. This trouble 
 is to be avoided by revolving the plane of commutation 
 about the shaft of the machine until a sufficiently strong 
 field acts upon the short-circuited coil to induce an opposing 
 E.M.F. of the same value as the E.M.F. of self-induction. 
 Both the self-induced E.M.F. and the E.M.F. due to the 
 rotation of the armature vary in magnitude during the 
 time that a brush is upon two adjacent segments. Their 
 
8o 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 \ 
 
 Fig. 66. 
 
 manners of variation need not be alike, and hence it may 
 be impossible in some cases to effectively oppose one 
 
 against the other. The 
 obvious remedy is to be 
 sought in more com- 
 mutator segments or a 
 change of shape of pole 
 shoe. 
 
 52. Cross-Magnetiz- 
 ing Effect of Armature 
 Currents. Indepen- 
 dent of field magnets 
 the current flowing in 
 the armature conductor 
 will magnetize the ar- 
 mature core. The poles thus produced will be in the 
 plane of commutation. Fig. 66 shows the magnetizing 
 effect of the armature turns 
 on a ring armature. Fig. 
 67 shows a cross-section of 
 a drum armature and its 
 windings with the resulting 
 magnetization. 
 
 Thus, when there is a 
 load on a dynamo and the 
 armature conductors are 
 carrying a heavy current, 
 there are two coexistent 
 magnetic fields. This condition results in a skewing of 
 the lines of force, as is shown in Fig. 68. As the lines 
 are skewed the neutral plane is shifted. To produce spark- 
 
 Fig. 6 7 . 
 
OPERATION OF ARMATURES. 
 
 81 
 
 less commutation the commutating plane must also be 
 shifted. This causes a further skewing of the lines. The 
 limit of this double interdependent shifting is reached 
 when the magnetic lines have 
 become so crowded in the 
 trailing-pole tips that they are 
 almost insensible to a further 
 shifting of the plane of com- 
 mutation. 
 
 This skewing is a source of loss in the operation of a 
 generator because it increases the magnetic reluctance in 
 two ways, (a) by saturating the iron at the horns, and 
 thus reducing the permeability, and (b) by lengthening the 
 paths, both in air and in iron, that the lines must follow. 
 
 Fig. 69 shows a 
 curve similar to Fig. 
 65 taken when the gen- 
 erator was under load 
 and the armature was 
 traversed by a heavy 
 current, the flux being 
 distorted because of it. 
 It is evident that the 
 angular displacement of 
 the neutral plane depends in magnitude upon the relative 
 number of armature ampere turns as compared with the ef- 
 fective field ampere turns. The use of a strong field and a 
 large air-gap length requires a large number of field ampere 
 turns. Both are much used in practice with great success. 
 
 53. Demagnetizing Effect of Armature Currents. It 
 has been shown that it is necessary to have the commu- 
 
 Fig. 69. 
 
82 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 tating plane in advance of the neutral plane. The angle 
 between them is called the angle of lag or lead. If an 
 axial plane be passed through the armature, making with 
 the neutral plane an angle equal to the angle of lag or lead, 
 but on the opposite side of the neutral plane from the corn- 
 mutating plane, then the angular space between this plane 
 and the commutating plane is called the double angle of 
 lag or lead. The armature conductors, which create a 
 magnetism that tends to skew the lines of the field magnets 
 as shown in the last article, are called the cross turns. 
 
 They lie outside the double angle of lead. Those armature 
 conductors which lie within the double angle of lead are 
 called the back turns, because, when carrying a current, 
 their magnetic tendency is to send lines in a direction 
 exactly opposite to the lines of the field magnets. They 
 neutralize in a certain measure the action of the field turns. 
 This action is clearer shown in Fig. 70, which is a cross- 
 section of a bipolar drum armature. At a there is a north 
 pole due to the back turns which lie in the double angle, 
 and at b there is the corresponding south pole. The effect 
 
OPERATION OF ARMATURES. 83 
 
 of these poles is to neutralize some of the useful magnetic 
 lines flowing from N to S. At c there is a south pole 
 due to the remaining or cross armature turns and at d is 
 the corresponding north pole. These poles skew the lines 
 flowing from N to S. Compensation for back turns is 
 easily calculated, since the number of back turns times 
 the current in them at any load multiplied by the coeffi- 
 cient of magnetic leakage at that load ( 47) gives the 
 number of additional field ampere turns necessary at that 
 load for compensation. 
 
 54. Sparking. As shown in 51, sparking can be 
 avoided by giving the brushes a lead sufficient to bring the 
 coils they short circuit into fields sufficiently strong to coun- 
 teract the effects of self-induction. Sparking in the opera- 
 tion of machines is generally due to the misplacement of the 
 brushes, though sometimes it is due to irregularities of the 
 commutator surface. A high bar passing from under a 
 brush will leave the latter suspended in air a moment, which 
 will break the whole current through 
 the brush and cause a bad spark or 
 arc. 
 
 A machine may also suffer melting 
 of the commutator bars without any 
 visible sparking. Suppose a coil of 
 low resistance to be short circuited 
 by a copper brush as in Fig. 71. 
 When the brush is chiefly on one 
 bar, and over-laps the other very 
 slightly, then a very considerable part of the resistance in 
 the circuit is the transition resistance at the small contact. 
 Under an E.M.F. of self-induction a current of sufficient 
 
84 DYNAMO ELECTRIC MACHINERY. 
 
 magnitude may flow to produce enough heat in the trans- 
 ition resistance to melt the surface of the commutator bar. 
 The E.M.F. may then disappear before the brush leaves 
 the bar, and there will be no spark visible. 
 
 Sparking may be due to excessive electromotive force 
 between the commutator segments undergoing commuta- 
 tion due to the self-induction of the coil and to mutual 
 induction between it and other coils undergoing commuta- 
 tion at the same time. To be able to determine the value 
 of this induced E.M.F. one must know both the self and 
 mutual inductances, and the time rate of suppression of the 
 current in the coil. Parshall and Hobart state that in 
 practice one may assume that a coil of a single turn when 
 traversed by one ampere produces and links with itself 
 20 c.g.s. lines per inch net length of armature lamination. 
 From this datum one can calculate the values of the self- 
 inductance and mutual inductance. 
 
 A coil which is undergoing commutation must have its 
 current changed from a maximum value in one direction to 
 zero and from zero to a maximum value in the other direc- 
 tion during the time that the two segments at its ends are 
 connected through the brush. This time is evidently 
 dependent upon the peripheral speed of the commutator 
 and upon the width of the brush. It is equal to the time 
 that it takes a point of the insulation between the seg- 
 ments to pass over the breadth of the brush ; that is, the 
 time in seconds is equal to the breadth of the brush in 
 inches divided by the peripheral velocity of the commutator 
 in inches per second. The reciprocal of this time gives the 
 number of commutations per second, or what is termed 
 the frequency of commutation. The frequencies found in 
 practice lie between 200 and 500 per second. While all 
 
OPERATION OF ARMATURES. 85 
 
 the current which traverses the coil is suppressed in one- 
 half the time taken for commutation, the manner of its 
 variation is unknown. Parshall and Hobart assume that 
 the current strength falls sinusoidally. An assumption of 
 a uniform decrease with the time yields results quite in 
 accord with practice. The value of the induced voltage 
 then will be equal to the product of the value of the corn- 
 mutated current and the sum of the mutual and self-induc- 
 tance divided by one-half the time occupied in completing 
 commutation. This value should not exceed 6 volts. 
 
 55. Prevention of Sparking The limit of the capacity 
 
 of a machine may be excessive sparking instead of exces- 
 sive heating, and therefore the suppression of sparking by 
 proper design of the machine is of utmost importance. 
 
 Sparking may be prevented : 
 
 a. By shifting the brushes till the short-circuited coil 
 is just under the fringe of the pole piece. This counter- 
 acts the effects of self-induction as explained in 51. The 
 reversal of the direction of flux in any but the short-cir- 
 cuited coils is to be avoided, since a loss of useful E.M.F. 
 would then occur. 
 
 b. By having a stiff field, that is, a field so strong as to 
 suffer very little skewing because of the armature cross 
 turns. There is then no lag. In practice, air-gap magnetic 
 densities vary from 2500 to 7500 lines per square centimeter. 
 The higher densities are to be found in the larger machines. 
 There is a general tendency to increase the density. 
 
 c. By nearly saturating the teeth of the armature core. 
 When the core teeth are nearly saturated, an increase of 
 load increases the reluctance very markedly, and the demag- 
 netizing effect of the back turns is restrained on increase 
 
86 DYNAMO ELECTRIC MACHINERY. 
 
 of load, because of the greater reluctance of the circuit. 
 This minimizes the shift of the commutating plane from no 
 load to full load, and is a device invariably employed on 
 railway generators and other machines that have to stand 
 severe changes of load without change of position of 
 brushes. 
 
 d. By using brushes of carbon, brass gauze, etc. In 
 machines of over 100 volts, carbon brushes are always 
 used. Besides their good wearing qualities, their resistance 
 prevents the flow of a large current in the short-circuited 
 coil in commutation, and thus a misplacement of the 
 brushes will not result in so violent a spark. In very low- 
 potential machines, as has already been said, carbon brushes 
 are impracticable, because their resistance causes a too 
 great fall of potential. So in these machines copper strip 
 brushes are employed when possible. When too much 
 sparking occurs with plain copper brushes, a brush of some- 
 what greater resistance is employed, such as copper gauze, 
 brass, brass gauze, etc., according to the requirements of 
 the case. 
 
 e. By slotting the pole pieces longitudinally. This in- 
 creases the reluctance offered to the lines due to armature 
 reactions, and so tends to prevent sparking. 
 
 f. By properly shaping the pole pieces. The distribu- 
 tion of flux should be such that a coil enters a weak field 
 first, and so gradually comes to the strongest part. If the 
 lines of force are allowed to crowd into the trailing-pole 
 tips, this gradual transition is impossible. If the horns are 
 farther from the armature surface than the body of the 
 pole face, then the air gap and consequently the reluctance 
 at the horns is increased, and the lines are compelled to 
 distribute themselves more symmetrically. A place suit- 
 
OPERATION OF ARMATURES. 87 
 
 able for commutation is then more readily found. One 
 may also resort to the shaping of the pole pieces by champ- 
 fering the corners, or by making the pole faces with a cir- 
 cle of greater radius than the armature. 
 
 The Sprague Electric Company, in its split-pole type of 
 the Lundell generator, avoids the distortion of the field 
 under full load, due to cross magnetizing turns, by making 
 use of a specially designed pole piece. Fig. 72 repre- 
 sents a cross-section of this generator, and shows the con- 
 struction of the pole piece. The magnetic flux which 
 enters the pole piece, divides between the two paths a and 
 b. Owing, however, to the greater span covered by the 
 shoe belonging to the part marked b, the magnetic reluc- 
 tance of that part is much smaller than that of the part 
 marked a. As a result, the flux does not divide itself 
 equally between the two paths. The part of the pole piece 
 marked b y under increasing excitation becomes saturated 
 before the part marked a. At normal excitation, the flux 
 density at b is above 16,000 lines per square centimeter, 
 while the flux density in a is but about 10,000 lines per 
 square centimeter. In other words, b is pretty well satu- 
 rated, while a has not been brought to a magnetization as 
 high as the knee of the magnetization curve. This satura- 
 tion of half of the pole piece is effective in preventing a 
 skewing of the field by the cross turns. This is shown in 
 Figs. 73 and 74, where Fig. 73 represents the development 
 of a 50 kilo- watt Lundell generator, and Fig. 74 shows the 
 distribution of flux along the line xy of Fig. 73. The 
 dotted line represents the distribution at no load, and the 
 heavy line the distribution at full load. This small dis- 
 torting effect of the cross turns permits the employment of 
 a small air gap without serious sparking. 
 
88 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 
 Fig. 73. 
 
OPERATION OF ARMATURES. 89 
 
 Ryan compensates for the magnetizing effects of the 
 armature winding by surrounding the armature with a sta- 
 tionary winding, which passes through perforations in the 
 pole faces. These stationary windings carry the whole 
 current of the machine. This method prevents all spark- 
 ing due to the distortion of the field, but it does not pre- 
 vent the sparking which is due to self-induction and mutual 
 induction of the armature coils. The latter sparking is 
 prevented to a certain extent by inserting a lug between 
 the pole horns, which is magnetized by a few series turns. 
 
 Pig. 74. 
 
 56. Energy Losses in Operation. Besides the energy 
 expended in exciting the field coils, there are losses of 
 energy in the armature and connections, as follows : 
 
 a. The bearing friction and the windage. This loss is 
 generally considered independent of load, but it is ques- 
 tionable whether the friction does not increase somewhat 
 under loads. This loss is from 1 5 per cent to 40 per cent 
 of the total loss. 
 
 b. The hysteresis loss in the iron of the core due to the 
 continued reversal of the direction of magnetism therein. 
 According to Steinmetz's Law, the hysteresis loss in watts, 
 
 h = 
 
90 DYNAMO ELECTRIC MACHINERY. 
 
 where Fis the volume of iron in cubic centimeters, & the 
 flux density, n the number of magnetic reversals per sec- 
 ond, and rj a constant depending in value upon the char- 
 acter of the iron. A table of values is given on page 29. 
 The value of & varies at different loads and at different 
 places, as was shown by Goldsborough, so this loss cannot 
 be said to be proportional to the speed or any power of 
 the voltage. The hysteresis loss is from 1 5 per cent to 
 40 per cent of the total losses. 
 
 c. Eddy currents in the iron and the copper conductors. 
 These might be expected to vary as the square of the 
 speed, but they do not for the same reason as in b. Be- 
 cause of the laminated structure of the core, and the 
 slight angular breadth of the conductors, this eddy loss is 
 of small magnitude, from 2 per cent to 10 per cent of the 
 losses. It may amount to 50 per cent of the losses in 
 the case of smooth-core armatures. Eddy currents in the 
 pole faces, which may be due to any variation in the re- 
 luctance encountered by the lines passing through the 
 poles, are reduced by an increase of air-gap length. They 
 are greatest with armature cores having slots with large 
 openings at the top, and least with armatures whose in- 
 ductors are threaded through inclosed channels in the core. 
 
 d. The armature resistance loss. This equals I^R, where 
 / is the total current of the machine, and R the resistance 
 of the armature measured between points rubbed by the 
 brushes which are drawing the current /. This is ex- 
 clusive of the transition resistance at the brushes. In 
 500 K. w. machines the I 2 R loss is about 2 per cent of the 
 total output. In 5 K. w. machines about 4 per cent, and 
 in smaller machines much greater. 
 
 e. The friction of the brushes against the commutator. 
 
OPERATION OF ARMATURES. 91 
 
 This loss varies about as the speed, and its importance is 
 generally underestimated. Carbon brushes press upon the 
 commutator with a force of from I to 12 pounds per 
 square inch of contact. Railway motors and similar ma- 
 chines have the larger value, while central-station gene- 
 rators have the smaller. The coefficient of friction between 
 carbon and copper varies from 0.28 to 0.32. 
 
 f. The resistance of the brushes and the transition re- 
 sistance of the brush contacts. The first loss varies as 
 the square of the current, and is of considerable magni- 
 tude in low-potential machines. The transition resistance 
 seems to vary inversely as the current, thereby always 
 causing a constant drop of voltage amounting to from i 
 to 1.5 volts per transition. 
 
 The heat produced by losses b, c, and d, being in the 
 armature itself, must be dissipated by the conduction, con- 
 vection, and radiation. Experience shows that from 2 to 
 2.\ watts can be radiated from every square inch of arma- 
 ture surface without causing a dangerous rise of tempera- 
 ture in the armature core. It is found that about 500 
 circular mils per ampere in the armature conductors brings 
 the loss d to such a point that, added to the losses c and 
 by they together give about 2 watts per square inch of 
 armature surface ; hence this value of 500 circular mils per 
 ampere is the mean of what is usually adhered to in winding 
 armatures of commercial machines. 
 
92 DYNAMO ELECTRIC MACHINERY. 
 
 CHAPTER VI. 
 
 EFFICIENCY OF OPERATION. 
 
 57. Efficiency. The following definition and discussion 
 of efficiency is taken from the report of the committee on 
 standardization of the American Institute of Electrical En- 
 gineers : 
 
 The "efficiency" of an apparatus is the ratio of its net 
 power output to its gross power input. 
 
 Electric power should be measured at the terminals of 
 the apparatus. 
 
 Mechanical power in machines should be measured at 
 the pulley, gearing, coupling, etc., thus excluding the loss 
 of power in said pulley, gearing, or coupling, but including 
 the bearing friction and windage. The magnitude of bear- 
 ing friction and windage may be considered as independent 
 of the load. The loss of power in the belt, and the in- 
 crease of bearing friction due to belt tension, should be 
 excluded. Where, however, a machine is mounted upon 
 the shaft of a prime mover, in such a manner that it cannot 
 be separated therefrom, the frictional losses in bearings 
 and in windage which ought, by definition, to be included in 
 determining the efficiency, should be excluded, owing to the 
 practical impossibility of determining them satisfactorily. 
 The brush friction, however, should be included. 
 
 Where a machine has auxiliary apparatus, such as an ex- 
 citer, the power lost in the auxiliary apparatus should not 
 
EFFICIENCY OF OPERATION. 93 
 
 be charged to the machine, but to the plant consisting of 
 machine and auxiliary apparatus taken together. The 
 plant efficiency in such cases should be distinguished from 
 the machine efficiency. 
 
 The efficiency may be determined by measuring all the 
 losses individually, and adding their sum to the output to 
 derive the input, or subtracting their sum from the input 
 to derive the output. All losses should be measured at, or 
 reduced to, the temperature assumed in continuous opera- 
 tion, or in operation under conditions specified. 
 
 58. Coefficient of Conversion. This has sometimes 
 been called the efficiency of conversion, but because of the 
 definition of the last paragraph it is better not to use the 
 word efficiency. The coefficient of conversion ft is the ratio 
 of the total electrical energy developed in the armature 
 winding to the total mechanical energy expended. 
 
 where P is the power expended in watts, /< the armature 
 current in amperes, and E t the E.M.F. in volts, developed in 
 the armature, ft is always less than unity, because of the 
 friction and windage of the armature, because of the eddy 
 currents in the core and conductors, and because of the 
 hysteresis of the core. 
 
 59. Economic Coefficient. (rj) This coefficient is equal to 
 the ratio of the useful electrical energy to the total electri- 
 cal energy developed in the armature circuit. It is always 
 less than unity because of the necessary loss of energy in 
 the exciting coils and in the armature coils. In the case 
 of a series dynamo, if we let 
 
94 DYNAMO ELECTRIC MACHINERY. 
 
 E t E.M.F. generated in volts, and 
 E = Terminal pressure in volts, then the economic coef- 
 ficient for a current of / amperes is 
 
 IE _ E 
 
 ^-JE,-^: 
 
 For shunt dynamos, 
 
 IE 
 
 Where /is the current in the outside circuit, /^the current 
 in the field coils, E the pressure at the terminals of the 
 machine, and E t the total pressure generated. 
 
 The efficiency of a machine e is evidently the product of 
 ft and rj. 
 
 For a series machine, / = / ( and 
 
 f t E t E IE 
 = /^=^r X t = -p' 
 
 For a shunt machine, / + I f I t) and 
 
 = , = = 
 
 -- 
 
 P (I+I f )E t ~ P 
 
 Hence the product of (3 and 77 for either machine is the 
 same, and corresponds to the definition of efficiency. 
 
 60. Separately Excited Dynamos. At a constant speed 
 and constant exciting current a nearly constant total pres- 
 sure (E t ) is generated ; and it is almost equal to the pressure 
 at the terminals at no load that is, on open circuit. This 
 follows from the equation for the average pressure, 
 
 where - is the number of revolutions per second, 5 the 
 oo 
 
 number of inductors, <j> the flux per pair of poles, and / the 
 
EFFICIENCY OF OPERATION. 
 
 95 
 
 number of pairs of poles. If the speed be varied, the pres- 
 sure will vary proportionally if no load is on the machine. 
 If, however, a current be taken off, then the demagnetizing 
 effects of the armature currents become evident in a 
 change of the value of <, and there will be a falling off of 
 pressure. The amount of this deviation is dependent upon 
 the composition and saturation of the magnetic circuit. 
 
 100 
 
 30 40 
 
 AMPERES 
 
 Fig. 75- 
 
 This effect is clearly seen in the curve in Fig. 75, where 
 the armature currents are measured in the X direction, and 
 the pressure in the Y direction, the conditions of speed and 
 exciting current remaining constant. 
 
 Let E t = the total volts produced, 
 
 E the volts at the terminals of the machine, 
 R a = the resistance of the armature, 
 R = the resistance of the external circuit, and 
 / = the current under these conditions. 
 
96 DYNAMO ELECTRIC MACHINERY. 
 
 Then for a separately excited machine, 
 
 E = IR, and 
 
 El PR R 
 
 and 
 
 In determining the efficiency of a separately excited 
 machine the energy lost in the exciting coils must be 
 charged against the coefficient of conversion. 
 
 The operation of any dynamo can best be studied by in- 
 spection of a curve which shows the relation existing be- 
 tween the current generated or supplied by the machine, 
 and the voltage under which it operated. Such curves 
 are called Characteristic Curves > and they are generally 
 plotted with currents for abscissae and volts for ordinates. 
 The characteristic curve for a separately excited dynamo 
 is that shown in Fig. 75. 
 
 61. Magnetos. A separately excited dynamo whose 
 field is maintained by a permanent magnet, instead of an 
 electric magnet, is called a magneto. These machines 
 from their similarity, both theoretically and practically, 
 should be mentioned together. Magnetos are, however, 
 generally alternating current machines with slip rings in- 
 stead of commutators. They are used in very great num- 
 bers in telephone subscribers' sets, and in many electrical 
 businesses for testing out the continuity of concealed con- 
 ductors, and in some cases for determining defective 
 insulation. To the armature is affixed a pinion, meshing 
 with a gear turned by hand. The alternating current pro- 
 
EFFICIENCY OF OPERATION. 
 
 97 
 
 duced is passed through the circuit whose continuity it is 
 desired to determine, and then passes through a polarized 
 bell which is caused to ring 
 These machines are manufac- 
 tured so as to ring through 
 an external resistance of as 
 high as 50,000 ohms without 
 undue effort at the handle. 
 The cut Fig. 76 shows a com- 
 mercial belt-driven magneto. 
 
 62. Series Dynamos. 
 Letting E, E 9 R, R a , and 7 
 have the same significance as 
 before, represent by R f the 
 resistance of the field winding, and by R b the resistance 
 of the brushes and transition contacts. Then 
 
 Fig. 76. 
 
 E = IR, 
 
 E t = 
 
 R f + R a 
 
 whence it follows 
 El 
 
 * - EJ- 
 
 The value of rj increases as R m R b , and R f approach zero. 
 R b is liable to be of greater importance than is imagined. 
 In low-tension machines all the resistances are small, and 
 care must be taken that R b does not unduly increase the 
 denominator of the expression for rj ; in other words, cop- 
 per brushes should be used on low-voltage machines. 
 
 The value of rj varies as R, but the load varies in- 
 versely as R ; hence rj is a maximum when the load is a 
 
9 8 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 minimum, and 17 = I when R = oo, or there is no load. 
 Fig. 77 is a curve showing relation between 17 and load. 
 
 1.0 
 
 LOAD 
 
 Fig. 77- 
 
 63. Characteristic Curve of a Series Machine. Fig. 
 78 shows the curves of a series dynamo. The curve of 
 total volts E t is very similar to the magnetization of a 
 
 magnetic circuit made 
 up of iron chiefly. It 
 falls below such a 
 curve (a) because 
 saturation causes in- 
 creased magnetic leak- 
 age, and hence the 
 value of <f> in the 
 
 equation^, = 
 
 1OAD 
 
 Fig. 78. 
 
 " is not proportional to 
 
 the total flux, and 
 (b) because of the demagnetizing and cross magnetizing 
 effects of the armature currents. The curve E, starts 
 above zero because of the residual magnetism in the cores 
 of the field magnets. If operated under constant load, a 
 
EFFICIENCY OF OPERATION. 99 
 
 series dynamo will give E, directly proportional to the 
 speed. 
 
 The straight line represents the loss or drop of potential 
 due to the resistances of the machine, R a , R b , and Rj. 
 Since drop of potential is proportional to the resistance, 
 this is a straight line, and must pass through the origin. 
 This loss line can be established by a point found by as- 
 suming the lost volts EI and solving for the current I from 
 the equation I (R a + R 6 + R 7 ) = Ej. For example, if the 
 resistances R a + Rj+ R/be assumed as 0.2 ohrn, then 10 
 volts would be lost in them only when 50 amperes were 
 flowing through them. A line drawn through the origin, 
 and a point on the characteristic curve diagram whose 
 coordinates were 10 volts and 50 amperes, would at every 
 point give the volts lost in sending the corresponding num- 
 ber of amperes. 
 
 The curve E showing the E.M.F. at the terminals of 
 the machine as a function of the current output is found 
 by subtracting the ordinates of the loss line from those of 
 E t and using the differences as the ordinates of E. In 
 practice E t cannot be directly found ; but the terminal volts 
 and the current can be measured, thus giving the curve E, 
 and from a knowledge of the loss line the curve E t can be 
 derived. 
 
 The operation of some special forms of series machines 
 will be discussed in the chapter on arc-lighting machines. 
 
 64. Power Lines. Where volts and amperes are used 
 as ordinates and abscissae, lines can be drawn connecting 
 points of constant product of the two, representing watts 
 or power. Fig. 79 shows such lines drawn for one, two, 
 and three kilowatts. If E be the external characteristic of 
 
100 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 a dynamo, then the curves make it apparent that the ma- 
 chine cannot generate 3 K.W., but that for most values 
 
 under 3 K. w. 
 there will be two 
 loads under which 
 the generator can 
 run and yield the 
 same voltage. 
 
 65. Shunt Dy- 
 namos. In 
 
 shunt-wound ma- 
 chines the cur- 
 rent in the arma- 
 ture is the sum of 
 the current in the 
 field coils and of 
 that in the ex- 
 ternal circuit, or 
 
 20 30 
 
 AMPERES 
 
 Fig. 79- 
 
 I a = If + / For sake of simplicity we will assume 7 a = /. 
 
 Practically this introduces but a small error under ordinary 
 
 conditions of load. 
 
 E* 
 R 
 
 IE, 
 
 IjE 
 
 + i* 
 
 , 
 
 R 
 
 + 
 
 R 
 
 A -L\-n A -- * 
 
 ~f? + ~& + J? * +~J?+~K 
 J\. J\ J\.j J\. J\.f 
 
 To determine what value of R will enable a given ma- 
 chine to operate with a maximum economic coefficient 
 
EFFICIENCY OF OPERATION. . , , ,, Ipl, 
 
 place the differential coefficient of rj, in respect to R con- 
 sidered as a variable, equal to o and solve for R 
 
 dR 
 
 The external resistance must be a mean proportional be- 
 tween R a and R fy and the maximum economic coefficient is 
 
 i + 2 V^ 
 
 66. Characteristic Curve of a Shunt Dynamo. In Fig. 
 80 the curve E is plotted from experimental results obtained 
 while the machine is running at various loads. To get satis- 
 
 factory results, one should begin with an infinite resistance 
 in the external circuit, which is then reduced step by step. 
 In some small machines it can be reduced to zero without an 
 extreme elevation of temperature due to excessive currents. 
 As a rule, only the upper and lower values of E, correspond- 
 ing to currents between o and a definite maximum value, can 
 
102 ' DYNAMO ELECTRIC MACHINERY. 
 
 be obtained. The loss line L is obtained by calculation as 
 before in the case of the series machine. The curve 
 showing the relation between external current and total 
 volts, E t > is obtained by adding the ordinates of L to those 
 of E. The drop in E is at first due chiefly to the drop re- 
 sulting from armature resistance. As the current increases, 
 the effects of armature reaction and saturation of the 
 magnetic circuit become evident. At the same time E is 
 affected by a decrease of the shunt-field current due to 
 the fall of potential at the terminals of the field circuit. 
 This soon becomes the predominating cause of drop, and to 
 such an extent that the curve turns back toward the origin. 
 When zero resistance is in the external circuit, of course no 
 current flows through the field, and the few volts then 
 produced are due to residual magnetism. It must be 
 remembered that while E is a double-valued function of / 
 it is a single-valued function of R. 
 
 The voltage of a shunt machine generally increases more 
 rapidly than the speed. An increase of speed not only in- 
 creases primarily the number of volts generated, but also 
 increases the armature flux < because of increased excita- 
 tion. The condition of the magnetic circuit as regards 
 saturation determines whether this secondary influence 
 shall be great or small. 
 
CONSTANT POTENTIAL DYNAMOS. 103 
 
 CHAPTER VIL 
 
 CONSTANT POTENTIAL DYNAMOS. 
 
 67. Constant Potential Supply The method of sup- 
 plying, at any point of usage, current at a constant poten- 
 tial irrespective of the load which is there or elsewhere, is 
 used in the distribution of electrical energy for purposes 
 of incandescent electric lighting, for consumption in con- 
 stant pressure motors, and for trolley-car propulsion. The 
 great sensitiveness of the candle power of incandescent 
 lamps to a change in voltage, the candle power varying 
 as the fourth power or more of the voltage, requires 
 that the pressure in lines used for lighting must not vary 
 by more than 3 per cent of its rated value. In street-car 
 work, where the load suffers tremendous variations, con- 
 stant potential supply is equally as imperative for satisfac- 
 tory operation. 
 
 68. Methods of Obtaining Constant Potential For 
 
 accomplishing this result many devices have been tried, 
 the more important of which are : 
 
 a Automatic variation of the resistance in the field cir- 
 cuit of shunt machines. 
 
 b Automatic change of the position of the brushes and 
 commutating plane. 
 
 c Automatic variation of armature speed. 
 
IO4 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 d Hand regulation of a resistance in series with a shunt 
 field coil. 
 
 e Self-regulation. 
 
 Of these the first three methods are no longer employed, 
 and either hand regulation or self-regulation or both to- 
 gether are relied upon to maintain the constant voltage 
 under varying loads. 
 
 69. Hand Regulation Inspection of the characteristic 
 
 curves of either the shunt or the separately excited dynamo 
 shows a drop in the voltage as the load increases. This is 
 due to the internal resistance of the armature and the 
 demagnetizing effect of armature reaction. In the formula 
 
 r- i jr. 
 
 for the E.M.F. of a machine, E = , the only quan- 
 
 tity that is practical to vary is </>. This can easily be 
 
 accomplished by regulating 
 the amount of resistance in 
 a rheostat, which is in series 
 with the field coils and which 
 therefore governs the amount 
 of current in them, as in 
 Fig. 81. 
 
 . In distributing current for 
 use among a number of con- 
 sumers the current is carried 
 to feeding-points which are 
 near the locality they supply, but may be distant from the 
 station. It is desirable to keep the pressure at these 
 points at a constant value, irrespective of the varying loss 
 of potential that is going on because of the resistance of 
 the conductors leading to them. To achieve this end the 
 
CONSTANT POTENTIAL DYNAMOS. 
 
