ClassT_El_jL5A Book . Ct a.. The D. Van Nostrand Company intend this booK to b3 sold to the Public at the advertised price, and supply it to the Trade on terms which will not allow of reduction ELEMENTS OF ELECTRIC TRACTION ELEMENTS OF ELECTRIC TRACTION FOR MOTORMEN AND OTHERS BY L. W. GANT LECTURER IN THE ELECTRICAL ENGINEERING DEPARTMENT OF THE LEEDS INSTITUTE TECHNICAL SCHOOL D. VAN NOSTRAND COMPANY 23 MURRAY AND 27 WARREN STREETS NEW YORK 1907 <.< ^^^ system as a whole, two diagrams are given, one of which illustrates diagrammatically the method of distributing such power, and the other illustrates also diagrammatically the parts of a car which are essential for the safe and effective utilization of the power. If the student does not understand all the terms used at the first reading of this chapter, he is advised to repeat the reading of it as he gets familiar with the later chapters. The source of energy employed in this country is the heat energy derived from coal, the coal being utilized for the generation of steam, engines being employed for driving large continuous-current multi- polar dynamos or generators, giving the requisite electrical pressure. This pressure, according to Board of Trade rules, must not exceed 550 volts, as delivered to the trolley wire. Distribution. — The electrical power so generated is first delivered to the main switchboard for distribution, regulation, etc. On the main switchboard will be found all the main switches, fuses, circuit breakers, instruments, etc., for the controlling and operating of the various dynamo and feeder circuits, thus each dynamo and feeder circuit is controlled and safeguarded. Each dynamo has its own main switches, automatic circuit breaker, shunt regulator, etc., with the necessary instruments such as voltmeter and ammeter for in- dicating the pressure and current. By means of these switches the various dynamos can be coupled up in parallel so as to feed the circuits, or be cut out as the case may require. The current from the machines is thus delivered to what are B 2 ELEMENTS OF ELECTRIC TRACTION called the main bus bars ; these consist of heavy copper bars which carry the whole output to the various feeder panels, where it is taken off for the various districts. Each district, or portion of such, as the fH case may require, is fed by means of a feeder cable, each feeder being controlled by its own set of main switches, circuit breaker, fuses, ammeter, etc., fitted on the main switchboard. INTRODUCTION 3 The feeder cables are led underground to the section feeder boxes, and from these the overhead trolley wire is fed, as indicated below. The trolley wire is divided up — Board of Trade rule — into sections not exceeding half a mile in length, each half-mile section of the trolley wire being insulated from one another by section insulators. The section feeder box on each route generally feeds one section of the trolley wire, and the various sections are suitably connected together by means of switches, fuses, connecting links, etc., placed in what are termed the feeder boxes. The overhead trolley wire, being so subdivided and so connected, is not liable to entire breakdown in the event of any accident to one section of the trolley wire, as that section can be cut out, and the other sections fed by suitably cross-connecting by means of the switches, etc., fitted in the feeder boxes. In connection with the main switchboard there will also be what is generally called the Board of Trade panel, containing the various recording voltmeters, ammeters, and other instruments required for the various Board of Trade tests and records. Fig. 1 diagrammatically illustrates the above arrangement, two dynamos, one feeder circuit, and four sections of the trolley wire, simply being shown. In this diagram the dynamos and switches, etc., are shown in diagrammatic fashion, thus showing the method of diagrammatically illustrating the various parts of an electric circuit. The current, after passing down the trolley and through the car, returns to the generating station through the rails, cables being led from the rails at various points back to the main negative bus bar, and in this way the current is provided with a path back to the generators, a complete circuit thus being provided. As the rails are earthed, that is, connected with earth and so having the same electrical pressure, such a system is said to have an earthed return. Car Equipment. — The essential parts of a car equipment can be classified as follows : — (1) The necessary switches, circuit breakers, fuses, controllers, rheostats, or resistance boxes, for regulating and controlling the electrical power supplied to the motors ; also (a) Some form of lightning arrester, for safeguarding the car from a lightning discharge. (b) In some cases an earth switch, whereby in an emergency the trolley wire can be safely and effectively earthed, as may be advisable in the event of a trolley wire breaking. (2) The necessary motors, complete with gear wheels, etc. (3) The necessary switches, fuses, lamps, etc., for the effective lighiing 9f the car. ELEMENTS OF ELECTRIC TRACTION (4) The necessary hand brakes, sand boxes, and gear, life-guard attachments, gongs, signalling bells, destination indicators, etc. Different engineers adopt slightly different arrangments in con- nection with the above switches, etc., that shown in Fig. 2 being one of many. As the different arrangements are, however, more a INTRODUCTION 5 matter of detail, the diagram given will enable the general purpose and principle to be understood. Following the path of the current by means of the diagram, it will be seen that after being collected from the trolley wire by means of the trolley wheel, it then passes, by means of a cable connection from the trolley head, down the trolley pole and standard to a main motor switch fitted at one end of the car under the canopy, within reach of the motorman or conductor as the case may be. From there it passes to the other end of the car to an automatic circuit breaker, this being similarly situated to the main motor switch. From the automatic circuit breaker the current then passes through a fuse, this being placed outside the car at one end, and from there through the choking coil to the main trunk leads which run under the car seats. The choking coil is connected with the lightning-arrester portion of the circuit, the purpose of which will be explained later. The arrester and choking coil are also often fixed under the seats of the car. The main trunk leads consist of two bunches of cables running from one end of the car to the other, and by means of these the controllers, motors, and rheostats are all connected ; the various cables being connected as they pass from controller to controller to the various armatures, fields, and rheostats. The particular path the current will take will depend upon the particular positions of the main and reverse drums of the controller in use. Assuming the power handle is on the first notch in series, and the reverse drum is in the forward position, the current will pass, not directly as here indicated, but by means of the various fingers and cables through the following path, the particular path indicated being that taken in a B18 (B.T.H.) controller. After leaving the choking coil the current will pass through the blow-out coil of the controller in use to the main trolley finger, and from there will pass by means of the various fingers and cables to K2 terminal on the resistance frame. The remainder of the connections then being such that E7 is connected to Al, AAl to Fl, El to A2, AA2 to F2, and E2 to earth wire, and so to frame, wheels, and rails. The current in this way, after leaving the choking coil, passes through the blow-out coil of one controller, and after passing through a certain amount of regulating resistance E2 to R7, it then passes through the armature Al to AAl and field Fl to El of No. 1 motor, and then through the armature A2 to AA2 and field F2 to E2 of No. 2 motor, and from there to earth wire, and so to wheels and rails, in this way finally completing its circuit back to the generating station. In some cases two circuit breakers are employed instead of one and a main switch, and sometimes the two circuit breakers are arranged in parallel; in this case, only one is employed at a time, the rear platform one being thrown off. 6 ELEMENTS OF ELECTRIC TRACTION In other cases the fuse is dispensed with, the circuit in such instances being simply protected by means of the circuit breaker. The characteristic difference between these two methods of protecting a circuit will be dealt with later on. Where both are employed a double safeguard is obtained. Lightning Arresters. — A lightning arrester has two main functions to fulfil — one of providing a suitable path for the light- ning discharge, and the other of preventing current flowing from the trolley through that same path direct to earth. This is often carried out as follows : — Two metal or carbon contacts are provided, one of which is con- nected to the trolley cable and the other to the w^heels and so to rails, namely earth ; these two contacts, however, do not quite touch, a small distance separating them ; there is thus a small air gap breaking the circuit. In consequence of this no current will flow, the pressure of 500 volts being insufficient to break down the resistance of the air gap. Very often the air gap is so arranged in traction work that at least a pressure of 2000 volts would be required to break down, or, in other words, to spark across the gap. While the trolley pressure is thus insufficient, the tremendous pressure obtained with a lightning discharge amounting to thousands of volts will be more than sufficient, and so the discharge when such takes place can bridge across the air gap, and so to earth, and in this way a path is provided. To prevent, however, the discharge also finding its way through a parallel path, namely, that of any portion of the motor circuit to earth — the discharge piercing the insulation — a choking coil is inserted, as shown in the motor circuit. This may consist of a few turns of insulated wire wound in the form of a spiral or solenoid on to a wooden core. The action of this, as well as other portions here mentioned, will be better understood after reading through the principles explained in the following chapters. A lightning discharge is of an oscillating nature, somewhat similar to an alternating current, and the effect of such a current passing through a coil is to set up within the core a magnetic field, first in one direction, then in the other. The changing magnetic field, cutting across the turns of wire in the coil, sets up a back electromotive force similar to that obtained in a motor. The result is that of the two paths provided, the high-resistance one, due to the air gap in the lightning arrester, offers less total resistance to the discharge than that of the choking coil, which, as its name implies, tends to choke down any alternating current or discharge that tends to pass through it. In this way not only is a path provided for the lightning discharge in the arrester, but the motor circuit is also protected by means of the choking coil. When once the resistance of the air gap has been broken down INTRODUCTION 7 by the discharge — the arc formed across the gap consisting of metal or carbon vapour having far less resistance than the air — a small pressure, comparatively speaking, will be able to maintain the arc. The result of this would be that after a discharge had taken place the trolley pressure would maintain the arc once formed, and in this way a dead short to earth would take place. The second function of the lightning arrester is to prevent this. This is often carried out electromagnetically : the electromagnet, being excited by means of the current flowing from the trolley, operates either a magnetic blow- out, or mechanically separates the two contacts. In this way the arc which is being maintained is ruptured, current ceases to flow, and the electromagnetic portion of the apparatus then ceases to act. If the contacts have been separated they will then return to their original position, and the operations will be repeated when the next discharge takes place. One method of carrying this out is shown in the diagram. A high non-inductive resistance — no back electromotive force is set up in a non-inductive resistance — is inserted in series with the air gap, and a portion of it is placed in parallel with the coil for operating the electromagnet. The lightning discharge takes the path through the non-inductive resistance to earth in preference to that through the coil, as this would have a choking effect. The current from the trolley, however, when such passes, being continuous in direction, prefers the path of least resistance to a continuous current, namely, that through the coil of the electromagnet. In this way, as soon as current begins to flow from the trolley through the arrester, the electromagnet automatically operates so as to break the circuit and prevent current from the trolley passing through the arrester to earth. Earth Switch. — An emergency-switching arrangement is some- times fitted to cars, so that in the event of a broken trolley wire the motorman can connect the trolley wire — the portion that is not down — direct to earth ; this causes a dead short which will blow the circuit breaker out at the power station, thus removing a source of danger from the streets, namely, a live trolley wire. This switch is entirely an emergency one, and is operated by the motorman breaking a glass, the breaking of the glass releasing a catch which holds the earth switch in its off position. Leak on Trolley Standard. — As a danger exists of the passengers getting a shock from the trolley standard in the event of the insulation of the cable or trolley lead breaking down, the trolley standard in this way becoming alive, the following method of detection for such is sometimes adopted : — Two lamps (tinted red) are coupled in series, one terminal is connected to the iron standard, and the other remaining terminal to earth. In the event of the insulation breaking down and the standard 8 ELEMENTS OF ELECTRIC TRACTION becoming " alive," the lamps light up, and warning is thus given of the state of affairs, and the conductor can then warn passengers to leave the top of the car. Lighting. — One advantage of electric traction is that the car can be lighted electrically, and so much more effectively. For this purpose incandescent lamps are employed, these generally being 100-volt lamps, and arranged five in series. With 500-volts pressure this means that each lamp has 100 volts across its terminals. The lighting cables are attached to the trolley side of the main motor switch, and in this way, by switching on the lights, the motorman can test whether the section is " alive " or " dead ; " that is, whether pressure is available for use or not. Each set of live lamps will be protected by a fuse and controlled by a suitable switch, the complete circuit also being protected by a main fuse. The lighting generally consists of interior lights, fore and aft signal, destination, dash, canopy, and roof lights. In an ordinary double-decker without roof these may amount to some seventeen lights, fifteen being in use at one time, one canopy and one dash-light only being lit at a time. This means three circuits, each circuit being composed of five lamps in series, one circuit being so arranged by means of switches that one canopy and one dash-light are always cut out. There are various arrangements of switches and lights adopted ; the one mentioned above will, however, indicate the general lines upon which these may be arranged. CHAPTER II PRINCIPLES OF MAGNETISM It has been found by experiment that only certain materials can be magnetized to an extent suitable for practical engineering purposes, and materials so magnetized are found to possess certain characteristic properties quite distinct and apart from any other properties they may possess. The word '' magnet " is supposed to have been derived from the place Magnesia, where it is supposed iron ores possessing similar properties were first discovered. The materials which can be magnetized are iron and steel and their alloys, though there are, it should be said, other materials which can be magnetized, such as nickel and cobalt, but these only to a very slight extent. The other materials in use in engineering work, such as copper, brass, gun-metal, zinc, and lead, can be treated as practically non- magnetic or non-magnetizable. It should, however, be added that certain alloys of iron can be made, such as a special mixture of manganese steel, which are only magnetizable to a very slight extent. At the outset it must be said there is nothing of an outward indication to show whether a body is magnetized or not, but if a straight bar of iron or steel be taken and magnetized in some way, that is, if an ordinary simple bar magnet be taken, it will be found on experimenting to possess the following magnetic properties : — That one end of the bar magnet will repel one end of another bar magnet, and attract the opposite end of the same magnet. Thus there is a mutual action either of attraction or repulsion set up between two magnetized bars, and depending upon this the follow- ing properties will also be found to exist on further experimenting — (a) That of setting or tending to set always in a certain direction, if carefully and delicately suspended, so as to be free to twist in any given direction. (b) That of attracting unmagnetized pieces of iron or steel, namely, magnetizable bodies ; strictly speaking, this is not quite true, as will lo ELEMENTS OF ELECTRIC TRACTION be seen later, the unmagnetized pieces of iron or steel really becoming ^magnetized by induction, so the real action is between two magnets, as stated above. These fundamental experiments will now be dealt with a little more in detail, and from these will be obtained some knowledge of the characteristic properties of magnets generally. If a simple bar magnet be dipped into a mass of iron or steel filings, it will be found that the filings are attracted and cling to the magnet chiefly in two clusters, the clusters being at opposite ends, thus giving a rough indication that the magnetism is chiefly con- centrated in two places, or poles, as they are called, one being called the north, and one the south pole, the reason for which will be seen later. If a piece of copper had been dipped into copper or brass filings, no such action would have taken place, and if an unmagnetized bar of iron or steel had been dipped into unmagnetized iron or steel filings, again no such action would have taken place; also if a magnetized bar of iron or steel had been dipped into copper or brass filings, no attraction whatever of the filings would have occurred. The action only takes place between any material that has been magnetized and materials which can be magnetized. This principle is employed in the magnetic separation of iron and brass filings ; it also explains the reason copper oil-cans are often employed in con- nection with dynamos, as the ordinary tinned iron might be attracted, and so damage some portion of the machine. The reason for calling one end of the magnet a north pole and the other a south, is because, if a magnet be suspended as indicated above, it will be found to always set in one direction, namely, approximately due north and south ; that is, the end which is called the north pole is the end that always tends to point towards the north pole of the earth. If now a simple bar magnet be suspended, it will set as indicated approximately due north and south. On bringing near to the north end the corresponding end of another magnet, it will be found that the north pole of the one repels the north pole of the other, and that the north pole of the one attracts the south pole of the other, this being one of the fundamental facts in magnetism that like poles repel, and unlike attract. Thus there is a force of attraction between a north and a south pole, but a force of repulsion between two north poles or two south poles. Earth's Magnetism. — It has been found that the earth itself is a magnet possessing a magetic north and south pole, and it may just be mentioned that the magnetic north pole is not the geographical north pole, consequently in navigation an allowance has to be made for this fact. It is the action of the earth's magnetism acting upon the PRINCIPLES OF MAGNETISM ii maguetized needle, or bar of iron, which causes it to twist, and so set in a given direction. As it often leads to confusion, it should be noted that if that magnetic pole of the earth which lies approximately toward the actual north pole is a magnetic north pole, then the pole of the compass needle which is attracted in that direction must, since opposite poles attract, be a magnetic south pole. This difficulty is got over by calling the end of the compass needle which points northward, the north-seeking end or pole, or simply the north pole or end. The action of the earth's magnetism upon a magnetic bar is simply that of twisting it in a certain direction ; it will not tend to move it bodily, as can be tested by floating a compass needle in a bath of water, the reason for this being that while one of the magnetic poles of the earth will attract one end of the compass needle, that same magnetic pole will also repel the other pole of the small magnet with practically the same force, the length of a small compass needle being exceedingly small compared with the distance of the earth's magnetic poles. Polarity. — The north and south poles of a magnet cannot be separated. If a simple bar magnet be broken in two it will be found that each half contains a north and a south pole ; therefore wherever there is a north pole in that same bar there must also be a south pole. Sometimes it is useful to imagine such a thing as a free north pole, that is, a magnet possessing only a north pole, but such a thing actually is an impossibility. It is possible, however, to have magne- tization without having poles ; thus, if an ordinary straight bar be taken and magnetized, and then bent into a circle so that the north and south poles touch one another, the effect of these two poles when close together will be to simply neutralize one another's action on any external piece of iron or steel, yet the iron ring will be in a state of magnetization. Magnetic Force. — The force with which a magnet attracts or repels another magnet or any piece of iron or steel is called the magnetic force. This force will depend not only on the degree of magnetization, but also upon the distance the two magnets, or the magnet and the piece of iron, are apart ; the nearer they are the much greater will the force of attraction or repulsion be. Magnetic Field. — The space all round a magnet pervaded by the magnetic force is called the magnetic field of that magnet. A magnetic field may be due either to a magnet or to a number of magnets, or some equivalent of a magnet in the shape of a coil of wire through which a current of electricity is flowing. Magnetic Induction. — This is one of the most important facts connected with magnetism, namely, that magnetism is induced in a bar of iron or steel, or any magnetizable material — such as the alloys 12 ELEMENTS OF ELECTRIC TRACTION of iron or steel — when the bar is brought within a magnetic field, that is, brought near, for instance, to another magnet. The nearer the iron to the magnet, or, in other words, the stronger the magnetic field it is placed in, the greater the inductive effect, that is, the more strongly willi the iron become magnetized ; the further away, or the weaker the field, the less the inductive effect, A north pole of a magnet will induce south magnetism in that part of the iron nearest to it, the corresponding north pole being in the end furthest away, and vice versa. This explains why a magnet attracts apparently unmagnetized pieces of iron or steel, magnetism being induced in the iron or steel, attraction then taking place between opposite poles, induction preceding attraction. Retentivity. — If a piece of very soft wrought iron be taken and magnetized inductively by placing it in a very strong magnetic field, it will be found that when either it is removed from the magnetic field or the magnetizing force be removed, the piece of soft iron retains or keeps only a very little magnetism, and what it retains it will very readily lose. If, however, a piece of steel be taken and magnetized in a similar way, it will be found that the steel retains more magnetism. This property of retaining is described by the word " retentivity," the steel having a greater retentivity than the soft iion ; the harder the steel, as a rule, the greater the retentivity. Compass needles, small horseshoe or bar magnets, are permanent magnets ; that is, their magnetism is that which they retain, and depends upon the retentivity of the steel from which they are made. These magnets are, comparatively speaking, weak, and for engineering purposes, where strong magnets are required, this retaining power is not solely depended on ; but by employing strong magnetizing forces in the shape of electric currents circulating through coils of wire surrounding the iron, very much more powerful magnets can be obtained, or, in other words, electro-magnets. In all magnetic experiments, then, it must be remembered that the permanent magnetism of a compass needle, for instance, is not very strong, and if a powerful magnet be brought very near a small compass needle, the magnetism which will be induced in the compass needle may overpower the permanent or residual magnetism, and thus a north pole may seemingly attract a north, whereas the strong north pole has induced a south, and the attraction is really between the north pole and the induced south pole. Methods of Magnetizing. — A bar of iron or steel can be magnetized either by means of permanent magnets, by stroking the bar, for instance, always in one direction with one end of a permanent magnet for a number of times, or electrically, as will be seen later when dealing with electro-magnets ; or again by placing a bar of iron or steel in a strong magnetic field, and so inducing magnetism, a PRINCIPLES OF MAGNETISM 13 certain amount of which will be retained when the magnetic force is removed. Tapping it while in such a field will facilitate the process. This brings us to the theory of magnetism. One theory is that every individual particle of a bar of iron or steel is a small magnet, and that when the iron is in an unmagnetized condition it is supposed that all these small particles are simply mixed up in such a manner that there is no combined action, and consequently apparently no magnetization. The act of magnetization is to cause all these small magnets to turn more or less in one direction, and thus the effect is that of a number of small magnets assisting one another. Hysteresis. — If a bar of iron or steel be rapidly magnetized, first in one direction and then in the other, so as to rapidly reverse its polarity, it will be found to get hot. This is due to two causes : one electrical, which will not be considered here, and one magnetic. The harder the steel, or the greater its retaining power, the greater will be the amount of heat developed in a given time. Owing to the retaining power of the iron or steel, the magnetism does not readily change with alteration either in amount or direction of magnetic force ; in fact, a small reversing force has to be applied in order to actually rob the iron of the magnetism it retains, that is, its residual magnetism. In this way energy has to be expended in forcing the iron to behave in the desired manner, and this energy appears in the form of heat. This lagging of the magnetic state is called hysteresis, and the loss in heat due to this cause is termed the hysteresis loss, being entirely due to the magnets not responding to the magnetic forces at work. The softer and more pure the iron, the more readily will it respond, and the less heat will be developed in a given time. Lines of Force. — If a piece of paper be placed over, say, a bar magnet, and fine iron filings be then sprinkled on the paper, it will be found, on gently tapping the paper, that each little filing, having become magnetized by induction, sets itself in a given direction, due to the magnetic forces at work upon it. The direction is that of the magnetic force at that point, due of course to both poles of the magnet. It must be clearly understood that the iron filings only indicate the direction of the magnetic force in the plane or surface of the paper, and that magnetic force is acting in a similar way all round the magnet. It will also be noticed that where the magnetic force is strongest, that is, near the poles, there the iron filings are very much more crowded together than where the field is weaker. In this way the magnetic field due to various combinations of magnetic poles can be plotted and so studied. This has led to a conventional way of speaking about magnetic force, which enables not only the strength of field at any particular place to be specified, but also enables one to picture the distribution of the magnetic force. The method is to 14 ELEMENTS OF ELECTRIC TRACTION represent force by lines — lines of magnetic force, as they are called — the greater the force the greater the number of lines in any given area, just as is graphically represented by the iron filings, a given force being represented by a given number of lines. This method of expression, although imaginary, is useful from many points of view. From this it will be understood how a certain number of lines are supposed to issue from the north pole of a magnet of given strength, and, after completing some sort of a path, re-enter the south pole of that same magnet. The particular direction the force is supposed to act has also been agreed upon, namely, from north to south, or that direction in which a free north pole would travel. When it is said the density of the magnetic lines is very great, or that the lines are very crowded together, it is meant that the magnetic force at that place is very strong. Where the lines are not dense, that is, where they are not crowded together, the magnetic force is not so great. If it is desired to employ the full strength of a magnet, then the full number of lines must be utilized as far as possible ; that is, lines must not be allowed to leak away along other paths than that desired, and if by any means lines can be concen- trated at the point where full use can be made of them, then the full magnetic force can be employed. This, it may be said, is a principle employed in dynamo and motor design. Electro-magnetism. — Experiments show that if a straight wire is held over and in line with a compass needle at rest, that is, in the magnetic meridian, as it is called, on passing a current of electricity through the wire, the compass needle is deflected, just as though a magnet had been brought near ; in brief, it is found that a current of electricity will produce a magnetic field, thus showing a certain relationship to exist between magnetism and electricity. A certain position of the wire with regard to the needle and a certain direction of current will give a certain deflection of the needle. The following rule will show what relation these have to one another : — Rule, — Take the right hand and place the thumb at right angles to the fingers ; now place the hand on the wire, fingers pointing in the same direction that the current is flowing, palm of the hand being toward the compass needle, then the thumb will point in the direction in which the north pole of the needle tends to move. This rule, also, by noting the deflection of the needle, enables the direction in which the current is flowing in a circuit to be determined, and so enables the polarity, say, of a dynamo or battery to be also determined, namely, which the positive and which the negative terminal. For an illustration of this, see Fig. 3. In the left-hand diagram the wire is over the needle nearest the reader ; in the right-hand diagram the wire is under the needle furthest from the reader. In each case current is supposed to be flowing from A to B, and the arrows PRINCIPLES OF MAGNETISM 15 indicate the direction the magnetic needle will tend to move, due to the action of the current. If now an insulated wire be wound into the form of a simple B Fig. 3. spiral or coil (solenoid, as it is sometimes called), as shown in Fig. 4, on passing a current of electricity through the coil it will be found on experimenting to act similarly to a bar magnet, one end of the Fig. 4. coil having north polarity and the other south. If the right hand with thumb at right angles be placed on the coil, fingers pointing in the direction i of current, the thumb will indicate which is the north end of the coil. i6 ELEMENTS OF ELECTRIC TRACTION Adopting the method already mentioned, a certain number of imaginary lines will be issuing from the north end of the coil, and, after completing some sort of circuit, enter the south. If now a bar of iron or steel, or, as is generally said, an iron or steel core, as shown by the dotted lines, be inserted inside the coil, it will be found that the bar of iron has become magnetized, and if the magnetizing force is at all a reasonable one, a far greater number of lines will now be issuing from the end of the coil than before. It is found that by these means magnetism can be induced to a much greater extent than any magnetism the steel will retain of itself, and in this way powerful electro-magnets can be made. If the current be stopped, the magnetizing force will cease, and the bar will lose nearly the whole of its magnetism, retaining, however, just a little, as previously explained. The magnetizing force, it has been found, in any given case, depends upon not only the number of turns of wire in the coil, but also upon the amperes flowing through the coil, the magnetizing force being proportional to what are called the ampere turns. If the number expressing the number of turns be multiplied by the number ex- pressing the amperes, a number will be obtained expressing the ampere turns. Thus, if in a given case the current be doubled and the number of turns be increased to three times the previous number, there will be 3 X 2, namely, six times as many ampere turns as before, and consequently six times the magnetizing force. Six times the magnetizing force, it must, however, be clearly understood, does not necessarily mean six times the number of magnetic lines, or, in other words, that the magnetic field has been increased to six times its original value. All this will depend, as will be seen later, upon the degree of magnetization of the iron. Permeability. — Having given, then, a certain magnetizing force, experiments show that iron or steel offers, as it were, far less resistance to the passage of these lines, than, for instance, air does, and con- sequently a far greater number of lines will be got with an iron core than with no core. This is expressed by saying the iron or steel is more permeable or has a greater permeability than air. If the number of lines obtained with, say, a certain specimen of wrought iron in the core be divided by the number obtained without the iron, a number will be obtained which will express the value of the permeability of this particular material with the particular magnetizing force employed. Example. — A long spiral of insulated wire is wound having fifteen turns of wire for each inch length of coil, 3 amps, are sent through the coil, thus giving 45 ampere turns per inch length of coil. With this particular magnetizing force, roughly speaking, there will pass through every square inch cross-section of the coil 150 lines, PRINCIPLES OF MAGNETISM 17 that is, there will be a density of 150 lines per square inch ; this is generally expressed by saying there is a flux density of 150 lines. The total number of lines, or total flux, would be got by multiplying 150 by the number of square inches cross-section in the core. A soft wrought-iron core is now made just to fit the inside of the coil, and is placed within it ; with this particular magnetizing force it will probably be found that 93,000 ]ines per square inch now pass through the core, therefore 93,000 divided by 150, namely 620, is the number expressing the permeability of this particular specimen of wrought iron with a magnetizing force due to 45 ampere turns per inch length of material. The more permeable, then, a material the more lines will be obtained, or, in other words, the stronger the magnets will be, with a given magnetizing force. The permeability of iron or steel is, how- ever, not a constant quantity, the permeability depending not only on the material, but also on the magnetizing force. There is a period reached, for instance, when any increase in the magnetizing force will not bring about any appreciable increase in the number of lines, the iron or steel having reached practically a state of magnetic saturation. Beyond this stage it would not be advisable to attempt to increase the magnetization, the return in the shape of increased lines of magnetization not warranting the expenditure in the shape of magnetizing force. Eoughly speaking, for ordinary soft wrought iron this state of practical saturation would be reached when there was a density of 100,000 to 120,000 lines or so per square inch. Higher densities than this can be obtained, but, as explained, it might be very inadvisable in practice to spend the energy in order to obtain such a density. Special makes of mild cast steel, it may be said, are now largely used in dynamo and motor construction. This steel will not give such good results as wrought iron for the lower densities, but gives equally good if not better for the higher densities, and is besides cheaper. As copper, brass, lead, and zinc are practically non-magnetic, the number of lines passing through a coil will be in no way altered, if a brass or copper core be inserted, from that originally obtained without anything in the core. The permeability of air, copper, brass, etc., is a constant quantity, and a little consideration of the above will show that in consequence of this, the ratio of the number of lines, say, with a copper core to the number of lines without the copper, is equal to 1 ; that is, the permeability of air, copper, brass, lead, and zinc has the numerical value 1. In Fig. 5 three curves are drawn — A for a specimen of cast iron, B for a specimen of cast steel, and C for a specimen of soft wrought iron. From this diagram it will be seen how the density of lines, or flux density, varies with the magnetizing force. The density of lines c i8 ELEMENTS OF ELECTRIC TRACTION is given as the number of lines per square inch. The magnetizing force is given as the number of ampere turns for every inch length of path of the particular material. The magnetizing force required to send a given number of lines through a given magnetic path will depend not only on the total number of lines or flux, but upon the length, the area, and the per- meability of the material or materials forming the magnetic path ; thus, the longer the path, the more ampere turns will be required, the larger the area — that is, the less the density — the less ampere turns Boax r- \-x "^ LfL K - 1 p — — = - IIOPOQ ^ . sA. 2-t H- 1 . _. ^ '^^^r- — 1 r KOHOC -J ■^ ^ — 1 — ■ — ' NR ou OH - y ^ ^ ,— . -— I 40 ~ 5 = 8 ; that is, the resistance will be 8 ohms. (3) What pressure will be required to send 5 amps, through a resistance of 8 ohms ? Since CR = E, 5 X 8 = 40 ; namely, 40 volts pressure will be required. The three examples given above are modifications of one and the PRINCIPLES OF ELECTRICITY 29 same case. In any one example two out of the three quantities are supposed to be known, the third being calculated. The last example indicates how the drop in pressure in any given resistance, or length of cable, for instance, can be calculated, the drop in pressure being defined as follows : — Drop. — The difference in pressure across any part of a circuit is the pressure required to drive whatever current is flowing through Vbt7M£TRP Ml 4oyoit5' mm A,8ofiMS VOLTIACTtR. B.C>OHf^s M\ 10 VOLTS. «- Maaaah /lMf»£R£ 3 equals j'^, and I equals ^2> ^ total of ||^ Thus the joint conductivity is }f that of 1 ohm. On the other hand, the resistance, that is, the combined resistance, is If of 1 ohm, namely, just under 1 ohm. Thus the joint resistance of 2, 3, and 4 ohms in parallel with one another is, roughly speaking, equivalent to a resistance of 1 ohm. The following are a few practical notes dealing with cables, con- tact surfaces, fuses, and automatic cut-outs : — - PRINCIPLES OF ELECTRICITY 37 Cables. — There are two methods eraployed in specifying the size of cables — 1. As cables are built up of a number of strands of wire, one method is to specify both the number of wires forming the strand of the cable, and also the gauge — thus a j-§ cable means that the cable is composed of nineteen wires of 16 gauge, the gauge taken being that of the standard wire gauge ; again, a -^-J cable would be a cable com- posed of thirty-seven strands, the gauge of each strand being No. 12. 