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'"^' 1
EWn. Ubrary
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ST TBS SAUK AUTHOSB
Elementar; Electro -Teclinlcil Series
LltenutlDC Blictric Curranta,
BIcctric HaatiDf.
Blcetromagactltm.
Blactridtjr In Bleetro-Thanptutlca.
Elactric Arc l,\t\Mnt.
:i«lije IncaDdcacant LightlBE.
Blectric Hotar*.
Electric Street lUilwarm.
ElBGtriE Tele phony.
Electric Telcgrapby.
Oloth, Moe p«T Tolnma, %\M.
THE W. J. JOHNSTON COMPANY
flW Bboadwai, Kcw You
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ELECTRO-DYNAMIC MACHINERY
FOR CONTINUOUS CURRENTS
EDWIN jt' HOUSTON, Ph. D. (Primcetoh)
A. E. KENNELLY, Sc. D.
NEW YORK
THE W. J. JOHNSTON COMPANY
as3 Broadway
.896
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THE W. J. JOHNSTON COMPANY.
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PREFACE.
Although several excellent treatises on machinery employed
in electro-dynamics already exist, yet the authors believe that
there remains a demand for a work on electro-dynamtc ma-
chinery based upon a treatment differing essentially from any
that has perhaps yet appeared. Nearly all preceding treatises
are essentially symbolic in their mathematical treatment of
the quantities which are involved, even although such treat-
ment is associated with much practical information. It has
been the object of the authors in this work to employ only the
simplest mathematical treatment, and to base this treatment,
as far as possible, on actual observations, taken from practice,
and illastrated by arithmetical examples. By thus bringing
the reader into intimate association with the nature of the
quantities involved, it is believed that a more thorough appre-
ciation and grasp of the subject can be obtained than would
be practicable where a symbolic treatment from a purely
algebraic point of view is employed.
In accordance with these principles^ the authors have in-
serted, wherever practicable, arithmetical examples, illustrat-
ing formulas as they arise.
The fundamental principles involved in the construction and
use of dynamos and motors have been considered, rather than
the details of -construction and winding.
The notation adopted throughout the book is that recom-
mended by the Committee on Notation of the Chamber of
Delegates at the Chicago International Electric Congress
of 1893.
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The magnetic units of the C. G. S. system, as provisionally
adopted by the American Institute of Electrical Engineers, are
employed throughout the book.
The advantages which are believed to accrue to the concep-
tion of a working analogy between the magnetic and voltaic
circuits, are especially developed, for which purpose the con-
ception of reluctivity and reluctance are fully availed of.
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CONTENTS.
CHAPTER I.
Defiollion of ElectTO-Djmunic Machineiy, General Lang of the Genera-
tion of E. M. F. in Dynamos. Electric Capability. Output. Intake.
Commercial Efficiency. Electrical Efficiency. Maximum Onlput.
Maximum Efficiency. Relation between Oulpal and Efficiency, t
CHAPTER 11.
STKUCTUKAL tLBMKNTS OF DVNAMO-KLBCTRIC MACHlNEa.
Aimatura. , Field Magneti. Ma^etic Flux. Commutator Bmtbet.
Conitaiit- Potential Machines. Constant-Current Machines, Magneto.
Electric Machines. Separately- Excited Machines. Self -Excited
Machines. Series-Wound Machines. Shun [-Wound Machines.
Compound-Wound Machhies. Bipolar Machines. Multipolar Ma--
chines. Quadripolar. Sextipolar, Octopolar and Decipolar Machines.
Number of Poles Required for Continuous and Alternating-Current
Machines. Consequent Poles. Ring Armatures. Drum Anoatures.
Disc Armatures. Pole Armatures. Smooth-Core Armatures. Toothed-
Core Armatures. Inductor Dynamos. Diphasers. Triphaiers.
Single Field-Coit Multipolar Machines. Commutatorless Continuous-
Cnirent Machines, 9
CHAPTER III.
MAGNBnC FLUX.
'Working Theory Outlined. Magnetic Fields. Direction, Inteniity, Dis-
tribution. Unifonnity, Convergence. Divergence. Flux Density.
Tubes of Force. Lines of Magnetic Force. The Gauss. Properties
of Magnetic Flux. M. M. F. Ahipcre-Tuni. The Gilbert. Flux
Paths aq
CHAPTER IV.
NOH-FSKRIC MAGNETIC aKCDITS.
Relnctance. The Oersted. Ohm's Law Applied to Magnetic Circuits.
Ferric, Non-Ferric, and Aero-Ferric Circuits. Magnetising Fone.
Magnetic Potential. Lam of Non-Ferric Circuits 4B
CHAPTER V.
FEUtIC HACNBTIC CIKCUIT.
tivity. Laws of Reluctivity.
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CHAPTER VI.
AERO-FERRlC UAGNETIC C
Magnetic Stresses. Laws of Magnetic Attraction. Leakage, • . .
CHAPTER VIL
LAWS OF BLECTKO-DVHAMIC INDUCTION.
Fleming's Hand Rule. Cutting and Enclosut« of Magnetic Flux, , .
CHAPTER Vlir.
BLECTRO-DYNAUIC INDUCTION IK DVNAUO ARMATURES.
Curves of E. M. F. Generated in Armature Windings. Idle-Wire,
CHAPTER IX.
ELECTROMOTIVE FORCE INDUCED BV MAGNETO GEHEKAraKA, I
CHAPTER X.
POLB AMfATURES, I
CHAPTER XI.
GRAMME-RING ARMATURES.
E. M. F». Induced in. Effect of Magnetic Dissymmetiy. Commnta*
tor- Brushes. Effect of Dissymmeliy in Winding. Best Cross-
Section of Armature,
CHAPTER Xn.
CALCULATION OF THE WINDINGS OF A CRAMHE-BING DYNAMO, I3S
CHAPTER XIII.
MULTIPOLAR GRAMUB-RING DYNAMOS.
Belt-Driven vrrsus Direct-Driven Generators. Reasons for Employing
Multipolar Field Magnets. Multipolar Armature Connections. Effect
of DissymmetiT in Magnetic Circuits of Multipolar Generators. Com-
putations for Multipolar Gramme-Ring Generator, .... 135
CHAPTER XIV.
CHAPTER XV.
ARMATURE JOURNAL B
Friclional T-owes of Eneigy in Dynamos, Sight-Feed Oilers and Self-
Oiling Bearings, I
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CONTUffTS.
CHAPTER XVI.
EDDY CVBRBNTS.
Methods of Lamination of Con. Tntosposition of Conductors,
CHAPTER XVIl.
C HySTEKESIS.
CHAPTER XVIII.
AT COHMUTATORS,
*me[er of CommutaCian, E. M. F. of Sel(-Induc(ioo. Inductance of
Coils. Crost-Maeneliiation. Bftck-Magnetixation. Leading and
Following Polar Edges, Lead of Brushes. Distortion of Field. Con-
ditioDi Favoring Sparking at Commutator. Conditions Favoring
Sparlilcss Commutation, Methods Adopted for Preventing Spaiking, 179
CHAPTER XIX,
HBATING OP DYHAUOS.
CHAPTER XX,
RBGULATION OP
Seriea-Wonnd, Shunl-Woand and Compound-Wound Geneiatots. Over-
compounding. Characteristic Curves of Machines. Internal and
External Characterislic. Computalion of Characteristics. Field
Rheostats. Series- Wound Machines and their Regulation. Open-Coil
and Closed-Coil Armatures, ..,,.... S06
CHAPTER XXI.
COMBINATIONS OF DYNAMOS IN SERIES AND FARALLEU
Generator Units. Series-Wound Machines Coupled in Series. Shunt-
Wound Machines Coupled in Parallel. Equalizing Bare. Omnibus
Bars, ' . aiO
CHAPTER XXII.
DISC-AKMATURBS A
CHAPTER XXni.
CO>raUTAT0Kt.ESS CONTINUOUS-CUB RE NT GENERATORS.
Disc and Cylinder Machines, , . . . . . .
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X CONTENTS.
CHAPTER XXIV.
ILICTIO-DYNAMIC FOBCB.
Fleming's Hand-Role. Ideal Etectro-dynemic Motor, .... 341
CHAPTER XXV.
MOTOR TOKQUe.
CHAPTER XXVI.
r OF MOTORS.
CHAPTER XXVII.
KSCULATION OF MOTORS.
Control of Speed and Torque under Various Condiljoni. Control of 5erie*>
Wound Hoton, sBo
CHAPTER XXVIII.
STARTING AND RIVERSIMG OF MOTOtS.
Starting Rheostati. Startiog Coils. Automatic Switches. Direction of
Rotation in Hoton, 397
CHAPTER XXIX.
UBTBR-UOTORS.
Condltioni aoder which Moton may act ai Meters 309
CHAPTER XXX.
MOTOR DYNAMOS.
Conilnidioa and Operation of Motor-Dynamos, 31S
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ELECTRO-DYNAMIC MACHINERY
FOR CONTINUOUS CURRENTS.
CHAPTER I.
* GENERAL PRINCIPLES OF DYNAMOS.
I, By electro-dynamic machinery is meant any apparatus
designed for the production, transference, utilization or
measurement of energy through the medium of electricity.
Electro-dynamic machinery may, therefore, be classified under
the following heads :
(i.) Generators, or apparatus . for converting mechanical
energy into electrical energy.
(2.) Transmission circuits, or apparatus designed to receive,
modify and transfer the electric energy from the generators to
the receptive devices.
(3.) Devices for the reception and conversion of electric
energy into some other desired form of energy.
(4.) Devices for the measurement of electric energy.
Under generating apparatus are included all forms of con-
tinuous or alternating- cur rent dynamos.
Under transmission circuits are included not only conduct-
ing lines or circuits in their various forms, but also the means
whereby the electric pressure may be varied in transit
between the generating and the receptive devices. This
would, therefore, include not only the circuit conductors
proper, but also various types of transformers, either station-
ary or rotary.
Under receptive devices are included any devices for con-
verting electrical energy into mechanical energy. Strictly
speaking, however, it is but fair to give to the term mechanical
energy a wide interpretation, such for example, as would per-
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a ELECTRO-DYNAMIC MACHINERY.
mit the introduction of any device for translating electric
energy into telephonic or telegraphic vibrations.
Under devices for the measurement of electric energy would
be included all electric measuring and testing apparatus.
In this volume the principles underlying the construction
and use of the apparatus employed with continuous-current
machinery will be considered, rather than the technique in-
volved in their application.
2. A consideration of the foregoing classification will show
that in all cases of the application of electro-dynamic machin-
ery, mechanical energy is transformed, by various devices,
into electric energy, and utilized by various electro-receptive
devices connected with the generators by means of conducting
lines. The electro-technical problem, involved in the practi-
cal application of electro- dynamic machinery, is, therefore, that
of economically generating a current and transferring it to the
point of utilization with as little loss in transit as possible.
The engineering problem is the solution of the electro-technical
problem with the least expense.
3. A dynamo-electric geturator is a machine in which con-
ductors are caused to cut magnetic flux-paths, under conditions
in whi6h an expenditure of energy is required to maintain the
electric current Under these conditions, electromotive forces
are generated in the conductors.
Since the object of the electromotive force generated in the
armature is the production of a current, it is evident that, in
order to obtain a powerful current strength, either the electro-
motive force of the generator must be great, or the resistance
of the circuit small.
Mlectromotive sources must be regarded as primarily producing,
not electric currents, but electromotive forces. Other things
being equal, that type of dynamo will be the best electrically,
which produces, under given conditions of resistance, speed,
etc., the highest electromotive force (generally contracted
E. M. F.). In designing a dynamo, therefore, the electromo-
tive force of which is fixed by the character of the work it is
required to perform, the problem resolves itself into obtaining
a machine which will satisfactorily perform its work at a given
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GENERAL PRINCIPLES OF DYNAMOS. 3
efficiency, and without overheating, with, however, the maxi-
mum economy of construction and operation. In other words,
that dynamo will be the best, electrically, which for a given
weight, resistance and friction, produces the greatest electro-
motive force,
4. There are various ways in which the electromotive force
of a dynamo may be increased ; viz.,
(i.) By increasing the speed of revolution.
(3.) By increasing the magnetic flux through the machine.
(3.) By increasing the number of turns on the armature.
The increase in the speed of revolution is limited by well-
known mechanical considerations. Such increase in speed
means that the same wire is brought through the same mag-
netic flux more rapidly. To double the electromotive force
from this cause, we require to double the rate of rotation,
which would, in ordinary cases, carry the speed far beyond
the limits of safe commercial practice.
Since the E. M. F. produced in any wire is proportional to
its rate of cutting magnetic flux, it is evident that in order to
double the E. M. F. in a given wire or conductor, its rate of
motion through the flux must be doubled. This can be done,
either by doubling the rapidity of rotation of the armature ; or,
by doubling the density of the flux through which it cuts, the
rate of motion of the armature remaining the same.
Since the total -E. M. F. in any circuit is the sum of the
separate E. M, Fs. contained in that circuit, if a number of
separate wires, each of which is the seat of an E. M. F., be
connected in series, the total E. M. F. will be the sum of the
separate E. M. Fs. If, therefore, several loops of wire be
moved through a magnetic field, and these loops be con-
nected in series, it is evident that, with the same rotational
speed and flux density, the E. M, F. generated will be pro-
portional to the number of turns.
An increase in E. M. F. under any of these heads is limited
by the conditions which arise in actual practice. As we have
already seen, the speed is limited by mechanical considerations.
An increase in the magnetic flux is -limited by the magnetk
permeability of the iron — that is, its capability of conducting
magnetic tlux — and the increase in the number of turns Is
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4 ELECTRO-DYNAMIC MACHINERY.
limited by the space on the armature which can properly be
devoted to the winding.
5. It will be shown subsequently that a definite relation
exists between the output of a dynamo, and the relative
amounts of iron and copper it contains — that is to say, the
type of machine being determined upon, given dimensions and
weight should produce, at => given speed, a certain output.
The conditions under which these relations exist will form the
subject of future consideration.
6. Generally speaking, in the case of every machine, there
exists a constant relation between its electromotive force and
resistance, which may be expressed by the ratio, — , where E,
is the E. M. F. of the machine at its brushes, in volts, and r,
the resistance of the machine; /'. e., its internal resistance, in
ohms. In any given machine, the above ratio is nearly con-
stant, no matter what the winding of the machine may be;
I. e., no matter what the size of the wire employed.* This
ratio may be taken as representing, in watts, the electric
activity of the machine on short circuit, and may be con-
veniently designated the electric capability of the machine.
For example, in a aoo, KW (200,000 watts) machine; ('. «., a
dynamo, whose output is aoo KW (about 367 horse power), the
value of the electric capability would be about to, 000 KW,
so that, since — = 10,000,000, if its E. M. F. were 155 volts,
its resistance would be 0.0024 ohm; whereas, if its E. M. F.
were 100 volts, its resistance would be approximately o.ooi
ohm.
7. Hitherto we have considered the energy absorbed by the
dynamo, independently of its external circuit — that is, we
have considered only the electric capabilit/ of the machine.
When the dynamo is connected with an external circuit, two
extreme cases may arise ; viz.,
* This iBllo would be constant if the ratio of insulation thickneu t
of wire remained constant through all siz«s of wire.
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GENERAL PRINCIPLES OF DYNAMOS. 5
(r.) When the resistance of the external circuit is very
small, so that the machine is practically short circuited. Here
all the electric energy is liberated within the machine.
(2.) When the external resistance is so high that the resist-
ance of the machine is negligible in comparison. Here practi-
cally all the energy in the circuit appears outside the machine.
The total amount of work, however, performed by the machine,
under these circumstances, would be indefinitely small, since
the current strength would be indefinitely small. Between
these two extreme cases, an infinite number of intermediate
cases may arise.
8. By the output of a dynamo is meafit the electric activity
of the machine in watts, as measured at its terminals; or, in
other words, the output is all the available electric energy, ,
Thus, if the dynamo yields a steady current strength of 500
amperes at a steady pressure or E. M. F., measured at its termi-
nals, of no volts, its output will be no X 500 = 55,000 watts,
or 55 kilowatts.
The intake of a dynamo is the mechanical activity it absorbs,
measured in watts. Thus, if the dynamo last considered were
driven by a belt, which ran at a speed of 1,500 feet-per-minute,
or 25 feet-per-second, and the tight side of the belt exerted
a stress or pull of 2,500 pounds weight, while the slack side
exerted a pull of 710 pounds weight, the effective force, or
that exerted in driving the machine, would be 1,790 pounds
weight. This force, moving through a distance of 35 feet
per second, would develop an activity represented by
1,790 X 25=44,750 foot-pounds per second; arid one foot-
pound per second is usually taken as 1.355 watts, so that the
intake of the machine is 60,630 watts, or 60.63 I^^-
By the commercial efficiency of a dynamo is meant the ratio of
its output to its intake. In the case just considered, the com-
mercial efficiency of the machine would be J^K - = 0.9072.
By the electric efficiency of a dynamo is meant the output,
divided by the total electric activity in the armature cir-
cuit. Thus, if the dynamo just considered had a total electric
energy in its circuit of 57 KW, of which 3 KW were expended
in the machine, its electric efficiency would be — = 0.965.
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6 ELECTRO-DYNAMIC MACHINERY.
9. The output of a machine would be greatest whea the
external resistance is equal to the resistance of the machine.
In this case, the output would be just one-quarter the electric
capability, and the electric efficiency would be 0.5, Thus,
the resistance of the dynamo considered in the preceding para-
graph would be, say, 0.008 ohm, and the electric capability of
: 1,512,500 watts, or 1,513.5 KW. If the
external resistance were equal to the internal resistance —
namely, 0.008 ohm, the total activity in the circuit would be
756.25 KW; the output would be 378.13 KW, and the electric
efficiency 0.5.
That is to say, in order to, obtain a maximum output from
a dynamo machine, the circumstances must be such that half
the electric energy is developed in the machine, and half in the
external circuit; or, in other words, the electric efficiency
can be only 0.5. In practice, however, it would be impossible
to operate a machine of any size under these circumstances,
since the amount of energy dissipated in the machine would
be so great that the consequent heating effects might
destroy it.
10. We have seen that whenever the resistance in the
external circuit is indefinitely great, as compared with that
of the machine, the electric efficiency of the machine will be
i.o or 100 per cent. It is evident, therefore, that in order to
increase the electric efficiency of a dynamo, it is necessary
that the resistance of the external circuit be made great, com-
pared with the internal resistance of the machine. For ex-
ample, if the external resistance be made nine times greater
than that of the internal circuit, then the electric efficiency
will be — - — = 0.9; and, similarly, if the external resistance
9 + 1
be nineteen times that of the internal resistance, the electric
efficiency would be raised to — ~~- = 0.95. Generally speak-
ing, therefore, a high electric efficiency requires that the
internal resistance of the machine be small as compared with
the external resistance, and, also, that the amount of power
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GENERAL PRINCIPLES OF D YlfAMOS. 7
«xpended in local circuits, as in magnetizing the field magnets
of the dynamo, be relatively small.
11. Care must be taken not to confound the electric
efficiency of a machine with its electric output The
electric output of a machine would reach a maximum
when the electric efficiency was 0.5 or 50 per cent.,
and the output would be zero when the electric efficiency
reached i.o.
The electric efficiency of the largest dynamos is very high,
about 0.985. Indeed, the electric efficiency of large machines
must necessarily be made high, since, otherwise, the libera-
tion of energy within them Would result in dangerous over-
heating.
The commercial efficiency of a dynamo is always less than
its electric efficiency, since all mechanical and magnetic
frictions, such as air resistance, journal-bearing friction,
hysteresis and eddy currents come into account among the
tosses. The commercial efficiency also depends upon the type
of machine, whether it be belt-driven, or directly mounted on
the engine shaft, since the mechanical frictions to be overcome
differ markedly in these two cases. The commercial efficiency
will also vary with the character of the iron employed in its
field magnets and armature, and with the care exercised in
securing its proper lamination. In large machines, of say
500 KW capacity, the commercial efficirticy may be as high
as 0.95. In very small machines, of say o,s KW, the highest
commercial efficiency may be only 0.6.
12. Although in the United States it is the practice among
constructors generally, to calculate, express and compare
lengths and surface areas in inches and square inches, when
referring to dynamo machinery, and although it might seem
therefore more suitable to adopt inches and square inches as
units of length and surface throughout this book; yet the fact
that the entire international system of electro-magnetic meas-
urement is based on the centimetre, renders the centimetre and
square centimetre the natural units of dimensions In electro-
magnetism. The authors have therefore preferred to base
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8 ELECTRO-DYNAMIC MACHIffERY.
the formulae and reasoning in this volume on the French
fundamental units, in order to simplify the treatment, well
knowing that once the elementary principles have been
grasped, the transition to English measurements is easily
effected by the student. The following data will, therefore, be
useful:
I inch = 3.54 cms. I cm. = o..')g37 inch.
I foot = 30.4S cms. I cm. = 0.03131 foot.
r sq. inch = 6.4;i5 sq. cms. i sq. cm. = 0.155 sq. in.
I cnbic inch = 16,387 c. c. I c. c. = 0.06102 c. in.
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CHAPTER II.
STRUCTURAL ELEMENTS OF DYNAMO-ELECTRIC MACHINES.
13. Dynamo-electric machines, as ordinarily constructed,
' consist essentially of the following parts; namely,
(i.) Of the part catted the armature, in which the E. M. F.
is generated. The armature is generally a rotating part,
although in some machines the armature is fixed, and either
the field magnets, or the magnetic field, revolve.
(2.) Of the part in which the magnetic field is generated.
This part is called th& fieid magnet &.aA provides a magnetic flux
through which the conductors of the armature are generally,
actually, and always relatively, revolved.
(3.) Of the part or parts that are employed for the pur-
pose of collecting and rectifying the currents produced by the
E, M, F. generated in the armature; /. e., collecting and
commuting them, and causing them to flow in one and the same
direction in the external circuit. This portion is called the
commutator.
(4.) Bundles of wire, metallic plates, metallic gauze, or
plates of carbon, pressed against the commutator, and con-
nected'with the circuit in which the energy of the machine is
utilized. These are called the brushes.
In addition to the above parts, which are directly connected
with the electric actions of the machine, there are the neces-
sary mechanical parts, such as the bearings, shaft, keys, base,
etc., which also require attention.
The particular arrangement of the different parts will neces-
sarily depend upon the type of machine, as well as on the char-
acter of the circuit which the machine is designed to supply.
It will, therefore, be advisable to arrange dynamo-electric
machines into general classes, before attempting to describe
the structure and peculiarities of their various parts.
14. Dynamos may be conveniently divided into the follow-
ing classes; viz..
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lO ELMCTRO-DVI<rAM!C MACHINERY.
(i.) Constant potential machines, or those designed to main-
lain at their terminals a practically uniform E. M, F. under
alt variations of load.
To this class belong nearly all dynamos for supplying incan-
descent lamps and electric railroads.
Fig. I represents a particular machine of the constant-
potential type. A, A, is the armature, whose shaft revolves
in the self-oiling bearings B, B. C is the commutator, and D,
D, are triple sets of brushes pressing their tips or ends upon
FIG. I. — CONTINOI
the commutator. F, F, are the field magnets, wound with
coils of insulated wire. T, T, are the machine terminals, con-
nected with the brushes and with the external circuit or load.
The whole machine rests on slides with screw adjustment for
tightening the driving belt.
Constant-potential generators are made of all sizes, and of
various types. .
(z.) Conslanl-eurrent machines, or those designed to main-
tain an approximately constant current under all variations of
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STRUCTURAL ELEMENTS. 11
Constant -current machines are employed almost exclusively
for supplying arc lamps in series.
Fig. 2 represents a form of constant-current generator.
This is an arc-light machine. It has four field magnets but
only two poles, /" and /", connected by a bridge of cast iron
at B. At R^ is a regulating apparatus for automatically main-
taining the constancy of the current strength, by rotating the
CONSTANT-CURRENT BIPOLAR GENERATOR.
brushes back or forward over the commutator, under the influ-
ence of an electromagnet M.
Constant-current machines are made for as many as loo arc
lights; /. e., about 10,000 volts and 9 amperes, or an output up
to 90 kilowatts capacity, but such large sizes are exceptional.
15. Constant-potential machines may be subdivided into
sub-classes, according to the arrangement for supplying their
magnetic flux — namely:
(a.) .it/iifff^/'i'-ir/irir/ri'f machines, in which permanent magnets
are employed for the fields.
The magneto-electric generator was the original type and
progenitor of the dynamo, or dynamo-electric generator — but
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I« ELECTRO-DYNAMIC MACHINERY.
has almost entirely disappeared. It is, however, still used in
telephony, the hand call beinga small alternating-current mag-
neto generator, driven by power applied at a handle. The
magneto- electric generator Is also used in firing mining fuses,
and in some signaling and electro-therapeutic apparatus.
Fig, 3 represents a form of magneto -electric generator.
M, is a triple group of permanent magnets, and A, is the
armature.
(b.) Separately-excited machines, in which the field electro-
magnets are excited by electric current from a separate elec-
tric source.
■m
FIG. 3. — ALTERNATING-CURRENT MAGNETO -ELECTRIC
A particular form of separately excited generator is repre-
sented in Fig. 4.
Here a generator A, has its field magnets supplied by a
small generator B, employed for this sole purpose. It is not
necessary, however, that the exciting machine be used exclu-
sively for excitation. Thus two generators, each employed in
supplying a load, and each supplying the field magnets of the
other, would be mutually separately excited.
In central stations large continuous-current machines are
occasionally, and alternating-current machines are usually,
separately excited.
(c.) Self-excited machines, or generators whose field magnets
are supplied by currents from the armature.
Fig. 5 represents a form of self-excited generator. M, M,
are the field magnets, P, ihe. ptht lampj i. e., a lamp connected
across the terminals of the machine, to show that the generator
is at work. S, the main circuit switch, J?, the rocker-arm
carrying the brushes £, B.
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STRUCTURAL ELEMENTS. 1%
l6. Self-exctted machines may be divided into three classes;
■viz.,
(i.) Series wound.
(a.) Shunt wound.
{3.) Compound wound.
Series-wound machines have their field magnets connected
in series with their armatures. The field winding consists of
—ALTERNATING- CURRENT
Stout wire, in comparatively few turns. Arc-light machines
are almost always series wound. Fig. 6 represents a particular
form of series-wound machine for arc-light circuits. Here the
current from the armature passes round the cylindrical mag-
nets M, M, through the regulating magnet m, and thence to
the external circuit. The machine in Fig. 2 is also series
wound.
Shunt-wound machines have their field magnets connected
to the main terminals, that is, placed in shunt with the external
circuit. In order to employ only a small fraction of the total
current from the armature for this purpose, the resistance of
the field magnets is made many times higher than the resist-
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14 ELECTRO-DYNAMIC MACHINERY.
ance of the external circuit This is accomplished by winding
the magnets with many turns of fine wire, carefully insulated.
A particular form of shunt-wound machine is represented in
Fig. 7.
Here the fine wire windings of the four magnets coils are
supplied in one series through the connecting wires \V, W, W,
from the main terminals of the machine, one of which is shown
at M. In order to regulate the strength of the exciting cur-
rent through the magnet circuit, it is usual to insert a hand-
regulating resistance box, called the field regulating box, in
series with them.
(d.) Compound-wound tnachines. These are machines that are
partly shunt wound and partly series wound.
It is found that when the load increases on a series-wound
generator, it tends to increase the pressure at its terminals ;
i.e., to raise its E. M. F, On the other hand, when the load
increases on a shunt-wound generator, it tends to diminish the
pressure at its terminals; /'. e., to lower its E. M. F. In order,
therefore, to obtain good automatic regulation of pressure
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STRUCTURAL ELEMENTS. i5
from a machine under all loads, these two tendencies are so
directed as to cancel each other ; this is accomplished by
employing a winding that is partly shunt and partly series.
