TK
ELECTRICAL ENGINEERING
LECTURE OUTLINES
HARRIS J. RYAN, Editor
CF17 1918
146
iN86
B 429372
ELECTRICAL ENGINEERING
A Course of Lectures Adapted to the Needs
of Non-Electrical Engineers
BY
HENRY H. NORRIS
Assistant Professor of Electrical Engineering
CORNELL UNIVERSITY
ITHACA, N. Y.
THE STEPHENS PUBLISHING COMPANY
1903
Copyright by Henry H. Norris
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OCT 1 7 1918
TK
है
146
N86
ELECTRICAL ENGINEERING
LECTURE OUTLINES
HARRIS J. RYAN, Editor
B 429372
ELECTRICAL ENGINEERING
A Course of Lectures Adapted to the Needs.
of Non-Electrical Engineers
BY
HENRY H. NORRIS
Assistant Professor of Electrical Engineering
CORNELL UNIVERSITY
ITHACA, N. Y.
THE STEPHENS PUBLISHING COMPANY
1903
Copyright by Henry H. Norris
+
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ELECTRICAL ENGINEERING
LECTURE OUTLINES
HARRIS J. RYAN, Editor
ELECTRICAL ENGINEERING
A Course of Lectures Adapted to the Needs
of Non-Electric Engineers
BY
HENRY H. NORRIS
Assistant Professor of Electrical Engineering
CORNELL UNIVERSITY
ITHACA, N. Y.
THE STEPHENS PUBLISHING COMPANY
1903
Copyrighted by Henry H. Norris. All rights reserved
TH
146
N86
Reclass
11-15-39
явив
TABLE OF IMPORTANT SYMBOLS AND
ABBREVIATIONS.
B. . . . . . density of magnetic flux or induction.
B...
C. . . . . . electrostatic capacity.
e. m. f. . .electromotive force in volts.
E...
e.
.e. m. f., effective value.
.e. m. f., instantaneous value.
F. . . . . . mechanical force.
f. . . . . . .frequency in cycles per second.
H...
.magnetomotive force in gilberts per cm.
I... . . . current in amperes, effective value.
i..
j. . .
.current, instantaneous value.
-I.
L. . . . . . inductance in henrys.
m. m. f. . magnetomotive force.
μ.
magnetic permeability.
P. . . . . . electric power.
Φ.
.total magnetic induction or flux.
Q. . . . . .quantity of electricity.
r, R.... electric resistance.
R.
t..
..
•
0 ..
W…..
x.
Z.
magnetic reluctance.
time in seconds.
angle of phase difference.
electric energy or work.
reactance.
. . impedance.
3
ELECTRICAL ENGINEERING.
INTRODUCTORY.
This outline gives roughly the form and order of a lecture course
planned to cover the essential features of electrical engineering for
the purposes of the mechanical or the civil engineer. The subject
is one which cannot, at present, be covered by a text book on
account of the rapid development along all lines of the application
of electrical machinery for engineering purposes. All engineers
must meet electrical problems and they must be able to apply to
these a judicial skill, in order to select properly the electrical
machinery or apparatus which is needed and to apply it in such a
manner as to bring about the most satisfactory results. For this
reason the course consists in the study of electrical problems from
the operating standpoint, including only such introductory matter as
is essential to a proper understanding of the subject as a whole.
This pamphlet is intended as a general outline upon which a
complete set of notes can be based. Its purpose is also to empha-
size certain fundamental points which must be correctly understood
and which should be committed to memory. It is advised that
note books of letter-sized paper be used and that the notes be
systematic and concise. A general index made up by the stu-
dent and placed at the end of the book is profitable in the mak-
ing, and, at the same time, renders the material of the book
immediately available. It is also advised that these note-books
be kept up-to-date by revision, as practice changes and fresh infor-
mation appears.
No bibliography is printed in the syllabus as books on this subject
are, as à rule, of a transitory nature. A number of very excellent
books will be referred to from time to time.
3.
The Electric Current.
In order of importance the uses of the electric current may be
roughly grouped as follows:
I. Transmission of mechanical power.
2. Production of light.
3. Electrolysis.
4. Transmission of intelligence.
5. Production of heat.
For these purposes current is available in three forms:
1. Alternating current.
2. Uniform continuous current.
3. Pulsating continuous current. (Rectified alternating current.)
Fields of Application of Alternating and Continuous Current
Although many classes of work can be performed equally well by
either alternating or continuous current, each is peculiarly adapted
´to certain ones. This division of the fields of application, while
not rigid, can be seen by a study of the following table.
Alternating Current.
1. Power transmission over large or small areas.
2. Incandescent lighting over large areas.
Transmitted at high pressure and transformed to low
pressure by means of constant potential static trans-
formers.
3. Arc lighting.
In many locations there is not enough arc lighting
business to warrant the use of continuous current arc
generators. In others the a. c. lamp is preferred
for its economy and convenience. For these cases
alternating current arc lamps, operated in series on
a constant potential circuit and with current regulat-
ing devices in series, are very satisfactory. The lamps
may also be operated in parallel in which case, a lamp
adjusted for operation on constant potential would
be employed.
4
4. Constant speed motors and in a few cases variable speed
motors.
5. Electrical furnaces.
Such as are used for the manufacture of carborundum,
graphite, etc.
Continuous Current.
1. Incandescent lighting.
Over small areas, especially in the crowded districts
of large cities, in which the pressure neccessary for
the lamps can be used economically in the transmis-
sion.
2. Arc lighting.
On
The continuous current is well adapted to arc light-
ing, pulsating current being generally used.
account of the superiority of the alternating current
from the transmission standpoint, the continuous cur-
rent arc lamp is being gradually driven from the field.
3. Variable speed motors.
Practically all of this line of work, which includes
electric traction, cranes, etc. is best served by the
constant potential series, continuous current motor.
4. Constant speed motors.
Such as would be used in factories for the driving of
tools and of line shafts and for general power pur-
poses in the crowded districts of large cities. While
called constant speed these motors may be varied in
speed by means of various devices, but when the
speed has been set thus, it is maintained regardless of
the load. In the previous class of motors mentioned,
the speed varies with the load.
5. Electrolysis.
The continuous current finds an excellent field in
the reduction of metals and the production of many
chemical compounds very cheaply. In the storage
battery a very useful adjunct to lighting and power
stations is found.
Historically the application of the continuous current to these various uses was first, but in importance
it may be said that now the alternating current is first. The latter is more easily produced as the generator
is simpler. It owes its superiority, however, to the fact that it may be transformed in pressure in station-
ary apparatus, with practically no loss. This important point makes possible the transmission of elec-
tric power over long distances and it has forced the manufacturing companies to develop lines of ap-
paratus, in all directions possible, which can be used on alternating current circuits.
5
Classification on the Basis of Pressures Employed.
Alternating Current.
I. 1,000 to 3,000 volts, constant potential, usually single phase.
Applicable to arc and incandescent lighting. For the
former a constant current is maintained by a constant
current transformer or a constant current regulator.
Or, either arc or incandescent lamps can be operated
in parallel on from 100 to 250 volts, constant poten-
tial, reduced from the line pressure in constant poten-
tial transformers.
2. 2,000 to 7,000 (or more) volts, constant potential, usually
three-phase.
This is used for power transmission purposes under
ten miles, particularly for power distribution for rail-
ways in large cities. For this purpose the alternating
current is transformed to continuous current in rotary
convertors located in sub-stations from which the
power can be economically transmitted at 500 volts
pressure.
3. 6,000 to 80,000 volts, constant potential, generated two-phase
or three-phase, but transmitted on a three-phase line.
Transformation from two-phase to three-phase is
easily accomplished in static transformers. These
high pressures are used for power transmission pur-
poses up to several hundred miles, the power being
generated at a moderate pressure of about 10,000
volts which is raised to the pressure desired for the
line by means of constant potential transformers.
4. Miscellaneous pressures used for electro-chemical and elec-
tro-metallurgical purposes.
This is usually generated at a fairly high pressure and
is then stepped down to any pressure desired and
transformed to continuous current for electrolysis.
This depends upon the process used and the num-
ber of tanks connected in series.
Continuous Current.
1. 6 to 350 volts, constant potential for electrolysis.
This includes the reduction of metals and the forma-
tion of various chemical compounds.
6
2. 50 to 550 volts, constant potential used for incandescent
lamps and constant potential arc lamps.
The standard pressures are from 104 to 250 volts for
the two-wire and from 208 to 500 for the three-wire
system.
3. 110 to 550 volts, constant potential used for motors of both
constant and variable speed.