 105 
 
 Edison system employs feeders to carry the current to the 
 feeding-points. Each feeder is accompanied by a pilot wire 
 imbedded in the insulation. At the feeding-point the pilot 
 wires are attached to the feeder terminals, and at the sta- 
 tion end are attached to a voltmeter, so that one can, in 
 the station, regulate the pressure not at the machine ter- 
 minals but at the distant distributing point. 
 
 70. Field Rheostats For varying the current in the 
 
 shunt fields of dynamos, it is usual to employ field rheostats 
 
 "L. 
 
 *MiiiiiniiiriunnH 
 
 Fig. 82. 
 
 which are mounted on the switch-board along with indicat- 
 ing instruments. A form of such rheostatic regulators is 
 the so-called Packed Card Rheostat, manufactured by the 
 General Electric Company. This derives its name from 
 
io6 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 the method of constructing it. A tube of asbestos, in- 
 closing a steel mandrel, is wound with a chosen amount of 
 German-silver wire or ribbon.- The tube is then removed 
 from the mandrel, and pressed into the form of cards as 
 shown in Fig. 82. These cards are then assembled, with 
 interposed asbestos, in sufficient numbers to make up the 
 required resistance of the rheostat. Iron plates, somewhat 
 
 Fig. 83. 
 
 wider than the cards, are introduced at intervals, and thus 
 increase the radiating surface. The whole is held together 
 by iron end plates and bolts, as shown in Fig. 83. Con- 
 tact bolts are connected with various points of the conduc- 
 ting part of the rheostat, and these bolts are connected 
 through a wiping-finger with the field circuit. Fig. 84 shows 
 a rheostat of this type built for regulating a railway gene- 
 rator and arranged to be placed on the back of a switch- 
 
CONSTANT POTENTIAL DYNAMOS. 
 
 ID/ 
 
 board with the regulating handle projecting in front. 
 For the largest generators resistances made of iron grids 
 supported in iron frames are employed. Both of these 
 constructions are fire-proof and easily repaired in case of 
 accident. 
 
 When large generators, such as are used in railroad work, 
 have their field circuits opened, the E.M.F. self-induced 
 by the disappearance of the flux in the fields is liable to 
 reach such a magnitude as to pierce the insulation of the 
 field coils and destroy their usefulness. To obviate this, 
 before the field circuit is broken, the field coils are con- 
 nected (Fig. 85) through a high discharge resistance, and 
 the current in them is allowed to die out slowly. It is 
 thus unattended with any destructive potential differences. 
 
io8 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 The Edison Electric Illuminating Company of New York 
 City, in the case of its Duane-street generators, allows the 
 field circuits to discharge themselves through an arc light. 
 Another form of field rheostat is the Carpenter Enamel 
 Rheostat, made by the Ward Leonard Electric Company. 
 In this rheostat the heat generated is not radiated directly 
 
 Binding Posts 
 
 Parallel Resistance" 
 
 Rheostat Switch 
 
 Pilot" Lamp Oc= 
 
 Tietd 
 
 /Irmatur-e 
 
 Fig. 85. 
 
 from the surface of the wire, but is conducted to a sup- 
 porting plate, which then becomes the radiating surface. 
 The resistance wires are surrounded with an enamel, which 
 attaches them to the supporting plates, insulates them 
 therefrom, and protects them from corrosion. Owing to 
 the increased radiating surface thus obtained, a shorter and 
 smaller wire can be used for a given volt-ampere capacity 
 
CONSTANT POTENTIAL DYNAMOS. 
 
 ICQ 
 
 than if the wire were merely exposed to the air. No con- 
 sideration of the mechanical strength of the wire enters 
 
 Fig. 86. 
 
 into the design of this resistance, since it is supported and 
 protected by the enamel. To further increase the radiat- 
 
 Fig. 87. 
 
 ing surface, the back of the plate is provided with raised 
 annular ribs. The'makers claim that this rheostat can radi- 
 
no 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 ate 5 watts for each square inch of one surface. Thus a 
 plate 10 by 10 inches will dissipate 500 watts. The 
 
 method of using iron radiat- 
 ing plates for purposes of 
 dissipating large amounts 
 of heat is to be found in 
 the rheostats of many man- 
 ufacturers. Wirt (Fig. 88) 
 incloses resistance wire or 
 ribbon in radiating plates, 
 insulating them from each 
 other by means of mica. 
 Other firms employ sand as 
 an insulating material. 
 
 7 1 . Self -Regulation . - 
 By far the most elegant 
 method of constant poten- 
 tial regulation is that in 
 which the main current of 
 the machine is utilized in 
 maintaining constant the magnetic flux < through the 
 armature. This is accomplished by passing all or the 
 greater part of the current produced in the armature a 
 few times around the field magnets, so that an increased 
 load on the armature increases the magnetizing ampere 
 turns of the field coils. These series turns, when rightly 
 proportioned, can be made to compensate for a part, for 
 all, or for even more than all of the drop. This device 
 can be used in connection with any other form of 
 excitation, as permanent magnets, separate excitation, 
 or shunt excitation. In the last case, the dynamo is 
 
 Fig. 88. 
 
CONSTANT POTENTIAL DYNAMOS. Ill 
 
 said to be compotmd wound, as described in 45. If 
 the machine is designed to maintain a constant pressure 
 at some distant feeding-point, instead of at the machine 
 terminals, the machine is said to be over-compounded, since 
 the potential at the terminals will rise on increase of load. 
 From 3 to 5 per cent over-compounding is frequent in 
 machines used to supply lighting circuits, and 10 per cent 
 over-compounding is usual in railway generators. 
 
 72. Economic Coefficient of a Compound Machine. - 
 To discover the value of 77 in this case, let R be the resis- 
 tance of the external circuit, R s the resistance of the series 
 turns, R s i the resistance of the shunt-field, and R a the 
 resistance of the armature. Then assuming that the cur- 
 rent in the armature is the same as in the external circuit, 
 an assumption which is warranted in the case of commer- 
 cial machines, 
 
 Considering R as a variable dependent on 77, and solving 
 for a maximum of 77 
 
 . -,-. + +_ 
 
 dR R &^ & 
 
 Hence it is seen that the maximum economic coefficient is 
 obtained, when the external resistance is the geometric 
 
I 12 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 mean between the shunt-field resistance and the sum of 
 the resistances of the series field and of the armature. 
 Under these conditions, 
 
 i 4-2 
 
 73. Efficiency of Compound Machines -- The efficiency 
 of a generator increases with the size, being quite low on 
 small machines, and sometimes very high on the larger 
 dynamos. Since the distribution of the magnetic and elec- 
 trical losses of a generator lies within the discretion of the 
 designer, it is possible to so design a machine as to have 
 
 its point of maximum effi- 
 ciency at full load or at a 
 smaller load, for instance, 
 at one-fourth load. The 
 two following cuts show 
 the relations between effi- 
 ciencies and loads on two 
 different machines. 
 
 100 
 
 
 44- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 90 
 80 
 70 
 60 
 50 
 40 
 30 
 20 
 10 
 
 
 
 -ft 
 
 X 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 M 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1 
 
 y 
 
 III EFFICIENCY CURVE 
 200 K.W V SIZE 224 
 DIRECT DYNAMO ~ \ 
 
 JM CROCK PE WHEELED ELEc'TBlc CO 
 
 - 
 
 o- 
 
 t 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ( 
 
 
 -PUT 
 
 Kl 
 
 0- 
 
 WA 
 
 FT, 
 
 
 
 4- 
 
 04 
 
 - 
 
 74. The Compounding 
 Rectifier --- The gradual 
 saturation of the fields of 
 a generator as full load 
 approaches causes the 
 E.M.F. of even a com- 
 
 80 ^20 160 ^00 m 
 Fig. 89. 
 
 pound-wound machine to sag at full load, or if the machine 
 is so heavily compounded that it maintains its potential at 
 
CONSTANT POTENTIAL DYNAMOS. 
 
 full load, its voltage will rise abnormally at some load less 
 than full load. To counteract this effect, the Crocker 
 Wheeler Company employs a device which is termed a 
 compounding rectifier. It consists of a suitable resistance 
 shunted across the ter- 
 minals of the series field 100 f 
 coils. The full armature 
 current therefore divides 
 between this rectifying 
 coil and the series coils. 
 As the load increases, 
 more current passes 
 through each, but the 
 coils are so designed 
 that this increase heats 
 the rectifier and causes 
 its resistance to increase, 
 while the resistance of 
 the series coils remains 
 practically unaltered. 
 
 Thus, as the load increases, a larger proportion of the whole 
 current passes through the series coils, and this compen- 
 sates for the sag in voltage that would otherwise have 
 existed. 
 
 70. Theory of Self -Regulation. To determine the 
 number of turns of wire necessary to be used in the series 
 regulating coils which are wound on the field magnets of a 
 compound machine, 
 
 Let n = number of shunt turns. 
 n' = number of series turns. 
 B = number of back turns. 
 
 20 40 
 
 80 100 120 140 160 
 Pig. 90. 
 
114 DYNAMO ELECTRIC MACHINERY. 
 
 X= number of cross turns. 
 J? a + s = the resistance of the armature plus that of the 
 
 series coil. 
 
 f sh = current in the shunt coils. 
 I = current in the armature and also in the series coils, 
 
 since they are practically the same. 
 E t = total pressure developed. 
 E = pressure at terminals. 
 A. = the coefficient of magnetic leakage. 
 (R = the reluctance of magnetic circuit when armature 
 
 is idle. Then 
 
 (H - = reluctance with current /in armature. 
 
 nl sh 
 
 Let <, <', <", = flux in the armature under different con- 
 ditions of working. 
 
 When no current flows in the armature, 
 
 , = * = 2C 
 
 io<RX ' 5 fltt" 
 
 When the current / flows in the armature, 
 
 (RX -X7 1 H- 
 
 / 
 
 where -= ^- Jl? f?, hence rt: represents the ratio of the 
 
 a nSsh 
 
 reluctance at no load to the reluctance with the load /. 
 The latter value is the greater because of the skewing 
 effect of the cross turns, a, therefore, is less than i . 
 
 The flux in the armature which is due to the shunt coils 
 only, when a current I flows in the armature circuit, is 
 
CONSTANT POTENTIAL DYNAMOS. 115 
 
 Thus under load the amature flux due to the shunt coils is 
 
 decreased in the ratio, 
 
 <}>" : <j> : : a : i. 
 
 The series turns must make up this loss, and also compen- 
 sate for the loss due to the back turns and for the electri- 
 cal losses due to the resistances of the armature and the 
 series coils. 
 
 Now , 
 and 
 
 For convenience let 
 * = 2 
 
 The first term of the right-hand member can be written 
 knl sh k (i a) nf sh , in which the expression knl^ repre- 
 sents the total voltage developed by the machine at no 
 load, which is therefore the terminal voltage at that load, 
 or in other words is the voltage for which the machine is 
 to be compounded. The equation for the terminal voltage 
 at the load / therefore becomes 
 
 E = knl sh k (i a) nl sh + \ka (n r B] l? a + s ] I. 
 Evidently, if E is to equal knl sh at any and every load, 
 
 - k(i -a) nl sh + \ka (n' - B) - a+s ] /= o, 
 whence 
 
 n = ~a 7" ~*~ + ka 
 
Il6 DYNAMO ELECTRIC MACHINERY. 
 
 Remembering that -= = -g? and also that the percentage 
 of electrical energy loss in the field/ = -j 100, 
 
 ' l ~ a A, R _L nI * R * + * 
 
 n = pn 4- JB -\ = 
 
 100 a 
 
 In this value for n' the first term gives the number of 
 series turns required to overcome the skewing due to the 
 cross turns ; the second term gives the series turns neces- 
 sary to compensate for the armature back turns ; and the 
 third term shows the number of series turns to balance the 
 loss due to the resistances of the armature and the series 
 coils. 
 
 The difficulty of applying this formula lies in finding a 
 suitable value for a. This differs in different machines, 
 having according to Jackson a value of from .75 to .85 at 
 full load. It is of course dependent on the load, and has a 
 value of unity for no load. 
 
 76. Views of the American Institute of Electrical 
 Engineers. The following statements concerning the 
 regulation of direct current apparatus are taken from the 
 report of the Standardization committee of the Insti- 
 tute : 
 
 The regulation of an apparatus intended for the gene- 
 ration of constant potential, constant current, constant 
 speed, etc., is to be measured by the maximum variation 
 of potential, current, speed, etc., occurring within the 
 range from full load to no load under such constant con- 
 ditions of operation as give the required full-load values, 
 the condition of full load being considered in all cases as 
 the normal condition of operation. 
 
CONSTANT POTENTIAL DYNAMOS. 117 
 
 The regulation of an apparatus intended for the gene- 
 ration of a potential, current, speed, etc., varying in a defi- 
 nite manner between full load and no load, is to be 
 measured by the maximum variation of potential, current, 
 speed, etc., from the satisfied condition, under such con- 
 stant conditions of operation as give the required full-load 
 values. 
 
 If the manner in which the variation in potential, cur- 
 rent, speed, etc., between full load and no load is not speci- 
 fied, it should be assumed to be a simple linear relation ; 
 i. e., undergoing uniform variation between full load and no 
 load. 
 
 The regulation of an apparatus may, therefore, differ 
 according to its qualification for use. Thus the regulation 
 of a compound-wound generator specified as a constant- 
 potential generator will be different from that it possesses 
 when specified as an over-compounded generator. 
 
 The regulation is given in percentage of the full-load 
 value of potential, current, speed, etc. ; and the apparatus 
 should be steadily operated during the test under the same 
 conditions as at full load. 
 
 The regulation of generators is to be determined at con- 
 stant speed. 
 
 The regulation of a generator unit, consisting of a gen- 
 erator united with a prime mover, should be determined at 
 constant conditions of the prime mover ; i. e., constant 
 steam pressure, head, etc. It would include the inherent 
 speed variations of the prime mover. For this reason the 
 regulation of a generator unit is to be distinguished from 
 the regulation of either the prime mover or of the gene- 
 rator contained in it, when taken separately. 
 
 In commutating machines as direct current generators 
 
Il8 DYNAMO ELECTRIC MACHINERY. 
 
 and motors, the regulation is to be determined under the 
 following conditions : 
 
 a. At constant excitation in separately excited fields, 
 
 b. With constant resistance in shunt-field circuits, and 
 
 c. With constant resistance shunting series fields ; i.e., 
 the field adjustment should remain constant, and should be 
 so chosen as to give the required full-load voltage at full- 
 load current. 
 
 In constant potential machines the regulation is the 
 ratio of the maximum difference of terminal voltage from 
 the rated full-load value (occurring within the range from 
 full-load to open circuit), to the full-load terminal voltage. 
 
 In constant current machines the regulation is the ratio 
 of the maximum difference of current from the rated full- 
 load value (occurring within the range from full load to 
 short circuit), to the full-load current. 
 
 In over-compounded machines, the regulation is the 
 ratio of the maximum difference in voltage from a straight 
 line connecting the no-load and full-load values of terminal 
 voltage as function of the current, to the full-load terminal 
 voltage. 
 
 77. Direct Driven Light Generators. The tendency of 
 modern engineering practice is to install lighting gene- 
 rators which are directly connected with the steam engines 
 which drive them. Owing to the inherent speed of engines 
 being smaller than that of generators, direct connected 
 armatures are designed to run at a lower speed than belt- 
 driven ones. Economical construction demands that they 
 be of the multipolar type. They require less floor space 
 per kilowatt than the belt-driven machines ; and this is a 
 question of considerable importance in many installations. 
 
CONSTANT POTENTIAL DYNAMOS. 
 
 119 
 
 They have a higher efficiency of operation consequent 
 upon the elimination of losses in belting and counter- 
 shafting. They also permit of operation of isolated plants 
 in residences and other places where the noise resulting 
 from belt-driven machinery would not be tolerated. 
 
 In order that standard generators may be easily con- 
 nected with engines of any make, and vice versa, commit- 
 tees from the American Societies of Electrical Engineers 
 and of Mechanical Engineers have recommended the 
 adoption of the following standard sizes, speeds, and arma- 
 ture shaft fits : 
 
 Sizes in K. W. Capacity . 
 
 
 5 
 
 7-5 
 
 10 
 
 15 
 
 20 
 
 25 
 
 35 
 
 Speeds in Rev. per Minute 
 
 
 45 
 
 425 
 
 400 
 
 375 
 
 35 
 
 325 
 
 310 
 
 Armature Fit in Inches . 
 
 
 3 
 
 3 
 
 3% 
 
 3% 
 
 4 
 
 4 
 
 4% 
 
 Sizes in K. W. Capacity . 
 
 50 
 
 75 
 
 IOO 
 
 125 
 
 J 5 
 
 200 
 
 250 
 
 300 
 
 Speeds in Rev. per Min. . 
 
 290 
 
 275 
 
 250 
 
 235 
 
 220 
 
 200 
 
 190 
 
 1 80 
 
 Armature Fit in Inches . 
 
 5 
 
 6 
 
 7 
 
 7X 
 
 8 
 
 9 
 
 10 
 
 ii 
 
 Fig. 91 shows a machine made by the Westinghouse 
 Electric Manufacturing Company in standard sizes of 100, 
 150, 200, 500, and 675 K. w., at 125 volts. The field 
 frame is circular and divided in a vertical plane. The pole 
 pieces are of laminated sheet steel, cast into the frame. 
 Projecting from the field frame are brackets, which hold 
 and carry the brush-holder mechanism. This consists of 
 a ring concentric with the axis of the armature. Upon 
 its rim is a gear, which engages with a worm operated by 
 a hand-wheel. The simultaneous shifting of the brush 
 can be accomplished by the turning of the hand-wheel. 
 The slotted armature disks are made of sheet steel, and 
 
120 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 are held together by cast-iron end plates. The disks and 
 end plates are mounted upon a cast-iron spider, which also 
 carries the commutator. The spider is fitted so as to be 
 pressed upon the engine shaft and keyed to it. The con- 
 
 Fig, gi. 
 
 ductors are bars of copper, which are forged into shape on 
 cast-iron formers wound and insulated with mica and ful- 
 lerboard. 
 
 Figs. 92 and 93 represent a front and rear view of a 
 
CONSTANT POTENTIAL DYNAMOS. 
 
 121 
 
 General Electric Company's Form L generator. The 
 frame, of a circular form, is divided in a horizontal plane, 
 and is made of soft cast iron. To it are bolted pole pieces 
 
 Fig. 92. 
 
 which are made of soft cast steel. A skeleton, circular, 
 disk-like brush-holder yoke is fastened to the frame by 
 means of three slots and bolts, and is capable of sufficient 
 angular rotation to permit of the proper adjustment of 
 
122 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 the brushes. The movement is accomplished by means of 
 a hand-wheel and pinion. The armature spider is so con- 
 structed that it receives the commutator as well as the 
 
 Fig. 93. 
 
 disks and the armature windings. It is open so as to offer 
 no obstruction to the free and thorough circulation of air 
 through it, which permits of a perfect ventilation. The 
 windings are of copper bars, and the end connections are 
 
CONSTANT POTENTIAL DYNAMOS. 123 
 
 supported by flanges which protect them from mechanical 
 injury. The commutator shell is pressed upon the arma- 
 ture spider. 
 
 Fig. 94. 
 
 The Crocker Wheeler Electric Company's direct-con- 
 nected and belt-driven generators differ from others which 
 have been described, chiefly because of the shape of the 
 field-magnet frame and the method of armature winding. 
 The field frame shown in Fig. 94 is circular in form, and is 
 
124 DYNAMO ELECTRIC MACHINERY. 
 
 divided in a horizontal plane. These frames are of cast 
 iron, and have short internal flanges on each side, which 
 mechanically strengthen the frame, and offer considerable 
 protection from mechanical injury to the field coils. The 
 round poles are of cast steel, cast-welded into the frame. 
 They are provided with removable cast-iron shoes, which 
 are clamped in place after the field coils have been put on. 
 The armatures, instead of being bar-wound, are wound 
 
 Fig. 95. 
 
 with solid copper wire of large sizes, which are triple 
 cotton covered. The conductors are threaded through 
 tubes which are placed one upon the other, and which are 
 made of micanite cloth and press-board rolled up on a 
 form and glued together. The brush holders and brush 
 rigging were shown in Figs. 48 and 49. 
 
 The Sprague Electric Company manufactures two types 
 of Lundell generators, both for direct connection and for 
 belt connection. They are, namely, the split-pole type, 
 which employs the principle laid down in paragraph 55 
 
CONSTANT POTENTIAL DYNAMOS. 
 
 125 
 
 for compensating for armature reaction, and the single-coil 
 type, which takes its name from the peculiar shape of the 
 field frame and poles, which permits of the use of but a 
 single field coil. Both frames are of the circular type, 
 
 Fig. 96. 
 
 the split-pole field being divided in a horizontal plane, 
 and the single-coil type being divided in a vertical plane 
 which is perpendicular to the axis of the armature. A 
 split pole, with its windings, is shown in Fig. 95. The 
 compound coil is placed nearer the shoe than the shunt 
 
126 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 coil, and both are kept in place by lugs, as shown in the 
 figure. Fig. 96 shows a 6-pole, single-coil type field- 
 
 magnet frame with its coil inclosed in the frame. The 
 brush holders which are employed on both types of ma- 
 chine are illustrated in Fig. 97, the brushes being of 
 
CONSTANT POTENTIAL DYNAMOS. 
 
 127 
 
 carbon used radially, and being perforated to receive a bolt 
 for clamping them to the holders. 
 
 The Bullock Electric Manufacturing Company's direct 
 connected generator, Fig. 98, has an external appearance 
 similar to that of the generators of other companies. It 
 
 is different from them, however, in having peculiarly con- 
 structed poles. These poles are made up of laminated 
 steel stampings, which are much thinner than are ordi- 
 narily used, and which have the peculiar shape shown in 
 Fig. 99. In assembling these stampings to form the pole, 
 every alternate one is reversed from the position which is 
 
128 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 indicated in the figure. The method of assembling is 
 shown in Fig. 100. After assembling, it will be seen that 
 the face of the pole for a short depth contains but one-half 
 as much iron as the main body of the pole. This results, 
 under normal excitation, in a saturated pole face. It has 
 the same effect in preventing distortion of the field under 
 the influence of armature reaction, as saturation of the 
 teeth of the armature core. The teeth can, therefore, be 
 
 Fig. 99- 
 
 Fig. 100. 
 
 operated at a smaller magnetic flux density. The hystere- 
 sis losses in the teeth can accordingly be made smaller. 
 The thinness of the stampings, and the ideally perfect 
 lamination of the pole face, permit the use of a smaller 
 ratio of tooth width to slot width, without the excessive 
 eddy current loss in the pole face which would occur in 
 ordinary machines. The possibility of using narrow teeth 
 results in a reduction of the inductances of the armature 
 coils. This facilitates effective commutation. 
 
CONSTANT CURRENT DYNAMOS. 129 
 
 -X X X X X- 
 
 CHAPTER VIII. 
 
 CONSTANT CURRENT DYNAMOS. 
 
 78. Direct Current Arc Lighting Generators. For 
 lighting by arc lights where considerable energy is e: - 
 pended at the points of illumination, and where these 
 points are separated from each other by considerable dis- 
 tances, it is sometimes economical and desirable to connect 
 the lamps in series and use a constant current. A single 
 line then completes a circuit of all the lamps (Fig. 101). 
 The line can be made 
 of much smaller wire 
 than in the case of a 
 constant pressure cir- 
 cuit, for on a constant 
 current circuit as the 
 load increases the power 
 
 or energy transmitted is Fig. 101. 
 
 increased by raising the 
 
 potential, the current remaining unaltered ; while in a con- 
 stant pressure circuit an increase of load is met by an 
 increase of current, and the wires of the line have to be of 
 sufficient size to safely carry the maximum. The size of 
 wire necessary is dictated, not by the energy transmitted, 
 but by the current flowing, hence a wire large enough to 
 supply just one lamp of a constant pressure circuit can 
 supply all the lamps of a constant current circuit. 
 
 O 
 
130 DYNAMO ELECTRIC MACHINERY. 
 
 The more general forms of arc lamps have what is 
 termed a spherical candle-power of 800, 1200, or 2000. 
 Lamps used in search-lights and in light-houses often ex- 
 ceed this in candle-power, and may consume many more 
 amperes. The arc lamp of 800 candle-power takes a cur- 
 rent of 4.5 amperes, that of 1200 candle-power 6.6 to 
 6.8 amperes, and that of 2000 candle-power 9.6 to 9.8 
 amperes. 
 
 An ordinary arc lamp, as it is trimmed and adjusted for 
 general use, requires between 45 and 50 volts to force its 
 rated current through it. A generator supplying a circuit 
 of say 2000 candle-power lamps with n such lamps in the 
 circuit must be capable of generating a constant current of 
 9.8 amperes. It must be able to regulate its pressure 
 between the limits of 50 and 50/2 volts. This is necessary 
 in order that it may operate all the lamps or any part of 
 the whole number. 
 
 The current of an arc-light machine must not exceed 
 nor fall below its normal value, no matter how suddenly 
 the load is varied ; for the slightest change, even for a very 
 brief instant of time, affects the quality of the light at the 
 lamps. It is obvious that some mechanical device could be 
 applied to an ordinary shunt -wound generator to cause it 
 to give constant current, either by changing the position of 
 the brushes or by varying the ampere turns of the field 
 coils. However, any such device would be slow of opera- 
 tion, and a sudden short circuit would cause a destructive 
 current to flow before the regulator completed its action. 
 It is, therefore, necessary to rely on the armature reactions 
 for regulation, since they vary simultaneously with the cur- 
 rent. All successful constant current machines are con- 
 structed on this principle. The machine is designed with 
 
CONSTANT CURRENT DYNAMOS. 13! 
 
 a field of very great magnetizing power, the armature re- 
 actions are very great, and thus the total flux effective in 
 producing E.M.F. is reduced. A slight increase of current 
 in the armature materially increases the armature reactions. 
 The effective flux is thus reduced, and the pressure falls 
 until the current returns to its normal value. Thus the 
 machine is completely and instantly self -regulating. Since 
 the field magnetization is kept constant and the machine 
 produces constant current the field coils are series wound 
 on all arc-light generators, and the cores of the field 
 magnets are worked at a very high magnetic density, since 
 the magnets are then more insensible to slight changes in 
 the magnetizing force. In commercial machines the den- 
 sities in the field cores are from 17,000 to 18,000 lines per 
 square centimeter for wrought iron or steel, and from 9000 
 to 1 1,000 lines for cast iron. 
 
 In the armature high magnetic density is also required 
 to prevent a sudden rise of voltage when the circuit is 
 broken. With no current in the armature, the total mag- 
 neto-motive force of the field magnets would be effective 
 in producing E.M.F., and a destructive rise of pressure 
 would result, since the total M.M.F. of the field magnets 
 is much greater than the normal effective M.M.F. But a 
 high magnetic density in the armature core leaves the latter 
 incapable of receiving such an increase of flux, and there- 
 fore destructive voltages are avoided. In practice the arma- 
 ture core is designed to have a density of from 1 5,000 to 
 20,000 lines per square centimeter at its minimum cross- 
 section. 
 
 A consideration of the foregoing theory of regulation 
 shows that the following conditions should obtain more or 
 less completely in a successful constant current generator : 
 
132 DYNAMO ELECTRIC MACHINERY. 
 
 (a) since the current is small, there must be a great num- 
 ber of armature turns ; (b) the magnetic field of the ma- 
 chine must be much distorted ; (c) the path of the lines of 
 force of the field coils must be long and of small area, so 
 the M.M.F. cannot be readily changed ; (d) the path of the 
 lines of force due to armature magnetization must be short 
 and of great area, so the M.M.F. of the armature will change 
 with the slightest change of current ; and (e) the pole piece 
 must be worked at a high density. 
 
 Evidently extreme difficulty is found in so designing the 
 different parts of the machine as to give proper considera- 
 tion to each of the conditions and yet produce a machine 
 that will regulate for constant current at all loads. This 
 leads to the introduction of automatic mechanical devices 
 for aiding in the regulation. These devices must not be 
 considered as being the sole regulators, for in every case 
 they are secondary to the natural self -regulating tendency 
 of the armature. In general they regulate for the gradual 
 and greater changes of load, while the armature reactions 
 take care of the smaller and more sudden fluctuations. 
 
 There are two general systems of regulating arc dynamos. 
 The first method is to cause the machine to develop an 
 E.M.F. in excess of that required for the load, and to then 
 collect an E.M.F. just sufficient for the load in hand. This 
 is done by shifting the brushes from the neutral plane (50). 
 In a closed-coil armature this causes a counter pressure to 
 be developed in those conductors lying between the neutral 
 and the commutating planes. This reduces the pressure 
 to the desired amount. In an open-coil armature the 
 brushes, when in the maximum position, connect to the 
 circuit those coils of the armature which at that instant 
 have the maximum E.M.F. generated in them. By shift- 
 
CONSTANT CURRENT DYNAMOS. 133 
 
 ing the brushes either way coils can be connected to 
 the circuit which have some E.M.F. less than the total 
 E.M.F. generated in them, and the amount of shifting 
 regulates the pressure on the line. 
 
 The second method of arc-light dynamo regulation is to 
 vary the magnetizing force in the field magnets just enough 
 to put the required pressure on the line. Since the mag- 
 netizing force is dependent on the ampere turns of the field 
 coils, it can be varied either by cutting out or short circuit- 
 ing some of the turns or by changing the current in them 
 by means of a variable resistance which is shunted across 
 the field terminals. In practice both these methods have 
 been used. 
 
 Whether regulation is effected by changing the position 
 of the brushes, or by changing the field excitation, sparking 
 will occur at the points of collection of the current if means 
 are not provided to avoid it. Non-sparking collection could 
 be obtained if the field were perfectly uniform all around 
 the armature. In general this condition is impracticable, 
 since it requires almost the whole armature to be covered 
 by the pole faces, and it requires the density in the gap 
 beneath them to be uniform. Considerations of magnetic 
 leakage and armature reaction render almost impossible the 
 satisfying of these conditions. Another and more prac- 
 tical method is to employ for collection at one terminal of 
 the machine two brushes connected in parallel. These are 
 moved in opposite directions, thus giving the effect of a 
 single brush of varying circumferential contact, whose 
 center can always be kept in the neutral plane. This 
 prevents bad sparking. The device is used quite success- 
 fully in practice. There is, however, some question as to 
 the advisability of resorting to it. 
 
134 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 9. The Brush Machine. Fig. 102 shows a standard 
 i6o-light Brush arc generator, made by the General 
 Electric Company. The armature revolves between the 
 
 Fig. 102. 
 
 opposed pole faces of two sets of field magnets. Like 
 poles are opposed to each other. The flux, therefore, 
 takes a path out of the opposing pole faces into the arma- 
 
CONSTANT CURRENT DYNAMOS. 
 
 135 
 
 ture core, and then circumferentially through the core and 
 out into the next pair of opposing pole faces. 
 
 Fig. 103. 
 