Fig. 8 represents a particular form of a compound-wound
machine.
Here there are two spools placed side by side on each mag-
net-core, one of fine wire in the shunt circuit, carrying a cur-
rent, and exciting the fields, even when no current is supplied ■
externally by the machine, and the other of stout wire making
FIG, 6.— SELF-EXCITED SERIES-WOUND CO NTINUO US-CUB RENT
comparatively few turns. This is part of the series winding
which carries the current to the external circuit. The excita-
tion of the magnets from this winding, therefore, depends
upon the current delivered by the machine; i. t., upon its
load.
Many generators for incandescent lamp circuits, as well as
many generators for power circuits are compound wound.
17. Besides the preceding classes, dynamo-electric machines
may be conveniently divided into other classes, according to
a variety of circumstances; for example, they may be divided
according to the number of magnetic poles in the field frame,
as follows :
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l6 ELECTRO-DYNAMIC MACHINERY.
(a) Bipolar machines, or machines having only two magnetic
field poles.
Bipolar machines may be subdivided, according to the num-
ber of separate magnetic circuits passing through the exciting
coils, into single-circuit bipolar, double-circvit bipolar machines,
and so on. Generally, however, modern bipolar machines are
not constructed with more than two magnetic circuits. Figs.
1, 2, 3 represent bipolar machines. Of these, Fig. r possesses
a single magnetic circuit, and Fig. 3 a double magnetic circuit.
(b) Multipolar machines, or machines having more than two
magnetic poles.
Fig. 9 represents a multipolar, diphase alternator of many
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STRUCTURAL ELEMENTS. i?
poles. This machine was employed at the World's Columbian
Exhibition. ,
l8. Multipolar machines may be divided into the following
sub-classes :
• Quadripolar, or those having four poles,
SexHpolar, or those having six poles.
Octopolar, or those having eight poles.
Decipolar, or those having ten poles.
Beyond the number of ten poles, it is more convenient to
omit the Latin prefix, and to characterise the machine by the
TlNVOl'S-Cl'RREST GESEHATOB.
number of poles, as, for example, a 14-pole, or i6-pole machine,
etc.
Quadripolar machines are common. Fig. 10 shows a quadri-
polar machine. This machine has four brlishes and is com-
pound wound. It is designed to supply from 500 to 600 volts
pressure at its brushes, and is surmounted by a group of six
pilot lights in series.
Fig. 7 also represents a quadripolar generator.
Fig. ir shows a form of continuous-current, self-exciting,
compound-wound, sextipolar machine, arranged for direct con-
nection to the main shaft of an engine. The machine is pro-
vided, as shown, with six collecting brushes.
Fig. 12 shows an alternating-current, self-exciting, octopolar
generator for arc circuits. Although this machine is an alter-
ftator ; i. e., supplies alternating currents, it, nevertheless.
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l8 ELECTKO.DYNAMIC MACHINERY.
supplies its field-magnet coils in series with continuous cur-
rents from the commutator C, at one end of its shaft. The
magnet M, forms an essential part of a short-circuiting device,
whereby the machine is automatically short-circuited, on the
external circuit becoming accidentally broken, in which case
the pressure generated by the machine might become so great
as to endanger the insulation of the armature.
Fig. 13 shows a decipolar alternator, separately excited, and
compensating. This machine is belt-driven, and it drives in
turn a small dynamo D, employed for exciting the ten field
magnets. The commutator, shown at C, is provided for the
purpose of automatically increasing the pressure at the brushes
of the machine with the load, so as to compensate for drop of
pressure in the line or armature. In other words, the machine
is compound- wound.
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STRUCTURAL ELEMEiVTS. 19
As we have already observed, bipolar machines may be sub-
divided into classes according to the number of magnetic
circuits passing through their exciting coils. In general,
multipolar machines may be similarly classified. But, as
usually constructed, there are as many independent magnetic
circuits as there are poles. Thus, a quadripolar generator has
usually four magnetic circuits, a sextipolar six, and so on. In
some cases, however, a double system of field magnets is pro-
vided, one on each side of the armature; in this case, the
number of magnetic circuits may be double the number of
poles.
19. In designing a continuous-current generator, the num-
ber of poles in the field is, to a certain degree, a matter of
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20 ELECTRO-DYNAMIC MACHINERY.
choice. In almost all cases, directly-coupled, continuous-cur-
rent dynamos are multipolar, while belt-driven dynamos are
frequently bipolar. Directly-coupled, continuous-current dy-
namos are usually multipolar machines, owing to the fact that,
in order to conform with engine construction, they have to be
made with a comparatively slow speed of rotation, and, since
FIO. II. CONTINUOUS-CURRENT SELF-EXCITED GENERATOR.
the E. M. F. generated depends upon the rate of cutting mag-
netic flux, if the speed of the conductor is decreased, the total
amount of flux must be correspondingly increased. This
necessitates a greater cross-section of iron in the field magnets
in order to carry the flux, and this large amount of iron is most
conveniently and effectively disposed in multiple magnetic cir-
cuits. To a certain extent the number of poles is arbitrary,
but usually, in the United States, the greater the output of a
direct-driven generator, the greater the number of poles.
In alternators, however, the case isdiflferent. Here, in order
to conform with a given system of distribution, the frequency
of alternation In the current is fixed, and, since the speed of
revolution of the armature is determined within certain limits.
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STRUCTURAL ELEMENTS. ai
by mechanical considerations, or by the speed of the drivings
engine, the number of poles is not open to choice, but is fixed
by the two preceding considerations. In any alternator, the
number of alternations of E. M. F. induced per revolution In
the coils of its revolving armature, is equal to the number of
FIG. la.— ALTERNATTSC-CURRENT SELF-KXCITED OCTOPOLAR GENERATOR.
poles. Consequently, an alternator producing a frequency of
133-^ ; that is a frequency of 133 complete periods or cycles per
second, delivers 266 alternations from each coil, and its arma-
ture must, therefore, pass 266 poles per second.
20. Fig. 16 shows a la-pole alternator. The wires a, a, are
in circuit with the field magnets, and serve to carry the current
which excites them, while the wires b, b, lead from the brushes.
21. Dynamo-electric machines may also be divided, accord-
ing to their magnetic circuits, into the two following classes:
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32 ELECTRO.DYNAMIC MACHINERY.
(a.) Those having simple magnetic circuits formed by a single
core and winding.
(b.) Those having consequent poles, or poles formed by a
double winding; that is, by the juxtaposition of two poles of
the same name. Dynamo-electric machines belonging to the
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STRUCTURAL ELEMENTS. ^3
first class are shown in Figs, i, 3 and 5, A type of machine
belonging to the consequent-pole class is shown in Figs. 14 and
15. The poles are shown at .A', jV, and S, S, in each case, the
field coils being so wound and excited as to produce consequent
poles.
22. Dynamo machines may also be classified according to
the shape of the armature, as follows; namely,
(a.) Ring armatures.
(b.) Cylinder or drum armatures.
(c. ) Disc armatures.
(d.) Radial ox pole armatures.
(e. ) Smooth-core armatures.
(f. ) Toothed-core armatures.
Figs. 2 and 11 represent examples of ring armatures.
Since Gramme was the first to introduce the ring type of
armature, this form is frequently called a Gramme-ring armature.
Figs, r, 5 and 14, show examples of cylinder or drum arma-
tures. Disc armatures are very seldom employed in the United
States. An example of a disc armature is shown in Fig. 19.
An example of a radial or pole armature is seen in Fig. 17.
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24 ELECTR0.DYNAM1C MACHINERY.
A smooth-core armature is one on which the wire is wound
over the cylindrical iron core, so as to cover the armature sur-
face completely; or, if the wire does not cover the surface com-
pletely, the space between the wires may either be left vacant
or filled with some non-magnetic metal. Such armatures are
represented in Figs, i, a, 5, 15.
A toothed-core armature, on the other hand, is one on which
GENERATOR.
the wire is so wound in grooves or depressions, on the surface
of the laminated iron core, that the finished armature pre-
sents an ironclad surface, but with slots containing insulated
copper wire.- Such an armature is shown in Fig. 18 and
also in Figs. 7, 10 and 11. It is frequently called an iron-dad
armature.
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STRUCTURAL ELEMENTS. *5
23. Djmamos may also be divided, according to the actual
or relative movement of armature or field, into the following
classes; namely,
(a.) Those in which the field is fixed and the armature
nc. 17,— DIAGEAM o
revolves. This class includes all the machines previously
described, except that represented in Fig. 19.
(b.) Those in which the armature is fixed and the field
revolves. An example of this type of machine is shown in
1 TOOTHED-CORE ARM AT V RE
Pig. 19 A and B, where two sets of field magnets, mounted on
a common shaft, revolve together around a tixed disc arma-
ture, shown in Fig. 19 B, which is rigidly supported vertically
in the space between them.
(c.) Those in which the field and armature are both fixed,
but the magnetic connection between the two is revolved.
These dynamos are usually called inductor dynamos.
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'° ELECTRO-DYNAMIC MACHUfERY.
24. Dynamo machines may also be divided, according to the
character of the worlc they are intended to perforin, into tlie
following classes ; namely,
■(a.) Arc-light generators.
(b. ) Tncandescent-light generators.
(c. ) Plating generators.
(d.) Generators for operating motors.
FIC. 1 9 A, — ALTERNATING-
(e. ) Telegraphic generators.
(f. ) Therapeutic generators.
(g. ) Welding generators,
25. Alternating-current generators may be divided, accord-
ing to the number of separate alternating currents furnished
by the machine, into the following classes; namely,
(a.) Uniphase alternators, or those that deliver a single alter-
nating current. To this class of machines belong all the
ordinary alternators employed for electric lighting purposes.
(b.) Multiphase alternators, or those that deliver two or more
alternating cinrents which are not in step.
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STRUCTURAL ELEMENTS. 27
Some multiphase alternators can supply both single-phase
and multiphase currents to different circuits.
Multiphase machines may be further subdivided into the
following classes; namely,
(i.) Diphase moikines, or those delivering two separate alter-
nating currents. These two currents are, in almost all cases,
quarter-phase currents, that is to say, they are separated by a
quarter of a somplete cycle. Although it is possible to employ
any other difference of phase between two currents, yet the
quarter-phase is in present practice nearly always employed.
Fig. 9 represents a diphase generator, or diphaser.
(2.) Triphase machines, or triphasers, are generators deliver-
ing three separate alternating currents. These three currents
are, in all cases, separated by one third of a complete cycle.
Uniphase machines are sometimes called single-phase machines,
and diphase machines are sometimes called tivo-phase machines
or tivo-phasers, while triphase machines are sometimes called
three-phase machines or three-phasers. The terminology above
employed, however, is to be preferred.
26. In addition to the above classification there are the fol-
lowing outstanding types :
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as ELECTRO-DYNAMIC MACHINERY.
(a.) Sii^le-field-coil multipolar machinest or machines in which
multipolar magnets are operated by a single exciting field
coiL
(b.) Ccmmutaiorless conHnutms^atrrent mackines, or so-called
unipolar mackines, in which the E. M. Fs. generated in the arma-
ture, being obtained by the continuous cutting of flux in a
uniform field, have always the same direction in the circuit,
and do not, therefore, need commutation. The term unipolar
is both inaccurate and misleading, as a single magnetic pole
does not exist.
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CHAPTER III.
MAGNETIC FLUX.
27, A magnet is invariably accompanied by an activity in
the space or region surrounding it. Every magnet produces a
magnetic field or flux, which not only passes through the sub-
stance of the magnet itself, but also pervades the space sur-
rounding it. In other words, the property ordinarily called
magnetism is in reality a peculiar activity in the surrounding
ether, known technically as magnetic ^ux.
By a simple convention magnetic flux is regarded as passing
out of the north-seeking pole of a magnet, traversing the space
surrounding the magnet, and finally re-entering the magnet at
its south-seeking pole. Magnetic flux, or magnetism, is cir-
•cuital ; that is, the flux is active along closed, re-entrant curves.
28. Although we are ignorant of the true nature of magnetic
flux, yet, perhaps, the most satisfactory working conception
we can form concerning it, is that of the ether in transtatory
motion ; in other words, in a magnet, the ether is actually
streaming out from the north-seeking pole and re-entering at
the south-seeking pole.
Since the ether is assumed to possess the properties of a
perfect fluid ; t. e., to be incompressible, readily movable, and
almost infinitesimally divisible, it is evident that if a hollow
tube, or bundle of hollow tubes, of the same aggregate dimen-
sions as a magnet, be conceived to be provided internally with
a force pump in each tube, and that such tube be placed in free
ether, then, on the action of the force pumps, a streaming would
occur, whereby the ether would escape from one end of each
of the tubes, traverse the surrounding space, and re-enter at
the other ends of the tubes. Moreover, if the stream lines,
through which the ether particles would move under such ideal
circumstances, were mapped out, they would be found to coin-
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3" ELECTRO'DYlfAMIC MACHINERY.
cide with the observed paths which the magnetic stream lines
take in the case of a magnet.
Similar stream hnes could be attually observed in the case
of a hollow tube provided internally with a pump, and filled
with and surrounded by water ; only, in this case, on account
of the friction of the liquid particles, both in the tube and
between themselves, work would require to be done and energy
expended in maintaining the motion, and, unless such energy
WATGK,
STREAM LINES
were supplied, the motion would soon cease. In the case of
the ether, however, there being, by hypothesis, no friction,
although energy would probably be required to set up the
motion, yet, when once set up, no energy would be required
to sustain it, and the motion should continue indefinitely.
This is similar to what we find in the case of an actual steel
magnet. The above theory is merely tentative. The real
nature of magnetism may be quite different ; but, for practical
purposes, assuming its correctness, since there is no knowledge
as to the pole of the magnet from which the ether issues, it is
assumed, as above stated, to issue from the north-seeking pole.
29. Fig. 3o represents, diagramatically, a tube provided at
its centre with a rotary pump P, and immersed in water. If
the pump were driven so as to force the water through the tube
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MAGNETIC FLUX. 3>
in the direction of the arrows; /. e., causing the water to enter
the tube at S^ and leave it at N, then stream lines would be
produced in the surrounding water, taking curved paths, some
of which are roughly indicated by arrows.
Fig. loA represents the application of this hypothesis to the
case of a bar magnet of the same dimensions as the tube.
Here the magneto-motive forte of the magnet corresponds to the
water-motive force of the pump in Fig. 20, and is hypothetically
assumed to cause an ether stream to pass through the magnet
in the direction indicated by the arrows ; namely, to enter the
magnet at the south pole and issue at the north pole. These
ether streams would constitute hypothetically the magnetic
flux, and would pass through the surrounding space in paths
roughly indicated by the arrows. The actual flux paths that
would exist in the case of a uniformly magnetized short bar
magnet are more nearly shown in Fig. ai. Here it will be
noticed that the fluz by no means issues from one end only of
the magnet, re-entering at the other end. On the contrary,
the flux, as indicated by chains of iron filings, issues from the
sides as well as from the ends of the bar. The reason for this
is evidently to be found in the fact, that each of the particles
or molecules of the iron, is, in all probability, a separate and
independent magnet, and therefore must issue its own ether
stream independently of all the rest. The effect is therefore
not unlike that of a very great number of minute voltaic cells
connected in series into a single battery, and the whole
immersed in a conducting liquid where side leakage can exist.
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3» ELECTRO-DYNAMIC MACHINERY.
30. The magnetic field, that is the space permeated by mag-
netic flux, may be mapped out in the case of any plane section
by the use of iron filings. For example, Fig. ai, before alluded
to, as representing the flux of a straight-bar magnet, had its
flux paths mapped out as follows : A glass plate, covered with
a thin layer of wax, was rested horizontally on a bar magnet,
with its wax surface uppermost It was then dusted over with
iron filings and gently tapped, when the iron filings arranged
themselves in chain-forms, which are approximately those of
FIGS. 32, A AND B.— MACNBTIC FIELDS BETWEEN PARALLEL BAK HAGNBTS.
the stream-lines of magnetic flux. A satisfactory distribution
having been obtained in this manner, the glass plate was gently
heated in order to fix the filings. On cooling, the filings were
sufficiently adherent to the plate to permit it to be used as the
positive from which a good negative picture can be readily
obtained by photographic printing,
31. A modification of the above process was employed in the
case of Figs. 22, A and B, shown above. Here a photographic
positive was obtained by forming the field, in the manner pre-
viously explained, on a sensitized glass plate in a dark room,
instead of on a waxed plate ; and, after a satisfactory grouping
of filings had been obtained under the influence of the field,
exposing the plate momentarily to the action of light, as, for
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MAGNETIC FLUX. 33
example, bj the lighting of a raatch. The filings are then
removed, the plate developed, and the negative so obtained
employed for printing.
32. Magnetic flux may vary in three ways; namely,
(i. ) In direction.
(2.) In intensity.
(3.) In distribution.
The direction of magnetic flux at any point can be readily
determined by the direction assumed at that point, by the
magnetic axis of a very small, delicately suspended compass
needle. The compass needle always comes to rest as if threaded
by the flux, which enters at its south pole, and leaves it at its
north pole, thus causing the needle to point in the direction of
the flux. Assuming that a compass needle may be represented
FIG. 33.— HYDRAULIC AKALOGUB SHOWING ATTKACTIOS OF OPPOSITE POLES.
by a little tube containing an ether force pump, the tube
would evidently come to rest when the flux it produced passed
through it in the same direction as the flux into which it was
brought. That is to say, if the needle be brought into the
neighborhood of a north pole, it will come to rest with its
south pole pointing toward the north pole of the controlling
magnet, since in this way only could a maximum free ether
motion be obtained. If, however, the compass needle be held
in the opposite direction; /. e., with its north-seeking pole
toward the north-seeking pole of the magnet, the two opposed
stream lines will, by their reaction, produce a repellent force.
These effects are generally expressed as follows :
Like magnetic poles repel, unlike magnetic poles attract.
Strictly speaking, this statement is not correct, since, what-
ever theory of magnetism be adopted, it is the fluxes and not
the poles which exerclsq attraction or repulsion.
33. Fig. 33 represents the action of the flux from a magnet
apon a small compass needle, as illustrated by the hydraulic
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34 ELECTRO-DYNAMIC MACHINERY.
analogy. The water is represented as streaming through the
tube O N, and issuing at the end N, in curved stream lines.
Suppose the small magnet, or compass needle S N, also has a
stream of water flowing through it, entering at S^^ and leaving
at N^. Then, if the compass needle be free to move about its
centre of figure, it will come to rest when the stream from the
large tube,<7 N, flows through the smaller tube from 5, to N^
that is, in the direction of its own stream.
If, however, the small tube 5', N^ is not free to move, but is
fixed with its end jV, toward the end N of the larger tube, as
no. 14.— HVDKAULIC
shown, in Fig. 24, then the opposite streams will conflict, and
produce, by their reaction, the effect of repulsion between the
tubes.
34, Magnetic flux possesses not only definite direction, but
also magnitude at every point; that is to say, the flux is
stronger nearer the magnet than remote from it. For example,
considering a magnet as being represented by a tube with an
ether force pump, the velocity of the ether flux will be a maxi-
mum inside the tube, and will diminish outside the tube as we
recede from it. The intensity of magnetic flux is generally
called its magtutU intensity or fiux density.
Faraday, who first illustrated the properties of a magnetic
field, proposed the term lines of magnetic force, and this term
has been very generally employed. The term, however, is
objectionable, especially when an attempt is made to conceive
of magnetism as possessing flux density, or as varying in
intensity at any point; for, in accordance with Faraday's con-
ception, the idea of an increased flux would mean a greater
number of lines of magnetic force traversing a given space.
While this might be assumed as possible, still the conception
that magnetism acts along lines, and not through spaces, is
very misleading. An endeavor has been made to meet this
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MAGNETIC FLUX. 35
objection by the introduction of the term tubts ef force. A
far simpler working conception is that of velocity of ether.
that is, increased quantity passing per second, as suggested
by the force-pump analogue. Here the increased flux density
at any point would simply mean an increase of ether velocity
at such point.
35, Intensity of magnetic flux is measured in the United
States, in units called ^fl«w«, after a celebrated German mag-
netician named Gauss. A gauss is an intensity of one line of
force, or unit of magnetic flux, per square centimetre of cross-
sectional area, and is an intensity of the same order as that
produced by the earth's magnetism on its surface. For ex-
ample, the intensity of the earth's flux at Washington is about
0.6 gauss, with a dip or inclination of approximately 70°.
Magnetic flux may be uniform or irregular. Fig. 25 A,
shows a uniform flux distribution, as represented diagrammati-
cally, by straight lines at uniform distances apart. That is to
say, uniform intensity at any point is characterized by rectan-
gularity of direction in path at that point. Irregular intensity
is characterized by bending, and the degree of departure from
uniformity is measured by the amount of the bending. Irreg-
ular flux density may be either converging, as at B, or diverging,
as at C. Convergent flux increases in intensity along its path,
and divergent flux diminishes.
36. When the flux paths are parallel to one another, the
intensity must remain uniform. Thus, in Fig. 35 at A, let the
afea, A BCD, be 1 square centimetre, then the amount of flux
which passes through it in this position, or, in our hydraulic
analogue, the quantity of water which would flow through it in
a given time, will be the same if the area be shifted along the
stream line parallel to itself into the position E F G H.
When the flux converges, as at B, in Fig. 25, then the
amount of flux passing through the normal square centimetre
J J K L, will, further on, pass through a smaller intercepting
area, say one-fourth of a square centimetre jV A'' O /", and
consequently, the intensity at this area would be four times
greater, and, in the hydraulic analogy, the same quantity of
water passing per second, flowing through a cross sectional
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30 ELECTRO-DYNAMIC MACHINERY.
area four times more constricted, will flow there with four
times the velocity.
When the flux diverges, as at C, the opposite effect is pro-
duced. Thus the flux shown in the figure as passing through
the area Q R S T, say one-fourth of a square centimetre,
lirtMuKl UnMryirf
would, at C" K WX, pass through one square centrimetre, at
four times less density, or, in the case of the hydraulic analogy,
at one-fourth of the velocity.
37. The existence of a magnetic flux always necessitates the
expenditure of energy to produce it. In the case of the ether
pump, assuming that energy is required to establish the flow
through the tube, this energy being imparted to the ether,
becomes resident In its motion, so that ether, plus energy
of motion, necessarily possesses different properties from ether
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MAGNETIC FLUX. 37
at rest. In the same way in the case of a magnet, the energy
required to set up the magnetic flux; /. e., to magnetize it, is
undoubtedly associated with such flux. Wherever the mag-
netic intensity is greatest, there the corresponding ether
velocity, according to our working hypothesis, is greatest, and
in that portion of space the energy of motion is greatest.
38. It is welt known, dynamically, as a property of motion,
that the energy of such motion in a given mass varies as the
square of the velocity, so that, by analogy, if magnetic flux
density corresponds to ether velocity, we should expect that
the energy associated with magnetic flux should increase with
WtBB CAKRYINC
the square of its intensity. This is experimentally found to
be the case. Thus if CB, represents the intensity of magnetic
flux, expressed in gausses, then the energy in every cubic cen-
timetre of space, except in iron or other magnetic material;
i. e., in the ether permeated by such intensity, is ^ ergs.
Thus, if a cubic inch of air (a volume of 16.387 cubic centi-
metres), be magnetized to the intensity of 3,000 gausses, the
energy it contains, owing to its magnetism, will be
.6.387 X 3,°°o >c 3,°°° ^ „.58,8 ^ ,„,„g,. ^ „.536S jouk.
39. Just as in the electric circuit,the presence of an electric
current necessitates the existence of an E. M. F. producing it,
BO in a magnetic circuit, the presence of a magnetic flux neces-
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3a ELECTRO-DYNAMIC MACHINERY,
sitates the existence of a magneto-motive force (M. M. F.)
producing it.
We know of but two methods by which a M. M. F. can be
produced, viz. :
(i.) By the passage of an electric current, the neighborhood
of which is invested with magnetic properties; t. e., surrounded
by magnetic flux;
(a.) As a property inherent in the ultimate particles of cer-
tain kinds of matter, possibly the molecules, of the so-called
magnetic metals.
The passage of an electric current through a long, rectilin-
ear conductor, is attended by the production of a magnetic
field in the space surrounding the conductor. The distribution
of flux in this field, is a system of cylinders concentric to the
conductor, and is directed clock-wise around the conductor, if
the current be supposed to flow through the clock from its face
to its back. This distribution is shown in Figs. 36, 37 and 28.
Fig. 36 represents the distribution as obtained by iron filings.
The density of the flux is roughly indicated by the density of
the corresponding circles.
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MAGNETIC FLUX. 39
40. Fig. 37 shows the geometrical distribution of the flux
paths around a wire carrying a current, which is supposed to
flow from the observer through the paper. Here a few of the
flux paths are indicated by the circles, i, 3, 3, 4 and 5, while
the direction is shown by the arrows. The distribution of the
flux is such that it varies in intensity, outside the wire, inversely
as the distance from the axis of the wire, and the total flux
between any adjacent pair of circles in the figure is the same,
no. a8.— DIAGRAM 09 RELATIVE DIRECTIONS OP MAGNETIC FLUX
AND ELECTRIC CURRENT.
for example, between i and a, or between 4 and 5. Or, in the
hydraulic analogue, the total flow of water per second, between
any pair of adjacent circles is the same, as between the circles
3, 3, or 4, 5, the velocity diminishing as the distance from the
axis of the wire.
Fig. 28 represents the direction of the flux round the active
conductor, the current flowing from the observer through the
shaded disc.
41, The physical mechanism of the magnetic flux produced
by a current is unknown, but if an electric current be assumed
to be due to a vortex motion of ether in the active wire, the
direction of which is dependent on the direction of the current
through the wire, then such vortex motion will be accompanied
by such a distribution of circular stream-lines in the ether, as is
actually manifested, and, when the direction of the current
through the conductor is changed, the direction of the stream-
lines outside the conductor will also necessarily be changed.
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40 ELECTRO-DYNAMIC MACHINERY.
As the strength of the current through the wire increases, the-
velocity of the ether surrounding the wire increases; t. e., the^
intensity of the magnetic field everywhere increases.
42. If a conductor conveying a current be bent in the form
of a circle as shown in Fig. 29, and a current, of say one
ampere, be sent through the conductor, there passes through
the loop so formed a certain number of stream-lines as repre-
sented dia grammatically. If now, the current in the wire be:
THRBADBD
doubled, that is increased to two amperes, the flux intensity
everywhere will be doubled. The same effect, however, can
be practically obtained by sending one ampere through the
double loop, shown in Fig. 30, provided the two turns lie very
close together. Magnetic flux through a loop, will depend,
therefore, upon the number of ampere-turns, so that, by wind-
ing the loop in a coil of many turns, the flux produced by
a single ampere through the coil may be very great. The
M. M. F. product by a current, therefore, depends upon the
number of ampere-turns.
43. The unit of M. M. F. may be taken as the ampere-turn,
and it frequently is so takeirfor purposes of convenience. The
fundamental unit, however, of M. M, P., in the United States,
is the gilbert, named after one of the earliest magneticians.
Dr. Gilbert, of Colchester. The gilbert is produced by — of a.
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MAGNETIC FLUX. 41
C. G. S. unit current-turn, and, since the C. G. S. unit of current
is ten amperes, the gilbert is produced by — ampere-turn (o.S
4 ff
approximately, more nearly 0.7958). It is only necessary,
therefore, to divide the number of ampere-turns in any coil of
FIO 31.— mSTttlBUTION OF FLUX IN PLA.SB OVER A HORSE-SHOB
wire by 0.8, that is to multiply the number of ampere-turns by
1.15, more nearly 1.257) '" obtain the M. M, F, of that coil
expressed in gilberts.