In addition to this classification upon a pressure basis there
should be mentioned the constant current system, both alternating
and continuous current. This is popular for arc lighting especially
for out-of-door purposes. From three to ten amperes are used for
this purpose. Occasionally constant current motors are used on
the continuous current circuit, but they are not satisfactory and
their use is to be disparaged except where no other source of cur-
rent is available.
7
PART I. FUNDAMENTAL FEATURES OF ELECTRIC
AND MAGNETIC CIRCUITS.
The Electric Circuit.
The electric circuit is a molecular kinematic connection between
the power generator and the receiver. Mechanical power can be
transmitted by this means over long distances, much greater than
would be practicable by any other means of connection.
The electric circuit is analagous to a belt transmitting power, the
velocity of the belt corresponding roughly to the current and the
tension to the pressure. For the transfer of power the circuit must
be complete, which it may be in one or more of the following
ways.
1. Through a conductor.
In this case the energy of the current is all dissipated
in heat unless some part of the conductor is the seat
of an electric pressure, when it will come under case 3.
2. Through an electrolyte and a conductor.
Here all of the energy of the current produces chem-
ical dissociation in the electrolyte, except that portion
lost as heat in the conductor. The dissociation is
accompanied by the presence of a counter-pressure in
the electrolyte.
3. Through a motor and a conductor.
In a motor the transformation of electrical into me-
chanical power is accompanied by the production of
a counter-pressure.
4. Through a dielectric and a conductor.
If a dielectric forms part of an electric circuit a cur-
rent will flow through the circuit until the dielec-
tric produces a counter-pressure exactly equal to the
line pressure. In order to maintain a continuous flow
8
of current through such a circuit either an alternat-
ing pressure must be used or the direction of connec-
tion of the dielectric in the circuit must be period-
ically reversed.
It will be noted from the preceding statements that, except in the
production of heat, the transformation from electrical into some
other form of energy is accompanied by the presence of a counter-
pressure which is the evidence of a resistance to the change of form.
In the case of heat there is no such resistance, as heat is the lowest
form of energy and the more highly organized forms tend to reduce
to this one.
Conditions and Properties of the Electric Circuit.
Electric circuits, which are composed of materials having certain
mechanical properties, and existing under definite conditions of tem-
perature, strain, etc., may also exhibit manifestations entirely dis-
tinct from these. For example, under certain conditions such a cir-
cuit will be urged from a space known as a magnetic field. Differ-
ently arranged the conditions of this circuit may be such as to pro-
duce chemical dissociation or it may become very hot from some in-
ternal source of energy. Evidently we must have some system for
defining and relating these conditions. The relations of these con-
ditions are the measures of the properties of the circuit. Each con-
dition must be studied from the peculiar results produced. While
several different kinds of results may be produced from the same set
of electrical conditions, we shall here treat the mechanical
effects as fundamental and others as secondary. For this pur-
pose it is necessary to base the proposed system upon a con-
dition of space known as the magnetic field of flux.
The Magnetic Field.
For all practical purposes the magnetic field is a space in
which a mechanical tension exists. While this tension is not
in itself the field it has a definite relation thereto. From other
considerations the unit of magnetic flux has been selected as
that condition of a space in which a tension of 1÷8″ dynes exists
over each square centimeter of cross-section. This tension
may be measured between its terminal surfaces known as poles
where the flux disappears in magnetic material and is known
9
therein as magnetic induction. The unit thus defined is known
as the Maxwell. By virtue of the form of other electrical defi-
nitions the pulling strength of the field is proportional to the
square of the density of the flux and to the cross-sectional area
of the field. That is,
F =
B2 × A
8 π
Where F is the tensile force in dynes
B is the flux density in maxwells per sq. cm.
A is the area of the flux in square centimeters.
A useful form of this expression is obtained by dividing by
445,000 the number of dynes in a pound, when
B2 × A
F' (in pounds)
8 π X
445,000
Conditions of the Electric Circuit.
Electro-motive force (e. m. f.)
When a conductor is moved across a magnetic field there
is produced in it a tendency to set up an electric current. This
condition is found to be proportional to the density of flux and
to the velocity of cutting.
The e. m. f. produced by the cutting of a field of flux density of one
maxwell per sq. cm., at a velocity of one cm. per second (in a direction
normal to the field and to the conductor) by one cm. of conductor, is the
unit of e. m. f. in the c. g. s. (centimeter-gram-second) system.
100,000,000 times this is the Volt.
Electric Current.
When a conductor is urged from a magnetic field, the con-
dition of the conductor under which this occurs is known as the
presence in it of an electric current.
In the c. g. s. system the unit of current exists when each centi-
meter of a conductor (located in a plane normal to the field) is urged
from a field of one maxwell per sq. cm. flux density with a force of
one dyne.
One-tenth of this unit is the Ampere.
Quantity of Electricity.
When a current flows for a length of time a definite amount
of effect is produced. This effect is proportional to current and
IO
time and this product is therefore known as the quantity of
electricity.
The ampere-second is the Coulomb.
A more practical unit for engineering purposes is the ampere-
hour.
Electric Power.
When current and e. m. f. are present in a circuit at the
same time it is found that a transformation of energy is taking
place, the value of which is proportional to the product of these
quantities. In fact, since (in a magnetic field) e. m. f. is pro-
portional to velocity, and current to force, this is already evi-
dent from the definitions of those quantities.
The volt-ampere is the Watt of which 746 are equivalent to
one mechanical horse-power.
The more usual form of the unit is the kilo-watt.
Electric energy.
The duration of electric power gives rise to electric ener-
gy, this being equal to the product of power and time.
The watt-second is the Joule.
The commercial unit is the kilo-watt-hour (K. W.-hr.), an
older form being the electrical horse-power-hour. (E. H. P.-hr.)
and in the use of very large quantities of energy the E. H. P.
-year is sometimes adopted.
Properties of the Electric Circuit.
Electrical properties are discovered and defined by certain
relations of conditions. There are three electrical properties as
follows:
1. That, by virtue of which a uniform current in a circuit
produces a proportionate loss in electric pressure.
This is known as Resistance. The volts lost per ampere are
the Ohms of resistance.
E
R =
I
Other commercial units-milli-ohm, megohm.
This property is analagous to mechanical resistance.
+
II
2. That, by virtue of which a changing current produces an
e. m. f. which is proportional to its rate of change.
This is called Inductance and the volts per ampere per second
are the Henrys of inductance.
E
L = di
dt
Another commercial unit is the milli-henry.
Inductance is closely analagous to mechanical inertia.
3. That, by virtue of which a changing e. m. f. produces a
current which is proportional to its rate of change.
This is the Capacity and the amperes per volt per second are
numerically equal to the Farads of capacity.
C
I
de
dt
The micro-farad is the only commercial unit as the farad is
entirely too large for practical purposes.
Capacity is almost exactly analagous to mechanical elasticity.
Electric Current Control.
The flow of current through a circuit is determined only
by the resistance and the counter e. m. f. present. All circuits
possess resistance which consumes e. m. f. in proportion to the
current.
I
E*
R
The e. m. f. consumed in overcoming a counter-e. m. f. is
exactly equal and opposite to it. Such counter-e. m. f. may be
produced in several ways and as resistance is always present
the total consumed e. m. f. in these cases is as follows:
In a motor (transformation to mechanical energy),
EE' IR
In an electrolyte (transformation to chemical energy,
E = E' + IR
*See equation under definition of resistance.
12
In a circuit containing inductance (transformation to and
from potential energy in the magnetic field),
E = L + IR
di
dt
In a circuit containing capacity (transformation to and
from potential energy in the dielectric),
E =
Sidt
+ IR
C
In a circuit containing inductance and capacity.
E = L
di _ Sidt
+ IR
dt
C
Note that the e. m. f.s produced in inductance and capac-
ity are opposed to each other.
It will be evident from the above that with the continuous
current we are not concerned to any extent with the e. m. f.s
produced by change of current. In alternating current work
these are of the greatest importance.
Alternating Current Control.
An alternating quantity is one which periodically reverses its
direction, reaching equal positive and negative maximum values.
By adapting the apparatus to the nature of these alternating
quantities, continuous effects may be produced. A familiar ex-
ample of this is found in the steam engine where the alternat-
ing pressure on the piston is made to produce continuous rota-
tion of the main shaft.
The series of values of an alternating quantity between positive
to negative maxima constitute an Alternation and two successive
alternations, a Cycle. The time of a cycle is a Period and the
number of cycles or alternations in a given time is the Freq
uency. The two common forms of frequency are cycles per
second (often written p. p. s. as an abbreviation for periods
per second) and alternations per minute. Commercial fre-
quencies range from 15 to 133 cycles per second in engineer-
ing practice.
Effective Values of Alternating Quantities.