2. Another method of specifying large cables is sometimes adopted, namely, to simply specify the area in square inches of the conductor — thus a 0'4 cable means a cable which has an area of 0*4 sq. inch, that is, 1% sq. inch ; this means that if the area of each small wire composing the strand be taken, and all the areas be added together, the total area of the conductor will be obtained, which in this case is j*q of 1 sq. inch. A common rule to determine the number of amperes which a cable or wire can carry is to multiply the number of square inches area of the cable by the number 1000. In other words, to allow 1000 amps, for every square inch of area — thus a 0'4 cable would carry 0*4 x 1000, namely, 400 amps. Where cables are specified in the former manner, in order to obtain the area, the area of the cable must be calculated from the number of wires and their gauge. As a matter of fact, however, cable-makers generally publish tables giving this and other information concerning the cables they manufacture. The above rule is only a rough one, and, in some cases, cables would be run at nothing like 1000 amps, per square inch if of con- siderable length, as a considerable drop of pressure might be taking place in the cable, and this very often is a decided objection in itself, apart from the question of loss of energy. From a heating point of view, small cables can be run at higher densities than large ones, as small cables have more cooling surface in comparison with their size. The heating effect and "drop" are the two main con- siderations which have to be taken into account in determining the size of cable to be employed. A good rule to remember is that when cables are run at 1000 amps, to the square inch, a drop of 2 J volts will take place in the cable itself for every 100 yards of actual length. It may be noted that the class of insulation for a cable, that is, the material of which it is composed, and its thickness, will depend upon the pressure that it will have to carry, and the use to which the cable is going to be put; the greater the pressure the better the insulation will have to be, that is, the insulation resistance will have to have a higher value. Contact Surfaces. — All contact surfaces for making electrical contact between any two parts should be not only of sufficient area, 38 ELEMENTS OF ELECTRIC TRACTION but should be clean and make good contact. If they are of insufficient area, or dirty, or not making good contact, resistance is introduced, and heat in consequence is developed. The fingers on a controller barrel, or the brushes on a commutator, or the blades of a switch, are examples of contact surfaces. It depends upon the kind of contact as to what area it will have to be in order to carry a given current. In some form of switches specially designed for making good contact, as many as 300 amps, to the square inch can be allowed, the material in this case being copper. In other forms of contact possibly only some 50 amps, or so to the square inch would be advisable. Fuses. — With the object of protecting the various parts of a circuit in case an excessive current should flow, fuses are inserted in the circuit. These generally consist of a copper, lead, or tin — some- times a special alloy of the last two — wire, or strip of such a cross- sectional area as to melt and so burn out and break the circuit before damage can be done to the remainder of the circuit. Special means have to be adopted in breaking heavy currents to " blow out " the arc formed. Automatic Cut-outs. — Automatic cut-outs or circuit breakers such as the canopy cut-out fitted at one end of a tramcar are for the same purpose, and generally consist of a special form of switch which automatically operates, say, when an overload occurs. The auto- matic action is often brought about by means of an electro -magnet. Here, again, special means have to be adopted to prevent arcing. In comparing these two methods of safeguarding a circuit, it should be remembered that the danger to a circuit depends not only on the excessive current that may be flowing, but also upon the time it is allowed to flow, as the heat developed with a given resistance depends upon both time and current. The difference, then, between an ordinai'y excess current, automatic cut-out, and a fuse, lies in the fact that a fuse intended to blow, say, at 50 amps., may carry 75 amps, if such an overload is only on for a few seconds, the heat developed, as already seen, depending upon the time. In this way the fuse resembles the rest of the circuit, such overloading for such a short time doing no damage, and the circuit is not needlessly broken ; in short, the damage done to the circuit depends upon the length of time the overload is carried, and the current the fuse will blow at will depend to a certain extent in a similar manner upon the length of time the current is flowing. An ordinary automatic cut-out, that is, one not fitted with some time element device, as it is generally termed, may, on the other hand, carry for any length of time a very heavy current, say just under what it will blow at, without operating ; time here not entering into the question at all. It, however, has the advantage that it is readily operated and easily replaced when once the circuit has been broken. PRINCIPLES OF ELECTRICITY 39 Examples (1) A 100-volt 16-c.p. incandescent lamp when connected across a 100- volt circuit takes 0*6 of an ampere. What is its resistance when so connected ? As R = 7i .*. R = TT-T = 166'6 ohms L U*b Therefore resistance of lamp when fully illuminated or resistance when hot = i66*6 ohms. It should be noted, as a matter of fact, that the resistance of the carbon filament when cold will be much greater than this, having possibly twice the above value. (2) A voltmeter reading up to 600 volts has a resistance of 40,000 ohms. What current will pass through the instrument when it is registering 500 volts ? As 0=1 .-. C = -4-0^0 = ^, or 0-0125 amp. Therefore the current that will pass through the instrument when registering 500 volts = 0*0125 amp. or 12-5 milliamperes. A milliampere = yooo P^^^ ^^ ^^ ampere. (3) A rheostat or regulating resistance for starting a motor has a total resistance of 6^ ohms. (a) What drop in pressure will take place in the total resistance when a current of 60 amps, is flowing through it ? (b) What pressure will be available for driving current through the remainder of the circuit if 500 volts is the pressure of supply ? (a) . . . As E = CR .*. E = 60 X 6-5 = 390 volts Therefore drop in pressure taking place in the resistance when 60 amps, are flowing = 390 volts. (6) Pressure of supply = 500 volts Drop of pressure in resistance = 390 volts .-. 500 - 390 = 110 volts Therefore pressure available for driving current through the remainder of the circuit =110 volts. (4) A Leclanche cell has an E.M.F. of 1*5 volts and an internal resistance of 1*5 ohms. What will be the P.D. across its terminals when sending current through a resistance also of 1'5 ohms ? 40 ELEMENTS OF ELECTRIC TRACTION E.M.F. = 1-5 volts Total res. of circuit = res. of cell -f- res. in external circuit .*. total resistance of circuit = 1'5 + 1-5 = 3 ohms As C = :p .-. current = -^ = J amp. " Drop " of pressure in cell = CE = J x 1-5 = 0*75 volt .-. P.D. = 1-5 - 0-75 = 0-75 volt Therefore P.D. across terminals when |- amp. is flowing = 075 volt. (5) What would be the P.D. in the above example if the resist- ance of the external circuit was — (a) 13-5 ohms ? {h) 148-5 ohms ? W . . . AsC=^^i^. = J|=J,-amp. • " Drop " of pressure in cell = j^ X 1*5 = 0'15 volt .'. P.D. across terminals = 1"5 -- 0"15 = 1*35 volts Therefore P.D. across terminals when ji^ amp. is flowing = 1-35 volts. (6) . . .AsC=j,^-^^ = J|^=TJoamp. "Drop" of pressure in cell = yJ^f, x 1'5 = O'OIS volt .*. P.D. across terminals = 1'5 — 0*015 = 1-485 volts Therefore P.D. across terminals when ^Vo ^^P* is flowing = 1*485 volts. Note. — The less the current, the more nearly does the P.D. equal the E.M.F. (6) Ten Leclanche cells, each having an E.M.F. of 1*5 volts, and an internal resistance of 1'5 ohms, are coupled up in series, eight assisting one another, and two assisting one another but opposing the eight. A resistance of 3 ohms is placed in the external circuit. {a) What current will flow through the circuit ? {Ij) What will be the P.D. across the terminals of the 10 cells ? (a) . E.M.F. due to the 8 cells = 8 x 1-5 = 12 volts E.M.F. due to the 2 cells = 2x1-5=3 volts Effective E.M.F. for driving) __ ^^ q _ n u current through the circuit 5 - J-^ - '^ - «^ volts Note. — The 3 volts are acting in opposition or as a back E.M.F. PRINCIPLES OF ELECTRICITY 41 Internal resistance of the 10 cells =^ 1*5 X 10 = 15 ohms Kesistance of external circuit = 3 ohms .*. total resistance in circuit = 15 + 3 = 18 ohms As C = g /. C = f^ = i amp. Therefore current flowing through the circuit = ^ or 0-5 of an ampere. (b) .... Eesistance of the 10 cells = 15 ohms As E = CR, .*. drop taking place in ) ^ ^ , the 10 cells = 1 X 15 ^ = V-5 volts Effective E.M.F. across the ) __ ^ , terminals of the 10 cells f ^ ^ ^^^^^ .*. P.D. across the terminals ) ^ - ,, of the 10 cells = 9 - 7-5 f = ^'^ ^^^^^ Therefore the P.D. across the terminals of the 10 cells when ^ amp. is flowing = i*5 volts. Note. — The 1"5 volts are required to send J amp. through the resistance of 3 ohms. (7) Four resistances, two of 8 ohms each, one of 4 ohms, and one of 6 ohms, are placed in parallel with one another. (a) What is their combined resistance ? (b) What drop would take place in each resistance if 4 amps, were passed through the combination ? (0) What current would pass through each resistance ? 3 _f- 3 4. 6 + 4 (a) Combined conductivity = 1+1 + ^ +i= 9T = 24 .*. combined resistance = f-| = IJ ohms Therefore the combined resistance of the set = 1^ ohms. (b) The same drop is bound to take place in each resistance when placed in parallel. Therefore if 4 amps, pass through the set, the drop taking place in any one resistance or in the set will be 4 x I'o = 6 volts. E (c) The current through each res. of 8 ohms, as C = p, will therefore = [: = f amp. 42 ELEMENTS OF ELECTRIC TRACTION Current through the res. of 4 ohms ) ^ will therefore f = I = H amps. Current through the res. of 6 ohms 1 « ^ will therefore f = t = 1 amp. Total current then passing being ) . l + f+li+l f=^ ^°^P^- Therefore current passing through each resistance of 8 ohms = f amp. Therefore current passing through the resistance of 4 ohms = ij amps. Therefore current passing through the resistance of 6 ohms — I amp. (8) A resistance of 8 J ohms is placed in series with the 4 resist- ances in parallel in the above example. What current would flow if a battery with an E.M.F. of 3 volts and an internal resistance of 2 ohms was employed to send current through this particular arrangement of resistances ? Total effective E.M.F. = 3 volts Combined resistance of the 4 resistances = 1*5 ohms Eesistance in series with above = 8*5 ohms Internal resistance of battery = 2 ohms .*. total resistance in circuit =12 ohms E As C = p .-. C = ^2 = a amp. Therefore current flowing through the above arrangement would be i amp. CHAPTER IV PRINCIPLE OF THE DYNAMO While there are various methods of generating electricity, the only practical and economical method of generating such electrical pressures as are required for engineering and traction purposes, is by means of the dynamo or continuous-current generator where continuous currents are employed, or by means of an alternating generator where alternating currents are employed. The principle underlying both is the same. Fundamental Principle. — When dealing with electromagnetism it was seen that a current of electricity flowing along a conductor caused a magnetic needle suitably situated to move, this being due to the magnetic field set up by the current. It has also been found by experiment that a reverse effect can be somewhat similarly produced, namely, that a moving magnet will set up a difference of electrical pressure in a conducting wire suitably situated, the difference of electrical pressure causing a current to flow if a complete path be provided. In the one case a current of electricity causes a magnet to move, in the other a moving magnet causes a current of electricity to flow. If the conducting wii^e be suitably moved instead of the magnet, a difference of pressure will also in like manner be set up. To put this in its simplest form, suppose two bar magnets be taken (see Fig 11), and that the north pole of one be placed opposite the south pole of the other, a small distance separating them ; between these two poles there will be a fairly concentrated magnetic field. If now, say, a copper wire be taken and passed down through this gap between the poles, the wire being at right angles to the magnets so as to cut across the imaginary magnetic lines which are passing from the north to the south pole, it will be found there is set up a difference of electrical pressure between the ends of the copper wire, when the wire is passing down the gap, or, in other words, when cutting across these imaginary magnetic lines ; and if a suitable complete circuit be provided, this difference of pressure will cause a 44 ELEMENTS OF ELECTRIC TRACTION current to flow through the circuit. This current would, however, be exceedingly small, as the magnetic field would only be a weak one, and would require some very delicate instrument in order to detect it. If, now, instead of passin g the wire down between the Fia. 11. poles it is moved, say, simply from north to south, sliding as it were along these imaginary lines, and not cutting across them, it will be found that no electrical pressure whatever is set up ; an electrical pressure only being set up when these imaginary magnetic lines are cut across, or in two, as it were. The above indicates the fundamental principle upon which all dynamos are based. It will thus be seen that there are two essential parts and one essential feature for the production of electrical pressure in every dynamo. Essential parts — conductors and magnets. Essential feature — relative motion, a certain direction of motion being essential. The relative motion is that which must take place between the conductors and magnets ; thus to take two of the simplest cases, either the magnets could move and tlie conductors be stationary, or the conductors could move and the magnets be stationary. In most continuous-current dynamos the latter is the general practice. Electromotive Force. — The pressure of electricity generated it is found depends, not simply upon the number of imaginary magnetic lines, but upon the rate at which these , lines are cut, namely, the PRINCIPLE OF THE DYNAMO 45 number cut per second ; consequently the E.M.F. of a dynamo depends primarily upon three things — 1. The strength of the magnetic field, that is, upon the total number of lines cut in one revolution. 2. The speed at which the wires, or conductors, as they are called, move or cut across the field, that is, upon the number of revolutions per second. 3. The number of conductors connected in series and which are engaged in cutting the lines. The reason for the last statement is, that as there are practical limits both to the speed and to the strength of the field, and also in order to obtain a more uniform pressure, it is the practice in all generators to have a number of conductors coupled up in series, so that the E.M.F. generated in one conductor is added to the E.M.F. generated in another, and so on. By this means, not only is a higher pressure more readily obtained, but also a more uniform one, as will be seen later. It will be noted in the simple experiment described, that the direction of magnetic force, namely, that pointing from north to south, direction of current (when there was a circuit for such), and direction of cutting motion, that is, either up or down, were all at right angles to one another, and experiments show that the exact relation between these three directions (see Fig. 12) can be repre- sented by means of the following right- hand rule : — Right - hand Rule. — Place the thumb and first two fingers of the right hand all at right angles to one another. Then place the thumb in the direction of the magnetic force, namely, pointing from north to south. Arrange the hand so that the second finger points in the direction in which the wire or conductor cuts the field, that is, either up or down, still keeping the thumb pointing from north to south, then the first finger will indicate the direction in which the electrical pressure generated tends to drive the current. Taking the simple case of the wire passing down between the north and south poles of two bar magnets, if the north and south poles be arranged as in Fig. 11, then, on passing a wire down between these poles, experiments show that an electrical pressure is generated, tending to send current along the wire from right to Fig. 12. 46 ELEMENTS OF ELECTRIC TRACTION left, as indicated by the arrow. If the wire be moved up the gap instead of down, the electrical pressure generated will tend to send current from left to right, namely, in the reverse direction. If, on the other hand, the direction of the field be reversed, namely, a north pole take the place of a south, and a south take the place of a north, and the wire be moved down, then current will tend again to flow in the opposite direction to that indicated by the arrow. Tlius reversing either the direction of motion, or reversing the direction of field, causes the pressure generated to be also reversed as regards direction. The finger rule given can in this way be checked. If now a complete rectangle of wire be taken, as shown in Fig. 13, and passed vertically down so as to cut across a uniform magnetic Fig. 13. field as shown, the conductors AB and CD will both cut these lines, and as they both cut them at the same rate, the same pressure will be generated in each. The wires AC and BD will not cut across the lines, but slide along them, consequently these would be of no use for generating electrical pressure, but would simply connect AB and CD together. Now in this case, as the pressure generated in AB would tend to send current one way round the rectangle, namely, from A to B, and the pressure generated in CD would tend to send current the opposite way round, namely, from C to D, no current would flow, seeing the pressures are equal and opposite. If, however, AB was cutting lines in a very dense field, and CD in a PRINCIPLE OF THE DYNAMO 47 very weak one, namely, if the field was not uniformly strong, the pressure generated in AB would be greater than the pressure generated in CD, and current would then flow in that direction in which the greater pressure tended to send it. If, however, the rectangle be rotated, say in the direction of the hands of a watch, as indicated by the dotted lines, instead of being moved vertically down, then when AB was cutting down across the lines, CD would be cutting up, and the directions of the pressures would not then oppose, but would assist each other, current then circulating from A to B, and from D to C round the rectangle. During the next half- revolution, namely, when AB is moving up, and CD moving down, current would circulate round the rectangle from C to D and from B to A. It will thus be seen that continuous rotation causes current to flow first one way and then the other round the rectangle. The next feature of importance in any dynamo is that connected with the collection of the current from the various moving conductors, so that the pressure generated may be applied to some external circuit, and a complete path be so provided that current may flow from the dynamo through the external circuit back to and through the dynamo. In the simple rectangle shown in Fig. 13, the circuit was simply that of the rectangle or coil itself. Simple Dynamo — In Fig. 14 is shown in skeleton form the simplest possible form of dynamo. This consists, as will be seen, of Fig. 14. a single coil ; that is, two conductors suitably connected together, one conductor being connected at one end to a metal ring (slip-rings, as they are generally called), and the other conductor being connected to a similar ring. The two rings are insulated from one another, and the whole is fnounted on, and insulated from, a suitable spindle, so 48 ELEMENTS OF ELECTRIC TRACTION that the coil can be rotated. Bearing on the rings are two contact fingers, or brushes, which conduct the current to the external circuit. When the coil is in the position shown, current will be passing out of the ring marked A, going through some external circuit, say, and returning back through the ring marked B, a complete circuit thus being provided not only from and to the dynamo, but also through the dynamo. By the time the coil has gone halfway round, or through 180°, it will be found that the conductor connected to ring A is cutting across the magnetic lines in the reverse direction ; also in a similar manner the one connected to B ; consequently, the pressure will be reversed, and hence the direction of current will also be reversed. In this way, current will flow first one way through the circuit, and then the opposite way. Such a current is called an Fia. 15. alternating current, and the arrangement shown in Fig. 14 is really the simplest possible form of an alternating generator. For most traction purposes continuous current is employed, and consequently a generator built on these lines would not be suitable for supplying current direct to the trolley wire. Referring now to Fig. 15, a different arrangement will be seen : one end of one conductor is connected to a half-metal ring, and the other conductor connected to another half-metal ring, the half-rings being insulated from one another. It will be seen from the figure that current is passing, when the coil is in the position sho^vn, from the half-ring A to the top brush, and from there through the external circuit back to the bottom brush, and so to the other half- ring B. On moving the conductors round halfway, namely, PRINCIPLE OF THE DYNAMO 49 through 180° just as before, it will be noticed that the conductor connected to A will, owing to its new position, have the current reversed in it ; but the half-ring A connected to this conductor now no longer touches the same sliding contact or brush, but the opposite one, namely, the bottom one. Thus, in the simple case illustrated, the top brush, for instance, is only connected to a conductor while current is flowing through that conductor in one direction, and current will be flowing through the opposite conductor in this same direction when that conductor comes in contact with this same particular brush. A little thought will show that in this way current will always flow the same way round the external circuit. In this way a current is obtained which flows continuously in one direction, and such a current is called a continuous current. Uniform Pressure. — When the coil is in the position shown in the figure, the two conductors, it will be seen, are cutting directly across the magnetic lines ; they will then, in consequence, at that instant — assuming the speed to be constant — be generating the maximum E.M.F. Later, for instance, when the coil is in a position at right angles to that shown, the two conductors will be sliding aloncr the magnetic lines, and no E.M.F. at that instant will be generated. In the intermediate positions they will partly cut and partly slide. In this way, it will be seen the pressure generated by a single coil rotating in a uniform field will not be constant in value, but will vary, for instance, during one half-revolution in the following manner : — Starting from the instant when the pressure generated is at the maximum value, it will be seen that the pressure gradually decreases in value until it becomes nothing at all, then it gradually increases in value until the maximum pressure is again reached. During the next half-revolution it will vary in a similar manner. This is on the assump- tion that the speed has not varied in any way during the revolution considered. It must be noted that while the pressure varies in value, the direction the pressure is tending to drive current through the external circuit is always the same, owing to the arrangement of the two half-rings, or simple two-part commutator, as it is called. It may here be mentioned that in alternating work, not only does the pressure drive current first one way, then the other, through the circuit, but also it varies in value in a similar manner to that indicated above. In continuous-current work, however, dynamos are designed to give practically a constant pressure. This is obtained in the following manner : — If, iustead of two conductors, eight are taken, as indicated diagrammatically in Fig. 16, and connected to a four-part commutator, it will be seen that, with such an arrangement, when four of the conductors are idle, owing to their position, the other four will be 50 ELEMENTS OF ELECTRIC TRACTION generating. In this way a more uniform pressure can be obtained. In actual practice, to obtain practically a constant pressure, a number of conductors are arranged all round the armature core, with a corresponding number of commutator segments. By this means a large number of conductors are continually at work generating au e '7 IN TH£ POSITION SHOWN I. 2. 5, and6 are ACTIVe CONDUCTORS. J,4,7,anp8v\re /dl£ conductors. Fig. 16. electrical pressure, and it will be noted when one conductor is passing into a worse position for cutting lines, another is passing into a better ; the result is that practically a constant pressure is obtained ; uniform direction being obtained, as previously explained, by means of the commutator. It should be particularly noted that all remarks previously made have been made with reference to continuous-current work, and not to alternating work. Ohm's law, for instance, as given, would not apply to alternating work. Construction. — The following description will indicate the most important features in the actual construction of a dynamo : — Commutator. — In actual dynamos, the two half-rings are replaced by a number of segments, owing to there being a number of conductors. This part of the dynamo is called the commutator, and the various segments, commutator bars or segments, the various segments being insulated from one another and from the rest of the PRINCIPLE OF THE DYNAMO 51 machine generally by means of mica. The same principle holds good with a number of segments, as in the simple two-part commutator shown in Eig. 15. Brushes. — The contacts which bear on the segments are called the brushes. These brushes consist generally of specially prepared carbon blocks attached to suitable holders. It should be noted that at the instant when the brush is bridging across the segments, and so shortcircuiting that particular coil, the coil when in that position is not cutting across the lines of magnetic force, and consequently that particular coil at that instant will not be generating any electro- motive force, and so no damage is done by such shortcircuiting. In actual practice, however, it must be said, a great many difficulties are here introduced. Magnets. — In order to obtain strong magnetic fields, electro- magnets are employed, the current required for magnetizing these being obtained, as a rule, from the dynamo itself. The different types of dynamos are described by the particular methods of connect- ing the field coils to the armatures. Thus — Series Dynamo. — If the whole of the current passing out of the dynamo passes through the coils which are used for exciting or magnetizing the field, that is, if they are in series with the armature, then such is termed a series dynamo. Shunt Dynamo. — If, however, only a small fraction of the total current coming from the dynamo be employed for purposes of excitation, that is, if a small fraction of the total current be shunted through the field coils on the magnets, then such is termed a shunt dynamo, the field coils being connected as a shunt across the armature. Compound Dynamo. — A combination of the above two methods of winding the field is very often adopted, and such is called a compound dynamo. Separately Excited Dynamo. — If the terminals of the field coils of a shunt dynamo be disconnected from the armature, and the current from the field be taken, not from the dynamo itself, but from some separate source, then such a dynamo is said to be separately excited, as compared with the self-excitation of other dynamos. The behaviour of such a dynamo is very similar to that of a shunt dynamo. In the shunt dynamo, as the exciting current is only small, a great many turns of wire are employed in order to get the requisite ampere turns, the wire in this case, having only a small current to carry, is only of small section. In the series dynamo, as the exciting current is the full current the machine is giving, less number of turns are required, and these of large section. Armature Core. — The core upon which the conductors are wound is called the armature core, and is generally made of soft iron, 52 ELEMENTS OF ELECTRIC TRACTION for the purposes of concentrating and providing an easier path for the magnetic lines. Now, just as an electrical pressure is set up in the conductors, so an electrical pressure would be set up in this rotating iron core, producing current, which would simply circulate down one side and up the other of the core. This would heat up the iron core, and energy would be wasted. To prevent this, the armature is not made of solid iron, but is built up generally of thin circular charcoal iron sheets, these being varnished or papered, and thus insulated one from the other. This building up of the armature by means of thin sheets or laminae is called laminating the armature core. The in- sulating of these various sheets one from the other directly prevents these eddy currents, as they are called, circulating through the core, and thus prevents energy being wasted. The core, however, will still provide a path for the magnetic lines of force, the laminating of the core in no way interfering with this. In tramway motors, and also in most large dynamos, slots are cut out of the armature, and in these the conducting wires are em- bedded. The benefit of tliis, from a mechanical point of view, is that the conducting wires are firmly fixed to the armature, as, in the case of a motor, for instance, it is the action between the field and the currents w^hich the conductors are carrying which causes the arma- ture, and hence the shaft, to rotate ; consequently the need for the conductors to be firmly attached in order to transmit the necessary driving force. Armature Conductors. — As already mentioned, a great number of conductors or coils are employed. These conductors generally con- sist of copper wire or bars, suitably insulated from the armature core and from one another, the conductors at one end being connected to the various commutator segments. Copper is employed because of its good conductivity, less heat in this way being generated in the conductor, and consequently less energy wasted. Characteristic Properties. — Investigations have shown that the various types of dynamos behave differently, or, as is generally stated, have different characteristics. On testing a dynamo to find out its characteristic, no attempt is made to regulate the dynamo, as the object is to see how the dynamo regulates itself; consequently, in taking the characteristic of any dynamo, it is run at a constant speed, and the shunt regulator in a shunt machine is not interfered with while the test is being made. The following notes will just briefly indicate the chief charac- teristics of the various types : — Shunt Characteristic. — A shunt dynamo, when tested, will be found to give its full voltage when no current is flowing through the external circuit ; in other words, when the dynamo is on open circuit. It should be noted the only current then passing through the dynamo PRINCIPLE OF THE DYNAMO 53 is the small current that will be passing through the field coils. On closing the external circuit, and gradually loading the dynamo up by reducing the resistance in the external circuit, so that the current is gradually increased, it will be found that as the current increases the volts decrease. This decrease, however, is not a very great one, within the ordinary limits of full load. In a well- designed shunt dynamo the drop in pressure will probably be some 10 per cent, of the full voltage. Such a dynamo is said to have a falling charac- teristic, as the volts fall as the current is increased. It may be mentioned that shunt dynamos, as a rule, are fitted with shunt regulators, and by means of these the current passing through the field coils can be regulated, and in this way the magnets can either be strengthened or weakened. By this means the volts caii either be increased or decreased within certain limits. Series Characteristic. — A series dynamo, when tested, will be found to give on open circuit only a very small voltage. The reason for this is that when this type of dynamo is on open circuit no current is passing through the field coils, as they are in series with the armature, and not in parallel, as is the case with the shunt dynamo. The voltage thus obtained with a series dynamo on open circuit is simply that due to the residual magnetism of the field magnets, and this will only be a very small part of the total generated later when on full load. On closing the circuit and gradually re- ducing the resistance in the external circuit, it will be found that the machine will not build up, as it is termed ; that is, no definite pres- sure will be maintained, and no appreciable current will flow unless the resistance be reduced to a certain amount. Finally, when the resistance is reduced so that an appreciable current begins to flow, it will be found that as the dynamo is loaded up — the current being increased by reducing the resistance — the volts, within certain limits, will also increase. Such a dynamo is said to have a rising charac- teristic, as the volts rise in value as the current is increased — that is, within certain limits. Compound Characteristic. — A compound dynamo, when tested, will be found to give practically a constant voltage at all ordinary loads. This is brought about by the combination of series and shunt windings. As a rule, such machines are designed either to give a constant voltage, or a slightly increasing voltage with increase of load. The following reasons will briefly indicate why the above results are obtained : — In a shunt dynamo the field becomes weaker the greater the current passing through the armature ; the result is, the greater the current the less the E.M.F. generated. This weakening of the field is due to two causes, one of which is brought about indirectly and one directly — 54 ELEMENTS OF ELECTRIC TRACTION 1. In every dynamo certain demagnetizing actions are set up, due to the current circulating round the armature core, which de- magnetizing action tends to weaken the main field. The greater the current passing through the armature the greater the demagnetizing action set up, and, consequently, the greater the current the weaker the field. 2. The current through the field coils, and hence the strength of the magnetic field, depends upon the P.D. across the terminals of the field coils. This P.D. depends not only upon the E.M.F. generated, but also upon the " drop " taking place in the armature, and the greater the current the greater will be this drop. The result of these two actions is, that not only is the field weakened indirectly by means of the demagnetizing action, but it is also weakened directly due to the decreased current through the field coils, which is the outcome of the lesser E.M.F. generated, and the increased drop taking place in the armature with increase of load. Within the limits of ordinary load the total drop is, however, not a great amount in a well-designed machine, as already mentioned. It should be noted that even if the E.M.F. generated were always constant, there would be a slight drop in the terminal volts as the current was increased, due to the drop taking place in the armature, this increasing with increase of current. In a series dynamo, as the current increases the field will also increase in strength, and while the demagnetizing action will tend somewhat to weaken it, the field as a whole will increase in strength, and the E.M.F. will correspondingly increase with increase of current. Later on, with heavy loads and with saturated magnets, the demagnetizing action may cause the volts to drop. The P.D. across the terminals in this, as in all other dynamos, will be less than the E.M.F. by the amount of the drop taking place in the armature, and in this case the field coils. In a compound dynamo sufiicient series turns are generally added to what is practically a shunt dynamo, to approximately give the necessary increase in volts to make up for the drop due to the various causes. In this case the field coils will consist of two sets, one taking a small fraction of the total current, and the other consisting of a few turns taking the whole of the current. It is on account of the magnetizing effects due to current in the armature that the brushes have to be moved round in a dynamo in the direction of rotation, and in motors in the reverse direction, in order to prevent sparking at the commutator. Tramway motors, however, are designed for a fixed position of the brushes. Finally, it must be remembered that all self-exciting dynamos have to depend upon the residual magnetism of the field magnets for generating an E.M.F. on starting up, as, until a current begins to PRINCIPLE OF THE DYNAMO 55 flow, the machine will not be excited electrically. The initial current is obtained by means of the weak residual magnetic field. The building up of the full field then takes place if the conditions are suitable ; that is, neither too large a resistance in the shunt circuit of a shunt dynamo, nor too large a resistance in the main circuit of a series dynamo. Motive Power. — It must always be remembered that power will have to be supplied to the dynamo in order to drive it, a force being required to pull the conductors round when they cut across the magnetic lines of the field, that is, when current is flowing. The greater the output of the dynamo the greater the power required to drive it. But it should be noted that when no current is flowing, no pressure or force will be required to move the conductors other than that required for overcoming the mere friction, etc., so that when no electrical power is being generated no power will be required to drive the dynamo, other than that required to overcome the friction of tlie bearings, brushes, etc. The force which will have to be exerted on the conductors to twist the armature of a dynamo round will depend primarily upon (1) the intensity of the field, (2) the current flowing through the armature conductors, and (3) the number and length of the conductors. While outwardly there is nothing to indicate there would be any resistance to twist the armature round, it must be remembered that the forces simply have no visible medium. CHAPTER V PRINCIPLE OF THE CONTINUOUS-CURRENT MOTOR Fundamental Principle. — When dealing with the principle of the dynamo, reference was made to a fundamental experiment in electro- magnetism. That same experiment will now again be referred to in dealing with the principle of the motor. The experiment showed that if a wire through which a current of electricity was passing was placed over and near a magnetic needle, the needle was deflected, that is, it moved under the action of some force, namely, a magnetic one, a magnetic field being set up by the current of electricity flow- ing through the wire. Now, the force between the magnetic needle and the wire carrying the current was a mutual one ; that is, that just as a force was at work tending to move the needle, so also an equal and opposite force was at work tending to move the wire. In the experiment referred to, the wire was held and the needle moved ; had the needle been held, and the wire quite free to move, then the wire in this particular case would have twisted or deflected in the opposite direction to that taken by the needle. A simple experiment will still further illustrate this — One end of a conducting wire, AB (see Fig. 17), was hooked on to a conducting and supporting wire leading from a set of batteries. The other end, B, of the wire dipped into a glass vessel containing water, to which a little acid was added to make the water conduct somewhat better. The circuit was completed by running another wire from the water back to the battery, a switch being placed in the circuit. In this way, current could be passed through the wire AB, which was free to swing. A couple of bar magnets were fixed as shown, thus placing a portion of the wire in a magnetic field. Experiments then showed that on passing a sufficiently large current through the circuit, the wire moved either towards or away from the reader, depending upon the direction current was passing through the wire, namely, up or down. Had the wire not been hinged at the end A, but free to slide altogether, it would have tended to move bodily PRINCIPLE OF THE CONTINUOUS-CURRENT MOTOR 57 either towards or away from the reader. Again, had the north pole of one of the magnets been placed dead over one end of the wire AB, and the south pole of the other bar magnet dead under the other end of the wire, no motion would have taken place. Motion of the wire only takes place when by so doing it cuts across magnetic lines of force. In the experiment as originally described above, the magnetic lines of force were cut as the wire moved. With the magnets placed dead e^ <9. N J S ^5= I t BATTERY Fig. 17. over and under the wire, that is, when the direction of the current and direction of field are exactly in line, any slight motion of the wire, it will be seen, either up or down, sideways, backwards, or forwards, only causes the wire to slide along the lines, and cutting does not take place, and hence in this case no motion would take place. It will therefore be seen that there are two essential parts, and one essential feature, in every continuous-current electric motor, namely — Essential Parts — conductors and magnets. Essential Feature — current must flow through the conductors, 58 ELEMENTS OF ELECTRIC TRACTION a certain position of the conductors with regard to direction of field being essential. The resulting motion could either be that of the magnets or that of the conductors. In actual practice the magnets are stationary, and the conductors move, the armature to which they are fixed rotating. It will thus be seen that the essential parts of a motor are the same as those of a dynamo, and an electric motor in its construction is identical in every respect with a dynamo. In the dynamo the conductors are moved and an electromotive force is generated. In the motor an electromotive force is applied so that current flows through the conductors, and a force is set up which tends to move the conductors. Any motor, if driven from some suitable source of supply, such as a steam-engine, will generate an electrical pressure, and any dynamo, if supplied with a suitable current, will run as a motor. Certain details, such as direction of rotation, will have to be seen to, etc., but these in no way affect the general principles laid down. Thus it will be seen, in the dynamo, mechanical energy is supplied and electrical energy generated ; in the motor, electrical energy is supplied and mechanical energy generated. The motion of a conductor carrying a current of electricity in a magnetic field can also be experimentally shown by means of a con- tinuous-current arc light. It must be explained that the excessive heat which is developed when an arc of any kind is formed, causes the metal — or carbon in the case of an arc light — to be vaporized and burn, also that the burning vapour acts as a conductor. This ex- plains why an arc once formed is so readily maintained. If the arc be placed in a suitable magnetic field it will be found that the conductor — in this case not a wire but the burning vapour — moves just as the wire did. This causes the arc to be displaced, and conse- quently lengthened, and in this way to be " blown out ; " the lengthening of the arc increasing the resistance, thus reducing the current, and consequently the heat, and, with the reduction of the heat, the reduction of the arc. The above is the principle adopted in the magnetic blow-out, a strong magnetic field being so arranged as to blow out any arc formed on breaking the circuit. Driving Force. — The force tending to rotate the armature of a motor, that is, the sum of the forces acting on all the conductors, it is found depends primarily upon three things — 1. The intensity of the magnetic field, that is, upon the density of the magnetic lines. 