 The armature, Fig. 103, consists of a number of coils or 
 bobbins placed on a ring core of greater radial depth than 
 breadth, and the pole faces cover the sides instead of the 
 
 Fig. 104. 
 
 circumference. The bobbins are protected by an insulat- 
 ing box, shown in Figs. 104 and 105, but are not surrounded 
 by any masses of metal. This fact, coupled with the fact 
 that the armature is of .such a shape as to cause great air 
 
136 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 disturbance, insures exceptional ventilation of the armature, 
 and tends to prevent the " roasting out " of the coils when 
 subject to an overload. This machine is of relatively slow 
 speed, the larger sizes running at only 500 R.P.M. 
 
 Fig. 105. 
 
 At a given instant of time, the different coils on the 
 moving armature have E.M.F.'s of widely different magni- 
 
 Fig. 106. 
 
 tudes induced in them. The commutator, Fig. 106, is so 
 designed that it connects the coils of highest E.M.F. in 
 series with each other to the external circuit, and con- 
 nects the coils of medium E.M.F. in multiple with each 
 
CONSTANT CURRENT DYNAMOS. 
 
 137 
 
 other to the external circuit, while those of smallest E.M.F. 
 are cut out entirely from the circuit. 
 
 The bearings are self-lubricated by means of rings. 
 Since the poles are on the sides of the armature, side 
 
 Fig. 107. 
 
 play in the bearings must be prevented. To this end the 
 commutator end of the shaft is turned with six thrust col- 
 lars, as seen in Fig. 107, which are engaged by correspond- 
 ing annular recesses in the brasses. 
 
13* 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 Regulation on these machines is effected by a variable 
 resistance in shunt with the field coils; and as the field 
 current is changed the position of the brushes is also 
 changed, not to collect current at a lower voltage as de- 
 scribed in 78, but to obtain sparkless collection. These 
 two operations are performed by a regulator (Fig. 108), 
 
 Fig. 108. 
 
 which is attached directly to the frame of the machine. 
 The mechanism consists of a rotary oil-pump driven by a 
 belt from the armature shaft, a balance valve of the piston 
 type, and a rotary piston in a short cylinder, which is 
 directly connected to an arm sweeping the contacts of the 
 field-shunt rheostat. The valve is operated by a lever 
 
CONSTANT CURRENT DYNAMOS. 
 
 139 
 
 actuated by a controlling electro-magnet which is energized 
 by the whole generator current. At normal current the 
 valve is centrally placed, and the oil from the pump flows 
 around the overlapping ports into the reservoir without 
 effect (See Fig. 109). If the current rises above the nor- 
 mal, the armature of the controlling magnet is attracted, 
 the balance valve moves up, and oil enters the cylinder, 
 
 Fig. 109. 
 
 moving the rotary piston in a clockwise direction. The 
 shaft of this piston moves the arm of the rheostat, cutting 
 out resistance and thus lowering the field exciting current. 
 At the same time a pinion on the shaft, seen in Fig. 1 10, 
 actuates a rocker arm which moves the brush holders to a 
 position such that the collection by the brushes will be 
 sparkless. When the current returns to normal the 
 adjusting spring, seen in Fig. 1 1 1, returns the lever and 
 balance valve to the central position. If the current falls 
 
140 DYNAMO ELECTRIC MACHINERY. 
 
 Fig. in. 
 
CONSTANT CURRENT DYNAMOS. 141 
 
 below the normal, these operations are reversed. The 
 tension of the adjusting spring can be regulated from the 
 outside of the dust-proof case by a hard rubber knob. 
 From the nature of the case the parts are always well 
 lubricated. 
 
 It is claimed for this regulator that it can bring the cur- 
 rent back to normal from a dead short-circuit in from 3 J to 
 4 seconds. 
 
 80. The Westinghouse Arc-Light Machine Fig. 1 1 2 
 
 shows a 75 -light direct current arc-lighting generator, 
 made by the Westinghouse Electric and Manufacturing 
 Company. It is of rigid construction, the bearing sup- 
 ports being cast integral with the frame. For facilitating 
 transportation and repairs the yoke parts in the middle 
 on a horizontal plane. The bearings are of the self- 
 oiling, self-aligning type described in 41. The armature 
 shown in Fig. 1 1 3 is of the open-coil type, which gives a 
 unidirectional but not absolutely continuous current. The 
 slight pulsations of the current thus set up, while not 
 affecting the steady mean value of the current, cause a 
 slight constant vibration in the mechanism of the lamps 
 that helps overcome any tendency to stick or a failure to 
 feed the carbons. A unique feature of this armature con- 
 sists in its having two separate sets of windings on the 
 same core, each set having its own commutator. The coils 
 and commutators are so arranged that while a set of coils 
 of one winding is being cut into or out of the curcuit a set 
 of the other winding is supplying current to the line. It is 
 claimed for this method of connecting open-coil armatures, 
 that it yields a more satisfactory current, and obviates the 
 vicious sparking at the commutator found in other types 
 
142 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 of open-coil machines. The armature is made of lami- 
 nated steel sheets punched with T-shaped teeth between 
 the winding slots. The armature coils are wound on 
 
 ; 
 
 Fig. 112. 
 
 molds, and insulated and mounted on the armature as 
 shown in Fig. 1 1 4. They are held in place by wooden 
 wedges forced into the loops left at the ends of the arma- 
 ture. This construction admits of removing one coil for 
 repairs without disturbing any of the other coils. 
 
CONSTANT CURRENT DYNAMOS. 
 
 143 
 
 This machine differs from 
 the general type of arc-light- 
 ing machines in that it is 
 separately excited, a small 
 auxiliary machine generating 
 current for the field coils at 
 100 volts. This obviates the 
 possibility of danger from a 
 too high pressure resulting 
 from an open circuit. 
 
 Regulation is obtained by 
 careful design, so that the 
 armature reactions cause the 
 voltage to vary in just the 
 proper proportions, as de- 
 scribed in 78. The exciting 
 field current is regulated to 
 give the proper excitation by 
 a series rheostat. By this 
 means the line current can 
 be raised or lowered slightly 
 if desired, without affecting 
 the self-regulation. 
 
 Fig. 1 1 5 shows the double 
 commutator of this machine. 
 The segments are easily re- 
 moved and replaced in case 
 they wear or burn out. 
 
 81. The Wood Arc Dynamo. Fig. 116 shows a Wood 
 constant current dynamo for lighting 125 2000 c.p. lamps. 
 This machine claims an efficiency of 90 per cent on full 
 
144 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 load. The bearings are self -oiling, and may be removed 
 for repairs or inspection without removing the armature. 
 The armature has large radiating surface and shallow wind- 
 
 Fig. n 4 . 
 
 ing, and its temperature does not rise more than 40 C. 
 above the temperature of the room. This armature is of 
 the closed-coil type, requiring a commutator of many seg- 
 ments with but a small potential between any two adjacent 
 
CONSTANT CURRENT DYNAMOS. 
 
 145 
 
 ones. This fact, and the use of two brushes in parallel, as 
 explained in 78, obviates all sparking. 
 
 This machine operates by generating full pressure at all 
 times, and by automatically setting the brushes to take off 
 just such potential as is necessary. This allows of "regu- 
 lation without making use of rheostats, separate ex- 
 citers, wall controllers, motors, or relays. The regulating 
 
 Fig. 115. 
 
 mechanism is set in operation by a sensitive and rather 
 powerful electro-magnet excited by the main armature 
 current. This attracts a lever which is restrained by an 
 adjustable coiled spring. A variation in the current 
 strength causes this lever to throw into train one or the 
 other of two oppositely revolving fiber friction cones, which, 
 acting through gears and levers, shifts the brushes the re- 
 
146 DYNAMO ELECTRIC MACHINERY. 
 
 quisite amount, and also varies their angular contact or 
 collecting extent. All the delicate parts of this mechanism 
 are inclosed in the pillar supporting the commutator end 
 of the armature shaft, and are thereby protected from 
 
 Fig. 116. 
 
 injury, dust, and grit. The wearing surfaces of this regu- 
 lator are all large and the speed is slow, so that wear is re- 
 duced to a minimum. Without any change of adjustments 
 this regulator will operate when run either way, which is 
 an advantage when two or more dynamos are run from one 
 engine, and economy of space is essential, or in case of 
 accident to a prime mover. 
 
CONSTANT CURRENT DYNAMOS. 
 
 147 
 
 82. The Excelsior Arc Dynamo. This machine, Fig. 
 1 1 7, is a closed-coil ring armature generator, having pole 
 faces that cover both the sides and the circumference of 
 the armature. The interesting feature of this machine is 
 the method of regulation. The proper potential is sup- 
 plied to the line by using both methods of control in con- 
 junction ; that is, sections of the field windings are cut in 
 or out of circuit, and at 
 the same time the posi- 
 tion of the brushes is 
 shifted. The proper 
 motion of the field regu- 
 lator arm and of the 
 brush holder is obtained 
 by means of a small 
 motor whose field is 
 " sneaked " from the 
 main magnets of the 
 machine. This motor 
 is operated by a device 
 shown in Fig. 118. The 
 whole device is inserted 
 in series with one of the 
 mains from the gener- 
 ator. The right-hand 
 lever is of insulating material, with the contact blocks 
 a and b properly placed upon it. The left-hand lever 
 is of conducting material, and is capable of being attracted 
 by the electro-magnet which is excited by the main 
 current. The magnet and spring are so adjusted that 
 when the normal current is flowing, both a and b are 
 in contact with the left lever, and the current flows in the 
 
 Fig. 117. 
 
DYNAMO ELECTRIC MACHINERY. 
 
 three shunt paths, R y R v and R z . There will be no cur- 
 rent in the armature of the regulating motor, since the 
 potential at brush x is equal to the potential at brush y. 
 If now the line current becomes too strong the magnet 
 attracts the left lever to it and the contact at a is broken. 
 
 KV >/l 
 
 To Line 
 
 From Dynamo 
 
 Fig. Il8. 
 
 Immediately the current flowing through b divides at the 
 brush x, part going through J\z and part through the motor 
 armature and F^. The motor will then revolve in a given 
 direction, and by simple mechanical devices will cut out 
 sections of the field windings, and will shift the brushes 
 until the normal current is flowing, when contact is again 
 
CONSTANT CURRENT DYNAMOS. 149 
 
 made at a and the controlling motor stops. If the line 
 current drops below normal, the spring pulls the lever 
 away from the magnet and the contact at b is broken. 
 Part of the current then flows from y to x through the 
 motor armature. It therefore revolves in a direction op- 
 posite to that which it had before. The brushes on the 
 dynamo are shifted back again, and more sections of field 
 winding are put into circuit. 
 
 In practice the levers and the magnet are mounted on 
 the wall or the switch-board, while the regulating motor is 
 mounted on the dynamo frame. 
 
 When the current is broken at a or b, there is no serious 
 sparking, since there are always two circuits in shunt with 
 the break. The whole current of the dynamo does not 
 exceed ten amperes ; and the resistances R, R v and R z are 
 so proportioned that only a small portion of that flows 
 through a or b. 
 
 83. The Ball Arc Generator. Fig. 119 shows a double 
 armature, automatic regulating constant current generator, 
 made by the Ball Electric Company. Two independent 
 circuits, each automatically controlled, can be operated 
 from the one machine, since it has two distinct armatures, 
 commutators, arid regulators. The advantage claimed for 
 this arrangement is that the pressure has to be but half as 
 high as if the two circuits were united and fed by a single 
 armature. Yet if it be undesirable to bring the ends of 
 two circuits into the power-house, they can be connected 
 in series, and fed by the two armatures also connected in 
 series, and then the voltage per armature will be half that 
 of a single armature machine giving like results. 
 
 The armatures are of the closed-coil ring type. The air 
 
150 DYNAMO ELECTRIC MACHINERY. 
 
CONSTANT CURRENT DYNAMOS. 151 
 
 gap between pole faces and armature is short in length and 
 great in area, requiring a minimum of magnetic excitation. 
 The commutator is built up of a great number of segments, 
 the potential between any two adjacent segments not ex- 
 ceeding fifteen volts. This assures sparkless commutation. 
 
 This generator is regulated by shifting the brushes until 
 pressure of a suitable magnitude is collected. 
 
 A magnetic body placed in a magnetic field will tend to 
 rotate until the longest axis is parallel to the magnetic lines 
 of force. This principle is applied to the Ball regulator as 
 follows : A magnetic portion of the brush carrier is made a 
 part of the magnetic circuit, and is placed in a recess of the 
 dynamo frame. It tends to assume an axial position with 
 a force varying as the flux through it. As the line current 
 increases the flux increases, and the brush holder, which is 
 mounted on ball bearings, rotates, shifting the brushes the 
 required amount. The impulse to regulate is applied 
 directly to the brush holder, instead of being communi- 
 cated to it by by a more or less complex mechanism. The 
 magnetic tendency to shift the brush holder is opposed by 
 gravity. 
 
 84. The Thomson-Houston Dynamo. The Thomson- 
 Houston arc generator is of a type entirely different from 
 the other machines here described, not only in appearance, 
 but also in method of armature winding and of regulation. 
 A view of this machine is given in Fig. 1 20. Each field 
 coil has for its core an iron tube, flanged exteriorly at each 
 end to form a recess for the windings, and fitted at the 
 armature end with a concave iron piece that surrounds part 
 of the armature. This tube, with the flanges and the cup- 
 shaped end., is cast in one piece. The farthermost flange 
 
152 DYNAMO ELECTRIC MACHINERY. 
 
 of each field core is bolted to a number of wrought-iron 
 connecting-rods which hold the magnets in place, protect 
 the field windings, and take the place of the yoke of other 
 machines in completing the magnetic circuit. The mag- 
 nets are mounted on a frame, including legs and bearing 
 supports for the armature shaft. 
 
 The armatures of the older machines of this type are 
 spheroidal in shape, while the more recent ones have ring 
 armatures which are more readily repaired or rewound. 
 The winding of either form of armature is peculiar in that 
 only three coils are employed, set with an angular dis- 
 placement from one another of 120 degrees. In the ring 
 armature no difficulty is found in properly winding these 
 coils ; but in the old spherical armature the following de- 
 vice was employed to secure the windings, and give each 
 of them the same average distance from the pole face. A 
 hollow spheroidal iron core was keyed on the shaft. The 
 core had three rows of externally projecting wooden pins. 
 Between these pins the coils were wound, half of coil A 
 being wound first, then at 1 20 degrees distance half of coil 
 B was wound, covering parts of coil A. Then at 120 de- 
 grees from both A and B all of coil C was wound. Over 
 this, but in its proper angular position, the other half of 
 coil B was wound, and finally the rest of coil A was put in 
 place. By this arrangement the average distance of each 
 coil from the pole face or from the iron core was the same. 
 In either type of armature the inner ends of the three coils 
 are joined to each other, and are not attached to any other 
 conductor, an arrangement unique in direct current dyna- 
 mos. The outside ends are connected to the segments of 
 a three-bar commutator, from which the current is collected 
 by four copper brushes connected in multiple. 
 
CONSTANT CURRENT DYNAMOS. 
 
 153 
 
 Regulation is obtained by shifting the brushes in the 
 following manner. Fig. 1 2 1 shows the two possible rela- 
 
 Fig. 120. 
 
 tions between brushes and commutator that may exist at 
 any instant. Both brushes of each set may rest on one 
 commutator bar, or the brushes of one set may span the 
 
154 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 break between two of the bars. These conditions are re- 
 peated three times at each brush for each revolution. If 
 the dotted line shows the position where the maximum 
 E.M.F. is generated in the coils, then in Fig. 121 a the 
 two most active coils are connected in series with the out- 
 side circuit, while the coil near the position of least activity 
 is out of circuit. In Fig. i2ib the two less active coils are 
 in multiple with themselves and in series with the most 
 active coil and the external circuit. In practice the 
 
 Fig. 121. 
 
 brushes of a set are 60 degrees apart, leaving 120 degrees 
 between the leading brush of one set and the following 
 brush of the other set ; and since 1 20 degrees is the angular 
 measure of the length of a commutator bar, there is no 
 coil out of circuit at normal load, two being always in 
 parallel and in series with a third. If the current rise 
 above the normal the leading brushes move a small angle 
 forward, while the following brushes recede through three 
 times that angle. This will shorten the time that a single 
 coil gives its whole E.M.F. to the circuit, and will place it 
 more quickly in parallel with a comparatively inactive coil. 
 But such a movement will reduce the angular distance be. 
 
CONSTANT CURRENT DYNAMOS. 
 
 155 
 
 tween tne nearest brushes of the opposite sets to less than 
 1 20 degrees, hence the machine will be short circuited six 
 times per revolution, since one brush of each set will touch 
 one segment of the commutator at the same time. If the 
 current in the line falls below normal, then the brushes 
 close together, and the time that a coil is in series is 
 lengthened, and the time that it is in parallel with an 
 inactive one is lessened. 
 
 field 
 
 The arrangement for moving the brushes is shown in 
 Fig. 122. The leading brushes are shifted forward on an 
 increase of current merely to help avoid sparking. The 
 brushes are moved by levers actuated by a series magnet 
 A. This magnet is normally short circuited by the by- 
 pass circuit. On an undue rise of current this circuit is 
 broken by the series magnet B. A then becomes more 
 powerful, and the levers separate the brushes. While the 
 machine is in operation the circuit-breaker C is constantly 
 vibrating, and brushes adjusting to suit the load. A high 
 
156 DYNAMO ELECTRIC MACHINERY. 
 
 carbon resistance is shunted across C to prevent sparking 
 at that point. 
 
 As might be expected, with but three parts to the com- 
 mutator and collection made with small regard to the 
 neutral point, the sparking of this machine is such as to 
 speedily ruin the commutator and the brushes, if means 
 are not taken to suppress it. A rotary blower is mounted 
 on the shaft, and is arranged to give intermittent puffs of 
 air, which at the right moment blow out the spark. The 
 insulation between the segments is air, considerable gap 
 being left between them, and through these gaps the 
 sparks are blown. 
 
 85. Western Electric Arc Dynamo. Fig. 123 represents 
 this machine, which is regulated by means of shifting the 
 brushes. The brush and rocker are connected by means 
 of a link and a ball and socket joint with a long screw. 
 This screw is held in position by a nut. When the current 
 is normal, both the nut and screw revolve at the same 
 rate, and consequently there is no end movement of the 
 screw. The brush, therefore, remains stationary. An 
 electro-magnet, energized by a coil which is in series with 
 the main circuit, attracts an armature whose movement 
 towards the magnet is opposed by the action of a spring 
 which is susceptible of regulation. When the current has 
 too high a value, the electro-magnet attracts its armature 
 more strongly than ordinarily. The latter moves toward 
 the magnet, and by its movement catches a stop on the re- 
 volving nut, and thereby prevents the revolution of the nut 
 until the resulting longitudinal movement of the screw has 
 shifted the brushes sufficiently to bring the current to its 
 normal value. If the current be too weak, the spring 
 which is attached to the magnet armature overpowers the 
 
CONSTANT CURRENT DYNAMOS. 157 
 
 electro-magnetic attraction. The resulting movement of 
 the armature stops the rotation of the screw and permits 
 the rotation of the nut. This results in a longitudinal 
 movement of the screw and a shifting of the brushes in 
 the opposite direction. The stopping and starting of the 
 nut and screw is accomplished through the medium of 
 
 Fig. 123. 
 
 small triggers controlled by the armature of the series 
 magnet. The triggers are fastened to a gear rotated from 
 the main shaft by a belt. They engage with stops on the 
 nut and screw respectively. 
 
 Fig. 124 gives a sectional view of the regulator, and the 
 
158 DYNAMO ELECTRIC MACHINERY. 
 
 Fig. 124. 
 
CONSTANT CURRENT DYNANOS. 159 
 
160 DYNAMO ELECTRIC MACHINERY. 
 
 trigger which engages with the screw is shown at n, and 
 the one which engages with the nut is shown at m. 
 
 Fig. 125 represents the details of the armature con- 
 struction. The latter is ring wound with a large number 
 of turns in each section. 
 
MOTORS. 161 
 
 CHAPTER IX. 
 
 MOTORS. 
 
 86. Principle of the Motor Any direct current dy- 
 namo will act as a motor if supplied with current from some 
 external source. This source may be a constant E.M.F. 
 system, a constant current system, or any other system. 
 The rotation of the armature is a direct consequence of the 
 conditions laid down in 20. It is evident that if the 
 negative and positive terminals of a dynamo be connected 
 with the corresponding terminals of some external source 
 of supply, the direction of flow in the armature will be 
 reversed. Irrespective of the multipolarity of the field or 
 of the method of armature winding, the electro-dynamic 
 actions between the field and all the currents in the in- 
 ductors will conspire to produce rotation in one direction. 
 
 87. Direction of Rotation To determine the direction 
 
 of movement of an inductor carrying a current of known 
 direction in a magnetic field of known direction, one may 
 employ a modification of Fleming's rule. Thus in a dynamo 
 the thumb and two first fingers of the right hand deter- 
 mine the direction of induced E.M.F. as shown in Fig. 
 1 26. But in a motor the thumb and two first fingers of the 
 left hand can be made to determine the direction of motion 
 as shown in Fig. 127. 
 
 If in a dynamo the direction of the field flux be not 
 
162 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 altered, and if the armature be supplied with a current flow- 
 ing in the same direction as when the machine was operated 
 as a dynamo, the direction of rotation will be reversed. 
 Thus, if the positive brush of a dynamo be connected to 
 the positive terminal of an external source of supply, and if 
 the negative brush be connected to the negative terminal, 
 then the direction of current flow in the armature will be 
 reversed. The direction of rotation of the armature, in 
 
 FICLO MAGNET 
 
 FIELD MAGNET 
 
 DYNAMoTlRIGHT HAND. 
 
 Fig. 126. 
 
 MOTOR LEFT HAND. 1 
 
 Fig. 127. 
 
 series-wound machines, since the field flux has its direction 
 also changed, will be reversed. In shunt-wound, separately 
 excited, and magneto machines, since these do not have 
 their fields reversed, the direction of rotation will be un- 
 altered. Compound-wound machines will have the same 
 or reversed direction of rotation, depending upon whether 
 the magnetizing effect of the shunt coils is stronger or 
 weaker than that of the series coils. In a compound gen- 
 erator the actions of the shunt coils and the series coils are 
 cumulative, i. e., in the same direction ; but when used as a 
 motor the actions are differential, i. e., opposed to each other. 
 Motors are also wound so as to have cumulatively acting 
 series coils. 
 
MOTORS. 163 
 
 88. Speed Conditions If an electric motor be supplied 
 
 with electrical energy, it will vary its rate of rotation until 
 it has attained such a speed as will produce an equality be- 
 tween the input of energy and the output of energy. The 
 latter appears both as useful work and as losses. In the 
 case of a motor, speed acts toward electrical energy like 
 temperature in the case of heat energy. Temperature 
 always rises until the heat energy which is produced is 
 equal to the heat energy which is disposed of by con- 
 duction, convection, and radiation. 
 
 The electrical energy which is communicated to a motor 
 is transformed, a, into useful mechanical energy, which is 
 taken from the armature shaft either by a belt or by direct 
 connection ; b, into friction at the bearings and at the 
 brushes; r, into windage; d, into foucault and eddy currents; 
 and finally e, into ohmic heat energy in the motor's electrical 
 circuits. The energy required per unit of time to overcome 
 friction, windage, hysteresis, and foucault and eddy currents 
 increases as the speed of rotation increases. Nearly all 
 practical loads put upon a motor machinery in one form 
 or another require an increase of power for an increase 
 of speed. Therefore, if a given amount of electrical power 
 be communicated to a motor under load, the armature will 
 assume some speed of rotation, so that a balance between 
 the input and the output of energy is maintained. 
 
 89. Counter E.M.F. If the variation of losses and useful 
 energy with the speed were the only conditions governing 
 the speed, then there would result in practice variations of 
 speed through enormous ranges. But there is another con- 
 dition affecting the speed. The armature, by varying its 
 speed, not only governs the rate of expenditure of energy, 
 but also governs the amount of electrical energy received. 
 
164 DYNAMO ELECTRIC MACHINERY. 
 
 The armature of a motor revolving in a field under the 
 influence of supplied electrical energy differs in no respect 
 from the same armature revolving in a field under the in- 
 fluence of supplied mechanical energy. There is an E.M.F. 
 generated in it which is determined by the speed and quan- 
 tity of flux. For the same speed and the same flux there 
 would be generated the same E.M.F. in the case of a motor 
 as in the case of a dynamo. The direction of this E.M.F. 
 is, however, such as to tend to send a current in a direction 
 opposite to that of the current flowing under the influence 
 of the external supply of E.M.F., according to 87. 
 Therefore this pressure which is induced in the armature of 
 a motor is called counter electro-motive force. The current 
 which will flow through the inductors of an armature is 
 therefore equal to the difference between the supply E.M.F. 
 and the counter E.M.F. divided by the resistance of the 
 
 armature, or _ _ 
 
 T E* E c 
 
 a ~^r 
 
 For example, an unloaded i K.W. shunt motor having an 
 armature resistance of I ohm, when connected to a con- 
 stant source of potential supply of 100 volts, would not take 
 a current of 100 amperes as dictated by Ohm's law, unless 
 its armature were clamped so as to prevent rotation. If 
 undamped, its armature would assume such a speed that it 
 would have induced in it a counter E.M.F. of say 97.5 
 volts. The current then flowing in the armature would be 
 
 ioo 07. c 
 
 - = 2.5 amperes. 
 
 The power represented by this current, viz., 2.5 X ioo 
 watts, would all be expended in overcoming the losses of 
 the machine. 
 
MOTORS. 
 
 I6 S 
 
 90. Armature Reactions Since in a motor, for a given 
 
 direction of rotation and flux, the current in the armature 
 flows in a direction contrary to that 
 
 which it would have as a dynamo, 
 therefore the effect of the motor 
 armature cross turns is to skew the 
 field against the direction of rotation, 
 as in Fig. 128. Tnis increases the 
 magnetic density in the leading pole 
 tip, and decreases it in the trailing 
 tip. This necessitates, for sparkless 
 operation, a backward lead, or a lag, 
 of the brushes. If the brushes were 
 in the same place as when the ma- 
 chine was operated as a generator, 
 the direction of armature current 
 having been reversed, then the de- 
 magnetizing or back turns of the 
 generator would become magnetiz- 
 ing turns for the motor ; but with the brushes shifted to 
 a position of lag, then the motor has also demagnetizing 
 or back turns. 
 
 9 1 . Efficiency In a compound- wound motor connected 
 to a constant pressure supply, 
 
 Let R& resistance of shunt field coils, 
 
 ^a+ 8 = resistance of armature plus that of the series field 
 
 coils, 
 
 V= number of revolutions per minute, 
 T = torque given off at the pulley in pound feet, 
 E supply voltage, 
 
 f n = no-load armature current in amperes, and 
 / = armature current when torque T is yielded. 
 
 Fig. 128. 
 
166 DYNAMO ELECTRIC MACHINERY. 
 
 useful power output 
 
 The efficiency = - : J =- -- , 
 electrical power input 
 
 hence, 
 
 The useful power can be expressed as the difference 
 between the power input and the losses. Now at no load, 
 when there is no useful power output, the following rela- 
 
 tions exist : ^ 
 
 EI n -f - = power input, 
 <K sh 
 
 and 
 
 I^R a+s = P r = power in the armature and series coils. 
 
 Assuming the friction, the windage, the foucault current, 
 and the hysteresis losses to be constant and the same as 
 at no load, we have for their value a constant loss = the 
 no-load power input the variable loss. 
 Hence, 
 
 The constant loss = P f = EI n + P sh - P' , 
 
 where ^ 
 
 P sh = loss in shunt coils = - 
 
 ^sh 
 
 The efficiency under load will therefore be 
 
 El + P A 
 
 In a shunt motor R a + s represents the armature resistance 
 
 only ; hence, 
 
 I 
 e = - 
 
 In a series motor P sh = o ; hence, 
 
MOTORS. 
 
 I6 7 
 
 In the first of these three expressions for efficiency, 
 solving for that value of / which will give a maximum effi- 
 ciency, we have 
 
 <& (EI+P sh } (E - 2 S a+s ) - (EI-P f -I* a+8 )E _ 
 
 dl 
 
 whence 
 
 EI+P sh 
 
 = o, 
 
 , 
 
 " E* " E' 
 
 Fig. 129 gives a set of curves indicating the perform- 
 ance of a motor whose fixed losses are large. Fig. 130 
 
 Fixed/Losses in Shunt Coils, friction, foucault, and hysteresis. 
 
 Pouter Input 
 
 Fig. 129. 
 
 gives a set of curves for an exactly similar machine save 
 that the fixed losses are smaller. They might be con- 
 sidered as taken from the same machine as the first, but 
 with journals better oiled, and hence with less friction loss. 
 The difference in the efficiency curves is noticeable. 
 
i68 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 Abscissae in all cases represent power input. The ordi- 
 nate of P at any given load shows the power input at that 
 load. The constant ordinate of F represents the power 
 consumed by the fixed losses, which is constant for all 
 loads. The ordinates of V, measured from F, follow the 
 
 Fixed Losses in Friction, foucau It, hysteresis and shunt coils* 
 
 Pouter Input 
 
 Fig. 130. 
 
 law PR a+sy and represent the variable loss at various loads. 
 Therefore the total loss for any load is represented by the 
 total ordinate of Fat that load. The difference between 
 the power input and the losses gives the useful power, 
 which is represented by the difference of the ordinates of 
 
 P and V. The values of the ratio power ^ ^^ 
 
 power input 
 
 load are plotted in the efficiency curve. Comparing the 
 curves of the two machines, it is seen that to get a high 
 efficiency at full load the variable loss must be kept small, 
 while to obtain a good efficiency at small loads, the fixed 
 
MOTORS. 
 
 169 
 
 losses must be made small. The shape of the efficiency 
 curve can be controlled by a proper adjustment of the 
 relation which exists between the fixed and the variable 
 losses. 
 
 92. Starting Rheostats When the armature of a motor 
 
 is at rest there is no counter E.M.F.; and at the instant of 
 closing the circuit a destructive current would flow if a re- 
 sistance were not first inserted in the circuit, except in the 
 case of very small motors whose armatures have small 
 moments of inertia. As the speed increases the resistance 
 can be lessened without allowing too severe a current to 
 flow, and when full speed is obtained the resistance must 
 all be cut out to avoid loss. In order that counter E.M.F. 
 may be generated from the start, the shunt field circuit 
 must first of all be closed. These ends are obtained by the 
 use of a starting-box or rheostat, the wiring of the ordinary 
 type of which is shown in Fig. 
 131. Its main feature is a con- 
 tact arm pivoted at its center, 
 and revolving through almost 
 1 80, making various contacts. 
 This arm is connected to one 
 terminal of the supply as shown. 
 As it is slowly turned on, one 
 end of it first makes a connection 
 which completes the shunt field 
 circuit. Then the other end 
 makes a contact which closes 
 the armature and series coils 
 through the maximum resistance 
 of the starting-box. As the 
 speed increases, the revolving p^ , 3 , 
 
 Shunt 
 
DYNAMO ELECTRIC MACHINERY. 
 
 arm is made to cut out the resistance, piece by piece, until 
 it is finally all out of the circuit and the machine is run- 
 ning independent of the starting rheostat. 
 