As it is essential that alternating e. m. f. and current be
13
measured it is necessary to select some value of these quanti-
ties which shall be suited to this purpose.
Naturally such values of these quantities have been chosen
that an alternating current ampere is such as will heat a wire
to the same extent as will an ampere of continuous current.
Similarly an effective alternating volt is such an e. m. f. that,
impressed upon the terminals of a circuit containing resistance only,
that circuit would be heated to the same extent as it would have
been by a continuous volt. An effective alternating volt will
send an effective alternating ampere through one ohm of resist-
ance.
Alternating Power.
The product of the effective alternating volts and am-
peres in
an alternating current circuit will not necessarily
be the power of the circuit, for it is possible that the maximum
values of current and e. m. f. may not occur at the same time.
In fact, when the two have the same frequency, as is generally the
case, it may happen that one will be always zero when the other
is maximum and in general no power would then be produced.
The quantity by which the effective e. m. f. and current in a cir-
cuit must be multiplied in order to give the watts developed, is
called the Power Factor.
P. F.
watts
volts × amperes
Here and hereafter whenever alternating volts and amperes
are referred to, the effective values will be understood.
Time Relations of Alternating Quantities.
When alternating quantities have the same frequency they
are said to be synchronous. When similar values of these
quantities occur at the same time they are in phase, otherwise
they are out of phase. When quantities are synchronous and in
phase their relation is often referred to as one of exact synchron-
ism.
Geometric and Algebraic Conventions.
For purposes of study, alternating quantities may be treated
by one or more of the following plans:
1. Graphically, "point-by-point.”
14
2. Graphically, by use of vectors, assuming sine law con-
necting variables.
3. Algebraically, by use of vectors, assuming as in (2).
4. Algebraically, by means of Fourier's Series.
All of these are in use and all are good, but the particular
plan to be used in any case must be selected with reference to
the circumstances and to the degree of accuracy desired.
1. Point-by-point method.
A simple and practically accurate handling of alternating
current problems consists in plotting the variables in rectangu-
lar co-ordinates, time being used for abscissæ. The laws of the
electric circuit may be applied to enough ordinates to insure.
reasonable acuracy.
2. Geometrical method.
Frequently it is found that diagrams plotted as above, corre-
spond closely with those of the forms resulting from the appli-
cation of the equation,
y= A sin x
where A is the maximum value of the alternating quantity and
where a value of x equal to 27 radians corresponds to one com-
plete cycle of the alternating quantity. The equation may
then be written
2T
y = A sin t
T
Where T is the period of the alternating quantity. As I÷T
if the frequency (f) we may write
y = A sin 2″ ƒ t
y may represent any variable electrical or magnetic quantity
such as current, e. m. f., magnetic flux, etc.
It is evident that this sine curve would be projected upon
an axis of reference by a rotating radius revolving with a con-
stant angular velocity of ƒ revolutions per second and with a
length equal to A.
Two or more such rotating radii of the same angular velocity
would project sine curves of which similar values would differ
in time by definite fractions of a period, which are known as
phase differences. Phase differences may be referred to either
15
in terms of a fraction of a period or as the corresponding angle
between the projecting radii.
In applying the above to alternating current problems, which
usually involve effective rather than maximum values we note
that there is a fixed relation between effective and maximum
values of any particular form of wave. For the sine curve this
Effective values may be represented by lines
I
is the ratio√
having magnitude and direction, for while an effective value
cannot have a definite direction in space it has a relative direc-
tion as compared with some other effective value of a quantity
of the same frequency. Hence for all practical purposes effec-
tive values are considered as vectors. Then for the purpose of
solving problems we may graphically represent effective values
by lines drawn to scale and differing in direction by the angles
which separate the generating radii of the curves. Upon such
lines all ordinary geometrical operations may be performed.
3. Algebraic method.
This is based upon the assumption made in method (2). The
vectors are decomposed into two rectangular components by
projecting upon reference axes. These may be arbitrarily
chosen but one usually coincides with one of the vectors and
this reference vector for convenience, is usually given a hori-
zontal direction. A vector may be represented by the formula,
a + jb
2
where its length is Va² + b² and the angle between the vector
and the axis upon which a is projected is
0 =
tani
b
a
The symbolj indicates a quadrature relation of a and b. By
convention, jor +j is prefixed to a quantity 90° ahead of
another in an anti-clock-wise direction, while -j is prefixed to
the reverse of this. The name complex quantity is given to one
containing the symbol j.
By means of these conventions all ordinary operations can be
performed upon vectors in their algebraic form and this is the
common practice at present.
1
16
4. Algebraic method with Fourier's Series.
The reliability of methods (2) and (3) depends upon the
accuracy of the assumption that sine curves may replace the al-
ternating quantities. If exact accuracy is desirable it may be
obtained by writing the equation of the alternating quantity in
terms of a series based upon the discovery that alternating quanti-
ties may be decomposed into component sine curves of different
frequencies. Any alternating quantity may be written in the form
+
1
3
y A₁ sin (x + α1) + A, sin (3x + a3) + A 5 sin (5x + az)
+ An sin (nx + αº)
where the subscripts indicate the frequencies of the harmonics
compared with that of the fundamental component, and the angles
21, α, etc., indicate the angular displacement of the components
of the irregular curve when x is xero.
For practical purposes but three terms are usually found neces-
sary for substantial accuracy. Evidently the various components,
being sine waves, may be treated by methods (2) and (3) and the
results can be combined to give the complete result.
It should be noted that while method (4) is the only absolutely
correct one, it is tedious of application and for commercial pur-
poses the preceding plans give good satisfaction. For purposes of
exact analytic study the last plan must be used.
Impedance and reactance.
In an alternating current circuit e. m. f. may be consumed in
other ways than in producing heat (in resistance) and in over-
coming mechanically or molecularly-produced e. m. f. s. Aside
from directly useful consumption of e. m. f., a certain. amount will
also be necessary to send current through inductance and capacity.
From the definitions of these properties it follows that
di
CL = L
dt
ic
de
C
dt
From the properties of the sine curves it follows that
eL
= 2π f Li
i
ec
2π ƒ С
17
The qualities 27 fl and I÷ 27 ƒ C are known as Reactance,
which is therefore numerically equal to the number of volts con-
sumed per ampere in inductance, in capacity or in both. From
the nature of the mechanism of these properties they produce
e. m. f. s. which oppose each other, hence they tend to neutralize the
effect of each other in a circuit.
Impedance is the combination of resistance and reactance.
With sine-form currents and e. m. f. s. reactance and resistance con-
sume e. m. f. s. a quarter period apart in phase position. Hence
Where
Z
2
√x² + x
z-impedance
r=resistance
x=reactance
2
In the solution of alternating current problems, impedances may
be combined in series, in parallel or in series-parallel. In the solution
of such problems care must be taken to include the effect of phase
relations upon the results, as these greatly affect the magnitude of
the absolute values. A large number of such problems should be
solved by a student of this subject and it is recommended that both
the geometric and the algebraic methods be used, as one forms a
check upon the other. It is also very difficult to perform division
of complex quantities algebraically, but by obtaining the absolute
values of the complex quantities and dividing these, much labor is.
saved and the quotient may be readily decomposed into its com-
ponents, forming the corresponding complex quantity.
Polyphase Circuits.
In engineering, for power purposes it is found most satisfactory
to use two, three or even more alternating currents which differ in
phase position. By this plan it is possible to employ self-starting
motors and at the same time copper in the line may be saved. Any
desired number of phases may be employed and these may differ in
phase position by any fraction of a cycle, but certain values are
found to be most useful commercially.
No. of wires used.
Number of phases
2*
3*
4
6
12
Phase displacement
1-4
1-3
1-4
1-6
1-12
*These are the currents used for power transmission purposes.
4 or 3
3
4
6
12
18
In this table the first current would be a "2-phase, quarter-
phased current," the second a "3-phase, third-phased current" etc.
Fortunately transformation from one number of phases to
another is accomplished easily and at high efficiency as will be de-
scribed in connection with transformers.
The copper economy of a transmission circuit varies with the
variety of current selected, the comparison being based upon equal
maximum pressures between wires. Three phases are almost in-
variably employed for long distance transmission.
The Magnetic Circuit.
A magnetic circuit is the path of the magnetic flux and induc-
tion. The direction of this path is controlled by the disposition
of the magnetomotive force (the name given to the cause of the
magnetic field) and to the nature and arrangement of the materials
composing it.
Sources of M. m. f.
There are two commercial sources of m. m. f.:
1. Certain substances known as magnetic materials. The
m. m. f. is an inherent property of the molecules of these sub-
stances. Ordinarily this m. m. f. is not externally apparent as it
usually neutralized within the mass of the material except when
controlled by a force externally produced.