2. The current flowing through the conductors. 3. The number and length of conductors actively employed. From this it will be seen that in any given motor the force PRINCIPLE OF THE CONTINUOUS-CURRENT MOTOR 59 tending to rotate the armature will primarily depend — as the number and length of conductors will always be the same — upon the strength of the magnetic field and upon the current passing through the arma- ture conductors. The greater the current and the more intense the field, the greater will be the force acting. It will be noted in the simple experiment described that the direction of magnetic force, namely, that pointing from north to south, direction of current and direction of motion were all at right angles to one another, and experiments show that the exact relation between these three directions can be represented by the following Left-hand Eule : — Left-hand Rule. — Place the thumb and first two fingers of left hand all at right angles to one another. Then place the thumb in the direction of the magnetic force, namely, pointing from north to south. Arrange the hand so that the first finger points in the direction in which current is passing through the conductors, still keeping the thumb pointing from north to south, then the second finger will indicate the direction in which the conductor will tend to move. The above left-hand rule enables the direction of motion of a conductor in a motor to be determined, if direction of field and direction of current be known, just as the right-hand rule enables direction of current through a conductor in the case of a dynamo to be determined, where direction of field and direction of motion are known. Referring now to Fig. 17, it will be seen that if current passes down the wire, that is, from A to B, the wire will tend to move towards the reader; if the connections to the battery be reversed, so that current passes up instead of down, the wire will tend to move away from the reader. If, now, the field be reversed, the north pole being placed at the right-hand side, and the south at the left, and current be passed down the conductor, its tendency will be to move away from the reader. Thus, reversing either the direction of current or direction of field causes the motion to be reversed. To reverse, then, the direction of rotation of a motor, either its field must be reversed, or direction of current through the armature conductors must be reversed. In dealing with the dynamo, a commutator was found to be neces- sary to enable the current passing through the conductors to flow always in one direction through the external circuit. This being due to the fact that with the armature rotating continuously in one direc- tion, current flows first one way and then the other through any one conductor, in consequence of it both cutting up and cutting down, across the lines of the magnetic field. In the motor, the commutator has to fulfil just the opposite 6o ELEMENTS OF ELECTRIC TRACTION purpose, namely, to enable the continuous current supplied to flow first up and then down any one conductor, depending upon its position in the field, and in this way enabling continuous rotation of the armature to be obtained. In the one case the commutator was required to obtain con- tinuous current with continuous direction of rotation, and in the other with continuous current to obtain continuous direction of rotation, constant direction of field being assumed as is actually the case with continuous-current motors and generators. A reference to Figs. 14 and 15, and an application of the above left-hand rule, will more definitely show this need of a commutator in order to obtain rotation in one continuous direction. Back Electromotive Force. — A most important point in con- nection with motors is that dealing with what is called the back electromotive force. Before attempting to explain this, the following facts will first of all be considered : — A small series motor was taken, and the resistance of the armature and field — the field in a series motor being in series with the armature — was measured and found to be, roughly speaking, about 1^ ohms. Now to send, say, for example, 3 amps, through a resistance of 1 J ohms, a pressure it will be seen of 3 X IJ, namely, 4^ volts, would be required. Actually, however, a pressure of 100 volts, about twenty- two times the above pressure, is applied to the terminals when the above motor is running under ordinary working conditions. With twenty- two times the applied pressure, if the resistance in the circuit — namely, under working conditions the resistance of the armature conductors and field winding — had only to be considered, twenty-two times 3 amps., namely, some 66 amps., would fiow through the motor circuit. As a matter of fact, however, experiments show that when the motor is running on a light load, only some 3 amps, pass through the circuit, but that if friction be applied to the motor pulley so as to reduce the speed, the current increases ; in fact, the current increases as the load upon the motor is increased. It will be seen from the above that something else besides the resistance of the motor and the applied voltage has to be considered in determining the current passing through a motor, and that some- thing depends upon the motion of the armature. So much does the current depend upon the motion, that if the armature of the above motor is held so that it cannot rotate, experiments show that a pressure of 4 J volts is quite sufficient to send a current of 3 amps, through the armature and field, thus indicating that a pressure of 100 volts under similar conditions would send the excessive current of 66 amps, through the armature and field, which, as a matter of fact, is nearly 4 J times the maximum current, namely, 15 amps., PRINCIPLE OF THE CONTINUOUS-CURRENT MOTOR 6i this particular motor is designed to carry. In contrast, then, with the current of 66 amps., one has to consider the 3 amps, passing through the motor under exactly the same condition as regards the resistance in circuit and applied voltage, the sole difference between the two cases being that in the one case the armature is at rest, and in the other the armature is in motion. The explanation of this is as follows : — When dealing with the principle of the dynamo, it was seen that two essential features, namely, conductors and a magnetic field, and one essential condition, namely, relative motion, were necessary for the production of electrical pressure. Now, in a motor in motion, all these conditions are fulfilled, so it is practically a dynamo in motion, the particular motive power causing rotation ; that is, v/hether Fig. 18. it is being rotated electrically, or by hand, or by whatever means, making no difference, the fact remains the armature is in motion and the field is excited. Consequently, the motor, when running, will generate a pressure of electricity just as any dynamo would, and this pressure will tend to send a current of electricity in a certain direction through the circuit. On taking the simplest possible case (see Fig. 18), it will be seen that if the brush marked A be connected to the positive terminal of some source of supply so as to send current through the conductors from brush A to brush B, current will pass from C to D and from E to F. The motor will then run, it will be found, on applying the left-hand rule in the direction shown, namely, in the same direction as the hands of a watch. If, now, the motor be 62 ELEMENTS OF ELECTRIC TRACTION looked upon as a dynamo, it will be found, on applying the right-hand rule, that when the conductors rotate in the direction shown — the field, of course, being still the same, as one is studying what is going on in the conductors at one and the same time — a pressure is being generated tending to send current in the reverse direction ; that is, from F to E and from D to C. The pressure generated, in short, will be found to oppose the applied pressure, and this opposing or back pressure is called the back electromotive force, briefly, the back E.M.F. The back E.M.F, will always be less than the applied E.M.F., because if they became equal, for instance, there would be no effective pressure, and no current would flow. The force driving the armature round would cease, and as the speed became reduced in consequence, so also would the back E.M.F. become reduced ; in other words, the back E.M.F. would not remain equal to the applied E.M.F. For the motor to generate a greater E.M.F. than the applied E.M.F., it must act no longer as a motor receiving electrical energy, but as a dynamo delivering electrical energy, power in this case being supplied from some source to drive it. To determine, then, the current in a circuit of this kind, the total effective pressure that is acting must be taken. In the case of a motor, the effective pressure is the difference between the applied and the back E.M.F., and it is this difference, divided by the resistance of the armatiu'e and field in the case of a series motor, which gives the value of the current. Thus in the above case, when 100 volts were applied to the motor terminals, and 3 amps, were flowing through the circuit, as only 4^ volts were required to send this current, a back E.M.F. of 100 minus 4^, namely, 95J volts, was being generated. The E.M.F. generated in any given motor, that is, the back E.M.F., will primarily depend, just as the E.M.F. of any given dynamo does, upon (1) the strength of field, (2) upon the speed of rotation, and (3) upon the number of conductors connected in series, and which are actively employed. The generation of a back E.M.F. raises up a very important point in connection with the starting of motors. When starting a motor, since the armature is at rest, no back E.M.F. will at the moment of starting exist, and consequently, were the full voltage to be applied directly to the terminals of the motor, and no other resistance in- serted in the circuit, an excessive current, as already seen, would flow. This excessive current would either blow the fuse or automatic cut-out placed in the circuit to protect it, or else, before the motor had time to get up speed and generate a back E.M.F., would cause serious overheating of some part of the circuit to take place, the out- come of which might either damage the insulation or burn the circuit out. When once a back E.M.F. is generated the current would be PRINCIPLE OF THE CONTINUOUS-CURRENT MOTOR 63 reduced, depending, of course, upon the value of the back E.MT., as this would reduce the effective E.M.F. For example, as already stated, the current that would flow in the case of the small series motor, which has already been considered, under these conditions would be about; 4^ times the full-load current. As the heating depends upon the square of the current, in a given time over 4i X 4 J, that is, over twenty times more heat, would be generated than with full-load current. In a large motor the armature and field resistance will be much less than is the case with a small motor, and the result will be correspondingly and detrimentally greater. In order, then, to keep the current within reasonable limits on starting, a starting switch is provided, by means of which resistance can be inserted in the motor circuit, the resistance being made sufficiently great to keep the current within reasonable limits. Thus, if in the example above, 5 J ohms be inserted in series with the armature and field, this 5 J ohms, together with the IJ ohms due to armature and field resistance, will make a total of 7 ohms. Now, 100 volts will only send a little over 14 amps, through a resistance of 7 ohms. In this way the starting current can be kept within any desired amount. As the motor gradually gets up speed, and so gradually generates a back E.M.F., so also can the resistance be gradually taken or cut out of the circuit by means of the starting switch, so that finally, when the motor has attained its full speed and so developed its full back E.M.F., all the resistance can and will be cut out. This does not necessarily mean the motor's highest speed, as in the case of tramway motors there are two speeds, one when the motors are in series, each getting only half-the-line voltage, and another when the motors are in parallel, each then getting the full-line voltage. In the intermediate positions, the current will be kept within reasonable limits, partly due to the resistance in circuit, and partly due to the back E.M.F. that is then being generated. In tramway work, the controller, besides fulfilling other functions, forms the starting switch for the motors. It is essential, then, that a motor should be provided with some suitable form of starting switch, and it is also equally essential in stopping a motor to see that the starting switch is moved back to its original or starting position, otherwise it may by carelessness be rendered of no use. Thus, if a motor is stopped by some other switch than the starting one — say by means of the main switch, the starting switch being left in its " ou " position — the next time the motor is started, the main switch might in all probability be inserted without attention being given to the starting one, in which case the starting switch would be rendered entirely useless for the purpose it was intended, being in its on position and having all the resistance in consequence cut out. 64 ELEMENTS OF ELECTRIC TRACTION In order to guard against this, and to prevent accidents due to such carelessness, the starting switch is often made automatic, so that it always flies back to its starting position if the circuit is broken either directly, by means of another switch, or by means, for instance, of a fuse blowing. It may be mentioned in connection with traction work, that previous to making connection with the trolley wire by means of the trolley pole, care should be taken to see that the power handle of the controller is in the off position, not only for reasons similar to that given above, but also as accidents have happened owing to the car being in this way violently started with no one on board. In the ordinary type of tramway controller, such as the KIO, B13 (B.T.H.) etc., there is no attempt made to make the action automatic in the exact way as above mentioned, but the reverse, and power handles (that is, the barrels which they operate) are interlocked so that when all is in order, a man is bound to bring his power handle to the *'off " position or starting end, looking at it from the point of view of a starting switch, before he can bring his reverse handle either into its off position or to its reverse position. It may be said it is only when the reverse handle is in the off position that the operating reverse handle can be removed. By means of this interlocking arrangement, a man cannot break the circuit by means of the reverse handle until the power handle is in its proper position, namely, the off position. In fact, as far as the controller is concerned, he is bound to stop the motor by means of the starting-switch portion of the controller. In a similar way the power handle cannot be moved and resistance cut out without having first completed the circuit by means of the reverse handle. In short, as far as the controller is again concerned, a man cannot first cut out resistance and then complete the circuit through some other switching arrangement, namely, the reverse barrel. In this way the operations are bound to be performed in the right order, but it should never be forgotten that while reverse and power handles are interlocked, power handle and either trolley pole, canopy cut-out, or main switch are not interlocked, and here care and thought must be taken for the prevention of accidents. As already mentioned, before connecting trolley pole to trolley wire, or before closing canopy cut-out or main switch, it should always be seen that the power handle, or, more strictly speaking, the power barrel, is in its off position. Another advantage of the above interlocking of the two handles is, that as the man has but one reverse handle as well as one power handle, he is bound to bring the power handle to the right position before he can remove for use at the other end of the car the reverse handle. This will leave the barrels in the one controller not in use PRINCIPLE OFTHE CONTINUOUS-CURRENT MOTOR 65 in their right position, and also prevents the controller barrels being tampered with. Cases are on record, where, owing to defective fitting, reverse handles have been removed when not quite in the off position ; the above remarks are on the understanding that all is in proper order. Examples (1) A pressure of 500 volts is applied to the terminals of a series motor. The resistance of the armature winding is 0*36 ohm, and the resistance of the series field winding is 0'76 ohm. What will be the back E.M.F. generated when a current of 50 amps, is passing through the motor ? Effective pressure required — As E = CR = 50 (0-36 + 0-76) = 50 (M2), namely, 56 volts .-. back E.M.F. = 500 - 56 = 444 volts Therefore the back E.M.F. generated under above conditions is equal to 444 volts. (2) What current would pass through the above motor if all starting resistance were cut out before motion took place ? Effective pressure in this case = 500 volts, as there is no back E.M.r. As C = ^ .-. current = pr^ = 446*4 amps., say 446 amps. Therefore the excessive current of 446 amps, would pass through the motor under such conditions, that is, until motion began to take place. (3) The armature resistance of a shunt motor is ^ ohm, the pressure of supply is 500 volts. What resistance must be inserted on starting to limit the current to 60 amps. ? E C Total resistance required = 8"3 ohms .*. res. to be inserted = 8*3 — 0*5 = 7'8 ohms Therefore resistance to be inserted on starting = 7*8 ohms. As R = =! .-. R = %o_o = 8-3 ohms (4) Two series motors are placed in series with one another and in series with a starting resistance of 3*2 ohms. The combined resistance of the armature and field of each motor is 093 ohm, and the pressure of supply is 500 volts. V 66 ELEMENTS OF ELECTRIC TRACTION What current will pass through the motors when each is generating a back E.M.F. of 150 volts ? Pressure of supply = 500 volts Total back E.M.F. = 150 + 150 = 300 volts /. effective pressure = 500 - 300 = 200 volts Total resistance in circuit = 3-2 + 0-93 + 0*93 = 5-06 ohms "p 200 As C = ^ .*. current = ^-:r^ = approximately 40 amps. Therefore the current passing through the motors in the above case will be 40 amps. (5) Two series motors exactly similar to above are placed in parallel with one another, but in series with a resistance of 1 ohm, the pressure of supply being as before — 500 volts. What current will pass through each motor if each generates a back E.M.F. of 350 volts ? Effective pressure driving current through the resistance and the two motors in parallel = 500 — 350 = 150 volts. As the current passing through the resistance divides, only half the total current will pass through each motor. .*. 2CE la " drop '* in resistance, where C = current through each motor and E = value of the starting resistance. Also Cr = " drop " in armature of one motor, where r = resistance of armature and field winding. .*. 150 volts = 2CE 4- Cr .-. 150 „ = 2C (1) + C (0-93) /. 150 „ = 2C + 0-93C = 2-93C = say 3C /. 150 = 3C /. C = 50 amps. Therefore current passing through the resistance = 100 amps. Therefore current passing through each motor = 50 amps. (6) What back E.M.F. would each motor generate in the above case if 80 amps, were passing down the trolley ? Pressure of supply = 500 volts " Drop " in starting resistance = CE = 80 X 1 = 80 volts .-. pressure across terminals of I goo _ 80 = 420 volts each motor j Pressure required to drive 40) ^>,-r, ^nvnno Q^7o i+ ,1^1 x^ . > = CE = 40 X 0'93 = 37 2 volts amps, through each motor ) /. back E.M.F. of each motor = 420 - 37-2 = 382*8 volts Therefore under above conditions each motor will generate a back E.M.F. of 382-8 volts. CHAPTER VI POWER AND POWER MEASUREMENT Before dealing with the actual measurement of either the electrical power supplied to a motor, or the mechanical power that the motor is capable of exerting, certain general principles of mechanics underlying the measurement of power generally will first of all be explained. Principle of Work. — If a weight of 1 lb. is lifted 1 foot high, 1 foot-lb. of work is said to have been done. This is also expressed sometimes by saying that 1 foot-lb. of energy has been expended in raising 1 lb. 1 foot high. Consequently, if a weight of 1 lb. be lifted, say, first 1 foot high, then another 1 foot high, and then again another 1 foot high, raising it in all 3 feet, a total of 3 foot-lbs. of work will have been done in raising the 1 lb. weight 3 feet high. If, instead of 1 lb., three separate weights of 1 lb. each be lifted 1 foot high, it will be seen that exactly the same amount of work will have been done in lifting these three weights as in the previous case, namely, 3 foot-lbs. If, now, the three separate weights be fastened together, and the whole three be lifted 1 foot high, exactly the same amount of work will have been done in raising the 3 lbs. all together as was done in lifting the three weights separately. Therefore the same amount of work is done in lifting 1 lb. 3 feet high as is done in lifting 3 lbs. 1 foot high. If, instead of lifting a weight of 1 lb., a small truck had been moved a distance of 1 foot, which resisted motion in the direction it was moved with a force equal to that of a 1 lb. weight, then, in like manner, 1 foot-lb. of work would have been done in so moving it. The energy expended, expressed as so many foot-lbs. of work done, can thus be calculated — Hide. — Multiply the distance in feet a body is moved by the number of pounds' force resisting motion in that direction, and a number will be obtained expressing the number of foot-lbs. of work ^one in so moving it. Conservation of Energy. — There is one great principle in 68 ELEMENTS OF ELECTRIC TRACTION connection with energy, which is called the conservation of energy, which acts as a guiding principle in all engineering work. Stated briefly, this principle is that energy can neither be created nor destroyed. Practically it means — to take a simple case — that if so many foot-lbs. of work have to be expended in raising a body, at least that same number of foot-lbs. of energy will have to be exerted. With regard to the creation of energy, it may be pointed out that, for instance, in the case of coal, the heat energy is stored up in the coal, this energy having been derived ages ago from the sun. With regard to the destruction of energy, it should also be pointed out that the energy expended in any given case may or may not be readily available for use again. In most cases it is not, the energy expended not having been destroyed, but lost as far as all useful purposes are concerned. For instance, energy spent in overcoming friction is generally converted into heat, and this heat energy generally becomes dissipated and lost. If, however, to take a very simple case, a 10 -lb. weight be lifted 10 feet high, the 100 foot -lbs. of work which will have been expended in so raising it — neglecting the work done in overcoming friction — will be stored up in the w^eight, as it were, simply by virtue of its position, and this energy will be available for use again. The weight, for instance, could be allowed to fall again, and in so doing raise other weights up, a certain loss of energy, however, taking place in the process, due to friction, etc. This principle is sometimes practically employed in cable traction systems, cars descending inclines assisting cars which are ascending. While energy cannot be created or destroyed, it may be transferred or transmitted from one body to another body. Also energy of one kind can be transformed into energy of another kind, as, for instance, in the case of a dynamo, where mechanical energy is transformed into electrical energy, or, as in the case of a motor, w^here electrical energy is transformed into mechanical energy. From the above principle it will be seen that to do 3 foot-lbs. of work, 3 foot-lbs. of energy must at least be expended, and if by any mechanical arrangement, such as a lever, to take a simple case, a large weight is lifted by means of a small one, the distance the large weight is moved is correspondingly less than the distance the small weight or force is exerted, the work done being equal to the work actually expended, provided there is no friction or loss of any kind ; and where there is friction, then the work actually done is less than the work or energy expended, as part of the energy will be expended in overcoming friction — that is, it will be transformed or spent in the form of heat, which is useless for the purpose under consideration. Energy, as a matter of fact, is always lost (not destroyed), that is, as far as useful purposes are concerned, either in its transmission from POWER AND POWER MEASUREMENT 69 one body to another, or in its conversion from one kind of energy to another, friction and other losses always occurring. Power or Rate of doing Work. — Up to the present, in dealing with work done, nothing has been said with reference to the time taken ; total work done has simply been dealt with, and not the rate or the speed, as it were, at which the work has been done, namely, the number of foot-lbs. of work done, say, in one minute. Now, the rate at which work is done expresses the power that is exerted. Hence the term '' power " means, not the energy expended, but the rate at which the energy is expended, twice as much power being developed where 100 foot-lbs. of work are done in 1 minute as in the case where 100 foot-lbs. are done in 2 minutes. Mechanical power is always expressed as so much horse-power, this term having come into use in the early days of the steam-engine, when engines were made chiefly to take the place of horses. It was then estimated that the horse was capable, on average working, of doing 33,000 foot-lbs. of work in 1 minute. This rate of doing work, namely, 33,000 foot-lbs. of work per minute, or, what is the same thing, 550 foot-lbs. per second, is accepted as the standard, the power so exerted being expressed as 1 horse-power, generally written 1 H.P. Thus, if 33,000 foot-lbs. of work are done in 1 minute, work is done at the rate of 1 H.P. If 66,000 foot-lbs. of work are done in 1 minute, work is done at the rate of 2 H.P. If 66,000 foot-lbs. of work are done in 4 minutes, work is done at the rate of one quarter the above rate, namely, at the rate of i H.P. The above estimate of doing work is, no doubt, a high one, but this is of no practical consequence, as the power of engines, motors, etc., is now always calculated, not from the number of horses that might have to be replaced, but from the number of foot-lbs. of work that have actually to be done in a given time. Thus, the figure 33,000 can be looked upon as simply a number which, when divided into the number of foot-lbs. of work done in 1 minute, gives a number expressing the horse-power. Before leaving this subject, it may be pointed out that a horse can and may exert a great deal more than 1 H.P. for a short space of time, the figure 33,000 being assumed to be an average, say, for a day's work. Thus, to replace a horse which is capable of pulling a car with a motor which is capable of giving out as a maximum 1 H.P. would not be altogether a fair substitution, the motor being rated at its maximum, and the horse at its average. While a motor can temporarily sustain a considerable overload, it may be said that the horse-power of motors for traction work, as 70 ELEMENTS OF ELECTRIC TRACTION generally given, is that maximum power which they are capable of exerting continuously with a given temperature rise — 75° Fahr. above the surrounding atmosphere being the usual temperature rise. The heating up of the armature and field of a motor, due to the current it carries, is one of the factors that determines the load it can safely carry for any length of time. Before leaving the above portion of the subject, the reader should carefully note the distinction between the three different quantities, namely — 1. Force simply expressed, say, as so many pounds. 2. Total work done, or energy expended, expressed, say, as so many foot-lbs. 3. Power or rate of doing work, expressed, say, as so many foot- lbs, of work done per minute, or as so much horse-power. As already seen, in an electric motor two forms of energy are dealt with, and consequently have to be measured, namely — 1. The electrical energy supplied. 2. The mechanical energy generated, or, more strictly speaking, transformed. Electrical Energy and Power. — No attempt will be made here to fully prove the following statements with regard to the measurement and calculation of electrical energy and power; the facts simply are stated, and an idea given as to the underlying reasons. The reader should again carefully note the distinction between electrical energy or work done, and the rate at which the work is done, or, in other w^ords, the electrical power. Now, in the simple case of a v^eight being lifted, just as the foot- lbs, of work done is obtained by multiplying the pounds weight lifted by the distance the weight is lifted in feet, so also in a similar manner the electrical work done, say, by a generator, is obtained by multiplying the coulombs quantity of electricity passing through the circuit by the difference in electrical level, as it were, or pressure that it has been raised through or to ; in other words, by the volts pressure generated. This, however, is not expressed as so many volt coulombs, but as so many joules — ^joule being a term used to represent a certain quantity of energy. Example. — If 10 couls. quantity of electricity pass through a circuit, across which there is a difference of pressure of 8 volts, 10 X 8, namely 80 joules, of electrical work will have been expended, that amount of work or energy having been supplied by some form of generator. The term "joule " thus corresponds in general meaning to the term " foot-lb. ; " that is, it is a measure simply of work done, 1 joule being equivalent, roughly speaking, to about |- of a foot-lb., more accurately, 0737 foot-lb. This term, however, is seldom employed in practical POWER AND POWER MEASUREMENT 71 work, a much larger standard or unit of measurement — as will be explained later — being employed. Now, just as the pounds weight lifted in a given time multiplied by the height in feet it has been lifted is a measure of the rate at which work is being done, and hence the power supplied, work at the rate of 33,000 foot-lbs. per minute, or 550 foot-lbs. per second, being equivalent to 1 H.P., so also the number of coulombs per second that are raised to a given pressure, multiplied by the pressure in volts, is a measure of the rate at which electrical work is beinTHRUST ON RAILS 800LBS FiQ. 26. than that actually measured on the armature shaft without the gearing, under the same conditions of current and voltage, the amount less being accounted for as indicated above. It should, however, be noted, with the introduction of gearing in this way, that while the speed of the driving axle is less than the spbed of the armature, the torque exerted on the driving axle will be correspondingly greater. The reason for this simply following from the principle of work, the greater torque at the lesser speed exerting the same power as the lesser torque at the higher speed. Thus, with a gear ratio of 5 to 1, while the driving wheels will only revolve with J the number of revolutions per minute that the armature does, the torque exerted on the driving axle will be approximately 5 times that exerted on the armature shaft — actually some 5 per cent, or 6 per cent, less than 5 times, owing to a torque or twisting effort being required to overcome the resistance set up by the extra friction introduced with the gearing. 102 ELEMENTS OF ELECTRIC TRACTION Relation between Torque and Draw-bar Pull. — In tramway work there are generally two motors, each being connected to a driving axle. In the following explanation, simply one driving axle, and the one motor connected with it, will be considered. Now, if the torque, say, in Ibs.-feet exerted on the driving axle be divided by the radius in feet of the driving wheel, then the total force in pounds that one motor exerts at the rim or periphery of the driving wheels tending to twist the driving axle round, will be obtained, since torque is force multiplied by leverage. Thus, if the torque exerted on the one driving axle were 1000 Ibs.-feet, and the driving wheels 30 inches in diameter, that is, 1^ feet radius, the total force exerted by the one motor at the rim of the driving wheels will be -^tj— ; that is, 1000 x f , namely, 800 lbs. As there are two wheels on each axle, this really means a force of 400 lbs. at the rim of each. To prevent confusion, however, the total force of 800 lbs. will simply be spoken of as though it were acting simply on one wheel, this one wheel simply then being considered. Now, the value of this force of 800 lbs. in this particular example is also the value of the force that is at work tending to propel the car forward ; in other words, it is the value of the horizontal effort as exerted by one motor. The reason for this can readily be seen on referring to Fig. 27. Assuming the car to be a fixture, and the rails the moving part, it will be seen that a force of 800 lbs., acting at the rim of the driving wheels in the dii^ection shown, will exert (assuming no slip of wheels) an equal and opposite force of 800 lbs., tending to move the rails back as shown. Now, the value of the force tending to move the rails back is also the value of the force at work tending to move the car forward. In other words, with rails fixed and a car free to move, a total force of 800 lbs. acting at the periphery of the wheels will produce a horizontal effort or draw-bar pull also equal to 800 lbs. It will now be seen that with driving wheels 30 inches in diameter, the value of the draw-bar pull in pounds (assuming no slip of wheels) will always be J that of the number expressing the value of the torque in Ibs.-feet exerted on the driving axle. Again, with a gear ratio of 5 to 1, and driving wheels 30 inches in diameter — since the torque at the driving axle will be practically 5 times that exerted on the armature shaft — the value of the draw- bar pull in pounds will be practically 5 x |, namely, 4 times that of the number expressing the value of the torque in Ibs.-feet exerted on the armature shaft. For example, see torques exerted as shown in Fig. 26. Thus there is always a definite connection between the number MECHANICS OF TRACTION 103 expressing the draw-bar pull, and the number expressing the torque exerted, say, on the armature shaft ; the smaller the driving wheels, or the greater the gear ratio, the greater will be the draw-bar pull for a given torque on the armature shaft, and the greater will be the number expressing the ratio between the value of these two quantities. Consequently, if in any given case the torque be doubled, the SOOlbs pulk Fig. 27. draw-bar pull will also be doubled, or if the torque be made ^ as great, the draw-bar pull will also be made ^ as great, and so on. As the torque exerted by a motor primarily depends upon the strength of current flowing through the armature, and upon the strength of field, so also will the draw-bar pull depend primarily upon the same two quantities. Thus an increased draw-bar pull can only be brought about, either by an increase in the value of the current passing through the armature, or by an increased field strength, or by an increase in both, and vice versa. It should also be added that, as in an ordinary series motor the field strength is primarily determined by the armature current, an increased draw-bar pull in any given series motor can only be brought about by an increase taking place in the current passing through the motor, and vice versa. With two motors attached to the car, it must be noted that the 104 ELEMENTS OF ELECTRIC TRACTION total draw- bar pull will be twice that due to one motor, assuming, of course, that each motor is doing an equal amount of work. Adhesive Force. — It will be seen from what has been said above, that whatever force there is at work tending to move a car forward, there is always at work an equal and opposite thrust on the rails, tending to move them back. In the example given above, with one motor exerting a draw-bar pull of 800 lbs., there would really be, due to this one motor, a force of 400 lbs. tending to drive each rail backward, assuming each wheel was doing its equal share of work. It may be noted here that in horse traction the thrust takes place, not on the rails, but on the setts, whereas in electric traction the thrust is taken by the rails, hence the wear and tear that takes place. It has already been seen that were it not for friction between the wheels and the rails, the wheels would slip and motion of the car would not take place, as no thrust on the rails could be obtained. The resisting force due to the friction existing between the wheels and the rails is termed the adhesive force. The total adhesive force, it will now be seen, must be greater than the total tractive effort. Thus, if the total adhesive force in the above case were 700 lbs., then a total tractive effort of 800 lbs. could not be exerted, the friction between the wheels and the rails not being able to sustain a thrust of anything greater than 700 lbs., any greater thrust than this causing the wheels to slip. The adhesive force existing between the wheels and the rails, it is found, depends not only upon the weight upon each driving wheel, but largely upon the condition of the surface of the rails. If the rails are only slightly wet, for instance, the adhesive force is very much reduced, the rails then being in a greasy condition. The application of sand in such cases causes the adhesive force to be increased. In railway work it has been found that the adhesive force varies roughly from 400 lbs. per ton weight on driving wheels under average conditions to under 200 lbs. per ton under bad conditions. Romapac System. — With regard to the wear and tear of tram- rails mentioned above, this is proving such a serious item in electric tramway practice that a compound tramrail is being put forward by the Romapac Tramway Construction Company, Ltd. In this system, the bottom portion of the rail consists of a steel girder somewhat similar to the bottom portion of the ordinary tram- rail, and this, together with the bonding, etc., forms the permanent portion of the track, and, when once laid, it is not interfered with. On the top of the steel girder, which is suitably shaped, is rolled by means of a special and patent machine, the grooved steel head portion MECHANICS OF TRACTION 105 of the rail, after the bottom portion of the track has been laid. Thus the rail as a whole consists of two portions. The top portion is rolled on so as to grip each side of the steel-girder portion of the track, and the grip obtained is of such a character that tests which have been made show that a force of some 24 tons is required to slide a 1 foot length of the top portion off the bottom portion. The head portion, it must also be added, is made to the same specification as the standard rails, and is therefore equally hard. When the rails become worn, the top portion is stripped off by means of the patent rolling-on and cutting-off machine, and the new head portion rolled on as before on site. The advantages claimed for this system are that, in the renewing of the track, only the worn-out portion of the rail has to be renewed. A saving of some 50 per cent, in the weight of renewable material, and also a saving of some 50 per cent, in the actual cost of renewing the track, is in this way claimed. There is also a considerable saving in time and labour, and less upset of road and traf&c in the process, as only the top setts on each side of the rail have to be taken up when the track is being renewed. A longer life is also claimed, as the joints of the top portion overlap those of the girder portion, and in this way a practically con- tinuous track is obtained, with an increased life in consequence. Measurement of Draw-bar Pull, etc. — Makers, when plotting in the form of curves the results of a complete test of a motor, such as is generally made, generally plot, in place of the torque exerted say on the driving axle, the equivalent draw-bar pull the motor would exert under the same conditions as regards current, etc. Also, they generally plot, in place of revolutions per minute say of driving axle, the equivalent speed in miles per hour the car would have when the driving axles were making this particular number of revolutions. As a rule, these tests are made by supplying the motor with a constant voltage, generally 500 volts, and noting the speed, also the amperes taken by the motor, etc., etc., when various draw-bar pulls are being exerted. Speed. — If the size of driving wheels and number of revolutions they make be known, it is simply a matter of calculation to find the speed a car would have if these wheels were attached, and. were making this given number of revolutions. Thus, if the circumference of the driving wheel in feet be multi- plied by the revolutions made in one hour, the number of feet a car would travel during the hour at this particular number of revolutions will be obtained, and this, divided by 5280,* will give the speed in miles per hour. From this the following approximate rule is obtained : — * 5280 feet = 1760 yards = 1 mile. io6 ELEMENTS OF ELECTRIC TRACTION Eadius of driving wheel in feet X revs, per min. of driving axle 14 = speed in miles per hour Draw-bar Pull. — From what has already been said, it will be readily seen how the actual draw-bar pull a motor is capable of exerting under any given condition of current, etc., can be experi- mentally determined from the motor itself. All that need be done is to test the motor in a similar way to that indicated in Chapter VI. for measuring the B.H.P. ; in fact, this would be done when making the complete test for B.H.P., speed, etc., etc., with varying values of current as mentioned above. Thus, if the brake be applied to a pulley on the driving axle, that is, if the B.H.P. be measured through the gear, the draw-bar pull can either be obtained direct from the particulars taken during the test, or it may be simply calculated from the B.H.P. the motor exerts under any given conditions. Thus, if the pulley on the driving axle is the same size as the driving wheel, the effective pull in pounds, that is, the difference between the pulls exerted on each side of the pulley, will give the value of the draw-bar pull in pounds that would be exerted under exactly the same conditions as regards current, etc., by this one motor. On the other hand, if the B.H.P. simply be known, the draw-bar pull exerted under the same conditions (assuming no slip of wheels) can readily be calculated thus — As— Torque in lbs. -feet at driving axle = rev. per minute of driving axle And as — -p. , n . 