44, Figs. 31 to 42 are taken from actual Aux distributions as
obtained by iron filings, and represent a series of negatives or
positives secured by the means already described. A study of
Fia. 3a. — DISTRIBUTION OP FLUX IN PLANE OVER A HORSE-SHOE
MAGNET.
such flux-paths assists the student to mentally picture the flux
distributions which occur in practice.
Figs. 31 and 32 are the respective positive and negative
photographic prints taken in the case of a horse-shoe magnet.
Here the filings are absent in a region outside the magnet in
the neighborhood of the poles N S. The cause of this is as fol-
lows : the fields were obtained by sprinkling iron filings over
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42 ELECTRO-DYNAMIC MACHINERY.
a smooth glass surface; the tapping of the surface necessary
to insure the arrangement of the filings under the influence of
the magnetic flux, has caused an accumulation of filings
around these poles at the expense of the gap immediately in
front of the poles which would otherwise be more fairly filled.
F FLUX BY IRON FILINGS IN PLANE C
45. The student should carefully avoid being misled by the
supposition that the relative attractive tendencies of the iron
filings in such diagrams represent the corresponding densities
of the magnetic flux, for the reason that in a uniform mag-
netic flux such as shown at A, in Fig. 25, there is no attrac-
tion of iron filings, whatever its intensity, although, of course,
PIG. 34,— DISTMBUTION OF FLUX BY CUT IRON WIRE IN PLAN!
a directive tendency still exists. In order that there should
be any attractive tendency, in contradistinction to a mere
directive tendency, it is necessary that the intensity of
the magnetic flux shall vary from point to point; or, in
other words, that the flux shall be convergent. The greater
the degree of convergence the greater the attractive force.
Consequently, variations of flux intensity as indicated by iron
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MAGNETIC FLUX. 43
filings always exaggerate the appearance of flux density.
Generally speaking, it is only the directions assumed by the
filings in such diagrams, as indicative of the directions of the
flux, which can be regarded as trustworthy. The neglect of
this consideration has given rise to a popular belief that
magnetic streamings occur with greater density at points,
than at plane or blunt surfaces, which is not the case. There
must necessarily be a rapid convergence or divergence of mag-
netic flux at points, although the maximum density may not be
very great. Owing to this convergence, iron filings, particles,
nails, etc., are attracted more powerfully at such points, even
though the uniform intensity of flux at plane surfaces in the
vicinity may be greater.
46. Fig. 33 shows the distribution of magnetic flux as
obtained by iron filings in a horizontal plane over the vertical
poles of an electro-magnet. Here the flux-paths pass in
straight lines between the nearest paints of the adjacent poles,
and in curved lines over all other parts of the plane. If we
imagine, following the hydraulic analogue, that water streams
proceed from minute apertures in one of the poles, and that
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44 ELECTRO-DYNAMIC MACHINERY.
the magnet is immersed in water, then the stream-lines so pro-
duced in the water as it emerges from pole N, and enters
through pole S, will be the same as is indicated b^ the iron
filings. Fig. 34 shows a similar distribution of flux over
the poles of the same electro-magnet, where short pieces
of fine soft iron wire were used in place of the iron filings.
Here the flux-paths have practically the same distribution as in
the preceding case.
Figs. 36 and 37 show the distribution of flux by iron filings
in a horizontal plane over the poles of the magnet represented
in Fig. 35, the magnet being presented vertically in Fig. 36,
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MAGNETIC FLUX 45
and faorizontally in Fig. 37, to the plane. Here the general
-distribution of flux between the polar surfaces is rectilinear.
DISSIMILAR POLES.
Fig. 38 illustrates the flux distribution attending the
approach of what are called unlike poles. Here the ether
K-PATHS BETWEEN SIMILAR POLES.
Streams we assume to issue from N, in entering the magnet S,
take the paths indicated.
Fig. 39 illustrates the flux distribution attending the
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46 ELECTRO-DYNAMIC MACHINERY.
approach of what are called like poles. Here the hypothetical
ether streams issuing from N, N, impingoi, as shown, and pro-
duce a neutral line, A A, corresponding to slack water in the-
hydraulic analogue.
—FLUX-PATHS U
Fig. 40 shows the distribution of flux in the case of two-
straight bar magnets laid side by side with like poles opposed.
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MAGNETIC FLUX. 47
The imaginary ether streams again oppose and the neutral
line B B, is produced as shown.
Fig, 41 shows the distribution of magnetic flux in the case
of two straight bar magnets, laid side by side, with unlike
poles opposed. Here, according to hypothesis, some of the
ether streams issuing from each magnet, pass back through
the other magnet, the remainder closing their circuit through
FIG. 4>. — FLUX-PATHS SURKOUHDING ANOMALOUS MAGNBT.
the air outside. A curious central region between the mag-
nets, bounded by curves resembling hyperbolas is shown at
C, where, by symmetry, no ether motion penetrates, and thus
corresponding, in the hydraulic analogue, to calm water.
Fig. 42 shows the distribution of flux over the surface of
what is commonly called an anomalous magtut, that is a magnet
having two similar poles united at its centre; or, in other
words, having two separate magnetic circuits. Here the dis-
tribution of flux is similar to that in Fig. 40, where like poles
are approached.
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CHAPTER IV.
NON-FERRIC MAGNETIC CIRCUITS.
47. As we have already seen, magnetic flux always flows in
closed paths, or forms what is called a magnetic circuit. The
quantity of magnetic flux in a magnetic circuit depends not
only upon the magneto- motive force, but also on the disposition
and nature of the circuit For example, it is not to be sup-
posed that the flux produced by the 12 ampere-turns (15.084
gilberts) in the right-handed coil or helix of Fig. 43, by one
ampere flowing through the twelve turns shown, would be
exactly the same, either in magnitude or distribution, as the flux
from a single turn carrying 13 amperes, although the M. M. F.
would be the same in each case. Just as in the case of an
electric circuit, the current produced by a given E. M. F.
depends on jhe resistance of the circuit, so in the case of a
magnetic circuit, the magnetic flux produced by a given M.
M. F. depends on a property of the circuit called its magnetic
reluctance, or simply its reluctance.
Magnetic reluctance, therefore, is a property corresponding
to electric resistance, and is sometimes defined as the resist-
ance of a circuit to magnetic flux.
The resistance, in ohms, of any uniform wire forming portion
of an electric circuit is equal to the resistivity, or specific resist-
ance, of the wire, multiplied by the length of the wire, and divided
by its cross-sectional area. In the same way, the reluctance, in
oersteds, of any uniform portion of a magnetic circuit, is equal
to the reluctivity, or specific magnetic resistance of the portion,
multiplied by its length in centimetres, and divided by its
cross sectional area in square centimetres. The reluctivity of
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NOff-FEX/ilC MAGNETIC CIRCUITS. 49
air, wood, copper, glass, and practically all substances except
iron, steel, nickel and cobalt, is unity. Strictly speaking, ttie
reluctivity of the ether in vacuous space is unity, but the dif-
ference between the reluctivity of vacuum and of all non-
magnetic materials is, for atl practical purposes, negligibly
small. Thus, the reluctance of a cylinder of air space of lo
cms. long and z sq. cms. in cross-sectional area, is 5 oersteds.
48. The reluctance of a circuit is measured in units of relucl-
ante called oersteds. An oersted is equal to the reluctance of
a cubic centimetre of air (or, strictly speaking, of air-pump
vacuum) measured between opposed faces.
Having given the reluctance of a magnetic circuit, and its
total M. M. F., the tlux in the circuit is determined in accord-
ance with Ohm's law; that is 4" = -^ where ^, is the flui in
webers, £F, is the magneto-motive force in gilberts, and <R, the
reluctance in oersteds. It may afford assistance to con-
corresponding magnetic expression, webers =
oersteds.
49. Tht unit 0/ magnetic fiux, in the United States, is called the
•weber, and is equal to the flux which is produced by a M. M. F.
of one gilbert acting through a reluctance of one oersted, cor-
responding in the above expression to the am/»-c,*the unit of
electric flux, which is the electric flux or current produced by an
E. M. F. of one volt through a resistance of one ohm. For
example, if an anchor ring of wood, such as is represented in
Fig. 44, have a cross section of 10 sq. cms. and be uniformly
wrapped with insulated wire, then when the current passes
through the winding, the magnetic circuit will be entirely con-
fined to the interior of the coil or solenoid, and no magnetic
flux will be perceptible in the region outside it. This is the
only known form of magnetic circuit in which the flux-paths
can be confined to a given channel. These fiux-paths are all
circular, and -possess the same intensity around each circle.
If the mean circumference of the ring be 60 cms., the reluct-
ance of the magnetic circuit will be approximately — =
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5°
ELECTRO-DYNAMIC MACHINERY.
6 oersteds, as in the similar case of electric resistance. If the
number of turns in the winding be 200, and the exciting current
steadily maintained at four amperes, the M. M. F. in the
magnetic circuit will be 80a ampere-turns, or 1,005.6 gilberts.
1,005. 6_
From this the total flu:
webers.
through the ring will be -
■ = 167.6
Fio. 44.— semoNs
WRAPPED WITH
50. Besides the case of the anchor ring, represented in Fig.
44, the magnetic circuit of which, being entirely confined to
the interior of the coil, permits its reluctance to be readily
calculated, and the flux to be thus arrived at, another case,
almost as simple, is afforded by a long straight helix of length
I cms., uniformly wrapped with «, turns per cm. or N = I n,
turns in all. Such a helix, when excited by a current of /
amperes, develops a M. M. F. of « /ampere-turns, or 1.257 « /
gilberts in each centimetre, or 1.357 JV/ gilberts, for the total
M. M. F.
The magnetic circuit of such a solenoid is roughly repre-
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NON-FERRIC MAGNETIC CIRCUITS. SI
sented in Fig. 30 A. An inspection of this figure will show that
flux passes through the interior of the helix in parallel streams,
until it reaches a comparatively short distance from the ends,
when it begins to sensibly diverge, and, emerging into the
surrounding space, is diffused through widely divergent paths.
That is to say, the magnetic circuit is characterized by two
distinct regions; namely, that within the coil, where the flux
is uniform, and, except near the ends, of a maximum intensity,
and that outside and beyond the ends of the coil, where the
flux is divergent and greatly weakened in intensity.
51. In the case of a long, straight, uniformly-wrapped helix,
the reluctance of the circuit may be considered as consisting
of two distinct portions; namely, a straight portion occupying
the interior of the coil and lying practically between the ends,
and a curved or diffused portion exterior to the coil. The
reluctance of the first, or interior portion, will be practically
— oersteds, where a, is the cross sectional area of the interior
of the coil in square cms. and /, the length of the coil in cms.,
or, more nearly, the reduced length of the non-divergent flux.
It will be seen, therefore, that the interior of the coil behaves
like a straight wire carrying electric flux, since it practically
confines the flux to its interior, and, this particular portion of
the magnetic circuit is similar to the case of the anchor ring
above referred to, where (he magnetic flux is confined to the
interior of the ring.
Since the external circuit is dUFused, its reluctance cannot be
so simply expressed. Its value, however, may obviously be
dealt with as follows : although the mean length of the flux-
paths outside the coil is greater than in the interior portion,
yet the area of cross section of the circuit is enormously
extended. It would appear, therefore, that in the case of an
indefinitely long straight coil, the external reluctance becomes
negligibly small compared with the internal reluctance, and
may be left out of consideration. In such a case, therefore,
the fiuz established becomes
_ 1.JS7 /« / , .
^ = — tS- = i.aS7 n J a webers;
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Sa ELECTRO-DY.VAMU: MACHINERY.
and, since, within the coil, this flux passes through a cross sec-
tional area of a square centimeters, the interior intensity will be
gausses.
Strictly speaking, therefore, this is the intensity of flux within
an indefinitely long straight helix, and is approximately the
intensity within helices which have lengths more than ao times
their diameter.
52. We have now discussed two cases of non-ferric circuits,
whose reluctance is readily calculated; namely, a closed cir-
cular coil and a long straight helix.
In all other cases, the reluctance of a magnetic circuit is
much more difficult to compute, although the fundamental
relations remain unchanged.
When the magnetic circuit is non-ferric, although the
reluctivity of the circuit always equals unity, yet, owing to the
difficulty of determining the exact paths followed by the diver-
gent flux, the reluctance is difficult to determine.
Most practical magnetic circuits, however, are composed
either entirely, or mainly, of iron. At first sight, the intro-
duction of iron into the circuit would appear to make the
reluctance more difficult to determine, because the reluctivity
of iron not only varies greatly with different specimens, but
also with its hardness, softness, annealing, and chemical com-
position. Moreover, the apparent reluctivity of iron varies
markedly with the density of the flux passing through it
Iron, when magnetically saturated, possesses a reluctivity
equal to that of air; while, as we have seen, at low intensities,
the reluctivity is much smaller, and may be several thousand
times smaller.
Since, however, ferric circuits, as ordinarily employed,
practically confine their flux-paths to the substance of the
iron, and, since the reluctance of the iron is so much less than
the reluctance of the alternative air path outside, the air flux
may usually be neglected. Even where, owing to the reluct-
ance of the air gaps In the circuit, such as in the case of
dynamos and motors, a considerable amount of magnetic leakage
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NON-FERRIC MAGNETIC CIRCUITS. SJ
or difumm may take place through the surrounding air, yet it
is preferable to regard this leakage as a deviation from the
iron circuit, which may be separately treated and taken into
account, and that the flux passes principally through the
iron. For these reasons, ferric or aero-ferric circuits, at least
in their practical treatment, are simpler to determine and
compute than non-ferric circuits, since, although their
reluctivity is variable at different points, yet the geomet-
rical outlines of the flux-paths can be regarded as limited,
and the reluctance of these paths can be readily determined
approximately.
53. Magnetising force may be defined as the space rate at
' which the magnetic potential descends in a magnetic circuit.
Since the total fall of magnetic potential is equal to the M. M. F.
in the circuit, just as the total ' ' drop " in a voltaic circuit is
equal to its E. M. F. Consequently, the line integral or sum of
magnetizing force in a magnetic circuit must be equal to the
M. M. F. in that circuit. In other words, if we multiply the
rate of descent in potential by the distance through which that
rate extends, and sum all such stages, we arrive at the total
descent of magnetic potential. For instance, in Fig. 44 the
total difference of magnetic potential is 1,005.6 gilberts, which,
by symmetry, is uniformly distributed round the entire circuit.
Since the mean length of this circuit is 60 cms. the rate of fall
of potential is ■ ' ^" ■ = i6.76gilberts-per-centimetreall round
the ring, and this is, therefore, the magnetizing force, or, as it
is sometimes called, the magnetic force. This magnetizing force
is usually represented by the symbol 3C, and, when no iron or
magnetic metal is included in the circuit, is numerically iden-
tical with the flux density (fi, so thatSC, is expressed in Alberts-
per-centimetre. The term magnetizing force was adopted
from the old conception of magnetic poles ; for, if a pole of unit
strength could be introduced intoaflux of intensity 3C gausses,
the mechanical force exerted upon the pole would be 3C dynes,
directed along the flux-paths. In any magnetic circuit, if we
divide the M. M. F. in gilberts, by the length of a flux-path,
we obtain the average value of the magnetizing force (or flux
density in the absence of iron). Thus, in Fig. ai, if the long
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54 ELECTRO-DYNAMIC MACHINERY.
beliz there represented, has a M. M. F. of 5,000 gfilberts, aad a
particular flux-path has a length of 500 cms., the mean maginet-
izing force, will be = 10 gilberts-per-centimetre, and the
mean flux density will be 10 gausses, if there is no iron in the
circuit. If there is iron, the m&.a prime flux lUnsity or magnet-
ising force, will still be 10 gilberts-per-centimetre, but the flux
density established in the circuit will be greatly in excess of 10
gausses.
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CHAPTER V.
FERRIC MAGNETIC CIRCUITS.
54. We will now proceed to study the phenomena which
occur when trrni is introduced into a magnetic circuit, as for
esample, into the circuit of the closed circular coil shown in
Fig. 44, the mean interior circumference of which is 60 cms.,
and the mean cross sectional area 10 sq. cms. We have
seen that if this ring be excited with 800 ampere- turns, or
tcw5.6 gilberts, the flux through the ring will be 167.6 webers;
or, since the cross section of the ring is ten square centimetres,
the intensity will be — — = 16.76 gausses, and this inten-
sity would remain practically unchanged if the substance of
the ring were copper, brass, lead, zinc, wood, glass, etc.
When, however, the ring is made of iron or steel, a very marked
change takes place ; the flux instead of being 167.6 webers,
becomes, say, 170,(^00 webers, with a corresponding increase
in intensity. This increase of flux in the circuit must either
be due to an increase in the M. M. F., or to a diminution in
the reluctance. It is usual to consider that iron conducts mag-
netic flux better than air; or, in other words, has a greate; mag-
tutic permeability than air. This idea corresponds to a reduc-
tion of reluctance similar to the reduction of resistance in an
electric circuit. Although generally accepted, this conception
is manifestly incorrect ; for if the increased flux, due to the
presence of iron in the ring, disappeared immediately on the
removal of the M. M. F., there would be no preponderance of
evidence in favor of either hypothesis. But the magnetic flux
does not entirely disappear on the cessation of the prime
M. M. F. On the contrary, in the case of a closed iron ring,
the greater portion of the flux remains in the condition called
residuai magnetism.
55, It is evident, therefore, since M. M. F. is necessary
to maintain the residual magnetic flux in the iron, that this
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Sfi ELECTRO-DYNAMIC MACHINERY.
M. M. F. is the cause of the increase in magnetic flux when the
prime M. M. F. is applied, and that, therefore, the increased
flux cannot be due, except, perhaps, in a very small degree,
to any change in the reluctivity of the medium, but to the
establishment of a M. M. F. in Ihe iron itself under the influ-
ence of the magnetizing flux. It is now almost certain that
the ultimate particles of the iron, the molecules, or the atoms,
are all initially magnets ; »'. e., inherently possess M. M. Fs.
and magnetic circuits. The origin of this molecular magnetism
in iron is, however, not yet known. In the natural condition,
all the separate magnets of which iron is composed, are dis-
tributed indifferently in all directions, so that their circuits-
neutralize one another and produce no appreciable external
effects. Under the influence of a magnetizing flux, these mole-
cular magnets tend to become aligned, and to break up their
original groupings. As they become aligned, and their M. M.
Fs. become similarly directed, they are placed in series, and
their efiFects are rendered cumulative, so that they exercise an
increasing external influence, and an extending external flux.
Or, taking the hydraulic analogue already referred to, and
regarding each separate molecular magnet as a minute ether
pump, as all the ether pumps are brought Ihto line, the streams
they are able to direct are increased in velocity, and are, there-
fore, carried further into the surrounding space. Conse-
quently, the flux produced in the magnetic ring shown in Fig.
44, when furnished with an iron core, may be regarded as aris-
ing from two distinct sources of M. M. F. ; namely,
(i.) The prime M. M. F., or that due to the magnetizing
current which produces the flux through the circuit and sub-
stance of the iron, the value of which is practically the same
as though the core were of wood or other non-magnetic
material. This flux may oe called the firimc Jfux and possesses
a corresponding prime intensity. In the case considered, the
prime intensity or magnetizing flux density is 16.76 gausses.
This magnetic intensity, acting upon the molecules of the iron,
produces :
(2.) The induced M. M. F., which may be called the aligned
or structural M. M. F. , and depends for its magnitude not only
upon the quality of the iron, but also upon the intensity of the
prime flux. The harder the iron, and the greater its mecbani-
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FERRIC MAGNETIC CIRCUITS. 57
cal tendency to resist molecular distortion, the greater must be
the prime intensity or the magnetic distorting power, in order
to bring about the full structural M. M. F. When the prime
intensity has reached such a magnitude that all the separ-
ate molecular magnets in the iron are similarly aligned, the
iron is said to be saturated, and the M. M. F. it produces is a
maximum, and, on the removal of the prime M. M. F. the
structural M. M. F. will, in the case of a closed ring, largely
remain, especially if the ring be of hard iron or steel. If, on
FIG. 45.— IRON RING PROVIDED WITH AIR-OAP, AND WOUND WITH WIRE.
the contrary, the ring be of soft iron, and have an air-gap cut
in it, the structural M. M. F. may largely disappear. The
relation between the structural M. M. F. and its flux, and the
prime M. M. F. and the intensity which produces it, is complex,
and can only be ascertained by experimental observation.
56. Fig. 45 represents the same iron ring with a saw-cut or
air-gap at A, having a width of 0.5 cm. The reluctance of
this air-gap, which, neglecting diffusion, has a length of 0.5
cms. and a cross-section of 10 sq. cms. is-^ = 0.05 oersted. If
the total structural M. M. F., established in the ring under
excitation, be 180,000 gilberts, then, immediately on the with-
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58 ELECTRO-DYNAMIC MACHWERY.
drawal of the prime M. M. F., the residual flux through the
circuit will be — ^ — = 30,000 webers. Where this flux
passes through the reluctance of the air-gap there will be
established a C. M. M. F., just as in the electric circuit where
a current of / amperes passes through a resistance of R, ohms,
there is established a C. E. M. F. oi I R volts. So that
the C. M. M. F. has in this case the value, F = ^ R =
30,000 X 0.05 = 1,500 gilberts. This C, M, M. F. represents
a mean demagnetizing force of ~ — = 35 gilberts-per-
centimetre, through the iron circuit. If this intensity of de-
magnetizing force is suflicient to disrupt the structural align-
ment of the molecular magnets, the residual magnetism will
disappear. If, however, the intensity be less than that which
the hardness of the iron requires to brealc up its structure, the
residual magnetism will be semi-permanent.
Even though it be admitted that the preceding represents
the true condition of affairs, and though it is the only existing
hypothesis by which the phenomena of residual magnetism can
■be accounted for, nevertheless, ' for practical computations ■
connected with dynamo machinery, it is more convenient to
assume that there is no structural M. M. F. in iron, and that
the difference in the amount of flux produced in ferric circuits
is a consequence of decreased reluctance in the iron; or, in
other words, that iron is a better conductor of magnetism.
We will, therefore, in future, adopt the untrue but more con-
venient hypothesis.
57, The reluctivity of iron may be as low as 0.0005, ^^^
varies with the flux density ; that is to say, the reluctance of a
cubic centimetre of iron, measured between parallel faces, may
be as low as 0.0005 oersted.
58. The fact has been established by observation, that in the
magnetic metals, within the limits of observational error, a
linear relation exists between reluctivity and magnetizing force.
That is to say, within certain limits, as the magnetizing force
brought to bear upon a magnetic metal increases, the apparent
reluctivity of the metal increases in direct proportion. Thus,
taking the case of soft Norway iron, its reluctivity, at a mag-
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FERRIC MAGNETIC CIRCUITS. 59
aetizing force of 4 gilberts-per-centimetre, or prime magnetic
intensity of 4 gausses, may be stated as 0.0005. Increasing
the magnetizing force, the reluctivity increases by 0.000,057
,
Ord
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» C^ l,J4K.l.~l||r-<l.l«J> ••Ix.JK
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I1V.' „ s<ifiirgn (sioMoi4r-<i!oo«+ oioowsc JC
-t-V. - Nomylran (R<»>ta>KO*»<l.«IKII + g.HMMJC
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3 FORCE KfPRrME FLUX DENSITY) GAUSSES
FIG. 46. — CURVES OF RELUCTIVITY IN RELATION TO MAGNETIZING FORCE.
per gauss, and this increase, plotted graphically, would be
represented by a straight line.
59. The accompanying curve sheet represents the results
of actual observations by different observers upon different
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6o ELECTRO-DYNAMIC MACHINERY.
samples of soft wrought iron and cast iron. It will be seen that
in the early stages of magnetization, below a critical magnetiz-
ing force, which varies with different samples from i to, perhaps,
15 gilberts-per-centimetre, corresponding to a prime magnetic
intensity of i to 15 gausses (the latter in the case of cast iron),
the reluctivity decreases with an increase in magnetizing force;
but, when the critical magnetizing force is reached, the direction
of the curve changes and the value becomes linear. Strictly
speaking, the linear relation of reluctivity and magnetizing force,
represented in the figure, is true only for the apparent reluc-
tivity of the metal itself, and is irrespective of the ether which
pervades the metal; for, were this relation strictly linear for all
values of the magnetizing force beyond the critical value, the
reluctivity would become infinite with an infinite magnetizing
force ; whereas, by observation, the reluctivity of the most
highly saturated iron never exceeds unity, that of the air pump
vacuum, or practically that of air. In point of fact we may
consider the magnetism as being conducted through two paths
in multiple ; namely, that of the magnetic metal proper, and
that (rf the ether permeating the metal. The first path may
be called the ferric path of metallic reluctivity, and has a reluc-
tance varying from a minimum at the critical magnetizing force,
up to infinity, by the linear relation. The second is the ether
path of reluctivity, and may be assgmed to have a constant
reluctivity of unity. The Joint reluctivity of the two paths will
be — ^ — = — -j— where v, is the reluctivity of the ferric path.
Since in actual dynamo machinery the value of the magnetiz-
ing force is never much more than 80 gilberts-per-centi metre,
the above consideration is of small practical importance, since
f is, always much less than unity, say o.oi, and the discrepancy
introduced by taking account of the multiple-connected ether
path, is only the difference between o.oi and — '■ = -^ —
I -\- O.OI 1. 01
or about i per cent., so that, for all practical purposes, we may
assume that the metallic reluctivity is the actual reluctivity of
the iron.
Beyond the critical magnetizing force, therefore, the value
of the metallic reluctivity may be readily obtained by the equa-
tion V = a ~\- bX, where a, is the reluctivity which would exist
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FERRIC MAGNETIC CtRCUtlS. 6i
at zero magnetizing force, if the linear relation held true below
the critical value, and b, is the increase in reluctivity per gauss
of prime magnetizing intensity expressed by 3C According to
the present accepted values of the C. G. S. system, reluctivity
is a numeric, and its value never exceeds unity ; thus for
wrought iron a = 0.0004, and b = 0.000,057.
60. If the ring shown in Fig. 44 be composed of wood, and
be excited by 1,000 ampere-turns = 1,357 gilberts, then, since
its mean length of circi^t (circumference) is 60 cms.| and cross
sectional area 10 sq. cms., its reluctance will be 6 oersteds, the
flux ' ■ = 309.5 webers, and the intensity — ^^ e= 30.95
pusses, so that the magnetic force has a rate of descent of mag-
netic potential, the uniform distribution of which is — j . ■■ =
30.95 gilbcrts-per-centimetre. Strictly speaking, the intensity
of the magnetic flux is not uniform over all portions of the area
of cross section of the core, being denser at the inner circum-
ference and weaker at the outer circumference. For example,
if the inner circumference, instead of being 60 cms., which is the
mean value, be 58 cms., the gradient of magnetic potential will
be uniformly ' ^ =31.67 gilberts-per-centimetre, and the
intensity, 31.67 gausses; while, if the outer circumference tJe
63 cms., the intensity at that circumference will be ■- } ■ =
20.37 gausses. Since, however, all such existing differences of
intensity can be made negligibly small, by suf&ciently increas-
ing the ratio of the size of the ring to its cross section, we
may, for practical purposes, omit them from consideration.
61. Suppose now the core of the ring be composed of
soft Norway iron instead of wood; then from the preceding
curves, or the equation,
V = 0.0004 + 0.000,057 3C,
we find that at this mean intensity of 3C = 30.95
V =; 0.0004 ~l~ O.OOII94 ■=. 0.001594,
or about ^^th of that of air. The mean length of the cir-
600 "
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63 ELECTRO-DYNAMIC MACHINERY.
cuit being 60 cms., and its area, as before mentioaed, 10 sq.
cms., its reluctance, under these circamstaDces, will be — x
0.001594 = 0.009564 oersted, and the iluz in the circuit
— L_iZ_ = 131,430 webers, with an intensity of =
0.009564 10
13.143 gausses.