2. The electric current. This produces m. m. f. in circu-
lar paths about the conductor, in planes normal to it. This
m. m. f. is proportional to the current. It is conveniently produced
by this means, which is the only practicable one for utilizing the
inherent m. m. f. of magnetic materials.
Varieties of Magnetic Circuit.
A magnetic circuit may be composed as follows:
a. Magnetic materials only, e. g., a transformer core.
b. Non-magnetic material only, e. g., a lightning arrestor
choke coil.
3. Magnetic and non-magnetic materials, e. g., generator
armature and field cores.
19
Conditions and Properties of the Magnetic Circuit.
As in the case of the electric circuit, so in the magnetic circuit
there are certain phenomena which are peculiar to itself. The
fundamental one is that condition known as the magnetic flux.
The space known as a magnetic field exerts mechanical tension
upon its terminals or poles. This same field has the ability to
produce e. m. f. in circuits which move through it. It remains to
define these conditions and the properties by which the relations
of the conditions are known.
Conditions of Magnetic Circuits.
Magnetic Flux and Induction.
While the phenomena of the electric and the magnetic cir-
cuits are closely connected it is possible to consider the flux apart
from the current only by means of the definition previously
given.
T
That space in air in which exists a mechanical tension of 1÷8π
dyne per square centimeter of normal cross-section is said to con-
tain a unit of magnetic flux per square centimeter, in the c. g. s.
system.
The c. g. s. and practical units in this case are the same. The
unit density of flux thus defined is called the Gauss and the unit
of flux the Maxwell. No distinction is made between the flux in
the air and the induction in the magnetic materials, the same
units being used for both.
Magnetomotive Force.
That condition by virtue of which a unit magnetic flux dens-
ity is produced in a unit length of path in air is the unit of mag-
netomotive force. The c. g. s. unit, which is also in practical
use, is the Gilbert. One ampere flowing in one turn of wire sur-
rounding a magnetic circuit produce a m. m. f. of 4-10 gilberts
(1.257 gilberts).
Properties of the Magnetic Circuit.
π
The only property of a magnetic circuit as a whole is the
ratio of the total m. m. f. consumed in the circuit to the total
resulting flux. This ratio is known as the Reluctance and there
is no name for its unit.
20
There are, however, certain properties of the materials mak-
ing up the circuit which will be considered under the subject of
Magnetic Materials.
Laws of the Magnetic Circuit.
The B-H Curve.
The magnetic circuit is very similar to the electric circuit in
that there is a relation between cause and effect, called resistance
or impedance in one case and reluctance in the other. The im-
portant difference is that the reluctance is not constant nor does
it obey any simple law. Hence it is necessary to have for each
circuit or for each part of that circuit in which the same material
is used, the relation of the m. m. f. to the flux produced. This
relation is usually plotted in the form of a curve with maxwells
per unit area for ordinates and either gilberts per centimeter or
ampere-turns per inch for abscissæ. One of these "B-H" curves
is needed for each kind of material employed.
Composition of Reluctance.
The reluctance of that part of a circuit in which the same
material is used is
1 × H
R
AX B
Where
I is the length along the magnetic circuit in centi-
meters.
H is the gilberts per centimeter along the circuit.
A is the area of circuit cross-section in square centi-
meters.
B is the flux density in maxwells per sq. cm.
For any particular value of H or B the corresponding value of
Bor H is found from the B -H curve.
The ratio, H÷B is called Specific Reluctance, that is, this ratio
is the reluctance when I and A are unity. Its reciprocal is the
Permeability.
Relation of M. m. f. and flux.
From the definition of reluctance it follows that
Φ
m. m. f.
R
21
Where total flux.
Writing in a different order
M. m. f. = § × R
As stated before the m. m. f. produced by a conductor carry-
ing one ampere is 1.257, (.47), gilberts. It makes no difference
whether this conductor is straight or forms a loop around the
magnetic circuit. If however, it loops the circuit several times
the m. m. f. proportionately increased. Hence the m. m. f. of a
solenoid or coil is
m. m. f. = .4 π I n
where n is the number of loops of the electric around the mag-
netic circuit.
22
PART II. MATERIALS OF ELECTRICAL
ENGINEERING
These may be classified as follows:
1. Those used for mechanical support of electric and
magnetic circuits.
The general principles of mechanics and machine design apply
to the support of electrical conductors. Whether these form
parts of electrical machines or of transmission circuits. These
applications involve the use of all of the materials of engineering
as will be brought out in connection with the particular cases.
2. Conducting materials of electric circuits.
(1.) Those used for conducting current economically,
copper, aluminum and iron or steel.
(2.) Those used for controlling flow of current (resist-
ance materials), iron or steel, german silver, manganin, etc.
3. Magnetic materials.
Iron and steel are the only materials commercially available for
the construction of magnetic circuits.
4. Dielectric or insulating materials.
Conducting Materials.
Properties of Conducting Materials.
Specific resistance.
This is the resistance between two opposite faces of a centi-
meter cube of the material. For practical purposes resistances
are compared by reference to wires of the materials one foot long
and of circular section one one-thousandth inch in diameter. The
area of such a wire is called a circular mil and the dimensions of the
wire are covered by the term circular mil-foot or simply mil-foot.
23
The resistance of any wire is
1 x Rst
R =
A
Where
I is length in feet
Rsp is the resistance of a mil-foot
A is the area in circular mils (square of the diameter is
mils.)
The reciprocal of the resistance is the Conductivity.
Temperature Co-efficient.
The resistance of all metals varies with temperature to a con-
siderable extent, the variation for pure metals being a rise of about
four-tenths per cent per degree C. The temperature coefficient,
or proportional (not per cent) rise in resistance per degree, may
be reduced by combining different metals. These facts are sum-
marized in the following table.
TABLE OF SPECIFIC RESISTANCES AND TEMPERATURE
Material.
Copper (soft, pure)
Aluminm (soft, pure)
Iron (soft, pure)
German Silver
Manganin
COEFFICIENTS.
Resistance of round
wire I ft long, .001
in. diameter at oo C.
9.612
16.02
60.00
127.00
291.00
Temperature
Coefficients.
.004284
.0039
.00453
.00044
10000*
Copper. Best conductivity, smallest surface, can be hard drawn
without great loss of conductivity. Used for indoor and outdoor
work where insulation is needed and where space must be econo-
mized, and for conductors in electrical machinery.
Aluminum. Light weight; used for transmission lines which are
not insulated.
Iron. Used sometimes in the form of steel as feeders and prac-
tically always as street railway return circuits on account of dura-
bility and large surface exposed. As street railways must have
the rails these are used for return as a matter of course.
As a resistance material iron is very satisfactory as it exposes a
considerable surface for radiation and is cheap.
24
German Silver. High specific resistance. Fairly low tempera-
ture coefficient. Used in resistance boxes to some extent.
Manganin. High specific resistance, low temperature coeffi-
cient. Used for resistances and shunts for measuringinstruments.
Carbon. High resistance. Used for resistances, and for
brushes for electrical machines.
Magnetic Materials.
Properties of Magnetic Materials.
1. Permeability.
This is numerically the number of maxwells per square centi-
meter produced per gilbert per centimeter. It is an abstract num-
ber for the above ratio may be stated as the proportionate in-
crease in the flux due to displacing air with magnetic material, as
by definition the permeability of air is unity. Permeability is not
constant and its values must be obtained by experiment for each
sample of material and for each flux density.
Commercially, values of µ range from upwards of 5,000 to 100
or even less. An average range would be from 500 to 1,000 for
steels and wrought iron and one-half this for cast iron. A large
number of considerations enter, sometimes dictating a high and
sometimes a low value of μ.
2. Hysteresis coefficient.
As a magnetic material is subjected to alternating magnetic
flux a certain amount of energy is lost per cycle in addition to
that dissipated in the electric currents set up within it. This loss
varies with the density of magnetic flux, increasing with the one
and six-tenths power of the increase of B. (Steinmetz's law.)
The coefficient is the energy loss per cycle per gauss or per
maxwell per square inch per unit volume, usually a cubic inch.
Only the best magnetic materials are available for alternating flux
circuits and for these the coefficient ranges between .001 and .002
or more ergs per cubic inch, per cycle, per maxwell per square
inch.
3. Retentiveness.
When the m. m. f. is removed from the magnetic circuit there
is always a certain amount of remnant flux which requires a de-
25
magnetizing or coercive force to remove, although jarring will en-
tirely remove it in some cases and partly in others. This feature
is made use of in the production of permanent magnets but it is
objectionable in most other cases.
Variations in Magnetic Properties.