11 torque in Ibs.-feet exerted at driving axle Draw-bar pull m lbs. = — ,. ^ , . . -, — :r--- — ?-t ■ ^ radms oi driving wheel m leet Draw-bar pull in lbs. = B.H.P. X 5252 revs, per min. of driving axle X rad. of driving wheel in feet In this way, if B.H.P. exerted at driving axle and revolutions per minute and radius of driving wheels be known, then the draw-bar pull that will be exerted under these same conditions can be calculated. Another modification of the above gives the following rule, or formula, for connecting total D.B. pull, speed, and total H.P. expended. Thus— D.B.P. X speed in miles per hour __ tt p ~375 -M.i. MECHANICS OF TRACTION 107 Example. — Thus, in the detailed example previously given, with a draw-bar pull of 2910 lbs., and a speed of 7^ miles per hour, the 2910 X 15 H.P. being expended on the car at that instant will be —^w^ o~ = 58-2 H.P. The H.P. calculated is the B.H.P. that will have to be exerted by the motors when this particular draw-bar pull is being exerted at this particular speed. The H.P. so calculated, it must be noted, is the actual H.P. exerted at the particular instant the draw-bar pull and speed have these particular values, for just as either pull or speed, or both, might vary from instant to instant, so also would the H.P. vary. In the above example it means, for instance, that if the pull and speed were kept constant for 1 minute, 58*2 x 33,000 foot-lbs. of work would be done during that minute. The following formulae, which have been deduced on similar lines simply from first principles, together with those already explained, will be found to be of use in dealing with traction problems : — A = amperes passing down the trolley pole. V = volts pressure at trolley wire. S = speed of car in miles per hour. K = radius of driving wheel in feet. D = diameter of driving wheel in feet. T = total torque in lbs. -feet as exerted by the motors on the driving axles. D.B.P. = total draw-bar pull in pounds as exerted by the motors. N = number of revolutions per minute of driving axle. H.P. = horse-power expended in traction. a N X E . , CI N X Pt S = -^j-, more accurately S =-j^:^^ S = H.P. = T D.B.P. = ^ N X D , , ^ N X D — 2g — , more accurately S = ^^.^^ D.B.P. X S 375 K The following have been worked out on an assumption of a total efficiency of 80 per cent. : — io8 ELEMENTS OF ELECTRIC TRACTION A X V A X V T = — ^;^: — X 5-6, more accurately T = — ^ — x 5-63 T = AxVxR ,,^ AxVxE rr-~^, , more accurately T = ,^ .^^ 2*58 -' 2-48S D.B.P. = ^ ^r. , more accurately D.B.P. = ^ ,^ 2-5S -^ 2-48!S H.P. = gg^ , more accurately H.P. = ^g^,^ Varying Draw-bar Pull. — At the beginning of this chapter it was stated that, apart from the force required to overcome frictional and gravitational effects, no force was required simply to maintain a car at any given speed. This should always be borne in mind in dealing with the varying D.B. pull that may be applied to a car. Consequently, when the D.B. pull becomes equal to, or just balances, the force required to overcome the various frictional resisting forces if the car is on the level, or the various frictional resisting forces and gravitational effect if the car is mounting a gradient, the car will either be at rest, or, if moving, will be travelling at some uniform speed. In short, no acceleration or retardation of the car will be taking place. If the car is in motion, and the D.B. pull exceeds the frictional and gravitational forces, then the car will be in a state of acceleration, that is, it will be gaining speed. To illustrate this, two simple cases will be taken, and the effects produced by varying D.B. pull considered — 1. That of a car running on the level. 2. That of a car mounting a gradient. 1. Car running on the Level, Friction neglected. — In order to show exactly the effects produced by friction in dealing with the motion of a car, an imaginary case will first of all be considered in which friction will be assumed not to exist. If no friction of any kind whatever existed, the slightest D.B. pull would set a car in motion, and would give to the car a small but definite acceleration, depending, as already seen, simply upon the total weight of car and upon the D.B. pull applied. If, now, in such a case, a D.B. pull of 1540 lbs. — see previous example — were applied to a 10-ton car starting from rest, the car would have given to it, as long as this D.B. pull remained constant, an acceleration of IJ miles per hour per second, and at the end of 10 seconds the car would have attained a speed of 15 miles per hour. The different effects which would be produced by varying the MECHANICS OF TRACTION 109 D.B. pull at this stage of the car's motion will now be considered. To make the case as simple as possible, it will be assumed that the changes in the D.B. pull take place not gradually, but instan- taneously. If now at this stage, that is, after the 10 seconds, the D.B. pull were suddenly increased to 3080 lbs., that is, just twice its previous value, the acceleration would also be suddenly increased from IJ to 3 miles per hour per second. With a constant D.B. pull of 3080 lbs., the car would gain during the next 5 seconds an increase in speed of 3 x 5, namely, 15 miles per hour, so that at the end of 15 seconds from the start, the car would have attained altogether 15 + 15, namely, a speed of 30 miles per hour. If at the end of the 10 seconds the D.B. pull had, instead, been suddenly decreased to 770 lbs., that is, just half its previous value, then with this accelerating force at work the car would still go on gaining speed, but only now at the rate of f miles per hour per second, and if the D.B. pull of 770 lbs. were kept constant, the car during the next 5 seconds would gain an increase in speed of f X 5, namely, 3| miles per hour, so that in this case the car would have at the end of the 15 seconds, 15 + 3|, namely, a speed of 18| miles per hour. Thus, while the acceleration in this case will depend with constant load simply upon the accelerating force at work, the speed will depend not only upon the value of the various accelerating forces, but also upon the length of time these various forces have been at work. If now, on the other hand, at the end of the 10 seconds the motors had ceased to exert a D.B. pull, then as the speed at that instant was 15 miles per hour, and no force would be at work after that, either accelerating or retarding the motion of the car, the car would simply go on travelling at the uniform speed of 15 miles per hour, and an opposing force of some kind would have to be applied to stop the car. Car running on the Level, Friction considered. — Taking now the actual case, unless a certain D.B. pull be applied, no motion of the car will take place owing to friction, and any force that may be applied over and above that required for friction will be available for accelerating the car. Thus, with a tractive resistance of 25 lbs. per ton, to give a 10 -ton car an acceleration of IJ miles per hour per second, will now require a D.B. pull of 250 + 1540, namely, 1790 lbs., a greater D.B. pull than in the previous case, owing to frictional resistance. Now, as long as this effective force of 1540 lbs., namely, the amount over and above that required for friction, is constant, so long also no ELEMENTS OF ELECTRIC TRACTION will the acceleration be constant, provided, just as before, no increase in the total weight takes place, and at the end of 10 seconds the car, starting from rest, will have attained with this D.B. pull of 1790 lbs. a speed of 15 miles per hour. If now the effective force of 1540 lbs. be increased, the acceleration will be increased, and if it be decreased, the car will go on gaining speed, but at a correspondingly lesser rate, just as in the previous case. The acceleration in this case will depend not only upon the D.B. pull and total weight of car, but also upon the frictional resistance. If at any instant the D.B. pull becomes just equal, that is, just balances the frictional resisting forces, the car will continue tra- velling at whatever speed it may have at the instant the forces become so balanced. Thus, in the above case, if at the end of the 10 seconds the D.B. pull is suddenly reduced to 250 lbs., the car will continue travelling at the rate of 15 miles per hour, as long as the D.B. pull of 250 lbs. total load carried and frictional resistances are not varied. It should, however, be noted that any increase that may take place in the frictional resistance, such as may arise due to increased air resistance, will have the effect of reducing the effective force. Where high speeds are adopted, as in railway work, an increase of friction in this way takes place, and this may reduce the effective force until all acceleration ceases. If the motors cease to exert a D.B. pull, the frictional resisting forces will act as a retarding force, and the car will gradually slow down. In this case the kinetic energy of the car is simply trans- formed, due to friction, into heat energy, which will be dissipated in various ways. The length of time it takes the car to come to rest after the D.B. pull is removed will depend upon the frictional resistance, and, to shorten this time, other frictional forces in the shape of brakes may be introduced. 2. Car mounting a Gradient. — In this case, before motion can take place, not only will a force have to be applied to overcome frictional resistance, but, in addition to this, a force will have to be applied to balance the particular gravitational pull due to the particular gradient the car is on, and any force applied over and above this will then be available for accelerating the car. Thus, with a tractive resistance of 25 lbs. per ton, as already seen, a D.B. pull of 2910 lbs. will be requii'ed to give a 10-ton car an acceleration of 1-^ miles per hour per second on a gradient of 2o-> a greater D.B. pull than in the previous case, owing to the car being now on a gradient. If the 250 lbs. required for friction and the 1120 lbs. required for balancing the gravitational pull be deducted from the total D.B. pull of 2910 lbs., a nett accelerating force, it will be seen, MECHANICS OF TRACTION iii of 1540 lbs. will be left, which force will give to the car an acceleration of 1^ miles per hour per second just as before. Now, as long as this effective force of 1540 lbs. is constant, so long also will the acceleration be constant, provided no increase in the total weight takes place, and at the end of 10 seconds the car, starting from rest, will have attained a speed of 15 miles per hour on this particular gradient with this particular D.B. pull of 2910 lbs. Any increase or decrease in the effective force of 1540 lbs. will bring about, just as before, an increase or decrease in the acceleration, and as long as this effective force remains constant, so long also will the acceleration remain constant, provided no increase in the total weight takes place. The acceleration in this case, it will be seen, depends not only upon the total D.B. pull, total weight of car, and frictional resistance, but also upon the value of the gradient. A comparison may now be made between the various D.B. pulls required in the three cases; in each case the same acceleration of 1^ miles per hour per second is given to a 10-ton car — On the level, friction neglected . . 1540 lbs. D.B. pull. On the level, friction included . . 1790 „ „ On a gradient of 20-^ friction included . 2910 „ „ If now at any instant the D.B. pull just becomes equal to the gravitational pull and the various frictional forces, the car will continue moving up the gradient at whatever speed it has at the instant the forces became so balanced, and it will neither gain nor lose speed as long as the balance exists. If, in the above case, for instance, at the end of 10 seconds the D.B. pull is suddenly reduced to 250 + 1120, namely, 1370 lbs., the car will go on travelling up this particular gradient at the speed of 15 miles per hour as long as the D.B. pull of 1370 lbs., total weight of car, gradient, and frictional resistance, do not vary. If the motors cease to exert a D.B. pull, not only will the frictional resistance tend to retard and so stop the car, but the force due to gravitation will also be at work, tending to stop the car. In this way, the car will come to rest in a shorter time on a gradient, depending upon the value of the gradient, than when on the level. The gravitational force, it must also be noted, will not only assist in retarding the motion of the car, but it will also, if the gradient be sufficiently steep, restart the car in motion in the reverse or downhill direction. In such cases, frictional resistance, in the shape of brakes, will have to be applied in order to hold the car on the gradient, and so prevent such action. In the above example, for instance, a force of 1120 — 250, namely, 112 ELEMENTS OF ELECTRIC TRACTION 870 lbs., would be available for accelerating the car in a downhill direction. In stopping a car in this way on an incline, the kinetic energy of the car is partly transformed into heat energy, due to frictional resistance, and part is transformed into potential energy, the car's own motion carrying it forward up the incline, and so raising the car some definite number of feet. Examples (1) A car travels 100 yards in 15 seconds. What is the speed of the car, expressed — (a) In miles per hour ? (h) In feet per minute ? (a) A rate of 100 yards per 15 seconds is equivalent to a rate of 400 yards per 60 seconds, also to a rate of 24,000 yards per hour ; .*. equivalent rate = -^f^(y =13*6 miles per hour Therefore the car is travelling at the rate of 13*6 miles per hour. As a rough rule, divide twice the number of yards by the number of seconds, and the result will give the approximate speed in miles per hour ; thus — — q-^ — = 13*3 miles per hour 15 (b) A rate of 100 yards per 15 seconds is equivalent to a rate of 400 yards per minute, also to a rate of 1200 feet per minute. Therefore the car, when travelling at a speed of 13*6 miles per hour, is travelling also at the rate of 1200 feet per minute. (2) A car is given a uniform acceleration of 2 miles per hour per second. (a) What speed will the car attain, starting from rest, in 6 seconds ? (b) What number of yards will the car travel during the 6 seconds ? (a) As increase of speed is 2 miles per hour every second, there- fore in 6 seconds the car will have increased its speed 6x2, namely, 12 miles per hour Therefore, starting from rest, the car in 6 seconds will attain a speed of 12 miles per hour. MECHANICS OF TRACTION 113 (b) As at the end of 6 seconds the speed is 12 miles per hour, the average speed has been ^-, namely, 6 miles per hour. .*. in 6 seconds the car will travel ^ ^ = yj^ part of a mile As yjfj mile = 17*6 yards, the car therefore travels 17*6 yards Therefore the car with the above acceleration will travel in the 6 seconds a distance of 17*6 yards. This could have been calculated much more easily from the approximate formula given in Chapter X. ; thus — AT^ If acceleration and time were simply known, then, as Y = —-p (see page 175) — Y = J =18 yards approximately or, if speed at end of 6 seconds and acceleration were known, as — S2 = 4AY /. 12 X 12 = 4 X 2 X Y .*. 8Y = 144 .-. Y = 18 yards, as before (3) A car is given an acceleration of 4 feet per second per second. "What is the value of the acceleration expressed in miles per hour per second ? An increase of 4 feet per second every second is equivalent to an increase of 4 x 3600, namely, 14,400 feet per hour per second. As 14,400 feet = -L4^%Q miles, namely 27 miles .*. 14,400 feet per hour per second = 2*7 miles per hour every second Therefore an acceleration of 4 feet per second per second is equivalent to an acceleration of 27 miles per hour per second. (4) A car travelling at 20 miles an hour is pulled up in 40 feet. What is the value of the retardation in miles per hour per second, assuming it to have been uniform ? Average speed in slowing down = -2^, namely, 10 miles per hour A speed of 10 miles per hour is equivalent to a speed of — 10 X 5280 , ^._„ ^ 60 y 60 ' ^^^^^y> 14^ i6^t per second 40 /. time taken to travel the 40 feet = yj;^ = 274 seconds approx. 20 .*. retardation = T^^ffi "^ ^'^ miles per hour per second approx. Therefore the retardation in the above case will be 7-3 miles per hour per second. I 114 ELEMENTS OF ELECTRIC TRACTION By means of the approximate formula given in Chapter X., this could be calculated much more easily thus — As 40 feet = 13-3 yards and as A = -r^ 4Y , , ^. 20 X 20 400 retardation = . ■ ■■ ..o o = ftttt 4 X 13-3 53*2 = approximately 7" 5 miles per hour per second (5) It is required to give an acceleration of 2 miles per hour per second to a 6-ton car mounting a gradient of 1 in 14. What draw-bar pull would be required, assuming a tractive resistance of 25 lbs. per ton ? As 102*6 lbs. per ton are required to give an acceleration of 1 mile per hour per second, .*. 205*2 lbs. per ton will be required to give an acceleration of 2 miles per hour per second. .-. force required for accelerating l ^ ^ purposes J force required to balance! ._ 6 x 2240 _ ^^^ gravitational effects J 14 ~" " force required to overcome \ _ c^f- n _i-n tractive resistance | - ^5 X b _ iDU „ .*. total draw-bar pull = 2341-2 Therefore the draw-bar pull required under above conditions = 2341 lbs. approximately. (6) Each of the two series motors on a car under certain conditions exerts 25 B.H.P. at the armature shaft when the armature is making 555 revolutions per minute ; the gear ratio is 4*78 to 1, and the driving wheels are 33 inches diameter. (a) What torque in Ibs.-feet does each motor exert on the armature shaft ? (b) What torque in Ibs.-feet does each motor exert on the driving axle (neglecting friction of gear) ? (c) What will be the D.B. pull in pounds due to each motor ? {d) What speed in miles per hour will the car have under the above conditions ? MECHANICS OF TRACTION 115 (a) . . As torque m Ibs.-ieet = -. ^ ^ rev. per minute 25 X 5252 /. the torque exerted by one motor = =W^-^ = 236"57 lbs. -feet 000 Therefore the torque exerted by one motor under above conditions on the armature shaft = 236*57 lbs. -feet. (b) With a gear ratio of 478 to 1— The torque exerted on the driving axle (neglecting friction of gear) = 236-57 X 4-78 = 1130-8 Ibs.-feet. Therefore the torque exerted by each motor on the driving axle under above conditions = 11 30*8 Ibs.-feet. (c) With driving wheels 33 inches diameter or 16 J inches radius, that is, 1*375 feet radius — The total force exerted at the rim of) 1130-8 the two driving wheels ) 1'375 •^o = 822-4 lbs. Therefore the D.B. pull exerted by each motor under above conditions = 822*4 ^bs. This could also be worked out from the formula — D.B.P. in lbs. = ?i^^^^*^^^ (See page 106.) 555 As N =-7yjn, namely, 116 revolutions per minute approx., D.B.P. in lbs. = -r^-^— — TTon^ = 823 lbs. approximately as before (d) As 33 inches diameter = 33 x 3-14, namely, 103-62 inches circumference or 8*63 feet circumference, The car will travel 8-63 feet for every revolution of the driving axles (assuming no slip). As the armature makes 555 revolutions and the gear ratio is 4-78 to 1, 555 The driving axles under these conditions will make 77=^, namely, 116 revolutions per minute approximately. .*. in 1 minute the car will travel a distance of 8-63 x 116, namely, 1002 feet approximately. 1002 feet per minute = rc^nr^ — miles per hour = 11-4 miles per hour approximately ii6 ELEMENTS OF ELECTRIC TRACTION Therefore under above conditions the car will travel at a speed of approximately 11*4 miles per hour. This could also be worked out more easily by the approximate formula — S='^^ (See page 107.) .*. S = .. ■ = 11-4 miles per hour approximately as before (7) A car is supplied with 50 amps, at a pressure of 500 volts, assuming an efiQciency of 80 per cent, for the motors and gearing. What D.B. pull under these circumstances will the motors exert when the car is travelling at 10 miles per hour ? As D.B. pull in pounds with an efficiency of 80 per cent. Ax V . - ^ 2-5 X s ^pp^o^'^i^^^t^iy /. D.B. pull = ^^^^-^- = m- = 1000 lbs. ^ 25 X 10 ^^ Therefore with an efficiency of 80 per cent., the motors under the above circumstances will exert a D.B. pull of 1000 lbs. approximately. (8) A 10-ton car is mounting a gradient of 1 in 12 at a speed of 8 miles per hour, and the resistance to traction under these circumstances is 30 lbs. per ton. (a) What D.B. pull will be required simply to maintain the car at the uniform speed of 8 miles per hour ? (h) What H.P. would the motors be exerting under these circum- stances ? (c) What amperes would the car be taking, assuming an efficiency of 80 per cent, and that the trolley pressure is 500 volts ? (a) D.B. pull required to overcome resistance to traction = 10 X 30 = 300 lbs. D.B. pull required to balance gravitational pull = = 2a^oo _. iSQQ it>s. approximately .-. total D.B. pull = 2166 lbs. Therefore D.B. pull required simply to maintain the car at the uniform speed of 8 miles per hour = 2166 lbs. MECHANICS OF TRACTION 117 (b) . . As H.P. =^:5iZ^lfiP2id • HP— ^ — 1TL3 2 8 _ Aa-0 , . n.r. _ ^f^ _ -^75- -- ^t> z Therefore the H.P. exerted by the motor under the above con- ditions would be 46 -2 H.P. A X V (c) As D.B. pull with an efficiency of 80 per cent. = -^7= — ^ D.B.P. X 2-5 X S Amperes = ^ — 2166 X 2-5 X 8 2166 x 20 ^^ ^ :. amperes = ^^ = ^^ — = 86'6 amps. Therefore the amperes taken by the car under the above conditions would be 86*6 amps. (9) A 10-ton car with wheels 30 inches diameter is descending a gradient of 1 in 10. Assuming a tractive resistance of 24 lbs. per ton — (a) What retarding draw-bar pull would the motors acting as dynamos have to exert to prevent the car being accelerated ? (h) What is the total retarding torque they would have to exert on the driving axles ? (c) What acceleration would be given to the car if the motors exerted no retarding force ? (d) What speed would the car attain with the motors exerting no retarding force in travelling 50 yards ? Answer — (a) 2000 lbs. (h) 2500 Ibs.-feet. (c) 1*94: miles per hour per second. (d) 19 '9 miles per hour. (10) A 10-ton car in descending a gradient of 1 in 10 is main- tained by means of the motors (acting as dynamos) at a speed of 4 miles per hour. Assuming an efficiency of 70 per cent., what horse-power would be available for being regenerated or sent back to the line if the motors are employed as dynamos for that purpose, assuming a tractive resistance of 24 lbs. per ton ? Ansiuer — 149 E.H.P., or its equivalent, 11*1 kilowatts. CHAPTER VIII CHARACTERISTIC PROPERTIES OF CON- TINUOUS-CURRENT MOTORS Classification of Motors. — Motors are classified and described by the method adopted of exciting the field exactly in the same way as for dynamos ; thus there are series, shunt (including those motors in which the shunt field is excited separately), and compound motors. These different types of motors have their own characteristic properties, depending on the method of exciting the field, as will now be seen. Shunt Motor. — A shunt motor is, roughly speaking, a constant- sp-eed motor, its speed in this way being independent of the load. Strictly speaking, however, the speed of a shunt motor drops slightly as the load is increased, and of course increases slightly as the load is decreased. The variation in speed, however, with a well-designed shunt motor is only very small, possibly at the most 5 per cent, or so, the variation depending upon the design and size of motor. A shunt motor can be run light, that is, without load, and it will then have its normal and also its maximum speed. If the field terminals of a shunt motor be detached from the armature or main terminals of the motor, and the shunt field winding be connected to some source of supply quite separately, that is, as a circuit distinct in itself, apart from the armature circuit, such a motor is sometimes said to be separately excited. The behaviour of such a motor is practically identical with that of a shunt motor, the speed being practically constant. Series Motor. — A series motor is a variahle-sipeed motor, its speed varying with, and in this way depending on, the load, running more slowly on heavy loads than on light ones. On a very light load it will run at an excessive speed, so much so that it would be dangerous to run a series motor of any appreci- able size without load, the danger being that with the excessive speed the centrifugal force would be so great as to possibly " burst " the machine. This excessive speed is due to the very weak field that exists, and which is the outcome of the light load. This danger, it may be said, also exists with any type of motor PROPERTIES OF CONTINUOUS-CURRENT MOTORS 119 on light load should the field be weakened in some accidental manner, as the breaking of the shunt circuit in a shunt motor. On account of the above, it is not advisable to transmit power from a series motor by means of a belt, as, in the case of the belt coming off or breaking, the machine is left without load ; with chain or gear driving the above danger is reduced. A series motor is very suitable for starting under heavy loads, and is consequently used in crane and traction work. Compound Motor. — A compound motor has a combination of series and shunt field windings. In dynamo construction the object of compounding, as already seen, is to obtain either a constant voltage, or a voltage which increases with increasing load ; consequently, the series winding is arranged to assist the shunt. In compounding a motor, the series winding may be arranged either to strengthen the main shunt field somewhat — a greater strengthening action taking place the greater the current — in which case the field is called a cumulative winding, the magnetizing effects being added ; or it may be arranged to weaken the main shunt field somewhat — a greater weakening action taking place the greater the main current — in this case it is called a " differential winding," the strength of field being due to the difference of the two magnetizing effects. Cumulative Compound Motor. — This type of motor is de- signed for the purpose of giving to a shunt motor some of the characteristics of the series motor, namely, to increase its torque at starting, or under- heavy loads, and so enabling it to cope with heavy overloads without the danger of being pulled up. The speed variation in this type is greater than in a shunt motor, resembling more the series. Differential Compound Motor. — This type of motor is de- signed for running at constant speed. Heavy overloads, however, with this type of motor should be avoided, the motor being liable to stop when so overloaded, owing to the serious weakening of the main field and the consequent reduction of the torque. Self-regulating Properties. — An electric motor, in certain re- spects, is self-regulating, and before dealing in detail with the various characteristic properties of the types used in traction work, the general way in which all continuous-current motors regulate them- selves will first of all be considered. If a series of experiments be made with any of the above types of electric motors, fitted, say, with a friction brake similar to that already described, so that the load upon the motor can be varied — increase of load being brought about by increase of friction; this being again brought about by adding additional weights to the 120 ELEMENTS OF ELECTRIC TRACTION belt on the opposite side to the spring balance, so that the greater the load the greater the pull the motor has to exert on the belt — two main facts will be noticed on running the motors under various loads — 1. Concerning the speed. 2. Concerning the electrical input. 1. Speed. — It will be found that an electric motor, if supplied with a suitable and constant voltage, will run and continue to run, when once it has been started, at some uniform speed as long as the load remains constant. Variations of load will bring about variations of speed in all the above types of motors to a greater or less extent, except those specially designed for running at a constant speed ; and where varia- tions of speed do take place it will be found that with each load the motor will run at some definite and uniform speed, whatever that speed may be, as long as that particular load remains constant. Consequently, in all these cases variation of speed will only be brought about under above conditions by variations in load, the motor always having a definite and uniform speed for each definite load. The load upon the motor in all these cases is assumed to be, of course, a suitable one ; that is, within the range and power of the motor. 2. Electrical Input. — It will also be found that the amperes passing through the motor under test are constant as long as the load is constant, and that the amperes increase and decrease — not necessarily in strict proportion — with increase and decrease of load. In short, a series of experiments will show that an electric motor is self-governing ; that is, the motor itself automatically regulates the electrical power put in, so as to correspond with the work done or power given out. Thus, it must be very clearly borne in mind that while a motor may not be as efficient at light loads as on heavier ones, it takes less electrical power, that is, less watts on light loads than on heavy ones, no external regulator as far as power is con- cerned being required. The consumption of energy always corre- sponds with the work done or energy given out by the motor. Thus, if the load on a motor is reduced, so also automatically is the power reduced, no waste of energy in this way taking place. The above experiments thus show that with variable-speed motors (whatever be the amount of variation), if the supply voltage be constant, a motor will have for every given load a corresponding speed and current. When a motor is designed for constant speed, it will have for every given load a corresponding current, the speed, of course, being practically the same for all loads. PROPERTIES OF CONTINUOUS-CURRENT MOTORS 121 The reasons for the above self-regulation will now be given, and to make the following explanation as simple as possible, those motors which vary their speed with variation of load will simply be con- sidered. These types, it might also be added, are the ones chiefly in use. In traction work the series motor is chiefly employed, although at the present time the shunt type is also being advocated. Speed and Power Regulation. — In Fig. 28 a simple case of a motor under load is shown, the motor being represented as hoisting a weight of 200 lbs. by means of a pulley fixed to the armature shaft. To merely hoist the weight, the motor would have to exert a force equivalent to a weight of 200 lbs. at the rim of the pulley, or, better still, if the pulley were 3 feet diameter, the motor would have to exert a torque or twisting effort of 200 X li, namely, 300 Ibs.-feet. In addition, however, to this, the motor would have to exert a force to overcome friction and various other resisting forces which are set up when an electric motor is in motion, and which will be dealt with later. To take figures which would be something like proportionate to the force under con- sideration, a force of 5 lbs., for instance, might have to be applied at the rim of the pulley to overcome friction, and a further force also of 5 lbs. might also be required to overcome the other resisting forces men- tioned. In all, then, the motor would have to exert — A torque of 200 lbs. x 1^ feet, namely 300 Ibs.-feet, to balance the load ; A torque of 5 lbs. x 1| feet, namely 7^ Ibs.-feet, to over- come the friction ; A torque of 5 lbs. X 1^ feet, namely 7^ Ibs.-feet, to over- come the various other resisting forces. Thus the motor would have to exert a total torque of 315 Ibs.- feet, or, if the reader prefers to deal in forces, a force equivalent to 210 lbs. — applied, it must be noted, at the rim of the 3-feet-diameter pulley. Now, the following principle, in dealing with the load and speed of a motor, must always be clearly borne in mind, just as in dealing with the motion of a car : — When the torque exerted by a motor just balances that due to load, friction, and the other resisting forces mentioned, set up in and due Fig. 28. 122 ELEMENTS OF ELECTRIC TRACTION to the motion of the motor, the motor will run at some uniform speed as long as the load, friction, etc., and torque exerted by the motor so balance one another, just as a car will continue to run on the level at a uniform speed when the draw-bar pull just balances the tractive resistance. Thus, in the above example, when the motor is running at some uniform speed under the above load it will just be exerting a torque of 315 Ibs.