 Fig. 132 shows such a starting-box as made by the 
 Crocker Wheeler Company. The brass contact points 
 
 and the arm are mounted 
 on a slate slab, which 
 serves as the top of an 
 open-work cast-iron box 
 which contains the resist- 
 ances in the form of spiral 
 coils of bare wire. The 
 wire is generally either of 
 German silver or of some 
 one of the special iron 
 alloy resistance materials. 
 A shunt motor may 
 have its armature coils 
 destroyed by an excessive 
 rush of current resulting from a dropping or ceasing alto- 
 gether of the supply voltage followed by a sudden renewal 
 after the speed of the armature has fallen. These condi- 
 tions may arise through accidents to mains or because of a 
 too heavy load on mains of insufficient cross-section. An 
 armature may also be burned out by an excessive cur- 
 rent due to overloading the motor. The resulting lower- 
 ing of its speed is accompanied by a corresponding lowering 
 of the counter E.M.F. Again, a too high supply voltage, 
 which might result from some cross or other accident might 
 cause a destructive rush of current. To meet these condi- 
 tions, starting rheostats are often made with attachments 
 for opening the circuit on no voltage or low voltage, and 
 
 Fig. 132. 
 
MOTORS. 
 
 I/I 
 
 Release Magnet) 
 
 Fig. 133- 
 
 Fig. 134. 
 
172 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 others with attachments for opening the circuit on overload. 
 Some have both attachments, but it is modern practice to 
 remove the overload attachment from the starting-box and 
 put it on the switchboard. Fig. 133 is a diagram of the 
 
 Armature 
 
 Details of Release Magnet 
 
 e e 
 
 Fig. 135- 
 
 wiring of a starting rheostat for a shunt motor with auto- 
 matic release and low-voltage attachment. Fig. 1 34 gives 
 a front view of this same instrument. When the starting- 
 
MOTORS. 
 
 173 
 
 handle is placed in the "on" position, the magnet in the 
 field circuit holds it there, although a spring tends to throw 
 it back. If now, because of low voltage, the current in field 
 and magnet becomes weak, the magnet is no longer able to 
 detain the handle, and the spring throws it to the " off " 
 position, where it stays until the motor is again turned on 
 by an attendant. 
 
 Figs. 1 3 5 and 1 36 show a view and a diagram of the 
 wiring of a rheostat with both release and overload attach- 
 
 Fig. 136. 
 
 ments. The former is similar to the one just described, 
 while the overload attachment consists of a magnet in the 
 armature circuit which on overload becomes strong enough 
 to attract to itself a pivoted iron arm supplied at its end 
 with a device which short circuits the field current around 
 
174 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 the release magnet. This causes the latter to let 
 starting-handle drop as in the case of low voltage. 
 
 the 
 
 93. Characteristic Curves of a Shunt Motor A shunt 
 
 motor, having a small R a and a large R sh , and having the 
 field well saturated, will give a fairly constant speed under 
 all loads, if supplied from a constant pressure circuit. 
 This is shown by the curves in Fig. 1 37, which are from 
 a bipolar, shunt-wound, 10 horse-power, 23<D-volt Crocker 
 
 Wheeler motor. 
 
 A shunt motor when 
 started cold on no load 
 quickly arrives at a speed 
 which then gradually 
 rises to a maximum. The 
 gradual heating of the 
 field coils increases their 
 resistance. This allows 
 less current to flow in 
 them, . and the resulting 
 magnetic flux is less. 
 Therefore the armature 
 must rotate faster to gen- 
 erate the same counter E.M.F., as explained in 89. 
 
 94. Compound- Wound Motors In silk-mills and other 
 
 textile factories where any slight variation in the speed 
 affects the character of the manufactured product, com- 
 pound motors give a satisfactorily constant speed. The 
 object of the compounding coils is to weaken the flux in 
 the armature as the load increases. If, at a given load, 
 under the influence of shunt excitation alone, the speed 
 
 EFFICIENCY, SPEED 
 & TORQUE CURVES 
 
 SIZE IOC. 230V. 
 BI-POLAR SHdNT MOTOR 
 
 CROCKER-WHEELER COMPANY 
 
 13 14 
 
MOTORS. 175 
 
 would fall a certain per cent of the speed at no load, then 
 the armature flux must be lessened by the same percentage 
 in order to bring the speed up to its original value. In 
 calculating the number of series turns, account must be 
 taken of the fact of 
 magnetic leakage, 
 since the regulating 
 coils are on the field 
 magnets and not on the 
 armature direct. Cu- 
 mulatively compound- 
 wound motors are used llu_ $&$ 
 
 in order to obtain a 
 
 large starting torque. Fig. i 3 s. 
 
 The influence of the 
 
 series coils is not very great after full speed has been 
 
 attained. 
 
 95. Hand Speed Regulation A rheostat placed in the 
 
 field circuit of a shunt motor can be used to vary the 
 speed of the motor at will, as in Fig. 138. An increase 
 of the resistance will decrease the current in the field 
 coils. As a result the armature magnetic flux will decrease 
 and hence the speed will increase. If the fields be pretty 
 well saturated, it will require a resistance of some con- 
 siderable size, say twice as large as the field resistance, to 
 cut the current down enough to materially reduce the 
 flux and increase the speed. Motors of older make seldom 
 had fields magnetized anywhere near saturation. There- 
 fore they are very susceptible to the slightest change of 
 resistance in their field circuits. If the demagnetizing 
 armature ampere turns be large, it is possible for a motor 
 
DYNAMO ELECTRIC MACHINERY. 
 
 Series ff. 
 
 to increase its speed under increase of load. This is due 
 to the decrease of armature flux. 
 
 96. Speed Regulation by Series Resistances. The 
 
 speed of a motor on a constant pressure circuit can easily 
 
 be varied over a wide range, 
 from rest to full speed, by 
 manipulating a resistance 
 in series with it. The use 
 of this method is not to 
 be advised save for ex- 
 perimental purposes, since 
 it is very wasteful of en- 
 ergy. The 7 2 R loss in 
 the regulating resistance 
 
 T 
 
 Source 
 
 QJMMiU 
 
 Field Coils 
 
 Fig- 139- 
 
 is sometimes considerably 
 more than the power actually used. Fig. 139 shows the 
 wiring for this style of regulation. 
 
 97. The Leonard System of Motor Speed Control This 
 
 system is especially advocated for use in operating elevators, 
 cranes, battleship turrets, and all equipments requiring a 
 thorough control of the speed and precision of stoppage. 
 Fig. 140 shows the arrangement of such a system. M is 
 a motor whose field is separately excited all the time from 
 a source of constant potential, E. G is a dynamo which 
 generates power for the armature of motor M. The arma- 
 ture of the dynamo G is maintained at approximately con- 
 stant speed by the prime mover S, which may be a steam 
 engine or a motor run by power taken from the source E. 
 The generator G is separately excited by current derived 
 from E and controlled by the reversing rheostat C. 
 
MOTORS. 
 
 177 
 
 When it is desired to start the motor, the field of the 
 generator is weakly excited by moving the controller so 
 that a high resistance is in circuit with the field. This 
 causes the dynamo to send current of low potential to the 
 armature of the motor M. The latter then starts to move 
 slowly. To accelerate the speed, more resistance is cut 
 out of the controller. The pressure of the current supplied 
 to the motor armature simultaneously increases and with 
 it the motor's speed. Since the power represented by 
 the current required to excite the field of G is at most 
 
 dDJ 
 
 Fig. 140. 
 
 but a small fraction of the useful power given out by the 
 motor, the 7 2 R loss in the resistance C is very much less 
 than would be the loss in a series regulating resistance as 
 described in the last section. It is claimed that the extra 
 first cost of this system is offset by the decreased cost of 
 repairs, since violent stresses and bad sparking are avoided. 
 
DYNAMO ELECTRIC MACHINERY. 
 
 98. Slow-Speed Motors. It is a practical advantage to 
 have a motor connected directly to the machine it is to 
 operate, without the intervention of belting or reducing 
 gears. Slow speed is also of advantage where absence of 
 jarring is desired or where many stops and starts are to 
 be made. Slow speed can always be obtained from an 
 electric motor ; but it is generally expensive, since the 
 natural speed of motors as well as of dynamos is high. In- 
 crease of magnetic flux and increase of armature diameter 
 is necessary to obtain slow speed. The increase of ma- 
 
 
 ::: P 
 
 Fig. 141. 
 
 terial increases both the first cost and the losses during 
 operation. 
 
 The power of a motor is its torque or turning moment 
 multiplied by the number of revolutions ; hence for the 
 
MOTORS. 
 
 179 
 
 same output of work, a machine making half as many 
 revolutions as another must have twice the turning mo- 
 ment. These conditions make it imperative that the ma- 
 terials of construction, both iron and copper, be worked at 
 maximum magnetic and current densities respectively, in 
 order to economize in first cost and weight. In general 
 the efficiencies of low- 
 speed motors do not 
 compare favorably 
 with those of motors 
 having a higher speed. 
 Fig. 141 is a cut of a 
 Crocker Wheeler eight- 
 pole, direct current 
 motor for direct connec- 
 tion. It will furnish 2 
 horse-power at 100 rev- 
 olutions per minute, 4 
 horse-power at 200 R. 
 P.M., and the quotient 
 of its speed per minute 
 by its full load horse- 
 power is equal to the 
 constant quantity 50. Its efficiency increases as the 
 speed according to the curves shown in Fig. 142. 
 
 99. Brake Motors. For cranes, elevators, and hoists, 
 where it is necessary to hold the load after raising it, and 
 for looms and printing-presses, where it is important to 
 secure a sudden and accurate stop instead of a gradual 
 slowing down, it is desirable to use motors with a brake 
 attachment. A brake operated by hand or foot requires 
 
 EFFICIENCY AT 
 VARIOUS 'SPEEDS OF 
 SIZE 2-100 MOTOR. 
 CROCKER-WHEELER 
 
 ELECTRIC CO. 
 AMPERE. N. J. 
 
 100 200 300 
 Fig. 142. 
 
 10J 600 600 
 
i8o 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 careful operation lest it be applied too soon and injure the 
 machine, or too late and allow the load to fall some ; hence 
 an automatic brake is desirable. 
 
 Fig. 143 shows the construction used by the Crocker 
 Wheeler Company. One of the pole pieces is pivoted at 
 its base, and thus has a slight motion to or from the arma- 
 ture. It is normally held from the armature by a heavy 
 coil spring, and in this position tightens the brake band. 
 The moment that current is allowed to pass through the 
 field coils, the poles attract each other, overcoming the 
 
 OFF 
 
 Fig. 143. 
 
 resistance of the spring, and the brake band is thus loosened. 
 The spring and band may be adjusted to allow a few revo- 
 lutions before stopping, or the armature may be clamped 
 the instant current is turned off. In the latter case, if 
 connected to heavy machinery, shafting or gearing may be 
 broken. 
 
 Strap brakes are cumulative in their action ; the friction 
 on the free end of the brake against the drum tightens 
 the whole brake, thus increasing its effect. This action is 
 
MOTORS. 
 
 181 
 
 only obtained when the motion of the drum is away from 
 
 the fixed end. To obtain powerful brake action, therefore, 
 
 on motors that run either way, 
 
 as in elevator motors, a reversing 
 
 brake is employed. This is op- 
 erated by the movement of one 
 
 pole piece as before, but the ends 
 
 of the brake band are attached 
 
 to a system of links and levers 
 
 so that either end may become 
 
 the fixed end. This construction 
 
 is shown diagrammaticallyin Fig. 
 
 1 44. When the brake is applied, 
 
 the friction causes the whole 
 
 band to follow the drum until the sliding link attached to 
 
 one or the other end of the band is held by the stud. The 
 
 other, or free, end of 
 the band, is tightened 
 by the pull of the lev- 
 ers on one of the 
 smaller straps attach- 
 ed to the brake band 
 as shown. Fig. 145 
 shows a one horse- 
 power Crocker Wheel- 
 er motor fitted with 
 reversing brake. 
 
 In multipolar ma- 
 chines it is imprac- 
 ticable to employ a 
 moving piece, and in 
 I45 . large bipolar ma- 
 
182 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 chines it is undesirable to interrupt the magnetic circuit 
 by a pivot joint ; hence a solenoid brake is employed. This 
 is simply a spring actuated friction brake kept norm- 
 ally in engagement. On current being supplied to the 
 machine a solenoid acts to release the brake. The opera- 
 tion of this type is clearly seen by inspecting Fig. 146. 
 
 Fig. 146. 
 
 An objection to this type of automatic brake is that it 
 consumes electrical energy all the time that the machine is 
 in motion. 
 
 loo. Recording Meters. The recording watt-hour meter, 
 Fig. 147, is coming into extensive use, both as a station in- 
 strument and as a measurer of the quantity of energy 
 supplied to individual consumers. It is a very delicately 
 adjusted compound-wound motor, having no iron in its 
 magnetic circuit. When a current flows, the time integral 
 
MOTORS. 
 
 183 
 
 of the watts or power is registered, by means of a train of 
 wheels operated by the armature, on a dial similar to that 
 of a gas-meter. The connec- 
 tions for such a meter are 
 shown in Fig. 148. The ar- 
 mature is connected in series 
 with a high resistance across 
 the service wires ; hence the 
 current flowing in the arma- 
 ture is proportional to the 
 volts of the supply. The 
 field coils are in series with 
 the service, giving a field 
 strength proportional to the 
 current, and the motor effort 
 is proportional to the product of the two or to the watts 
 supplied. The shunt field coils are added to compensate 
 for the friction of the moving parts. Since a small cur- 
 rent is always flowing in the armature, as well as in the 
 
 Fig. 147. 
 
 Fig. 148. 
 
 shunt field coils, the motor is always slightly excited, and 
 by regulating the number of shunt turns the amount of 
 
184 DYNAMO ELECTRIC MACHINERY. 
 
 this excitation is adjusted so that at no load on the service 
 wires the armature almost, but not quite, moves. If it 
 were not for this constant excitation, a small, though 
 continuous, current could be drawn off the mains without 
 operating the recording mechanism because of its friction. 
 To control the speed a copper disk is mounted on the 
 armature shaft and between the poles of two or more 
 adjustable and permanent horseshoe magnets. When the 
 armature revolves, Foucault currents are set up in this 
 plate, and cause the proper retardation. By moving the 
 poles of these horseshoe magnets from the center to the 
 circumference of the disk, a variation of about 16 per cent 
 in the speed for a given watt consumption can be effected. 
 Advantage is taken of this fact in adjusting the instru- 
 ments. The more important bearings are constructed of 
 jewels, such as are used in watches, and the whole machine 
 is carefully protected from dust. When the instrument 
 is in a position where it is subject to jars or vibrations 
 that reduce the friction of standing to such a point that 
 the constant excitation causes the armature to revolve a 
 little, the machine is said to "creep." The remedy is 
 to mount on a rubber or other non -vibrating' base, or to 
 reduce the number of shunt field turns. 
 
SERIES MOTORS. 185 
 
 CHAPTER X. 
 
 SERIES MOTORS. 
 
 101 . Series Motors. When subjected to a heavy load on 
 starting, that is when there is a heavy current at a very low 
 speed, a series-wound machine is far superior to one that is 
 shunt-wound. For work that requires good effort at widely 
 different speeds the series motor is particularly adapted. 
 For this reason series-wound machines are used on electric 
 railways, for crane motors, for ammunition and other hoists, 
 for mill motors, and in all other places where a good effort 
 is required at a varying speeds. A series-wound machine 
 can be used on either a constant current circuit or on a con- 
 stant potential circuit ; but a series motor is seldom run on 
 a constant potential circuit unless it is directly or very 
 solidly coupled with its load, as in the case, for instance, 
 of a railroad motor. If connected by means of a belt, 
 and if the belt should break off or slip off, the motor would 
 race and damage might result. This difficulty does not 
 present itself when series motors are used on a constant 
 current circuit. 
 
 102. Series Motors on Constant Potential Circuits. As 
 in the case of a shunt motor, on a constant pressure 
 circuit, the armature speed of a series motor will increase 
 until it reaches a value where the counter E.M.F. cuts 
 down the armature current to such a point that the total 
 
1 86 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 electric power, (IE), received by the motor, is equal to the 
 sum of the fixed losses, the variable losses, and the useful 
 mechanical power. With a shunt-wound motor, a very 
 small variation of speed is sufficient to compensate for a 
 wide variation of load. A series motor tends to increase 
 its speed on removal of the load, as in the case with shunt 
 motors. It in this manner increases the counter E.M.F. 
 The resulting decrease of current results, however, in a 
 
 weakening of the field, 
 and as a consequence ad- 
 ditional speed is required 
 to maintain the E.M.F. 
 Thus a small change in 
 load results in a wide 
 change of speed in a series 
 400 g motor. For a series- 
 
 o 
 
 300 wound mill motor, the 
 
 Ul 
 
 200 1 relations which exist be- 
 tween speed, current, and 
 useful torque (turning mo- 
 ment) are shown in Fig. 
 149. There is also given 
 a curve of the efficiency of the machine including gear- 
 ing. It is evident that if, while the motor is at rest, 
 the circuit be closed, an enormous rush of current would 
 occur, giving an enormous torque. Destructive heating 
 and sparking would probably result. To prevent damage 
 it is therefore necessary, in the operation of these motors, 
 to insert a series resistance at the start which may be cut 
 out after the speed has risen enough to give a sufficient 
 counter E.M.F. In practice controllers are used as de- 
 scribed later. 
 
 EFFICIENCY, SPEED AND 
 TORQUE CURVES 
 
 SIZE 14 280 V. 
 MILL MOTOR SERIES WOUND 
 CROCKER-WHEELER COMPANY 
 
 10 SO 30 40 50 60 70 
 j?ig. 149. 
 
SERIES MOTORS. 187 
 
 103. Railroad Motors Experience has shown that 
 
 series motors operating on a constant potential circuit of 
 550 volts, furnish a very satisfactory motive power for the 
 propulsion of trolley street-cars and electric railroad motor- 
 cars. At the time of this writing there are nearly two 
 million horse-power of street-car motors in service in this 
 country. The railway motor has been developed to a high 
 degree of perfection during recent years, and is reasonably 
 well fitted to meet the many requirements that are found 
 in this service. A railway motor must be mechanically 
 strong to withstand the excessive hammering to which it 
 will be subjected when in service. Rough tracks and bad 
 switches are usual in trolley road beds. When satisfactor- 
 ily geared to the wheel axle, the motor can be suspended 
 by springs on one side only, the other side being of neces- 
 sity mounted directly on the axle. Railway motors are 
 also subject to abuse at the hands of the motormen. The 
 series resistance is often cut out rapidly before the car has 
 an opportunity to accelerate. As a result there is an enor- 
 mous current and torque with little speed. This severely 
 strains the motor and is particularly liable to disturb the 
 armature windings. The motor must be either weatherproof 
 of itself or incased in a weatherproof shell, because of the 
 mud, the water and the slush through which cars must 
 often run. Furthermore a railway motor should permit of 
 quick, convenient, and accurate alignment of parts and 
 adjustment of the intermediate driving mechanism. 
 
 The method of suspending the motors from the trucks 
 is a matter of considerable importance. In practice four 
 styles of suspension are used, viz., the side bar, the cradle, 
 the nose, and the yoke suspensions. In every case one 
 end of the motor frame contains bearings which run on 
 
188 DYNAMO ELECTRIC MACHINERY. 
 
SERIES MOTORS. 
 
 189 
 
190 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 the wheel axle and keep the pitch circle of the armature 
 shaft pinion always tangent to the pitch circle of the gear 
 which is mounted on the axle. The side-bar suspension, 
 shown in Fig. 150, consists of two parallel side-bars which 
 are mounted on the truck through heavy springs, and 
 which support the motor in the line of its center of gravity. 
 The motor-axle bearings are thus relieved of the weight of 
 the motor, and the latter is held without undue strains. 
 
 Fig. i53. 
 
 The cradle suspension, Fig. 151, is very similar to the side 
 bar, the difference being that the two side bars are replaced 
 by one U-shaped piece. This, at its curved end, is mounted 
 flexibly on a part of the truck frame which in turn is 
 mounted on the truck through springs. The nose suspen- 
 sion, Fig. 152, does not hold the machine at its center of 
 gravity, but part of the weight is thrown on the motor- 
 axle bearings. The rest is suspended from a spring- 
 mounted member of the truck by a link, bolted to a " nose" 
 
SERIES MOTORS. 
 
 191 
 
 cast in the motor frame. The yoke suspension, which is 
 the least flexible of all, differs from the nose suspension in 
 that the link is dispensed with and the spring-supported 
 member of the truck is bolted rigidly to the motor frame. 
 The cradle-suspension type is advocated by the Westing- 
 house Company, the yoke or nose by the General Electric 
 Company. The size or style of truck frequently requires a 
 particular type of suspension. 
 
 Fig. 154- 
 
 A GE-6/ railway motor, made by the General Electric 
 Company, is shown in Figs. 153 and 154. This machine 
 will develop 38 horse-power when operated on a 5<DO-volt 
 circuit without heating more than 75 C. above the sur- 
 rounding atmosphere after one hour's run. The magnet 
 frame is hexagonal, with rounded corners, and is cast in 
 two pieces from soft steel of high permeability. The 
 parts are hinged together so that the lower part may be 
 
192 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 swung down for inspection or repairs (Fig. 155). The up- 
 per part has cast on it two lugs, shown clearly in Fig. 153, 
 pierced with two holes each for bolting to the yoke. Nose 
 suspension can, however, be substituted. A covered opening 
 over the commutator permits removal of the brushes with- 
 out disturbing the rest of the machine. The bearings, 
 both for armature shaft and for axle, consist of cast-iron 
 
 Fig. 155- 
 
 rings or shells, with Babbitt metal swaged into them, and 
 are arranged for lubrication by both oil and grease. The 
 oil is supplied to the shafts by felt wicks leading from oil- 
 wells. The grease is fed through a slotted opening in the 
 top of each bearing from a grease-box directly over each 
 bearing, and means is provided for the passage of the 
 lubricant from the bearing after it has been used. The 
 armature bearings are 3" x 8" at the pinion end and 2f" x 
 
SERIES MOTORS. 193 
 
 6\" at the commutator end. The motor-axle bearings are 
 each 8" long. The pole pieces are built of soft iron lam- 
 inations, riveted together, and are securely bolted into 
 place on the magnet frame. The coils are spool wound, 
 and are held in place by steel flanges. These magnetically 
 imperfect constructions are rendered necessary by the 
 severe service the machine is expected to stand. 
 
 The armature is built up of thin, soft iron laminations, 
 japanned, and keyed to the shaft. At each end is a cast- 
 iron head, also keyed to the shaft. The core is hollow, 
 ventilation being effected by air which enters at the pinion 
 end and passes out through ventilating ducts left in the 
 laminations. The winding is of the series drum type, 1 1 1 
 coils being used, which are connected to a commutator of 
 1 1 1 segments. The number of turns to a coil depends 
 upon the class of service the motor is to render. The 
 coils are made up of sets of three, each set being sepa- 
 rately insulated before being placed in the slots. The 
 coils are firmly secured in place by tinned steel wire bands 
 held by chips and soldered together. Where the windings 
 cross the ends of the core, they are protected by canvas. 
 On the pinion end, a projecting flange protects the wind- 
 ings from injury by careless handling. The brushes slide 
 radially in finished ways in a brass brush holder, and 
 are held in contact by independent pressure fingers. All 
 the leads to the motor pass through rubber-bushed holes 
 in the front of the magnet frame. The pinion has a taper 
 fit on the armature shaft. It, as also the gear on the axle, 
 is made of steel, and has teeth of 4^" face and 3" pitch. 
 When mounted properly on a truck with the ordinary 33- 
 inch wheels, there is 4-J-" clearance between the bottom of 
 the motor and the top of the rails. The shapes of the 
 
194 DYNAMO ELECTRIC MACHINERY. 
 
 different parts of this motor are well shown in the exploded 
 Fig. 156. 
 
 A motor for railway service, very similar in design to 
 the one just described, is No. 49, made by the Westing- 
 house Electric and Manufacturing Company. This motor, 
 shown in Fig. 157, has a weather-proof cast -steel frame, 
 hinged to open in a horizontal plane through its center, 
 and having a hand-hole above and one below the com- 
 mutator. The upper half is cast with lugs for side-bar or 
 cradle suspension, and also with a lug for nose suspension. 
 The pole pieces are of laminated soft iron, are four in 
 number, and are secured to the frame by having the latter 
 cast around them. The lathe-wound field coils are secured 
 on the pole pieces by brass castings, which are bolted to 
 the frame. 
 
 The armature is of the slotted drum type, having a 
 laminated core with three ventilation passages parallel to 
 the shaft. The coils are wound on formers, insulated in 
 sets of two, and then applied to the core. This armature 
 is constructed as light and as small in diameter as is prac- 
 ticable, for the double reason of decreased centrifugal 
 strain on the armature coils and decreased wear on the 
 parts in stopping the car. When the motor is started, 
 energy is stored in the armature and other revolving parts, 
 as in a fly-wheel ; and when the car is stopped, this energy 
 is wasted, and causes wear and tear on the pinions and 
 bearings. In street-car service, where stops are frequent, 
 this loss and this wear is by no means inconsiderable. 
 Hence the armature of a street-railway motor should not 
 be built with a great fly-wheel capacity. The high speed 
 of car-motor armatures makes the operating expenses for 
 car acceleration and retardation considerable. 
 
SERIES MOTORS. 
 
 195 
 
196 DYNAMO ELECTRIC MACHINERY. 
 
SERIES MOTORS. 197 
 
 104. Controllers. It is general practice to equip each 
 trolley car with at least two motors, and to regulate 
 the speed of the car in the following manner : First, the 
 two motors and a resistance are connected in series. The 
 resistance is then cut out step by step until the two motors 
 are operating in series on 500 volts. Since, with all the 
 
 MOTOR 1 . MOTOR 2. 
 
 NOTCH 1 7 ' ROUE * R 1 R 2 R 3 ARMA ' F 'E<-> ARMA. FIELD 
 
 ^AW w, m ow ow^ CK 
 
 RUNNING). 
 
 NOTCH * 
 
 RUNNING), 
 NOTCH I 
 
 ^^A^WVF^VV^ 
 
 NOTCH 7 
 
 RUNNING 
 
 UNNING) 
 
 NOTCH i* 
 
 Fig. 158. 
 
 resistance cut out, there is no unnecessary I^R loss, this is 
 called a running connection, and the controlling mechanism 
 is said to be upon a running point. To further increase 
 the speed, the motors are placed in parallel with a resist- 
 ance in series with both. This resistance is then cut out 
 step by step until the motors are each operating on 500 
 volts. This, again, constitutes a running connection. A 
 further change is sometimes effected by placing a small 
 
198 DYNAMO ELECTRIC MACHINERY. 
 
 resistance in shunt with the fields when all the series re- 
 sistance is out. This reduces the field flux, and causes a 
 higher armature speed to maintain the necessary counter 
 E.M.F. A car governed in this way has four running 
 connections. On heavy cars, such as are used in elevated 
 railway or inter-urban service, four motors are used on 
 each car. In this case, the motors are governed in two 
 series-parallel combinations, as if there were two separate 
 cars governed by one controller. The connections for a 
 two-motor car having nine speeds, a three-part series re- 
 sistance, and a field-shunt resistance, are shown diagram- 
 matically in Fig. 158. 
 
 The different connections are made by a motorman, who 
 operates a handle on top of a controller. Each different 
 combination is called a point or a notch. A pointer affixed 
 to the controller handle indicates at what notch the car is 
 running. Running points are indicated on the controller 
 top by longer marks than the resistance points. A con- 
 troller is almost invariably placed at each end of the car. 
 
 Fig. 159 shows the interior of a General Electric 
 Company's k-io series parallel controller. The wires 
 from the trolley, from the fields, from the armature, and 
 from the different terminals of the series and shunt re- 
 sistances are brought up under the car to terminals on a 
 connecting-board in the bottom of the controller. On 
 this connecting-board there are also switches, one for each 
 motor. These enable one to cut out an injured motor 
 without interfering with the operation of the other motor 
 or motors. From the connecting-board conductors are 
 run to terminals, called fingers or wipes. Mounted on an 
 insulating cylinder, which may be revolved by the con- 
 troller handle, are insulated contact pieces, which at various 
 
SERIES MOTORS. 
 
 199 
 
 angular positions of the cylinder make electrical connec- 
 tions between various wipes, and give the proper con- 
 nections for the various "points" or " notches." A 
 
 Fig. 159. 
 
 smaller cylinder connected to a reversing-lever, is situated 
 to the right of the main cylinder. This has contact pieces 
 which are arranged so as to enable the motorman to re- 
 verse the direction of rotation of both motors or to cut 
 them out entirely. Interlocking devices are supplied so 
 
200 DYNAMO ELECTRIC MACHINERY. 
 
 that the reversing handle cannot be moved unless the con- 
 trolling handle is in such a position that connection with 
 the trolley is broken. The controlling handle also cannot 
 be moved, if the reversing handle is not properly set either 
 to go forward or to go backward. The reversing handle 
 cannot be removed from the controller, save when the 
 smaller cylinder is in the position that cuts out both motors. 
 As serious arcs are liable to develop upon breaking a 
 circuit of 500 volts, the contact pieces and wipes are sepa- 
 rated from adjacent ones by strips of insulating material 
 which are fastened to the inside of the controller cover, 
 and which fold into place when the cover is closed. These 
 are to be seen at the right of the figure. The power 
 should never be turned off by a slow reverse movement of 
 the controller handle, as destructive arcs are liable to 
 occur upon a slow break. To lessen the speed of a car, 
 the power should be completely and suddenly shut off. 
 Before the car has slackened its speed too much the con- 
 troller handle can be brought up to the proper point. The 
 arcs, which form upon disconnection at the fingers, are 
 pretty effectively blown out by the field of an electro- 
 magnet whose coil is above the connecting-board at the 
 right. 
 
 105. Motors For Automobiles. For electric automo- 
 biles the series-wound motor is invariably employed. A 
 storage battery of 40 or 44 cells is the customary source 
 of power for these motors. The use of these cells affords 
 a convenient and economical means of speed control. In 
 the case of a single motor, for the first controller notch, 
 the cells may be connected in four-series groups of 10 or 
 II each, giving about 22 volts, the four groups being con- 
 
SERIES MOTORS. 2OI 
 
 nected in parallel. Other notches would correspond to 
 other series parallel combinations, and finally the last and 
 highest speed notch would correspond to a connection of 
 all the cells in series. By this arrangement one cell is 
 used just as much as any other, and they are discharged at 
 equal rates. As the voltage supplied to the motor is 
 varied without recourse to a series regulating resistance, 
 there is no useless I^R loss in starting or running at less 
 than full speed. Often a series parallel control is employed 
 when two motors are used. It is also common to use two 
 37^ volt motors connected permanently in series and con- 
 trolled as one motor. 
 