62. If the core of the ring instead of being of soft Norway iron
be made of cast iron, the reluctivity, at 3C = ao.95, would be
approximately, 0.0046, and the reluctance of the circuit 0.0376
oersted, making the total flux 45)54o webers, with an intensity
of 4,554 gausses, or about three times less than with soft Nor-
way iron. The practical advantages, therefore, of construct-
ing cores of soft Norway iron, rather than of cast iron, is man-
ifest, when a high intensity is required.
63. It is important to remember that the entire conception
of metallic reluctivity is artificial, and that although very con-
venient for purposes of computation, yet as already pointed out,
it is incompetent to deal with the case of residual magnetism.
Thus, if the prime M. M. F. from an iron ring be withdrawn, we
should expect the flux to entirely disappear, whereas we know-
that a large proportion will generally remain. Since, however,
electro-dynamic machinery rarely calls residual magnetism into
account, the reluctivity theory is adequate for practical pur-
poses beyond critical magnetizing forces.
64. As another illustration, let us consider a very long rod
of iron, wound with a uniform helix. Here, as we have already
seen, disregarding small portions near the extremities, the
intensity may be regarded as uniform within the helix.
Since the reluctance of the external circuit may be neglected,
this flux density is 1.257 n i, gausses, where n, is the number
of loops in the helix per centimetre of length, and »', is the
exciting current strength in amperes. Or, regarding the
intensity as being numerically equal to the gradient of mag-
netic potential, which changes steadily by 1. 257 n i, per centi-
metre (this being the number of gilberts added in the circuit
per centimetre of length, the fall of potential or drop in the
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FERRIC MAGNETIC CIRCUITS. 63
external circuit being negligible), the gradient, within the helix,
is i.as7 n i gausses as before. A rod of Norway iron i
metre long and a cnis.-in-diaii»eter,hWOund with twenty turns
of wire to the centimetre, carrying a current of i ampere,
would, at this magnetizing force, have an intensity in it of
approximately 1.357 X 10 x i = a5-'^4 gausses. The reluc-
tivity of Norway iron would be by the preceding formula
*• = 0.0004 + 0.000,057 X 25-14 = 0.001833 or about — th
500
of air. The length of rod being 100 cms., and its cross section'
3. 1416 square cms., the reluctance would be approximately
-^° ^ > X 0.001833 = 0.05836 oersted. The total M. M. F.
3.1416
being 100 X »o X i = a.ooo ampere-turns = 3,514 gilberts.
The flux in the circuit, assuming that the reluctance of the air
path outside the bar may be neglected, is, approximately,
— ■ -. = 43,070 webers, with an intensity of — — - = 13,710
0.05836 3-1416
gausses.
65. In cases where the flux is confined to definite paths, as
in a closed circular coil, or in a very long, straight, and uni-
formly wrapped bar, the preceding calculations are strictly
applicable. When, however, an air-gap is introduced into the
closed ring, that is, when its circuit becomes aero-ferric, the
results begin to be vitiated, partly owing to the influence
of diffusion, and partly to the results of the C. M. M. F.
which is established at the air-gap. As the length of the air-
gap increases, the degree of accuracy which can be attained
b)«the application of the formula diminishes, but in dynamos,
the aero-ferric circuits are in almost all cases of such a char-
acter, that the degree of approximation, which can be reached
by these computations, is sufficient for all practical purposes;
for, while it is impossible strictly to compute the magnetic
circuit of a dynamo by any means at present within our reach,
yet the E. M, F. of dynamos, and the speed of motors, can be
predicted by computation within the limits of commercial
requirements.
66. If the ring of Fig. 45 be provided with a small air-gap of
0.5 cm. in width, the intensity in the circuit, before the intro-
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*4 ELECTRO-DYNAMIC MACHINERY.
duction of the iron core, will be practically unchanged by the
existence of the gap, that is to say, with the same i,ooo ampere-
turns, or i,as7 gilberts of M. M. F., the prime inteasity exist-
ing in the ring will be practically .30.95 gausses. In the air-
gap itself, the intensity will be less than this, owing to lateral
diffusion of the flux; but, neglecting these influences, we may
consider the intensity to be uniform. Now, introducinga soft,
Norway iron core into the ring, the iron is subjected to an
intensity of approximately, 30.95 gausses throughout the cir-
cuit. The reluctivity of the iron at this intensity, is, as we
have seen, 0.001596. The length of the circuit in the iron
will be 59.5 cms., and its cross section 10 sq. cms,, making the
ferric reluctance-5^^ X 0.001594 = 0.009484 oersted. The
reluctance of the air-gap, neglecting the influence of lateral
diffusion, will be — x i = 0.05 oersted, and the total reluct-
ance of the circuit therefore, will be 0.009484 + 0.05 =
0.059484 oersted. The flux in the circuit will be — ' ' ■ =
0.059454
31,130 webers, and the intensityin the iron, 2,113 gausses.
The existence of the air-gap has, therefore, reduced the flux
from 131 kilowebers to 31 kilowebers.
67. In practical cases, however, the problem which presents
itself is not to determine the amount of flux produced in a
magnetic circuit under a given magnetizing force, but rather
to ascertain the M. M. F., which must be impressed on a cir-
cuit in order to obtain a given magnetic flux. ' When the total
required flux in a circuit is assigned, the mean intensity of fkix
in all portions of the circuit is approximately determinable,
being simply the flux divided by the cross section of the circuit
from point to point. What is required, is the reluctivity of iron
at an assigned flux density and this we now proceed todetermine.
From the equations, »- = <j -|- ^ K, and & = — ■ , correspond-
ing in a magnetic circuit, to i = — in the electric circuit, 1,
being the electric flux density or amperes-per-sq. -cm. and p,
the resistivity, we obtain, f := — —
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FERRIC MAGNETIC CIRCUITS. 65
This equation jfives the reluctivity of any magnetic metal
for any value of the flux density <5> passing through it, when
the value of the constants a and b, have been experimentally
determined. The values of v, so obtained are only true for
reluctivities beyond the critical value, where the linear relation
expressed in the equation f =: a -|- ^ 3C commences.
68. The following table gives the values of the reluctivity
constants a and b, for various samples of iron :
Samfli. a i Otttrmr,
Soft Iron, o.ooo.a 0.000,056 Stoletow.
Hamj lion 0.000,1 0.000,059 KovUnd.
Sheet Iron, o.ooo,a375 0.000,059s FMsenden.
" " 0.000,3375 0.000,0654
■ " " 0.000.3335 0.000.064
•' •' o.ooo,ai3 0.000,05605
CastSleel 0.000,45 0.000,05135
" " 0.000.314 0.000,0563
Mkis Iron 0.000,15 o«»,oS7S
Cut Iron 0.001,031 0.000,139
Impioved Cast Iron, . 0.000,9035 0.000.I06 "
Wrought Iron 0.000,33 0.000,058 Hopkinson,
Djmuno Wrought Iron, 0.000,4 0.000,057 KenneUy.
" Cut Iron, . 0.003,6 0.000,093 "
Annealed Norway Iron, 0.000,3 0.000,057 "
69. Fig. 47 shows curves of reluctivity of' various samples
of iron and steel at different flux densities. The descending
branches are of practically little importance in connection with
dynamo-electric machinery. They are included in the curves,
however, in order to bring these into coincidence with actual
observations. It will be seen, that while the reluctivity of
Norway iron is only 0.000,5 ^^ ^ kilogausses, that of cast iron
is commonly about 0,010, or twenty times as great.
70. In order to show the application of the above curves of
reluctivity, we will take the simplest case of the ferric circuit;
namely, that of a soft Norway iron anchor ring, shaped as
shown in Fig. 44, of 10 square centimetres cross section and
60 cms. mean circumference, uniformly wrapped with insulated
wire. If it be required to produce a total flux of 80 kilowebers
in this circuit, the intensity in the iron will be iJ kilogausses.
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ELECTRO-DYNAMIC MACHINERY.
'
F-
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4
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FIG. 47- CURVES O
and, by following the curve for Norway iron, in Fig 47, it will
be seen that its reluctivity at this density is 0.000,5. ''^^^ '^
luctance of the circuit, therefore, will be — x 0.000,5 = 0.003
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FERRIC MAGNETIC CIRCUITS. 67
oersted, and the M. M. F. necessary to produce the requisite
magnetic flux will be fF = 4^ (R = 80,000 x 0.003 = ^4"^ S*''
berts, or 340 x 0.7958 = 191 ampere-turas.
71. If, however, the ring be of cast iron, instead of soft Nor*
way iron, its reluctivity at this density would be say 0,010,
and its reluctance ■ — X o.oio = 0.06 oersted, from which the
required M. M. F. will be 80,000 X 0,06 = 4,800 gilberts =
3,830 ampere-turns. The importance of employing soft iron
for ferric magnetic circuits, in which a large total flux is re-
quired, will, therefore, be evident
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CHAPTER VI.
AERO-FERRIC MAGNETIC CIRCUITS.
72. We will now consider the case of the aero-ferric magnetic
circuit. Fig. 48 is a representation of a simple ferric circuit
consisting of two closely fitting iron cores, the upper of which
is wrapped with a magnetizing coil M. The polar surfaces
are made to correspond so closely, that when the coil M, has
a magnetizing current sent through it, the magnetic attraction
• between the two cores will cause them to exclude all sensible
air-gaps. The general direction of the flux-paths is shown by
the dotted arrows, and a mechanical stress is exerted within
the iron along the fluz-paths.
These stresses cannot be rendered manifest, so long as the
iron is continuous. In other words, the continuous anchor ring,
as shown in Fig. 44, would give no evidence of the existence of
stress along its flux'paths. In the case shown in Fig. 48, the
stress is rendered evident by the force which must be applied to
the two magnetized cores in order to separate them. Theamount
of this force depends upon the magnetic intensity in the iron
at the polar surfaces, and, if (B, represents this intensity in
gausses, the attractive force exerted along the flux-paths at the
polar surfaces; i. e., perpendicularly across them, will be ^-^
dynes-per- square- centimetre of polar surface. The dyne is the
fundamental unit of force employed in the system of C. G. S.
units universally employed in the scientific world, and ts equal
to the weight of 1.0203 milligrammes at Washington; that is
to say, the attractive force which the earth exerts upon one
milligramme of matter, is approximately, equal to one dyne.
73. If the magnetic circuit shown in Fig. 48 has a uniform
area of cross section of iz square centimetres, and the mag-
netic intensity in the circuit be everywhere 17 Icllogausses,
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AESO-FERRIC MAGNETIC CIRCUITS. 69
then the attractive force exerted across each square centi-
metre of the polar surfaces at ^„ and J?„ will be
I X 17,000 .
1,500,000 dynes,
8 X 3.1416
)r 11,500,000 X 1.0203 = 11,730,000 milligrammes weight =r
11,730 grammes weight = 25.86 lbs, weight.
As there are twelve square centimetres in each polar surface^
m^ 0^^^
- >1^^ R. ^iJ-
the total pull across each gap will be ts x 35.86 = 310.32 lbs.
weight; and since there are two gaps, the total pull between
the iron cores will be 620.64 I^^- weight, so that, if the whole
magnet were suspended in the position shown in Fig. 48, this
weight should be required to be suspended from the lower core
(less, of course, the weight of the lower core) in order to effect
a separation; or, in other words, this should be the maximum
weight which the magnet could support
74. In order to ascertain the M. M, F. needed to produce'
the required intensity of 17 kilogausscs through the circuit in
order to cause this attraction, we find, by reference to Fig. 47,
that the reluctivity of Norway iron at this intensity is 0.0073;
73
that of air. The reluctance of the magnetic cir-
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70 electho-dynamic machine/iy.
cult will, therefore, be — X 0.0073 = 0.03042 oersted. The
total fluz through the circuit will be 17,000 x 12 = 304,000
webers, and the M. M, F. required to produce the flux, there-
fore, will be 304,000 X 0.03043 = 6,3o6 gilberts, or 6,206 x
0.7958 = 4,937 ampere-turns. If, then, the coil M, has 3, 000
turns, it will be necessary to send through it a current of
3.469 amperes, in order to produce the flux required.
The electric circuit analogue of this case is represented in
the same figure, where E, represents the E. M. F. in the electric
circuit as a voltaic battery, and the amount of this E. M. F.
necessary to produce a current of strength. 1, amperes, when the
total resistance of the circuit is r, ohms, will be A = i r volts.
75. So far we have considered that no sensible reluctance
existed at the polar surfaces Ji^, and R^. Practically, how-
ever, it is found, that, no matter how smooth the surfaces may
be, and, therefore, how closely they may be brought into con-
tact, a small reluctance does exist, owing, apparently, to the
absence of molecular continuity.
This reluctance has been found experimentally, in case of
very smooth joints, to be equivalent to the reluctance of an
air-gap, from 0.003 to 0.004 cm. wide (o.ooia" to 0.0016").
Taking this reluctance into account we have at R^, and at R^,
an equivalent reluctance of air path, say 0.0035 cm. long and
13 cms. in* cross-sectional area. Since the reluctivity of air
is unity, the reluctance at each gap becomes ~ — ~ X 1 =
0.000,39 oersted, and the reluctance of the circuit has, there-
fore, to be increased by 0.000,58 oersted, making a total of
0.03042 + 0.000,58 = 0.031 oersted, and requiring the M. M.
F. of 304,000 X 0.031 or 6,324 gilberts = 5,033 ampere- turns,
or an increase of current strength to 2.516 amperes.
76. It is evident, since the attractive force exerted across a
.square centimetre of polar surface is equal to — dynes, that
doubling the intensity at the polar surface will quadruple the
attraction per square centimetre. Therefore, all electro-
magnets, which are intended to attract or support heavy
weights, are designed to have as great a cross-sectional area
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AERQ.FERRIC MAGNETIC CIRCUITS. 71
of polar surface as possible, combined with a high magnetic in-
tensity across these surfaces. If, however, the increase of the
Area of polar surface is attended by a corresponding diminii-
tion of flux density, the total attractive force across the surface
will be diminished, because the intensity, per-unit-area, will be
reduced in the ratio of the square of the intensity, while the
pull will only increase directly with the surface. It is evident,
therefore, that soft iron of low reluctivity is especially desira-
ble in powerful electro- magnets.
If, for example, cast iron was employed in the construction
of the magrnet of Fig. 48, instead of soft Norway iron, and the
same M. M. F., namely, 6,324 gilberts were applied, the mean
magnetizing force would be this M. M. F., divided by the mean
length of the circuit in cms., or K = — ■ = 136.48 gilberts-
per-centimetre.
At this magnetizing force, a sample of cast iron would have a
reluctivity represented by the formula r = (a + iK), where a,
may be 0.0037, ^^^ ^t 0.000,09, ^° ^^^^ '^^ reluctivity at 133.92
gilberts per centimetreofmagnetizingforcewould be (0.0027 -|-
0.000,09 X 136.48) = 0.01407. The reluctance of the cast iron
circuit, including the small reluctance in the air-gaps, would be
— X 0.01407 = 0.05863 oersted, and the flux in the circuit would
be — - 5 - - - = 108,700 webers, or an intensity of 9,058 gausses.
The magnetic attraction between the surfaces per-square-
centimetre, would, therefore, be ^^ ^L_l __ 3^264^000
dynes, or 3,331 grammes wei^t, or 7.343 lbs. weight; and, since
the total polar surface amounts to 24 square centimetres, the
total attractive force exerted between and across them is
176.3 lbs. weight. The effect of introducing cast iron instead
of wrought iron into the magnetic circuit, keeping the dimen-
sions and M. M. F. the same, has, then, been to reduce the
total pull from 630.64 lbs. to 176.3 lbs., or 71.6 per cent.
77. If now an air-gap be placed in the circuit at X^, and Ji^,
of half an inch (1.37 cm.) in width, as in Fig 49, two results will
follow; viz..
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7* ELECTRO-DYNAMIC MACHINERY.
(i.) A greater reluctance will be produced in the circait
(2.) A leakage or shunt path will now be formed through the
air between the poles A'' and S. Strictly speaking, there wilt be
some leakage in the preceding case of Fig. 48, but with a ferric
circuit of comparatively short length, it will have been so small
as to be practically negligible. In Fig. 49, however, the reluc-
tance of the main circuit between the poles including the air-
gaps will be so great as to give rise to a considerable difference
of magnetic potential between the poles iVand S^ so that appre-
ciable leakage will occur between these points. The reluctance
of the leakage-paths through the air will usually be very com-
plex, and difiScult to compute, but, in simple geometrical cases,
it may be approximately obtained without great difficulty. In
this case we may proceed to determine the magnetic circuit
first on the assumption that no leakage exists, and second on
the assumption of the existence of a known amount of leakage.
Assuming that the cores are of soft Norway iron, and that
it is required to establish a total flux of 204,000 webers
through the circuit, then the flux density in the iron will be 17
kilogausses and its reluctivity 0.0073. The reluctance of the
circuit, so far as it is composed of iron, will be 0.03042 oersted,
while the reluctance of each air-gap will be — — X i = ©■ 1058 ;
or, in all, 0.2016 oersted. The total reluctance of the circuit
will, therefore, be 0.23203 oersted, and the M. M. F. required
will be 204,000X0.23202 = 47,330 gilberts = 37,660 ampere-
turns; or, with 2,000 turns, 18.83 amperes. The attractive
force on the armature will be 620 lbs. as in the previous
case.
78. Considering now the effect of leakage, we may assume
that the reluctance of the leakage path through the air ^„ is
0.5 oersted, and thatafluxof 108 kitowebers has to be produced
through the lower core; the length of mean path in the lower
core being 20 cms., and in the upper core 30 cms., it is required
to find the M. M. F., which will produce this flux through the
lower core.
The intensity in the lower core will be — '^=9,000
gausses, at which intensity the reluctivity of Norway iron will
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AERO-FERRIC MAGNETIC CIRCUITS. 73
be, by Fig. 47, 0.000,6, so that the reluctance of the lower core
will be — X 0.000,6 =s 0.001 oersted, and this added to the re-
luctance of the two air-gaps, 1. 27 cms.in width, = 0.2016 -{-o. 001
= 0.2026 oersted. The magnetic difference of potential in this
branch of the double circuit will, therefore, be ioS,ooo x o. 2026
= 11,880 gilberts. This will also be the difference of magnetic
potential between the terminals of the leakage path R^ and the
leakage flux will, therefore, be — ' = 43)7^0 webers. The
total flux in the main circuit through the upper core will be the
sum of the flux in the two branches, or 108,000 + 43,760 =
151,760 webers, making the intensity in the upper core ' —
= 12,647 gausses, at which intensity the reluctivity is 0.00121,
so that the reluctance of the upper core is *- X 0.0012 =
0.003 oersted. The drop of potential in the upper core will,
therefore, be 151,760 x 0.003 = 455 gilberts, and the total
difference of potential in the circuit, or the M. M. F., will be
21,880 +455 =22,335 gilberts = 17,775 ampere-turns, or
8.89 amperes at 3,000 turns.
79, It is obvious that the results obtained by the preceding
method of calculation cannot be strictly accurate, since no
account has been taken of any magnetic leakage except that
which occurs directly between the poles N and S. Also we
have assumed that the flux density remains uniform through-
out the lengths of the two cores. When a greater degree of
accuracy is desired, corrections may be introduced for the
effects of these erroneous assumptions, but the examples illus-
trate the general methods by which the magnetic circuits of
practical dynamo-electric machines may be computed with fair
limits of accuracy.
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CHAPTER VII.
LAWS OF ELECTRO-DYNAMIC INDUCTION,
80. When a conducting wire is moved through a magnetic
flux, there will always be an E. M. F. induced in the wire,
unless the motion of the wire coincides with the direction of
the flux; or, in other words, unless the wire in its motion does
na. 50.— CONDUCTO* perpendicular to uniform uagnbtic flux, aiw
MOVING AT BIOHT ANGL^ TO SAME.
not pass through or cut the flux. Thus, if, as in Fig. 50, a
straight wire A B,ot I cms. length, extending across a uniform
flux, be moved at right angles to the flux, either upwards or
downwards, to the position, for example, a 6, or a' b', it will
have an £. M. F. induced in it, the direction of which will
change with the direction of the motion.
81. A convenient rule for memorizing the direction of the
E. M. F. induced in a wire cutting, or moving across, magnetic
flux, is known as Fleming's hand rule. Here, as in Fig. 51, the
right hand being held, with the thumb, the forefinger and the
middle finger extended as shown, the thumb being so pointed
as to indicate the direction of motion, and the /orefinger the
direction of the magnetic /lux, then the widdle finger jrlll indi-
cate the direction of induced E. M. F. For example, if,' as in
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LAIVS OF ELECTRO-DYNAMIC INDUCTION. 75
F'K- 5^1 * wire be moved vertically downwards from A B, to
a b', and the thumb be held in that direction, the forefinger
pointing in the direction of the flux, the £. M. F. induced in
the wire will take the direction a' b\ during the motion, follow-
ing the direction of the middle finger. If, however, the wire
be moved upwards through the flux, an application of the same
FIG. 51.— FLEMINGS HAND RULE.
role will show that the direction of the induced E. M. F., as
indicated t^ the middle finger, is now changed.
82. The induction of electromotive force in a conductor,
moving so as to pass through or cut magnetic flux, is called
electro-dynamic induction. The value of the E. M. F. induced in
a wire by electro-dynamic induction depends,
(i.) On the density of the magnetic flux.
(3,) On the velocity of the motion, and
(3.) On the length of the wire.
This is equivalent to the statement that the E. M. F., in-
duced in a given length of wire, depends upon the total amount
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16 ELECTRO-DYNAMIC MACHINERY.
of flax cut by the wire per second in the same direction; or,
e=<S,lv C. G. S. units of E. M. F.
Where (fi, is the intensity of the flux in gausses, /, the length
of the conductor in cms., v, the velocity of motion in cms.-per-
second, and e, the induced electromotive force as measured in
C. G. S. units. Since one international volt is equal to
100,000,000 C. G. S. units of E. M. F., the E. M. F. induced
in the wire will be
e = volts.
83. The preceding equation assumes that the wire is not
only lying at right angles to the flux, but also that it is moved
in a direction at right angles to the direction of the flux. If
instead of being at right angles to the flux, the wire makes an
angle /?, with the perpendicular to the same, as shown in Fig.
52, then the length of the wire has to be considered as the
virtual length across the flux, or as its projection on the
normal plane, so that the formula becomes,
volts.
If the motion of the wire, instead of being directed perpendic-
ularly to the flux, is such as to make an angle a, with the per-
pendicular plane, the effective velocity is that virtually taking
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LAIVS OF ELECTRO-DYNAMIC INDUCTION. 77
place perpendicular to the flux, or v cos a, as shown in Fig. 53,
so that the formula becomes in the most general case,
100,000,000
84. It will be seen that in all cases the amount of flux cut
through uaiformly in one second, gives the value of the E. M. F.-
induced in the wire, and that the value of theE. M. F. does not
<lepend upon the amount of flux that has been cut through, or
that has to be cut through, but upon the instantaneous rate of
-cutting. The E. M. F. ceases the moment the cutting ceases.
85. If the loop A B C D, Fig. 54, be rotated about its
axis O C, in the direction of the curved arrows, then, while
the side C D, is ascending, the side A B, is descending; con-
sequently, the E, M. F. in the side C D, will be oppositely
directed to the E, M. F. in the side -4 B. Applying Fleming's
hand rule to this case, we observe that the directions of these
E. M. Fs. are as indicated by the double-headed arrows, and,
regarding the conductors CD and A B, as forming parts of
the complete circuit C D A B,'\t'\.% evident that the E, M. Fs.
induced in A B and C D, will aid each other, while, if they
are permitted to produce a current, the current will flow
through the circuit in the same direction.
86. We have seen that no E. M. F. is induced in a wire
unless it cuts flux. Consequently, the portions B Cand A D,
of the circuit which move in the plane of the flux, will con-
tribute nothing to the E. M. F. of the circuit.
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78 ELECTRO-DYNAMIC MACHINERY.
If the dimensions of the wires forming this loop shown in
the figure, are such that CD and A ^.having each a length
of la cms., while A B and'i* C, are 4 cms. each., the circumfer-
ence traced by the wires A B and C D, in their revolution
about the axis, will be 3. 1416 x 4 = 13.567 cms.; and, if the
rate of rotation be 50 revolutions per second, the speed with
which the wires A B and C D, revolve will be 628. j cms. per
second. If the intensity of the magnetic flux B, is uniformly
5 kilogausses, the E. M. F. induced in each of the wires A 3
FIG. 54.— RECTANGULAR
and C D, will be, 5,000 x i3 X 628.32 ^37, 6<)<), 200 C. G. S.
units of E. M. P., or 0.377 ^olt. This value of the E. M. F.
only exists at the instant when the loop has its plane coincident
with the plane of the flux, and the sides cut the flux at right
angles. In any other position, the motion of these sides is
not at right angles to the flux, so that the E. M. F. is reduced.
87. In order that the E. M. F. induced in a wire may estab-
lish a current in it, it is necessary that such wire should form
a complete curcuit or loop, as indicated in Fig. 55. When
such a conducting loop is moved in a magnetic field, some or
all portions of the loop will cut flux, and will thereby contribute
a certain E. M. F. around the loop. If the loop moves in it&
own plane, in a uniform magnetic flux, there will be no resultant
E. M. F. generated in it. For example, considering a circular
loop, we may compare any two diametrically opposite segments,
when it is evident that each member of such a pair cuts through
the same amount of flux per second, and will, therefore, gener-
ate the same amount of E. M. E., but in directions opposite
to each other in the loop. At the same time, it is clear that
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LAiVS OF ELECTRO-DYNAMIC INDUCTION. 79
the total amount of flux in the loop does not change; for,
while the flux is being left by the loop at its receding edge, it
is entering the loop at the same rate at its advancing edge, and,
since these two quantities of flux are equal, the total amount
of flux enclosed by the loop remains constant.
88. The cutting of flux by the edges of a moving loop, there-
fore, resolves itself into the more general condition of enclos-
ing flux in a loop. The value of the E. M. F. induced around
the loop does not depend upon the actual quantity of flux
enclosed, but on the rate at which the enclosure is being
made. If, as we have already seen, the loop is so moved
that the total flux it encloses undergoes no variation, the
amount entering the loop being balanced by the amount leav-
ing it, although E. M. Fs. will be induced in those parts of
the loop where the flux is entering and where it is leaving, yet
these E, M. Fs. being opposite, exactly neutralize each other,
and leave no resultant E. M. F. Consequently, the value of
the E. M. F. induced at any moment in the loop by any
motion, does not depend upon the flux density within the loop,
but on the rate of change of flux enclosed.