-feet. If, now, half the load were "thrown off" — the motor now only having a weight of 100 lbs. to hoist — the torque, exerted by the motor, would, at this particular instant, before any re-adjustment of current took place, exceed that due to the decreased load, etc. The balance of torques would in this way be upset, and, roughly speaking, a little less than half the torque the motor was exerting at the particular instant the load was thrown off, would be available for acceleration. The motor would consequently speed up, and in speeding up would gradually generate a greater back E.M.F., thus reducing the current passing through the motor, and so gradually reducing the torque until a balance was reached, namely, when the torque due to load, and that required for friction, etc., was just balanced by the now reduced torque exerted by the motor. The motor would then continue to run at whatever speed it had at the instant the torques became so balanced ; that is, in the above case, at some higher but uniform and definite speed, corresponding with the reduced load. The reduction of current would in this way bring about a reduction in the electrical power supplied, thus corre- sponding with the reduced load. If, on the other hand, the load were increased, yet still within the range of the motor, the torque due to the load, etc., would, for the time being, be greater than the torque exerted by the motor. The motor in consequence would slow down to a greater or less extent, depending upon the type of motor. In this way the back E.M.F. would be gradually reduced, current and torque would in consequence be gradually increased, until, as before, the torque due to the increased load, etc., was just balanced by the increased torque now exerted by the motor, the motor then continuing to run at some slower but uniform and definite speed, corresponding with the increased load. The increase of current would bring about an increase in the electrical power supplied, thus corresponding with the increased load. If a considerable overload were put on the motor, there would be a corresponding reduction of speed — the actual reduction depending upon the value of the load, and upon the type of motor employed — and a corresponding considerable increase in the current PROPERTIES OF CONTINUOUS-CURRENT MOTORS 123 and the consequent torque, due to the decreased back E.M.F. In this way the motor always endeavours to increase its torque to suit that of the load. If the load were increased to such an extent as to be finally beyond the effort of the motor, the motor in such a case, not being able to exert a torque equal to that due to the load, etc., would slow down and finally stop revolving. The excessive current that would flow under the above circum- stances would, of course, ia practice never be allowed, as the excessive generation of heat, and consequent excessive rise in tem- perature of the windings, if such current were allowed to flow for any length of time, would be sufficient to burn the motor out. To protect the motor from such excessive overload of current, fuses and circuit breakers are generally inserted, as already mentioned. Load. — In varying the load in the above experiments, it will be seen that the torque or twisting effort exerted by the motor had to increase in a corresponding manner with increase of load, the greater the pull the motor exerted, the greater being the torque exerted. Thus, in the sense the term " load " is here used, the torque will always be a measure of the load ; that is, a motor exerting a constant twisting effort or torque will be said to have a constant load, and consequently a varying load will mean a varying torque, the torque simply increasing and decreasing with increase and decrease of load. As applied to traction work, this will mean, in a similar way, that a motor fitted to a car with a given ratio of gear wheels, and a given size of driving wheels, will be said to have a constant load as long as it exerts a constant draw-bar pull ; constant load in this case meaning not only constant torque, but also constant draw-bar pull. As the horse-power developed by a motor depends not only on the torque, but also upon the speed, so it also depends, in like manner, not only on the load put on a motor, but also upon the speed of the motor. Thus distinction must be made between load and power, as, for example, the load — in the sense that it is used here — may be so great as to prevent the motor working, or, in other words, developing any power. If the load is so excessive that the motor fails to revolve, then all the electrical energy supplied will simply be transformed into heat, electrical energy only being transformed into mechanical energy when motion takes place, that is, when a back E.M.F. is being generated. The following analysis will now enable the characteristic properties of motors to be dealt with a little more in detail. To simplify matters as much as possible, it will be assumed that the 124 ELEMENTS OF ELECTRIC TRACTION torque the motor exerts is simply that required for the actual load put on the motor. The motor, of course, really has to exert a greater torque than this, in order to overcome friction and the various other resisting forces set up. As the resisting forces due to friction, etc., only form, however, a small portion of the total load — except when a motor is on light load, when they form the sole load — they will, in order to simplify matters, be ignored for the present. Motor Analysis. — A varying load, as already seen, means that a varying torque has to be exerted by the motor. Now, the torque exerted by any given motor, that is, a motor already constructed, depends simply upon the strength of armature current and strength of field. The torque, therefore, of a motor can be varied by the varying of either — (1) Strength of field ; (2) Armature current ; or a suitable variation of both. (1) Strength of field variation can be brought about by a variation in the value of the current passing through the field coils, this being again brought about by a variation of either — {a) The resistance in the field circuit, (6) The volts across the terminals of the field — in a shunt motor this will be the supply volts — or a suitable variation of both. (2) Armature current variation can be brought about by a variation of either — (a) The supply volts. (fS) The back E.M.F. ; this, with a given motor, depending simply upon strength of field and speed. (c) The resistance in the armature circuit ; or by a suitable variation of any two or all three. It should be noted that the torque exerted by a motor depends not only upon the strength of field, or, more definitely, upon the intensity of the field, that is, upon the number of lines say per square inch passing out of the magnets, and upon the armature current, but also upon the number and length of the armature conductors, and also upon the distance of the conductors from the centre of the shaft. As the number and length of the conductors and their distance from the centre of the shaft cannot be altered when once the armature has been constructed, the only way the torque can be altered, as far as the armature is concerned, is by a variation in the current flowing through the conductors taking place. Again, the intensity of the field strength will depend not only upon the magnetizing force at work, but also upon any demagnetizing force there may also be at work. Now, owing to current circulating round the armature core, PROPERTIES OF CONTINUOUS-CURRENT MOTORS 125 certain demagnetizing actions are set up which tend to demagnetize and so weaken the main field. The intensity of the field strength, apart from the above demagnetizing force, depends not only upon the current passing through the field coils, that is, upon the excitation current, but also upon the number of turns of wire in the field coils, and upon the length and permeability of the materials forming the magnetic path. Here, again, when once a motor has been constructed, the only way the torque can be altered, as far as the intensity of the field strength is concerned, that is, apart from the demagnetizing force at work, is by a variation in the value of the current passing through the field coils taking place. Variation in the value of the current passing through the field coils, and hence variation of field, is brought about, as shown in the above analysis, by a variation of either the resistance in the field circuit, or the pressure of supply across the field terminals, or by a suitable variation of both. In this way, the field strength in any given motor will depend, not only upon the pressure of supply across the field terminals, and upon the resistance of the field winding, but also upon the amount of demagnetizing action taking place. To simplify matters, however, consideration of the demagnetizing effects will be left until later. Shunt Motor. — To apply this analysis to a shunt motor, let it be assumed that the motor is supplied with constant pressure, and that the resistance in circuit is constant, as would be the case under most working conditions, that is, when all starting resistance has been cut out. If load be varied, a variation in the torque will be brought about by a variation in either — (1) Strength of field ; (2) Armature current ; or a suitable variation of both. Strength of Field. — As the supply volts are constant, and also as the resistance of the field winding will be practically constant, the field strength will be constant. Therefore, in this case, varying load will simply mean a variation in the armature current. Armature Current. — As the supply volts and the resistance in the armature are both constant, it will be seen that the back E.M.F. will have to vary in order for there to be any variation in the armature current. Again, as the field is constant, the speed will have to vary in order for there to be any variation in the back E.M.F. Consequently, with a shunt motor, under ordinary working conditions, variation of load means variation of speed. Extent of Variation of Speed. — It only remains now to be seen to what extent the speed must vary under the above conditions 126 ELEMENTS OF ELECTRIC TRACTION to bring about the requisite change in the value of the current. This speed variation will be found to be very small. It must always be remembered in dealing with the current passing through a motor, that it is the effective E.M.F. in the motor circuit which determines the value of the current. As the resistance in the armature circuit after all starting resistance is cut out is small, being in shunt motors only the resistance of the armature, and in series motors the resistance of armature and field — this in machines of any appreciable size will only be the fractional part of an ohm — the effective E.M.F. need only be, comparatively speaking, small, in order to send a large current through the circuit. As the effective E.M.F. is the difference between the applied and the back E.M.F., a slight decrease in the value of the back E.M.F. will bring about a much greater increase in the effective E.M.F. Thus, let the line AB (see Fig. 29) represent say the 100 volts of supply to a motor, and let the length BC represent the back E.M.F., then AC, the difference, will represent the effective volts. Now, to double the effective E.M.F. so as to double the current does not ACD Fig. 29. mean reducing the back E.M.F. represented by BC to half its value, but only to a value represented by BD, the length AD being equal to twice the length AC. The effective E.M.F. is now represented by AD. As the speed (the field being constant) is proportional to the back E.M.F., it will be seen that a small drop in speed corresponding to the slight drop in the back E.M.F. will be sufficient to increase the current to twice its original value. Thus a small drop in speed in a shunt motor brings about a much greater difference in the effective E.M.F., and so a much greater difference in the value of the current. Thus a small variation in speed with a large varia- tion of load. Example. — To take an actual case, the armature resistance of a small 100-volt shunt motor is 0'2 of an ohm, its full-load current being 10 amps. When running at quarter-full load, namely, when 2^ amps, are passing through the armature, an effective pressure of half a volt is required. The back E.M.F. is therefore in this case 99^ volts, and the speed, treating the field as constant, proportional to 99^. To increase the current to four times its value, that is, to 10 amps, (the full load), four times the effective pressure is required, PROPERTIES OF CONTINUOUS-CURRENT MOTORS 127 namely, 2 volts, the back E.M.F. now being 98 volts, and the speed in this case proportional to 98. Thus, as the load is increased from quarter-full load to full load, the speed drops proportionately from 99 J to 98, about IJ per cent. Thus the shunt motor, while it has, strictly speaking, a definite and different speed for each different load, has a speed variation so small that for many practical purposes it can be said to be a constant-speed motor, and that the speed is thus independent of the load. Variation of Torque with Current. — It has already been shown that the torque depends upon strength of field, and strength of armature current ; the stronger these are, the greater the torque. In a shunt motor, as the field is practically constant, the torque will simply vary, with the armature current increasing and decreasing practically at the same rate. It will be found that if the matter be now looked at from a power point of view — the input and output of the motor being considered quite separately — the results concerning variation of torque and speed with variation of current are in entire agreement. Input. — If the current in any given case be doubled, the voltage being constant, the electrical power put in will be doubled, and therefore the H.P. output should also be approximately doubled. Output. — The output of a motor, as already seen, depends upon load and speed. If with double the current double the torque is obtained, then under these circumstances it means that the load has been doubled. With double the load, and speed remaining practically constant, double the output will be also approximately obtained, the output thus corresponding with the input. Thus, roughly, in a shunt motor, if in any given case — The load is made ..... twice as great. In order to obtain — A torque ....... twice as great, The current will become .... twice as great ; The H.P. output will be . . . . twice as great. The speed being practically . . . constant. Series Motor. — To apply the analysis now to a series motor, let it be assumed that the motor is supplied with constant pressure, and that the resistance in the armature circuit is constant, as will be the case when all starting resistance has been cut out just as before. If load be varied, a variation in the torque will be brought about by a variation in either (1) strength of field ; (2) armature current ; or a suitable variation of both. Strength of Field. — As the field winding is in series with the armature, whatever current passes through the one will pass through 128 ELEMENTS OF ELECTRIC TRACTION the other, unless some device be adopted of shunting a portion of the main armature current through some other path than that of the field. It will be assumed, however, that no such device is adopted, and that an ordinary series motor such as is used in the ordinary series parallel system for traction work is being considered. Varying torque, therefore, in this case can only be brought about by a variation taking place both in the field strength and in the armature current, as the one cannot be varied without the other, seeing, for instance, that increased armature current means also an increased field strength, and vice versa. Consequently, it must be borne in mind that an increase of load will bring about an increase in the field strength, and a decrease in load a decrease in field strength. The value of the current in the series winding, and the consequent field strength, will in this case not be determined by considering the volts across the terminals of the series winding, or its resistance, but by considering simply the armature current. Armature Current. — As the supply volts and the resistance in the armature circuit are both constant, it will be seen that the back E.M.F. will have to vary in order for there to be any variation in the armature current. Variation in the back E.M.F. may be brought about by a variation in the speed, or the field strength, or a suitable variation of both. Now, as increase of field strength takes place with increase of load, it will readily be seen that a decrease must take place in the speed if an increased current is to be obtained, and that especially so with an increased field strength. The decrease in speed would be very small, just as in the case of a shunt motor were the field constant, but with an increased field strength a greater reduction in speed must take place, even if the same back E.M.F. is to be generated, and the same current to flow as originally, therefore to obtain a still greater current a still further slight reduction of speed will be required. Thus, the chief cause of the great reduction in speed in a series motor with increase of load is not so much the increased current required, but is chiefly due to the increase in field strength which is the outcome of the increased load. In a similar way, to decrease the current the back E.M.F. must be increased, and this with a weakened field means a considerable increase in speed. To sum up, with a series motor under ordinary working conditions an increase of load will bring about a considerable decrease in speed, and a decrease in load a considerable increase in speed. It remains now to be seen to what extent the torque, speed, and current vary with varying load. PROPERTIES OF CONTINUOUS-CURRENT MOTORS 129 Variation of Torque, Speed, and Current with Varying Load — At the outset a difficulty arises, owing to the fact that doubling the current may or may not mean doubling the strength of the magnetic field, and so on, all depending upon the degree of magnetization of the field magnets, this again depending upon the particular design. To form a rough initial idea, however, it will be assumed that doubling the current does double the strength of the magnetic field.. Assume, then, that a given series motor be supplied with some constant voltage, and that a given load be applied. It will take a given number of amperes, run at some definite speed, and in so doing will exert a certain definite torque. If now the load be made four times as great, four times the torque will at the least be required. If the current be doubled, and the field in consequence be doubled in strength by so doing, then the torque will be increased to four times its value, which is the value required. Thus, on the above assumption, doubling the current has increased the torque to four times its value. In a similar way, with half the current only, one quarter the torque will be exerted. This rapid increase or decrease in torque value, with an increase or decrease in current value, is one of the characteristic properties of the series motor. Now, before the load was increased, the motor was developing a certain back E.M.F., and had the field with increase of load remained constant, a very little variation in the speed, as already explained, would have been required in order to obtain double the current. Owing now to the field being doubled in strength, the speed will have to be half its original value, in order to obtain the same back E.M.F. and the same current as originally, and consequently a little less than half to obtain double the current. Thus, on the above rough assumption concerning strength of field, if in a series motor in any given case — The load is made . . . four times as great, In order to obtain — A torqiic, . . . . . .four times as great, The currc7it need only be . . . twice as great ; The H.P. output will be also . . twice as great, The speed now being . . . half as great. In a similar way, if — The load be made .... one quarter as great, K 130 ELEMENTS OF ELECTRIC TRACTION 111 order to obtain — A torque .'..,. one quarte?' as great, The current will be . . . . half as great ; J The JET.P. output will also be . . half as great, J The speed now being .... twice as great. It will be found, just as before, that if the matter be considered from a power point of view, the results are again in entire agreement. Input. — If the current in any given case be doubled, the voltage being constant, the electrical input will be doubled, and therefore the horse-power output should also be approximately doubled. Output. — If with double the current four times the torque is obtained, then under these circumstances the load has been increased to four times its original value, but with speed reduced to half its original value, the horse-power output is only twice its original value, thus corresponding with the electrical input. Thus, roughly, in a series motor with — Twice the H.P., Twice the current, but Half the speed and Four times the torque. And, in a similar way — Half the U.V., Half the current, but Twice the speed and One quarter the torque. This will give an approximate idea as to the variation that takes place in a series motor. Actually, however, it may be said the magnetic field of an ordinary series motor, when under ordinary working conditions, will not increase and decrease in strength at the rate indicated above, namely, double the current double the magnetic field (the magnets approaching more or less the state of saturation), but will increase at some lesser rate ; consequently, the speed will not decrease, and the torque will not increase at such a rapid rate — that is, as the B.H.P. output is increased, or, vice versa, as the B.H.P. output is decreased. The more the magnets are saturated (the field in con- sequence becoming more or less constant), the less rapid will be this rate of variation, and the more will the behaviour of a series motor begin to resemble the behaviour of a shunt motor. On the other hand, if with reduced load the magnets become only slightly magnetized, variations in the current may bring about PROPERTIES OF CONTINUOUS-CURRENT MOTORS 131 even a greater rate of variation in the strength of the magnetic field, and consequently with reduced current, due to reduced load, a greater rate of increase of speed; hence the excessive increase of speed on very light loads, and the consequent danger already referred to. The results tabulated on p. 132, obtained from a Dick Kerr motor, will give some idea as to the results obtained in actual practice. The speed is not given in revolutions per minute, but the equivalent in miles per hour, as already explained. The efficiency, it may be said, is the efficiency per cent., and includes gear losses. From this data, curves can be plotted, say, with amperes on the horizontal line, and efficiency, miles per hour, horse-power, and horizontal effort (that is, tractive effort or draw-bar pull) in pounds, on the vertical; such curves, if plotted, will show how the various quantities vary throughout the whole range given, and in this way the complete behaviour of the motor is shown in one diagram. These same particulars are shown plotted in this way in Fig. 30, page 146. From these results, it will be seen that the draw-bar pull increases and the speed decreases as the B.H.P. output increases, and vice versa, but the rate of increase and decrease is not as rapid as that indicated in the rough example taken, for reasons already explained. Speed Regulation. — There are two methods by which the speed of a motor can readily be varied, that is, within certain limits, and so regulated if required — 1. By varying the field strength, more definitely, by varying the current passing through the field coils. 2. By varying the pressure at the terminals of the motor. 1. Varying Field. Field weakened. — In a general way, if the field strength of a motor — pressure of supply and load being constant — be weakened, the speed will be increased. Field strengthened. — If the field, under similar conditions, be strengthened, the speed will be reduced. 2. Varying Pressure. Pressure increased. — In a general way, if the pressure at the terminals of a motor be increased — load and field being constant — the speed will be increased. Pressure reduced. — If the pressure, under similar conditions, be reduced, the speed will be reduced. The above, of course, assumes that the variations either in the field strength, or in the pressure, are suitable ones for the motor under consideration. The reasons for the above behaviour will now be considered more in detail. 132 ELEMENTS OF ELECTRIC TRACTION CO , O) rH .a o O i=l ■^ ^ l^ c3 O o p H >^ II r^ O CO bo kr-( ^ o ^ l-H ^H ^ O 1^ u -M -M C3 T— 1 02 o t- W o o" (D a> PP a c3 c3 O) •l-H n o »o Hw i- -1« O CO LO l:- i^ oo l:- 1— < o lO CO o CM 1—1 CO- LO CO CO oc o rH I:- o CO CO o r—i iH 00 o uo Oi Hci IJO OJ ■ IC oq 00 1—1 t- o lO tile* CO 1 O o 1—1 1—1 o 00 o o 00 o i •-^l-* '-'H O Hc« . rH o CTi o oq 1—1 i:- 00 CO «|Tf< 00 1— i o CO o 00 o CO CO 1—1 cq 1—1 o o uo lO cq CO 1— i CO rH o CO oo 00 1:- o O CO o lO (M T— 1 tH i:- Jr- o CM rH o 1—1 o o rH CO . CO 1— < ^ • • fl , . . ir' 05 ^ o O ^ ^ ^ ^ o o O) O) ^ Ph (> »— 1 »> ^ c3 >^ CO o o -+-> o p^ Ph o PI CO s •«v. ^S J S^ J p 3^ 1 *" C^ t E S5 t^ \^^ t L t^ i^ *^^ Q 1 V Uv 3 > ^ t S - - " Wj 11 ^ k^ xjj 1 \- ^ ^ ^4 S. ^^ r ^^ A ^ ^ 5 ^ \ Q ^ 5 V p.4 ^ ^ X >r^ - %i C> 5 S. k^I ^ ^^ - Vr >^ 3t i^ Oc^ - 5 5 iit - •o i^ . L. ^^S '*• r^ ir 5 it^ ^ ^ J^ V ^^r- fr* q; -^ j^ 3 ^-^s, -i ^ ^ C? It '^ S. ^ K. j ' Ok r#» ?^ CK " 5 '^ _ 7 S' to Uj ,j. i,. 3- s: I K K /K ^ - -f\r 3 "g ^^ I- io r-v Li S ^L .r *" ^S t V ?J f^ At t\^ ^S^ t ^» ^ •; QC -,, A> t^ ■^> 1 5 > 2 5= ^l ?3 ^^S / ^. ^ ^ c" - -il 5dS: '^^ i 9 f^ ^ r^ ^^ ^-^^ ■^£='5 i M^ v: Q /^ -It ^^ !V. 3 ^ L - v£ u -5 I fti ^ K^ K m ^s S s: '^ ^ ^ if) J: 1 ^^^ ^^^ - *o I 1 Ui $- "S \ I '^ -^ r^ c^ i^. Z^\^ v/ CQ i^ ^ ir ^ I Ul Li 3^ !V ^V^ ^ w-i >: v^ :i' - 4^ -5^_ _ X. ^ ^ ^r^ SC -i ^ *^ c^ ^ • ^ ^-t \ t ^ c^ ^-t > ' > fW- /VN ^1-4 X s _ ^ u^ QQ ^ C^ ^^ OVJ ^ ^ S ^ tA V- 2- s^ ^.^ ^ \ J ^^ X ^ u. 4 _ J- S _ ^^ _ P A- 2 ^s. ^ "^^ X V 5 3 ft- ^r- ^•^ S^ ^ J< t Z \^ A ^ - ^^ 2 -^^ k- '^ K S 2 ^ \ "^ 1 . % 2 !V 4 ^ < ^ 7 1 'J .s 1 7 4(> level APPLICATION OF MOTORS TO TRACTION 153 On studying the above figures it will also be seen that the draw- bar pull exerted by the two motors in parallel is much greater than that exerted by the two motors in series, if the speed is the same in •each case. Additional Speed Regulation. — In some forms of controllers, it may be mentioned, such as the K2 (B.T.H.), additional speed regulation is obtained after moving on to full parallel by varying the field strength, this in turn being obtained by shunting a portion of the armature current so that the whole of it does not go through the series winding, thus weakening the field and so increasing the speed. These notches can be used for fast running on the level, but as ■weakening the field reduces the torque, armature current remaining the same, these notches are not intended for use where a large draw- bar pull is required. A car fitted with two series motors and arranged for series- parallel control is sometimes said to be a two-speed car ; strictly speaking, there are two running notches, the full series and the full parallel. It is a matter of opinion as to whether this meets the whole of the needs of any given system. In many cases it may be found that the full series is far too slow, in which case the full-series notch simply develops into a mere speeding-up notch, the driver practically never halting on it, and so seldom using it as a permanent running notch. On the other hand, in many cases the full parallel is too fast, the driver in this case simply regulating the speed by shutting off and coasting. It is a matter of interest to note that in the two regenerative systems already mentioned, and which are now being brought forward, greater speed variation is given to the driver — in short, more running notches. Current Economy. — One of the advantages of the series-parallel •system is, that when the motors are in series, the same draw-bar pull is obtained as would be obtained with double the current with the motors in parallel. For example, if 65 amps, are passing down the trolley when the motors are in series, each motor will have passing through it a current of 65 amps. ; now, to get the same draw-bar pull when the motors are put in parallel, 65 amps, would have to pass through each motor, just the same as before, and this means that a total current of 130 amps, would be passing down the trolley, or a current of just double the previous value. Thus, quite apart from the type of motor employed with the series-parallel system, not only is the series speed more adapted for starting purposes, but a more economical use is thus made of the current at starting. In the table just given, the various values of current and B.H.P. are given for the different conditions, and these should be carefully studied. 154 ELEMENTS OF ELECTRIC TRACTION It should, however, be borne in mind that if the motors were put into parallel at the start, unless the wheels slipped an excessive current would flow, " blowing " the canopy cut-out, as the motors would not speed up sufficiently quickly to enable the requisite back E.M.F. to be generated. This excessive current would produce, for the time being of course, an excessive draw-bar pull — the resistance provided being insufficient of itself to keep the current within reasonable limits — but a time would elapse before the car could sufficiently respond, and in the mean time the cut-out would in all probability " blow." The above, however, shows that with a given current passing down the trolley, a greater starting effort can be obtained with the motors in series than with the motors in parallel. Controllers. — The controller is that portion of a car's equipment which enables all the operations in connection with the regulation of the motors to be performed in their proper order. It may be described as a somewhat complicated switch, by the operation of which the necessary and various connections can be made. There are various types and makes of controllers now in use. No attempt will be made here to describe the various types, but rather to explain those underlying principles which are common to all. A motorman cannot be too strongly advised to study the par- ticular types in detail which he is called upon to use, so as to know exactly the whereabouts of such things as motor cut-outs, etc., also as to what fingers are the best to be detached to replace a broken trolley finger, for instance, and in general to make himself perfectly familiar with the construction and working of the types he has to handle. The controller has not only to act as a starting switch, by means of which the necessary resistance may be inserted until the motor speeds up, and so develops the requisite back E.M.F., but it has also to perform the necessary operations, when so required, of placing the motors either in series or in parallel, reversing the motors, and also to place them in parallel as dynamos, where the controller is designed for electric braking. Taken in order, these various steps are as follows : — (1) To connect the motors in series with a certain amount of regulating resistance in circuit. See Fig. 31, a, (2) To gradually cut out the resistance, placing the motors finally in full series. See Fig. 31, h. (3) To connect the motors in parallel with a certain amount of regulating resistance in series with them. See Fig. 31, c, (4) To gradually cut out this resistance, placing the motors finally in full parallel. See Fig. 31, d. The controller also enables the direction of rotation, and hence APPLICATION OF MOTORS TO TRACTION 155 the direction of the car, to be reversed, either by reversing' the armature current or by reversing the fields, and enables the above O^OUEY WHEEL 00 U2 Rlf^^ G BLoy^Oi/r Coil Ficld RHEOSTfiTS RfilLS \ ljLIULIlJU X "Nnrn.. JCPw\J' 'klPv\M' TTJfT 15" iKMAAi kiM/VW ?CF?W^' ?C7M /^/i^ST" BHAKE Nortu R7J R/iiLS Fig. 31. 156 ELEMENTS OF ELECTRIC TRACTION four operations to be performed in the order given for the new direction of rotation. See Fig. 31, e. The particular connections shown correspond with those shown in Fig. 31, a, namely, the first notch in series, but, for a reversed direction of rotation, to that shown in Fig. '^l,a. Reversal in this case is brought about by reversing the direction of the current passing through the armatures, the fields being kept the same. Also, where designed for electric braking, to couple the motors in parallel as dynamos, see Fig. 31,/, and to regulate them as such by means of the rheostats, whether the car be travelling backward or forward.* This will be dealt with in detail in the next chapter. Reversal. — With reference to the reversing operations, it may be said in order to reverse a motor, a simple application of the left- hand rule will show that in order to reverse the direction of rotation, either the field, that is, the direction of the current through the field, or the direction of the current through the armature must be reversed. In some form of controllers, such as the B.T.H. B13, B18, and KIO, or Westinghouse 90M and 210 types, the fields are always kept the same, and the current through the armature is reversed. In others, such as the Dick-Kerr D.B.L form, C type, reversal of one of the motors is brought about by reversing the current through the field winding, the direction of the armature current being kept the same ; the other motor being reversed by reversing the current through the armature, the current through the field of this motor not being altered as regards its direction. In the ordinary type of controller, there are generally two drums, or barrels as they are termed, one the main barrel, and the other the reverse barrel. Fixed on these barrels are various contacts suitably connected to or insulated from one another, as the case may require. The various connecting cables from the motors and rheostats are led to a terminal board fixed in the controller case, and connecting cables are led from the terminal board to various " contact fingers," as they are termed, which fingers press on the desired contacts fixed on the above barrels when these are suitably operated. Thus, when the barrels are rotated, the various contacts are also rotated, and in this way are brought into contact with the correct contact fingers, and by this means the various and necessary connections are suitably made and broken as required. To prevent excessive arcing, a magnetic blow-out is very often fitted to blow out any arc that may be formed when breaking the circuits. Reverse Barrel. — This is a small barrel, by the operating of which the motors can be reversed, as regards their direction of rotation. Tliis barrel is generally so interlocked with the main * The couacctioQS showu correspond for direction of motion with Fig. ol, e. APPLICATION OF MOTORS TO TRACTION 157 barrel that the main barrel cannot be operated until the reverse barrel is moved into one of its running positions, that is, its ahead or astern position. Also, in a similar way, the reverse barrel cannot be operated when once it has been moved until the main barrel has been put to its " off" position. The reason for this interlocking has already been dealt with. Main Barrel. — This is a larger barrel than the reverse one, and by the operation of this, the various connections are made for in- serting resistance in the motor circuit, putting the motor into series or into parallel, etc., as required. The handle for operating the main barrel is generally termed the power handle, in contradistinction to the handle employed for operating the reverse barrel, which is generally termed the reverse handle. These handles are removable, the reverse one only so when in its off position. Motor Cut-outs. — In addition to the above, in most controllers for traction work will be found, in some shape or other, two switches which enable either one or the other of the motors to be cut out. Thus, in the event of one motor breaking down, that motor can be cut out, and the car handled simply by means of the other. These motor cut-outs, in some forms of controllers — some of the B.T.H. types, for instance — are so interlocked with the controller barrel that in the event of one motor being cut out, the power handle cannot be moved past the full-series notch. In other words, when one motor is cut out, the other motor is simply operated on the series notches. In other types, such as the Brush Company's controllers, and some of the Dick-Kerr controllers, the remaining motor is worked only on the parallel notches. In the B.T.H. B13, B18, and KIO types the motor cut-outs consist of small separate switches. In the Westinghouse, No. 90M and No. 210 types, the motors are cut out by operating one of the reverse drums, a small brass knob being fitted for suitably discon- necting such. In the Dick-Kerr D.B.I, form, C type, the motors are cut out by pulling back one of the fingers at the side of the reverse drum, these fingers, numbered 1 and 2, being provided with catches to hold them off when so required. Controller Operations. — In order to form some idea as to what happens when a car is set in motion, and the function fulfilled by the controller, a concrete case will be dealt with in the simplest way possible, by assuming certain values of the current, and considering what takes place under a given set of conditions. It should be noted that any sudden increase in the value of the current means a sudden increase in the value of the draw-bar pull. 158 ELEMENTS OF ELECTRIC TRACTION and the effect of this tends to give the car a sudden jerk, depending, of course, upon the amount of the increase. Whether starting or stopping a car, all such sudden jerking should be avoided as much as possible, not only for the comfort of the passengers, but to avoid any sudden straining of the motors and gearing. On putting the power handle on the first notch, the motors being quite stationary, the current flowing through the motors will depend upon the resistance placed in series with them. The current will set up a certain draw-bar pull, which should start the car in motion unless the resistance of the circuit is much increased, as might be the case with very dirty rails, for instance. In cases where the resistance is such as to cause a heavy current to flow on starting, the sudden jerk can be lessened by the motorman " easing off" the hand brake at the same time he puts on to the first notch in series. Putting into Full Series. — To make the matter as simple as possible, it will be assumed that a 10-ton car is fitted with two motors similar to the one of which particulars have already been given, and that the resistance is such that when the power handle is on the first notch 60 amps, pass down the trolley, and consequently 60 amps, pass through each motor. Each motor will then exert a draw-bar pull (see Fig. 30) of some 1400 lbs., making a total draw-bar pull of some 2800 lbs. Taking the resistance to traction as 25 lbs. per ton, 250 lbs. will be required to overcome frictional resistance, and assuming the car to be on a gradient of 1 in 22, a force of some 1018 lbs. will be required to balance the gravitational pull ; thus a total pull of 1018 -I- 250, namely, 1268 lbs., will be required either just to move or to keep the car in uniform motion when once a speed has been obtained. Thus a nett force of 2800 — 1268, namely, 1532 lbs., will be available for acceleration, that is, 1532 lbs. per ton, and this will give, roughly, an acceleration of 1^ miles per hour per second. As the car gets up speed a back E.M.F. is developed which tends to reduce the current ; if, however, as the back E.M.F. is developed, resistance be cut out at a suitable rate, the current can be kept fairly constant. The increasing back E.M.F. tends to decrease, and the decreasing resistance tends to increase the current value, and in this way the two actions can be made to approximately balance one another. Consequently, as the car gets up speed, by passing the power handle from notch to notch, the motorman can, if resistance be suitable, keep the average value of the current, and consequently the draw-bar pull, fairly constant. On reaching the full-series notch all the regulating resistance will be cut out, and the full pressure of 500 volts will then be across the outer terminals of the two motors, and as they are in series this means that each motor will have 250 volts across its terminals. APPLICATION OF MOTORS TO TRACTION 159 N'ow, on referring to the diagram given, it will be seen that when 60 amps, are passing through the motors, with 500 volts pressure across the terminals of each, the car will have a speed of slightly over 8 miles per hour. With half this pressure across the terminals of each motor, the motors being in series, the car will have approxi- mately half this speed. Hence when in full series the car will have a speed of some 4 miles per hour, and as the current up to now has been kept constant, the total draw-bar pull of 2800 lbs. will also have been kept constant, and an accelerating force of 1532 lbs. will have been continuously at work. Putting into Full Parallel. — If now the controller handle be moved on to the first notch in parallel, and the resistance in the circuit is such that the current passing through each motor is the same as before, namely, 60 amps., that is, 120 amps, down the trolley, the draw-bar pull will remain constant, and the car will continue accelerating at the same rate as before, namely, at the rate of li^ miles per hour per second. If now the resistance be such that on gradually passing from notch to notch, as the back E.M.F. rises, the current can still be kept constant, then the accelerating force will also be kept constant, and in this way the car will speed up with the constant acceleration of IJ miles per hour per second, until finally the full parallel position is reached. When on full parallel, as all regulating resistance will then be cut out, each motor will have the full 500 volts across its terminals, and consequently the actual speed given in the diagram ; that is, the car will be running at a speed slightly over 8 miles per hour, and the motors will be exerting a total draw-bar pull of 2800 lbs. After this, the car with an accelerating force still at work will continue to increase its speed, and, since no resistance is now being cut out, the current and draw-bar pull will gradually decrease. However, as long as there is an accelerating force at work the car will continue to speed up, but at a gradually decreasing rate, owing to the decreasing value of the accelerating force. When a speed of 11 miles per hour is reached, it will be seen that each motor will exert a draw-bar pull of some 634 lbs., making a total of 1268 lbs. As this is just sufficient to balance the resistance to traction and gravitational pull due to the gradient of 1 in 22, the motors will cease to accelerate, and will continue to maintain the speed of 11 miles per hour, the current then passing through each motor being 35 amps., a total of 70 amps, passing down the trolley. The rheostats in actual practice are, in a great many cases, adjusted so that practically a uniform acceleration can be main- tained, as indicated above, until the full parallel position is reached. i6o ELEMENTS OF ELECTRIC TRACTION Running on Full Series. — If, instead of moving into parallel, the power handle had been kept on the full-series notch, a similar action would have taken place on the full series as that which took place on the full parallel. Previously, on just reaching the full series the car had, as shown, a speed of 4 miles per hour, and the motors were exerting a total draw-bar pull of 2800 lbs. If, however, one had continued to run on the full-series notch, the motors would have continued to accelerate at some decreasing rate just as before, due to the decreasing current consequent upon the increasing back E.M.F., until a speed of approximately 5^ — one-half of 11, due to the motors being in series — miles per hour had been reached, the total draw-bar pull then of 1268 lbs. being just sufficient to balance the frictional and gravitational forces. The motors would then have ceased to accelerate, and the uniform speed of 5^ miles per hour would have been maintained, the current then passing down the trolley and also through the motors being 35 amps. It will thus be seen that if the car is allowed to run either on the full-series or the full- parallel notch, some uniform speed is finally reached, and all accelerating effort ceases, unless new condi- tions arise in some way or other to alter matters. If, from any cause, the total pull required to maintain motion increases, such as would be brought about by an increase in weight or an increase of gradient, the car will slow down, the draw-bar pull and power increasing, until again some balance is reached, the car then maintaining some uniform but lesser speed. If, on the other hand, the total pull required decreases, the car will speed up, the draw-bar pull and power decreasing until again some balance is reached, the car then maintaining some uniform but higher speed. Late Paralleling. — If, after running on the full-series notch, the power handle be moved over so as to pass into parallel, it is interesting to note the kind of action that will then take place. Previously, when full series has been reached just previous to passing into parallel, 60 amps, were passing through the motors, and the car was travelling, for the time being, at a speed of some 4 miles per hour, 250 volts being across the terminals of each motor. In the case that is now being considered, after running on full series 35 amps, are passing through the motors, and the car is running at a uniform speed of some 5^ miles per hour, 250 volts, as before, being across the terminals of each motor. In consequence of this higher speed, a greater back E.M.F. is being generated, as indicated by the decreased current of 35 amps., as against the 60 amps, in the previous case. On account of this higher back E.M.F., when the motors are put into parallel, a less current than 60 amps.. APPLICATION OF MOTORS TO TRACTION i6i will flow through each motor, but, on the other hand, there will be a greater current than 35 amps., the reason of this being as follows : — In the first case that was considered, as the current did not alter its value when the motors were put into parallel — the back E.M.F., roughly speaking, on full series being very little different from that when on first notch in parallel — the drop in pressure in the resistance had to be such as to keep the pressure across the terminals of the motors approximately the same as when they were in series, namely, 250 volts, a drop of 250 volts therefore taking place in the resistance. This took place when 120 amps, were passing through the regulating resistance in circuit. If, now, in the case under consideration, some less current than 60 amps, passes through each motor, a less current