 The advantage of using two motors on an automobile is 
 that each may drive a wheel, allowing independent rota- 
 tion on turning curves, while if one motor only is used 
 some form of differential gear must be employed to allow 
 for sharp turns. But the efficiency of one motor is in 
 general greater than the efficiency of two motors of half 
 the power, and the gain in efficiency by using one motor 
 more than balances the cost and complication of a differ- 
 ential gear. 
 
 The question of efficiency in these motors is of great 
 importance, for practice has shown that it is profitable to 
 purchase I per cent efficiency, even at the cost of 10 per 
 cent increase of motor weight. This is because the ratio 
 of the battery weight to the motor weight is such that a 
 decrease of i per cent in the capacity of the battery re- 
 duces its weight more than 10 per cent of the motor 
 weight. Since lightness is a prime object, only the very 
 best materials can enter into the construction of a suc- 
 cessful automobile motor. The magnetic circuit must be 
 of material of the highest permeability. Ball bearings are 
 
202 DYNAMO ELECTRIC MACHINERY. 
 
 not infrequently used in the shaft bearings, but their lia- 
 bility to wear and the consequent regrinding is an objec- 
 tion. 
 
 It is general practice to rate these motors at 75 volts, or 
 37 \ volts if two are used. Since 40 or 44 cells of battery in 
 series can fall to 75 volts without injury, this is the lowest 
 pressure on which the motors will be expected to run for 
 any length of time at full speed. Hence this voltage is 
 used as the basis for rating. For the best motors the 
 rating is for a temperature rise of 50 or 60 C. on an in- 
 definite run. A motor so rated will carry 100 per cent 
 overload for half an hour, 150 per cent for ten minutes, and 
 a momentary overload of 400 or 500 without overheating 
 or damage to the insulation. 
 
 The battery of 40 or 44 cells is well adapted to automo- 
 bile purposes. It can conveniently be made to have the 
 required capacity, and it may be charged from any 1 1 5- 
 volt direct current, incandescent lighting circuit with very 
 little resistance in series and hence a small I^R loss. 
 
 Although the voltage of these motors is somewhat low 
 for the use of carbon brushes, the necessity of reversal of 
 direction and the liability of sparking on over-load make 
 their use desirable. Soft carbon brushes of low resistance 
 can, however, be obtained, and they are to be recom- 
 mended. 
 
 Fig. 1 60 illustrates a motor which is used on automo- 
 biles and manufactured by the Eddy Electric Manufacturing 
 Company. It is a four-pole machine. The frame is ring 
 shape and made of cast steel. The pole pieces, also made 
 of cast steel, are fastened to the frame by bolts and 
 steady pins. The armature is wound with formed coils 
 which are cross connected, and therefore require but two 
 
SERIES MOTORS. 203 
 
 sets of brushes. These brushes are made accessible by 
 the existence of a window in the end plate. A pinion 
 which is mounted upon the armature shaft meshes with 
 an inside gear placed upon the wheel of the vehicle. A 
 recess in the exterior of the magnet frame is fitted to re- 
 ceive some part of the frame of the vehicle. Clamps for 
 fastening to this frame are provided to suit the character of 
 the vehicle. The motor is intended to be operated on 75 
 volts, and is rated at 1.6 horse-power, at the speed of 1400 
 
 Fig. 160. 
 
 revolutions per minute. Its weight is 142 Ibs., and it 
 has an efficiency of 79^ per cent at full load. At 100 per 
 cent over-load it has an efficiency of 76^ per cent, and at 
 150 per cent over-load an efficiency of 73 per cent. 
 
 1 06. Mill Motors. For many kinds of mill work re- 
 quiring great torque at low speed, reversibility, and wide 
 variation of speed, the series-wound motor is well adapted. 
 Since mill motors are to be used in places where dust, 
 grit, and small particles of metal are apt to be floating in 
 the air, it is necessary, to insure good continuous operation, 
 
204 DYNAMO ELECTRIC MACHINERY. 
 
SERIES MOTORS. 
 
 205 
 
 B 
 
206 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 that they be inclosed after the fashion of railway motors. 
 Mill motors differ from shunt-wound machines in that they 
 are capable of giving a turning-power, when slowed down 
 or started from rest, many times as great as that given 
 at full speed. 
 
 Fig. 161 shows a Crocker Wheeler mill motor, and Fig. 
 162 shows the same disassembled. It is a bipolar drum 
 armature machine, designed for about 800 R.P.M., and 
 giving without overheating an intermittent horse-power of 
 
 14 or a continuous horse-power of 5. 
 It is rated in this way, since fre- 
 quent stops and starts are expected 
 in the use of such a motor. The 
 hotter a motor gets during an in- 
 terval of use the more it will cool 
 off during an interval of rest. Of 
 course an inclosed motor such as 
 a mill motor heats up much more 
 rapidly and severely than does an 
 open motor where the air circulates 
 around the fields and the armature. 
 Since these motors are reversi- 
 ble the brushes can have no lead. 
 Sparkless running is accomplished 
 by a long air gap. Being series 
 wound the field increases with load 
 and the speed is reduced corre- 
 spondingly, hence commutation is readily effected. 
 
 These motors are controlled by a variable series resist- 
 ance, the various connections being made in a controller, 
 such as is shown in Fig. 163. The circuits are made by con- 
 tact pieces on a cylinder coming in contact with fingers or 
 
 Fig. 163. 
 
SERIES MOTORS 2O/ 
 
 wipers which are mounted on a board forming the back of 
 the controller. The controller illustrated is also a reverser. 
 The motor can be run in either direction by moving the 
 controller handle to the right or to the left of the central 
 position. 
 
208 DYNAMO ELECTRIC MACHINERY. 
 
 CHAPTER XI. 
 
 DYNAMOTORS, MOTOR-GENERATORS, 
 AND BOOSTERS. 
 
 107. Dynamotors. A dynamotor is a transforming 
 device combining both motor and generator action in one 
 magnetic field, with two armatures or with an armature 
 having two separate windings. They are generally sup- 
 plied with a commutator at each end, which are connected 
 to the two windings respectively. Either winding of the 
 armature may be used as a motor winding, and the other 
 as the dynamo winding. These machines occupy the same 
 position as regards direct current practice as is occupied 
 by transformers in alternating current practice. That is, 
 they enable one to take electrical energy from a system 
 of supply at one voltage, and deliver it at another voltage 
 to a circuit where it is to be utilized. They cannot, how- 
 ever, be constructed so as to operate with the same high 
 efficiency as a transformer does. As the currents in the 
 two armatures flow in opposite directions, and' the machines 
 are so designed as to have practically the same number of 
 armature ampere turns when in operation, there is practi- 
 cally no armature reaction. The field, therefore, is not 
 distorted so as to require a shifting of the brushes, nor is 
 there sparking present as a result of a change of load. 
 These machines are more efficient than motor generators, 
 which will be described later, as they have but a single 
 
DYNAMOTORS. 209 
 
 field. They cannot be compounded so as to yield a con- 
 stant E.M.F. at the dynamo end. A cumulative series 
 coil would tend to raise the E.M.F. at the dynamo end, 
 but it would lower the speed of the armature as a motor 
 by a corresponding amount. 
 
 1 08. The Bullock Teaser System. Dynamotors are 
 used extensively by the Bullock Electric Manufacturing 
 Company in their so-called Teaser system of motor-speed 
 control. This system is used in driving large printing- 
 presses from supply circuits, which are at the same time 
 used for lighting and other purposes. Large printing- 
 presses contain very many sets of gears, and possess very 
 large moments of inertia. These large machines require 
 an unusually large torque on the part of the motor to start 
 them. Sometimes it is as much as five or six times that 
 torque which the motor is called upon to produce at full 
 load. Now, the torque which is exerted by a motor is de- 
 pendent upon the current which flows through its arma- 
 ture, while the speed at which this torque is applied is 
 dependent upon the impressed electromotive force. As 
 the current, which is required to produce the normal run- 
 ning torque is already of considerable strength, it is desira- 
 ble that some direct current electrical transformation be 
 employed to avoid the excessive starting current. The 
 Teaser system accomplishes this by making use of the 
 dynamotor. The motor winding is designed for five times 
 the electromotive force of the dynamo winding. Its field 
 winding is excited directly from the supply mains. The 
 negative brush of the motor side is connected with the 
 positive brush of the dynamo side. The two armature 
 windings are connected in series with a regulating resist- 
 
2IO 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 ance to the supply mains. At starting, the main motor, 
 which drives the press and which is generally a cumu- 
 latively compound-wound motor, is supplied with current 
 from the dynamo end of the dynamotor. The voltage 
 with which it is supplied is somewhat less than one-fifth 
 that of the main supply, depending upon the magnitude 
 of the resistance in series with the dynamotor. This low 
 
 voltage permits of the application of a proper amount of 
 torque at a low speed. Furthermore, the drain of current 
 from the supply mains is but about one-fifth that which 
 passes through the main motor. By manipulating the dy- 
 namo regulating resistance, the electromotive force sup- 
 plied to the main motor is raised, and with it the speed. 
 The highest speed of the main motor which can be attained 
 by this arrangement is such, that, when attained, the mo- 
 tor's connections may be transferred to the supply mains 
 
DYNAMOTORS. 
 
 211 
 
 AMAMAA/VV\ 
 
 /XAAAAA/WJ 
 
 through another series regulating resistance without any 
 excessive drain of current from those mains. The arrange- 
 ment of the apparatus is shown in Fig. 165, and the 
 amount of current which is 
 taken by the main motor as 
 compared with the amount 
 of current which is drawn 
 from the supply mains is re- 
 presented in Fig. 164. Re- 
 gulation of the resistances 
 and changes of connection 
 are accomplished through the 
 aid of a controller. The 
 different speeds are secured 
 by the manipulation of a 
 
 single hand-wheel on the con- 
 Fig. 165. 
 
 troller, and thus the press- 
 man has at his command a means of manipulating the press 
 which is not complicated. 
 
 109. Dynamotors for Electro-Deposition of Metals. In 
 large electro-plating establishments, it is common to in- 
 troduce a dynamotor, whose two armature circuits are 
 exactly similar, and under ordinary excitation give 5 or 10 
 volts. The commutators, brushes, collecting devices, and 
 leads are of necessity quite massive. The leads are gener- 
 ally so arranged that the two armatures may be placed in 
 series with each other, or in multiple. The low voltage 
 of platers makes it impracticable to have a machine self- 
 exciting. It is common practice in cities to excite these 
 machines from no-volt lighting circuits, with a regulating 
 rheostat whose resistance is of such a magnitude as to 
 
212 DYNAMO ELECTRIC MACHINERY. 
 
 permit of the variation of the voltage of the machine over 
 a range of 25 per cent of its full-load value. Fig. 166 
 shows a dynamotor constructed by the Eddy Electric 
 Manufacturing Company for the electro-deposition of cop- 
 per. Each armature winding gives 10 volts and 4,500 
 
 tig. 
 
 amperes. It is designed to be belt -driven through a large 
 pulley at one end of the armature shaft. A small pulley 
 upon the other end is for the purpose of receiving a belt 
 connected with a small separate exciter. The large split 
 clamps connected with the leads are for the reception of 
 the terminals of the main conductors. 
 
 no. The Eddy Company's Rotary Equalizer --- This is 
 a dynamotor having a single field which is excited from a 
 22O-volt circuit, and a single armature core upon which is 
 wound two distinct 1 1 o-volt armatures. One armature 
 
DYNAMOTORS. 
 
 213 
 
 has its commutator on one end of the shaft and the other 
 at the other end. The machine is used in connection with 
 a 22O-volt generator, to enable one to use it for supplying 
 energy to a three-wire, no-volt, incandescent lighting sys- 
 tem. The principle of its action can be seen from an 
 inspection of Fig. 167. When the system is unbalanced, 
 
 Fig. 167. 
 
 that side which has the smaller load has the lesser drop, 
 and therefore the higher difference of potential. The 
 armature winding of the dynamotor which is connected 
 with that side acts as a motor, runs the armature, and 
 causes the other armature winding to act as a generator in 
 raising the pressure of the heavier loaded side. Obvi- 
 ously the employment of this system can, in some cases, 
 result in a considerable saving of copper. 
 
 in. Other Applications of Dynamotors. The Crocker 
 Wheeler Company manufactures a special line of dyna- 
 motors (Fig. 168) for use in telegraphic work. The motor 
 is designed to be supplied with electrical energy from street 
 service mains, or from the house-lighting mains in the 
 case of isolated plants. The generator end furnishes cur- 
 rents at a constant potential, which is different in the case 
 
214 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 of different machines. These machines are designed to 
 take the place of batteries of a large number of gravity 
 cells such as were used, in large quantities, a few years ago. 
 The cost of operation of a dynamotor for this service is 
 about one-fifth of what it is in the case of the gravity 
 cells. The space which the machine occupies is but 
 
 79 
 
 Fig. 168. 
 
 that of the cells. They are to be preferred to bat- 
 teries also on the ground of cleanliness. Their reliability, 
 when supplied by electric energy from large city service 
 mains is equal to that of the cells. The same cannot be 
 said in the case of small towns. The telephone companies 
 are also rapidly adopting the dynamotor for the purpose 
 of charging storage cells. With some forms, the charging 
 of the cells can go on continuously, they being at the same 
 
DYNAMOTORS. 215 
 
 time used for telephone purposes. Dynamotors also fur- 
 nish a convenient and satisfactory means of heating 
 surgeons' electro-cauteries. Cautery knives take from 3 
 to 8 amperes at 5 volts, while dome cauteries take from 
 1 5 to 20 amperes at the same voltage. 
 
 112. Motor-Generators. A motor generator is a trans- 
 forming device consisting of two machines, a motor and 
 generator, mechanically connected together. They have 
 the advantage over dynamotors in that the voltage of the 
 
 Fig. 169. 
 
 dynamo armature can be made to assume almost any 
 value within limits by means of a resistance placed in 
 series with its field-winding and capable of variation. 
 They can furthermore, besides being separately excited, 
 be shunt wound or compound wound. They are used 
 quite extensively in the Ward-Leonard system of motor 
 speed control, which was described in paragraph 97. They 
 are also used for charging storage batteries. In this case 
 
2l6 DYNAMO ELECTRIC MACHINERY. 
 
 they are almost always shunt wound. They are also used 
 in electro-plating establishments. In this case they are 
 separately excited, as the voltage generally employed in 
 such places is too small to give satisfactory self-excitation. 
 For general laboratory work on tests which require large 
 current at a low voltage or a small current at a high 
 voltage, motor generators are of inestimable value. 
 
 113. Boosters A booster is a machine inserted in series 
 in a circuit to change its voltage, and may be driven either 
 by an electric motor, or otherwise. In the former case it is a 
 motor-booster. This machine is used very extensively on 
 Edison three-wire incandescent lighting systems which 
 supply current at a constant potential. Feeders which run 
 to feeding-points at a great distance, if supplied by current 
 from the same bus bars as shorter feeders, will have too 
 small a difference of potential at the feeding-points to give 
 satisfactory service. A booster with its field and armature 
 windings in series inserted in series in the feeder will add 
 E.M.F. to the feeder which in magnitude is proportional to 
 the current flowing in the feeder, that is, as the current in- 
 creases the field excitation will increase and with it the 
 E.M.F. produced by the armature. The machine may, 
 therefore, be so designed as to just compensate for any 
 drop which is due to the resistance of the feeders and to 
 the current flowing through them. As all the current of the 
 feeder must pass through the booster armature, the collect- 
 ing devices must be massive and must be designed to carry 
 these heavy currents. The rating of a booster is of course 
 determined by the voltage which it produces, and the total 
 current which passes through it and the feeders. Boosters 
 are also used in the central stations of trolley companies to 
 
DYNAMOTORS. 2I/ 
 
 raise the voltage which is supplied to the feeders connected 
 with distant sections of the line. They are also being in- 
 troduced in office buildings in connection with electric 
 elevator service. When the elevator motors are supplied 
 from the same generators as the lights and fans in an office 
 building they give to the generators what is called a lumpy 
 load. The excessive currents demanded by the elevator 
 motors on starting produce wide fluctuations of voltage in 
 the mains. A booster inserted in these mains may be 
 made to add E.M.F. to the mains on these occasions. 
 
218 DYNAMO ELECTRIC MACHINERY. 
 
 CHAPTER XII. 
 
 MANAGEMENT OF MACHINES. 
 
 114. Connections for Combined Output of Dynamos. 
 
 In general a dynamo is much more efficient when operated 
 at its full load than when operated at one-half or one-quarter 
 load. It is usual to install in central stations, which, as a 
 rule, have to supply different quantities of electrical energy 
 at different times of the day, a number of smaller units 
 rather than one unit large enough to supply the total 
 energy. By this means any load can be handled by a 
 machine or a number of machines all operating at about their 
 maximum of efficiency. It is well, therefore, to consider 
 the methods of combining two or more machines on one 
 load. The simplest and most usual method of connecting 
 dynamos is that employed in incandescent light generating 
 stations. Here a number of constant pressure machines 
 are arranged as in Fig. 1 70, to act in parallel on one pair of 
 bus bars. The figure shows shunt machines with hand 
 regulators. The various external circuits are connected in 
 parallel to the bus bars. This practice is frequently 
 modified by separating those machines which supply the 
 circuits that deliver at the more distant points from those 
 that operate the shorter circuits. This is because the main- 
 taining of a constant and uniform pressure at all distribut- 
 ing points requires a higher pressure on the station ends of 
 the longer mains than on the shorter. When a machine 
 
MANAGEMENT OF MACHINES. 
 
 219 
 
 is to be thrown into circuit on to bus bars already in opera- 
 tion, it is first brought up to speed, the field magnetization 
 is then adjusted till the machine gives the same pressure 
 as exists between the bus bars, and the main switch is then 
 
 H-BUS 
 
 + BUS 
 
 closed, which puts the machine in circuit. The proper 
 pressure at which to throw in the new machine may be 
 roughly determined by comparing the relative brightness 
 of its pilot lamp with that of the lamps operating on the 
 circuit. 
 
 m ic 
 
 A more exact way is to 
 compare the readings of a 
 volt-meter across the ter- 
 minals of the machine with 
 one across the bus bars. 
 The most convenient way is 
 to use a "cutting-in galvano- 
 meter." Of these there are 
 two forms, the zero galvanometer and the differential gal- 
 vanometer. The zero galvanometer, shown with connec- 
 tions in Fig. 171, has a single coil of high resistance. 
 When the pressure of the machine is not exactly that of 
 the bus bars a current will flow one way or the other, and 
 
 Fig. 171. 
 
220 DYNAMO ELECTRIC MACHINERY. 
 
 the needle will be correspondingly deflected. When there 
 is no deflection, the machine may be thrown into circuit. 
 This instrument is simple and cheap, but it requires that 
 one terminal of the machine be permanently connected to 
 a bus, which is not always desirable. The differential gal- 
 vanometer, Fig. 172, has two high resistance coils, one in 
 shunt across the bus bars, and one in shunt across the 
 machine terminals. 
 
 When equal pressures are ___^ - - BUS 
 
 impressed on each of the I 
 
 coils, they, by their differ- 
 ential action, hold the needle 
 in equilibrium, but when one 
 coil is subject to more pres- 
 sure than the other a deflec- 
 tion occurs. This instrument 
 
 i i Fig. 172. 
 
 is more costly and more 
 
 complex than the last, but it has the advantage that a two- 
 pole switch may be used to cut in or out the machine. 
 
 When shunt machines are connected in parallel, it is 
 expected that they will all be kept at the same pressure. 
 If they are not, no serious damage is likely to occur, since 
 the lower pressure machine merely fails to take its full 
 share of the load. If the pressure of one machine falls so 
 low that it is overpowered and run as a motor, still no 
 damage will result, save perhaps the blowing of a fuse, 
 since the direction of rotation for a shunt machine is the 
 same whether it be run as a dynamo or as a motor. If it 
 be desired to regulate a number of machines together by 
 one regulator, it may be accomplished by bringing the 
 positive ends of the field coils to one side of the regulator 
 and connecting the other side to the negative bus. 
 
MANAGEMENT OF MACHINES. 
 
 221 
 
 Shunt machines may be operated in series by connecting 
 the positive brush of one machine to the negative brush of 
 the next, and connecting the extreme outside brushes with 
 the line wires. When this is done each machine can be 
 regulated separately to generate any portion of the pressure. 
 If it be desired to regulate all the machines thus connected 
 uniformly and as a unit, the field coils of all the machines 
 may be put in series with one regulating rheostat, and 
 shunted across the extreme brushes of the set. In the 
 Edison three-wire system two ii5-volt direct-connected 
 shunt machines are mounted on one engine shaft. The 
 dynamos are connected in series as described above, the 
 neutral wire being connected to the united brushes, as in 
 Fig. 173. 
 
 Series-wound dynamos may be operated in series with- 
 out any difficulty, though it is not customary to do so. 
 Series generators are used almost exclusively on constant 
 current (arc light) circuits, and it is usual to have as many 
 machines as there are external circuits, each machine being 
 of capacity enough to operate that circuit alone. A new 
 
 form of Brush generator supplies 
 several series circuits from its 
 terminals, and regulates for all 
 of them. If it be attempted to 
 operate series dynamos in paral- 
 lel, the following difficulty occurs : 
 If the machines start with a 
 proper distribution of load among 
 them and one does not generate 
 
 just its full pressure, then this one does not continue to take 
 its full share of the load ; and, since it is series wound, 
 a decrease in load is followed by a decrease in pressure. 
 
 Fig. 173- 
 
222 DYNAMO ELECTRIC MACHINERY. 
 
 The conditions become always more uneven until the 
 machine is overpowered and it turns into a motor. Since 
 the direction of rotation of a series-wound motor is oppo- 
 site to its direction when run as a dynamo, serious results 
 may occur. The only remedy for this trouble is to arrange 
 the field coils so that the magnetization in any one machine 
 will remain the same as in the other machines, even though 
 its pressure falls below that of the others. To accomplish 
 this the series fields must all be placed in parallel. This 
 may be done by means of an equalizer, which is a wire of 
 small resistance connected across one set of brushes, and 
 by placing the fields in parallel, as shown in Fig. 1 74. Two 
 
 Fig. 174. 
 
 series dynamos may be run in parallel without an equalizer 
 by resorting to mutual excitation, that is, by letting the cur- 
 rent of one armature excite the field of the other. In this 
 case if the pressure of one machine falls and its load there- 
 fore decreases, the magnetization of the other is reduced, 
 compelling the first to maintain its share of the load. 
 Series dynamos are never operated in parallel in practice, 
 but this discussion is introduced because of its application 
 to compound-wound dynamos. 
 
 The use of compound generators for constant pressure 
 circuits is very common. Since these have series coils 
 they cannot be run in parallel without special arrange. 
 
MANAGEMENT OF MACHINES. 223 
 
 ments. It is usual to fit the series coils with an equalizer, 
 as in Fig. 175. The desired end might be accomplished 
 in the case of two ma- 
 chines by making the 
 series coils mutually ex- 
 citing. 
 
 115. Connections of 
 Motors for Combined 
 
 Output Any number 
 
 of shunt motors may be placed in parallel across mains 
 of a constant pressure, and their operation will be sat- 
 isfactory whether each has a separate load or whether 
 they be connected through proper ratios to one shaft. 
 Shunt motors will operate in series on a constant pressure 
 circuit when positively connected together ; but if con- 
 nected to the same shaft by belts, and one belt slips or 
 comes off, that motor will race, and rob its mates of their 
 proper portion of the voltage. This arrangement is not 
 common. 
 
 Series motors will operate satisfactorily on constant 
 pressure circuits : but when two or more such machines, 
 that are arranged in parallel on a constant pressure circuit, 
 are connected to one shaft an equalizing connection is 
 sometimes used. Series motors in series on constant 
 pressure mains will operate satisfactorily, dividing up the 
 total voltage between them according to the load each is 
 carrying. If it be desired to make them share a load 
 equally they must be geared together so that each rotates 
 at the speed corresponding to its share of the voltage. 
 Series motors only are used on constant current circuits. 
 Any number of these may be placed in series on such a 
 
224 DYNAMO ELECTRIC MACHINERY. 
 
 circuit individually or connected to a common shaft. A 
 series motor on a constant current circuit may be over- 
 loaded until it stops without harm, since a constant current 
 flows at any speed. 
 
 Compound-wound motors are coming into quite general 
 use, and they are invariably operated on constant pressure 
 circuits, and each machine has its own load. 
 
 In ordinary electric railroad practice, as has been stated, 
 there are two series-wound motors to a car, operating 
 either in series or parallel, according to the position of the 
 controller handle, on a constant pressure of 500 volts. 
 Each of these motors is geared to a separate pair of driv- 
 ing-wheels. Since under ordinary conditions the rate of 
 rotation of the two motors is the same, the E.M.F. sup- 
 plied to each is the same when they are in series, and 
 since the current is common they divide the work evenly. 
 When in parallel the pressure on each is 500 volts, and 
 since the rotations are the same the currents will be the 
 same and the load will be divided evenly. It often occurs 
 that the back platform of a car is so loaded that the front 
 drivers slip when the power is applied at starting. This 
 occurs when the motors are in series and the current 
 is common to the two. But the higher rate of rotation of 
 the front motor causes it to generate a greater counter 
 E.M.F., thus lowering the pressure acting on the rear 
 motor. Thus more electric energy is consumed in the 
 front motor, and the surplus of work turns into heat from 
 the friction between the slipping wheels and rails. When 
 the car gains such a velocity that the front wheels bite the 
 rails, the work is again evenly distributed between the two 
 motors. It should be remembered that this occurs only 
 when the motors are in series. 
 
MANAGEMENT OF MACHINES. 225 
 
 116. Care and Operation of Machines In what follows 
 
 on the operation of motors and dynamos, it is assumed 
 that the machine is properly designed and of sufficient 
 capacity for the work it is called upon to perform. For 
 satisfactory operation, the machine must be connected with 
 an appropriate circuit and one of the voltage or amperage 
 for which the machine was designed. Further it is 
 assumed that the mere mechanical details have been looked 
 to, such as proper foundation, proper alignment with shaft- 
 ing, and good lubrication. Only electrical trouble will be 
 treated. 
 
 If trouble be detected, a machine should be at once 
 stopped to prevent further trouble. In central generating 
 stations, one of the most positive rules is not to shut 
 down while any possible means is left to keep running. 
 In such plants there are always one or two units held in 
 reserve, and one of these may be started and substituted 
 for a machine developing a fault so that the latter may be 
 shut down and its fault remedied. 
 
 Sparking at the brushes is the most general trouble, and 
 it has more causes than any other. The brushes must 
 make good contact with the commutator, they must be 
 true, and have good contact surface. The commutator 
 must be clean. Any collection of carbonized oil is sure to 
 cause sparking. A very thin layer of good oil, free from 
 dust, is advantageous. On a bipolar machine the brushes 
 must be diametrically opposite, on a four pole exactly 90 
 apart, etc. This condition must be attained while the 
 machine is at rest, either by actual measurement or by 
 counting the commutator bars between each brush. If 
 the brushes of one set are "staggered" they may cover 
 too much armature circumference and cause sparking. 
 
226 DYNAMO ELECTRIC MACHINERY. 
 
 The brushes must be set at the proper point. This is 
 accomplished while the machine is in operation under its 
 required load. The rocker arm, which carries all the 
 brush holders, is moved carefully back and forth until the 
 point of minimum sparking is found. Sometimes there is 
 quite an arc of movement in which sparking is not 
 observed. The brushes should then be set at the center 
 of this arc, since heating occurs when the brushes are off 
 the proper commutating point, even if sparks be not seen. 
 
 Sparking may be due to fault in the commutator. A 
 high-bar, a low-bar, or flat, projecting mica, rough or 
 grooved surface, eccentricity, or any condition of surface 
 which causes the brushes to vibrate and lose contact with 
 the commutator will surely cause sparking. If sparking 
 be allowed to go without correction, it will pit the commu- 
 tator and aggravate these conditions. If the irregularity 
 of surface be slight, it may be cut down by sandpaper 
 (never emery) held in a block cut to fit the commutator. 
 If the surface be very bad, it must be cut down by a 
 machine. A small armature may be swung in a lathe ; but 
 a large one must be left in its own bearings, and a tool 
 held against the commutator by some special device. A 
 perfectly true commutator may act eccentrically toward 
 the brushes because of wear in the shaft bearings. New 
 bearings will remedy this fault. 
 
 If a coil of the armature be short-circuited, periodic 
 sparking may result. The coil is liable to burn out if the 
 machine is not immediately stopped. The short circuit 
 may occur from breakdown of the insulation within the 
 armature, in which case rewinding is necessary ; or it may 
 be caused by metal chips or the like at or near the com- 
 mutator, in which case the cause can be easily removed, 
 
MANAGEMENT OF MACHINES. 22/ 
 
 When a coil is broken very violent sparking occurs, since 
 half the armature current is broken every time the com- 
 mutator bar connected to the broken coil passes from under 
 a brush. Such a break may occur within the armature, 
 requiring rewinding ; but it is more likely to occur where 
 the coil end is attached to the commutator bar lug. If 
 the break be at this place, the wire needs but to be screwed 
 or soldered to the lug and the machine is repaired. 
 
 If the field of a motor is too weak, sufficient counter 
 E.M.F. is not generated, and excessive current flows and 
 causes sparking. The weakening of the fields may occur 
 from a short circuit in the field coils or two or more 
 grounds between the field coils and the pole piece, or by a 
 broken field circuit (shunt coils) which reduces the mag- 
 netism to almost zero. In any case, unless the trouble is 
 to be found external to the coils, rewinding is necessary. 
 
 Heating of machines is another frequent source of 
 trouble. The limit of temperature that may be allowed in 
 the bearings depends on the flashing-point of the lubricant 
 used, but a well designed and lubricated bearing ought 
 always to run cooler than the commutator or armature. 
 The limit of temperature that may be allowed in the arma- 
 ture depends on the " baking "-point of insulation used, 
 and also on the melting-point of the solder used if the 
 coil ends are soldered to the commutator lugs. A good 
 general rule is this : If you can hold your hand on any 
 part of the machine for more than a few seconds, that 
 part is not dangerously hot. Of course metal feels warmer 
 than insulating cotton for the same temperature, and 
 allowance should be made for this. If a burning smell or 
 smoke comes from a machine, the safe temperature limit 
 has been far exceeded, and the machine should be shut 
 
228 DYNAMO ELECTRIC MACHINERY. 
 
 down at once. This indicates a serious trouble a short 
 circut or a hot bearing probably. 
 
 If the trouble arises from the bearings the ordinary me- 
 chanical precautions of cleaning, aligning, lubricating, etc., 
 will generally cure it. Never use water to cool hot bearings. 
 If water gets into the windings of either the field or the 
 armature, short circuits will occur and ruin the machine. 
 It must not be assumed that because one part of a 
 machine is hot the trouble lies with that part. Heat is 
 quickly conducted all over a machine ; and when heat is 
 detected in one place the machine should be felt all over, 
 the hottest part probably being the part at fault. The 
 brushes of a machine should not be set too tight, for, be- 
 sides reducing the efficiency greatly, they cause much 
 heat from friction. The commutator should not be more 
 than 5 C. hotter than the armature. 
 