1. The presence of impurities.
Some metals when added to iron produce increased μ and
decreased hysteresis loss. Carbon and other such impurities as a
rule increase hysteresis loss and retentiveness and decreases μ.
2. Physical treatment.
Hardening decreases and increases hysteresis, while
annealing has the reverse effect. At temperatures above 200 C,
hysteresis loss is less and a greater up to 700 C. It is impor-
tant that magnetic material be not subjected to temperatures
above 60° to 80° C, for above these points hysteresis loss in-
creases with time (Aging.)
3. Magnetic Flux Density.
µ
Both and hysteresis loss vary with B. is large at small
μ
values of B. reaches a maximum and finally decreases. It is sel-
dom advisable to use material at maximum μ as the circuit is too
bulky and expensive. As before stated hysteresis increases as
the 1.6 power of B.
Losses in Magnetic Materials.
Hysteresis as described above.
Wh
nx B 1.6 x V xf
107
Eddy currents or eddies. (Foucault currents).
These flow in all magnetic materials subjected to alternating
flux in accordance with the laws of the electric circuit. They
are reduced by making the structure of sheets, the lamination be-
ing in planes parallel with the direction of the field.
Where
W. =
b x B ² x V x fxt
107
W-watts lost in each case.
B=maxwells per square inch.
V volume in cubic inches.
f=frequency.
t=thickness of sheets in inches.
n and b are coefficients.
26
Features of Various Materials.
Cast iron. Cheap and easily moulded but bulky and me-
chanically weak. Used only with steady flux as in dynamo fields.
Cast steel. Expensive but has good magnetic properties and
is mechanically excellent for steady flux as in motor and dynamo
field frames, rotating field structures, etc.
Electrical Steel. Only material now used for laminated
structure such as armature cores, transformer cores, field poles,
etc. Excellent magnetic properties, very ductile and mechani-
cally strong enough for its purpose. It is practically pure iron.
Wrought iron. Excellent mechanically and magnetically but
not easily formed for magnetic circuits. Hence, little used ex-
cept as electrical steel.
Nickel steel.
Fine in every way for steady flux but very
expensive and only used where the great mechanical strength and
reliability combined with good electrical properties are required
and where forging is possible.
Dielectric Materials.
In order to prevent passage of current between wires which
have e. m. f. between them, it is necessary to have the space
filled with dielectric materials. The mechanical properties of
these substances are of vital importance and will be treated
under the head of construction of Electric Circuits.
Electrical Properties of Dielectrics.
1. Specific resistance.
A matter of very little importance as all dielectrics have
such a high resistance that the leakage of current is negligible.
2. Specific Inductive Capacity.
taken up by
The ratio of the quantity of electricity taken
a given volume of the dielectric to that taken up by air under
the same pressure. A very important property for a. c. pur-
poses.
3. The dielectric flux constant.
The coulombs absorbed by an inch cube of the material
with one volt pressure between opposite faces. As the con-
stant for air is 2.244 x 10 −13 coulombs, that for any other ma-
terial can be found if the specific inductive capacity is known.
27
4. Electric pressure rupturing gradient.
The number of volts per inch of thickness necessary to rup-
ture the material. This is the most important propertý
of dielectric materials.
The following tables show clearly the electrical properties
of the various commercial dielectric materials. In addition the
other physical properties must be given due weight. Each sub-
stance must be selected for its particular purpose with the fol-
lowing considerations taken into account:
a. Mechanical fitness, including strength, fluidity (if fluid),
ease of working, etc.
b. Heat resisting properties.
c. Durability under action of weather or of chemicals.
TABLE OF RUPTURING PRES-
SURE GRADIENTS
TABLE OF SPECIFIC INDUC-
TIVE CAPACITIES.
Dielectric
Kilovolts per
Dielectric
inch.
Air, less than .05 inch.
250
Vacuum
Air, .5 to 1 inch..
Specific inductive
capacity
.9996
28
Air.......
Air, 1 to 2 inches
25
Glass, old hard
1.00
6.96
Air, 2 to 5 inches
18.8
Glass, new hard.
2.II
Air, 5 to 10 inches....
15
Glass, clear
Glass, hard 13 m.
3.31
300
Glass, extra dense flint.....
9.9
Mica
800
Paraffined paper.
Glass, lowest value..
2.8
860
Potcelain
Paraffin, melted.
4.4
200
Shellac
2.74
Linseed oil, boiled
200
Turpentine oil
Sulphur
3.0
160
Copal varnish.
Rubber, pure
2.12
76
Lubricating oil, crude
Rubber, vulcanized
2.69
46
Rubber, hard
2.25
Vulcabeston
91
Paraffin..
Linseed oil paper
2.0
1000
Wax
1.86
Linseed oil cloth
500
Olive oil...
Micanite cloth..
Micanite paper...
3.08
440
Turpentine
2.25
325
Sperm Oil
3.09
Petroleum, crude.
2.07
Petroleum, "headlight". 2.II
Vaseline oil...
2.17
28
PART III. CONSTRUCTION AND INSTALLATION OF
ELECTRIC CIRCUITS.
Electric circuits may be classified as,
1. Outdoor transmission circuits.
2. Indoor distribution circuits.
3. Electrical machinery windings.
The selection of conductors and methods of insulation
for these classes are entirely different as this is largely settled
by the environment. Of equal importance in a circuit are:
a. The wires.
b. The method of support.
c. Switching and protective devices.
The National Electric Code.
All indoor wiring and to a certain extent outdoor work as well
are regulated by the rules of the National Board of Fire Under-
writers. These rules, known as the "National Electric Code"
are modified annually at the suggestion of the National Electric
Association. The National Board maintains in Chicago a lab-
oratory for the testing of all kinds of electrical supplies and
publishes from time to time a list of such as are approved and
only such "approved" supplies are "passed" by local inspectors.
The Bureau also collects and distributes information about elec-
trical fires and in this way emphasizes the importance of a care-
ful following of the rules of the code. The local inspectors are
appointed by various associations of insurance companies.
Wires.
a. Rubber-covered, for use in all high pressure indoor cir
cuits, in knob and tube work, in conduit work, for optional use
on indoor exposed wiring. These have a single braid except in
conduits where it is double.
29
b. Slow-burning weather-proof, consisting of a fire-proof and
a weather-proof coating. These consist of braid impregnated
with compounds having the qualities mentioned. Used for
most interior work not requiring rubber-covered wire.
c. Slow-burning, having a fire-resisting covering and not
used except in special cases where the other insulation would
not endure, or where the flames due to the combustion of the
weather-proof coating would in case of fire cause great damage.
d. Weather-proof, having three braids saturated with mois-
ture-proof compound and with outer surface "slicked" smooth
and hard. This wire is for outdoor use only.
e. Flexible cord for lamps and portable apparatus is rub-
ber-covered and braided and for special purposes is further pro-
tected by additional outside rubber and braid.
f. Armored cable. This is a convenient form of wire and
conduit combined and is treated as such.
When used in underground conduits the rubber-covered wires
are frequently encased in lead, thus affording adequate protec-
tion against moisture. This is essential when paper or yarn im-
pregnated with insulating compound displaces the rubber.
1. Out-door transmission circuits.
Matters of importance are:
Material for conductors.
Insulation of wire, if any.
Support: pole line or conduit.
Insulators and details.
Switch boards at terminals and switches along line.
Lightning protection.
Outdoor lines must be erected for durability and reliability.
Not only is the continuous working of the plant important but
the danger to life and the disastrous result of crossing with other
circuits must be considered.
The Code emphasizes such points as:
Insulation of wires and forms and materials for insulators.
Location with reference to other wires and to buildings.
Guard or protecting wires or screens.
Joints.
Grounding.
Section switches.
Method of entering buildings.
Location and installation of transformers.
$
30
2. Indoor Distributing Circuits.
Two general classes of wiring
a. Open.
b. Concealed.
1. Knob and tube.
2. Conduit, lined or unlined.
3. Molding.
Open wiring is cheap, is accessible for inspection and repair, is
easily modified to suit changing conditions.
Concealed knob-and-tube wiring is cheap, is flexible, is invisible.
Conduit wiring is safe, is permanent, requires least room, in
common with "knob-and tube" work it is invisible. Conduit is
not always concealed but is often used in exposed work on ac-
count of safety and permanence.
Molding work is cheap, is fairly well protected, is easily in-
stalled in old buildings. It is rapidly going out of favor.
Essential Features of Interior Wiring.
The dimensions, material and construction of insulators,
switches, fuse cut-outs and circuit-breakers are so specified that
fire-risk may be permanently avoided and these devices are made
in a substantial manner to maintain the "risk" a safe one.
Conduits are made fire-tight so that the effects of any short-cir-
cuit may be confined to the tube or fittings. These are also to a
limited extent water-tight.