than 120 amps, will pass through the resistance, and, in consequence, a less drop than 250 volts will take place, and therefore a higher pressure than 250 volts will be across the motor terminals, and hence a greater current than 35 amps, through each motor. In short, as stated above, a current will flow having a value somewhere between 35 and 60 amps. Assuming a likely value, say, of 45 amps., it will be seen that after running on series and passing into parallel, the draw-bar pull will be increased from a total of 1268 lbs. to a total of 1880 lbs., an increase of 612 lbs. thus taking place with the increase of 10 amps, through each motor; this means there will now be at work an accelerating force of 612 lbs. A jerk might be noticed, due to this sudden increase, but as this increase is only small compared with the 1532 lbs. accelerating effort, when the car is started the jerk will be correspondingly less. The accelerating effort of 612 lbs. will give an acceleration of some j% mile per hour per second, and if resistance can be cut out at some correspondingly slow rate, so as to keep the current of 45 amps, fairly constant, an acceleration of ^^ miles per hour per second will take place, until finally, when all resistance is cut out, the car will have obtained a speed of some 9 J miles per hour. The car will then continue to speed up at some decreasing rate, until a speed of 11 miles per hour is reached as before. With the slower rate of acceleration a longer time will be taken in obtaining a given increase in speed. In actual practice, what will in all probability take place, is, that the motorman will cut out resistance at a greater rate, thus increasing the 42 amps., say, up to 60, and in this way obtaining finally the previous rate of acceleration of IJ miles per hour per second, pro- vided no alteration has taken place in the gradient. In this case the draw-bar pull will gradually increase, if resistance be cut out, fairly uniformly from 612 lbs. to 1532 lbs. ; the average rate of acceleration in passing from first notch in parallel to full M i62 ELEMENTS OF ELECTRIC TRACTION parallel will be higher than in the case just considered, and the car will speed up in quicker time. The rate of acceleration will not, however, be as uniform, and the average accelerating force will not be as great as in the original instance, where 1532 lbs. accelerating effort was continuously at work, and consequently the time taken will be longer for any given increase in speed. The practical outcome shows that staying too long on series, paralleling too late, as it is termed — that is, when it is not desirable to run on the series notch — only causes a reduction of the current, a liability to some slight jerking action when moving into parallel, due to the varying value of the accelerating force, and a longer time taken to accelerate to a given speed. On the other hand, moving into parallel too early at starting only wastes energy. As already indicated, the current passing down the trolley can be increased to double its value with the motors in parallel without any increase being made in the draw-bar pull. From above it will be seen that anything that causes the current to be suddenly increased, such as unsuitable resistance, too few notches, or cutting out resistance too quickly, that is, at a greater rate than the back E.M.F. is increasing, will cause the draw-bar pull to be suddenly increased, and a jerking effect and variable accelerating effort will be the outcome. • Regenerative Systems. — Regeneration. — Regeneration, as ap- plied to electric traction, means the regenerating of electrical energy, or the converting again of the energy that has been imparted to a car into electrical energy, with the idea of utilizing the energy so recovered for useful purposes. In imparting motion to a car, a certain amount of the energy expended becomes converted into heat, due to friction of one kind and another. As the heat will have been dissipated in various ways, the recovery of the energy expended in this direction is quite out of the question. On the other hand, a certain amount of energy has had to be expended in giving to the car its kinetic energy ; that is, the energy the car possesses by virtue of its motion. Also a certain amount of energy has had to be expended in giving to the car its potential energy ; that is, the energy the car possesses simply by virtue of its position, where the car has mounted a gradient. Now, the energy expended in giving to a car both its kinetic and also its potential energy becomes stored up in the car, as it were, and can therefore be recovered to a certain extent, less, of course, the losses which are bound to take place in the recovering process. Before a car can be brought to rest — as energy cannot be destroyed — it will have to be robbed of its energy of motion, and when so robbed, it will come to a standstill. This energy of motion APPLICATION OF MOTORS TO TRACTION 163 will depend not only upon the speed the motors may have given to the car, but also upon any additional speed the car may have obtained in descending a gradient, the energy due to this additional speed being derived from the potential energy ; that is, the energy previously spent in moving the car up the incline. Therefore, before a car can be brought to rest, the kinetic energy that it possesses at any particular moment, whether on a gradient or on the level, will either have to be converted into some other form of energy or it will have to be transferred to some other body. In the ordinary way, to bring a car to rest, or to control its speed, friction brakes, or electric brakes of some kind or another, are generally employed. When friction brakes are adopted, the energy stored up in the car is converted directly into heat, and in this way is wasted. When ordinary rheostatie electric braking is employed, the energy stored up in the car is converted into electrical energy, but the electrical energy so generated is simply wasted in heating up the resistance placed in the circuit. In regenerative systems the object is to convert the energy that can be recovered into electrical energy, and to utilize the electrical energy so generated. For this purpose the pressure generated is employed to send current back to the trolley wire, and in this way assists the main generating plant in supplying electrical energy to the system. This means, also, that an electric braking action is obtained when regeneration takes place, the motors having to be driven as dynamos. The power regenerated in this way, it is claimed, not only relieves the generating station, but also enables size of cables, feeders, etc., to be reduced. The saving in electrical energy claimed varies from 15 per cent, to 30 per cent., and more, depending upon the nature of the line, etc. In Birmingham, where some tests have been made with Mr. Kaworth's regenerative system, the consumption averaged 0*97 Board of Trade unit per car mile, the usual figure under similar conditions with the ordinary series system being 1'2 units per car mile, this representing a saving of some 19 per cent. A further saving is also claimed in repairs, wear and tear of wheels, brake-blocks, etc. Here again in Birmingham the saving in repairs on the cars^ amounted to no less than six-sevenths of the ordinary cost, as ascertained by comparative tests. The saving in braking is due to the fact that the driver has much greater control over the speed of the car, and instead of slowing a car down by means of the hand brake, can either run very slowly along, or even be regenerating when moving through congested traffic. Naturally, the best results with these regenerative systems are obtained in hilly districts. i64 ELEMENTS OF ELECTRIC TRACTION From what has already been seen of the characteristic behaviour of dynamos, it will be understood that in order to send current back to the trolley, some form of dynamo must be employed which will give a fairly constant pressure, whether on light or heavy loads, that is, whether sending back a small or a large current. Thus a shunt or compound dynamo would be suitable for such a purpose. In one of these regenerative systems a shunt motor is employed, which also acts as a shunt dynamo when regenerating. In the other, by a special field- changing device, the motors, which are arranged to act as series motors when driving the car, are converted into a type of compound dynamo when employed for regenerating purposes. Whatever type of dynamo is employed, the pressure generated must exceed that of the trolley wire if current is to be sent back, the trolley pressure acting as a back E.M.F. to the motors then acting as dynamos. In this way, regenerative systems have the benefit that the motors, when acting as dynamos, cannot readily be overloaded, as is often the case when they are employed for ordinary electric braking. Braking. — Another advantage claimed for regeneration is in connection with braking. In many cases where ordinary electric brakes are fitted to a car, they are not employed for general braking purposes, but only in cases of emergency. The reason for this is, that serious overloading of the motors when acting as dynamos is apt to take place when such braking is employed. The result of tliis is, the motorman does not become perfectly familiar with, and dependent upon, the use of such brakes. Also, there is the danger of these brakes being out of order with such occasional use unless daily examination is made. In the regenerative systems, every time a man retards the car's motion, regeneration to a greater or less extent takes place ; in other words, an electric braking action goes on. The motorman in this way learns to rely upon this means of retarding the motion of a car, and any failure in the retarding or braking action is at once noted. The wheels cannot, of course, be brought to a standstill by re- generative braking, because, when the speed becomes reduced to a certain value, the pressure generated falls below that of the trolley, and the trolley pressure then being the greater, current is supplied to the motors. In short, the wheels will always revolve as long as the circuit is complete, unless they are jammed by means of the hand brake, and to stop the car mechanical or rheostatic braking of one kind or another has in consequence to be adopted. Consequently a more perfect and complete control of the car under all ordinary conditions is obtained in the regenerative systems, as a car when not receiving electrical energy is either restoring energy or is actually being brought to a standstill. APPLICATION OF MOTORS TO TRACTION 165 As already stated, two regenerative systems at the present time are being put forward in this country. One by the Raworth's Traction Patents, Ltd., and the other by the Johnson-Lundell Traction Co., Ltd. A brief description of these two systems will now be given. Raworth's Regenerative System. — In this system a special design of shunt motor is employed, which, when regenerating, acts as a shunt dynamo. Speed variation is chiefly obtained by varying the field strength, this being brought about by means of a suitable shunt regulator, which enables resistance to be either inserted or cut out of the shunt circuit as desired. In this way the exciting current is varied, and, in consequence, also the field strength. A certain loss of energy, in addition to that taking place in the field, will, of course, take place in the regulating resistance, the pro- portion of energy expended in each being proportionate to their respective resistances. This method of varying the speed, it will be seen, is of a very simple character, and variation can in this way be obtained from 4 to 14 miles per hour, or from 6 to 21 miles per hour. On starting a car the field circuit is completed first, that is, before the armature circuit, and as all regulating resistance is cut out of the shunt circuit at this stage, the field has its maximum strength, and the car is given its minimum speed. To increase the speed of the car the field is weakened, resistance for this purpose being inserted in the shunt circuit by means of the shunt regulator. It should be noted here that with every variation in the strength of field there will be a variation in the speed. To retard the motion of the car at any time — unless the speed is such a very low one that regeneration cannot be carried out, in which case mechanical or rheostatic braking is employed — the field is simply strengthened. With the increased field strength, the back E.M.F. now generated by the motor will exceed that of the supply — that is, until the car's speed becomes reduced to its corresponding value — and current will be sent back to the trolley, regeneration thus taking place. The motors during this stage act as dynamos, and, in consequence, a retarding or braking action takes place when so regenerating. The stronger the field is made, or the greater the speed, the greater will be this retarding or braking action, also the greater will be the E.M.F. generated, and, in consequence, the greater will be the value of the current sent back. Regeneration in this system is also automatic in the following sense : With a given strength of field there will be practically a given speed, whether the car is mounting or descending a gradient. In mounting the gradient, the motors will be receiving energy from i66 ELEMENTS OF ELECTRIC TRACTION the trolley, the energy being expended in propelling the car up the gradient, whereas in descending the gradient the motors will be delivering electrical energy — in other words, regenerating. This will take place without the controller handles being operated, and, consequently, in this sense, the regeneration is automatic in character. The reason for the above is as follows : — As already seen, a shunt motor is practically a constant-speed motor under all loads, that is, with a given strength of field and a given voltage of supply. Under these conditions, very little variation in the speed with varying load is requisite to bring about the corre- sponding small change in the back E.M.F. for the required change in the value of the armature current. On very light loads the back E.M.F. generated will be almost equal to the voltage of supply, and if the load be entirely taken off, and the motor be driven — ^this corresponding to the case of a car descending a gradient — a slight increase in the speed will cause the E.M.F. generated to exceed that of the supply, and current in this way will be delivered to the trolley. In short, with a shunt motor with a given strength of field and a given terminal voltage, whether taking or delivering energy, the variation in the speed will not be great, and consequently, when applied to traction, the car's speed under these conditions will be practically uniform. Some of the advantages claimed for the shunt motor as applied in the above system, and as compared with the ordinary series motor in the series parallel system, are as follows : — Independence of Speed with regard to Load. — The speed of a shunt motor, the field being practically constant, varies very little, as already seen, with variation in the load — that is, whether developing a large or small horse-power. Applied to traction work, this means that a shunt motor, whether mounting a gradient or running on the level, will practically main- tain the same speed without any regulation, due to the fact that very little variation takes place in the speed with varying values of draw-bar pull and horse-power. Consequently, with a shunt motor with a given field strength, one uniform speed will practi- cally be obtained on all conditions of gradient, and variations in the field strength — which, as seen, can very readily be obtained in the above system — will bring about corresponding changes in the speed of the car. A Lesser Decrease in Draw-bar Pull with Increase ot Speed. — In the above system, a saturated field is employed, and as the maximum field strength is practically obtained when saturation is reached, the maximum draw-bar pull that a motor of given dimensions can exert, will, under these conditions, also be obtained — that is, as APPLICATION OF MOTORS TO TRACTION 167 far as the field strength is concerned. In short, whatever be the means of excitation, the iron cannot be worked to more than its full limit, and consequently, by employing a saturated field, Mr. Raworth obtains the same maximum draw-bar pull with shunt excitation as can be obtained with series excitation. In fact, in some cases series motors have been taken and rewound as shunt motors. If reference be made to the actual figures of the series motor previously given, it will be seen that at the lower speeds, that is, when the motor is exerting its maximum draw-bar pull, there is a rapid drop in draw-bar pull with increase of speed. This result is due to the fact that in ordinary series motors, field strength and armature current are dependent upon one another. Thus, in a series motor a weakened field means also a weakened armature current. With shunt motors regulated in the manner indi- cated above, a weakened field does not necessarily mean a weakened armature current, field and armature currents in this case being independent. The result of this is, at the higher speeds a greater draw-bar pull can be obtained with a shunt motor than with the equivalent ordinary series motor at the same speed. Thus, a higher speed can be maintained on a given gradient without increasing the maximum speed of the motor, and in this way a greater range of speed regulation is obtained. With the higher speed on a given gradient, a greater armature current will be taken and a greater amount of power developed — in fact, with shunt motors so regulated, more power can be obtained at the higher speeds than with the ordinary and equivalent series motor. In this way a better average accelerating effort is obtained over a wide range of speed. A motorman has, in consequence, either with this system or the Johnson-Lundell System, much greater control over the speed of a car, and has not simply to take one of two speeds which the motors may give. It must be pointed out here that in the Johnson-Lundell System, similar speed variation is obtained with series motors. In this system, however, the field is also made, to a certain extent, inde- pendent of the armature current, and, consequently, similar results can be obtained, as in the above. The arrangement, however, as will be seen, is not so simple, and to convert the series motors into compound dynamos for regenerating purposes an additional and automatic controller has to be added to the equipment. Controller. — With regard to the controller, originally Mr. Raworth employed a different type from that in general use in the series-parallel system. The controller handles had a fore and aft motion, one being on each side of the controller. They were inter- locked in a similar way to the ordinary controller, and to suit the 1 68 ELEMENTS OF ELECTRIC TRACTION special requirements of the shunt motors, such as exciting the field before closing the armature circuit, etc. Mr. Eaworth has, however, now adopted, in addition to the above means of regulating the speed, that also of series-parallel control for use with the shunt motors. A new form of controller has also been designed similar to the general type in outward appearance, the handles now being at the top and rotated in the usual way, each position of the regulating handle also corresponding to a certain speed of the car. Speeds in this way on the series notches can be obtained from 5 to 10 miles per hour, and on the parallel notches anything up to 20 miles per hour. When the speed is insufficient to generate a sufficiently high E.M.F. to send current back to the trolley, a braking action can be obtained by shortcircuiting the motors through a resistance ; in this case a rheostatic braking action is obtained independent of the line current. In the event of the trolley pole leaving the wire all regenerative braking ceases. In the later equipments a switch is added, which in such cases — when the E.M.F. generated by the motors exceeds a given amount — automatically closes the circuit through a resistance, thus bringing about automatically a rheostatic braking action. This also meets a difficulty which has arisen in these instances due to the E.M.F. generated bursting the car lamps, etc. The Johnson-Lundell Regenerative System. — In this system, as already indicated, series motors are employed. Speed variation is obtained partly by series-paralleling the motor circuits, and partly by varying the field strength. The circuits are paralleled twice, thus necessitating the employment of four instead of two armature circuits. These may consist of either four separate motors, or simply two motors, each motor having two separate windings on the same armature core, and two commutators on the same armature shaft to correspond. In the latter case one field is common to the two armatures. By means of this double series paralleling in combination with strong magnetic fields, which are also to a certain extent variable, main circuit resistances in this system are dispensed with, and nine out of the ten notches on the controller are running notches, the first being merely a starting notch. On some tests made at the above company's works, speeds ranging up to 17 miles per hour were obtained, and regeneration commenced at once when desired, and was continued down to speeds not more than 2 miles per hour. The motors, when acting as such, namely, when driving the car, are series excited ; the main current, however, in this system does not simply pass through one set of field coils, as is the case in an ordinary series motor, but passes through a number of coils, some of APPLICATION OF MOTORS TO TRACTION 169 which are fixed-series turns, and some of which are in parallel with one another, the whole, however (treating it as such), forming a series winding. The coils, which are in parallel with one another, it may be said, are fine-wire coils, and when regenerating, these coils are placed in series with one another, and so form the shunt portion of the compound winding. When placed as indicated above, in parallel, they form a portion of the series winding for the motors. Variation of the field strength is obtained by diverting a portion of the main current through diverting resistances. On the first three running notches, all four armatures are in series with one another, together with the two fields. Variations in speed are obtained by varying the field strength, this being obtained by diverting a portion of the main current through diverting resistances, as mentioned above, so that the whole of the armature current in these cases does not pass through the series windings. In this way the field becomes slightly weakened, and the speed correspondingly increased. On the next three notches, two of the armatures are placed in series with one another and their corresponding field, the whole being in parallel with the other two armatures and their field. Further variations in speed are obtained by again diverting a portion of the field current. On the next and last three notches, two of the armatures are placed in parallel with one another and in series with their field winding, the whole being in parallel with the other two armatures and their field, further variations in speed being obtained as previously. When regenerating, that is, when braking, those field coils which were placed in parallel previously, and formed part of the series winding, are now placed in series, and form the shunt portion of the compound winding. The series portion of the compound winding acts in opposition to the shunt winding, and thus its tendency is not to keep the voltage constant, as is attempted in an ordinary compound dynamo, but to reduce the voltage slightly on heavy loads. For this purpose the series windings are only few in number, and, in addition, are shunted to a certain extent. In practice this arrangement has been found to be of advantage in preventing sudden heavy rushes of current when regenerating. The motors, when acting as dynamos, go through a similar but reverse cycle of changes, from full parallel to finally full series, thus a variable braking effect is obtained such that regeneration, it is claimed, can take place when the car is moving no faster than a slow walk. Thus, every time a car is stopped, checked, or restrained on an incline, a certain amount of electrical energy is recovered. 170 ELEMENTS OF ELECTRIC TRACTION The above various field changes are obtained by the employment of a special device, which is called the "field changer," this being operated automatically, the motorman bringing it into operation by thumb pressure on a lever fixed on the controller handle. In operating the controller, the various changes of the circuits, etc., which are effected by the forward movement of the controller handle for the purpose of acceleration, are made in the reverse order when the handle is moved backward, but are without effect for regenerating, if the lever on the controller handle is not operated. If, however, this be operated, the field-changing device automatically brings about the requisite changes — the field winding now being a compound one — and so enables the motors to develop a retarding or braking effect as the handle is moved backwards in the same graduated way as they developed an accelerating effect when the handle was moved forward. In this system, regeneration is not automatic in the same way as in the previous system, the motorman having to operate by thumb pressure a small operating lever, which allows the requisite field changes to be automatically made, and which converts the series motors into compound dynamos for regeneration. In this system, when the speed falls to one mile per hour or so, regeneration is not attempted, but an automatic mechanical braking device is substituted for finally stopping the car and holding it. In the event of the electrical circuit being broken in any way whatever, the above mechanical brake is automatically brought into operation without the intervention of the driver. CHAPTER X BRAKES Brakes as applied to ordinary tramway traction have, roughly speaking, two functions to fulfil — 1. To enable the car to be brought to rest, or its speed to be reduced. 2. To enable the car to be held at rest, on any gradient it may be on. To satisfactorily perform these apparently simple requirements under all the varied conditions and circumstances which arise in practice, is, however, by no means an easy or a simple matter, and various types of brakes — some mechanical and some electrical — have been adopted. In this chapter no attempt will be made to describe the con- structional details of any particular type of brake, but rather to explain the principles which underlie all braking actions, and the actions which take place when the various types of brakes are employed for the purpose of retarding the motion of a car, and more particularly the actions which take place when the electric motors are employed in some form or another for retarding or braking purposes. General Principle. — The principle of the conservation of energy, viz., that energy cannot be destroyed, has a very definite and direct application in the stopping of a car in motion. The energy expended in imparting motion to a car, as previously pointed out, becomes converted chiefly into three forms of energy — 1. Heat energy — due to friction. 2. Potential energy — the energy a car possesses by virtue of its position. 3. Kinetic energy — the energy a car possesses by virtue of its motion. As the energy expended in overcoming friction becomes converted into heat, and as the energy expended in raising a car up a gradient becomes stored up in the car simply by virtue of its position, the only energy that need be considered in the stopping of a car is simply 172 ELEMENTS OF ELECTRIC TRACTION that which the car possesses at any moment by virtue of its motion, that is, its kinetic energy. The car's kinetic energy is therefore the energy which must be converted either into some other form of energy, or else be transferred to some other body — as it cannot be destroyed — before the car can be brought to rest. Where mechanical or electrical brakes, as at present adopted, are employed — with the exception of the systems where regeneration is carried out — the kinetic energy that the car possesses always becomes directly or indirectly converted by means of the brakes into heat, and in consequence is dissipated in various ways. The kinetic energy in such cases is not in consequence utilized — whether this energy is utilized or not is, however, simply a matter of economy — but the faster this kinetic energy is converted into some other form of energy, the quicker will the car, of course, be brought to rest. This converting of the kinetic energy, then, is the object of all brakes of whatever form or type. Having brought a car to rest, if it is on a gradient, a force of some kind will have to be employed — unless the gradient is a very slight one, when the tractive resistance of itself will be sufficient — to balance the gravitational pull, and so prevent it running down the incline. In other words, brakes will again be required, to hold the car at rest. In this case, if a force were not applied, the potential energy would become partially converted — as friction has to be over- come — into kinetic energy, or, in other words, the car would descend the gradient gaining speed, and so energy of motion. Thus, for example, a 10-ton car starting from rest, on descending a gradient of 1 in 10, would, it may be said, gain of itself, that is, without the motors being employed in any way — assuming a tractive resistance of 25 lbs. per ton — a speed of over 28 miles per hour in travelling a distance of 100 yards down such a gradient, and in travelling the 100 yards some 266 foot-tons of potential energy would be converted into kinetic energy ; in fact, as the motors would not have imparted any motion to the car, its entire kinetic energy at the instant under consideration would have been derived from the car's potential energy, in other words, from energy previously expended in raising the car up to its present level. If the car started at some uniform speed down the gradient, then, if it were not to increase its speed, the potential energy, less that required for overcoming frictional resistance, would have to be converted by means of the brakes into some other form of energy, otherwise some of this energy would be expended in accelerating the car. If a car were mounting a gradient, the gravitational pull would assist the braking action, as in this case some of the kinetic energy BRAKES 173 would be expended in raising the car up the incline ; that is, would be converted into potential energy. Kinetic Energy. — The value of a car's kinetic energy depends not only upon the total weight of the car (passengers included), but also largely upon the speed ; in fact, it varies as the square of the speed. Thus in any given case, to double the speed of a car means that the kinetic energy would be increased to four times its original value ; three times the speed means that the energy would be increased to nine times its original value; and so on. The fact that the kinetic energy increases as the square of the speed, indicates the danger that arises when a car gets beyond control coming down an incline, as with a gain in speed there is a still greater gain in energy, and hence the increasing difficulty in success- fully braking the car, as the greater amount of energy has to be converted into some other form of energy before the car can be brought to rest. The actual value of the kinetic energy can easily be calculated if the weight and speed of the car be known. When dealing with draw-bar pull, it was seen how, in the case of a car starting from rest, the kinetic energy could be calculated if the accelerating force, and the distance the force was exerted through, were known. The following approximate rule, which is generally more applic- able, is a simple modification of that same method of calculating the kinetic energy : — Kinetic energy I _ (weight of car in tons) x (speed in miles per hour)^ in foot-tons j ~ 30 This can be still further simplified when the car weighs 10 tons, the kinetic energy in foot-tons then being obtained by dividing the square of the speed in miles per hour by 3. Example. — The kinetic energy in foot-tons stored up in a 10-ton 12 X 12 car travelling at a speed of 12 miles per hour is equal to — ^j , namely, 48 foot tons, or 48 x 2240, namely, 107,520 foot-lbs. If the car were brought to rest in 10 seconds, or J minute, this energy would have been converted into some form or other, at the rate of 107,520 foot-lbs. per J minute ; that is, at the rate of six times 107,520, namely, 645,120 foot-lbs. per minute. If this energy were converted, say, into electrical energy, and utilized, assuming a total efficiency of say 70 per cent., useful work would have been done at the rate of ittttt , namely, 451,584 foot-lbs. 100 j> ^ per minute, and this would be equivalent to -'sVo^oIt' iiamely, 13 '6 E.H.P. 174 ELEMENTS OF ELECTRIC TRACTION That is, energy stored up in the car in this case would be capable of supplying electrical energy at the average rate of 13"6 E.H.P., or lO'l kilowatts for the short period of time during which the car was being pulled up, namely, J minute ; in other words, electrical energy of the value of 0028 B.O.T. unit. As it is useful to be able to estimate the speed a car will attain on descending a gradient — the excessive speed attained in a given time in some cases hardly being realized — the first two of the follow- ing formulae have been specially drawn up to enable the speed a car wall attain to be approximately calculated, as indicated below : — (1) The first formula enables the approximate speed to be readily calculated that a car starting from rest will attain, in any given number of yards, on descending any given gradient, (2) The second enables the approximate speed to be readily calculated that a car starting from rest will attain, in any given number of seconds, on descending any given gradient. The speeds are those which would be attained simply due to gravitational effects, and not to any speed the motors might give to the car in addition, and on the assumption, of course, that no braking action is taking place, the only retarding force being that due to tractive resistance. The rule for obtaining the kinetic energy stored up in a car is also given in formula form, and various other formulse are added for enabling various calculations to be made, as briefly indicated below. Most of these formulae are simply modifications of well-known formulae used in mechanics, which have been modified specially for direct application to traction work : — W = w^eight of car in tons. S = speed in miles per hour. Y = distance in yards. A = acceleration in miles per hour per second. F = accelerating force in lbs. G = distance in feet a car travels up a gradient in order to rise 1 foot. T = time in seconds. (1) S^ = Y — p based on an assumption of a tractive resist- ance of 25 lbs. per ton. Example. — What speed will a car attain starting from rest on travelling 100 yards down a gradient of 1 in 15 ? S2 = 100 ^^^^^ = 1^ X 5f = 500 .-. S2 = 500 BRAKES 175 Therefore the speed attained will be approximately 22'3 miles per hour, 22'3 being the square root of 500. (2) S = ^^ "" based on an assumption of a tractive resistance of 25 lbs. per ton. Example. — What speed will a car starting from rest attain in 15 seconds on descending a gradient of 1 in 15 ? (90- 15) 60 S = ^^^.-^^ X 15 = !-§ X 15 = 18| Therefore the speed attained in 15 seconds will be approximately 18f miles per hour. WS^ WS2 (3) Kinetic energy in foot-tons = -ok~' more accurately K(pJ^ (A) Kinetic energy in foot-lbs. = WS^ X 75, more accurately WS2 X 75-28. (5) F = A X W X 102, more accurately F = A x W x 102'66 F X Y F X Y *(6) S2= ~^^ , more accurately S^ = ^^.^^ ^ (7a) S2 = 4AY, more accurately S^ = 4-09 AY (7b) A = -^y, more accurately A = ttaqy (8) S = AT AT^ AT^ (9) Y = -J—, more accurately Y = ^Taq (5) enables the accelerating force to be calculated, if weight of car and acceleration required be known. (6) enables the speed to be calculated that a car starting from rest will attain in a given number of yards, if the accelerating force and weight of car be known. (7a) enables the speed to be calculated that a car starting from rest will attain in a given number of yards, if the acceleration in miles per hour per second be known. (7b) enables the retardation in miles per hour per second to be calculated, if the speed a car is travelling at at any instant, and the number of yards it takes the car to pull up in, be known ; the letter A then representing the retardation in miles per hour per second. (8) enables the speed to be calculated that a car starting from rest will attain if the acceleration in miles per hour per second, and the time the acceleration is taking place, be known. * FormulflB 6 to 9 are based on the assumption that the acceleration or retardation, as the case may be, is of constant value. y 176 ELEMENTS OF ELECTRIC TRACTION (9) enables the distance a car starting from rest will travel in a given time, if the acceleration and time be known. The actual means of dealing with the kinetic energy of a car will now be considered, that is, the various methods of braking. The brakes as used in tramway work can be roughly divided into two classes — 1. Mechanical brakes. 2. Electrical brakes. Mechanical Brakes. — These can be again divided into two classes — 1. Wheel-brakes, or brakes which operate on the wheels or driving axles of the car. 2. Track-brakes, or brakes which operate directly on the rails. The ordinary wheel-brake is generally operated by hand, while mechanical track-brakes are sometimes operated by hand and some- times by means of compressed air. Electrical Brakes. — These can be divided into three classes — 1. Rheostatic brakes. 2. Electro-magnetic brakes. 3. Regenerative braking. In all the three types here considered the motors are employed as dynamos when braking. The rheostatic and electro-magnetic brakes are those generally fitted when series motors are employed, in the series parallel system, and this particular application of these types will simply be considered. Rheostatic Brakes. — In this type the series motors are em- ployed as series dynamos when braking, in this way bringing about a retarding action on the wheels, as will be seen in detail later. The current in this type of brake is not utilized, but simply heats up the regulating resistance inserted in the circuit. Electro-magnetic Brakes. — These brakes employ the series motors as series dynamos, and the current in addition is employed to operate electro-magnetically, either track, or wheel, or both types of brake. In both the above types the energy generated, however, is finally wasted. Regenerative Braking. — In regenerative systems, as already seen, the current is sent back to the trolley, and in this way the electrical energy generated is utilized. The retarding action in this case is due to the fact that the motors are employed as dynamos, energy having to be expended to rotate these as such, just as in ordinary rheostatic braking. In dealing with the various types of brakes, distinction must also be made between brakes which enable a graduated braking action to be obtained to suit the various needs and requirements, and those BRAKES 177 which, strictly speaking, act only as emergency stops, and which do not permit of any graduation in the retarding or braking effect. Before dealing with the action of these various brakes, in order that the reader may understand the present position and practice concerning tramway brakes, the following extracts from the B.O.T. Rules (1906) with reference to brakes for electric tramways are here given — Board of Trade Regulations. — " 1. Every motor carriage used on the tramways shall comply with the following requirements, that is to say — "(b) The wheels shall be fitted with brake blocks, which can be applied by a screw, or by other means, and there shall be in addition an adequate electric brake." (Note. — Where, for a considerable distance, the gradients are 1 in 15 or steeper, the following will be added to this regulation : " and a slipper-brake or other track-brake approved by the Board of Trade for use on the tramways.") (Note. — Where for a considerable distance the gradients are 1 in 15 or steeper, the following regulation will be imposed : " VL The slipper-brake or other track-brake should be applied in descending all gradients of 1 in 15 or steeper.") From these regulations, it will be seen that the Board of Trade require each car to be fitted with a wheel-brake, and an adequate electric brake, and where the gradients are 1 in 15 or steeper, a slipper or other track-brake of approved design is also required. The majority of cars fitted with series motors for ordinary series parallel control, at the present time are in consequence fitted with either one or the other of the following combinations : — Hand-operated wheel-brake and a rheostatic brake — in some cases an electrical emergency brake taking the place of the rheostatic. Hand-operated wheel-brake and a slipper-brake operated electro- magnetically. Hand-operated wheel-brake" and a combined slipper and wheel- brake operated electro-magnetically. While other combinations are to be found, for instance, a mechanical track-brak^, operated either by hand or by power, in some cases taking the place of the electro-magnetic slipper-brake, the above are fairly representative combinations. The action of the different types already mentioned will now be briefly considered. Mechanical Brakes. — Wheel-brakes. — These generally consist of cast-iron brake-shoes, which, whan the brakes are operated, are forced by suitable brake gear agaiijigt th^ rims of the car- wheels. 