 Machines that operate on constant pressure circuits are 
 liable to overheat because of too much current flowing 
 through some parts of them. This may result from over- 
 loads, in which case the remedy is obvious, or because of 
 short circuits in the machines, in which case rewinding is 
 generally necessary. In the case of constant potential 
 generators a short circuit of the mains will produce a sud- 
 den and severe overload, which can only be remedied by 
 tracing out the lines and removing the short circuit. 
 
 When a machine makes an unwarranted amount of noise 
 it usually indicates the need of attention. Carbon brushes 
 chatter and spark sometimes when the commutator is 
 sticky, the action being something like a bow on a violin 
 string. Cleaning the commutator will cure this. Hum- 
 ming and vibration result when the revolving parts are 
 not revolved about their center of gravity. This may be 
 
MANAGEMENT OF MACHINES. 229 
 
 because of faulty construction or warping after completion. 
 If the fault be with the pulley, it may be turned out or 
 counterweighted. If the shaft be sprung it may be 
 straightened or a new one used. If the armature core or 
 windings be out of balance, there is not much help for it. 
 Slower speed will reduce the noise from this cause. 
 
 Noise may occur from the armature rubbing or striking 
 against the pole faces. This is a serious matter, and if not 
 immediately attended to results in the destruction of the 
 armature. It is caused generally by wear in the shaft 
 bearings, in which case new brasses will remedy the 
 trouble. Sometimes it results from a sprung shaft, in 
 which case the shaft must be either straightened or re- 
 placed. A rattle produced by loose collars, bolts, nuts, or 
 connections would indicate that these parts needed setting 
 up or adjusting. 
 
 If a motor revolves too slowly, it may be because of an 
 overload of mechanical work, because of excessive friction 
 in the machine, or because of the armature rubbing against 
 the pole face. A variation in the pressure supplied to a 
 motor causes a variation in speed. If the field magnetism 
 be too weak the motor will revolve too fast when not 
 loaded, and too slow when under full load, and will take 
 excessive current. A weak field may be caused by a short 
 circuit which cuts out some or all of the field turns, or by 
 a broken field circuit. If the load be removed from a 
 series motor on a constant current circuit it will race badly 
 unless its field coils are shunted. Practically such a motor 
 should not be used in any position where it may be sud- 
 denly relieved of its load, as by the slipping of a belt. A 
 shunt motor, whose fields are not excited, will run either 
 forward or backward when a current is allowed to flow in 
 
230 DYNAMO ELECTRIC MACHINERY. 
 
 the armature, according to the relative magnitudes and 
 directions of the residual magnetism and the armature re- 
 actions. Ordinarily, however, if a motor runs backward, it 
 may be assumed that the connections are wrong. Usu- 
 ally changing the connections to the brush holders, so that 
 the brushes change their signs without changing any other 
 connections, will make the motor change its direction of 
 rotation. A series motor also may be made to change its 
 direction by changing the direction of current flow in 
 either field coils or armature, but not in both. 
 
 On starting up, a self -exciting dynamo is supposed to 
 build up its voltage to normal, having at first no excitation 
 save that of residual magnetism. After standing some 
 time, or in proximity to other dynamos, or after being 
 hammered, the magnet frame may have lost all its residual 
 magnetism. In this case the machine does not build up 
 when revolved. By passing a current from another 
 machine through the field coils the dynamo will generate 
 as a separately excited one. Then the exciting current 
 may be thrown off and the self-excitation thrown on, when 
 the machine will build up satisfactorily. If the residual 
 magnetism becomes changed in direction, or the separately 
 exciting current be passed in the wrong direction, then 
 what little voltage may be generated will, when connected 
 for self-excitation, send the current in such a direction as 
 to tend to demagnetize the field, and building up will be 
 impossible. A shunt machine builds up better the less 
 the outside load, since at no load the terminal voltage is the 
 greatest and the most likely to send a magnetizing current 
 in the field coils. A series machine builds up better when 
 the outside load is increased. Such a machine may even 
 be momentarily short circuited to make it build up. For a 
 
MANAGEMENT OF MACHINES. 231 
 
 given voltage resulting from residual magnetism, the current 
 in the field coils is greater the less the resistance in the 
 circuit. If the connections to one of the field coils in a 
 bipolar machine be reversed, causing two poles of the same 
 polarity, the machine will of course fail to generate. This 
 condition may be detected by the use of a compass needle. 
 Small machines sometimes generate at starting a few volts, 
 showing proper connections and the presence of some 
 residual magnetism, but refuse to build up beyond this 
 point. It is sometimes convenient to materially increase 
 the speed of such a machine, whereupon it will build up 
 rapidly, and the speed may then be reduced to normal, and 
 the dynamo will continue to generate at its normal pres- 
 sure. 
 
232 DYNAMO ELECTRIC MACHINERY. 
 
 CHAPTER XIII. 
 
 THE DESIGN OF MACHINES. 
 
 117. Different Methods of Design. It is impossible to 
 lay down a fixed set of rules to be followed in the design 
 of dynamo electrical machinery. This is because the 
 specified conditions of operation and construction are 
 seldom alike in two cases. A designing engineer may be 
 called upon to design a machine of a given output at a given 
 voltage, the field frame, however, to be chosen from one of a 
 set already in stock ; and again it may be required that the 
 machine shall be direct connected, the output, the voltage, 
 and the speed of rotation being given ; still, again, the 
 capacity, the maximum gross weight, and the efficiencies of 
 operation at various loads, may be specified, as in the case 
 of an automobile motor ; or he may be called upon to design 
 a machine of a given output and voltage, which shall 
 operate at a satisfactory efficiency, and which shall have a 
 first cost which will enable the manufacturer to successfully 
 compete with others in the sale of his products. Through- 
 out the calculations the engineer is obliged to refer to his 
 experience or the experience of others in determining the 
 values of different quantities which must be assumed before 
 there can be any further progress on the design. Further- 
 more, after having assumed certain values, results which 
 are arrived at later on in the work will necessitate the re- 
 jection of these values and the assumption of new ones. 
 
THE DESIGN OF MACHINES. 233 
 
 Oftentimes what one might desire as a value for one quan- 
 tity is undesirable, because it conflicts with the adoption of 
 a value for some other quantity which is more desirable. In 
 the following paragraphs a method is given for designing 
 a machine under the conditions specified. 
 
 118. Specifications. The following specifications are 
 given and must be complied with : 
 
 The type of machine as regards the shape of its field 
 frame, its bearings, and the method of its being driven ; its 
 output in kilowatts ; its terminal voltage at full load and at 
 no load ; the materials from which are to be constructed its 
 field frame, its pole pieces, its armature core, its brushes, 
 its shaft, its bearings, its armature spider, and its con- 
 ductors ; and the insulation throughout its various parts. 
 
 119. Preliminary Assumptions. The design will be 
 based upon an assumption of the values of four different 
 quantities. 
 
 The first assumption is that of the value of the flux den- 
 sity in the air gap, which will be represented by ($> g . The 
 value which will be chosen will depend somewhat upon the 
 method to be employed for obviating armature reaction. 
 Almost all designers rely upon a stiff y bristly fie Id \.Q assist 
 in preventing a distortion of the field when under load, and 
 therefore higher flux densities are being used now than 
 were a few years ago. Higher densities are used when the 
 pole pieces are made of wrought iron or of cast steel than 
 when they are made of cast iron. The densities are greater 
 in the case of multipolar machines than in the case of 
 bipolar ; and they increase, within limits, with the size of 
 the machine. A value between 4000 and 7500 should be 
 chosen. 
 
234 DYNAMO ELECTRIC MACHINERY. 
 
 The second assumption is a value for the peripheral 
 velocity F' of the armature in feet per minute. The com- 
 mon assumption in the case of drum armatures for all sizes 
 above five K.W. is 3000 feet per minute. High-speed ring 
 armatures have a higher value, ranging between 4000 and 
 6000. The larger value is to be used in the case of large 
 machines. 
 
 The third assumption is a value for the current density 
 in the armature conductor at full load. Inspection of a 
 large number of machines shows the use in many of them 
 of from 500 to 800 circular mils per ampere. Sometimes 
 as small a cross-section as 200, and in other cases as large 
 as 1 200 circular mils per ampere, have been found. The 
 low value is used in the case of machines subjected to 
 periodic loads of short duration. This is the case with 
 elevator motors, pump motors, sewing-machine motors, 
 dental drill motors, and motors on special machinery. The 
 high value is used on machines to be used in central stations 
 for lighting or power purposes. The specified output in 
 kilowatts divided by the full-load terminal volts gives the 
 total current output of the machine at full load. This, 
 divided by the number of armature circuits, gives the cur- 
 rent which must be carried by each conductor at full load. 
 This current multiplied by the assumed value of the number 
 of circular mils per ampere gives the cross-section of the 
 conductor in circular mils. Oftentimes a single armature 
 conductor is made up of several wires in multiple. The 
 multiplicity of wires affords pliability in winding, and 
 obviates, to a certain extent, eddy currents. Again, the use 
 of copper bars for windings is common, they being in- 
 sulated by the use of micanite, fuller board, or other 
 sheet insulating materials. A cross-section sketch of a 
 
THE DESIGN OF MACHINES. 235 
 
 single conductor should be made in which the dimensions 
 are given of the copper and of the insulating material. 
 
 The fourth assumption is the value s to be given to the 
 polar span, s represents the percentage of the armature 
 circumference which is covered by the faces of the poles. 
 This value varies considerably within narrow limits, but 
 unless there is some special reason for the assumption of 
 another value 0.8 may be taken. 
 
 120. Design of the Armature 1. To determine the 
 
 specific induced E.M.F. in volts per foot of active con- 
 ductor. y f 
 
 E' = j^> x jffi, X 30.5 ^- 8 volts. 
 
 where the first term in the right-hand member represents 
 the velocity of the moving conductor in centimeters per 
 second, the second term represents the average induction 
 density of the flux which enters the armature, and the 
 third term consists of constants to reduce feet to centi- 
 meters, and c. g. s. units to volts. 
 
 II. To obtain the length of active conductor I 1 in feet. 
 
 I' = -= X number of armature circuits. 
 
 L 
 
 III. To obtain the number of active conductors S upon 
 the armature. 
 
 Let ly = the number of layers in the armature winding. 
 
 p = the assumed ratio of the length to the diameter of the 
 
 armature core. 
 
 d = the mean winding diameter of the armature in inches. 
 
 w = the specific peripheral width of one armature conductor 
 
 in inches. (In the case of a smooth-core armature 
 
 w represents the width of the armature conductor 
 
236 DYNAMO ELECTRIC MACHINERY. 
 
 plus the double thickness of its insulation, both in 
 inches, while in the case of tooth armatures w rep- 
 resents the width of one tooth plus the width of 
 one slot divided by the number of conductors in 
 one slot in one layer.) 
 
 The length of the armature core = pd inches = ft. 
 
 and the circumference of the core = ird inches. The 
 
 _ vet 
 
 IV 
 
 "* 
 
 number of armature conductors in one layer = hence 
 
 w 
 
 the total number of armature conductors >S = 
 Since 
 
 In practice the width of the tooth ranges from 50 per 
 cent to 80 per cent, the width of the slot. In some cases 
 it has a width equal to that of the slot. The value for 5 
 yielded by this formula must, in nearly all cases, be altered 
 by either the addition or subtraction of a few conductors 
 in order to make it possible to employ the type of winding 
 which it seems desirable to adopt. The change may neces- 
 sitate a slight alteration of one of the assumed values, and 
 as a result the values derived from it. 
 
 For machines whose speed is prescribed, as is the case 
 with direct connected machines, one may use the form of 
 
THE DESIGN OF MACHINES. 237 
 
 the formula 5 = , where d is to be obtained as de- 
 
 w 
 
 scribed in the end of the next paragraph. 
 
 IV. To obtain the diameter of the armattire d in inches. 
 
 Sw 
 
 d = -2 inches. 
 
 7T 
 
 In case the speed of the armature in revolutions per 
 minute Fbe prescribed, as is the case with direct connected 
 machines, the preliminary assumption of the peripheral 
 velocity V 1 immediately gives a value for the armature 
 diameter. yt ^, ^ 
 
 d = ft. = inches. 
 Vv Vtr 
 
 V. To determine the length of the armature I in inches. 
 
 VI. To determine the internal diameter of the armature 
 core d' in inches. In determining this quantity a value for 
 the flux density in the armature core (B a must be assumed. 
 Wiener states that in incandescent dynamos, in railway 
 generators, in machines for power transmission and distri- 
 bution, and in stationary and railway motors, the density 
 varies from 5,500 to 15,500. Ring armatures have higher 
 densities than drum armatures, low-speed machines higher 
 densities than high-speed machines, and bipolar machines 
 have larger densities than multipolars. (B = 8000 is a 
 good assumption. 
 
 If the machine have / pairs of poles, the flux which 
 enters the armature through one pole 
 
238 DYNAMO ELECTRIC MACHINERY. 
 
 that is, the surface of the armature in square centimeters 
 times the average gap density divided by the number of 
 poles. Considering that but 75 per cent to 80 per cent of 
 the length of the armature core is made up of iron, the 
 rest being due to the spaces between the laminations and 
 the width of the ventilating ducts, the radial depth of the 
 armature core is 
 
 d-d r = 
 
 ( 2 -54) 2 
 
 & a /0. 75 (2. 54 ) 2 
 
 VII. To determine the armature losses. The armature 
 as already determined would theoretically operate satis- 
 factorily, but there is a possibility of its heating excessively 
 when running under full load. There are the two constant 
 supplies of heat, namely, that due to ohmic resistance 
 and that due to hysteresis and eddy currents. There are 
 also two avenues for the escape of heat, namely, radia- 
 tion and air convection. An equilibrium is established when 
 that temperature is reached which will make the escaping 
 heat per unit of time equal to the amount of heat gen- 
 erated in the same time. Concerning the escape of heat 
 by radiation, it should be borne in mind that the watts 
 radiated vary as the difference in temperature between the 
 radiating body and the surrounding atmosphere and as the 
 emissivity and the area of the radiating surface. There 
 is also on starting a conduction of heat to neighboring 
 bodies. After a short time, however, a static temperature 
 condition will be established. The power loss in hysteresis 
 in the armature is 
 
 i V 
 
 P h = _ -np >a*-r- v watts. 
 
 IO DO 
 
THE DESIGN OF MACHINES. 239 
 
 where ^ equals the hysteretic constant of the iron (0.002), v 
 equals the volume of the armature core in cubic centimeters. 
 The assumption is made that the flux density in the" arma- 
 ture core is uniform. This is not true for the main core, as 
 was shown by Goldsborough, and in the teeth the density is 
 much greater. When the volume of the latter is a relatively 
 large amount of the total core volume, a correction should 
 be made. When making many designs, in which the same 
 quality of iron is to be used, it is much easier to get the 
 hysteresis loss per cubic inch at various densities from 
 tables made up to suit the iron. The power loss due to 
 ohmic resistance 
 
 =( " 
 
 \number of ar 
 
 
 number of armature circuits 
 
 where I max is the full-load current of the machine in am- 
 peres, and R a is the resistance of all the armature con- 
 ductors arranged in series. Before getting P h and P r one 
 must determine the values in VIII. to XL 
 
 VIII. To obtain the armature speed V in revolutions per 
 minute. This quantity is prescribed in the case of direct 
 connected machines. In other cases in may be determined 
 by the formula 
 
 IX. To obtain tJie volume of the armature core v in cubic 
 centimeters. 
 
 where z is a coefficient which represents that part of the 
 armature core length which is occupied by iron. In ordi- 
 nary laminations the space occupied by air and insulating 
 
240 DYNAMO ELECTRIC MACHINERY. 
 
 oxide on the plates amounts to 10 per cent, therefore under 
 these circumstances z = 0.9. The introduction of ventilat- 
 ing ducts reduces this value by an amount which can be 
 readily determined. 
 
 X. To obtain the resistance of the armature wire in 
 ohms. The total length of the armature wire, 
 
 /, = f X k, 
 
 where k is a constant greater than unity, which takes into 
 account the amount of dead wire employed in making the 
 end connections. This value depends upon the value of 
 p and upon the method of winding. In the case of formed 
 coils its value may be determined from measurements upon 
 a single coil. This value is generally slightly greater 
 than 2. Considering that the resistance of a hot mil foot is 
 11.5 ohms, the resistance of the armature 
 
 11.5 I'* 
 
 cross-section in circular mils. 
 
 XI. To obtain the area of the armature radiating surface 
 A in square inches, 
 
 XII. As 2 to 2\ watts can be radiated per square inch 
 of armature surface without excessive heating, the value of 
 p _j_ p 
 r determines whether the armature is properly de- 
 
 yi 
 
 signed or not. If the fraction is less than 2, the armature 
 is needlessly large, and should be redesigned. If the frac- 
 tion is greater than 2\, the armature will heat excessively, 
 and should also be redesigned. 
 
THE DESIGN OF MACHINES. 241 
 
 121. Design of the Field. XIII. Dimensions of the 
 poles and field frame. The design of a field requires judg- 
 ment and experience on the part of the designing engineer, 
 and an acquaintance with the various machines of the type 
 being designed. One must assume values for the following 
 quantities : the flux density in the poles fl^, the flux 
 density in the magnet frame % the coefficient of magnetic 
 leakage A, and the ratio of the length of a pole to its diam- 
 eter in case it has a circular cross-section, or to some other 
 dimension in case it is not circular. The assumption is 
 made here that the field frame is of a circular type, and 
 that the pole is of circular cross-section. It is customary 
 to choose such a value for (& p that the magnetization will 
 be carried over the knee of the magnetization curve. In 
 the case of (By-, however, it is customary to choose a value 
 somewhat below the knee. The coefficient of magnetic 
 leakage for this type of machine is 1.4. A careful design 
 really requires a knowledge of the distribution of the leak- 
 age flux. Long experience enables one to make allow- 
 ance for this. From these assumed values one gets a 
 value for the cross-section of a pole, 
 
 <M 
 A = sq. centimeters, 
 
 <Bp 
 
 whence it follows that the diameter of the pole in inches 
 d p = 2.54 V/ ^ inches, and the cross-section of the frame, 
 
 T 7T 
 
 <M 
 A f = -?- sq. centimeters. 
 
 2 (jjf 
 
 XIV. Reluctance of the magnetic circuit. After mak- 
 ing a provisional scale-drawing of the field-magnet frame 
 with its poles and the armature core, exercising judgment 
 derived from experience or from the inspection of other 
 
2 4 2 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 drawings, determine the average length in centimeters of 
 the path of the magnetic lines in the frame, in the poles, 
 in the air gap, in the teeth, and in the armature core. 
 
 Fig. 176. 
 
 Represent by l f , l p , l g , l t and l t the length in centimeters 
 of the parts marked in Fig. 176. From the assumed 
 values of the flux density, and from the magnetization 
 curves of the metals from which the various parts of the 
 magnetic circuit are constructed, one can get the respec- 
 tive permeabilities. The reluctance may then be calculated 
 as follows : 
 
 Reluctance of the pole (R p = 
 
 of 4- section of field frame (R/- = - . 
 
 / 
 
 of the air gap (R = 
 
 of \ section of the armature core, 
 
THE DESIGN OF MACHINES. 
 
 243 
 
 To determine the reluctance offered by the teeth and 
 winding-slots, it is convenient to assume that the total flux 
 is carried by the teeth alone. Owing to the fringing of 
 the field at the pole tips, not merely the teeth immediately 
 under the pole face carry the flux from that pole, but, with 
 very short air gaps, an extra tooth takes part in the trans- 
 
 TABLE OF TOOTH-DENSITY CORRECTIONS. 
 
 CORRECTED IRON 
 
 DENSITIES ON THE ASSUMPTION THAT THE IRON TRANSMITS 
 
 DENSITY. 
 
 THE ENTIRE FLUX. 
 
 LINES PER SQ. CEN- 
 
 TOOTH WIDTH = 
 
 TOOTH WIDTH = 
 
 TOOTH WIDTH = 
 
 TIMETER. 
 
 SLOT WIDTH. 
 
 | SLOT WIDTH. 
 
 i SLOT WIDTH. 
 
 17050 
 
 17200 
 
 17380 
 
 17510 
 
 18000 
 
 18450 
 
 18600 
 
 18800 
 
 19050 
 
 19680 
 
 2OOOO 
 
 2O2OO 
 
 2OOOO 
 
 21050 
 
 2I3OO 
 
 21850 
 
 2I02O 
 
 22200 
 
 23OOO 
 
 23700 
 
 22OOO 
 
 24000 
 
 24800 
 
 25500 
 
 23100 
 
 26OOO 
 
 26800 
 
 28400 
 
 
 
 1 
 
 TOOTH DENSITY CORRECTION CURVES 
 
 23 
 
 2 
 
 ! 22 
 I* 
 
 I 20 
 
 z 
 o 19 
 
 jr 18 
 
 
 
 
 
 T-WI 
 S =WI 
 
 )TH OF 
 )TH OF 
 
 TOOTH 
 SLOT 
 
 
 
 
 ^H 
 
 4=-75 
 
 
 
 
 
 
 
 
 
 ^ 
 
 S^ 
 
 -S-* 
 
 
 
 
 
 
 /, 
 
 <^ 
 
 S^- 
 
 
 
 
 
 
 
 
 A 
 
 ^ 
 
 
 
 
 
 
 
 
 > 
 
 V 
 
 '/ 
 
 
 
 
 
 
 
 
 /6 
 
 ^ 
 
 
 
 
 
 
 
 
 
 ^ 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
 17 18 VJ 20 21 22 23 M. tt !M 
 
 UNCORRECTED DENSITY' KILOLINE8 PER SQ. CM. 
 
 Fig. 177. 
 
244 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 mission. With large air gaps two or three extra teeth may 
 take part. The value of the permeability obtained from 
 the flux density which is calculated upon the above as- 
 sumption would be too small. The value of the reluc- 
 tance based upon it would in consequence be too large. 
 The flux density arrived at will have to be corrected by 
 reference to the table on page 243. 
 
 The permeability, ^ tt corresponding to the corrected dens- 
 
 23000 
 
 21000 
 
 SB 
 20000 
 
 19000 
 
 18000 
 
 17000 
 
 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 
 10 20 30 40 50 60 70 80 90 100 110 120 130 .140 150 '160 [i 
 
 Fig. 178. 
 
 ity and to be obtained from Fig. 178, should be inserted in 
 the formula for the reluctance of the teeth, 
 
THE DESIGN OF MACHINES. 245 
 
 where A t is the net iron cross-section of the teeth under 
 one pole corrected for fringing. The reluctance, the flux 
 through which must be maintained by the field-windings 
 on one pole, is made up of a bi-parallel path in the arma- 
 ture and a bi-parallel path in the field frame, both arranged 
 in series with the pole, the gap, and the tooth reluctances. 
 This reluctance is equal to 
 
 XV. Magneto-motive force. The ampere turns per 
 pole nl^ necessary to produce the flux <f> a in the armature 
 at no load is equal to 
 
 These ampere turns are furnished by the shunt coil on 
 one pole. 
 
 XVI. SJiunt Coils. Assuming that E b volts are con- 
 sumed in the field regulating rheostat, 
 
 n E E h ' I A 
 
 -"*= 
 
 circular mils' 
 Whence, 
 
 The cross-section in circular mils = II '5 // * ^ 2 / > t 
 
 E E b 
 Where n number of turns in shunt coil, 
 
 7 sA = the current in the shunt at no load, and 
 ! sh = the mean length of one field turn in feet. 
 
 Assuming 1,000 circular mils per ampere in the shunt coil, 
 
 _ circular mils 
 
 , and 
 
 n = 
 
 1000 
 
 1000 n 
 
 circular mils 
 
246 DYNAMO ELECTRIC MACHINERY. 
 
 From a wire table the space occupied by the n turns can 
 be attained ; and, with due allowance for insulation, refer- 
 ence to the preliminary drawing will enable one to deter- 
 mine whether the assumed length of the pole l p is too 
 small or too great. Space must be left for the compound 
 coil. This occupies about one-half as much space as the 
 shunt coil. If l p seems of unsuitable length it should be 
 altered, and the calculation should be again gone over. 
 
 XVII. Compound Coils. The method of calculating 
 the number of compounding turns is so similar to that in 
 the case of shunt coils that it need not be gone into in 
 detail. The compound coils have to compensate at full 
 load for drop in the armature, for drop in the series coil, 
 for drop in the line in case of overcompounding, for the 
 demagnetizing armature ampere turns, and for changes in 
 reluctance due to skew by saturation. The back armature 
 ampere turns, when multiplied by the coefficient of mag- 
 netic leakage, give the series ampere turns necessary to 
 compensate for them. It should be borne in mind that 
 the maximum possible lead brings the brush no farther 
 than the pole tip. To compensate for a drop of a certain 
 percentage requires that the density in the air gap be 
 raised by that same percentage. This necessitates an 
 increase of all the densities. The increase of each reluc- 
 tance and the increase of each corresponding flux must be 
 cared for by the series windings. The coefficient of mag- 
 netic leakage varies with the load. The manner of its 
 variation may be unknown. The reluctances, into which 
 it enters, are such a small per cent of the total, that its 
 variation may often be neglected. 
 
 The following blank form, to be filled in by students in 
 designing, is self-explanatory. 
 
THE DESIGN OF MACHINES. 
 
 247 
 
 POLYTECHNIC INSTITUTE OF BROOKLYN. 
 
 DEPARTMENT OF ELECTRICAL ENGINEERING. 
 
 Data sheet to be filled in by students taking Electrical Engineering 8, and 
 covering the work of the first semester. This must be accompanied by the 
 following scale drawings : end elevation, longitudinal cross-section, plan, and 
 important details. A diagram of the armature-winding must also be given. 
 Assumed values are to be entered in red ink. 
 
 ....Designer Submitted 
 
 SPECIFICATIONS. 
 
 1. Type of Machine 
 
 2. Number of poles 
 
 3. Capacity in kilowatts 
 
 4. Terminal volts at no load 
 
 5. Terminal volts at full load 
 
 6. Amperes at full load 
 
 7. Revolutions per minute 
 
 MATERIALS. 
 
 8. Armature core 
 
 9. Armature spider 
 
 10. Armature end plates 
 
 n. Armature shaft 
 
 12. Commutator segments 
 
 13. Commutator spider 
 
 14. Magnet frame 
 
 15. Pole piece 
 
 1 6. Pole shoe 
 
 17. Brushes 
 
 1 8. Brush-holders 
 
 19. Brush-holder yoke 
 
 20. Commutator insulation 
 
 21. Armature conductor insulation 
 
 22. Field-coil insulation 
 
 DIMENSIONS. 
 Armature. 
 
 23. Diameter over all 
 
 24. Diameter at bottom of slots 
 
 25. Internal diameter of core 
 
 26. Length over conductors 
 
 27. Length of core over laminations 
 
 and ducts 
 
 28. Net length of iron 
 
 29. Number of ventilating ducts 
 
 30. Width of each ventilating-duct 
 
 31. Thickness of sheets 
 
 32. Number of slots 
 
 33. Depth of slots 
 
 34. Width of slot at root 
 
 35. Width of slot at surface 
 
 36. Width of tooth at root 
 
 37. Width of tooth at armature 
 
 face 
 
 38. Size and shape of bare con- 
 
 ductor 
 
 39. Size of conductor insulated 
 
 40. Pitch of winding, No. of teeth 
 
 41. Arrangement of wires or bars 
 
 per slot 
 
 42. Number in parallel per slot 
 
 43. Number in series per slot 
 
 44. Total insulation between con- 
 
 ductors 
 
 45. Thickness insulation between 
 
 conductors 
 
 Air Gap. 
 
 46. Length in center 
 
248 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 47. Length maximum 
 
 48. Bore of field 
 
 49. Minimum clearance 
 
 Pole Shoe. 
 
 50. Length parallel to shaft 
 
 51. Length of maximum arc 
 
 52. Length of minimum arc 
 
 53. Minimum thickness 
 
 Poles. 
 
 54. Length of pole 
 
 55. Width or diameter of pole 
 
 56. Lenpth parallel to shaft 
 
 Magnet Spool. 
 
 57. Number of spools 
 
 58. Length over all 
 
 59. Length of winding-space 
 
 60. Depth of winding-space 
 
 Magnet Frame. 
 
 61. External diameter 
 
 62. Internal diameter 
 
 63. Thickness 
 
 64. Diameter over ribs 
 
 65. Thickness of ribs 
 
 66. Length along armature 
 
 Commutator. 
 
 67. Diameter 
 
 68. Number of segments 
 
 69. Width of segment at commu 
 
 tat or face 
 
 70. Width of segment at root 
 
 71. Useful depth of segment 
 
 72. Thickness of mica insulation 
 
 73. Available length of surface of 
 
 segments 
 
 74. Total length of commutator 
 
 75. Peripheral speed 
 
 Brushes. 
 
 76. Number of sets of brushes 
 
 77. Number in one set 
 
 78. Length 
 
 79. Width 
 
 80. Thickness 
 
 81. Area of contact one brush 
 
 ELECTRICAL. 
 Arn ature. 
 
 82. Voltage at no load 
 
 83. Total voltage at full load 
 
 84. Total current 
 
 85. Number of sections 
 
 86. Turns per section 
 
 87. Number of layers 
 
 88. Total number of inductors 
 
 89. Type of winding . . . circuits 
 
 90. Style of winding 
 
 91. Circular mils per ampere 
 
 92. Mean length of one turn 
 
 93. Total length of arm wire 
 
 94. Resistance of armature cold at 
 
 20 C. . . . ohms 
 
 95. Resistance of armature hot at 
 
 70 C. . . . ohms 
 
 Shunt Coils. 
 96 Size of wire, No. B. & S. Gauge 
 
 97. Turns per layer 
 
 98. No. of layers 
 
 99. Turns per spool 
 
 100. Mean length of one turn 
 
 101. Total turns 
 
 102. Total length of wire 
 
 103. Total weight of wire 
 
 104. Total lesistance at . . . 20 C. 
 
 ohms 
 
 105. Total resistance at ... 70 C. 
 
 . . . ohms. 
 
 1 06. Volts allowed for rheostat 
 
 107. Maximum current . . . am- 
 
 peres 
 
 1 08. Total ampere turns 
 
 109. Circular mils per ampere 
 
THE DESIGN OF MACHINES. 
 
 249 
 
 Series Coils. 
 
 1 10. Size and shape of conductor 
 in. Number of conductors in mul 
 tiple 
 
 112. Arrangement 
 
 1 13. Turns per layer 
 
 114. Number of layers 
 
 1 1 5. Turns per spool 
 
 1 1 6. Mean length of one turn 
 
 117. Total turns 
 
 1 1 8. Total length of conductor 
 
 119. Total resistance at . . . 20 C. 
 
 . . . ohms. 
 
 1 20. Total resistance at . . . 70 C. 
 