Mechanical continuity is assured by the Code provision that the
wires shall be drawn into conduits only after the installation of
the latter is complete.
Electrical continuity of the conduit system is essential as great
reliance is placed on grounding to eliminate fire risk from leakage
of current to ground through high resistance.
Choice of size of wire is determined from one or more of these
considerations:
a. Allowable drop in pressure as dictated by the sensitive-
ness of the apparatus to be used.
b. Safe carrying capacity determined by the nature of the
insulation.
31
c. Mechanical strength which debars wires smaller than No.
12 except for special cases, and which dictates the presence of a
protecting strip under surface wires smaller than No. 6 to avoid
injury.
Joints in circuits, such as those in wires, between wires and
fittings and between parts of switches and cut-outs are sources
of most of the annoying electric light interruptions.
32
PART IV. ELECTRIC GENERATORS.
a. Essential Elements.
The electric generator is a device for the transformation of
mechanical into electrical power.
Its essential elements are:
1. A magnetic circuit for maintaining magnetic flux
through which electric conductors can be moved conveniently,
with a source of magneto-motive force for the same. This "field
magnet" may be stationary or it may revolve.
2. A set of electrical conductors suitably connected and
mounted on a support, preferably part of the magnetic circuit.
Provision must be made for maintaining relative motion of the
'armature" conductors and the magnetic flux.
(C
3. Auxiliary devices for conveying current from or into
the rotating part of the machine.
The armature of the continuous-current 'machine invariably
revolves, current being conducted from the winding through a
'commutator" as in all constant-potential and in a few constant-
current generators, or through a "rectifier" as in most constant-
current machines. In the alternator," either armature or field
may revolve and the current is conducted through "collecting
rings" to or from the moving member in either case. "Fly-
wheel-effect" often makes motion of the field magnet desirable.
b. Methods of Excitation.
The source and nature of the magneto-motive force for the
magnetic circuit of a generator determines to some extent the
nature of the armature output. The possible sources of m. m. f.
are as follows:
1. Separate Excitation from an auxiliary circuit as used in
most alternators except such as are mentioned under class (4).
The continuous current dynamo supplying the field current is
known as the "exciter.'
2. Shunt Excitation, that is by connection of the field cir-
cuit across the armature terminals. This plan is used in many
constant potential, continuous current lighting generators.
33
3. Series Excitation, the line current passing through the
field circuit and used on constant continuous current dynamos only.
4. Compound Excitation, a combination of series and
shunt or of series and separate excitation. Compound excitation
is used either in continuous or alternating current generators
where either a perfectly uniform pressure is desired or where
the pressure should rise with increase of load. The latter fre-
quently receives the name, "over-compounding."
In compounding alternators all or part of the line current is
rectified and passed through the series field coils, such an arrange-
ment being usually designated "composite wound." A compound-
ing effect is also produced very successfully without the aid of a
series winding by sending the alternating current through the arm-
ature of the exciter, modifying its field flux and e. m. f. and in
turn the field current of the alternator. This combination of ex-
citer and alternator is called a "compensated" alternator and the
method provides for variable power factor as well as variable load.
c. Structural Features.
These may be summarized under three heads:
Armature, consisting of the support for the laminated
structure, the magnetic circuit, the winding, the devices for con-
necting to the line circuit, such as commutators, brush-holders, etc.
Field Magnet, including mounting for the magnetic circuit,
the field cores, the winding, the circuit connections.
whole.
Mechanical Mounting of armature and field magnet as a
d. Generator Accessories.
Accompanying each generator there are a number of "extras"
necessary for the proper operation of the machine. These in-
clude field rheostats, generator switchboard panels with all fittings,
etc.
e. Generator Characteristics.
From the operating standpoint a most important feature is the
relation of the various elements of the generator output. These
relations are given the above name as they represent the inherent
qualities of the machine from the electrical side.
34
PART V. ELECTRIC MOTORS AND THEIR APPLI-
CATIONS.
Motor Classification.
Alternating Current types (constant potential.)
Synchronous,
rator.
that is, with armature running at same polar speed as gene-
Induction, or non-synchronous.
Either of the above may be either single, or polyphase.
Series,
with armature and field in series. Has commutator.
Continuous Current types (constant potential.)
Series,
with armature and field windings in series.
Shunt,
that is with field circuit taken directly from the brushes.
Compound,
combining the features of series and shunt,
Only constant potential motors have been listed above as con-
stant current motors are seldom used and they are not satisfac-
tory.
Classes of Motor Service.
Traction. Series c. c. motors used almost exclusively. In-
duction a. c. used somewhat and synchronous and series
a. c. proposed.
Manufactories. Shunt c. c. and induction a. c. motors used
and both are in favor for machine drive. For cranes and
elevators, series c. c., compound c. c. and specially wound
induction a. c. motors are preferred.
35
Sub-stations. In sub-stations motors are used as secondary
sources of power. A. c. motors, both synchronous and in-
duction, are used for driving electric generators. The
rotary convertor is a combined a. c. synchronous motor and
c. c. generator in one machine. The other combinations
are known as motor-generator sets.
Miscellaneous. For general uses of electric power in small units
the motor is always selected to suit the available power.
Manufacturers of motors can now supply reasonably satis-
factory motors to operate with any variety of current
supply.
Motor Characteristics.
Synchronous, a .c., single-phase.
Absolutely uniform speed, [adjustable power factor, not
self-starting.
Synchronous, a. c., polyphase.
As above, except that it is self-starting when unloaded
although it is not always desirable to utilize this property.
Induction, a. c., single phase.
Not inherently self-starting, but can be made to start un-
der light load by special winding, rather low power factor, runs at
fairly uniform speed.
Induction, a. c., polyphase.
Starts well under load, runs at practically uniform speed
but by use of high resistance secondary speed may be made to
vary with load, fair power factor, speed fairly independent of pres-
sure changes.
Series a. c., single-phase.
Good starting torque, high power factor, commutator is
main objectionable feature.
Series c. c.
Large starting torque, speed variable with load, pressure
and field strength adapting it perfectly to traction conditions.
Shunt c. c.
Practically constant speed independent of load at a given
pressure but speed may be changed by varying field strength and
pressure thus adapting it to shop conditions.
36
Compound, c. c.
A compromise between the large starting torque of the
series and the steady running of the shunt types. Series coil can
be cut out when motor has come to speed.
Structural Features.
Treated as under Generators.
Auxiliaries.
Field rheostats, starting rheostats, overload and underload
circuit-breakers, series-parallel controllers, fuses, switch-board
panels, etc.
37
1
PART VI. TRANSFORMERS AND THEIR
APPLICATIONS.
A transformer is a device for changing the form of electric
power. The requirements of engineering practice cover the fol-
lowing varieties.
Classes of Transformation.
a. Transformation of constant alternating pressure at a fixed
ratio, employing the constant potential transformer.
b. Transformation of alternating current at a fixed ratio, em-
ploying the series transformer.
c. Transformation from constant alternating pressure to con-
stant alternating current, employing the constant current trans-
former.
d. Transformation from constant alternating to constant
continuous pressure, employing the rotary transformer or convertor.
The word "constant" as used in this connection does not mean
that the quantity so designated is absolutely constant as there is
always some variation due to imperfections of the machinery.
Features of the Various Transformers.
Constant Potential and Series Types.
1. The coils must be so related to each other that no
magnetic flux passes through one which does not also pass
through the other.
2. The insulation of the coils must be such that no electrical·
connection can exist between the coils or between a coil and the
core.
3. The heat which is necessarily generated in the coils must
be conveyed away without causing undue rise of temperature in
any part of the transformer.
4. The losses of energy in coils and core must be kept low
in order to insure high efficiency. This is especially true of the
core which must be magnetized at all times whether the coils are
carrying useful current or not.
38
Constant Current Type.
1. The coils must be so related that the magnetic leakage
will vary in such a way as to prevent the rise of the current.
2, 3 and 3. As in constant potential type.
Rotary Transformer or Convertor.
Requirements are the same as for synchronous motors and
continuous current generators.
Structural Features.
The methods adopted in order to meet the requirements out-
lined may be summarized thus:
1. Magnetic Leakage. In order to insure the presence of
the same flux in both coils, the latter are divided into sections and
these sections are alternated in such a way that the turns are
practically interlaced. In the constant current transformer the
magnetic leakage is varied by allowing the coils to move under the
action of the repulsve force between them.
2. Insulation. The insulation of the coils is made as perfect
as possible as follows: The wires are individually insulated and
the layers are separated from each other with proper material of
great dielectric strength. Between primary and secondary coils
are placed sheets of insulating material which will withstand the
combined action of heat and electric pressure. In addition to all
of this the transformer is usually immersed in oil which permeates
all parts and greatly assists in improving the insulating properties
of the other materials.