89. If ^, be the total flux in webers contained within a
single loop, such as shown ^.i A B C, in Fig. 55, the mean rate
at which this flux is changing during any given period of time,
will be the quotient of the change in the enclosure, divided by
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So ELECTRO^YNAMIC MACHINERY.
that amount of time, so that if ^, changes by >o,ooo webers in
two seconds, the mean rate of change during that time will be
10,000 webers per second, and this will be the E. M. F. in the
loop expressed in C. G. S. units. But, during these two seconds
of time, the change may not have been progressing uniformly,
in which case only the average E. M. F, can be stated as being
equal to the 10,000 C. G. S. units. Where the change is not
uniform, the rate at any moment has to be determined by
taking an extremely short interval, so that if dV, represents
this indefinitely small interval of time, and (/<P, the correspond-
ing change in the flux enclosed during that interval in webers,
the rate of change will be -^— - webers-per-second, and this will
be the value of the induced E. M. F. at each instant,
90. If a small square loop of wire A S C D, one cm. in
length of edge, placed at right angles to the flux as shown in
Fig. 55A, contains a total quantity of flux amounting to 10,000
webers, the mean flux density at the position occupied by the
square, will be 10,000 gausses. If now, the loop be moved
uniformly upward in its own plane to the position a b(d, so
as to accomplish the journey in the — th part of a second,
and if the flux enclosed by the loop at the position abed,
be 1,000 webers, then 9,000 webers will have escaped from the
loop during the motion. Assuming that the distribution of
flux density in the held was such that the emission took
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LAWS OF ELECTSO-DYNAMIC INDUCTION. 8i
place uoiformly, the E. M. F. in the loop, during the passage,
will hare been,
At ~
91. If, however, the rate of emptying, during the motion,
were not uniform, 0.009 volt would be the average E, M, F.,
and not the E. M. F. sustained during the interval; or, in
other words, the instantaneous value of the £. M. F. in the
loop would vary at difiterent portions of this short interval of
time, or at corresponding different positions during the jour-
ney ; but, in all cases, the time integral of the E. M. F. will
be equal to the change in # ; thus, the change in ^, is, in this
case, 9,000 webers. If the motion is made in — th of a
100
second, the E. M. F., will be 900,000 C. G. S. units of E. M. F.,
which, multiplied by the time {0.01 second), gives 9,000 webers.
If, however, the motion were uniformly made in half a second,
the E, M, F. would have been 18,000 C. G. S. units, which,
multiplied by the time, would give as before 9,000 webers;
and under whatever circumstances of velocity the change were
made, the sum of the products of the instantaneous values of
E. M. F. multiplied into the intervals of time during which
they existed, would give the total change in flux of 9,obo
webers. Or in symbols,
Smce,= -^^-
fedt= A^
The first equation simply expresses that the E. M. F., e, is
the instantaneous rate of change in the flux enclosed, and the
second equation shows that the difference in the enclosure
between any two conditions of the loop is the time integral of
the E. M, F,, which has been induced in the loop during the
change, assuming of course, that the change continues in the
same direction ; 1. e., that the flux through the loop has con-
tinually increased or decreased.
92. If a circuit contains more than one loop, as, for example,
when composed in whole, or in part, of a coil, the turns of
which are all in series, the E. M. F, induced in any one turn
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82 ELECTR0J3YNAMIC MACHmERY.
or loop of the coil, may be regarded as being established inde-
pendently of all the other loops, so that the total E. M. F. ia
the circuit will be the sum of all the separate E. M. Fs. exist-
ing at any instant in the loops, and may, therefore, be regarded
as the instantaneous rate of change in the flux linked with the
entire circuit. A coil, therefore, may be regarded as a device
for increasing the amount of flux magnetically linked with an
electric circuit, so that by increasing the number of loops of
conductor in the circuit, the value of the induced E. M. F.
corresponding to any change in the flux, is proportionally
increased, and if the coil or system of loops forming the cir-
FIC. 56.— CLOSED CISCULAK HELIX LINKED WITH A LOOP OF WIRE.
cuit, contains in the aggregate ^ webers of flux linked with it,
taking each turn separately and summing the enclosures, then
the time integral of E. M. F. in the circuit will be the total
change in *, and this will be true, whether the loop is chang-
ing its position, or whether the flux is changing in intensity or
in direction.
53. It is evident from the preceding, that there are two
different standpoints from which we may regard the produc-
tion of electromotive force in a conducting circuit by electro-
dynamic induction ; namely, that of cutting magnetic flux, and
that of enclosing magnetic flux. These two conceptions are
equivalent, being but different ways of regarding the same
phenomenon. The amount of flux enclosed by a loop can
only vary by the flux being cut at the entering edge or edges
at a different rate to that at the receding edge; or, in mathe-
matical language, the surface integral of enclosing is equal to
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LAirs OF ELECTRO.DYNAMIC INDUCTION. 83
the line integral of cutting, taken once round the loop. This
statement is equally true whether the flux is at rest and the
conductor moving;, or the conductor at rest and the fluz mov-
ing, or whether both conductor and flux are in relative motion.
94. Cases of electro-dynamic induction may occur where the
equivalence of cutting and enclosing magnetic flux apparently
fails. On closer examination, however, the equivalence will be
manifest. For example, in Fig. 56, let .4 ^ C Z? be a wooden
anchor ring uniformly wound with wire^ as shown in Fig. 44,
and a b c d,A circular loop of conductor linked with the ring.
It has been experimentally observed that when a powerful cur-
rent is sent through the winding of the anchor ring, no appreci-
able magnetic flux is to be found at any point outside the ring,
although within the core of the ring a powerful magnetic flux
is developed. Nevertheless, both at the moment of applying
and at the moment of removing the exciting current through
the winding of the ring, an E. M. F. is induced in the loop
abed, whose time integral in C. G. S. units, is the total
number of webers of change of flux in the ring core. It might
appear at first sight that this E. M. F. so induced in the loop
cannot be due to the cutting of flux by the loop, but must be
due to simple threading or enclosing of flux. It is clear, how-
ever, that the mere act of enclosure will not account for the
induction of the E. M. F., since the passage of flux through
the centre of the loop cannot produce E. M. F. in the loop
itself, unless activity is transmitted from the centre of the loop
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84 ELECTRO.DYNAMIC MACHINES Y.
to its periphery. In other words, action at a distance, with-
out intervening mechanism of propagation, is believed to be
impossible.
Could we see the action which occurs when the current first
passes through the ring-winding, we should observe flux
apparently issuing from all parts of the ring and passing into
surrounding space, at a definite speed. The loop abed,
would receive the impact of flux from the adjacent portions of
the ring before receiving that from the more distant parts of
the ring, and, in this sense, would actually be cut by the flux.
As soon as the flux has become established, and the current in
1 UNIFOKM FLUX.
the winding steady, it is found that the flux from any particu-
lar portion of the ring is equal and opposite to that from the
remainder of the ring, and is, therefore, cancelled or annulled
at all points except within the ring core. It is evident, there-
fore, that we may regard the E. M. F induced in the loop
a b c d d,s due either to the cutting of the boundary by flux, or
to the enclosure of flux.
95, Let us consider the case of a square conducting loop
A B C D, Fig. 57, having its plane parallel with the uniform
magnetic flux shown by the dotted arrows. If this loop be _
rotated about the axis O O', which is at right angles to the
magnetic flux, and symmetrically placed with regard to the
loop, 50 that A D, descends, and B C, ascends, these sides,
which cut flux during the rotation, will have E. M. Fs. gene-
rated in them, in accordance with Fleming's hand rule already
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LA IVS OF ELECTRO-D YNAMIC INDUCTION. 85
described in Par. 81, and in the directioD shown by the
double arrows. The sides A B and D C, which do not cut flux
during the motion, will add nothing to the E. M. F. generated.
The figure shows that while the sides A D and CB, have oppo-
sitely directed £. M. Fs., yet regarding the entire loop as a
conducting circuit, these E. M. Fs. tend to produce a current
which circulates in the same direction.
96. As already pointed out, the value of the E. M. F. gene-
rated in the sides A D and C B, of the loop, by the cutting of
the flux, wilt depend upon the rate of filling and emptying the
loop with flux, and it is evident that this rate is at a maximum
when the loop is empty; i. e., in the position it occupies in
Fig- 57, when the plane of the loop coincides with the direc-
tion of the flux, and the motion of its sides is at right angles
thereto; for, when the loop reaches the position shown in Fig.
58, namely, when it is full of flux; or, when its plane is as
right angles to the flux, then at that instant the rotation of
the loop neither adds to nor diminishes, the amount of flux
enclosed, so that the E. M. F. in the loop is zero.
97. Continuing the rotation of the loop in the same direc-
tion, the £. M. F. generated will increase from this position
until the position shown in Fig. 59 is reached, where the plane '
of the loop is again coincident with the plane of the flux, but
in which the side A D, has moved through 180°, or one-half
a revolution from the position shown in Fig. 57, and the direc-
tions of E. M. Fs. in the wire, as shown, will be changed so far
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86 ELBCTRO-DYl^AMIC MACHINERY.
as the wire is concerned, being now from A to D, instead of
from D to A, in the conducting branch A D; and from C to B,
instead of from S to C, in the conducting branch B C. The
direction of E. M. F. around the loop, will, therefore, be
UMIFOKU FLUX.
reversed. Consequently, the loop A B C D, during its first
half revolution as shown in Figs. 57 to 59, has an E. M. F. in it
in the same direction; and, during the remaining half-revolu-
tion, has its E. M. F. in the reverse direction, as shown.
i^y-
FIG. 61. — FLUX OBUQUB TO PLANS OF KOTATING LOOF.
98. The value of the E. M. F. generated in a loop, during
its rotation, depends upon the flux density, on the area of the
loop, and on the rate of rotation.
Assuming the side of the loop CD, to occupy the position
shown in Fig. 61, making an angle oi, with the direction H K, of
the flux, then the E. M. F, generated in the loop at this instant
is the rate at which flux is being admitted into the loop. If
/ cms., be the length of the side of the loop or the length of
A D, in Fig. 57, the amount of flux embraced at this insUnt
will be/(B X 2 D K. During the next succeeding small interval
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LAIVS Of ELECTRO-DYWAMIC INDUCTION. 8?
of time dt, if the angular velocity of the loop, a> radians per
second, carries it to the position C'jy, the amount of flux
admitted during that time will be / (& X s /> Z. But DL=:
I? ly y, cosine of angle D'D L, and this angle is equal to the
1. 62.— FLinC COINCIPBNT WITH PI.«re OF ROTATING LOOP.
angle a, so that D L =i> D' xcos a, and D !>, will be ~a>dt
cms. in length, since the radius O D ■=— \ consequently, the
flux admitted into the loop during this brief interval of time
di, will be
i/4> = 3/x~(K(U cos a dt, or /' <B a> cos ct dt
= ^ a> cos a dt
d {■
so that ~ = ^ OD cos a.
Thus, at the instant of time in which the loop has reached the
, d' D
PIG. 63.— FLUX PUtPBNDICULAR TO PLANS OF ROTATIKQ LOOP.
position O D, if a, be the angle which the loop makes at any
time with the direction of the flux, the E. M. F. e, the instan-
taneous rate of increase in the flux, or will be generally ex-
pressed in C, G. S. units by
■ (T = 4> ft) cos a
t, being the maximum amount of flux in webers (/* (B), which
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88
ELECTRO-DYNAMIC MACHINERY.
the loop can embrace. When the plane of the loop coincides
with the direction H K, of the flux, as shown in Fig, 6a, D ly,
is brought into coincidence with D L, ot the cosine of a is i.
So that the £. M. F. e, in the loop has a maximum value, and
FIG. 64- — CURVE OF
ROTATING LOOP.
is equal to ^ o^, while when the loop is at right angles to the
flux, or as shown in Figure 63, D ly, the succeeding small
excursion of the loop, is at right angles to D L, or cosine a = o
so that e = 0.
99. If &, as in the case represented by Figs. 57 to 60, be two
kilogausses, and/= 100 cms., then *= 100 x 100 x 2,000 = ao
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LAIVS OF ELECTRO-DYNAMIC INDUCTION. 89
megawebers. If the loop be rotated in the direction shown at
an angular velocity of 50 radians per second ( ^— revolutions
per second), the E. M. F. «,, will be
t = 30,000,000 X 50 X cos a, or 100,000,000 cos a
= I cos a volt.
The E. M. F. generated by the loop, therefore, varies
periodically between i, o, — i, o, and 1, If these values be
plotted graphically as ordinates, to a scale of time as abscissas,
the curve shown in Fig. 64 will be obtained, where the distance
A 0, represents the time occupied by one half revolution of the
loop, the E. M. F. being positive from O to A, and negative
from A to B. If now, the speed of revolution be doubled;
/. e., increased to 100 radians per second, the time occupied in
each revolution will be halved, and 0'A\ Fig. 65, will be half
the length of O A, but e, will be doubled as shown. The
shaded area C C A', in Fig. 65, is equal to the area O C A,rA
of Fig, 64. The E. M. F. generated by the loop is alternating,
being positive and negative during successive half revolutions,
but, by the aid of a suitable commutator, the E. M. F. can be
made unidirectional in the external circuit, as represented
in Fig, 66, where the curve P S Q, corresponds to OCA, in
Fig. 64 and Q T £, Xo A D B.
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CHAPTER VIII.
ELECTRO-DVNAHIC INDUCTIOH IN DYNAUO ARMATURES.
100. The type of curve represented in Figs. 64, 65, and 66,
showing the E. M. F. generated by the rotation of a conduct-
ing loop in a uniform magnetic flux, may be produced by,tbe
rotation of the coil represented in Fig. 67. Here a number of
circular loops, formed by winding a long insulated wire upon
f KBVOLUTION
a circular wooden frame, are capable of being rotated by the
handle, in the uniform magnetic flux of the earth. If the
mean area of the loops be 1,000 sq. cms., the number of loops
500, and the intensity of the earth's magnetic flux threading
the loop 0.6 gauss, then the E. M, F. generated by rotating the
loop will depend only on the speed of rotation. Assuming this
to be 5 revolutions-per-second, or an angular velocity of
5 X 3 T = iS-7°8 radians- per- second, the E, M. F, will vary
between -|- # o) and —^a>, in each half revolution. Here 0,
the total flux linked with the coil is 500 x 1,000 x 0.6 == 300,000
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INDUCTION IN DYNAMO ARMATURES. 91
weberB, and w = 15.708, so that the maximum value of the
E. M. F. generated in the coil will be 4,713,400 C. G. S
units = 0.047 volt, or roughly ^th volt This corresponds
to the peaks C and D, of the waves of induced E. M. F. shown
in Fig. 64.
lOI. In practice, however, continuous-current generators
do not produce this type of E. M. F. Fig. 6S represents, in
cross-section, a common type of generator armature, situated
between two field poles N, and S. A type of generator,
armature and field poles, similar to this, is seen in Fig. t.
The flux from these poles passes readily into and out of the
armature surface as indicated by the arrows. In other words,
PIG. as.— CKOSS-SKCTION OF BIPOLAK DKUU AKMATUKX.
the flux cuts the surface of the armature at right angles, while,
in the cases shown in Figs. 57 to 60, the conducting loop is
only cut by the flux at right angles in two positions 180° apart,
so that the curve of E. M. F. is peaked at these points, and
descends rapidly from them on each side.
102. Suppose in Fig. 68 that the difference of magnetic
potential, maintained between ^and S, is 2,000 gilberts, that
the diameter of the armature core g ok, is 40 cms., that its
length is 100 cms., and that the air-gap or entrefer is i cm.;
then, if the reluctance of the iron armature core be regarded
as negligibly small, the magnetic potential between the polar
surfaces and the armature surface on each side, that is between
e N e and A g S, also between d Sf and A k B, will be 1,000
gilberts. The magnetic intensity in the air may be obtained
in two ways-
(i.) By considering the total reluctance of the air-gap and
obtaining, by this means, the total flux. Thus the polar surface
represented is 55 cms. in arc x 100 cms. in breadth = 5,500
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ga ELECTRO.DYNAMIC MACHINERY.
sq. cms. The reluctance of the air-gap on either side of the
armature is, therefore, oersted, and the total flux passing
5.500
vebers. This flux, divided by the area through which it passes,
gives the intensity, or 5i5 — ;££. _, i^ooo gausses.
(3.) The magnetic intensity is, as we have seen (Par. 53),
numerically equal to the drop of magnetic potential in air, or
other non-magnetic material, per centimetre, so that the drop
nc. 69.— DIAGRAM
of potential heing here 1,000 gilberts in i cm. of distance in air,
the intensity must be 1,000 gausses. Representing the in-
tensity graphically, as shown in Fig. 69, it will be seen that
the intensity is uniform from c to e, Fig. 68, and then descends
rapidly to zero at B, where it changes sign and becomes
negatively directed, and is then uniform from f to d, falling
again to zero at A. The flux direction, therefore, changes
sign twice in each revolution.
103. If a wire A B, be wound as a loop around the armature,
it will, when the armature revolves, cut this flux at right
angles, and will, therefore, have induced in it an E. M. F.
which must be of the same type graphically as the curve in
Fig. 69. Thus, if the surface of the armature moves at a rate
of 50 cms. per second, the £. M. F. induced in the loop will
be 2 T' / CB, the factor 2 being required, since both sides of
the loop are cutting flux, one at A, and the other at £; or,
3 X 50 X 100 X 1,000 = 10,000,000 C. G. S. units = 0.1 volt.
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INDUCTION IN DYNAMO ARMATURES. 93
«xcept at the motncDt when the wires emerge from beneath
the pole pieces. This curve is represented in Fig. 70,. where
the distance O F, represents the time of one complete revolu-
tion of the armature, and the elevation of A, corresponds to
0.1 volt. If the armature be set revolving at twice this
«peed, the time occupied in a revolution will be halved, but the
E. M. F. being proportional to the rate of cutting flux, will
i. 70. — DIAGRAM
be doubled, as represented in Fig. 71, where the E. M. F.
is alternately 0.2 volt in each direction. By the aid of
a suitably adjusted commutator, the E. M. F. instead of
changing sign, can be kept unidirectional in an external cir-
cuit, following the curve ab c k l/g hj.
104. We may regard the E, M. F. of the loop as being in-
<luced either by the cutting of the flux by the wire at the arma-
AT DOUBLED SPEED OF ROTATION.
ture surface, or by the enclosure of the flux by the loop. The
flux enclosed by the loop is represented by Fig. 72, where at
the initial position at A £, the loop encloses 5,500,000 webers.
As the armature is rotated counter-clockwise, so that A, is
■carried toward N, the flux enclosed by the loop diminishes,
until, when it reaches the horizontal position, the flux through
the loop is zero. As the rotation continues, the flux re-enters
the loop in the opposite direction, and becomes 5.5 mega-
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94 ELECTRO-DYNAMIC MACHINERY.
webers at a position 180° distant from the initial position A B.
The rate of change of flux enclosed, or the gradient of the
curve, shown in Fig. 7a, is uniform, since the curve is uni-
formly steep, except near the position of maximum flux, where
the gradient is considerably reduced, and the E. M. F. cor-
respondingly reduced as already observed in Figs. 70 and 71.
FIG. 73.— BEPRESENTIXa DIAGRAM OF FLUX ENCLOSED
105. When,*however, the wire instead of being on the sur-
face of the armature is buried in a groove in the iron, as in a
toothed-core armature (Par. aa), and as shown in Fig. 73,
it is often TOore convenient, for purposes of calculation, to con-
sider the E. M. F. as due to enclosing, rather than to cutting
flux. The following rule, will, therefore, be of assistance in
FIG. 73. — ARMATUKB LOOP ROTATING IN BIPOLAR FIELD.
determining the direction of the E. M. F. induced in a loop.
Bearing in mind the fact that a watch dial is visible, to an ob-
server who holds it facing him, by the light which proceeds in
straight lines from the watch to his eye, then the direction of
the E. M. F. induced in the loop, regarded as the outhne of
the watch face, can be remembered by the following rule.
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INDUCTION IN DYNAMO ARMATURES. 95
The E. M. F. induced in tie loop hat the same direcHen as the
motion of the hands of the viateh, when the fiux enUring the locp
has the same direction as the iighl.
106. Flux entering the loop in the opposite direction, or from
the observer, will induce an E. M. F. in the opj>osite direction to
the hands of the watch, that is, counter-clockwise.
Emptying a loop of flux produces in it ao E. M. F. in the
opposite direction to that produced by fiUingit
FIG. 74.— 1
107. Fig, 68, shows a single loop of wire wound upon a drum
armature, which by its rotation in the flux, has an E. M. F.
induced in it of the same type as is graphically repre-
sented in the curve of Fig. 69. Supposing that the speed of
revolution is such as to produce an E. M. F. of say one volt,
in this conducting loop, during its passage beneath the pole
faces, then if two turns of wire be wound on the armature at
right angles, as shown a^t, A B and CD, Fig. 75, they will each
generate E. M. F. of the same value, in their proper order, as
they pass through the flux, and if the E. M. F. from A B, is
represented by the curve oiab e d efg, of Fig. 74 A, and the
E. M, F, in the loop CD, be represented simultaneously by the
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96 ELECTRO-DYNAMIC MACHINERY.
curve of kijklmn o, of Fig. 74 B, then, by properly adding
and co-directing the £. M. Fs. so produced, by the aid of a
suitable commutator, we obtain an E. M. F. of two volts, as
shown in Fig. 75, C, by the curve pgrsfuvtaxyt^ «".
Moreover, while the £. M. F. produced from one wire alone
I TWO TUKNS OF WISE AT RIGHT
fluctuates between o and i volt, four times per revolution, the
E. M. F. produced by the combination fluctuates between i
and a voLtSr eight times per revolution.
108. If now, instead of two loops being wound on the arma-
ture, there are six loops, as shown in Fig. 76, the E. M. F.
, WITH CORftE-
generated in these, added and co-directed by the aid of a suit-
able commutator, will be represented by the curve in the same
figure, and while the E. M. F. generated in any one of the
conducting loops fluctuates between o and i volt, four times
per revolution, the total E. M. F. produced under these con-
ditions would vary between 5 and 5,5 volts, 34 times per revolu-
tion. In the same manner, if instead of 6 conducting loops
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INDUCTION IN DYNAMO ARMATURES.
97
being placed on the armature, there are 12 such loops, as
shown in Fig. 77, the total E. M. F., if added and co-directed"
by a suitable commutator as before, would vary between 10.6
and 10.8 volts, 48 times per revolution, as shown by the curve.
109. An inspection of the preceding curves of E. M. F. will
show that, while the total E, M. F. capable of being produced
from a combination of conducting loops, is less than the sum
of the maximum E. M. Fs. in each separately, yet their com-
bined E. M. F. is much more nearly uniform than their sepa-
rate E. M. Fs., and tends to become constant as the number of
loops is increased, the curve of the total £. M. F. tending to
become more and more nearly a horizontal straight line.
110. It must be carefully remembered that the E. M. F.
generated in any single turn does not necessarily continue uni-
form during the passage of the turn beneath the pole; or, in
other words, that the crests of the waves of E. M. F. are not
necessarily straight lines, such as are indicated in Fig. 69, 70, 71,
and 74. These crests will be straight lines, only if, as hitherto
assumed, the intensity in the air-gap remains uniform over the
entire polar surface. In practice this is rarely the case. The
intensity may be either greater or less at the centre of the pole-
face than at the edges, but is usually greater, the flux tapering
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98
ELECTRO-DYNAMIC MACHINERY.
off toward the polar edges. This is owing to the fact that the
relucUnce in the magnetic circuit is usually a minimum, at or
near the polar centre, with a consequent increase in intensity
in that region. The same rules apply, however, even when
the wave form of E. M. F., as generated by the wires singly,
is complex. The effect of winding a number of turns around
FlC. 78.— DKim ASUATUmE WITH TWENTY-POUB TUBUS M BIPOLAR FISLD.
the annatnre, and uniting their E. M. Fs., is to produce an
aggregate E. M. F. that is much more nearly uniform than the
E. M. F. in each separate turn.
Thus Fig. 78 represents a drum armature with twenty-four
complete loops, or forty-eight wires, lying over its surface and
uniformly dispersed. If this armature be rotated in a bipolar
field which is of such strength and distribution that each turn
no. 79.— B. M. F. DIACEAll
has induced in it an E. M. F., such as is represented in Fig. 79,
that is to say, no E. M. F. at the point a, about 0.7 volt at b,
a maximum of about 0.95 volt at c, and no E. M. F. at e ;
then, if with the aid of a suitable commutator, these loops are
connected together so as to unite their E. M. Fs. into two
equal series, the E. M. F. of the machine as obtained from the
brushes on the commutator is represented during half a com-
plete rotation by the curve in Fig. 80, the corresponding
points of which are marked a b c de. It will be observed that
there are twenty-four undulations in this curve, each undula-
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INDUCTION IN DYNAMO ARMATURES, 99
tion corresponding to the step between the entrance of each
turn ttnder the pole-pieces.
Ill, Moreover, the shape of the polar edges must necessarily
influence the rise and fall of the E. M. F. induced in each
separate wire as it passes beneath the pole. For example, if
i- v\An/?n/v/\/>A/\/\/\A/Vv\/'
FIG. So.— E. H. F. DIAORAM O
COUBINDfO K. IC n. FBOH
SKPAKATB TURNS.
the area of the pole-face be represented by the shaded area A
in Fig. 81, the wires passing in succession beneath this pole,
will have an E. M. F. induced first in a portion of their length,
and finally throughout their entire length, so that the E. M. F,
wave for each wire will rise gradually. If, however, the polar
FIG. Bl.— niAGKAHS OF POLAR 'FACES
DIFFIRENT OUTUNI, OVXI.
area be such as is represented at B, the wires enter the polar
flux more suddenly, and the E. M. F. wave of each wire, at the
beginning and end, will be rendered more abrupt. As regards
continuous-current generators, there is but little advantage to
be gained by variations in the shape of the pole-faces, since the
aggregate £. M. F. of such a machine is rendered nearly uni-
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loO ELECTRO-DYl^AMIC MACHINERY.
form by the superposition of the E. M. Fs. in the various wires.
Eddy currents in the conductors and iron core are, however,
diminished by tapering the pole pieces, as at A.
112. In studying the arrangement of the wires on the surface
of the armature in a generator, with the view of determining
the E. M. F. generated by the revolution of the armature, it
is necessary to observe that the E. M. F. developed does not
depend directly upon the length of the armature wire which
cuts magnetic flujt, but does depend directly upon the amount
of flux enclosed by the conducting loops during their revolu-
FIO. 83. — TYPE OP AKMATURE HAVINO COMPARATIVELV LITTLE " IDLE "
tion. It is a common error to regard all the wires on the free
surface of an armature which do not pass through the mag-
netic flux as idle wires ; and, consequently, detrimental to the
efficient operation of the machine. This error comes from
regarding the E. M. F. as produced alone by the cutting of
flux, whereas in such a case, as for example, a pole armature
(Fig. 17), none of the wire cuts the magnetic flux, and, conse-
quently, would, by the preceding definition, be regarded as
idle wire.
In reality, the generation of the E. M. F. is dependent on the
embracing of flux by the loops, and since the so-called idle wire
is necessary to form a part of the loop, it cannot properly be
regarded as idle. It is, of course, to be remarked that in the
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INDUCTION JN DYNAMO ARMATURES. loi
event of the conducting loop having a fairly considerable part
of its length formed of the so-called "idle" wire, in order
to permit the loops to embrace a considerable amount of fluz
during their revolution, the rate of cutting flux by the parts
that do cut, requires to be correspondingly increased, thus
requiring a greater density of magnetic flui.
That this consideration is correct may be seen from an
inspection of Figs. 83 and 83.
113, Fig. 8z represents a machine in which the armature is
almost completely enclosed by polar surfaces, so that, even
FIG. S3.— TYPE OF AKUATURB HAVING COUrAKATIVELV MUCH " IDLK "
allowing for the free wire on the sides of the armature, sixty
per cent, of the length of the wire is always in the magnetic
flux, and forty per cent, is "idle." Fig. 83 shows a type of
armature in which only about twenty-five per cent, of the
length of the wire is at any time in the magnetic flux, so that
about seventy-five per cent, is "idle." Yet, with equally ad-
vantageous circumstances as regards the cross-section of the
iron core, speed of revolution, and the number of turns of wire,
the E. M. F. from the machine shown in Fig. 83 is fully equal
to, if not greater than, that developed in the armature of Fig. 82.