 . . , ohms. 
 
 121. Maximum current . . . am- 
 
 peres 
 
 122. Total ampere turns 
 
 123. Circular mils per ampere 
 
 HEATING. 
 Armature. 
 
 124. Area of drum radiating sur- 
 
 face . . . sq. in. 
 
 125. Area each end radiating sur- 
 
 face . . . sq. in. 
 
 126. Total radiating surface . . . 
 
 sq. in. 
 
 127. /'/'full load . . . watts 
 
 128. Hysteresis . . . watts 
 
 129. Eddy currents . . . watts 
 
 130. Total . . . watts 
 
 131. Total J"R and core loss at 
 
 full load . . . watts 
 
 132. Watts per sq. in. radiating sur- 
 
 face, full load 
 
 133. Estimated rise of temperature 
 
 at full load . . . C. 
 
 134. Friction of windage and bear- 
 
 ings . . . watts 
 
 Field Coils. 
 
 135. Radiating surface (heads) . . . 
 
 sq. in. 
 
 136. Radiating surface (periphery) 
 
 . . . sq. in. 
 
 137. Total radiating surface . . . 
 
 sq. in. 
 
 138. 1*R shunt coils and rheostat 
 
 . . . watts 
 
 139. I*R series . . . watts 
 
 140. Total J*R . . . watts . . . % 
 
 141. Watts loss per sq. in. . . . radi- 
 
 ating surface 
 
 142. Estimated rise of temperature 
 
 at full load . . C. 
 
 Commutator, 
 
 143. Brush friction . . . watts 
 
 144. Brush contact . . , watts 
 
 145. Other commutator losses . . 
 
 watts 
 
 MAGNETIC. 
 
 146. fyi at open circuit 
 
 147. fa a< full load 
 
 148. Leakage coefficient 
 
 149. <fcf at open circuit . . , </" at 
 
 full load 
 
 150. Ampere turns required for 
 
 shunt 
 
 151. Ampere turns required for 
 
 series 
 
250 RELUCTANCES AND AMPERE TURNS PER POLE. 
 
 H 5 | 
 
 
 dVO'l 
 
 ON 
 
 O'tf 
 
 3 
 
 fa CJ 
 
 s 
 ^ 
 
 o '2 * 6 
 
 I > IS 
 
 tfi S 
 
 . H CJ W 
 
 ^ CO Tf tr> 
 
 U \O ^O MD 
 
 ^2 i* c 
 ^ .S 3 
 
 - ^ 
 
 a "3 ts 
 
 Hs 
 
 Q\ O M 
 io \O vo 
 
TESTS. 
 
 251 
 
 CHAPTER XIV. 
 
 TESTS. 
 
 122. Determination of a (B-OC Curve. The most exact 
 laboratory method of rinding this curve is by the ballistic 
 galvanometer or ring method. Fig. 179 shows the arrange- 
 ment of apparatus for this 
 method. X is the test piece in 
 the form of an annular ring, hav- 
 ing a mean circumference of / 
 centimeters and a radial cross- 
 section of a sq. centimeters. It 
 is wound uniformly with n prim- 
 ary turns of wire. Over these 
 three or four secondary test turns 
 of wire lead off to the ballistic 
 galvanometer G. A series cir- 
 cuit is formed of a storage bat- 
 tery or other suitable source of 
 
 E. M. F., B, a variable resistance R, the primary coil of 
 the test piece, an ammeter A, and a com mutating switch C. 
 The last is used for reversing the direction of the current 
 in the primary coil. 
 
 If R be adjusted to give a current 7, then the magne- 
 tizing force, or 5C, by 2 1 is represented by the formula 
 
 Fig. 179. 
 
252 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 If now the current be suddenly commutated, all the lines of 
 force will be withdrawn, and as many more set up in the 
 opposite direction. Each of these lines will induce, upon 
 commutation, a pressure in each turn of the test coil. 
 This induced E.M.F. furnishes a means of measuring the 
 flux of lines in the test piece or the flux density (B. By 
 application of the formula given in 16, one may obtain 
 the expression for this quantity, 
 
 Where 
 
 J? t = the resistance of the test coil, the galvanometer, and 
 
 the secondary circuit ; 
 a = the area of a radial section of the test piece in 
 
 square centimeters ; 
 
 2 = the number of turns in the test coil ; 
 k = a constant of the galvanometer; and 
 = the throw of the galvanometer which accompanies the 
 
 commutation of the primary current. 
 
 Though the most accurate method, the ring method is 
 not generally employed in commercial practice because of 
 
 the cost and the time re- 
 quired in preparing a test 
 piece. 
 
 The divided-bar method 
 admits of the use of a bar 
 of iron of ordinary shape as 
 
 TEST 
 COIL 
 
 V TO V--MEAN_CENGTH 
 L OF TEST PIECE 
 
 l8 - 
 
 into two pieces. A heavy 
 wrought-iron yoke, Fig. 180, has a magnetizing coil wound 
 inside of it. Through snug-fitting holes in the ends of 
 the yoke, the two halves of the test piece are inserted, 
 
TESTS. 
 
 253 
 
 one being secured, and the other being fitted with a 
 handle. The test coil is so mounted on springs as to 
 fly suddenly to one side when the test pieces are slightly 
 separated by a pull on the handle. It thus cuts all 
 the flux in the piece, and affords a means of measuring 
 it. The yoke is so massive, and has such a small reluc- 
 tance as compared with that of the test piece, that the 
 
 formula 3C = is practically true, where / is the mean 
 
 10 / 
 
 length of the test piece which is traversed by magnetic 
 lines. For (B the formula is twice what it was in the 
 ring method, since the test coil cuts the flux but once, or 
 
 tkO. 
 
 The method of reversals could be used equally well with 
 this apparatus, requiring the formula used in the ring 
 method. 
 
 The permeameter is a machine for measuring the flux in 
 a test piece by measuring the force necessary to detach it 
 from another part of the magnetic cir- 
 cuit. Fig. 1 8 1 shows in simple such a 
 machine. The magnetizing force is 
 supplied by the coil C the same as in 
 the divided bar method. The test coil 
 and galvanometer are done away with. 
 The bottom of the yoke Y is surfaced 
 to receive flatly the end of the test rod 
 T. When the proper current is flow- 
 
 r Fig. x8x. 
 
 ing in the coil, the force necessary to 
 
 separate the test piece from the yoke is found by means of 
 
 the spring-balance S. Since the force required to break 
 
254 DYNAMO ELECTRIC MACHINERY. 
 
 any number of lines of force varies as the square of that 
 number, it is easy to calculate the flux, and since 
 
 < = area X , 
 
 the magnetic density is readily found. The value of 3C is 
 obtained as in the preceding case. 
 
 123. Determination of the Ballistic Constant. The 
 
 standard condenser affords the most convenient and accu- 
 rate means of determining the constant of a ballistic gal- 
 vanometer. If the capacity of the condenser C, and the 
 voltage at which it is charged E, be known, then the quan- 
 tity of electricity that will flow when the circuit is closed 
 through the galvanometer is also known. It is equal to 
 EC. By observing the galvanometer throw 0, the value of 
 the constant k is determined from 
 
 The coil of a d' Arsonval ballistic galvanometer moving in 
 its field has an E.M.F. induced in it, which tends to send 
 a current in a direction opposite to that of the current that 
 produces the throw, and which, therefore, shortens the 
 throw or damps the galvanometer. The magnitude of this 
 damping current depends, of course, on the resistance of 
 the galvanometer circuit ; hence the constant k should be 
 determined with the same external resistance in the gal- 
 vanometer circuit as there will be when the test for the 
 value of & is being made. 
 
 To accomplish this an arrangement of apparatus such as 
 is shown in Fig. 182, may be employed, the particular fea- 
 ture of which is the quadruple contact key. This key is 
 
TESTS. 
 
 255 
 
 normally held up against a contact. In this position the 
 galvanometer circuit is open, and the condenser is in series 
 with a charging battery. 
 As the key is pressed down, 
 three things occur. First 
 the battery circuit is broken, 
 then the condenser is dis- 
 charged through the gal- 
 vanometer, and lastly the 
 galvanometer circuit is 
 closed through an appro- 
 priate amount of resistance 
 in the rheostat. 
 
 The ballistic constant may also be determined by the 
 use of a long solenoid, with a few turns about its center 
 for a test coil. A series circuit is formed (Fig. 183) of 
 a battery, a variable resistance, an ammeter, the solenoid, 
 
 fomrnmsm 
 
 ^ 
 
 Fig. 183. 
 
 and a key. The ends of the test coil are attached to the 
 galvanometer through proper resistance. On closing the 
 circuit the current / sets up a field at the center of the 
 solenoid, whose intensity, 
 
256 DYNAMO ELECTRIC MACHINERY. 
 
 where / is the length of the solenoid in centimeters, and n 
 the number of primary turns. 
 
 If A be the area in square centimeters of a cross-section 
 of the solenoid perpendicular to the axis, then 
 
 If n 2 be the number of turns in the test coil, and R be 
 the resistance of the galvanometer circuit in ohms, then 
 upon closing the circuit and upon establishing the flux <f>, a 
 quantity of electricity will pass around the secondary cir- 
 cuit which is equal to 
 
 n - w - n ^ 
 
 ~ 8 
 
 where is the throw of the galvanometer corresponding to 
 the current /. Therefore, 
 
 If the solenoid be less than ten diameters long, this result 
 is not accurate, owing to the influence of the ends of the 
 solenoid upon the value of JC. 
 
 There have been described numerous methods for deter- 
 mining k which depend upon a constant and definite inten- 
 sity of the earth's magnetic field. Nowadays the fact 
 that the earth's field is constantly changing, both in direc- 
 tion and magnitude, due to the prevalence of iron and steel 
 buildings, and the extensive use of electric currents for 
 trolley, lighting, and other purposes, makes these methods 
 practically worthless. 
 
 124. Determination of the Hysteretic Constant. The 
 
 hysteresis curve for any sample of iron may be found most 
 accurately by the step-by-step method. The arrangement 
 of apparatus is in all respects similar to that of Fig. 179, 
 
TESTS. 257 
 
 save that the rheostat R must be so designed that the cir- 
 cuit does not open, even for an instant, in passing from 
 one resistance point to another. The method of operation 
 is as follows : The rheostat is set for a maximum current 
 strength which is determined by means of an ammeter. 
 The rheostat handle is then quickly moved back one point. 
 This reduces the current and the dependent magnetizing 
 force proportionately. There is an accompanying decrease 
 of flux in the sample. This decrease is determined by the 
 galvanometer throw, the formula being as before, 
 
 Change in <& = ^-^- kO. 
 an^ 
 
 The ammeter current is again read ; and, as soon as the 
 galvanometer comes to rest, the resistance is increased by 
 another step, and the throw of the galvanometer is observed. 
 After the current has been reduced step by step to zero, it 
 is then commutated, and increased by steps until the maxi- 
 mum magnetization is obtained in a direction opposite to 
 that at the beginning. The current is again cut down by 
 steps to zero, is afterwards commutated for a second time, 
 and is again increased until the magnetic condition of the 
 iron which prevailed at the start is again attained. Giving 
 to (B a plus or minus sign, according to the direction of the 
 galvanometer throw, the algebraic sum of all the changes 
 of (B must equal zero. Therefore the algebraic sum of 
 all the galvanometer throws should equal zero. A simple 
 addition serves as a check on all observations. In practice, 
 the sum of the plus throws may differ from the sum of the 
 minus ones by three per cent without seriously affecting the 
 final result. Having the maximum value of (B, the (B's cor- 
 responding to each can readily be found by subtracting the 
 
2 5 8 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 changes in (fc from the maximum <B. Upon plotting a cyclic 
 curve of the various values of (fc and the corresponding 
 values of 5C, one obtains a hysteresis loop, as in Fig. 184. 
 The area of this loop in (BX units, when divided by 4?r, gives 
 the ergs loss of energy in carrying one cubic centimeter of 
 the iron under test through a cycle of magnetization be- 
 tween the limits of + > max and ($> max . According to the 
 Steinmetz formula, 
 
 h = rj^ max , 
 
 where h is the loss by hysteresis in ergs per cycle per cubic 
 centimeter. Hence, to find the hysteresis constant 17 of 
 the sample used in the foregoing test, one uses the formula 
 
 where A h =. area of hysteresis curve expressed in (B3C units, 
 and V '= volume of iron in cubic centimeters. 
 
 U ague fixing 
 
 Fig. 184. 
 
TESTS. 
 
 259 
 
 A much less laborious method of measuring rj, and one 
 which does not introduce the errors attending the measure- 
 ment of the area of a curve, is the wattmeter method. Since 
 the iron to be tested is generally for use in alternating cur- 
 rent apparatus, this method has the additional advantage 
 that the test occurs under the conditions which the iron 
 will meet in its working. 
 
 If the ring be made of annular stampings of sheet metal 
 well shellacked before assembling, then the loss due to eddy 
 currents will be negligible. The arrangement of apparatus, 
 shown in Fig. 185, consists of a source of alternating cur- 
 rent, a wattmeter, an alternating current ammeter, an alter- 
 nating current voltmeter, and the test ring, all connected 
 as shown. 
 
 Fig. 185. 
 
 Let R the resistance of the coil on the test ring ; 
 n = number of turns in this coil ; 
 W = the watts indicated by the wattmeter ; 
 /= the current indicated by the ammeter; 
 E = the pressure indicated by the voltmeter ; 
 V= the volume of the iron in cubic centimeters; 
 A = the area of a radial cross-section in square centi- 
 meters ; then, assuming the current to be sinus- 
 oidal, and of frequency y cycles per second, 
 
 Vf 
 
260 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 E wing's machine for hysteresis tests is shown in Fig. 186. 
 Its chief advantage lies in the fact that the test piece needs 
 to consist of but half a dozen pieces of sheet iron f " by 3". 
 This test piece is made to rotate be- 
 tween the poles of a permanent mag- 
 net, which is mounted on knife edges 
 on an axis coincident with the axis of 
 revolution of the test piece. The re- 
 sulting angular displacement of the 
 magnet, as marked by a pointer on a 
 divided scale, is proportional to the 
 hysteresis loss in the specimen. A 
 calibration curve is plotted by using 
 two different specimens having known 
 hysteretic constants. It is found that 
 small variations in the thickness of the test piece do not 
 affect the results, and that no correction need be made 
 for such variations. The machine yields but comparative 
 results. 
 
 Fig. 186. 
 
 125. Determination of Leakage Coefficient. The ratio 
 of the total flux generated by a field magnet to the flux 
 passing through the armature, that is, the leakage coefficient, 
 which is always greater than unity, may be found with an 
 arrangement of apparatus as shown in Fig. 187 where the 
 machine is a yoke-wound bipolar. A test coil of a few 
 turns is passed around the center of the field magnet, and 
 through it all the lines generated may reasonably be as- 
 sumed to pass. A similar coil is passed around the arma- 
 ture, in a plane perpendicular to the direction of the flux, 
 through which all the armature flux must pass. In the 
 case of a small machine, normal exciting current is passed 
 
TESTS. 
 
 261 
 
 through the field magnet, with arrangements for rapid com- 
 mutation. In this case, if one test coil have its ends 
 attached to a galvanometer or a low-voltage voltmeter, and 
 if the current in the field coil be commutated, a deflection, 
 which is proportional to the change of flux, will be observed. 
 The same will happen if the other coil have its terminals 
 connected to the galvan- 
 
 If B f and O a 
 
 ometer. 
 
 the deflections 
 
 with the field 
 
 armature test coils 
 
 spectively, then, as 
 
 fore, 
 
 be 
 
 observed 
 and the 
 
 2 n ' 
 
 re- 
 be- 
 
 and 
 
 2 ;/ 
 
 k6 a . 
 
 Fig. 187. 
 
 If the two test coils be constructed alike as regards number 
 of turns and resistance, then the values of R, n# and k are 
 the same in both equations, and we have the leakage coef- 
 ficient 
 
 Hence the ratio of the galvanometer throws gives the co- 
 efficient without further calculation. 
 
 This method may be employed to obtain the flux in any 
 part of the magnetic circuit, and it serves to locate the 
 points of greatest leakage. It may also be modified to apply 
 to any type of machine. In the case of large machines, 
 whose field currents cannot be commutated, a cyclic in- 
 crease and decrease of exciting current can be produced 
 by means of cutting out and in of resistance in the field 
 
262 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 circuit. Even then the time constants of large field coils 
 are so great as compared with the period of swing of ballis- 
 tic galvanometers, that the method is impracticable. 
 
 126. Magnetic Distribution in the Air Gap. Since ar- 
 mature reactions distort the magnetic field, it is desirable 
 to know the actual distribution of the flux. This may be 
 
 666 6 664 
 
 LOAD 
 
 Fig. 188. 
 
 determined by the use of a pilot brush, as shown in Fig. 
 1 88. A voltmeter is connected between one of the main 
 brushes and the pilot brush, and the latter is moved 
 through equal angular intervals until the opposite brush 
 is reached. The difference in the voltage of any two 
 consecutive readings is proportional to the magnetic flux 
 within the angular distance moved over between those 
 two readings. 
 
 Two pilot brushes may be used as in Fig. 1 89. In this 
 case the voltage is proportional to the flux corresponding 
 to the angular distance between the two brushes. By 
 
TESTS. 
 
 263 
 
 applying these brushes at successive intervals through 
 1 80 the flux distribution can be determined. 
 
 127. Measurement of Resistance a. By voltmeter alone. 
 For insulation resistances, or any resistances lying between 
 about 5000 and 100,000 ohms, a fairly accurate result may 
 be obtained by arranging the unknown resistance x and a 
 0-150 voltmeter in series with a source of constant potential 
 of about 115 volts. The reading is noted. The resist- 
 ance is then short-circuited and the deflection & noted. If 
 R be the resistance of the voltmeter, then 
 
 Maximum accuracy is obtained when x R. 
 
 AWIAMWI 
 
 *-^ 
 
 l 
 
 Fig. igo. 
 
 Fig. 191. 
 
 b. By the Method of Wheatstone's Bridge. If an un- 
 known resistance :r, two known resistances a and b, and 
 a known adjustable resistance R be connected as shown 
 in Fig. 191, with a galvanometer G and a battery cell B, 
 a Wheatstone's bridge is formed ; and, if the resistance R 
 be so manipulated as to prevent a flow of current through 
 the galvanometer, then the following relation is true ; 
 
 a \b\\R\x. 
 
264 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 It is usual to make the ratio a : b equal to some multiple 
 or submultiple of 10. In this case the value of X is read 
 directly from R with the decimal point suitably placed. 
 
 This method permits of great accuracy. 
 
 C. By ammeter and voltmeter. Resistances of ordinary 
 magnitudes are most conveniently measured by measuring 
 the pressure impressed on the resistance and the current 
 caused to flow thereby. This is the most practical method 
 for finding the resistances of armature and field-windings 
 of dynamos. 
 
 It is a method so rapid that the value of hot re- 
 sistances may be found, and fields can be measured even 
 
 Fig. 192. 
 
 while the machine is in operation. Fig. 192 shows an 
 arrangement of apparatus for measuring the resistance of 
 an armature, including the brush and contact resistances. 
 If / be the ammeter reading, and E be the voltmeter read- 
 ing, then by Ohm's law 
 
 128. Test of Dielectric Strength -- In order to test the 
 voltage necessary to break down a sample sheet of insulat- 
 ing material, the sample is placed between two flat metal- 
 lic surfaces, which are connected respectively with the two 
 terminals of a high-voltage transformer, whose voltage can 
 be varied at will. An air gap between needle-point ter- 
 
TESTS. 
 
 265 
 
 minals which can be adjusted in length is connected in 
 parallel between the two terminals. The distance between 
 these points serves to limit the voltage which can be im- 
 pressed upon the conductors on each side of the insulating 
 material. For small variations of gap length the voltage 
 necessary to produce an arc between the needle-points is 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ,^- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^ 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 x*" 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 t 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 I 
 
 <l f 
 
 S'' 
 
 
 2 
 
 6 
 
 2 
 
 8 
 
 3 
 
 3 
 
 I 
 
 3 
 
 1 
 
 3 
 
 G 
 
 3 
 
 8 4 
 
 x 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 . 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ,x 
 
 __ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^ 
 
 ^x 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 t 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 f 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 2J 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 6 .2 4 .6 .1 
 
 1. 1.2 1.1 1.6 l.S 2 
 
 Fig. 193- 
 
 nearly proportional to the length. The following table, 
 taken from the Standardization Report of the American In- 
 stitute of Electrical Engineers, shows the relation which 
 exists between air-gap length and the voltage necessary 
 to produce a disruptive discharge. The relations are also 
 exhibited in the curve of Fig. 193. 
 
266 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 TABLE OF SPARKING DISTANCES IN AIR BETWEEN 
 OPPOSED SHARP NEEDLE-POINTS, FOR VARI- 
 OUS EFFECTIVE SINUSOIDAL VOLTAGES, 
 IN INCHES AND IN CENTIMETERS. 
 
 KlLOVOLTS. 
 
 DISTANCE. 
 
 KlLOVOLTS. 
 
 DISTANCE. 
 
 Sq. Root of 
 Mean Square. 
 
 Inches. 
 
 Cms. 
 
 Sq. Root of 
 Mean Square. 
 
 Inches. 
 
 Cms. 
 
 5 
 
 0.225 
 
 0.57 
 
 60 
 
 4.65 
 
 1 1.8 
 
 10 
 
 0.47 
 
 I.I 9 
 
 70 
 
 5.85 
 
 14.9 
 
 15 
 
 0.725 
 
 1.8 4 
 
 80 
 
 7-1 
 
 1 8.0 
 
 20 
 
 I.O 
 
 2-54 
 
 90 
 
 8-35 
 
 21.2 
 
 25 
 
 1-3 
 
 3-3 
 
 IOO 
 
 9 .6 
 
 24.4 
 
 30 
 
 1.625 
 
 4.1 
 
 1 10 
 
 10.75 
 
 27-3 
 
 35 
 
 2.O 
 
 5- 1 
 
 120 
 
 11.85 
 
 3 O.I 
 
 40 
 
 2-45 
 
 6.2 
 
 130 
 
 12-95 
 
 32.9 
 
 45 
 
 2-95 
 
 7-5 
 
 I4O 
 
 13-95 
 
 35-4 
 
 50 
 
 3-55 
 
 9.0 
 
 150 
 
 15.0 
 
 38.1 
 
 In carrying out the test, the needle-points are adjusted 
 at a certain minimum distance apart. The voltage im- 
 pressed upon the terminals is raised until a spark passes 
 between the points. The air gap is then increased in 
 length, and the operation repeated until the sample breaks 
 down, and the spark passes through it instead of across 
 the air gap. The break-down voltage is then taken from 
 the table or curve corresponding to the last position of the 
 needle-points. 
 
 The sample should project considerably beyond the 
 edges of the compressing surfaces. Owing to surface 
 leakage a spark will pass over a very much greater distance 
 of the surface of an insulator than it will in free air. 
 
 For the purpose of obtaining a voltage any form of high- 
 potential transformer may be used, the primary being sup- 
 
TESTS. 
 
 267 
 
 plied by an alternating current. Fig. 194 illustrates a 
 10,000 volt transformer manufactured for this purpose by 
 the General Electric Co. Its core is of the H type, and 
 
 Fig. 194. 
 
 upon one branch of it is wound the low-tension circuit, 
 while upon the other is wound the secondary, consisting of 
 four coils, each wound and insulated independently. The 
 four coils are assembled upon a sleeve of heavy insulating 
 
268 
 
 DYNAMO ELECTRIC MACHINERY. 
 
 material. The transformer is immersed in oil, and its 
 primary is wound in two parts so that it may be used upon 
 a 52 or a 104 volt circuit. The adaptation to either of 
 these circuits is rendered possible by means of a porcelain 
 series multiple connection board which is placed inside the 
 inclosing case. On the top of the apparatus is a box with 
 a glass window, which incloses a micrometer spark gap, 
 which is connected in shunt across the high-potential 
 
 Fig. 195. 
 
 terminals. This box or cover carries 
 four long contact studs which fit into 
 sockets. In the transformer box the 
 apparatus is so arranged that the lifting 
 up of this cover for the purpose of ad- 
 justing the spark gap entirely discon- 
 nects the spark gap from the high-potential circuit. The 
 connections of this apparatus to a sample under test are 
 shown in Fig. 195. 
 
 This apparatus may also be employed in determining 
 whether a given sample of insulation will withstand an im- 
 pressed electromotive force without breaking down. The 
 
TESTS. 
 
 269 
 
 length of the gap is set so as to represent the value of the 
 prescribed electromotive force, and the sample is subjected 
 to the pressure which maintains a spark across the gap. 
 In case of break-down the spark at the gap will cease. 
 
 In case it is desired to test the dielectric strength of the 
 sample at some other than normal temperature, the sample 
 may be pressed between two surfaces of the apparatus 
 shown in Fig. 196, which was described by Mr. Charles F. 
 
 Fig. 196. 
 
 Scott. Two carefully faced blocks of cast iron are re- 
 cessed so as to receive coils D and D' of asbestos-wound 
 wire. These coils are supplied with alternating current 
 which raises the temperature of the disks by means of 
 eddy currents and hysteresis losses. Upon shutting off 
 the current, the disks and insulating material soon assume 
 a uniform temperature, which can be measured by means 
 of a thermometer whose bulb is inserted in a hole in the 
 upper disk. The two disks are made the terminals of the 
 high-tension circuit. Connections with the circuit which 
 is used for heating purposes must of course be removed 
 during the test. 
 
270 DYNAMO ELECTRIC MACHINERY. 
 
 The report of the committee on standardization of the 
 American Institute of Electrical Engineers gives the fol- 
 lowing : " The dielectric strength or resistance to rup- 
 ture should be determined by a continued application of an 
 alternating E.M.F. for five minutes/' 
 
 " The test for dielectric strength should be made with 
 the completely assembled apparatus and not with its indi- 
 vidual parts, and the voltage should be applied as fol- 
 lows : ist, Between electric circuits and surrounding 
 conducting material, and 2d, between adjacent electric cir- 
 cuits where such exist." 
 
 The report further recommends for apparatus, not in- 
 cluding switchboards and transmission lines, the following 
 testing voltages : 
 
 RATED TERMINAL VOLTAGE. CAPACITY. TESTING VOLTAGE. 
 
 Not exceeding 400 volts Under 10 K.W. . . 1000 volts. 
 
 Not exceeding 400 volts 10 K.W. and over . 1500 volts. 
 
 400 and over, but less than 200 volts Under 10 K.W. . . 1500 volts. 
 
 400 and over, but less than 800 volts 10 K.W. and over . 2000 volts. 
 
 800 and over, but less than 1 200 volts Any 3500 volts. 
 
 1 200 and over, but less than 2500 volts Any 5000 volts. 
 
 ( Double the normal 
 
 2500 and over Any . . < 
 
 ( rated voltages. 
 
 Synchronous motor fields and fields of converters started 
 
 from the alternating current side 5000 volts. 
 
 The values in the table above are effective values, or square roots of 
 mean square reduced to a sine wave of E.M.F. 
 
 When machines or apparatus are to be operated in series, so as to em- 
 ploy the sum of their separate E.M.F?*, the voltage should be referred to 
 this sum, except where the frames of the machines are separately insulated 
 both from ground and from each other. 
 
 129. Determination of the Magnetization Curve of a 
 
 Shunt-Dynamo To find the relation between the exciting 
 
 current and the no-load terminal volts of a shunt machine, 
 
TESTS. 
 
 2/1 
 
 excite tne shunt fields, Fig. 197, from an external source, 
 first passing the current through a variable resistance and 
 
 Fig. 197. 
 
 an ammeter. Run the machine at a constant speed through- 
 out the test. If a voltmeter be placed across the armature 
 terminals a pressure can be read corresponding to each 
 exciting current, and a curve can be plotted using volts as 
 ordinates and amperes as abscissae. Because of residual 
 
 HELD CURRENT 
 
 Fig. 198. 
 
 magnetism there are some volts with no exciting current, 
 and hence the curve, Fig. 198, does not pass through the 
 origin. 
 
 If the voltmeter be read while the current is increasing 
 by steps to the maximum, and again while the current is. 
 
2/2 DYNAMO ELECTRIC MACHINERY. 
 
 decreasing, step by step, the two curves will not coincide ; 
 the descending curve will lie above the other as in Fig. 1 99. 
 This is because of the hysteresis or magnetic retentivity of 
 the iron of the magnetic circuit. 
 
 Fig. 199. 
 
 130. Efficiency of Dynamos and Motors The efficiency 
 of these machines can be determined by any one of the 
 following methods : 
 
 a. Run the machine at its proper speed as a separately 
 excited motor. Let the excitation be normal. By means 
 of ammeter and voltmeter readings determine the electrical 
 input, the motor having no load upon it. The arrange- 
 ment of apparatus is shown in Fig. 200. The power put 
 in represents the PR losses in the armature and the field 
 plus the losses which are generally considered as constant 
 at al 1 loads. These constant losses are those due to fric- 
 tion, hysteresis, Foucault currents, and windage. They 
 are equal to the no-load input minus the no-load PR arma- 
 ture and field losses. The PR losses can be calculated at 
 
TESTS. 
 
 273 
 
 any useful load. The efficiency at that load is equal to 
 the load divided by the load plus the sum of the constant 
 
 SERVICE MAIN 
 
 ADJUSTABLE 
 RESISTANCE 
 
 SERVICE MAIN 
 
 Fig. 200. 
 
 losses and the load PR losses. The machine at the time 
 of no-load test should have the same temperature as it 
 would have under the load for which the efficiency is being 
 calculated. 
 
 b. Run the machine as a motor at its 
 rated speed and temperature. Measure 
 the electrical input by a voltmeter and an 
 ammeter. Measure the mechanical output 
 by a Prony brake. Then the efficiency, 
 
 _ watts at brake 
 watts input 
 
 There are many kinds of brake or ab- 
 sorption dynamometers that may be used 
 for this test. The most satisfactory one 
 for motors of small size is the strap-brake shown in 
 Fig. 20 1. A piece of leather belting and two spring 
 balances are all that is necessary. The formula for the 
 
 absorbed power is, 
 
 2TrrV(P-P'} 
 
 Watts = ^ X 746, 
 
 33000 
 
 where r= the radius of the pulley in feet ; 
 
 Fig. 201. 
 
2/4 DYNAMO ELECTRIC MACHINERY. 
 
 V= number of revolutions per minute ; 
 (P P') = the difference of the two-scale readings in 
 pounds. 
 
 Fig. 202 shows a form 
 of brake applicable to lar- 
 ger machines. The for- 
 mula for the power ab- 
 sorbed is, 
 
 2 irrVP 
 
 Watts = - - X 746, 
 33000 
 
 where r is the perpendicular distance from the center of 
 the pulley to the line of action of the scale in feet, P the 
 scale reading in pounds, and V the number of revolutions 
 per minute. The brake should be so poised as to give no 
 reading on the spring at no load. 
 