3. Ventilation. The oil, by circulation, carries heat from coils
and core to case, which has walls as thin and of as large surface as
is consistent with mechanical strength. This is sufficient venti-
lation in small size transformers. The radiating surface per pound
is evidently less as the transformer is larger. Hence, in large
sizes it is necessary to immerse in the oil, coiled pipes carrying.
cold water. In some cases the oil is dispensed with and air is
forcibly circulated about the coils and core.
4. The core and copper losses are kept down as in the arma-
tures of generators and motors by proper selection and applica-
tion of magnetic and conducting materials.
Installation of Transformers.
Transformers are treated as a considerable fire risk and for
39
this reason are placed preferably out-of-doors. When necessarily
installed in-doors they are preferably mounted in a brick or other
fire-proof chamber with ventilation to the out-door air. An ex-
cellent plan is the placing of the transformers in out-door man-
holes with underground service connections.
Polyphase Transformatoon.
Transformation from one number of phases to another be-
comes readily possible by means of constant potential transform-
ers, largely through the possibility of shifting the phase position
of any one of the component pressures through an angle of 180°
simply by reversing the terminals of the secondary coils.
The most useful phase transformations are:
Two phases to three, requiring two transformers. (Scott
system).
Three phases to two, the above reversed.
Two phases to one, requiring two transformers.
Three phases to six, requiring three transformers.
Six phases to three, above reversed-
Two phases to two, requiring two transformers.
Three phases to three, requiring either three or two trans-
formers, etc.
40
PART VII. ELECTRIC LIGHTING AND POWER
STATIONS AND SUB-STATIONS.
The present development of alternating current electric pow-
er generation and transmission allows the placing of the station
at a point most convenient from the standpoint of power supply.
Sub-stations are then necessary to transform the current to the
varieties needed for the local service. Two general forms of
practice in stations may be noted:
1. The use of separate generators for the different varieties
of current needed.
2. The present tendency to generate a standard form of pow-
er which is afterward transformed to the different varieties re-
quired.
Elements of Central Stations and Sub-stations.
The elements of all stations are:
I. The Source of Power. This may be boiler and engine,
water-wheel, gas or oil engine, synchronous motor or induction
motor. The proper one to use is that which, in the end, will de-
liver energy most cheaply at the receiving apparatus.
The Electric Generators. The different varieties have been
discussed. The particular one for a given purpose is that which
gives satisfaction most economically. The apparatus to be sup-
plied with current dictates the generators to be selected.
3. The Power Auxiliaries. The matter of utilizing the
various devices which are upon the market for increasing station
economy is one requiring experience and judgment. Economiz-
ers, feed heaters, etc., etc., are used or omitted as fuel is expen-
sive or cheap.
4. The Switch-boards and Fittings. These are most essen-
tial features of a station, and the arrangement of generator, feeder
and transfer switches, ammeters, voltmeters and wattmeters, cir-
cuit breakers, fuses, etc. must be carefully determined. As a
4I
general consideration it may be said that the switch-board must
be designed to protect the machinery, to provide for transfer of
feeder and generator circuits, both to avoid shut-down and to
properly distribute the load. It should also be provided with
such measuring instruments as will enable machines te be econ-
omically operated and to give a measurement of the daily output.
5. Battery Auxiliary. In modern stations the installation of
a battery is always considered, and frequently a battery is in-
stalled for one or more of the objects laid down under accumu-
lators. A battery properly installed may have a marked influence
upon the economy and reliability of a station.
Station Records. In a well-managed station such records are
kept as will enable the superintendent to determine at least the
following items:
a. Total daily energy output,
b. Average daily power output,
c. Maximum daily power output,
d. Minimum daily power output,
e. Total daily water consumption,
f. Total daily fuel consumption,
g. Hours during which each machine is in operation.
From these data may be deduced:-
a. Pounds of coal per K. W.-hour,
b. Pounds of water evaporated per pound of coal.
By familiarity with the standard values of these quantities for
his style of station the superintendent can know the rating of
his plant as a power producer.
42
PART VIII. ELECTRIC LIGHTING AND HEATING.
Electric Lighting.
The production of light by means of the electric current con-
sists in rendering certain substances incandescent or phosphor-
escent. The commercial sources of electric light at present may
be summarized as follows:
The Arc Lamp
a. Open form.
b. Enclosed form.
The Incandescent Lamp
with incandescent filament in vacuum.
The Nernst Lamp
with incandescent filament in air.
The Vapor Lamp
with phosphorescent vapor at low pressure.
The Arc Lamp
General features include a pair of carbon rods separated by a
short space, with mechanism for first placing them in contact and
then separating them a short distance. The arrangement of
coils gives rise to two general types of lamp: differential and
shunt. The former uses series coils for raising, a ndshunt coils
for lowering the carbons. In the shunt type the shunt coils only
are employed, the separation of the carbon points being effected
by means of a spring. Series arc lamps are provided with an
automatic device, or cut-out, for short-circuiting them in case of
failure of the regulating mechanism. By special adjustment of
the coils and the insertion of a "ballast" in series with the arc,
arc lamps may run successfully in parallel.
Open arc lamp has short arc with shadows, high efficiency,
irregular distribution, great luminous intensity, short life of
carbons.
Enclosed arc lamp has long arc, less efficiency on account of en-
closing globe, good illumination, long life of carbon with saving
in carbons and trimming.
43
The Incandescent Lamp.
A complete lamp consists of the following parts :-bulb, filament,
base (including support for filament), wires connecting base and fila-
ment (of platium). A good vacuum is maintained in the bulb.
The manufacture of incandescent lamps consists of these processes :
(1.) Forming the filament.
(2.) Carbonizing the filament.
(3.) Flashing the filament.
(4.) Blowing the bulb.
(5.) Mounting the filament in the bulb.
(6.) Exhausting the air from the bulb.
(7.) Testing and rating the lamp.
Operating features of the incandescent lamp.
Useful life is the length of time in which the lamp can be used
economically, usually the time required for it to lose about 25 per
cent. of its candle-power. The gradual deposit of carbon on the
bulb is the cause of this loss, in addition to the increased resistance
of the carbon filament as it loses size by evaporation.
Candle-power. The mean spherical candle-power is the average
in all directions. The candle power in any one direction is effected
by the form of the filament.
the
Efficiency. The candle-power produced per watt, or more usually
watts per candle-power, is the quantity used for comparing lamps.
Standard values of the latter are from 2.5 to 5 watts per c. p.,
higher values being used with circuits with poor pressure regulation
and in lamps of low c. p. or high voltage.
Smashing point. It is most profitable to discard lamps when the
total cost of operation including energy and renewal is minimum.
This often dictates a comparatively short useful life for a lamp.
Incandescent lamps are usually run in parallel but are sometimes
connected in series for street lights. They require good pressure or
current regulation in the two cases respectively.
The Nernst Lamp.
By the use of a highly refractory filament which is a non-conductor
when cold, the necessity of employing a vacuum is avoided.
essential parts are :
(1.) Glower or filament.
The
(2.) Heater, in circuit only until the filament becomes a conductor.
44
(3.) Heater cut-out.
(4.) Ballast, of iron wire, which controls the flow of current.
The Nernst lamp has a high efficiency with an excellent quality of
light in which pigments exhibit their natural colors.
The Vapor Lamp. (Cooper-Hewitt.)
A most efficient lamp is obtained by passing current through mer-
cury vapor. The essential parts are :
(1.) Tube for vapor with electrodes.
(2.) Vessel of mercury to maintain supply of vapor.
(3.) Transformer for producing a very high starting pressure.
The main objection made to the lamp is the quality of the light,
which has a green color. Its actinic properties are excellent. The
luminous intensity is low so that a large surface must be used.
Other Lamps.
Experiments are constantly being made to obtain greater luminous
efficiency than is now possible. The directions of progress are: the
addition of substances to the carbons of arc lamps to give color to
the flame; the employment of more refractory substances for the
filaments of incandescent lamps so that higher temperatures can be
used; and the use of phosphorescence of vapors.
Electric Heating.
Heat is produced for engineering purposes from the passage of a
current through a resistance. This process has an efficiency of unity.
Viewed from the financial standpoint alone it is very inefficient when
the electric current is generated from coal, therefore electric heat is
usually used only when considerations other than economy prevail.
These may be; convenience, cleanliness, safety, etc. As examples
of application we may cite the following:
a. Electric heaters, as in street railway cars.
b. Electric furnaces, as in the production of carborundum and
graphite.
C. Electric laundry irons, etc.
d. Electric ovens, as used at Niagara Falls.
e.