178 ELEMENTS OF ELECTRIC TRACTION Whether operated by hand or by power, their action is the same, namely, that of retarding by frictional means the rotation of the wheels, and hence the motion of the car. It will thus be seen that their action depends upon the rotation of the wheels. The greater the pressure with which the brake-blocks are forced against the wheels, the greater will be the friction, and the greater in consequence the retarding or braking action, that is, as long as the wheels are kept revolving. The kinetic energy of the car in this type of brake is simply converted by the friction into heat. In the ordinary hand type, the brakes are generally operated by means of a handle fitted with a ratchet attachment, so that the operating handle need not be turned continually round, but simply worked backwards and forwards ; in this way the motorman is always able to obtain the full leverage of the operating handle. The brake spindle when the brakes are being operated coils up a chain, which in turn operates levers, which force the shoes carried by the brake beams up against the rims of the wheels. A pawl or catch attach- ment operated by foot enables the brakes to be kept on when required, and spiral springs are arranged to pull the brakes off when the catch is released. While the ordinary hand wheel-brake is easily operated and applied, the great disadvantage with all wheel-brakes, whether operated by hand or by power, is the liability when put on hard of locking the wheels, and so causing the wheels to skid, the friction between the wheels and the brake-blocks is then so great that the wheels slip or skid on the rails without revolving. Flat places, or " flats," as they are called, are in this way formed on the rims of the wheels, the greasier the rail the more likely is this skidding action to take place. In skidding the retarding force is simply that due to the friction, which, under the circumstances, exists between the wheels and the rails due to the weight of the car. This friction is generally so small, especially if the wheels are in at all a greasy condition, that practically no braking action, such as is required, takes place, the car under these circumstances simply sliding along the rails despite the fact that the brake is hard on. In all cases of skidding it will therefore be seen that the brakes are really not acting at all, and it cannot be too clearly pointed out that it is essential in all wheel- braking that the wheels should be kept just revolving if a braking action is to take place, and that when the wheels cease to revolve all braking action, so far as the brake-shoes are concerned, ceases, as all friction of motion between the wheels and the brake- shoes then ceases, the two simply being locked together. In operating wheel-brakes, therefore, if a skid occurs, the motor- man should release the hand-brake somewhat, drop sand in front of BRAKES 179 the wheels if travelling forward, behind if travelling backward, say, down a gradient ; and after the wheels have regained their motion to again apply the brake gradually. In the ordinary type of hand-brake, the brakes can only be effectively operated from one platform at one time, and it is necessary therefore, to slacken the chain at one end if the brake is to be applied from the other platform. On all these points, however, as will be again referred to later, the motorman cannot, on the one hand, be too clearly instructed, and, on the other hand, cannot make himself too familiar with the con- struction and working of the particular type of brakes he is called upon to handle. Track-brakes. — In some towns the slipper, or other approved track-brake, takes the form of a mechanical slipper-brake, which is operated in some cases by hand, a hand-wheel then very often being employed in place of an ordinary handle for operating, and in other cases by means of compressed air (Hewitt and Ehodes air track- brake). The slipper is generally faced with wood, and when the brakes are operated, these slippers are forced by a suitable arrangement of levers, etc., on to the rails. The retarding force in this case, as in the previous one, is that due to friction, the distinguishing difference being that in the wheel-brake the motion of the wheels is retarded by frictional means, whereas in the slipper-brake the motion of the car itself is retarded. The action of the brake does not therefore depend upon the rotation of the wheels, but simply upon the motion of the car. The kinetic energy of the car in this case also is converted, by means of the friction between the slipper and the rails, into heat. The action of a mechanically operated slipper-brake tends to relieve the wheels of the weight of the car, but it should never be sufiBcient to actually lift the car off the rails on account of the possibility of derailment. The mechanical slipper-brake has this great advantage over the wheel-brake, that not only is skidding of the wheels avoided, but also its action is independent of the rotation of the wheels, and therefore of any such skidding. The mechanical slipper-brake has also the following advantages over electro-magnetic slipper-brakes : — First, that if the car is derailed, the slipper-brake can still be applied, whereas the slipper portion of an electro-magnetic brake is dependent on the car being on the rails ; this will, however, be again referred to later. The second advantage is .that there is not the same liability for a mechanical track-brake to get out of order, as is the case where i8o ELEMENTS OF ELECTRIC TRACTION brakes are operated in any way electrically, such as, for instance, may occur with a bad or broken connection. On the other hand, these risks with electrically operated brakes can be considerably reduced where regular examination is made, and when the brakes are allowed to be in constant use. Hand-operated track-brakes, however, throw a considerable strain upon the motorman, as considerable force is generally required to operate these, and also the length of time it takes a man to operate is of no small importance, especially when the speeds are at all high, such as are likely to occur with ordinary electric traction. On the other hand, when operated by compressed air, the increased weight and upkeep of the equipment is a matter that has to be considered. Electrical Brakes. — Before dealing with the different types of electrical brakes, the following general explanations, which apply to all forms of electrical brakes, that is, where the motors are made to act as generators, are given — It has already been seen that a motor, if suitably connected and driven, that is, provided with mechanical energy, will run as a dynamo, and partially convert the mechanical energy supplied into electrical energy, the remainder of the energy being converted in various ways into heat. This principle is now largely used for braking purposes, and, broadly speaking, all such braking is electrical braking, the car's kinetic energy in these cases being converted in this way into electrical energy. In this way the car is robbed, as it were, of its kinetic energy, and by being so robbed is brought to rest. Another way of considering the action of an electrical brake is as follows : — When the motors are coupled up to act as dynamos, and a suitable circuit arranged so that current can flow, a force will have to be applied to drive the motors, and hence the wheels round. Now, the fact that a force is required to drive the wheels round shows that the action of the dynamos (calling the motors now dynamos, since acting as such) tends to resist the motion of the wheels, thus tending to stop them, and so braking the car. Difficulty is sometimes experienced in grasping the above fact, namely, that a force has to be applied to drive a dynamo round, and that either the greater the current, or the greater the field strength, or the greater both these are, the greater will be the force required. This can possibly be best illustrated by treating the whole matter from a magnetic point of view, just as was done in the case of the motor. In Fig. 32, let A represent the armature of a dynamo revolving in the direction shown, with current flowing down the conductors on the left-hand side of the armature core, and up on the right. In BRAKES i8i consequence of the current so circulating round the armature core, the core becomes magnetized in such a. way that the bottom portion is always of north polarity and the top south. Now, to revolve the armature in the direction shown means that it will have to be driven in opposition to the forces of repulsion and attraction, due to the magnetic poles in the armature core and those of the main field. The result is that to drive the dynamo round, a force will have to be DYNAMO Fio. 32, applied to overcome the resisting forces. The greater the current the dynamo is delivering (assuming field to be either of constant or increasing strength), the greater will be the force required to drive the armature round, as, with an increased current, the more strongly will the armature be magnetized, and the stronger the poles, the greater the force that must be overcome. The retarding force, or, more accurately, the retarding torque, exerted by each motor when driven as a dynamo will therefore depend upon the strength of field, and upon the armature current, an 1 82 ELEMENTS OF ELECTRIC TRACTION increase in either or both of the above quantities bringing about an increased retarding or braking effect. The actual current flowing through the armature in any given case will depend upon the field strength, speed, and resistance in the armature circuit, the field strength and speed determining the E.M.F. generated. Therefore, the stronger the field, or the higher the speed, or the less the resistance in the armature circuit, the other quanti- ties remaining the same, tlie greater will be the retarding or braking effect produced. Where ordinary series motors are employed for braking purposes, as the armature current determines the field strength, the retarding effect will simply depend upon the speed at which the wheels revolve, and the resistance in the armature circuit. The higher the speed with a given resistance, or the less the resistance in circuit with a given speed, the greater will be the retarding effect produced. In short, the greater the speed of the car the greater will be the braking action with a given resistance in cii'cuit ; that is, with the power handle on a given brake notch. From this it will be seen that in all electric braking, that is, where the motors are employed as dynamos, the braking action is automatic in the sense that the greater the speed, and therefore the greater the retardation required, the greater is the retarding effect produced, provided the wheels are kept from skidding. A most important feature to be noted in connection with all electric braking, that is, when the motors are employed as above, is, that if the wheels cease to revolve, the dynamo ceases to generate an electrical pressure, current will then cease also to flow, and the resisting force, in consequence, also ceases ; in short, the braking action ceases. If skidding, therefore, takes place, all electric braking action ceases entirely. An electric brake, therefore, automatically does what a man does when he takes off his hand-brake as the wheels start to skid, and applies it again when they start to revolve, the electric brake ceasing to act when the wheels skid, and immediately acting when they begin to revolve. Strictly speaking, it must not be said, however, that the electric brake prevents skidding. What the electric brake does is to auto- matically take off and put on a braking action just as the wheels skid or revolve, as the case may be. The sudden application of the electric brake, however, may cause the wheels to be puUed up so suddenly that a skidding action may be started (especially if rails are at all " greasy ") unless sand be dropped, and the wheels thus started skidding may continue, despite the fact that when the wheels are skidding the brake is automatically " off," as the wheels are not revolving. If the wheels skid, just as with hand-braking, BRAKES 183 the releasing of the brake and the application of sand enables them to regain their motion, and when the wheels once more revolve the brake can then be gradually applied again. The "greasier " the rail, the more tendency there is, of course, for the wheels not only to start but to continue skidding. It must be, therefore, remembered that all electrical brakes of the above description depend entirely for their action upon the rotation of the wheels, and therefore, when' applying such brakes, the wheels should be kept from skidding both by the use of sand if the rails are at all greasy, and the careful application of the brake. An electrical brake, it must also be noted, will not effectively hold a car on a gradient, that is, where brakes are at all necessary, a gradual slipping of the car, due to the gradient, with an electrical brake taking place, as no retarding force is exerted until an E.M.F. is generated and current flows through the circuit. In such cases a mechanical brake is therefore necessary for holding the car at rest on a gradient. One advantage of electric braking is that practically no physical effort is required on the part of the motorman in order to apply the brake, and the intensity of the braking action is independent of the man's physical strength. Another advantage is that wear and tear of wheels and brake- blocks is saved, but it must not be forgotten that the strain of electric braking is thrown on to the motors and gearing. In all the various types, the disconnection from the trolley wire, and the various connections which are required, are made, it may be said, by means of the controller when suitably operated. Rheostatic Brake. — In this form of electric brake, the pressure generated by the series motors, acting as series dynamos, is employed to send current through a variable resistance, the amount of resist- ance in circuit being varied by means of the controller, that is, when it is operated on the brake-notches. For this purpose the main barrel of the controller is so designed, additional contacts, etc., being added, that after the brake-handle has been brought to its " off " position, it can then be moved on to what are termed the " brake- notches." When on these notches, the, motors are coupled up so as to act as dynamos in parallel, a certain amount of regulating resist- ance also being inserted in the circuit. On moving the power handle from brake-notch to brake-notch, this resistance can be gradually cut out, till finally, when on the last brake-notch, all regulating resist- ance is cut out of the circuit, the dynamos then being simply short- circuited on themselves. By this means a large or small current can, with a given speed, be allowed to flow through the circuit, and a graduated braking action can in this way be obtained ; the greater 1 84 ELEMENTS OF ELECTRIC TRACTION the current the greater being the braking effect, the greatest braking effect being produced when all regulating resistance is cut out. When the wheels cease to revolve, the dynamo ceases to generate, and all braking action then ceases, regardless, of course, of whatever brake-notch the handle is on. As the current, and hence the brak- ing effect, is regulated by means of a regulating resistance, or " rheostat/' as it is sometimes termed, the term " rheostatic brake " has thus been adopted to describe this particular type of brake. As examples, the B18, B13, British Thomson-Houston, and the rm BLOWOin" COIL r' RHEOSTATS Al ^MATura^ i — •-i^ EJ Field VJiNDiNq y^JsMr^: /fA/LS V\\VV Fig. 33. D.B.I., form C, Dick-Kerr, are types of controllers designed for rheostatic braking. Fig. 33 shows in simple diagrammatic form the connections that are made in the B18 B.T.H. type of controller when the power handle is placed on the first of the brake-notches, the two motors then being coupled up to act as dynamos in parallel, as indicated. It may be said here, that in some cases the above arrangement is somewhat modified when the controllers are arranged, as they easily can be, for magnetic slipper-braking. By moving the power handle from brake-notch to brake-notch, an increased retarding or braking effect can with a given speed be obtained, as already mentioned. If, on the other hand, the power handle be moved rapidly over the brake-notches on to the last brake- notch, a very powerful and instantaneous braking effect will take place, the effect then being in the nature of an emergency stop. A large current under these circumstances will flow through the circuit, that is, if the speed is at all an appreciable one, and the wheels, of BRAKES 185 course, kept from skidding. A very heavy strain in consequence is thrown in this case both on the motors and gearing, and the use of such stops is limited solely to emergency cases. As the sudden application of such braking action has a tendency to cause the wheels to skid, sand should always be dropped when such braking action is being applied. In rheostatic braking pure and simple, the electrical energy generated is wasted as far as all useful purposes are Concerned, the current simply heating up the resistances, armature, and field wind- ings, etc., in circuit, the kinetic energy of the car in this way being converted indirectly into heat. B.T.H. Magnetic Slipper or Track Brake. — In this type of brake, the series motors are employed as series dynamos, as in the ordinary rheostatic brake just considered, and the current is regulated in a similar manner by means of resistance, the controller also being practically identical with the one employed for rheostatic braking. The current, however, in this case also passes through magnetizing coils which are inserted in the circuit, and which operate the slipper, or brake-shoes, in the manner described below. On an ordinary single truck there are generally two shoes, one shoe being attached to each side frame. Each shoe is placed opposite and near to the rails, and is held clear of the rails when the brake is not being operated by means of springs. The coils for magnetizing the shoes are coupled in parallel, so that in the event of any injury to either one shoe or coil, the other shoe is still capable of operating. The coils are arranged so that when current passes through them, the steel shoes become strongly magnetized. When so magnetized, each shoe induces the opposite magnetic polarity in that part of tlie rail opposite the shoe, and the result of this is that when the brake is applied — the wheels revolving — the shoes are drawn hard on to the rails, the springs only being sufficient to hold the shoes oft' when the brake is not applied. Friction in this way is set up between the shoes and the rails, and an additional braking action thereby introduced. By this means, it will be seen, a slipper braking action is brought about by magnetizing the shoes electro-magnetically by means of the current generated. The braking action, it will therefore be seen, in this type of brake is twofold — 1. The braking action due to the motors acting as dynamos, an action which tends to stop the wheels revolving, practically a rheo- static braking action. 2. The slipper braking action, due to the friction between the magnetized shoes and the rails. The two actions, however, it should be noted, are entirely de- pendent upon the wheels revolving. 1 86 ELEMENTS OF ELECTRIC TRACTION The kinetic energy of the car in this case, just as in the previous one, is ultimately converted in various ways into heat, partly, for instance, by heating up the resistances, armature windings, field windings, etc., in circuit. The action here, as with all electric brakes, is automatic in the following sense, namely, the greater the speed with a given resistance in circuit, the greater will be the current, and the greater, in con- sequence, the attraction between the shoes and the rails (that is, of course, within certain limits, depending upon the degree of satura- tion), and, consequently, the greater the braking effect not only due to the shoes, but also due to the retarding effect of the motors acting as dynamos. A graduated or emergency braking action can also be obtained with this type of brake just as in the case of the rheostatic brake. When the car is brought to a standstill, no current will flow, and the shoes will then no longer be magnetized, that is, to any extent, the only magnetism remaining being the residual magnetism, the springs already described then pulling the shoes clear of the rails. This type of brake, it may be said, can very readily be applied to any car fitted for rheostatic braking, and it also has the following advantages : — Less current, it is claimed, is needed for a given braking action than with the ordinary rheostatic type, consequently there is less heating of the motors, etc., and less wear of gear and pinions. With an ordinary mechanical track-brake the wheels are relieved somewhat of the weight of the car when the brake is applied, and unless suitably provided for, there is, of course, with these brakes, the consequent possibility of derailment. In the magnetic slipper type of track-brake, the reverse is the case, the adhesion between wheels and rails being increased when the brake is applied, there is, therefore, no risk whatever of the car being derailed due to this cause. There is also no additional increase in weight, or cost of extra equip- ment, as there is when slipper- or track-brakes are operated, by means say of compressed air. In some experiments that were made with the above type of brake, the following results, which were at the time published in the electrical press, were obtained with a four-wheel double-deck car fitted with two motors : — Travelling at 25 miles an hour down a gradient of 1 in 13 to 14, the car was pulled up in 25 yards, 4 seconds being the time taken. The same car travelling at 14 miles per hour on the level was pulled up in a little less than 5 J yards, the time taken being 1*6 seconds. In each of the above cases the brake was simply used as an emergency stop. BRAKES 187 The following tests were also made for graduated braking when coasting down a gradient : — A speed of 5 miles per hour was maintained on a gradient of 1 in 13 to 14, the current per motor then being 4 amps. A speed of 5 miles per hour was maintained on a gradient of 1 in 17, the current per motor then being 3*5 amps. A speed of 5 miles per hour was maintained on a gradient of 1 in 45, the current per motor then being 2 amps. In the case of a car provided with this type of brake leaving the rails, provided the wheels are still revolving and the brake is applied, an E.M.F. will be generated, and current will flow magnetizing the shoe coils as before, but as magnetism cannot be induced in the setts, there will be no mutual attraction between the shoes and the setts, and the shoes consequently will not operate; this part of the braking action will therefore be useless. In this case, therefore, the rheostatic braking action will be the only one in operation, and that, as already pointed out, provided the wheels are revolving, the energy being spent in driving the current through the resistance in circuit. Westinghouse Magnetic Brake. — This type of brake is similar to the last, with the exception that when the magnetized shoes are drawn on to the rails, they also force brake-blocks by means of a suitable arrangement of levers up against the rims of the wheels. The braking action therefore, in this case, when the magnetic brake is applied, is threefold — 1. A rheostatic braking action, due to the motors acting as dynamos. 2. A slipper braking action, due to the friction between the magnetized slipper and the rails. 3. A wheel braking action, due to the pressure of the brake-blocks against the rims of the wheels, this latter being brought about at the same time the slipper is drawn on to the rails. Compared with the magnetic slipper-brake just described, it will be seen that in the Westinghouse type an additional braking action is obtained by means of a wheel-brake operated as indicated. The braking action, of course, just as in the B.T.H. magnetic slipper, or in the ordinary rheostatic type of brake, can be either of a graduated, or of an emergency nature, as required, the emergency braking action being obtained by putting the power handle directly on to the last brake-notch. In all such cases, sand should, of course, be dropped while applying the brake, as it is essential the wheels should be kept revolving, and the sudden application might start the wheels skidding, in which case, of course, all three braking actions would be rendered useless. 1 88 ELEMENTS OF ELECTRIC TRACTION The kinetic energy in this case also is ultimately converted into heat, just as in the previous case. The general behaviour, action, and advantages claimed for this brake are almost identical with the one just considered, and the reader is referred to the description given for a detailed explanation of the general behaviour and action. Application of Rheostatic or Magnetic Slipper-brake. — In such types of controllers as the B13 or BIS B.T.H., whether adapted for simple rheostatic or magnetic slipper-braking, the action of applying the brake is as follows : — If the car is travelling forward, the reverse handle then being in its forward position, the brake is applied in the following manner — . (1) Bring the power handle to its off position. (2) Bring the power handle on the brake-notches, the reverse handle being left in its forward on position. It should be noted, however, that in the case of a car ascending a gradient, if by any chance it should begin to travel backwards, due to the trolley coming off, or the power going off, the brake would have to be applied in the following manner — (1) Bring the power handle to its off position. (2) Bring the reverse handle to the reverse or backward position to correspond with the reversed motion of the car. (3) Bring the power handle on to the brake-notches. The reason for this is as follows : — When the motors (referring to the series motors) are employed as dynamos, the only field magnetism they have to rely on in starting is the magnetism which the field magnets retain, namely, the residual magnetism. The residual magnetism is generally sufficiently great to generate a small E.M.F., which, in turn, with a suitable resistance in circuit, will cause a small current to flow, and if the field-coil con- nections are suitable, the magnetizing effect, due to the field coils, will strengthen the residual field, and in this way the field will be gradually strengthened, or the field will be " built up," as it is termed. When employing the motors as dynamos, the connections are such that this building-up action takes place, that is, for instance, when the car is travelling forward and the reverse handle is in its forward position. If, however, the car is travelling backward, as in the case referred to, the motion of the wheels under these circumstances will be reversed, and the E.M.F. generated will send current the reverse way through the field coils, unless the reverse handle be operated as indicated. The result of this would be the residual field would be weakened instead of strengthened, and in this way the field would gradually be lost, and the dynamos under these circumstances would then fail to build up, and no braking action would in consequence BRAKES 189 take place. By bringing, however, the reverse handle to the back- ward position, before placing the power handle on the brake-notches, the connections to the field coils will be reversed, and the reversed direction of current will then strengthen the residual field as desired, and a braking action in this way will be obtained. As different types of controllers vary in their construction and arrangement, some types being so designed as to automatically apply the electric brake in the event of such an accident as that described, the motorman cannot be too careful in obtaining definite information on all such points concerning the particular type he has to use. Motor Cut-out. — The cutting out of one motor, in most cases where rheostatic or magnetic slipper-brakes are employed, does not prevent a braking action being obtained by means of the one remaining motor, the retarding action, however, in this case, is less, being that due simply to one motor. This applies, it may be said, to such types of controllers as the B13, B18 B.T.H., 90M Westing- house, but upon all such matters the motorman should again obtain definite information of the particular type employed. Emergency Brake. — Some types of controllers are designed simply for emergency braking action only, and in these types con- sequently no graduated braking action can be obtained. Examples — the KIO B.T.H., and the D.E.I., form B, Dick-Kerr, are types of controllers designed for emergency braking only. In the KIO type, when the emergency stop is employed, each series motor is simply shortcircuited on itself, the only resistance then in each circuit being the resistance of the armature and field windings and the resistance of the various connections. The connections in the above case, it will be seen, are somewhat similar to those employed in the B18 B.T.H. rheostatic type, when the power handle is placed on the last brake-notch, the actual difference being that in the rheostatic type mentioned, the two motors (dynamos) are^in parallel and shortcircuited on themselves, whereas in the KIO emergency type each series motor (dynamo) is short- circuited on itself. As the resistance in each circuit is very small, a very powerful braking action is in this way obtained, and, as pointed out when dealing with the rheostatic brake, the strain thrown upon the motors and gearing when such braking is employed is very heavy. On the other hand, such powerful braking action soon stops the car, and consequently the excessive current due to the shortcircuiting of the motors now acting as dynamos does not flow for any considerable time, otherwise the heating effect would be serious. As it is essential in all emergency braking to avoid skidding, sand should, of course, be dropped when applying such brakes. In the KIO type the brake is operated by means of the reverse handle, there being two emergency notches provided for this purpose, 190 ELEMENTS OF ELECTRIC TRACTION one in advance of the forward-running notch, and the other in the rear of the reverse notch. If the car is travelling forward, the reverse handle being in its forward position, the brake is applied in the following manner : — (1) Bring the powder handle to its off position. (2) Pull the reverse handle back to the far or rear emergency- notch, in this way passing the reverse notch. If the car in ascending a gradient should begin, due to some accidental cause, to travel backwards, the brake would be applied in the following manner : — (1) Bring the power handle to its off position. (2) Push the reverse handle to the front emergency notch. The reason for the above action is similar to that already described when dealing with the application of rheostatic and magnetic slipper-brakes. Motor Cut-out. — If one motor be cut out in either of the above types of controller, a braking action is obtained by means of the other motor, but the retarding force is less, being simply that due to one motor. Operating Electric Brakes. — Before concluding this part of the subject, it should always be remembered that the motors are acting as dynamos when they are employed for electric braking, and that the E.M.F. generated will be sending a current of more or less considerable value through the circuit, and, consequently, in the emergency type of brake, the reverse handle must not be moved from the emergency notch until the car has come to a standstill, otherwise the circuit may be broken while a large current is flowing, and a considerable amount of arcing may take place inside the controller. In a similar manner in ordinary rheostatic braking, the power handle should not be moved from the first brake-notch to the off position until the brake has exerted its full effect on that notch, in other words, until the speed has become reduced to its full extent, otherwise considerable arcing may take place inside the controller just as before, due to the breaking of the circuit when a considerable current is flowing. Regenerative Braking. — In the two regenerative systems which have already been described, and to which the reader is referred, the motors are employed as dynamos when regenerating, and the retard- ing action is similar to that obtained in ordinary rheostatic braking, the retarding effect being due to the fact that the motors are employed as dynamos. As the question of braking is also largely the question of speed control, there is much to be said quite apart from any question of economy for these systems, as in these the speed of a car can be BRAKES 191 efficiently controlled over a wide range. These systems also possess the advantage that the retarding effect is practically in regular operation, and, consequently, the slightest failure is at once notified to the motorman. Skidding is to a large extent also reduced, as the motors in the ordinary way are always in actual contact with the trolley wire, and must therefore be either acting as dynamos and regenerating, or as motors, and in consequence being driven. Brake Combinations. — In the above descriptions the individual action of the various types of brake has simply been considered. The combination of mechanical and electrical brakes will now be considered. The majority of cars for ordinary tramway traction are fitted with a hand-operated wheel-brake, as well as some form of electrical brake. While on the one hand each brake has its own advantages when considered quite separately, on the other hand the combination of wheel-brake and electrical brake is by no means a good one. Combination of Wheel and Electrical Brakes. — A wheel- brake, as already seen, tends to lock the wheels, and, as an electrical brake depends entirely for its action upon the wheels revolving, if both brakes are operated together there is always the strong likelihood of the one neutralizing the action of the other. In some towns the use of the electric brake is limited, a motor- man only being allowed to use this either in emergencies, or on gradients which demand the use of a slipper-brake. One result of this is, a motorman is liable in all cases from sheer habit to turn first of all to the brake he is most accustomed to handling, namely, the hand-brake. In many cases, either due to the above habit or having failed to pull a car up by means of the hand-brake, he has then applied the electric brake, but only possibly after locking the wheels by means of the hand-brake. The electrical brake has in this way been rendered entirely useless. The proper course of action would have been to release the hand-brake, drop sand, and apply the electric brake after the wheels have regained their motion ; but the liability for such an occurrence as the one mentioned above is one of the great disadvantages and dangers of the above combination. While the hand-operated wheel-brake is no doubt useful for all ordinary work on the level, or for holding a car on a gradient, the ideal conditions can only be fulfilled by having some combination in which the action of one l3rake cannot neutralize the action of the other. Again, in the event of a failure of the magnetic slipper, due, say, to a bad or broken connection, the hand-brake is not always powerful enough to deal both with steep gradients and greasy rails, especially if the car has in any way got momentarily beyond the control of the driver. Combination of Track and Electrical Brakes. — As a 192 ELEMENTS OF ELECTRIC TRACTION mechanical track-brake does not lock the wheels, the combination of mechanical track and electrical brake has the great advantage that the one cannot in this way neutralize the action of the other. On the other hand, as a mechanical track-brake tends to relieve the wheels somewhat of tlie weight of the car, the adhesive force is con- sequently somewhat reduced, and the driving effect of the wheels, and consequently of the motors acting as dynamos, is in this way somewhat interfered with. By limiting, however, the pressure that the track-brake can exert, or simply employing it as a reserve brake in case the electrical brake fails, the above difficulties can be over- come, the track-brake of itself being sufficiently powerful to deal with all requirements. At the present time, in consequence, many engineers favour the adoption either of a regenerative system, with some form of mechanical track-brake as a reserve, or some form of magnetic slipper-brake for regular use with a mechanical track-brake as a reserve. Two other methods of braking or pulling a car up by means of the motors will now be considered — 1. By reversing the motors and gradually applying the power. 2. By reversing the motors and passing into full parallel instantly. In this second method, the braking action, as will be seen in detail later, is due to the fact that one motor becomes employed as a dynamo, and supplies current to the other motor, tending to reverse that motor's direction. At the outset, it should be mentioned that the following explana- tions apply to those cases where, on reversing the motors, the direc- tion of current through the field coils is always kept the same ; that is, the motors are reversed by reversing the direction of armature current, as is the case with such types as the B13, B18, KIO B.T.H.; D.E.L, form B, Dick-Kerr; 90M and 210 Westinghouse. In cases where the fields are reversed, or where one field is reversed, instead of the direction of the armature current, the action and explanations are very similar. 1. Reversal of Motors. — If when a car is travelling down, say, a slight incline, or at a fair speed on the level, the motors are reversed (the motorman, for this purpose, after bringing the power handle to the "off" position, and the reverse handle to the reverse position, again applies the power by means of the power handle, reversing the motors, that is, as far as the position of the controller barrels is concerned), one of two things will happen: either the motors will respond to the reverse action, or they will not. Whether they respond to the reverse action or not will depend upon the speed and weight of the car, or the tendency there is at work — as in the case of a car descending an incline — for the car to continue moving BRAKES 193 in its original direction, and hence for the wheels also to continue revolving in their original direction. To reverse the motion of the motors, or, what is the same thing, the wheels, seeing they are geared together, the motors must first slow down and stop, if only for an instant, before their motion can be reversed. Responsive Action. — If now, on applying the power after reversing, the motors slow down, stop, and then reverse their motion — in other words, respond to the reversing action — the wheels will then reverse, and either they will slip, speeding quickly up, as they have very little load under such conditions, especially if the rails are at all greasy (the application of sand will remedy this), or they will, by their reversing action, pull the car up. The car having thus been brought to a standstill, the power handle should then be moved back to the *' off" position ; otherwise, an excessive current would then begin to flow, depending upon the notch the power handle happened to be on, causing the canopy cut-out in all probability to blow, as the car would then be starting up in the reverse direction, of course, with the power handle not in its starting position. In the above case it has been assumed that in the act of reversing the motors, namely, at the instant the motors stopped, previous to reversing their direction, the wheels, owing to the motion of the car, have not started to skid, as it must be remembered in this case current is passing through the motors, and so there is a force at work tending to turn the wheels, which if sufficiently great should prevent skidding, and set them in motion, namely, in the reverse direction. The tendency to slip, as already mentioned, however, will exist, and the remedy — sand. If the wheels skid here, as in other cases, the braking action will cease, or, if they slip, the braking action will in all probability cease to be of any value. If the power handle be moved from notch to notch, thus cutting resistance out, the motors will tend to speed up in the reverse direction, or if they do not speed up, there will be a greater current and a greater torque, and so a greater reversing or braking effect obtained, provided the adhesion is good — that is, the wheels do not slip. A too-sudden cutting out of resistance will, however, in all probability, start the wheels slipping. Non-responsive Action. — If the motors, on the other hand, do not respond to the reverse action, owing to the car's momentum, the energy stored up in the car carrying it forward, the wheels continuing also to revolve in their original direction in spite of all reversing tendencies — a car coming down a steep gradient would be very liable to do this — the following action will then take place : — 194 ELEMENTS OF ELECTRIC TRACTION If, just previous to reversal, the motors were being driven by means of the trolley pressure, and current was flowing through the armatures, as shown in Fig. 34, from Al to AAl, and from A2 to BACKE.M.F < Al r >sAAI FI TRoLLEy WHEFL El j^.o.i nsBmi uu it: BLOW OUT U U , COIL PHEOSTKTS J^C.0. 2 '". *■ MJtO.S, FIELD \" HELD MX. 0.2, WiNDiNQ (ARMATUI ^TOBrH)cafrf!OLLERS Fig. 34. AA2, then on reversing and applying the power, current will pass through the armature (see Fig. 35) from AA2 to A2, and AAl to Al, BMH EMF \&}vW\ .