 The brake may be made with a metal strap having 
 spaced blocks on its under surface that screw down against 
 the wheel, and for the spring balance one may use a plat- 
 form scale having a prop extending to the lever arm of the 
 brake. For large machines the heat generated by the 
 absorption of considerable power at the face of the pulley 
 causes an excessive rise of temperature. It is necessary 
 to find some means of carrying the heat away. This is 
 generally accomplished by flanging the inside of the brake- 
 wheel, forming a trough in which water is kept running. 
 Centrifugal force throws the water against the internal 
 circumference of the wheel and prevents spilling. The 
 water is removed either by a properly placed scoop, or it 
 may be allowed to boil out. 
 
 c. A convenient method of finding and separating the 
 losses of a machine is one which makes use of a rated 
 
TESTS. 275 
 
 motor, i.e., a motor whose mechanical output is known for 
 any given electrical input. By reading the volt-ampere 
 input of the motor the power expended on the machine to 
 be tested can be found. Run the machine by the rated 
 motor at the proper speed. If the brushes be removed 
 from the machine, and no current be flowing in the field 
 coils, then the power expended on it is the loss due to 
 friction at the bearings and to windage. Now let the 
 brushes be set, then the power expended is the loss due to 
 windage, bearing friction, and brush friction. By sub- 
 traction the brush-friction loss is found. This is greater, 
 particularly in small machines, than is generally supposed. 
 Now let the fields be separately excited by the normal 
 current, and the losses due to hysteresis and eddy currents 
 are included in the power expended on the machine. 
 From a knowledge of the hot resistances of the machine, 
 one can calculate the I 2 R loss for any useful load in both 
 armature and field windings. This useful load divided by 
 the sum of the useful load and all the losses, gives the 
 efficiency of the machine at that load. 
 
 d. The methods a, b, and c all require some outside 
 electric power. This requirement can be avoided by the 
 use of a transmission dynamometer to measure the power 
 input of any machine, and the power output can be read by 
 a voltmeter and an ammeter. This method is seldom re- 
 sorted to, since transmission dynamometers are often unre- 
 liable, they are expensive to set up, and some forms have 
 but a limited power range. 
 
 Professor Goldsborough has recently devised a very 
 ingenious dynamometer which consists simply of a coiled 
 or helical spring with the center line of the helix corre- 
 sponding with the center of the shaft. This spring con- 
 
2/6 DYNAMO ELECTRIC MACHINERY. 
 
 nects the driving and driven members. Readings are made 
 by means of two instantaneous contact points mounted, 
 one on the driven and one on the driving-shaft, which are 
 connected in series with each other and with a battery 
 and telephone receiver. As the spring becomes deflected 
 by a load, the contact on the driven shaft falls back, 
 and the corresponding brush must be set back by the 
 same angle in order to obtain a click in the telephone. 
 This angle is a direct measurement of the torque, and can 
 be calibrated at standstill. 
 
 e. The efficiency of direct connected units can be found 
 by using the indicator card from the steam-engine to deter- 
 mine the power input, and by using a voltmeter and an 
 ammeter for determining the electrical output. Even if 
 the engine losses were exactly known, the measurements 
 yielded by an indicator card are hardly exact enough to 
 afford a fair basis for testing the efficiency of a generator. 
 In other than direct-connected units it is not frequent that 
 one finds a generator driven by an engine that does no 
 other work. 
 
INDEX. 
 
 Air-gap, 69, 8 1, 85. 
 
 distribution in, 262, 87. 
 Ampere-turns, 21, 245. 
 Angle of lag or lead, 82. 
 Armatures, 34, 47, 51. 
 
 Back-turns, 82. 
 
 Bearings, 64. 
 
 Boosters, 216. 
 
 Box, starting for motors, 169. 
 
 Brake on motors, 179. 
 
 prony, 273. 
 
 solenoid, 182. 
 
 strap, 1 80. 
 Brushes, 60, 61, 86, 90. 
 
 Candle-power of arc lamps, 130. 
 of incandescent lamps, 103. 
 Capacity of a dynamo, 36. 
 Cauteries for surgeons, 215. 
 Chord winding, 52. 
 Circuits, divided, 7, 8. 
 Coefficient : 
 
 economic, 93. 
 
 of series dynamo, 97. 
 of compound dynamo, 1 1 1 . 
 of shunt dynamo, 100. 
 of conversion, 93. 
 of magnetic leakage, 73, 260. 
 of self and mutual induction, 
 
 17, 18. 
 of temperature, 5. 
 
 Coercivity, 28. 
 Coil: 
 
 armature, 46, 49. 
 
 compound, 71, 246. 
 
 field, 71. 
 
 formed, 55. 
 
 Collection of currents, 61. 
 Commutation, 84. 
 Commutator : 
 
 construction of, 59. 
 
 losses at, 59. 
 
 principle of, 32, 77. 
 
 segments of, 33. 
 Compounding : 
 
 in motor, 174, 224. 
 
 windings for, 71, in. 
 Conductivity, 5. 
 
 of copper, 6. 
 Connections : 
 
 for combined output, 218. 
 
 of motors, 223. 
 
 series dynamos in series, 221. 
 
 shunt dynamos in parallel, 220. 
 
 shunt dynamos in series, 221. 
 
 cross in armature, 48. 
 Constant : 
 
 determination of ballistic by 
 condenser, 254. 
 
 determination of ballistic by 
 long solenoid, 255. 
 
 determination of ballistic earth's 
 field, 256. 
 
 277 
 
2/8 
 
 INDEX. 
 
 Constant : 
 
 determination of hysteretic, 256, 
 259, 260. 
 
 hysteretic, 29. 
 Contact of brushes, 60. 
 Controller : 
 
 for mill motors, 206. 
 
 for street-railway motors 197. 
 
 fingers for, 198. 
 
 reversing lever, 199. 
 
 wipes, 198. 
 Core, armature, 37, 57. 
 
 field, 67. 
 
 Correction for tooth densities, 243, 
 Cross-magnetization. 80. 
 Current density, 72. 
 
 in armature, 234. 
 
 Currents, foucault or eddy, 38, 90. 
 Curve, B-H., 24, 25, 244. 
 
 characteristic, 96, 98, 101, 174. 
 
 determination of B-H, 251. 
 
 determinization of magnetiza- 
 tion, 270. 
 
 Demagnetization, 82, 246. 
 Density of flux, 22. 
 
 in air-gap, 233. 
 
 corrections for, in teeth, 243. 
 
 determination of distribution 
 of, 262. 
 
 in field, 131, 241. 
 Design, data sheet for, 247. 
 
 different methods of, 232. 
 
 of armature, 235. 
 
 of field, 241. 
 
 preliminary assumption for, 233 
 
 specifications for, 233. 
 Diameter of armature, 237. 
 
 of shaft, 64, 119. 
 Dielectric test of insulation of, 268. 
 
 test of strength of, 264, 270. 
 
 Direction of induced E. M. F., 16. 
 
 of rotation of a motor, 161. 
 Drop, 4. 
 Drums, 34, 51. 
 Dynamo : 
 
 arc lighting, 129. 
 Brush, 134. 
 Westinghouse, 141. 
 Wood, 143. 
 Excelsior, 147. 
 Ball, 149. 
 
 Thomson-Houston, 151. 
 Western Electric Co., 156. 
 definition of, 31. 
 direct-driven, 118. 
 
 Bullock Electric Co., 127. 
 Crocker-Wheeler, 123. 
 General Electric Co., 121. 
 Lundell, 88, 124. 
 Sprague Electric Co., 88, 124. 
 Westinghouse Co., 119. 
 Dynamotors, 208. 
 
 armature, reaction in, 208. 
 for Bullock teazer system, 209. 
 for electrometallurgy, 211. 
 as rotary equalizer, 212. 
 for telegraphic work, 213. 
 Dyne, definition of, i. 
 
 Efficiency, 92. 
 
 of compound dynamo, 112. 
 
 of compound motor, 165. 
 
 determination of, 272. 
 
 of direct connected units, 276. 
 
 of motors for automobiles, 202. 
 
 of shunt motors, 166. 
 
 of series motors, 166. 
 E. M. F. : constant supply of, 
 103. 
 
 counter in motors, 163. 
 
 direction of induced, 16. 
 
INDEX. 
 
 279 
 
 E. M. F. : 
 
 in shunt dynamos, 100. 
 
 in separately excited dynamos, 
 
 95- 
 
 in series dynamos, 97. 
 
 of induction, 14. 
 
 of self and mutual induction, 
 17, 79, 84. 
 
 of eddy currents, 38. 
 
 principle of production of, in 
 armature, 32. 
 
 unit of, 3. 
 Energy, i, 89. 
 Equalizer, bus, 222. 
 
 rotary, 212. 
 Erg, i. 
 Excitation, mutual, 222. 
 
 separate, 94. 
 
 of fields, 69. 
 
 Fall, of potential, 4. 
 Feeding-points, 104. 
 Field, magnetic, 12, 21, 67. 
 Fleming's rule, 16. 
 Fluctuation of E. M. F., 34. 
 Force, magnetizing, 22. 
 
 magnetomotive, 20, 26, 245. 
 
 units of, i. 
 Foot-pound, i. 
 
 Frequency of commutation, 84. 
 Friction of bearings, 89. 
 
 of brushes, 60. 
 Fuses, 10. 
 
 Gap, air, 69, 81, 85, 233. 
 Generators [see dynamo], 31. 
 
 Heat of current, 9. 
 Heating of armatures, 39. 
 Holders for brushes, 61. 
 Hysteresis, 27. 
 
 Hysteresis : 
 
 losses by, 89. 
 
 Inductance, 17, 18, 84. 
 Induction: 
 
 electro-magnetic, 13. 
 
 mutual, 1 8. 
 
 self, 17, 79. 
 Inductors, 34, 45. 
 Input, 92. 
 Intensity of magnetic field, 12. 
 
 Jig, for filing brushes, 62. 
 Joints in magnetic circuit, 76. 
 Joule, definition of, i. 
 
 Lag, 82. 
 
 of brushes in a motor, 165. 
 Lamination, 38. 
 Law of Steinmetz, 29. 
 Lead, 82. 
 Leakage, magnetic, 73. 
 
 determination of, 260. 
 
 in compound motor, 175. 
 Length of armature, 237. 
 
 of active conductor, 235. 
 Lines of force, 12. 
 Link, fuse, 10. 
 Losses, 92, 93. 
 
 armature, 238. 
 
 commutator, 59. 
 
 fixed, 167. 
 
 I 2 R, 9. 
 
 in operation, 89. 
 
 variable, 167. 
 Lubrication, 65. 
 
 Magnet, field, 35, 69. 
 Magnets, 70, 96. 
 Materials, insulating, 6. 
 magnetic, 24, 68. 
 
280 
 
 INDEX. 
 
 Matthiessen, standard of, 6. 
 Measurement of temperature rise, 42. 
 Melting of commutator bars, 83. 
 Meters, recording watt-hour, 182. 
 Mica, 7, 59. 
 Mil, circular, 6. 
 
 -foot, 6. 
 Motor, brake, 179. 
 
 compound wound, 174. 
 
 counter E. M. F. of, 163. 
 
 direction of rotation of, 161. 
 
 for railways, 187. 
 
 for automobiles, 200. 
 
 for mills, 203. 
 
 principle of, 161. 
 
 rated, 274, 
 
 series, 185. 
 Motor-generators, 215. 
 
 Oilers, 65. 
 
 Operation, care in, 225. 
 Output, 92. 
 
 Over-compounding, in. 
 coils for, 246. 
 
 Permeability, 22. 
 Permeameter, 253. 
 Permeance, 25. 
 Plane : 
 
 commutating, 78. 
 
 neutral, 78. 
 
 Point, running on controller, 197 
 Pole, magnetic, 12. 
 Pole pieces, 67, 75. 
 Potential, magnetic, 19. 
 Power, lines of, 99. 
 
 of electric current, 8. 
 
 unit of, 2. 
 Pressure, 4. 
 
 Prevention of sparking, 85. 
 Process of commutation, 77. 
 
 Rating of machines, 39. 
 Reaction of dynamo armature, 80. 
 
 of dynamotor armature, 208. 
 
 of motor armature, 165. 
 
 Rectifier, compounding, 112. 
 Re-entrancy, 46. 
 Regulation, arc dynamo, 132. 
 
 hand, 104. 
 
 self, 110-113. 
 
 (see speed). 
 Reluctance, 25. 
 
 of dynamo-magnetic circuit, 242. 
 Reluctivity, 26. 
 
 Report of Standardization Commit- 
 tee of the American Insti- 
 tute of Electrical Engineers : 
 
 on efficiency, 92. 
 
 on regulation, 116. 
 
 on spark-gap voltages, 266, 
 
 on temperature elevation, 40. 
 
 on testing dielectric strength, 
 
 270. 
 Resistance, armature, 240. 
 
 brush contact, 60, 90. 
 
 measurement of, 263. 
 Resistivity, 5. 
 Retentivity, 28. 
 Rheostats, Carpenter enamel, 108. 
 
 overload, 172. 
 
 packed card, 105. 
 
 starting, 169. 
 
 Ward Leonard Electric Co., 
 108. 
 
 Wirt, no. 
 
 Rise of temperature, 41, 91, 60. 
 Rockers, 61, 64. 
 Rule, Fleming's, 16. 
 
 Saturation of teeth, 85. 
 Shafts, 64. 
 
INDEX. 
 
 28l 
 
 Shape of pole pieces, 86, 
 
 Sheet, data for design, 247. 
 
 Shell, magnetic, 20, 21. 
 
 Shifting of brushes, 85. 
 
 Shoe, pole, 75. 
 
 Short-circuit of armature coil, 226. 
 
 Skewing of field, 81. 
 
 Slip-rings, 31. 
 
 Slotting of poles, 86. 
 
 Solenoid, 21. 
 
 brake, 180. 
 Span, polar, 69. 
 Spark, voltage of, 266. 
 Sparking, 61, 83, 85, 225, 
 Spectrum, magnetic, 13. 
 Speed, armature, 239. 
 
 motor, 163. 
 
 slow, 178. 
 
 hand regulation of, 175. 
 
 Leonard system of regulation, 
 176. 
 
 series resistance regulation, 1 76. 
 Steinmetz, law of, 29. 
 Strength, test of dielectric, 264. 
 Surface, for radiation in armature, 
 240. 
 
 Teazer for Bullock system of speed 
 
 control, 209. 
 Teeth, 85. 
 Temperature, critical, 25. 
 
 measurement of, 42. 
 
 rise of, 41, 43, 60, 91. 
 
 Tension of brush springs, 62. 
 Theory of self-regulation of dyna- 
 mos, 113. 
 Torque, 2. 
 
 Transformer for high potentials, 266. 
 Turns, series, no. 
 
 Unit, absolute and practical, 2. 
 mechanical, i. 
 of current, 3. 
 of potential difference, 3. 
 of resistance, 3. 
 pole, 12. 
 
 Velocity, peripheral of armature, 234. 
 Ventilation, ducts for, 39. 
 Vulcabeston, 7. 
 
 Watt, definition of, 2. 
 Wheatstone's bridge, 263. 
 Windage, 89. 
 Winding chord, 52. 
 
 closed-coil, 45. 
 
 cross-connected, 48. 
 
 drum, 51. 
 
 open-coil, 44. 
 
 ring, 47. 
 
 series, 70, 97. 
 
 short-connection, 49. 
 
 shunt, 70, 97. 
 Wires, binding, 55. 
 Work, i, 9, 19. 
 
 Yoke, 36, 67. 
 
LIST OF WORKS 
 
 ELECTRICAL SCIENCE 
 
 PUBLISHED AND FOR SALE BY 
 
 D. VAN NOSTRAND COMPANY, 
 
 23 Murray & 27 Warren Street, 
 
 NEW YORK. 
 
 ATKINSON, PHILIP. Elements of Static Electricity, with full de- 
 scription of the Holtz and Topler Machines, and their mode of operating. 
 
 Illustrated. 12mo, cloth $1.50 
 
 The Elements of Dynamic Electricity and Magnetism. 12mo, cloth. $2.00 
 Elements of Electric Lighting, including Electric Generation, Measure- 
 ment, Storage, and Distribution. Ninth edition, fully revised and new matter 
 added. Illustrated. 8vo, cloth $1.50 
 
 Power Transmitted by Electricity and applied by the Electric Motor, including 
 
 Electric Railway Construction. Third edition, fully revised and new matter 
 added. Illustrated. 12mo, cloth $2.00 
 
 BADT, F. B. Dynamo Tender's Hand-book. 70 Illustrations. 16mo, 
 cloth $1.00 
 
 Electric Transmission Hand - book. Illustrations and Tables. 16mo, 
 
 cloth $1.00 
 
 Incandescent Wiring Hand-book. Illustrations and Tables. 12mo, cloth. 
 
 $1.00 
 Bell Hanger's Hand-book. Illustrated. 12mo, cloth $1 .00 
 
 BIGGS, C. H. W. First Principles of Electricity and Magnetism. A book 
 for beginners in practical work. With about 350 diagrams and illustrations. 
 Illustrated. 12mo, cloth $2.00 
 
 BLAKESLEIT, T. H. Papers on Alternating Currents of Electricity, for 
 the Use of Students and Engineers. Third edition, enlarged. !2mo, cloth. 
 
 $1.50 
 
ii OTTO MS, S. R. Electrical Instrument-making for Amateurs. A Prac- 
 tical Hand-book. Sixth edition. Enlarged by a chapter on " The Tele- 
 phone." With 48 Illustrations. 12mo, cloth $0.60 
 
 Electric Bells, and all about them. A Practical Book for Practical Men. 
 
 With over 100 Illustrations. 12mo, cloth $0.50 
 
 The Dynamo : How Made and How Used. A Book for Amateurs. Sixth 
 
 edition. 100 Illustrations. 12mo, cloth $1.00 
 
 Electro-motors : How Made and How Used. A Hand-book for Amateurs 
 
 and Practical Men. Illustrated. 12mo, cloth $0.50 
 
 BUBIER, E. T. Questions and Answers about Electricity. A First Book for 
 Beginners. 12mo, cloth $0.50 
 
 CARTER, E. T. Motive Power and Gearing for Electrical Machinery; a 
 treatise on the theory and practice of the mechanical equipment of power 
 stations for electric supply and for electric traction. Illustrated. 8vo, 
 cloth $5.00 
 
 CROCKER, F. B., and S. S. WHEELER, The Practical Management of 
 Dynamos and Motors. Illustrated. 12mo. cloth $1.0C 
 
 CROCKER. F. B. Electric Lighting. Volume I., The Generating Plant 
 8vo, cloth. Illustrated $3.00 
 
 DESMOND, CHAS. Electricity for Engineers. Part I. : Constant Cur- 
 rent. Part II. : Alternate Current Revised edition. Illustratedo 12mo, 
 doth $2.50 
 
 DU MONCEL, Count TH. Electro-magnets : The Determination of the 
 Elements of their Construction. 16mo, cloth. (No. 64 Van Nostrand's 
 Science Series.) S - 50 
 
 DYNAMIC ELECTRICITY. Its Modern Use and Measurement, chiefly 
 in its application to Electric Lighting and Telegraphy, including : 1. Some 
 Points in Electric Lighting, by Dr. John Hopkinson. 2. On the Treatment 
 of Electricity for Commercial Purposes, by J. N. Schoolbred. 3. Electric 
 Light Arithmetic, by B. E. Day, M. E. 18mo, boards. (No. 71 Van Nos- 
 trand's Science Series.) $0.50 
 
 EAVING, J. A. Magnetic Induction in Iron and other Metals. Second 
 Illustrated. 8vo, cloth $4.00 
 
 FLEMING, Prof. A. J. The Alternate-Current Transformer in Theory 
 and Practice. Vol. I. : The Induction of Electric Currents. 500 pp. New 
 edition. Illustrated. 8vo, cloth $5.00 
 
 Vol II. : The Utilization of Induced Currents. 594 pp. Illustrated. 8vo, 
 
 cloth.' $5-00 
 
 FOSTER, HORATIO A. (with the collaboration of eminent specialists), Elec- 
 trical Engineers' Pocket-book. Illustrated with many cuts and diagrams. 
 Pocket size, limp leather with flap. (In Press.) 
 
 GORDON, J. E. H. School Electricity. 12mo, cloth $2.00 
 
 GORE, Dr. GEORGE. The Art of Electrolytic Separation of Metals 
 (Theoretical and Practical). Illustrated. 8vo. cloth - - $3.50 
 
HA SKINS, C. Bt. The Galvanometer and its Uses. A Manual for Elec. 
 
 tricians and Students. Fourth edition, revised. 12mo, morocco $1 .50 
 
 Transformers : Their Theory, Construction, and Application Simplified. 
 
 Illustrated. 12mo, cloth $1.25 
 
 HAWKINS, . ., M.A., A.I.E.E., and WALLIS, F., A.T.K.K. 
 
 The Dynamo, its Theory, Design, and Manufacture. 190 Illustrations 
 8vo, cloth $3.00 
 
 HOBBS, W. R. P. The Arithmetic of Electrical Measurements. With 
 numerous examples, fully worked. New edition. 12mo, cloth $0.50 
 
 HOSPITALIER, E. Polyphased Alternating Currents. Illustrated. 8vo, 
 cloth $1.40 
 
 INDUCTION COILS t How Made and How Used. Fifth edition. 16mo, 
 cloth. (No. 53 Van Nostrand's Science Series. ). . $0.50 
 
 INCANDESCENT ELECTRIC LIGHTING t A Practical Descrip- 
 tion of the Edison System, by H. Latimer. To which is added, The Design 
 and Operation of Incandescent Stations, by C. J. Field ; a Description of 
 the Edison Electrolyte Meter, by A. E. Kennelly ; and a Paper on the Maxi- 
 mum Efficiency of Incandescent Lamps, by T. W. Howells. Illustrated. 
 16mo, cloth. (No. 57 Van Nostrand's Series.) $0.50 
 
 KAPP, GISBERT, C.E. Electric Transmission of Energy and its 
 Transformation, Subdivision, and Distribution. A Practical Hand-book. 
 Fourth edition, revised. 12mo, cloth $3.50 
 
 Alternate-Current Machinery. 190 pages. Illustrated. (No. 96 Van 
 
 Nostrand's Science Series.). $0.50 
 
 Dynamos, Alternators, and Transformers. Illustrated. 8vo, cloth . . $4.00 
 
 KEUIPE, M. R. The Electrical Engineer's Pocket-book ; Modern Rules, 
 Formulae, Tables, and Data. 32mo, leather $1.75 
 
 KENNEL LIT, A. E. Theoretical Elements of Electro-dynamic Machinery. 
 Vol. I. Illustrated. 8vo, cloth $1.50 
 
 KILGOUR, HE. H., and S WA V, M., and BIGGS, C. H. W. Electri- 
 cal Distribution : Its Theory and Practice. Illustrated. 8vo, cloth $4.00 
 
 L (X'K WOOD, T. D. Electricity, Magnetism, and Electro-telegraphy. 
 A Practical Guide and Hand-book of General Information for Electrical 
 Students, Operators, and Inspectors. Revised edition. 8vo, cloth. Pro- 
 fusely Illustrated $2.50 
 
 L.ODGE, PROF. OLIVER J. Signalling Across Space Without Wires : 
 being a description of Hertz and his successors. Third edition. Illustrated. 
 8vo, cloth $2.00 
 
 L.ORING, A. E. A hand-book of the Electro-magnetic Telegraph. 16mo, 
 cloth. (No. 39 Van Nostrand's Science Series.) $0.50 
 
 HORROW, J. T., and REID, T. Arithmetic of Magnetism and Electricity. 
 12mo, cloth $1.00 
 
MUNRO, JOHN, C.E., and JAHIIESON, ANDREW, C.E. A 
 Pocket-book of Electrical Rules and Tables, for the use of Electricians and 
 Engineers. Thirteenth edition, revised and enlarged. With numerous diagrams. 
 Pocket size, leather $3.50 
 
 NIPHER, FRANCIS E., A. M. Theory of Magnetic Measurements, with an 
 Appendix on the Method of Least Squares. 12mo, cloth $1 .00 
 
 OHM, Dr. G. S. The Galvanic Circuit Investigated Mathematically. Berlin 
 1827. Translated by William Francis. With Preface and Notes by the Edi- 
 tor, Thos. D. Lockwood. 12mo, cloth. (No. 102 Van Nostrand's Science Se- 
 ries) $0.50 
 
 OUDIN, MAURICE A. Standard Polyphase Apparatus and Systems, contain- 
 ing numerous photo-reproductions, diagrams and tables. Second edition, 
 revised. 8vo, cloth illustrations $3.00 
 
 PLANTE, GASTON. The Storage of Electrical Energy, and Researches 
 in the Effects cieated by Currents combining Quantity with High Tension. 
 Translated from the French by Paul B. ElweL. 89 Illustrations. 8vo . . $4.00 
 
 POPE, F. L. Modern Practice of the Electric Telegraph. A Hand-book for 
 Electricians and Operators. An entirely new work, revised and enlarged, 
 and brought up to date throughout. Illustrations. 8vo, cloth $1 .50 
 
 POOL.E, J. The Practical Telephone Hand-book. Illustrated. 8vo, cloth. 
 
 $1.50 
 
 PREECE, W. H., and STUBBS, A. J. Manual of Telephony Illus- 
 trated. 12mo, cloth $4.50 
 
 RECKENZ AUN, A. Electric Traction. Illustrated. 8vo, cloth $4.00 
 
 RUSSELL, STUART A. Electric-light Cables and the Distribution of 
 Electricity. 107 Illustrations. 8vo, cloth $2.25 
 
 SALOMONS, Sir DAVID, M.A. Electric-light Installations. Vol. I. 
 Management of Accumulators. A Practical Hand-book. Eighth edition 
 revised and enlarged. 12mo, cloth. Illustrated...., $1.50 
 
 Vol. H.: Apparatus $2.25 
 
 Vol. III.: Application $1.50. 
 
 SCHELLEN, Dr. H. Magneto-electric and Dynamo-electric Machines: 
 Their Construction and Practical Application to Electric Lighting and the 
 Transmission of Power. Translated from the third German edition by N. 
 S. Keith and Percy Neymann, Ph.D. With very large Additions and Notes 
 relating to American Machines, by N. S. Keith. Vol. I., with 353 Illustra- 
 tions. Second edition $5.00 
 
 SLOANE, Prof. T. O'CONOR. Standard Electrical Dictionary. 300 
 
 Illustrations. 8vo, cloth ,. $3.00 
 
 THOM, C., and JONES, W. H. Telegraphic Connections, embracing 
 recent methods in Quadruples Telegraphy. 8vo, cloth. Twenty colored 
 plates - $1-50 
 
THOMPSON, EDWARD P. How to Make Inventions; or, Inventing as 
 a Science ana an Art. Au Inventor's Guide. Illustrated. 8vo, paper..$1.00 
 
 THOMPSON, Prof. S. P. Dynamo-electric Machinery. With an Intro- 
 duction and Notes by Frank L. Pope and H. R. Butler. Fully Illustrated. 
 (No. 66 Van Nostrand's Science Series.) $0.5C 
 
 Recent Progress in Dynamo-electric Machines. Being a Supplement to 
 
 "Dynamo-electric Machinery." Illustrated. 12mo, cloth. (No. 75 Van 
 Nostrand's Science Series.).. . . $0.50 
 
 The Electro-magnet and Electro-magnetic Mechanism. 213 Illustrations. 
 
 8vo, cloth $6.00 
 
 TREVERT, E. Practical Directions for Armature and Field-magnet 
 Winding. 12mo, cloth. Illustrated $1.50 
 
 TUMLIRZ, Dr. Potential, and its application to the explanation of Elec- 
 trical Phenomena. Translated by D. Robertson, M.D. 12mo, cloth $1.25 
 
 TUNZELMANN, G. W. de. Electricity in Modern Life. Illustrated. 
 12mo, cloth $1.25 
 
 URQUHART, J. W. Dynamo Construction. A Practical Hand-book for 
 the Use of Engineer Constructors and Electricians in Charge. Illustrated. 
 12mo, cloth.... $3.00 
 
 WALKER, FREDERICK. Practical Dynamo-building for Amateurs. 
 How to Wind for any Output. Illustrated. 16mo, cloth. (No. 98 Van Nos- 
 strand's Science Series.) $0.50 
 
 WALKER, SYDNEY F. Electricity in our Homes and Workshops. A 
 Practical Treatise on Auxiliary Electrical Apparatus. Illustrated. 12mo, 
 cloth $2.00 
 
 WERB, H. I*. A Practical Guide to the Testing of Insulated Wires and 
 Cables. Illustrated. 12mo, cloth $1.00 
 
 WEYMOUTH, F. MARTEN. Drum Armatures and Commutators. 
 (Theory and Practice.) A complete treatise on the theory and construction 
 of drum-winding, and of commutators for closed-coil armatures, together 
 with a full resum6 of some of the principal points involved in their design; 
 and an exposition of armature reactions and sparking. 8vo, cloth. Illus- 
 trated $3.00 
 
 WORMEL&, R. Electricity in the Service of Man. A Popular and Prac- 
 tical Treatise on the Application of Electricity in Modern Life. From the 
 German, and edited, with copious additions, by R. Wormell, and an Intro- 
 duction by Professor J. Perry. With nearly 850 Illustrations. Royal 8vo, 
 cloth. .........$3.00 
 
 *** A General Catalogue -SO pages of Works in all brandies 
 of Electrical Science 1 iiruished gratis on application* 
 
YC 33460 
 
 990644 
 
 S6 
 
 E_iA^cr\v (c o, 
 
 THE UNIVERSITY OF CALIFORNIA LIBRARY 
 
i i 
 
 S S 5 5 
 
 OS O< * W LO . C O X 
 
 WtO H>0 O O ' 
 
 : O C i O O O O O O 
 
 ' o o i <o o x ?: to o 
 
 i O tO h- OSXOtOW.XtOCSM 1 
 
 itot^xtKtoto^tsxlsooiwc' 
 
 00000 000 = 00 O : 
 X X O O to ! 51 ^ X O tO ' 
 
 O O 
 
 
 ): fill 
 
 O O O i 
 t^. >t* rf- i 
 
 M M tO tO tO 
 
 M A 5 otosotf'O ooxo-505 
 
 |g 
 
 H* M to tocc*-i^o x 
 
 toc*5t^O' 
 S S P M 
 
 sppi^psfe^p 
 
 g;iSii 
 
 i 
 
 HH 
 QC 
 
 tzj 
 
 O 
 
 *. O X <> 
 01 * g 5 
 
 irt 
 X X 
 
 w bi oo o 
 
 05 05 tO Hi Hi l _ l( 
 
 g g 
 
 to X p 51 
 
 i- 1 . . . . 
 
 to x H> 05 
 
 
 X 3S to to 3S (fc O 
 
 to 34 1 *k O -a HI O XW 
 
 to to WMCOOSO.. cew 
 
 ^ 
 
 Pd 
 
 t^S io^fe^l^sSISi g- | ^o,M^ | co 
 
 
 i 4"- S5 O O tO 
 
 to o; o w x 51 oc to M p : p p p p p p p p p p 
 
 poop p p p p p o