If, for example, the polar surface in Fig. 8i were reduced by
cutting it away along the lines ab, cf a^xiA. de, thus removing the
polar edges, and shortening the polar arc by about fifty per
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I03 BLECTRO-DYNAMIC MACHINERY.
cent, the E. M. F. developed by the generator wonld not be
redaced if the same total quantity of flux were forced through
the armature as before. The change effected would be that
the reluctance of the air-gap, between poles and armature on
each side, would be increased, since the cross-sectional area of
the air-gap would be diminished, and a greater M. M. F. would
therefore be needed on the field magnets in order to produce
the same flux through the circuit as before, but if this flux
were reproduced, the amount enclosed with each turn of the
armature by its revolution would be the same, and the total
E. M. F. induced in the armature would be the same; or,
regarding the question from a different standpoint, the inten-
sity of flux in the air-gap would be increased about one hun-
dred per cent., so that the wires would generate twice as
much E. M. F. as before, but would only be generating
E. M. F. about half the time in each revolution.
In other words, provided the armature core is traversed by
a given magnetic intensity, it is a matter of indifference how
much of its surface is covered by pole-pieces or how much
left exposed with "idle wire, "except as regards the amount of
M. M. F. which will be needed to force the flux through the
armature.
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CHAPTER IX.
ELECTROMOTIVE FORCE INDUCED BY MAGNETO GENERATORS.
114. One of the earliest types of operative dynamos was that
in which the field consisted of a permanent magnet, and the
armature was of the Siemens, or shuttle-wound type. This
armature consists essentially of a single coil of many turns
of wire, wrapped in a deep longitudinal groove, formed on
opposite sides of an iron cylinder. Owing to its simplicity,
this early type of magneto-electric machine has survived in its
competition with more advanced types, for such purposes as
signal calls in telephony, and for firing electric fuses in mines.
A machine of this type is shown in Fig. 84. The magnets
M, M, are usually compound ; i. e., consist of separate bars of
hardened steel, with their like poles associated as shown in the
side view. The magnets are thus combined to form a single
magnetic circuit through the armature, by means of soft iron
pole-pieces A'"' and S'. The armature core A A, was originally
formed of a single piece of soft iron, but is now usually
laminated, that is, formed of sheets of soft iron, laid side by
side. The armature winding is in the form of a single coil or
spool, and the ends of the coil are brought out to the insulated
segments of the two part commutator C C, Figs. 85 to 88.
115. In order to determine the E. M. F. capable of being
produced by a generator of this type and of given dimensions,
it is necessary first to ascertain the total quantity of flux which
passes through the armature in the different positions it
assumes during rotation. As shown in Fig. 85, the armature
core lies at right angles to the polar line, and, consequently,
no flux passes directly through its winding. When, during its
motion, the armature reaches the position shown in Fig. 86,
where the end A, has approached the north pole, the flux is
threading through the armature in a direction from the north
pole N, to the south pole S. In Fig. 87, the armature core is
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I04 ELECTRO-DYNAMIC MACHINERY.
shown as lying directly between the pole-pieces. In this posi-
tion the armature gives passage to the maximum amouat of
flux. In Fig. 88, the armature core is shown as moved beyond
this position, and is now reducing the amount of Hux threading
through its core. Continuing rotation until the completion of
a half turn, the position shown in Fig. 85, is reached, but now
FIG. 84.— MAONBTO GENERATOR WITH SHUTTLE ARMATURE.
in the reverse direction; ('. e., with the end A, lowest instead
of uppermost; and here the coil is emptied of flux as before.
116. It is evident, from a consideration of the preceding
iigures, that the amount of flux passing through the armature
in any position depends upon the M. M. F. produced by the
steel magnets; 1. e., upon their dimensions and shape, and on
the reluctance of the air-gap, that is, on the dimensions and
shape of the pole-pieces, as well as on the entre/er or air-gap
lying between the poles and armature.
For practical purposes, a steel magnet may be regarded as
producing a uniform difference of magnetic potential between
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MAGNETO GENERATORS. lOJ
its poles, except when the flux passing through the circuit
represents an intensity greater than one kilogauss in the steel.
We may practically consider that ordinary hard magnet steel
mainuins a permanent M. M. F. of lo gilberts- per-centimetre
of its length, independently of its cross-section, and at the
same time possesses a reluctivity of If, then, the magnets
shown in Fig. 84, are 30 cms. long and have a total cross-
FIGS. 85, 86, 87, AND 88.— SHUTTLB- WOUND AKUATURB IN BIPOLAK FIBLD.
section of 13 square centimetres, the M. M. F. they produce
will be 300 gilberts, and their reluctance will be — x — = ^
oersted. Neglecting leakage, the flux which will pass through
the armature will, therefore, be — webers, where iJl, is
,- -I- <R
the reluctance of the two air-gaps in series. If, then, we plot
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ELECTRO-DYNAMIC MACHINERY.
the total lenffth of air space in cms. (twice the length of the
air-gap), for different angular positions of the armature, and
divide by the area of the armature beneath one pole in sq.
cms., we obtain the reluctance A, and, substituting its value in
the above equation, we may determine, approi^imately, the
magnetic flux through the armature for all positions during
rotation.
117. Proceeding in this manner we obtain such a curve as is
shown in Fig. 89, which represents the flux passing through
no. 89.— DIAOKAH OF V
the armature core at different positions of angular displacement
from the initial position shown in Fig. 85, from actual measure*
ments of a particular shuttle-wound machine of this type. An
inspection of this figure will show that at 30° displacement the
flux through the armature will amount to above 40 kilowebers,
while at 90° displacement, the position of maximum flux, it
will reach about 93 kilowebers. From this position the flux
decreases until its value is zero at 180", the position assumed
by the armature when it has completed one half of a rotation
and is again in the position represented in Fig. 85, but in the
reverse direction. From this position onward, the direction of
flux is reversed, the maximum flux being reached at an angular
displacement of 270% or ^ of an entire rotation, completing a
cycle at 360",
118. Having thus obtained the value of the flux passing
through the armature, it is a simple matter to determine the
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MAGNETO GENERATORS.
107
E, M. F. at any speed of rotation; for, we have only to recon-
struct the flux diagram of Fig. 89, to a horizontal scale of time
in seconds, instead of angular displacement. This is shown in
Fig. 90, for an assumed rate of rotation of 1.5 revolutions per
second, or 94 revolutions per minute, the horizontal distance
of m, being taken as one second, and the vertical scale
being taken for convenience smaller than in Fig. 89.
N
^
V
1
vv
.»/k
" -f
: 'X
4
^/
ju
I
s
9
FIG. 90. — DIAGRAU O
The E. M. F. produced in any single loop or turn around
the armature will be the rate of increase in the flux passing
through the armature. If at the position O, commencing the
curve, we continue the curve along the dotted tangent of O O',
for one second of time, we reach the ordinate »/ O', of 770
Icilowebers, and this is the rate at which flux is entering the
loop at that moment; for, if the rate at O, were continued
uniformly for an entire second, we should evidently reach the
point O'. The E. M. F. existing at the moment of starting is.
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io8 ELBCTRO-DYMAMIC MACHINERY.
therefore, 770,000 C. G. S. units (of which 100,000,000 make
one volt) or 0.0077 volt, and, if the number of turns around
the armature core be 1,000, the total £. M. F. in the armature
vinding will be 7.7 volts. Again, if after a lapse of }^th of a
second, the flux curve ab c d efg h i kl m n,bt examined,
it will be found that the curve has reached the point b, or its
maximum positive value when it commences to descend toward
g, so that the tangent is horizontal, representing that the rate of
change of flux is zero, or similar to the condition of slack water
in a tide-way. At this point, therefore, the E. M. F. in each
turn on the armature is zero, and the curve of £. K.Y. A
BCD, etc., touches the zero line at this point B.
Again at the point q, on the flux curve, if the change of flux
were to continue for one second uniformly at this rate, we
should follow the dotted line or tangent g /, which reaches
the ordinate —400, or 500 below g', so that the rate of change
at the point g, on the curve is 500 kilowebers, represented by
the point Q, on the E. M. F. curve at that ordinate. Con-
tinuing in this way we trace the E. M, F. curve O A BC D, etc.,
showing that an alternating E. M. F. is produced in the
armature, varying between +7.7 and —7.7 volts. At the
rate of rotation assumed; namely, 15^ revolutions per second,
there will be three alternations of E. M. F. per second, or
twice the number of revolutions in that time,
119. Having now examined the means for determining the
value of the E. M. F. developed in the armature, we will con-
sider the effect of the commutator. It will be seen by refer-
ence to Figs. 85 to 88, the brushes B, B', resting on the
segments of the two-part commutator, that the direction of E.
M. F. from the armature toward the external circuit is reversed
at the moment when the core passes the position of maximum
contained flux, as indicated by the change in the direction of
the dotted loops C D' E' and L' M' N', relatively to the
horizontal line. The E. M. F. generated by the armature as
produced at the brushes B, B', will be represented by the
pulsating E. M. P., O W ^ C D' E F G H I K L M' N'.
It is evident that had we selected a higher rate of rotation, the
E. M. F. of the machine would have been correspondingly
increased.
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MAGNETO GENERATORS. 109
J20. The preceding considerations can only determine the
value of the E. M. F. at the brushes, while the external circuit
is open. As soon as the circuit of the armature is closed, the
£. M. T. at the brushes is reduced, for the following reasons;
viz.,
(i.) The current in the armature always produces an M. M. F.,
counter, or opposite to the M. M. F. of the field magnet, and,
therefore, diihinishes the flux through the magnetic circuit,
thus causing a corresponding diminution in the value of the
E. M. F. produced. Indeed, this opposing M. M. F. may,
under certain circumstances, assume a magnitude sufficient to
neutralize and destroy the permanent M. M. F. in the field
magnets. This is one of the reasons why magneto generators
are not employed on a large scale in practice.
(a.) The current through the armature produces in the
resistance of the armature, a drop in the E. M. F. If, for
example, the current through the armature at any instant be
one ampere, and the resistance of the armature be 10 ohms,
then in accordance with Ohm's law, the drop of E. M. F. pro-
duced in the armature, will be 1 x 10 = 10 volts.
(3.) The current through the armature not being steady, but
pulsating, the variations in current strength will induce
E. M, Fs. in the coil opposed to the change and, therefore,
reducing the effective E. M. F.
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CHAPTER X.
POLE ARMATURES.
121. The form of armature, which stands next la order of
complexity to the shuttle-wound armature last described, is the
radial oj f ale armatvre, represented in Figs. 91 and ga. Here
the armature coils c, c, are wrapped, usually by hand, around
radially extending laminated pole-pieces, formed from sheet
iron punchings laid side by side. This type of machine is
rarely found in continuous current generators, but is some-
times adopted in very small motors. The winding of such an
armature is carried out as represented in Pig. 93, where the
pole-pieces are shown at P J', and J^ J''. Starting the wind-
ing at the point M, the coil A, is wound from A to B, as
shown; the coil C, is then wound from £, through C to £>; the
coil £, from J), through £ to F; the coil G, from F, through
G to Hj the coil _/", from JI, through _/ to JC; the coil Z, from
A", through Z to M, finally connecting the last end of the coil
M, to the first end of the coil A, thus making the elosei-coit
winding shown in the figure. The connections of this winding
to the six-part commutator will be seen from an inspection
of the figure. The points J/, £, I>, F,ff3Lnd K, are branches
connected to the separate insulating segments of the commu-
tator, brushes being provided in the position shown on a line
connecting the centres of the pole-pieces. This commutator
is shown in cross-section at P, Fig. 92. It will be seen that,
owing to the conical boundaries of each armature coil, the
winding is difficult to arrange. This type of generator i&
always operated by an electro-magnetic field.
122. Since the dimensions of machines with pole or radial
armatures are always small, the reluctance of the circuit is
practically wholly resident in the air spaces between the poles
and armature projections, provided care be taken that the iron
in the armature is not worked at an intensity above 10 kilo-
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POLE ARMATURES. Ill
gausses, or above 7 kilogausses in the field magnet, if the latter
be of cast iron. If S, be the area of the polar face of a radial
armature projection in square centimetres, and d, be the clear-
entrefer over each armature projection. Since there are four
FIG. 91. — POLK
such air-gaps tn multiple-series the total reluctance of the cir-
cuit provided in the case represented by Fig. 91, will be
PIG. 93.— SECTION OF POLK ARUATUKB THROUGH AXIS.
—^ oersteds, assuming that the reluctance existing in the iron
is neglected.
123. The distribution of the flux through the armature is
diagrammatically represented in Fig. 95. If the cross-section
of each armature core be s, square centimeters, then at no
time will there be less than two radial projections carrying the
total flux, and if 10 kilogausses be the limit permitted by the
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iia ELECTRO-DYNAMIC MACHINERY.
reluctance of the air-gap, the total flux to be forced through
the armature will be j j X 10,000 = 20,000 j, webers. The M.
M. F, necessary on the field magnets will be ao,ooo j x -~- gil-
berts. For example, if j = 1.3 sq. cms., rf = o.»cm., s = 10 sq.
cms., the M. M. F. required will be 26,000 x 0.03 = 530 gil-
berts = 416 ampere-tUFns, and this must be the total excitation
included on the limbs of the elect ro-magneL
124. In order to determine the amount of flux passing
through a single projection, let the armature be considered as
slowly rotated counter-clockwise. Starting with the core i.
no. 93.— DIAGKAU SHOWING CONKBCTIONS OF COIL WITH COMMUTATOB.
Fig- 95, the magnetic flux passing through it will be found by
dividing half the M. M. F, by the reluctance of the air-gap over
its face, or — = 13,000 webers. As it moves counter-clock--
wise towards 2, no appreciable change is effected in the amount
of flux it carries, until the advancing edge of 3 emerges from
beneath the polar face N^. The flux through i, rapidly dimin-
ishes until before i becomes halfway between the pole faces
N^ and i',, it is entirely deprived of flux. When the position
3 is reached, the flux re-enters the coil of i, but in the
opposite direction, and when it passes position 3, the total
maximum flux of 13 kilowebers is in the reverse direction. The
curve. Fig. 94, commences at 13 kilowebers in the position
corresponding to i, Fig. 91, falls steadily from B to C, and,
after a short pause, from C to />, where the coil lies midway
between the poles, falls again from D to E, until the flux is 13
kilowebers negative, corresponding to the position 4. Con-
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POLE ARMATURES.
"3
tinuins: at this value to F, it rises to G, corresponding to the
position 5, and then pauses at the zero line, in the gap between
the poles, rising finally to J, corresponding to the ori^nal
position I, at K.
125. The E. M. F. established in any turn of the coil is found
by ascertaining, from the speed of rotation, the rapidity with
which the flux, threading through the coil, changes in value.
If, for example, the armature be driven at a speed of 1,500
revolutions per minute, or 25 revolutions per second, cor-
responding to the time of 0.04 second per revolution, the £.
M. F. will evidently be zero at the positions represented by the
straight line A B, CD, E F, G H, and / A" of Fig. 94, since
here, the rate of change in the flux is practically zero, and the
E. M. F. will be nearly uniform during the periods repre-
sented hj £ C,2> E, FG, and If/, since the rate of change is
nearly uniform in one direction or the other during those
periods. As shown in Fig. 97, the E. M. F. in the single turn
on the projection commencing at the position i, is zero from
<» to i. From ^, through i' to f, the flux diminishing at the rate
of 13,000 webers in 0.00433 second, and, therefore, at the rate
of 3,000,000 webers (3 megawebers) per second, and since 100
megawebers per second correspond to an E. M. F. of one volt,
the E. M. F. in a single turn is —0.03 volt Assuming 10 turns
of wire on each armature projection, the total E. M. F. will
be —0.3 volt at this period, and the ordinate M, represents
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114 ELECTRO-DYNAMIC MACHtNERY.
—0.3 volt in Fig. 97. At ^d, corresponding to the position
CD, Fig. 94, the E, M, F, is zero, falling again to — 0.3 volt
from d to 1^, corresponding to a change in flux from .D to ^
Fig. 94. After o.oa second has elapsed, the £. M. F. re-
verses in direction and becomes positive, tracing the carve ^
gg' hh'jf k.
By the aid of the commutator, the E. M. Fs. in the coils,
as soon as they change their direction, are reversed relatively
FLUX AND E. M. F. AT KISITION SHOWN.
to the external circuit, and, therefore,- preserve their direction
externally, as can be seen by examination of Fig. 93.
126. We have thus far traced the E. M. F. as developed in a
single polar projection, and so resulting from the variation of
flux passing through it. During the time that the E. M. F. is
being generated in this coil, a similar E. M. F. is being gener-
rated in the other coils, displaced, however, in time, by por-
tions of a revolution. As shown in Fig. 96, the six coils on
the armature have E. M. Fs. developed in them, being con-
nected with the external circuit through the brushes in two
parallel series, each of 3 series-connected coils. Each coil is,
therefore, acting in its circuit for one half of a revolution
before it is transferred to the opposite side, and while Fig. 97
represents the E. M. F. generated in any half .revolution of
one coil, we have to consider the E. M. Fs. coincidently
being generated in its next neighbor on either side. This is
shown in Fig. 98, where the E. M. F. of all three coils is de-
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POLE ARMATURES. "5
veloped independently on parallel lines one above the other,
each E. M. F. being a repetition of that in Fig. 98, but dis-
placed the |th of a complete revolution. Fig, 99 represents
. ....:nn
"lr"[s| f 5 ''• "■
•*h ( — 1 r
u-
W-M \ rJy
u -
7 1
1"
m
TIGS. 97, 98, AND 99-—^ M- r. 1
1 POLE AKKATUKE.
the effects of combining or summing these three separately
generated E. M. Fs. in the same circuit, and it will be seen
that the E. M. F. pulsates between o,z and 0.6 volt
127. If the resistance of the wire on each coil be r ohms,
then the resistance of the three coils on each side of the arma-
ture will be 3 r, and the resistance of these two sides in parallel
will, except at changes of segments, be i.j r, so that, neglect-
ing the resistance of the brushes and brush contacts, the resist-
ance of the armature will be 1.5 r ohms.
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1 1 6 ELECTRO-D YNAMIC MA CHINEJt Y.
The current strength which should be maintained by the
generator, when on short circuit, would, therefore, reach
0.6
— — - amperes, but in reality, the current will not reach this
amount, owing, among other things, to the effect of self-in-
duction in the armature, which, under load, tends to check
the pulsations, and, consequently, renders them more nearly
uniform, thus reducing the mean E. M. F.
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CHAPTER XI.
GRAUHE-RING ARMATUR£S.>
Z38. The armature of the dynamo-electric machine which
comes neit in order of complexity, is that devised by
Gramme, and now known generally as the Gramme-ring arma-
ture. This armature, as its name indicates, belongs to the
type of ring armatures, and consists essentially of a ring-shaped
laminated iron core wound with coils of insulated wire. In
3 AkMATUB.E IN BIPOLAR P
the Gramme-ring armature shown in Fig. too, the core is a
simple ring of iron, wound with 34 separate turns of wire,
placed so as to be able to revolve about its axis in the bipolar
field N, S. Considering the ring to be first at rest, the turns
6, 7, 8, 18, 19 and 10 are represented as being linked with the
total flux passing through the cross-section of the ring. If the
total flux entering the armature at the north pole and leaving
at the south pole, that is, passing from Jf to S, be two mega-
webers, then one megaweber passes through the upper half of
the ring, and one megaweber through the lower half. The
loops 5, 9, 17 and 21 are diagrammatic ally represented as hav-
ing 900 kilowebers passing through them. The loops 4, 10,
16 and 33 carry 700 kilowebers ; 3, 11, 15 and 23 carry 500
kilowebers ; 7, 12, 14 and 24, 300 kilowebers ; while 1 and ij
carry no flux.
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Il8 ELECTRO-DYNAMIC MACHINERY.
129. Suppose now, the ring be given a uniform rotation of
one revolution per second, in the direction of the targe arrows.
It is evident, that at any instant there is no change in the
amount of flux linked with the turns occupying the positions
6, 7, 8, iS, 19 and ao ; so that, although these contain a maxi-
mum amount of flux, they will have no E. M, F. generated in
them. Loops 5 and 9, however, are in a position at which the
flux they contain is changing ; that is to say, the amount of
flux that is passing through them at each instant has neither
reached a maximum nor minimum ; and the same is true with
regard to the loops 17 and 31. In 5, the flux is increasing,
and in 9, it is decreasing ; consequently, the E. M. F. in 5 is
directed oppositely to that in 9, and, according to rule, is in-
dicated by the curved arrows (Par. 105); for, if coil 5 be
regarded by an observer facing it from S, the flux, as the ring
moves on, will thread the loop in the opposite direction to
that of light coming from the face of the loop, cojisidered as a
watch dial, to the observer, and the E. M. F. generated in the
loop wilt be directed counter-clockwise, while the E. M. F. in
the loop 9 must have the opposite direction. Moreover, simi-
lar reasoning will show that all the coils to the left of the line
B B', that have E. M. Fs. generated in them, will have these
E. M. Fs. similarly directed ; i. e., outwards, as shown, while
all on the left-hand side of the line, will have the E. M. Fs.
also similarly directed, but inwards. Loops i and 13, which
lie parallel to the direction of the flux, will, in the position
shown, have no flux threading through them, but during rota-
tion, the rate of change of flux linked with them Is a maxi-
mum ; consequently, the E. M. F. induced in them is a
130. Instead of conceiving separate conducting loops to be
wound on the surface of the armature, as shown in Fig. 100,
let us suppose a continuous coil is wound on the surface of the
armature as shown in Fig. loi, the first and last ends of the
coils being connected together so as to make the winding con-
tinuous; then it is evident that the E. M. Fs. so acting being
similarly directed on each side of the vertical line B B", might
be made to produce continuously an E. M. F. in the conduct-
ing wire. Moreover, if two wires, or collecting brushes, were
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GRAMME-RING ARMATURES. "9
employed in the positions £, B', the E. M. Fs. from the two
halves of the ring would unite at the brushes B, B'.
Such a condition finds its analogue in the E. M. Fs. pro-
duced by two series-connected voltaic batteries connected as
shown in Fig. 102, with their positive poles united at B, and
their negative poles united at B". The figure shows two bat-
teries each of 9 cells connected in series. Here, as indicated,
all the cells have equal E. M. F. This condition of affairs
need not, however, exist in the Gramme-ring analogue, since
the only requirement is that the sum of all the E. M. Fs.
BIFOLAK FIELD,
generated in the coils on the right-hand side be equal to the
sum of those on the left-hand side. In point of fact, as
already observed, the E. M. Fs. are not the same in each of
the coils, those at i and 13 having a maximum E. M. F., and
those at 7 and 19 having zero E. M. F. Since these oppositely
directed E. M. Fs. balance each other, no current will be pro-
duced in the armature unless an external circuit be provided,
by joining the brushes B, B'.
131. Figure 100 shows no difference between the amount of
flux threaded through the coils 6, 7 and 8 ; or 18, 19 and ao,
and, consequently, according to theory, a total absence of
induced E. M. F. in these coils. In practice, however, owing
to leakage (Par. 77) and other causes, no coil is entirely free
from having E. M. F. generated in it
Moreover, the difference in the E. M. F. generated in coils
13, 13, II and 10, is not as great as might be inferred from
their angular position on the armature, owing to the fact that
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i«> ELECTRO-DYNAMIC MACHINERY.
(Par. loo) the flux enters the amuiturc core nearly uniformly
all around its surface.
In order to determine the total E. M. F, generated in such
an armature as is represented in Fig. loi, it is (irst necessary
to determine the E. M. F. generated in a single turn. Let u»
consider a turn starting from the position 7, and therefore,
generating no E. M. F., being carried by the uniform rotation
of the armature in the direction of the arrows to the position
19, in a time t seconds. During this time the flux threading
through it changes from — webers in one direction, to —
webers in the opposite direction, and, therefore, the change
in flux linkage will be $ webers, $, being the total flux pass-
ing from A' into S, through the armature. Whatever may be
the distribution of flux through the armature, and in the air-
gap, the average E. M. F. generated in the coil during this
<f
time will be — C. G. S. units of E. M. F. If the number of
/
revolutions made by the armature per second be «, then one
revolution takes place in the — th of a second, and a half revolu-
tion in the th of a second, so that t = — , and the average
E. M. F. is
— = a« «>
132, If, for example, the armature be revolved at a speed
of 600 revolutions per minute, or 10 revolutions per second,
n = 10, and since 4^, has been assumed to be 3 megawebers,
the average E. M. F. generated in any loop in passing from
the position 7, to the position 19, will be 20x2,000,000 =
40,000,000 C. G. S. units, or 0.4 volt (Par. 82). The same
E. M. F., oppositely directed, however, will exist on the
average in any turn on the right-hand side of the line B B".
If the ring were wound with only four turns, say i, 7, 13 and
19, the E. M. F. generated in these turns when placed in
series and connected to the brushes B and B, would evi-
dently fluctuate considerably; since, when the coils occupy the
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GRAMME-JUNG ARMATURES. 121
position sliown, the^E. M. Fs. would be a maximum in i and
13, and zero in 7 and 19, while, after ^th of a revolution, all
four coils would be active. If, however, numerous turns are
wound on the coil, it is evident that the total E. M. F.
between the brushes £ and £', will be very nearly uniform^ ■
since the only fluctuation which can take place is that coin-;
cident with the transfer of a single turn beneath the brush;
consequently, in order to determine the total E. M. F. gener-
ated by the rotation of a Gramme-ring armature, it is only
necessary to multiply the average E, M. F. in each turn by
half the number of turns on the armature; ('. e., by the number
%^
via. toa.— VOLTAIC analogue op e. h. Pa. generated in geamme biho.
of turns active between B and B', on each side, so that if to,
be the number of turns on the armature, counted once around,
— will be the number of turns active between brush and brush,
and the total E. M, F. on each side of the armature will be
2 100,000,000
If w = 24, as in the case represented, then the total E. M.
F. will be 3,000,000 X 10 X 24 = 480,000,000 = 4.8 volts,
133. There is only one method, in practice, of connecting the
separate coils of a Gramme-ring bipolar armature; namdy,
their continuous looping around the ring in a closed coil, as
shown in Fig. ror.
Suppose that it is desired to utilize the generated E. M. Fs.
for the purpose of supplying a current to an external circuit;
it is then only necessary to apply suitable brushes, or con-
ductors, at B and B', so as to rub continually against the
external surface of the turns as they revolve, making the
brushes sufficiently wide to maintain continuous contact
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133 ELECTRO-DYtfAMIC MACHINERY.
Under these circumstances, during the rotation of the
armature, a steady current will flow through the circuit main-
tained externally between i'and B', B, being the positive pole
of the machine, and S, the negative pole. Reversing the
direction of the armature rotation will, of course, reverse the
polarity of the brushes, as will also the reversal of the direc-
IC, 103. GRAMME-RINC SEXTIPOLAR
tion of the magnetic flux. If, therefore, it be required to
change the polarity of the brushes without changing the
direction of rotation, it is only necessary to reverse the
magnetic flux through the armature. Fig, 103 shows a
Gramme-ring sextipolar generator, with the commuUting
brushes bearing directly on the metallic surface of the turns
of conductor on the surface of the armature. This method,
however, of commuting the current from a Gramme-ring
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CRAMME-RIIfG ARMATURES. "3
armature is not the one in most frequent use; for, not only
are the conductors upon thi surface of the armature usually
too small to bear brush friction without destructive wear, but
also the relative amount of friction offered by brushes, placed
upon so large a diameter, is considerable, except in the case
. 104. — COMMUTATION C
of very large machines. In order to avoid this, as well as for
other reasons, it is usual to employ a special form of commu-
tator, as represented diagrammatically in Fig. 104, where each
turn is connected by a special conductor to a separately insu-
lated segment of a commutator. This commutator, therefore,
contains as many separate segments as there are turns on the
FIG. 105. — FORMS OF COMMUTATOHS.
armature. Usually, however, there are many turns of wire on
the armature to each segment of the commutator.
134. It is customary, in practice, to give a considerable length
of free surface to the commutator bars, so as to increase the
surface of contact and thus diminish the pressure that has
to be applied. Fig. 105 shows two forms of such commutator.