Electric cautery knives, used by surgeons.
45
PART IX. ELECTRICAL MEASURING INSTRUMENTS.
The engineering requirements in this line may be divided into
two classes:
I. The operator's requirements.
2.
The manufacturer's requirements.
Operating instruments may also be divided into permanent or
switch-board instruments and instruments for standardization.
Switch-board Instruments.
Ammeters.
For alternating currents either electro-dynamometer or soft-core
instruments are used. In the former the current reverses in fixed
and moveable coils at the same time. In the latter the induced flux
in the core reverses with the current in the coil, which is fixed.
These instruments may be also used for direct current, but those
employing the D'Arsonval galvanometer are preferred. The amme-
ter in common use is a milli-voltmeter, connected across the terminals
of a shunt-block through which passes the current. The same milli-
voltmeter may be used with a great variety of shunts.
Voltmeters.
Remarks about forms of ammeters also apply here. Some electro-
static instruments are used but mainly as ground detectors. By the
insertion of a series resistance or "6
multiplier," the same instrument
may be used for a variety of voltages.
Wattmeters.
Indicating wattmeters are not used extensively as they would only
be necessary in determining the instantaneous power factor. Inte-
grating or recording wattmeters are essential. They are motors so
arranged as to have a speed proportional to the power passing.
Miscellaneous Instruments.
Power factor indicators, phase indicators, ground detectors, etc.,
are employed in special cases. Where a station supplies power
46
through cables, it must also be provided with a set of cable testing
instruments which will indicate the condition of the insulation of a
cable quickly and accurately.
Standardizing Instruments.
Where the size of switchboard warrants the expense it is desirable
for a station to be provided with a potentiometer and standard cells,
standard resistant units and laboratory galvanometer for checking the
calibration of ammeters, voltmeters and wattmeters. Standard
ammeters and voltmeters may be kept on hand in addition to or in
place of the instruments mentioned. There are standardization
bureaus where such instruments may be checked from time to time.
Manufacturer's Instruments.
Instrument makers have devised convenient and practical instru-
ments for testing materials before and after making up into finished
products. Among these may be mentioned the hysteresis tester, the
conductivity bridge, cable testing sets and such devices.
47
PART X. ENGINEERING APPLICATIONS OF ELECTRO-
CHEMISTRY.
The engineer is practically interested in those results of electro-
chemical development which bear directly upon his work, rather than
those which result in the production of chemical compounds which he
does not use. Hence it is here possible to treat only such topics as :
The accumulator or storage battery,
a.
b. The reduction of metals,
C.
The electrolytic destruction of pipes and cable coverings,
d. The primary battery.
Essentials of Electrolytic Processes.
All electro-chemical operations involve the following essential ele-
ments:
I.
An Electrolyte, "a liquid which permits the current to flow only
by means of the decomposition of the liquid, the 'decomposition that
ensues being called electrolytic decomposition. (Houston.) The
liquid may be a fused salt or a salt in solution.
2.
,,
An Anode, through which the current enters the electrolyte
and which is therefore connected to the negative terminal of the source
of pressure. The anode attracts oxygen, chlorine, etc.
3.
A Kathode, through which the current leaves the electrolyte
and upon which hydrogen and metals are deposited.
The Accumulator.
Features of Accumulators.
The chemical storage of energy is possible by producing cumula-
tive changes in an electrolyte, in electrodes, or in both, by means of the
electric current. This stored energy may be reclaimed by connecting
the electrodes through an electric circuit. Various substances have
been proposed as electrodes and as electrolytes, the cells in most
general use having lead electrodes and sulphuric acid electrolyte.
When charged, the positive "active material" is lead peroxide and
the negative, spongy lead.. The Edison cell consists of nickel and
iron electrodes and caustic potash electrolyte.
positive
48
Storage cells give practically a constant e. m. f., the regulation
varying with the current drawn and the charge remaining. Capacities
are rated in discharge ampere-hours and these vary greatly with the
rates of charge and discharge.
Applications of Accumulators.
I. Storage battery traction.
This is of increasing importance,
particularly for electric locomotive use.
2. Isolated work, such as in connection with small power units,
launches, train lighting, etc.
3.
In central stations
a. To straighten the load curve.
b. To keep up voltage on heavy load.
C.
ن
To carry entire load at times.
d. To improve engine and generator operation.
e.
f.
41 50
g.
To regulate voltage fluctuations due to variation in
engine speed.
In sub-stations.
For transformation and sub-division of voltage.
Reduction of Metals.
This is but one of the many important electro-chemical industries.
The method is used both for the reduction of metals from their ores
and for the refining of metals. The metal is deposited from the elec-
trolyte upon the kathode in a practically pure state. Of greatest
importance to engineers are the reduction of copper and of aluminum,
although he also uses large quantities of lead. As examples of these
processes we may cite the refining of copper, where purè copper is
deposited upon a copper kathode from a copper sulphate solution;
and aluminum reduction, employing carbon electrodes and a solution
of alumina (prepared from bauxite) in fused cryolite.
Electrolysis of Pipes.
Stray current from the return circuits of electric railways (which
are usually the rails) cause destruction of pipes when the latter is an
anode and when the soil is an electrolyte. The former may be an
anode when its electric potential is a volt or more above that of the
rails, and the latter may be an electrolyte when it contains soluble
salts and a liquid to dissolve them. The remedies consist in keeping
49
the current away from the pipes or in the pipes by making them part of
the return circuit.
The Primary Battery.
This is a most useful device for certain purposes requiring little
electrical energy storage capacity. In addition to the elements of the
accumulator (also known as the secondary cell) the primary cell must
also be supplied with a "depolarizer," to absorb the hydrogen depos-
ited upon the kathode. Unless so absorbed this would form a non
conducting layer, greatly increasing the resistance of the cell. At
present for many purposes a "dry" cell is preferred, in which the
electrolyte is suspended in a porous support. Naturally the capacity
of such a cell is less than the wet cell but it is compact, light, cheap
and convenient.
1
50
PART XI. TRANSMISSION OF INTELLIGENCE.
Telegraphy and telephony have many points in common while
differing in essentials. Each involves the following four elements.
(1.) Sending apparatus.
(2.)
Switching and protective devices.
(3.) Transmission system.
(4.) Receivers.
Telegraphy.
The various systems of telegraphy may be classed in several ways
as follows:
(a.) With or without wires for transmission,
(b.) Manually operated or automatic sending apparatus,
(c.) Messages received by sound or by recording devices,
(d.) One or more messages on the same wires at the same time.
A simple telegraph outfit consists of keys, relays and batteries in
series, with sounders on the local relay circuits. Multiplex teleg-
raphy requires the use of different kinds of currents for the different
operators using the same line, with selective devices for separating
the messages. In wireless telegraphy, Hertzian waves are set up by
the stream of sparks from a powerful Ruhmkorff coil, signals being
made by varying the duration of these sparks. A coherer of finely
divided metal in a small tube has its resistance varied by the electri-
cal waves and thus reproduces the signals in a local circuit. Auto-
matic telegraphy consists in the manual production of a sending tape
which is fed into a machine for transmission. Such messages may
be received by a syphon recorder, by a printing machine, or if not
too rapid it may be taken by sound.
Telephony.
Telephony has now become a most important branch of engineering.
At the present time only telephoning with wires is practicable, although
wireless telephony is in the experimental stage. The essential feat-
ures of a modern telephone system are as follows:
SI
The subscriber's outfit consists of (a) call circuit with condenser
using alternating current; (b) talking circuit with microphone, and the
magneto-telephone, employing continuous current, and (c) switch.
In some systems the microphone current is provided by a dry bat-
tery at each subscriber's station, while in others all energy is supplied
from the central station. The calling current in all modern plants
comes from the central station.
The central station switchboard is a device for connecting together
the different subscribers quickly and accurately. In the multiple
board any operator can connect a subscriber to any other on the
system. With other boards it is necessary to cross-connect the cir-
cuits from panel to panel. The latter plan is cheaper; the former is
quicker in operation. By means of the "supervisory" system, in
which a lamp on the switchboard is extinguished when a subscriber is
talking, the operator keeps track of the duration of conversations
without the troublesome operation of "listening in."
The transmission circuit consists of a metallic (two-wire) circuit of
copper in cities and iron on toll line. This is mounted on standard
pole line, the poles being spaced farther apart than an electric light
and power work. At the station and at the subscribers' ends are
lightning arrestors and fuses for preventing unduly large currents from
entering buildings. These devices are arranged to discharge at 500
volts so that a dangerous power current as well as a lightning discharge
will be kept out. The insurance code is very specific in this line of
work.
52
UNIVERSITY OF MICHIGAN
3 9015 06708 7273

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