^»-/\ "^iM/uir^ M.C.o. I M.C.O S f£rfW BACK EMF. MCOi Fig. 35. 777777 MILS namely, in the same direction the back E.M.F. was previously tending tfo send it. BRAKES 195 As the direction of the field has not been altered, and as the dii-ection of rotation of the motors has not altered — the motors not having responded to the reverse action— the E.M.F. generated by the motors will still tend to send current the same way as before, through the armatures, namely, from AAl to Al, and from AA2 to A2. The result of this will be that the E.M.F. generated by the motors now no longer opposes but assists the trolley pressure ; an excessive current will in consequence flow, and the circuit breaker will be operated. If the car is going at a good speed when it is reversed in the way indicated, it will probably be found that the canopy cut-out will break the circuit when the power handle is moved on to the first or second notch after reversing. To replace it, of course, under these circumstances would be futile, and that brings us to the second method mentioned above of braking a car. 2. Reversal of Motors. — Following the previous explanation, Al V' nn rWn -*. <' fQr'rW- Fig. 36. if, after the canopy cut-out has broken the circuit, the power handle be moved over to " full parallel," the connections between the motors, etc., will be that shown in Fig. 36. By this means a powerful braking action is brought about, of which the following is an explanation : — The motors, being disconnected from the trolley, can no longer act as motors, and on studying Fig. 36, it will be seen that the two motors are coupled up in parallel, but that there is no external circuit. If, therefore, they act as dynamos, the pressure generated by the one will oppose the pressure generated by the other. The reason for this is that as the fields were previously excited alike, the residual magnetic field of each will still be alike, and therefore, as both motors are revolving the same way, the pressure generated by each will 196 ELEMENTS OF ELECTRIC TRACTION tend to send current the same way, that is, in opposition to one another. The residual magnetic field, it must be remembered, is the magnetism which has to be depended upon in starting a dynamo, the weak magnetic field due to the residual magnetism being sufficient to start the action. In the case of a series machine, the magnets will not be fully excited electrically until current flows through the armature circuit. Now, if the pressure generated by the one machine is exactly equal to that generated by the other, no current whatever will flow through the circuit, and, it may be added, no braking action of any kind will take place. This state of affairs is, however, not likely to take place, as the residual magnetic field of one motor will, in all probability, be not exactly of the same strength as that of the other ; also the speed of one motor, owing either to the wheels of one slipping slightly, or to one set of wheels not being exactly the same diameter as the other, will in all probability not be exactly the same as the speed of the other. The result is that one dynamo will in all probability generate a slightly higher pressure than the other. If No. 1 machine generates a slightly higher pressure than No. 2, as the pressure generated will tend to send current the same way the back E.M.F. did previously, a small current, depending upon the difference of pressures generated, will flow from AAl to Al, from Fl to El, from E2 to F2, from A2 to AA2, and from AA2 to AAl. The current passing through the field of No. 2 in the reverse direction will weaken its residual field, causing a lesser pressure to be generated in that machine, and a greater current in consequence to flow. In this way No. 1 machine will gradually build up as a dynamo, and will force current through the field of No. 2 machine in the reverse direction, thus reversing the field, but, owing to the reverse con- nections, the current will still pass through the armature of No. 2 machine in the same direction as originally — that is, when driven by means of the trolley pressure. Eeversing the field and keeping the direction of the armature current the same, means that No. 2 will endeavour, as a motor, to reverse its direction. The whole effect summed up is, that if No. 1 is on the front wheels, and No. 2 on the rear wheels of the car, No. 1, acting as a dynamo, will tend to stop the motion of the front wheels, and No. 2, acting as a motor, with its direction reversed, will not only tend to stop the rear wheels, but also tend to reverse their motion, and so the direction of the car, the whole bringing about a powerful braking effect. If for any reason an electric brake fails to act, there is at hand in the above method an emergency means of braking the car indepen- dent of the ordinary electric brake. BRAKES 197 When once the circuit breaker has broken the circuit, of course no pressure is available for reversing a car in the ordinary way, and if the above method is to be adopted, the power handle should then be moved into full parallel at once. If by one of those peculiar circumstances the above failed to act, that is, if the dynamos did happen to generate exactly equal pressures, then the breaking of the circuit and the making of it again by means of the power handle would in all probability upset such delicate equilibrium, and bring about the desired effect. If desired, before applying power after reversing by means of the reverse handle, the circuit breaker could be "tripped" by hand, instead of allowing it to come out automatically. In emergency cases, tripping the circuit breaker by hand in this way is possibly the quickest and surest plan to adopt, the power handle then being placed into full parallel immediately — that is, after placing the reverse handle, if this has not already been done, in its correct position. Summing up, it will be seen that the above braking action takes place when the trolley circuit is broken by means of the circuit breaker or canopy cut-out — assuming the trolley is still on — and in the event of the car travelling forward the handles being placed in the following position : — Eeverse handle in the rear reverse position ; Power handle on full parallel ; or, if the car is travelling due to some accidental cause backward down a gradient, the handles being placed in the following position, circuit breaker being tripped as before : — Eeverse handle in the forward position ; Power handle on full parallel ; the reverse handle in each case, it will be noted, being in the reverse direction to that of the car's motion. Application of sand here, as in other cases of electrical braking, is essential, as the whole of the action depends upon the rotation of the wheels. Motor Cut-out. — In the event of one motor being cut-out, no braking action would of course take place. It should also be noted that had the canopy cut-out been "tripped" so as to disconnect the trolley pressure, and the motors (neither being cut out) put in parallel without reversing, that is, when the car is travelling forward, no braking action would have taken place. In this case, neither machine would build up as a dynamo, as will be seen on studying Fig. 37. If No. 1 machine generates, due to residual magnetism, a higher voltage than No. 2, current would then tend to flow from AAl to Al, from A2 to AA2, from r2 to E2, and note from El to El. The E.M.F. generated in No. 2 machine would, of course, tend to send current in the reverse direction to this. 198 ELEMENTS OF ELECTRIC TRACTION It will now be seen there are two causes at work tending to reduce this small initial current passing through the circuit. First, the E.M.F. generated by No. 2 is opposing that of No. 1, and, secondly, the direction of current through the field of No. 1 — now acting as a dynamo — is such as to weaken its own residual field, being in the reverse direction. The consequence is, No. 1 will fail to build up as a dynamo, the small pressure generated due to the residual field getting less and less as this becomes weakened, and finally both motors will cease to operate entirely either as motors or AI/''">sAM Fl El ^v l^n m ru": fcQ-«?-5%£ TTrtTTJ Fig. 37. dynamos. In the event of a car travelling backward, no braking action would take place if the reverse handle was placed in the rear reverse position, this position of the reverse handle being really the correct one for running the car backward if so desired, and hence not the reverse position for this particular direction of motion, the forward position being the reverse position for the backward direction of motion. While dealing with the above method of braking, it may be said that when series dynamos have to be coupled in parallel to supply an external circuit, a similar difficulty arises in getting the two dynamos to give exactly the same pressure, and to overcome this, a connection is generally made from the armature of one dynamo to the armature of the other, namely, from C to D. See Fig. 38. This cross-connection provides a path so that current is able to flow from the armature of the machine with the higher pressure across to and through the field of the machine generating the lesser pressure, and in this way, by means of this connection, the field of the weaker machine is strengthened. The pressures in this way are BRAKES 199 automatically adjusted until they become equal, which enables the dynamos to be run in parallel. Such cross-connections, or their equivalent, will be found to exist wherever series dynamos are run in parallel, as, for instance, when they are arranged for rheostatic electric braking. See Fig. 33. In some of the Dick-Kerr controllers, each motor, when acting as a dynamo, excites the other's field, and the cross-connection referred to above is in this way dispensed with in this particular arrangement. Finally, with regard to all braking, with the speeds that are now adopted — or sometimes attained — in electric tramway traction, it is essential that no time should be lost by the motorman in all AAAAAAAAAMAA Fig. 38. emergency cases in effectively applying the brakes. The need for prompt action will be more apparent when it is remembered that a car travelling at 10 miles an hour covers a distance of some 15 feet every second, and at a speed of 20 miles a car will travel some 30 feet — in this case practically its own length — every second. It is therefore essential that a motorman should know beforehand exactly what to do in the event of any emergency, and for this purpose should be carefully instructed in the working of the various types of controllers he has to handle. Where brakes are not in regular use, not only should daily examination be made of these, but opportunities should at least be given the motorman for obtaining the necessary skill and confidence in the use of all braking apparatus. 200 ELEMENTS OF ELECTRIC TRACTION Examples (1) An 8 -ton car is travelling at a speed of 10 miles per hour. (a) What is the value of the kinetic energy in foot-tons stored up in the car under these conditions ? (h) If in reducing the speed of the car 60% of the above energy is regenerated, what will be its value expressed in B.O.T. units ? (a) As kinetic energy (K.E.) in foot-tons = -^^ approximately, ,\ K.E. = ^ = ^oQqO- = approximately 2 6 6 foot-tons Therefore kinetic energy stored up in the moving car = 26-6 foot- tons. (b) As 60% of the kinetic energy is assumed to be regenerated, .*. energy regenerated = 2 6 '6 x ^^qq = 15*96 foot-tons ap- proximately. As 1 B.O.T. unit = 1000 watt hours, and as 746 watts = 1 E.H.P., .-. 1 B.O.T. unit = i^£^- = approximately 1-34 E.H.P. hours. Energy supplied at the rate of ^f^£ E.H.P. for 1 hour = \%^ X 33,000 X 60 foot-lbs. = 2,654,155 foot-lbs. approximately = 1184-89, say 1185 foot-tons = 395 yard-tons approximately .•. 1 B.O.T. unit is equivalent to 1185 foot-tons In other words, if 1185 foot-tons of mechanical energy were all converted into electrical energy it would be equivalent to 1 B.O.T. unit. 15*96 .-. energy regenerated is equivalent to = 0-013 B.O.T. unit Therefore energy regenerated in above case would be equivalent to 0-013 B.O.T. unit. In dealing with problems similar to above, it is useful to re- member that in supplying power, 1 B.O.T. unit is equivalent, with an efficiency of conversion of 85%, to approximately 1000 foot-tons, and that in regenerating at an efficiency of 85%, 1400 foot-tons of energy are approximately equivalent to 1 B.O.T. unit. BRAKES 201 (2) An 8-ton car starting from rest descends a gradient of 1 in 17-5. Assuming a tractive resistance of 25*4 lbs. per ton — (a) What speed will the car attain in 10 seconds ? (h) What number of yards will it travel in the 10 seconds ? (c) How much of its potential energy will have been expended ? (d) What would be the value of the car's kinetic energy at the end of the 10 seconds ? 2240 (a) Gravitational pull per ton weight = y^ttf = 128 lbs. Tractive resistance per ton weight = 25 '4 lbs. .'. accelerating force per ton weight = 128 — 25'4 = 102*6 lbs. As 102*6 lbs. per ton will give to the car an acceleration of 1 mile per hour per second, .*. in 10 seconds the car will attain a speed of 10 miles per hour. Therefore speed attained in 10 seconds = lo miles per hour. (b) Average speed during the 10 seconds, as the acceleration has been constant — = 5 miles per hour 1760 X 5 , = ocf)r) — yards per second .'. distance travelled in 10 seconds = ttttt^ ooOO = 24-44 yards = 73-32 feet Therefore distance travelled during the 10 seconds = 24-44 yards. The distance travelled could also have been calculated from the formula — AT 4-09 (c) Vertical height descended by car = ?^ = 1*39 yards = 4-17 feet .*. potential energy expended = 2240 x 8 x 4-17 = 74,726-4 foot-lbs. Therefore potential energy expended during the 10 seconds = 74,726-4 foot-lbs. 202 ELEMENTS OF ELECTRIC TRACTION {d) The car's kinetic energy will equal the potential energy expended, less that required for overcoming tractive resist- ance. Energy expended in overcoming tractive resistance = 25*4 X 8 X 73-32 = 14,898-624 foot-lbs., .'. kinetic energy possessed by the car at tlie end of the 10 seconds = 74,726-4 - 14,898-624 = 59,827-77 foot-lbs., ap- proximately = 26-7 foot -tons Therefore kinetic energy stored up in the moving car at the end of the 10 seconds = 26-7 foot-tons approximately. Note. — This same result could have been obtained by calculating the kinetic energy direct from speed and weight of moving car. See previous example, and compare. The two results would be found to agree exactly if calculations were worked out more accurately. (3) A car starting from rest descends a gradient of 1 in 1 2. Assuming a tractive resistance of 25 lbs. per ton — {a) What number of yards would the car travel in 5 seconds ? (&) What number of yards would the car travel in attaining a speed of 15 miles per hour ? This problem could be worked out from first principles similar to the above. As the tractive resistance is assumed, however, to be 25 lbs. per ton, it can be more readily calculated from the approximate formulae — S=(90-G)t and ^^ = Y^^^ 4G G Thus— /^ A.Q (90 -G) (a) As S = — ^^ i . . . r 1 90 -12 ... , speed attamed m 5 seconds = j^rrjo -^ 1 ~ 48 ^ 48 = 8-125 miles per hour (90 - G) 4G 90-12 5 78 . 390 And as S2 = Y G .-. 8125 X 8-125 = y(90 " l^) 12 78Y .-. 66 approximately = -zr^ Y = — =-^ — = 101 yards approximately 7o BRAKES 203 Therefore distance travelled in the 5 seconds = lo-i yards approximately. The above two approximate formulae can easily be combined, and the result is that — (90 -G) 16G This enables the above result to be more readily calculated. Thus— _ (90 - 12) _ 78x25 _ ^ ~ 16 X 12 192 ~ '■"^ ^^ ''"°'^^ (*) AsS^ = y(50-G) • • ~ (90 - G) . . ,, , 15 X 15 X 12 2700 .,, . .-. yards travelled = ^^ __ ^^ = -^ = 34-6 Therefore distance travelled in attaining a speed of 15 miles per hour = 34-6 yards. (4) What speed would a car attain in travelling 50 yards down a gradient of 1 in 12, if it commences to descend the gradient at a speed of 2 miles per hour, assuming the tractive resistance to be 25 lbs. per ton ? Answer — 20 miles per hour approximately. (5) What gradient would be necessary so that a car, in descending such without the aid of the motors on the one hand, and without the application of the brakes on the other, would just maintain a speed which has previously been given to it, on the assumption that the tractive resistance for that speed is 17 lbs. per ton ? Answer — 1 in 131-7 (6) A car travelling at 25 miles per hour is pulled up by means of an emergency brake in 4 seconds. (a) What is the average value of the retardation expressed in miles per hour per second ? (h) What number of yards will the car travel in the 4 seconds if the retardation is of constant value ? 204 ELEMENTS OF ELECTRIC TRACTION Answer — (a) 6J miles per hour per second. (h) 25 yards approximately. (7) A car travelling at 14 miles per hour on the level is pulled up by means of an emergency brake in 5 J yards. (a) Assuming the retarding force has been of constant value, what is the value of the retardation expressed in miles per hour per second ? (h) What is the time taken in seconds to stop the car ? Ansiver — (a) 8"7 miles per hour per second. ih) 16 seconds approximately. (8) An 8-ton car is travelling at a speed of 6 miles per hour. Assuming a tractive resistance of 20 lbs. per ton — (a) How far would the car travel on the level if no brakes of any kind were applied ? (b) How far would it travel up a gradient of 1 in 20 if no brakes were applied ? (a) As kinetic energy in foot-lbs. = WS^ x Tf) approximately, .-. K.E. stored in car = 8 x 6 x 6 x 75 = 21,600 foot-lbs. approximately. Eetarding force due to friction = 20 x 8 = 160 lbs. As K.E. in this case is expended simply in overcoming frictional resistance, and as work done in foot-lbs. in overcoming frictional resistance = 160 lbs. x distance travelled in feet, .*. K.E. in foot-lbs. = 160 lbs. x distance travelled in feet, .-. 21,600 „ = 160 lbs. X •'• -Mu"^ >> = distance travelled = 135 feet. Therefore the distance the car would travel = 135 feet. Note. — A rough rule (which is based on an assumption of a tractive resistance of 25 lbs. per ton) is that the (speed in miles per hour)^ = yards travelled on the level without brakes of any kind. (&) Eetarding force due to friction =160 lbs. 8 X 2240 Retarding force due to gravitation = — ^ = 896 lbs. Total retarding force = 1056 lbs. .-. distance travelled = ^^S>^~ = 20*45 feet BRAKES 205 Therefore the distance the car would travel up the gradient = 2045 feet approximately. (9) A 10-ton car, fitted with two motors and wheels 30 inches diameter, in descending a gradient of 1 in 13 at a speed of 25 miles per hour is pulled up by means of an emergency brake in 25 yards. Assuming the tractive resistance is 22 lbs. per ton — (a) What is the average retarding draw-bar pull exerted by the motors during the time the car is being stopped ? (h) What is the average retarding torque exerted by each motor on the driving axle during that time ? The average retarding draw-bar pull can be calculated, knowing the distance the car travels in being pulled up, and the tractive resistance, from the kinetic energy the car possesses and the potential energy expended on the car. Assuming, on the other hand, that the retarding force is of constant value, it can be calculated as follows : — g2 (a) . As A = -jy approximately Ketardation = -^ — ^ = 6j miles per hour per second approx. As 102*6 lbs. per ton are required to give an acceleration of 1 mile per hour per second, So also 102"6 lbs. per ton are required to give a retardation of 1 mile per hour per second. .*. to give above retardation, the retarding force required per ton = 102-6 X 6-25 = 641-25 lbs. .*. to give above retardation, retarding force required = 641-25 X 10 = 6412-5 lbs. Ketarding di^aw-bar pull required to balance gravitational effects 10x2240 ,^00 -.1 = ^ = 1723 lbs. Frictional retarding force = 22 x 10 = 220 lbs. As friction assists in retarding the car, .*. nett retarding draw-bar pull = 6412*5 -}- 1723 - 220 lbs. = 7915*5 lbs. approximately Therefore average retarding draw-bar pull exerted by the motors under above conditions = 79i5'5 lbs. approximately. (h) Average retarding draw-bar pull exerted by each motor = "^i^'^ = 3957-7 lbs. 2o6 ELEMENTS OF ELECTRIC TRACTION With driving wheels 30 inches diameter, or 1-25 feet radius, average retarding torque exerted by each motor = 39577 X li = 4947 Ibs.-feet approximately Therefore average retarding torque exerted by each motor on the driving axle = 4947 Ibs.-feet approximately. (10) A 10-ton car fitted with two motors and wheels 30 inches diameter is ascending a gradient of 1 in 50 at a speed of 10 miles per hour, and is pulled up by means of an emergency brake in 4 yards. Assuming a tractive resistance of 22 lbs. per ton — (a) What will be the average retarding draw-bar pull exerted by the motors ? (b) What will be the average retarding torque exerted by each motor on the armature shaft, the gear ratio being 4' 5 to 1. Answer — (a) 5744"5 lbs. approximately. (h) 797'7 Ibs.-feet approximately. (11) A 10-ton car is descending a gradient of 1 in 10 at a speed of 20 miles per hour, and is slowed down by regenerative control to 10 miles per hour in 100 yards. Assuming that under the above conditions the dynamos have an efficiency of 70%, and that the tractive resistance is 25 lbs. per ton, what will be the value of the B.O.T. units restored ? WS^ As kinetic energy in foot-tons = -^^r approximately .*. K.E. possessed by car when travelling at 20 miles per hour 10 X 20 X 20 30 = 133*3 foot-tons approximately As the car descends a vertical height of 10 yards or 30 feet in travelling 100 yards down the gradient. Potential energy expended on car during that time = 10 X 30 = 300 foot-tons Energy expended in overcoming tractive resistance 25 X 10 X 300 ooA^^. = fSTTTT^ — - — = 33*4 loot-tons 2240 K.E. possessed by car when travelling at 10 miles per hour _ _ — = 33-3 foot-tons BRAKES 207 Initial K.E. and potential energy expended on car = 133-3 + 300 = 433-3 foot-tons Final K.E. and energy expended in tractive resistance = 33-3 + 33-4 = 66-7 foot-tons .*. total energy absorbed in slowing down = 433-3 - 66-7 = 366-6 foot-tons With dynamo working at an efficiency of 70%, energy regenerated = 366-6 X xm) = 256-62 foot-tons Equivalent value of energy regenerated 256-62 1185 = 0-2ia B.O.T. unit Therefore the value of the electrical energy regenerated = 0*216 B.O.T. unit. (12) A 10-ton car is maintained at a speed of 4 miles per hour for a distance of 200 yards in descending a gradient of 1 in 13 by regenerative control. Assuming the dynamos under these conditions have an efficiency of 60%, and that the tractive resistance is 22*4 lbs. per ton — (a) What would be the value of the B.O.T. units regenerated ? (b) Assuming the voltage at the terminals of the dynamos is 520 volts, what would be the value of the current sent back to the line? (a) Vertical height descended by car = ^^- = 15*38 yards = 46-14 feet .*. Potential energy given to car in travelling 200 yards = 10 X 4614 = 461*4 foot-tons Energy expended in overcoming tractive resistance 22-4 X 10 X 200 X 3 2240 = 60 foot- tons .*. Energy delivered to dynamos = 461*4 — 60 foot-tons = 401*4 foot-tons With an efficiency of 60%, energy regenerated = 401-4 X -fo^o = 240-84 foot-tons 2o8 ELEMENTS OF ELECTRIC TRACTION Equivalent value of electrical energy in B.O.T units = ,^^ = 0-203 B.O.T. unit 1185 Therefore the value of the electrical energy regenerated = 0-203 B.O.T. unit. (b) A rate of 4 miles per hour is equivalent to a rate of 7040 yards per hour .*. time taken to travel the 200 yards = w7vtf\ = tt^^-t^ tour '^ 7040 35'2 0-203 B.O.T. unit = 203 watt hours .*. watts supplied during ^^hour = 203 x 35-2 = 7145-6 watts As pressure at terminals = 520 volts 7145-6 . ^ ^ .*. current m amps. = ^^^ = 13 7 amps. Therefore value of the current restored to line during the time considered = 137 amps. (13) A 10- ton car travelling at a speed of 20 miles per hour on the level is slowed down by regenerative braking to 10 miles per hour in 100 yards. Assuming the dynamos under these conditions have an efficiency of 70%, and that the tractive resistance is 25 lbs. per ton — • What would be the value of the B.O.T. units restored ? Ansiver — 0-0393 B.O.T. unit (14) Assuming the maximum adhesive force between wheels and rails is 400 lbs. per ton, and the tractive resistance is 25 lbs. per ton, what is the shortest distance a car can be pulled up in under these conditions on the level at any given speed when rheostatic braking is simply and solely employed ? As maximum adhesive force is 400 lbs. per ton, .'. maximum retarding draw-bar pull motors can exert when acting as dynamos = 400 lbs. per ton. Resistance to traction = 25 lbs. per ton. Kinetic energy stored in car = S^ X 75 approx. foot-lbs. per ton. BRAKES 209 As K.E. is expended in overcoming tractive resistance and retarding draw-bar pull due to the motors acting as dynamos, .*. K.E. in foot-lbs. = total resisting force in lbs. X distance travelled in feet. .-. S2 X 75 = 400 4- 25 X feet travelled. /. distance travelled in feet = -tf^ — = c 7.7^ 425 5-66 Therefore under above conditions the shortest distance in feet a car can be pulled up in on the level is obtained by dividing the square of the speed in miles per hour by 5 '66. A rough rule is to divide the square of the speed in miles per hour by 6. The above example indicates what is the shortest distance a car can be pulled up in on the level by rheostatic braking under what is practically the best condition of rails. The use of sand increases the adhesive force, and hence enables the distance as calculated above to be reduced. INDEX [Numbers in italics refer to pages of examples on the subject.} Acceleration, 88, 113 and speed, 88, 90, 108-111, 112, 117, 161, 175, 201, 202, 203 conditions requisite, 90, 108, 109, 110, 144 constant, 91, 108, 110, 111, 159 equivalent values, 89 formulae, 92, 175 value depending on, 91, 92, 109, 110, 111 with given gradient, 97, 117, 201, 202, 203 Accelerating force, 90, 96, 97, 112, 114, 201 and speed, 109, 175 Adhesive force, 104, 192, 193, 203 Alternating current, 48 Alternator, simple, 48 Ammeter, 23, 25, 34 Ampere, 21 hour, 72 turns, 16, 17, 18, 51 Arcing inside controller, 190 Armature, conductors, 52 conductors' pull on, 55, 58 core, 51 B Back E.M.F., 40, 60, 65, 66 value depending on, 62 Balance of forces in traction, 108, 110, 111, 144, 159, 160 torques, 76 torques in motor, 121, 133, 135, 136 Blow-out coil, 5, 156 Board of Trade panel, 3 Board of Trade Regulations, brakes, 177 current density in rails, 31 drop in rail return, 30 Board of Trade Regulations — contd., trolley-wire pressure, 1 trolley- wire sections, 3 Board of Trade unit, 72, 82, 83, 84, 163, 200, 206, 207, 208 Bonding of rails, 31 Brakes, air-operated, 176, 179, 180 application of emergency, 189 rheostatic, 188 automatic, 168, 170, 189 classification of, 176 combination of, 177, 191 electric, 176, 180 electro-magnetic, 176, 185, 187 emergency, 189 function of, 171 hand, 176, 177, 178, 179, 182, 191 magnetic slipper, B.T.H., 185 Westinghouse, 187 notches, 183 on gradient, 171, 183 operating electric, 190 rheostatic, 176, 183 track, 176, 177, 179, 185, 187, 191 wheel, 176, 177, 187, 191 Braking, action with one motor, 189, 190, 197 advantages of electric, 183 bv reversal of motors, 192, 195 electric, 156, 180, 190 emergency action, 177, 184, 186, 187 examples on, 203-208 general principle, 171 regenerative, 164, 176, 190 retarding force in electric, 176, 180, 181, 182, 183, 186, 190, 191 retarding force in mechanical, 178, 179 rheostatic, 163, 168, 184, 187 Brake Horse Power, 73, 82, 83, 106, 141 and current, 76, 82, 86 and torque, 79, 85, 106, 114, 123 measurement of, 74, 75, 106 Brushes, 48, 51 Bus bars, 2, 3 212 INDEX c Cables, 37, 86, 142 Canopy cut-out, 3, 5, G, 38, 193, 195, 197 Car equipment, 3 Car mile, 82, 84, 163 Car on level, 108, 109 gradient, 110 Centrifugal force, 118, 134 Characteristic properties, 52, 118 compound cumulative motor, 119, 138 differential motor, 119, 138 dynamo, 53, 54 separately excited dynamo, 51 series dynamo, 53, 54 motor, 118, 127 shunt dynamo, 52, 53 motor, 118, 125 Chemical effect, 20, 25 Choking coil, 5, 6 Circuit breaker, automatic, 1, 2, 3, 5, 6, 7, 38, 193, 195, 197 Circumference, 75 CoeflBcient of friction, 93 Commutator, 49, 50, 59 Compound, dynamo, 51, 54 motor, cumulative, 119 differential, 119 Conductors, and non-conductors, 22 armature, 52, 55, 58 in magnetic field, 5G, 58 Connections, terms employed, 31 Conservation of energy, 67 Contact surfaces, 37 Continuous current, 49 Controller, arcing inside, 190 barrels, 156 blow-out coil, 5, 156 diagram of connections, 155, 194, 195, 198 functions of, 154 handles, interlocking of, 64, 156 motor cut-outs, 157, 189, 190, 197 Controller operations, 157 paralleling, early, 162 late, 160, 162 passing into full parallel, 159 full series, 158 running on full series, 160 Controller types, B.T.H. B13..64, 156, 157, 184, 188, 189 192 B.T.H. B18, 5, 156, 157, 184, 188, 189, 192 B.T.H. K2, 153 B.T.H. KIO, 156, 157, 189, 192 Dick-Kerr D.B.I., 156, 157, 184 Dick-Kerr D.E.I., 189, 192 Controller types — contd., Westinghouse 90M., 156, 157, 189, 192 Westinghouse 210.. 156, 157, 192 Coulomb, 21, 70, 72 Cumulative winding, 119 Current of electricity, 20, 21 alternating, 48 complete circuit for, 3, 20, 48 conditions for flow of, 20 continuous, 49 density in cables, 37, 86 rails, 31 effects due to, 20, 23, 24, 25 Curves of magnetization, 18 Cut-outs, automatic. See Circuit Breaker motor controller, 157, 189, 190, 197 D Demagnetizing action, 54, 125 effects in dynamos, 54 motors, 141 Derailed car, 179, 187 Diagram of, car circuit, 4 controller connections, 155, 194, 195, 198 traction circuit, 2 motor test, 131, 132, 145, 146 Dielectric, 22 Difference of potential, 21, 39, 40 Differential winding, 119 Direction of current, 31 test for, 14 Distinction between, distance, speed, and acceleration, 90 force, energy, and power, 70 load and power, 123 torque and power, 123 Distribution, general principle, 1 Draw-bar pull, 90, 97, 99, 114, 116, 117, 143, 205, 206 and B.H.P., 98, 106. 114, 116 and current, 103, 108, 116, 143, 147, 150, 152 and load, 123, 144 and torque, 102, 106, 114, 117, 205, 206 formulae, 106, 107, 108 measurement of, 105 under varying conditions, 108, 109, 110, 111,144, 147,150, 152 value varying with, 103, 143, 147, 150, 152 with series motors, 145-162 with shunt motors, 151, 166 Driving force in motor, 58 Drop, 29, 39, 40, 41 in armature, 30, 54 in batteries, 30, 39, 40 in cables, 37, 142 in rail return, 30 INDEX 213 Dynamo, E.M.F. value, 44 E.M.F. and P.D., 30, 54 essential features, 44 in parallel, 32, 33, 156, 198 magnetically considered, 180 polarity of, 14, 31 principle of, 43 right-hand rule, 45 simple, 47 types of, 51 Dynamo, characteristic properties of, 52 compound, 53, 54 separately excited, 51 series, 53, 54 shunt, 52, 53 E Earth a conductor, 23 Earth switch, 3, 7, 32 Earth wire, 5 Earthed return, 3 Earth's magnetism, 10 Eddy currents, 52 eflfects due to, 142 loss due to, 140 Efficiency, 72, 76, 82, 83, 86, 116, 117, 200, 206, 207, 208 Electricity, conditions for flow, 20 conductors and non-conductors of, 22 current, pressure, and quantity of, 21 generator of, 21 resistance to flow, 21 Electrolysis, 25, 31 Electromotive force, 30 and P.D., 30, 39, 40, 54 back, 40, 60, 62, 65, 66 of dynamo, 44 Electro-magnet, 12, 16, 19, 51, 176, 185 Electro-magnetism, 14 Energy, 67, 82, 83 conservation of, 67 consumption of, 82, 84, 163 conversion of, 68, 99, 110, 112, 162, 171, 172, 186, 188 converted in motor, 123, 138, 139, 141 electrical, 70, 82, 83, 84, 163, 200, 206, 207,208 expended in traction, 90, 162, 171 Energy, kinetic, 99, 171, 173, 200-208 calculation of, 98, 173, 174, 175 conversion of, 110, 112, 172, 186, 188 recovery of, 162, 200, 206, 208 Energy, potential, 99, 112, 117, 162, 16:-, 171, 172, 201, 205, 206, 207 recovery of, 117, 162, 172, 206, 207 Excitation of field, 51, 118 Examples, 39, 65, 82, 112, 200 . F Feeder boxes, 3 cable and panel, 2 return, 31 Field, building up of, 53, 55, 188, 194, 196, 198 changing device, 164, 170 circuit, making and breaking, 137, 145 coils and magnets, 31, 51, 53, 188 excitation of, 51, 118 intensity, 17, 55, 58, 124, 125 magnetic, 11 residual, 53, 54, 188, 195, 196, 198 Field strength, efiects of varying, 53, 131, 133 weakening, 119, 128, 131, 133 method of varying, 53, 125, 135, 165 Flow of electricity, 20 Flux, magnetic, 17 Foot-lbs. of work, 67, 71, 82, 83 and H.F., 69, 70 Formula, 28, 92, 96, 106, 107, 108, 139, 173, 174, 175 Friction, 92 coefficient of, 93 effects of, in traction, 109, 110, 111 energy expended in, 68, 94, 98, 99, 110, 112, 162, 171,202-208 rolling, 94 sliding, 92, 94 value varying with, 93 Frictional losses in motor, 138, 140 resistance to traction, 94, 95, 114, 116, 143, 201-208 Fuses, 38 G Gearing, 100 Gear ratio, 100, 103, 114, 206 Generator, electric, 21 Gradient, 96 for given acceleration, 97 speed attained on given, 97, 117, 174, 201, 202, 203 work done in mounting, 96, 98, 99 Gravitational force, 95, 96, 98, 112, 114, 116, 117, 201 H Heat, generated in braking, 110, 112, 172, 176, 178, 180, 185, 186, 188 generated in traction, 99, 110, 112, 162, 171 quantity and temperature, 24 Heating effects, in dynamo and motor, 63, 70, 123, 138, 142, 185, 186, 189 p 3 214 INDEX Heating effects — contd., of a current, 20, 23, 32, 176 watts expended in, 84, 139 Horse-power, 69 and watts, 71, 82 brake, 73, 74, 79, 82-86, 106, 114, 141 electrical, 71, 72, 76, 82, 83, 86, 117 expended in traction, 98, 106, 107, 108, 114, 116 load and torque, 123 Horizontal effort, 100, 102, 143 Hysteresis, 13 effects due to, 142 loss due to, 140 Incandescent lamp, 24, 39 Inductive windings, 137 Input of motor, measurement of, 72 self-regulation, 119 Instruments, measuring, 23, 25, 34, 72 Insulating materials, 23 Insulators, 22 JoHNSON-LuNDELL regenerative system, 165, 167, 168 Joule, the, 70 K Kilowatt, the, 71, 72 Kinetic energy, 99, 171, 173, 200-208 calculation of, 98, 173, 174, 175 conversion of, 110, 112, 172, 186, 188 recovery of, 162, 200, 206, 208 Lamination of armature core, 52 Late paralleling, 160, 162 Leakage, magnetic, 14 to trolley standard, 7 Left-hand rule, 59 Lighting of car, 8 Lightning arrester, 3, 5 action of, 6 Lightning discharge, 6 Lines of force, 13 Live wire, 7, 8, 20 Load and power, 123 Losses in motor, 138 M Magnet, bar, 9 electro-, 12, 16, 19, 51, 176, 185 poles, 10 Magnet — contd., permanent, 12 properties of, 9 Magnetic, attraction and repulsion, 9, 10 blow-out, 5, 58, 156 circuit, 18 effects due to a current, 14, 56, 58 field, 11 flux, 17 force, 11, 13 hysteresis, 13, 140, 142 induction, 10, 11, 16, 185 leakage, 14 lines of force, 13 materials, 9 permeability, 16 polarity, 11 retentivity, 12 saturation, 17, 54, 130, 137, 148, 166, 186 screening, 19 Magnetism, earth's, 10 electro-, 14 residual, 12, 53, 54, 186, 188, 196, 197 theory of, 13 Magnetization, curves of, 18 methods of, 12 Magnetizing force, 16, 18, 124 Megohm, 22 Milliampere, 89 Motor, continuous-current, action of, 80 analysis, 124 back E.M.F., 60, 62, 65, 66 constant-speed, 119 construction of, 58 controller cut-outs, 157 driving force in, 58 effective H.P. and torque, 141 energy converted in, 123, 138, 139, 141 essential features, 57 left-hand rule for, 59 load and power of, 123 load-limiting factors, 142 losses, 138 main switch for, 5, 63 measurement of, brake H.P., 74, 75, 106 input, 72 principle of, 56 simple, 61 starting of, 62 Motor, test of series, 75 traction, 131, 132, 145, 146 torque of. See Torque Motors, continuous-current, characteristic properties of, compound, 119, 138 series, 118, 127 shunt, 118, 125 INDEX 215 Motors, continuous-current — contd., classification of, 118 compound, 119 series, 118, 127, 145-162 shunt, 118, 125, 151, 166 efifects due to, field variation in, 131, 133 pressure variation in, 131, 135 secondary actions in, 141 in parallel, 151, 152, 153, 154, 159, 160, 195, 197 in series, 151, 152, 153, 154, 158, 160 rating of traction, 70, 147 reversal of, 59, 154, 156, 192, 195 self-regulation of, 119 speed regulation of, 131, 151, 165, 168 compound motor, 135 series motor, 135, 136 shunt motor, 135, 137 speed variation in, compound motor, 119, 138 series motor, 118, 127 shunt motor, 118, 125 types of, 118, 119 Motors, traction, Dick-Kerr, data of, 131, 132, 145, 146 series, 145-162 shunt, 151, 166 types employed, 145 N Negative booster, 31 Non-conductors, 22 Non-inductive resistance, 7 North-seeking pole, 1 1 O Ohm, 21 Ohmmeter, 23 Ohm's law, 25, 26, 39-42, 65, 66 Open circuit, 32 Parallel, 31 batteries in, 33 dynamos in, 32, 33, 156, 198 full, 151, 152, 153, 154, 159, 162, 192, 195, 197 motors in, 151, 152, 153, 154, 159, 160, 195, 197 notches, 154, 157, 159, 161 resistances in, 34, 41 Paralleling, late, 160, 162 early, 162 Path of current, from generating station, 3 through car, 5 Permeability, magnetic, 16 Polarity, dynamo test for, 14 magnetic, 11 Poles, magnetic, 10, II positive and negative, 31 Potential diflference, 21, 39, 40 energy, 99, 112, 117, 162, 163, 171, 172, 201, 205, 206, 207 recovery of, 117, 162, 172, 206, 207 Power, 69 and work performed, 70 electrical, 70, 72, 76, 82, 83, 84, 86, 117, 139 Pressure, electrical, 21 Principle of distribution, 1 dynamo, 43 motor, 56 Q Quantity of electricity, 21, 72 of heat, 24 R Rails, as conductors, 3, 30 bonding of, 31 thrust on, 102, 104 wear of, 104 Rating of traction motors, 70, 147 Raworth's regenerative system, 165 Regeneration, examples on, 117, 200, 206, 207, 208 Regenerative braking, 164, 176, 190 systems, 145, 162, 165, 168 Residual field, 53, 54, 188, 195, 196, 198 magnetism, 12, 53, 54, 186, 188, 196, im Resistance, and heat, 23, 32, 63, 84, 138, 176, 185, 186, 189 and temperature, 26 incandescent lamp, 39 internal, 30 non-inductive, 7 to flow of electricity, 21 to traction, 94, 95, 114, 116, 143, 201- 208 Resistances in series and parallel, 34, 41, 42 Retardation, 89, 113, 175, 203, 204 Retarding force, 91, 109, 110, 111, 117, 179, 181, 205, 206 in electric braking, 117, 176, 180, 181, 182, 183, 186, 190, 191, 205, 206 in mechanical braking, 178, 179 2l6 INDEX Retentivity, magnetic, 12 Return feeder, 31 Reversal of motors, 59, 154. 156, 192, 195 Rheostats, 3, 5, 156, 159, 184 Right-hand rule, 45 Romapac tramrail, 104 S Sand, application of, 178, 182, 183, 185, 187, 189, 191, 193, 197 Saturation magnetic, 17, 54 130, 137, 148, 166, 186 Secondary actions in motor, 141 Section feeder box, 3 insulators, 3 Separate excitation, 51 Series, 31 connecting batteries, etc., in, 32, 33 dynamo, 51, 53, 54 dynamos in parallel, 156, 198 full, 151, 152, 153, 154, 157, 158, 159, 160, 161 motor, 118, 127, 145-162 motors in, 151, 152, 153, 154, 158, 160 notches, 154, 157, 158 parallel control, 151, 167, 168 resistances in, 34, 42 Short circuit, 7, 22, 32 Shunt, 31 dynamo, 51, 52, 53 motor, 118, 125, 151, 166 regulator, 53, 135, 165 Skidding, 178, 179, 182, 183, 185, 187, 189, 191 193 Slip of wheels, 94, 102,' 193, 196 Slip rings, 47 Solenoid, 6, 15 Speed, 87, 112 and acceleration, 88, 90, 108-111, 112, 117, 161, 175, 201, 202, 203 attained on given gradient, 97,117, 174, 201, 202, 203 uniform in traction, 108, 109, 110, 111, 144, 150, 159, 160, 166 control, 163, 164, 167, 190 equivalent values of, 88 formulse, 106, 107, 174, 175 power and D.B.P. of series motors, 145- 162 power and D.B.P. of shunt motors, 151, 166 Speed regulation of motors, 131, 151, 165, 168 compound motors, 135 series motors, 135, 136 shunt motors, 135, 137 Speed and wheel revolutions, 105, 107, 114 Speed variation in, compound moture, 119, 138 Speed variation in — contd., series motors, 118, 127 shunt motors, 118, 125 Starting a car, 157 a motor, 62 resistance, 63, 63, 66, 154 switch, 63, 154 Switchboard main, 1 Tables of traction motor data, 132, 147, 150, 152 Temperature rise, 24, 123, 142 in traction motors, 70, 146 Terms, " dead," 8, 20 " live," 7, 8, 20 open circuit, 32 parallel, 31 positive and negative poles, 31 series, 31 short circuit, 7, 22, 32 shunt, 31 Test for direction of current, 14 Test of series motor, 75 traction motor, 131, 132, 145, 146 Time element, in braking, 199 in fuses, etc., 38 Theory of magnetism, 13 Torque, 76, 85, 181 and B.H.P., 79, 85, 106, 114, 123 and D.B. pull, 102, 106, 114, 117, 205, 206 and load, 121, 123 at driving axle, 101, 114, 117, 205 effective, 79, 121, 124, 141 measurement of, 79, 80 of a motor, 78, 103, 124 Torque of series motor, 79, 129 shunt motor, 127 Torques, balance of, 76 balance of, in motor, 121, 133, 135, 136 Traction, balance of forces in, 108, 110, 111, 144, 159, 160 consumption of energy in, 82, 84, 163 D.B. pull required in, 90, 97, 108, 109, 110, 114, 116, 117, 143, 147, 150, 152 energy expended in, 09, 162, 171 H.P. expended in, 98, 106, 107, 108, 114, 116 load on motors in, 144 series motors applied to, 145-162 shunt motors applied to, 151, 166 types of motors, 145 Tractive effort, 94, 95, 100, 102, 104, 143 Tractive resistance, 94, 95, 114, 116, 143, 201-208 INDEX 217 Trolley standard, leakage on, 7 Trolley wire broken, 7 " dead " and " live," 7, 8, 20 pressure at, 1 section, 3 Trunk leads, 5 U Uniform pressure in dynamo, 49 Uniform speed with motor, 120 Uniform speed in traction, requisite conditions, 108, 109, 110, 111, 144, 159, 160 with series and shunt motors, 150, 166 Volt, 21 Voltmeter, 23, 34 W Water a conductor, 22 Water resistance, 26 Watt, the, 71 hour, 72 Wattmeter, 72 Watts expended, calculation of, 72, 76, 82, 83, 84, 86, 117, 139 measurement of, 72 Windage, 141 Work, and power, 69, 70, 71 principle of, 67 rate of doing, 69, 82, 83 Work expended in, accelerating, 98, 99 mounting gradient, 96, 98, 99 overcoming friction, 94, 98, 99, 202-208 THE END PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLKS. 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