The separate segments are insulated from each other by mica
Strips. In order to provide for the connection of the wires
from the armature to the separate commutator segments or-
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H4 ELECTRO-DYNAMIC MACHmERY.
bars B, metal projections or lugs L, attached to the bars, are
provided. The bars, after being assembled, arc held rigidly
in place by the nut TV.
Various forms of brushes are provided to maintain contact
FIG. I06.— FORM OF GEHEKATOR BKltSM.
with the commutator bars. One form, consisting of wires and
strips in alternate byers, is shown in Fig io6,
135* I" tf'fi armature so far considered, it has been supposed
that the condition as regards distribution of dux and the con-
sequent generation of E, M. F. is symmetrical. It is possible,
however, that in the construction of the machine this symme-
try may not be secured. For example, in Fig. 107, the pole-
piece S, is represented as being considerably further from the
armature at its lower than at its upper edge, thereby increasing
the reluctance of the air-gap at the lower edge, and producing
magnetic dissymmetry, as represented by the distribution of flux
arrows. It will be found, however, on examination, that
despite this magnetic dissymmetry, the average E. M. F.
produced in the coils would remain the same, although the
distribution of this E. M. F. among the different turns neces-
sarily varies. Thus if #, be, as before, the total flux through
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GRAMME.RING ARMATURES. laS
the armature, the lower half of the armature may take a cer-
tain fraction n ^, where n, is less than 0.5, while the upper
half takes the balance (i — ti) ^. The total change in flux
linkage in passing from the position 7, to the position 19, will
be M* — {i — n) (P= — S>, as before, so that the average
E. M, F. will not be altered by the dissymetry. It might be
supposed, since the total flux passing through the armature
remains the same, that no loss exists in an armature whose air-
gap is thus widened, but a little consideration will show that
the increased reluctance in the magnetic circuit necessitates
a greater M. M. F. to drive the same amount of flux through
I. 108.— DlACaAM BSFKESBNTIKG
the circuit, and, consequently, if the M. M, F, in the magnetic
circuit remains the same, the total E, M. F. of the armature
■will be diminished. In addition to magnetic dissymmetry,
a dissymmetry 0/ armature -winding may exist, such as shown in
Fig. 108, where the right-hand half of the armature is seen to
be wound with six turns while the opposite half is wound with
five. In this case, supposing the armature to be rotating,
there will be, at the moment represented, a greater E. M. F.
in the right-hand half of the winding than in the left-hand half,
and a current will therefore tend to flow through the armature
under the influence of the resulting F,. M. F., even when no
external circuit is provided. When the armature has made
half a revolution from the position shown, the left-hand half
will be generating a greater E. M. F., thus tending to force
the current backward. Under these circumstances there will
be produced in the armature an oscillating E. M. F,, the
number of oscillations in a given time being the same as the
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I»6 ELECTRO-DYNAMIC MACHINERY.
number of poles passed by any part of the armature in that
time. That is to say, in a bipolar machine the frequency of
the double oscillations will be equal to the number of revolu-
tions of the armature per second. In a quadripolar machine it
would be equal to twice the number of revolutions, and so on.
These oscillations of current heat the armature winding and
waste energy in it. Consequently, although symmetry is
everywhere desirable in a machine, symmetry of armature
winding is of greater importance than symmetry of magnetic
flux distribution.
136. The armatures represented above are shown diagram-
matically as rings of circular cross- sect ion. In practice, how-
- A
m
L...;i,f 1,1.1
tVa. 109. CROSS- SECTIONS OF CRAtlME-RlNG ARUATURBS.
ever. Gramme-ring armatures always have a rectangular
cross-section, as represented in Fig. 109, We have seen
that the E. M. F. of a Gramme armature, depends upon the
number of turns of wire wound upon its surface, the flux
passing through it, and the number of revolutions per second.
The electric capability of a machine is expressed by — (Par.
6) ; that is to say, its capability increases directly with the
square of the E. M. F. and inversely with the resistance.
For a given E. M. F. of the armature, it is, therefore, desir-
able to reduce the resistance as far as possible, in order to
increase the electric capability of the machine. The shorter
the length of the winding; (. e., the shorter each turn, and the
greater the cross-section of the wire, the less the resistance of
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GRAMMB-RINC ARMATURES. n;
armature. If R ohms be the resistance of all the wire on the
armature, as measured in one length, then the resistance of
winding are in parallel; consequently, the resistance of the
armature will depend upon the shape of its cross-section, since
on this depends the length of each turn of conductor. A, B,
and C, Fig, 109, represent the cross- sections of three different
armature cores having the same area. Calling the length of
one turn around A, unity, the length of a turn around B, will
be 7 per cent greater, and around C, 40 per cent, greater.
Consequently, two armatures having respectively the cross-
sections of A and C, and wound with the same size and
number of turns of conductor, would have the same E. M, F.,
if driven at the same speed, when traversed by the same flux,
but the armature C, would have 40 per cent, more resistance
than the armature A, and its electrical capability would be
about 30 per cent, less, ( — 1. It is, therefore, desirable in
designing a Gramme-ring armature, to retain a nearly square
cross-section. On the other hand, the section shown at C,
offers for a given polar arc, a larger surface, and, con-
sequently, a lower reluctance to the passage of the flux in the
air-gap or entrefer, than in the case of the section A, so
that it may be sometimes desirable to employ an armature of
the type B, in order to reduce the air-gap reluctance, and, at
the same time, not greatly to increase the length of winding.
It has been aptly remarked that a dynamo is a combination
of compromises, since no single desideratum in its design can
be completely realized.
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CHAPTER Xn.
CALCULATION OF THE WINDINGS OF A GRAMME-RING DYNAMO.
137. In order to show the application of the foregoing
principles to the calculation of the E. M. F. produced in an
armature of the Gramme type, wc will take the case of a
bipolar Gramme-wound armature from dimensions given by
Messrs. Owen and Skinner in a paper read before the
American Institute of Electrical Engineers, May 16, 1894, to
which paper the reader is referred for fuller particulars of
construction and results.
Fig. no, reproduced from the paper referred to, shows a
vertical and a longitudinal cross-section of the machine, which
is a bipolar, constant-current, Gramme-wound generator, of the
Wuod type, intended for the supply of any number of arc
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WINDINGS OF A GRAMME-RING DYNAMO. la?
lamps in series up to 25, and, therefore, capable of supplying
a total £. M. F. of approximately 1,200 volts at terminals,
with a current strength of approximately, 10 amperes and an
external activity of about 12 KW.
This machine, when complete, closely resembles the gener-
ator shown in Fig. iii. Referring to Fig. 110, the field
magnet frame of cast iron is shown at M, M, M, M, the field
coils being wound on spools and filling the spaces indicated.
The shaft of the machine is supported in bearings B, B, and
riG. III. GRAMME TYPE ARC MACHISB.
space is left on the shaft for a commutator, at C, and a driv-
ing pulley at P". The bipolar field poles, produced by
the M. M. F. of the magnet coils M, M, M, M, are shown at
F F, P' P'. The Gramme-wound ring armature is shown at
AAA. The dimensions of the machine are indicated in
inches on the figures.
138. The field winding consists of 100 lbs. of No. 10 B. & S.
gauge, single cotton-covered copper wire, the total resistance
of the four coils in series being 15.75 ohms hot. The arma-
turecore is composed of soft charcoal iron wire of the cross-
ly GoOglc
13° ELECTRO-DYNAMIC MACHINERY.
section shown. It is wound in 15 layers of No. to B. & S.
gauge, and contains about 9,450 wires, each having a cross-
section of 0.00817 square inch, or a total cross-section of
77.2 square inches = 498.1 sq. cms. The armature is
wound in 100 sections of No. 14 B. & S. gauge, double cotton-
covered copper wire, in 57 turns each, or 5,700 turns, making
a total of 115 lbs. of wire, with a total resistance of 38.8 ohms
hot, but which, being connected in two parallel halves, as repre-
sented in the figure, has a joint resistance between brushes of
7,2 ohms. Assuming 10 amperes to flow through the machine,
the drop in the armature will be 72 volts, and the drop in the
field magnets 157. s volts, making the total drop in the machine
229.5 volts. When, therefore, the pressure at the machine
terminals is 1,200 volts, the E. M. F. generated by the machine
is practically 1,430 volts, or 1,430 X 10' = 1-430 X 10" C. G. S.
units of E. M. F.
139. The formula for determining the E. M. F, generated
by a bipolar armature is
£ = * w w C. G. S. units (Par. 131).
Consequently, * = ■
The speed of this generator is stated to be 1,000 revolutions
per minute, or 16.67 revolutions per second, and w, is 5,700,
therefore, * = . ~ : ^^ - ~ =1.505 x 10' The total
16.67 X 5.700 = ' ^
flux through the armature is, therefore, 1.5 megawebers.
140. Assuming that the M. M. F. required for this machine
were not known, it could be calculated in the following way:
We first determine the flux density in the various parts of the
circuit, and from that the reluctivity and reluctance of the
various portions.
The cross-section of the armature core, as already stated,
is 498 sq. cms. and if the flux goes through each side or
cross-section of the armature, the intensity in the armature
is, therefore, -^ — = = 15,060 gausses. The arc covered
49*
by each pole-piece is, approximately, 55 cms., and the effective
breadth 6.5" = 16.5 cms., so that the area of the polar surface
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WWD/NGS OF A GRAMME-RING DYNAMO.
131
is, approximately, 55 X 16.5 = 907.5 sq. cms. The total flux
passes through this surface, and the mean intensity in the
air-gap is — = 16.58 gausses. ,
907-5
141. Fig. 1 12 represents diagranimatically the arrangement
of magnetic circuits through the machine, where M, M, M, M,
represent the field magnet cores, P, P' the pole-pieces and A A
UAcNETic ciRcurr.
the armature. Fig. 113, represents diagrammatically the
voltaic analogue of the magnetic circuits, where M^ jV, M^ M^
are four batteries, whose £. M. Fs. correspond to the M. Id. Fs.
of the field-magnet coils. M^ and M^, form one circuit through
the field frame, a certain mean length of the pole-pieces, and a
mean length in the armature a, together with the two resist-
ances J?, R^ in the air-gaps. A similar circuit is provided for
the E. M. Fs. M^ and J/„ through the air-gap resistances JP,
R,, and the mean lengths of armature and pole-pieces. The
equivalent arrangement of circuits is represented in Fig. 114,
where M, M, arc E. M. Fs., each equal to M, in the preceding
figure, while the resistanc'e of the double circuit through the
field frame is one half of that of either of the resistances repre-
sented in Fig. 113.
142. The flux through the field cores will be greater than the
flux through the armature by reason of a certain leakage which
occurs over the surface of the magnetic circuit. This leakage
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13* ELECTRO-DYNAMIC MACHINERY.
is represented diagraramaticaUy in Fig. 113, as taking place in a
branch circuit or dotted semi-circte around the field coils, but,
in reality, the leakage takes place in an extended system of
branched or derived circuits between the polar surfaces and
portions of the entire field frame. The calculation of the vari-
ous reluctances in the air-path offered to leakage is very com-
plex, and it is preferable, rather than to attempt such calcula-
tion, to refer to experimental data alread}^ acquired with
machines of similar type. The leakage factor, or the ratio of
total flux through the field magnet cores to the total flux pass-
Flo. 113.— VOLTAIC ANALOGUE OF UAGNETtC CTRCUIT.
ing from them through the armature, for a machine of this type,
is approximately 1.7 ; so that, since the useful Aux passing
through the armature from each circuit M^ Af, and M^ M„ Fig.
113, is 0.75 megaweber, the flux through the field cares may be
taken as 0.75 x 1.7 = i.a7S megawebers. The cross-section
of the cores is found to be 176.8 sq. cms., so that the inten-
'-375 X 1
sity in them is, approximately.
176.8
: 7,311 gausses.
143. The reluctivity of the soft wrought iron armature at a
density of 1,5 kilogausses, is, approximately, 0.0045 ('^'S- 47).
the mean length of the flux paths through the armature 38 cms.,
and the cross section 498 square cms. The reluctance of each
side of the armature a. Figs. 113 and 114 is, therefore,
00045 .
498
3.000343 oersted. The joint reluctance of the
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WINDINGS OF A GRAMME-RING DYNAMO. 13J
armature will, therefore, be 0.00017 oersted ; and, since the
armature does not consist of continuous sheets of iron, but of
wires, and the flux has to penetrate from wire to wire down-
ward through small air-gaps, the total effective reluctance of
the armature will be approximately o.oot oersted. The length
of the air-gap or entrcfer, is 1.22"= 3.1 cms., and the area as
already determined, 907. 5 sq. cms. so that the reluctance in
each air-gap will be -^^ — =0.003416 oersted, the total reluc-
tance in the air, as seen in Fig. i io,will then beo. 006833 oersted.
The reluctivity of the cast iron in the field frame at a mean
intensity of 7,111 gausses, may be taken as 0.009 (^ig- 47)-
The length of the mean path in the field on each side of the
machine is, approximately, 153.4 cms., and its cross-sectional
area 176.8 sq. cms. ; so that the reluctance in each half of the
field will be, approximately, -^t-q X 0.009 = 0.00776 oersted.
The total flux being divided between the two sides of the field,
the joint reluctance, as represented in Fig. 114, will be 0.00388
oersted.
The drop of magnetic poten-
tial in the reluctance of the cuttrit.
armature (9 Ji) will be, , . . 1.5 x 10' X 0.001 = 1,500
The drop of magnetic poten-
tial in the reluctance of the
the air, 1.5 X lO* X 0.00683a = 10,348
The drop of magnetic poten-
tial in the reluctance of
the field, 3.55 x lo' x 0.00388 = 9,894
Total 21,641
Since one gilbert = o. 7854 ampere-turn, the total M. M. F.
in the circuit will have to be very nearly 17,000 ampere-turns,
or 8,500 ampere-tums on each of the spools M, M, M, M.
144. The preceding calculation is open to errors from
several sources in the absence of definite experimental data,
namely :
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>34
E.LECTRO-DYNAMIC MACHlffERY.
(i.) The assumed leakage factor may be inaccurate.
(2. ) The mean lengths of the flux paths in various portions of
the circuit may be inaccurate.
(3.) The assnmed increase in the reluctance of the armature
114. — DIAOKAM
due to its being formed of wires instead of solid sheets may
be inaccurate.
(4.) The reluctivity of the cast iron employed in the machine
may not be that of the sample of cast iron assumed.
In this, as in all constant- current machines, means are pro-
vided for maintaining a nearly constant current strength in the
circuit, despite changes in the toad, but a consideration of such
means, and of the requirements of the magnetic circuit to per-
mit such regulation, will preferably be postponed until arma-
ture reaction has been studied.
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CHAPTER XIII.
MULTIPOLAR GRAMME-KING DYNAMOS.
145. A given type of bipolar Gramme machine having proved
satisfactory as regards efficiency, ease of running and cost, at
a full-load output of say 10 KW, it may have to be determined
whether it would prove advantageous to maintain the same
design for a machine of a greater output, say 80 KW, Let us
assume that the linear dimensions of the lo-KW machine are
doubled, with the same speed of revolution, say 1,000 revolu-
tions per minute, maintained in the larger machine. Then,
assuming the same magnetic intensity in the armature, the
electromotive force will be four times as great, since the area
of cross-section of the armature, and, consequently, the total
useful flux, will be increased fourfold. The resistance of the
armature will be halved ; for each turn, though twice as long,
will have a cross-sectional area four times greater.
The electric capability pf the smaller machine being ex-
pressed by — (Par, 6), that of the greater will be ~-^ =
^2 — , or 32 times greater than in the lo-KW machine ; and, if
the same relative efficiency is maintained in the larger machine
the output will be 32 times greater. The weight of the larger
machine would, of course, be eight times that of the smaller,
and the output per pound of weight would, therefore, be four
times greater in the larger machine. In reality, however, such
&. result is impracticable, as will now be shown.
146. Dynamo machines are either M/-iiriT'en or direct-driven.
In the case of direct-driven generators, the speed of the
generator is necessarily limited by the speed of the engine,
and this, for well-known constructive reasons, has to be main-
tained comparatively tow, and the larger the generator the
slower the speed of rotation that has to be practically adopted.
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136 ELECTRO-DYNAMIC MACHINERY.
Thus, while a loo-KW generator is commonly driven direct
from an engine at a speed of about 250 revolutions per minute,
a aoo-KW generator is usually direct driven at about 150, and
a 400-KW generator at about 100 revolutions per minute. In
the case of belt-driven generators, the speed of belting is
usually limited, except when driving alternators, to about 4,500
feet per minute ; and, since larger generators require larger
pulleys, their speed of rotation has to be diminished. While
no exact rule can be applied for determining their speed, yet
roughly, in American practice, the speed varies inversely as
the cube root of the output, so that, when one generator has
eight times the output of another of the same type, the speed
of the greater machine would roughly be half that of the
smaller.
If no other limitation existed besides efficiency, the effect of
doubling the linear dimensions of any generator, even taking-
the reduced rotary speed into account, would result in pro-
ducing about sixteen times the output for eight times the total
weight; but targe machines must necessarily possess a higher
efficiency than small machines, not only owing to the fact that
they would otherwise 'become too hot, the surface available for
the dissipation of heat only increasing as the square of the
linear dimensions, while the weight and quantity of heat
increase as the cube of the dimensions, — but also because large
machines are expected to have a higher efficiency from a com-
mercial point of view.
147. Taking into account, therefore, the reduced i -o ta ry
speed of larger machines, their limits of temperature elevation,
and their necessity for an increased efficiency, the output only
increases, approximately, as the cube of their linear dimen-
sions ; and, consequently, the output of the larger machine,
per pound of weight, remains practically the same as that of
the smaller. The output of belted continuous-current genera-
tors is commonly six watts per pound of net weight, and of
direct-driven multipolar generators about eight watts per pound
of net weight.
148. We have already seen (Par. 132) that the E. M. F.
generated by a Gramme-ring armature, is (Pbw C. G. S. units,.
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MULTIPOLAR GRAMME-RING DYNAMOS. 137
or ^ — i volts, and the resistance of the armature will be —
ohms, if R, be the resistance of the winding measured all the
way round. Suppose now, that instead of employing a bi-
polar machine, we double the number of poles and produce a
four-pole or quadripolar machine, as shown diagrammatically in
Fig. 115. If we employ the same total useful flux 9, through
each pole, the average rate of change of tluz through the turns
on the armature will be doubled, since the flux through any
turn is now completely reversed in one-half of a revolution,
115. — DIAGRAM OF MAGNETIC CIRCUITS IN QUADRIPOLAR GRAMME
instead of in one com[^te revolution as before. The average
E. M, F, in each turn will therefore be doubled. In Fig. 115
the magnetic circuits of a quadripolar Gramme generator are
shown diagrammatical I y by the flux arrows. Here, as will be
seen, four distinct magnetic circuits exist through the armature,
instead of the two which always exist in the armature of a
bipolar generator. In this type of field frame four magnetizing
coils must be used. These may be obtained in one of two
ways ; namely,
(i.) By placing the magnet coils directly on the field magnet
cores, as shown in Fig. ti6; or,
(3.) By placing one coil on each yoke, as represented in
Fig. 117.
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T38 ELECTRO-D YNAMIC MACHINER Y.
149. In the same way, if we employ a field frame with six
magnetic poles, as shown in Fig. 118, the flux will be reversed
through each turn of wire three times in each revolution, and,
consequently, the average E. M. F. in each turn will be in-
creased threefold over that of a bipolar armature. In Fig.
118 there are six magnetic circuits through the armature.
Considering any segment of the armature underneath a pole
FlC. 116. — QIIADKIPOLAR
as, for example, between «, and /, the turn occupying the posi-
tion at »„ is filled with flux in an upward direction. As the
armature advances in the direction of the large arrows, the flux
through this turn will be diminished, and, when it reaches the
middle of the pole piece 5„ it will be completely emptied of
flux. The E. M. F. in the loop, during this portion of the
revolution, will be directed outward on the ring, as shown by
the double-headed arrows. After passing the centre of the
pole piece 5„ the flux through the loop begins to increase, but
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I40 ELECTRO-DYNAMIC MACHJNERy.
now in the opposite direction, the flux passing downward
through the loop instead of upward as before, and, as we have
already seen, flux entering a loop in one direction produces
the same direction of E. M. F. around the loop as fiux oppo-
sitely directed withdrawing from the loop (Par. 105), Conse-
quently, the E. M. F. is still directed outwards on the ring, as
indicated by the double-headed arrows, until the turn reaches
the position/,. In other words, the E. M. F. in a loop is simi-
larly directed during its motion toward and from the same pole ;
1. e., during its passage past a pole. When, however, the turn
begins to approach the pole N^, after being completely filled
with the downward flux at/,/ 1. e., as the flux in it begins to
decrease, the direction of the E. M. F. in it reverses, as shown
by the double-headed arrows, and this direction of the induced
E. M, F. continues until the turn reaches the position n,. By
tracing the directions of the induced E. M. Fs. in the various
turns of the ring, as shown, it will be seen that the positions
A' A' ^""^ A> 3''* points at which the E. M. F, is positive, or
directed outwards, while the positions «„ «„ and «„ are points
at which the E. M. F. is negative, or directed inwards. There
will be no current passing through the armature in the con-
dition represented, if the winding of the armature be sym-
metrical, since the E. M. Fs. in the various segments must be
equal and opposite. If, however, brushes be applied to the
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MULTIPOLAR GRAMME-BING DYNAMOS. 141
surface of the armature at the positions/,, /„ /„ and n„ n„ »„
any pair of these, including one positive and one negative
brnsh, will be capable of supplying a current through an ex-
ternal circuit.
150. When, therefore, an ordinary Gramme-ring winding is
employed, there will be one brush placed between each pair of
poles, or, in all, as many brushes as there are poles. Fig. 119
represents the connections employed to unite the various seg-
mental E. M. Fs. The E. M. F. of the armature is equal to that
of one of its segments, but the resistance of the armature is in-
versely as the number of segments and poles, and if R, be the
resistance between brushes, for there will be / sections in
parallel, each of which will have — ohms. Consequently, in
a six-pole armature, there will be six segments in parallel,
each having a resistance of -y-, making the joint resistance
R R
Fig. lao represents the mechanical arrangement for rigidly
supporting the armature of a direct-driven octopolar Gramme-
ring generator with eight sets of brushes pressing upon one
side of the armature, thus dispensing with the use of a separate
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142 ELECTRO-DYNAMIC MACHINERY.
commutator. The central driving pulley PPP, supports upon
its arched face two rings R,R'. These rings clamp between
them the armature core, and are clamped together by 14 stout
bolts. Where the supports ss, interfere with the winding of
the conductor inside the armature, the conductors are carried
on the supports as at i^ ^ f and d.
FIG. IK>. — CHAMME-BING
151. It is not absolutely necessary, however, to employ six
brushes in a sextipolar machine ; for, since in a machine of this
type the three separate Circuits are connected in parallel, con-
nections may be carried within the armature between the
various segments, permitting of the use of a single pair of
brushes. Thus Fig. iji represents a Gramme-ring armature,
wound for a sextipolar field, with triangular cross-connections
between its turns. In this case, the corresponding points/,,
A> A' •1"'^ "i' "v ">• ^^ ^'S- ' 1^1 instead of being connected to-
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MULTIPOLAR GRAMME-RING DYNAMOS. 143
gather by brushes externally as in Figs, iigor lao.are connected
together by wires internally. It is not, of course, necessary
that every turn on the armature should be so cross-connected,
but that the coils or group of turns which are led to the com-
mutator should be cross-connected, so that each of the 36 turns,
shown in Fig. tzi, may represent a coil of many turns.
Although the brushes are shown in Fig. 121, as beingplaced on
adjacent segments, yet they may be equally well placed
diametrically opposite to each other.
Fig. 122 represents the corresponding cross-connections for
a quadripolar Gramme generator, employing a single pair of
brushes. The advantage of cross-connections is the reduction
in the number of brushes. The disadvantage of cross-connec-
tions lies in the extra complication of the armature connections.
In targe machines it is often an advantage to employ a number
of brushes in order to carry off the current effectively.
152. Fig. 123 is a representation of a sextipolar generator
whose magnetic field is produced by three magneto-motive
forces, developed by coils placed as shown. The flux paths
are represented diagrammatically by the dotted arrows at A.
Each M. M. F. not only supplies magnetic flux through the
segment of the armature immediately beneath it, but also con-
tributes flux to the adjacent segments in combination with the
neighboring M. M. Fs.
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144 ELECTRO-DYNAMIC MACNlNBRy.
153, From the preceding considerations it is evident that
while it is possible to design a bipolar generator for any desired
output, yet, in practice, simple bipolar generators are not
employed for outputs exceeding 150 KW, and, in fact, are
seldom employed for more than looKW, ^ince their dimen-
sions become unwieldy and their output, per pound of weight,
smaller than is capable of being obtained from a well-designed
multipolar machine.
In the same way, a quadripolar generator can be made to
possess any desired capacity; but, in the United States,
PIG. 133. — CROSS-CO KNBCTIONS FOR QUADRIPOLAR CRAMHB-RIKG V
practice usually increases the number of the poles with an
increase in the output of the machine. Thus, it is common to
employ a four-pole or six-pole generator for outputs of from 25
to 100 KW, and 8 to 12 poles for a generator of 400 KW,
capacity.
154, Should the armature of a multipolar generator not be
concentric with the/o/ar bore ; i. e., if it is nearer one particu-
lar pole than any of the others, the reduction in the length of
the air-gap opposite such pole, will reduce the reluctance of
that particular magnetic circuit, and by reason of the increased
flux through the armature at this point, induce a higher
E. M. F. in the segments of the armature adjacent to the pole
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MULTIPOLAR GrAmME-RING DYNAMOS. 145
than in the reoiaining segments. If the armature be not inler-
tonnected ; t. e., if it employs as many, pairs of brushes as there
are poles, these unduly powerful E. M. Fs. can send no cur-
rent through the armature as longas the brushes remain out of
contact with the conductors; for an inspection of Figs, 118
and iig will show that no abnormal increase of E, M. F. can
exist in a single segment, but must be simultaneously generated
in adjacent segments, and that such pairs of E. M. Fs. will
counterbalance each other. When, however, the brushes are
brought into contact with the armature conductors, thereby
bringing the various segments into multiple connection with
PIC. 133.-~SEXTIFOLAK GRAUUS-BIKG
one another, a tendency will exist for the more powerful
E. M. F. to reverse the direction of current through the
weaker segments.
155. Whether this tendency will result in an actual reversal
of current depends upon the difference of E. M. F. between
the segments, their resistance, and the external resistance or
load.
Let A and B, Fig. 124, represent the E. M. Fs. of any two
segments in a multiple-connected Gramme-ring armature, and
let the E, M. F., £, of A, be greater than the E. M. F., E",
of B. Owing to drop of pressure in the internal resistance r,
the pressure e, at the terminals /, y, will be less than the
E. M. F,, E, of the stronger segment A. If c, is greater
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146 ELECTRO-DYl^AMK MACHINERY.
segment B, in the direction opposite to that in which its
E. M. F. acts. If t, be equal to E, there will be no current
through the segment B, while if e, be less than E\ a current
will be sent through B, in the direction in which its E. M. F.
acts, but of strength less than that supplied by segment A.
Thus, in Fig. 135, the E. M. F,, E, of the stronger segment A,
FIG. I24-— niAGRAH OP E. M. Fa. IN ADJACENT ASMATUKE SBCUEHTS.
is represented by the ordinate e -\- d. Owing to the resist-
ance r, in the segment A, a drop of pressure d, will take place
within it, and the pressure at its terminals will be e volts.
If E' be less than e, the stronger segment A, will send a cur-
rent back through the segment B, while if E\ be greater
than *, both segments will contribute current through the
external load resistance R ohms.
For example, a separately -excited quadripolar generator of
say 100 K.W capacity, supplying 1,000 amperes at 100 volts
E. M. Ft. IN ADJACENT ARHATURE SEGMENTS.
terminal pressure, has a resistance in each of its four armature
segments A, B, C, D, of — th ohm; then, provided its four
magnetic circuits are balanced or equal, the full load on each
segment will be 350 amperes, and the drop in each 2.5 volts;
so that the four E. M. Fs. will be, Fig. ia6 ;
Waiitd
E. M, P. Drafi. Currtnl. Ptwrr.
Velti. ftlit. Amptra. WatU.
A = 103.5 3.5 350 63S
B = toS.S 3.S 350 63S
C = ioa.s 3.S 350 635
D = to3.s 3.S 350 63S
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MULTIPOLAR GRAMME-RING DYNAMOS. 147
The power expended in each segment of the armature by
the current as /'J?, will be -^ ■ ■■-■ - = 6as watts, and the
total PR loss in the armature, 2,500 watts.
156. Considering one of the segments, say C, as normal, and
that A, owing to the magnetic dissymmetry, gives an E. M. F,
two volts in excess; B, one volt in excess; and D^ one volt in
deficit; the excitation necessary .for 1,000 amperes total ont-
put will produce (Fig. 127) the following conditions; namely,'
157. The effect of magnetic dissymmetry in the segments,
under the assumed difference of three volts, will produce, at
N THE SEGMENTS
full load, a difference of output among the segments, ranging
from 100 to 400 amperes, while the total power wasted in the
armature winding will be increased 30 per cent.; namely.
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148 ELBCTRO-DYNAMtC MACHINERY.
from 3,500 to 3,000 watts. The armature will, therefore, be
raised to a higher temperature, owing to the magnetic dis-
symmetry, but this increase in temperature will not be
localized, since, although at one moment a greater amount of
heat is being produced in certain segments than in others, yet,
owing to the rotation of the armature, the portions of the
armature constituting these segments are constantly chan^png.
158. Suppose now the exteroal circuit be entirely removed,
the brushes remaining in contact with the conductors (Fig. isS)
so that the circuits through the armature segments are com-
plete ; then the following conditions will hold :
An inspection of these values shows that a difference of
three volts between the E. M. Fs. of the four segments, pro-
duces a reversal of current through C and D, at no load, with
a useless expenditure of 500 watts. Consequently, between no
load and full load, there will be a change from an expenditure
of power with reversal of current in the weaker segments, to
an excessive drop and expenditure of power without reversal of
current.
159. Although this difficulty, arising from the unbalanced
magnetic position of the armature, does not, in practice, give
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UULTIPOLAS GRAMMB-RING DYNAMOS. I4»
rise to any serious inconvenience, when mechanical construc-
tion is carefully attended to, yet windings have been devised
by which it may be altogether avoided. For example, if all the
turns be so connected that their £. M. Fs. are placed in series,
then a single pair of brushes will be capable of carrying the
current from the entire armature, which will only be divided
into two circuits; or, the segments may be so interconnected
that turns in distant segments may be connected in series so as
to obtain a more general average in the total E. M. F. Such
windings are always more or less complex, and the reader is
referred to special treatises on this subject for fuller details.
160. The formula for determining the E. M. F. of a multi-
polar Gramme generator armature is,
E = d^nw C. G. S. units, where #, is the useful flux in
webers, or the flux entering the armature through each pole, n,
the number of revolutions per second of the armature, and w,
the number of turns on the surface of the armature counted
once around. If, however, the armature be series connected,
so that instead of having p, circuits through it between the
brushes, where/, is the number of poles, there are only two
circuits, then the E. M. F. will be £ = £ ^tmi, while if, as in
some alternators, the circuit between the brushes be a single
one, the mean E. M. F. of the armature wtll bep^nw.
161. Fig. 129 represents the magnetic circuits of an octopolar
generator, the dimensions being marked in inches and in centi-
metres. The field frame is of cast steel, and the armature
core is formed of soft iron discs. Let us assume that there
are 768 turns of conductor in the armature winding, and that
the speed of rotation is 172 revolutions per minute, or 2.867
per second.
Assuming an intensity of 9,500 gausses in the armature, it
may be required to determine the £. M. F. of the machine.
The cross-section of the armature is 31.1X13 = 404.3 sq.
cms., but allowing a reduction factor of 0.93 for the insulating
material between the discs, the cross-section of iron is 372
sq. cms. The total flux passing through the cross-section of
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ISO ELECTJIO-DYNAAIIC MACHINERY.
the armature will, therefore, be 373 x 9,500 = 3>534|000
webers.
The useful 6ux through each pole will be twice this amount,
or 7,068,000 webers, so that the E. M. F. of the generator
will be :
E = ^taa = 7,068,000 X 8.867 X 768 = 1.537 X 10" =
155.7 volts.
This will be the E, M. F. of the generator, provided all the
armature segments are connected in parallel, as shown in Fig.
IIS- If. however, the armature winding be so connected that
only a single pair of brushes and a single pair of circuits exist
through the armature, the E. M. F, would be 4 times as great,
while if the armature could be connected in a single series, the
E. M. F. would be 8 times as great.
162. In order to determine the M. M. F. necessary to drive
this flux through the armature we proceed as follows: viz..
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MULTIPOLAR GRAMME-klNG DY If AMOS.
ISI
We first determine the cross-section, the mean length, and
the intensity in each portion of the magnetic circuits. One of
the eight magnetic circuits through the armature is represented
by the dotted arrows at A (Fig. 129). We may assume that the
flux through the cores is 7,068,000 x i-3 = 9,188,400 webers;
1.3, being the approximate leakage factor for a machine of
this type; in other words, of all the flux passing through the
cores — X 100 = 76.9 per cent, approximately, may be
assumed to pass through the armature, half through each cross-
section. Consequently, we have the following distribution :
Sf..m.. "
WOtr,.
Gaiuui.
Field core, .
644
9.188,400
"3.430
Yoke, . .
354
3,534.000
9.980
Armature,
254
3,534.000
9,500
50
The entrefer, or gap, of copper, air and insulation, existing
between the iron in the armature and in the pole faces, is 1.5
centimetres in length, while the polar area is 41 cms,
X 34 cms., or 1,400 sq. cms, in cross-section. From these
data, the reluctance in the magnetic circuit through the
armature is
Field core.
Yoke. .
Entrefer, .
40
40
76
1-5
13.430
13.430
9.980
\
ArniBtnre,
SO
9.S<»
O.C
203.1
363.1
000.304
000,304
3S4.0
000. 21 s
003,14a
003.143
373
□
000,107.5
005.314.5
The M. M. F. required to drive a total flux of 3,534,000
webers through this circuit will be
5 1 8. 430 gilberts.
14,66s ampere-li
7.333 ■mpere-iu.
)n each spool.
With tioo turns on each spool, the current would be i
amperes.
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CHAPTER XIV.
DRUM ARMATURES.
163. The drum armature was first introduced into electrical
engineering by Siemens, in the shape of the shuttle armature,
and was modified by Hefner-Altenecic in 1873. Tiie drum
armature was subsequently modified in this country by the .
introduction of a laminated iron armature core, consisting of
discs of soft iron, called core discs, provided with radial teeth
or projections. This armature core, when assembled, had
MC. 130.— TOOTHED-CORE
space provided between the teeth for the reception of the
armature loops on its surface, a completed armature showing,
when wound, alternate spaces of iron and insulated wire, and
formed what is called a toothed-core armature. Next followed
the smooth-core drum armature, that is, a drum armature com-
posed of similar core discs in which the teeth were absent, so
that the completed armature had its external surface com>
pletety covered with loops of insulated wire. Fig. 130 shows
a common type of toothed-core armature in various stages
of construction. The laminated iron core is shown at A, as
assembled on the armature- shaft ready to receive its winding
of conducting loops in the spaces between the radially project-
ing teeth. At £, is shown the same core provided with wind-
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DRUM ARMATURES. I53
ings of insulated wire. At C, is shown a completed armature.
The detailed construction of a laminated armature core is
illustrated in Fig. 131, which shows a portion of a drum arma-
ture core already assembled by the aid of large bolts passing
FIG. 131.— TOOTH SD-COKE ARMATURB IN PROCESS OK ASSEMBLING
through holes in the core-discs. On the right are other
core-discs ready to be placed in position on the shaft
164. Fig. 139 shows a laminated armature body of the
smooth-core type. Here the separate core-discs are formed
of sheet iron rings assembled on the armature shaft as shown.
These discs, after being assembled, are pressed together
hydraulically. The end rings are heavy bronze spiders, held
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154 ELECTRO-DYNAMIC MACHINERY.
together internally by six bolts shown in the figure. When the
armature body is subjected to compression, these bolts are
tightened on the spiders, which are firmly keyed to the
shaft, so that on release of the hydraulic pressure, the lami-
nated iron core forms one piece mechanically. Fig. 133 shows
the same armature completely wound,
165. In the drum armature, the conducting wire is entirely
.coDfined to the outer surface, and does not pass through the
FIG. 134. — TYPICAL KORM OK SMALL SIZE l)»
interior of the core. In this respect, therefore, it differs from
the Gramme-ring armature, already described, in which the
winding is carried through the interior of the core, lying,
therefore, partly on the interior and partly on the exterior.
The armature core, or body, of a Gramme-ring machine differs
markedly in appearance from the armature body of a drum
machine, when both are in small sizes, since then the drum core
is practically solid, having no hollow space, so that it would
be impossible to wind it after the Gramme method. Such a
drum-wound armature is shown in Fig. 134. When, however.
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DRUM ARMATURES. '55
the drum armature is increased in size, so as to be emplo7ed
in multipolar fields, the form of the core or body passes from
a solid cylinder to that of an open cylinder or ring, as is
shown in Figs. 132 and 135, so that it would be possible to
place a conducting wire on such a core either after the drum
or Gramme type of winding. The tendency, however, in
modern electrical engineering is, perhaps, toward the produc-
tion of drum-wound rather than Gramme-wound generators.
It MULTIPOLAR FIELD.
This tendency has arisen, probably more from mechanical
and commercial reasons than from any inherent electrical
objections to armatures of the Gramme-ring type.
166, The windings of drum armatures are numerous and
complicated in detail, but all may be embraced under twft typi-
cal classes ; namely, lap-winding and wave-winding. In lap-
winding, the wire is arranged upon the surface of the armature
in conducting loops, the successive loops overlapping each
other, hence the term; while in wave-winding, the conducting
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IS6 ELECTRO-DYNAMIC MACHINERY.
wire makes successive passages across the surface of the
armature, while being advanced around its periphery in the
same direction,
167. Lap-winding is applicable particuUrly to bipolar arma-
tures, while wave-winding is applicable only to multipolar
armatures.
Fid. 136. — SIMPLE BIfOLAK
The simplest form of lap-winding is shown in Fig. 136, where
the separate paths taken by the turns a, b, c, d, and e, /, g, h,
across the outside of the bipolar armature core, and their con-
nections to the commutator, are represented as shown. If the
. 137. — SIMPLE BIPOLAR.
entire winding of the armature be completed, it is evident
that any attempt to represent the winding graphically by the
method adopted in this figure would lead to great complexity.
For this reason it is customary to represent the armature sur-
face as unrolled, or developed upon the plane of the paper, as
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DRUM ARMATURES. 157
shown in Fig. 138. For example, the winding already shown
in Fig. 136 becomes on this development represented as in
Fig. 138, Here it is clear that each loop overlaps its prede-
i^
no. 138,— DEVELOPMENT OF LAP-
cessor, and, consequently, it is evident that the simplest form
of drum-winding is a lap-winding.
Fig- '37 represents thesame winding as Fig, 136, except
Ijg. — QtlADRIPOLAK WAVB-WINDING.
that the connections with the commutator are given a lead of
90 degrees, requiring a correspondingly altered position of the
brushes of the machine.
Fig. 139 represents a number of conductors, ab, (d, ef, gh.
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IS* ELECTRO-DVyAMiC MACHINERY.
etc., wound on the external surface of a drum core in the
winding of the wave type. Here it will be seen that the
conducting wire, after crossing over from one side of the
armature core to the other, advances progressive!]' over
its surface in the form of a rectangular wave. The corre-
sponding development is shown in Fig. 140. The winding
shown is applicable only to multipolar fields ; for, an inspec-
FIG. 140.— DEVELOPMENT OF QUADRIPOLAR WAVB-WINDIKO.
tion of this particular arrangement of wave-winding will show-
that when conducting wires ab and </'are passing north poles,
the conducting wires cd and gh, are passing south poles, and
the direction of the induced E, M. F. while opposite in succes-
sive conductors, as regards the separate conductors ab, cd, ef,
and gh, is, nevertheless, unidirectional, so far as the entire cir-
cuit a, b, c, d, e, /, g, A, i, _/', Jt, is concerned. In the same
manner a wave-winding for an octopolar machine is required
to be spaced in accordance with the successive distances
between alternate poles.
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CHAPTER XV,
ARUATURE JOURNAL BEARINGS.
168. Even in the best designed types of electro -dynamic
machinery, there are certain losses of electric energy which
necessarily occur in the operation of the machine. These
losses may be grouped under the general head of frictions,
and include mechanical, electric, and magnetic frictions.
Since in well-designed types of large machines the commercial
efficiency may be as high as 95 per cent., it is evident that the
tosses from all these causes combined can be kept within a
small percentage of the total output.
169. This high efficiency, however, can only be obtained in
the case of large machines. In those of smaller output, the
proportion of the losses may be much greater. It is, there-
fore, advisable to examine the causes of these various losses,
■ their variation with the output of a machine, and the means by
which they are commercially reduced.
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i6o ELECTRO-DYNAMIC MACHINERY.
Considering first the mechanical losses : these may exist
as friction in the bearings of the moving parts of the generator,
friction arising from the pressure of the brushes on the com-
mutator, or contact parts, and friction from air churning.
The journal bearings are lubricated either by sight-feed
oiling, or self-oiling devices. In sight-feed oiling devices, a glass
oil cup, filled from time to time with oil, allowsoil to drop slowly
on the journal bearings, but requires to have its outlets opened
by hand, when the machine commences to run, and also to be
stopped when the machine stops.
Fig. 141 represents an end view and longitudinal section of
such a bearing. The oil cup C C, is provided with a head ff,
by the rotation of which an outlet in the base is adjusted.
The oil descends by gravity to the shaft S S, where, by the
movement of the shaft, it is mechanically carried through
spiral grooves on the inner surface of the babbitt-metal sleeve
B B, passing finally, from the ends on the bearing, into the
pans PP, whence it is drawn off at intervals and filtered.
The upper pan, P, is intended to catch any overflow of oil
that may occur during the process of filling. The box XX,
enclosing the babbitt-metal sleeve, is capable of rotation within
small limits, about a vertical axis, upon the spherical surfaces
ZZ. This play admits of the true alignment of the bearings to
the shaft S S. As soon as the shaft has been introduced and
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ARMATURE JOURiVAL BEARINGS. loi '
becomes self-atigned, any further undue play in the bearing is
prevented by tightening the nuts N N.
170. Sight'feed lubricating bearings necessitate, as already
observed, the opening and closing of the oil cup at the start-
ing and stopping of the machine. They have been, conse-
quently, almost entirely replaced by self-oiling bearings, which
require no such attention; here the oil is automatically fed to
the revolving shaft by its rotation. A self-oiling bearing of
this description is represented in Fig. 143. The oil is supplied
to the bearing into the oil well O O, through a screw hole h._
FIG. I43.~L0NCITUDINAL SECTION OF SELF-OILING BEARING.
During the rotation of the shaft S S, two rings r, r, which rest
upon the upper surface of the shaft, and dip into the oil within
the well, are set in rotation, and carry oil on the surface of the
shaft, where it is spread over the bearing along suitable
grooves in the babbitt-metal sleeve, as in the previous case.
Grooves are made in the upper surface of the babbitt-metal
sleeve for the reception of the rings, and the rings are pre-
vented from leaving the grooves by the screw clips m, m. The
rings are carried around by the friction caused by their weight
as they rest on the shaft, and, therefore, do not necessarily
rotate as rapidly as the surface of the shaft. The babbitt-
metal sleeve, which holds the shaft, is contained in a cylindri-
cal box with a spherical bolt £, at its centre. A pin or pro-
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l6a ELECTRO-DVNAMIC MACHINERY.
jecdon/, at the bottom of this box, engages in a hole in the
framework, thus preventing the box from rotating with the
shaft, but enabhng the shaft to align itself freely in the sleeves.
Nuts n, of which onl)r one is seen, clamp the box B, in position.
FIC. 144. — SLEEVE OF BABBITT METAL IN JOUR]
A draw-off cock is provided at d, for withdrawing the oil from
the well at suitable intervals.
171. Fig. 143 represents a longitudinal cross-section of a
similar bearing employed in machines of larger size. Here
PIC. 145.— DETAILS OF LARGE SELF-OILINC JOURNAL BEARING.
oil is fed through two openings/, /, and accumulates in the
lower part of the hollow cast-iron support S S. The rings r, r,
by their revolution upon the shaft, carry the oil into the
babbitt-metal sleeve didi, as before. The shaft is supported
upon the bracket//, which forms part of the pedestal or sup-
port S S, and is hollowed spherically so as to permit of the
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ARMATURE JOURNAL BEARINGS. 163
alignment of the babbitt-metal' sleeve and its box. Fig. 144
shows a general view of the babbitt-metal sleeve with grooves
for the reception of the oil rings, and with lugs L, L, L, for
assisting in the aligning. Fig. 145 represents partly in eleva-
tion, and partly in longitudinal section, a simitar bearing some-
times employed in still larger machines, differing from the last
described only in details of construction. The weight of the
shaft is taken directly upon the lower half of the bearing
B B B, which has its lower surface bowl-shaped, and fitting
into a pedestal or support 55, in such a manner that the bear-
ing can be readily aligned and finally tightly secured in place
by suitable bolts. The gauge glass T, enables the level of the
oil in the bearing to be clearly discerned.
172. The amount of energy expended as friction in journal
bearings varies with the weight supported on the bearing, the
accuracy of the workmanship, the correctness of the alignment,
the nature of the lubricating material, the character of the
surfaces in contact, the speed of rotation and the diameter of
the shaft.
Other things being equal, the energy expended is propor-
tional to the diameter of the journal in the bearing. In order
to keep the friction as low as possible, the diameter of the
journal is always kept as low as is consistent with ample
mechanical strength.
The power expended in brush friction depends upon the
number of brushes and the pressure with which they bear upon
the commutator. It also increases with the diameter of the
commutator and with the speed of rotation of the armature.
This waste of energy is often an appreciable fraction of the
total waste in a small machine, but is usually quite negligible
in a large one.
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CHAPTER XVI.
EDDY CURREHT&
173. During the rotation of the armature of a dynamo-
electric machine through the flux produced by its field magnets,
electromotive forces are not only generated in the conducting
loops on the armature, by the successive filling and emptying
of these loops with flux, but they are also generated in all
masses of metal revolving through the flux; in other words,
the iron in the armature core and the copper of the conductors
will also be the seat of E. M. Fs. Though these E. M. Fs.
may be locally very small, yet, since the resistances of their
circuits are generally exceedingly small, the strength of the
currents set up may be very considerable.
Such currents are generally known as eddy currents. They
are necessarily alternating in character, their frequency de-
pending upon the speed of revolution and upon the number
of poles.
Not only is the energy expended in eddy currents lost to the
external circuit, since these purrents cannot be made to con-
tribute to the output, but such currents also unduly limit the
output of the armature, by raising its temperature, independ-
ently of the increase of temperature due to the passage of the
useful armature current through the conducting loops. Losses
of energy due to eddy current are of the type /' R (in watts),
/, being the strength of the local current in amperes, "and JF,
the resistance of the local circuit in ohms.
174. It is evident that a dynamo machine can never be
designed so as to be entirely free from eddy currents; for, con-
ducting loops must be placed on the armature, and, moreover,
in nearly all the types of practical dynamo machines, iron arma-
ture cores are employed.
All that can be done is to reduce these losses as far as is
commercially practicable. In the case of the iron core, for
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EDDY CURRENTS. 165
example, the advantage arising from its use; namely, the
decrease in the reluctance of the magnetic circuit, can be
retained, provided the material of the core is laminated, i. e.,
made continuous in the direction of the magnetic flux paths,
and discontinuous at right angles to this direction.
175. If a piece of metal be revolved in a magnetic field, it
will enclose magnetic flux. A distribution of E. M. Fs. will
be established in it according to the rate at which the enclosure
takes place, and depending upon the shape ot the piece. These
E. M. Fs. will produce eddy currents in the moving metal.
The rate of expending work in eddy currents will be, for a
given flux intensity in the metal, in direct proportion to the
conductivity of the material. A piece of revolving copper
will have much more work expended in it by eddy currents than
a piece of lead or German silver. If, however, we divide the
mass of metal into a number of segments or smaller portions,
• the total E. M. F. at any instant will be divided into a num-
ber of parts, one in each segment, and the resistance of each
segment to its E. M. F. will be much greater than the ratio
of the resistance of the entire mass to the total E. M. F. in
such mass. The energy wasted in the mass will therefore be
reduced. For this reason, the iron core of the armature is
divided into sheets or lamin%, in such a manner that the sheets
afford a continuous path to the magnetic flux, but no circuit is
provided for. eddy currents across the sheets. The magnetic
flux is conducted through the entire length of the sheet, but
the circuits of the eddy currents are all in the cross- sections
of the sheet. The division of the armature core does not,
therefore, increase the magnetic resistance, or reluctance of the
armature, but enormously increases its resistance to eddy
currents.
176. Fig. 146 represents at D, an armature core of solid iron
capable of being revolved in a quadripolar field A'"', ■?■, N*, S*,
the arrows indicating the general directions of the flux paths.
The cross-section of the armature is shown at A, and the arrows
represents diagrammatically the distribution of the eddy cur-
rents set up in the solid mass of iron diiring the rotation of the
armature. At £, the cross-section is represented with lamina-
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i66
ELECTRO-DYNAMIC MACHINERY,
tions, parallel to the axis of the armature, as, for example, when
the armature core is composed of a spiral winding of sheet-iron
ribbon. Here the eddy currents are limited to the cross-
section of each band or lamina. *rhe magnetic flux, however,
has to penetrate all the discontinuities between the bands, in
order to penetrate to the deepest layer, unless the flux be
admitted to the armature on its sides, as shown in Fig. 8.
At C, the armature is laminated in planes perpendicular to
the axis, or is built up of sheet discs. Here the eddy currents
are confined, as in the last instance, to the section of each disc,
but the flux passes directly along each sheet.
While, therefore, the methods of construction indicated at
B and C, are equally favorable to the suppression of eddy
currents, B, tends to increase the reluctance of the armature,
and to magnetically saturate the outer layers of the core, with
a corresponding sparsity of flux in the inner layers, except
when the field poles cover the sides of the armature.
177. Taking a single lamina of the armature core, it is clear
that if the intensity in the core is, say, 13 kilogausses, each
square centimetre of cross-section in the lamina is linked with
13 kilowebers, first in one direction and then in the opposite
direction, as the armature moves from one pole to the next.
The value of the E. M. F. round the cross-section of the
lamina, considered as a loop, depends upon the speed with
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EDDV CUnttENTS. 167
-vrhich the linkage takes place, and, therefo^'e, on the intensity
iB the speed of rotation and the number of poles. The aver-
age E. M. F. in a lamina, rotating at a given speed through a
quadripolar field of intensity (B = iz,ooo, would be four times
as great as when passing through a bipolar field of intensity
ffl = 6,000. The rate at which an E. M. F, of e volts expends
energy in a resistance of r ohms, being ~ watts, the average
wasteful activity in eddy currents depends upon the square of
the speed of magnetic reversal in the core, and also upon the
square of .the intensity. If, then, we double the speed of
revolution in an armature core, we quadruple the eddy current
waste of power. The higher the intensity of magnetic flux in
the armature, and the more rapid the reversal, the more
important becomes the careful lamination of the armature, but
the eddy-current-loss in armature cores is usually very small
when the plates have a thickness not exceeding o.oz".
Moreover, when powerful eddy currents are present, the
M. M. F. they establish has such a direction as opposes the
development of magnetic flux by the field, so that the existence
of powerful eddy currents in an armature core tends to shield
the interior of the core, or its laminee, from magnetic flux,
thereby reducing the effective cross-section of the armature,
or increasing its apparent reluctance. This effect is usually
small in revolving armatures at ordinary speeds of rotation,
but becomes appreciable when the frequency of reversal is
high and the degree of lamination insufficient
178, It used to be the universal practice to separate adjacent
sheets of iron by thin sheets of paper, when assembling the
cores of armatures, so as to ensure the complete insulation of
the separate laminse. This introduction of paper into the core
had the disadvantage of reducing the effective permeance of
the armature core, or in other words, of increasing the flux
density in the iron. It has been ascertained experimentally,
however, in recent times, that the paper could usually be dis-
pensed with, as the superficial layer of oxide on the iron sheets
formed a layer of sufficient resistance to effectually insulate
the laminse against the feeble E. M. Es. in the eddy current
circuits.
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i68 ELECTRO-DYNAMIC MACHINERY.
179. As we have seen, eddy currents -are not limited to the
iron core of an armature, but are also set up in the conductors
wound on the armature.
In this case, eddy currents are set up in their substance by
revolution under the poles, but the conditions differ slightly in
detail. A Gramme-ring armature, for example, has no eddy
currents set up in the conductors except upon the outer sur-
face of the armature, since the flux passes through the wire at
the outer surface and not through the wire on the inner sur-
face. Similarly, a drum armature has no eddy currents set up-
in the wire upon the ends of the drum, if we may neglect such
leakage flux as may pass through the ends of the core. Again,
the amount of eddy-current-toss will depend upon the distribu-
tion of the magnetic flux over the surface of the armature. If
the flux entering the armature terminates sharply at the edge
of the pole-pieces, so that the wire suddenly enters or suddenly
leaves a powerful magnetic fleld in the air-gap, the rate of
change of the flux enclosed in the substance of the wire will
rapidly vary, inducing a brief, but powerful, E. M, F. in its
substahce, and the total expenditure of energy by eddy cur-
rents will be considerably greater than if the gradient of
magnetic intensity in the neighborhood of the polar edges is
less abrupt, and the E. M. F. smaller in amount but more
prolonged.
iSo. The eddy-current-loss for a given size of machine is-
apt to be considerably greater with l«w pressure than with
high pressure armatures, since the former require few -massive
copper conductors, while the latter require many, separately
insulated, conductors. The plan is, therefore, frequently
adopted of winding' low-pressure smooth-core armatures with
multiple conductors, each main conductor being composed of
a cable of separately insulated wires. Even when this is done,
an additional precaution is necessary, namely, to transpose the
conductors or twist them through 180 degrees, halfway across
the armature surface, in order to prevent any pair of wires
from acting as a loop for the generation of the E. M. Fs. This
is illustrated diagrammatically in Fig. 147, where the multiple
conductor CC, consisting of Ave insulated wires, laid over
the surface of the armature core A A A A, is reversed in the
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EDDY CURRENTS. i«9
centre, so that the advancing wire at one end becomes the re-
ceding wire at the other, and vice-versa.
It is sometimes found that the insertion of a sheet iron
cylinder of the form outlined in Fig. 148, closely fitted into
the polar bore, and forming a tube within which the armature
revolves, greatly diminishes the waste of energy in eddy
currents. This is for the reason that the edges of the pole-
pieces are removed, and the flux through the entrefer gradu-
ally varies between zero and full intensity as we advance round
the field. The effective area of the polar surfaces is for the
same reason increased. The objection to the introduction of
such a cylinder lies in the magnetic leakage it introduces; for,
FIG. 147.— DIAGRAM INDICATING THE TRAWSPOaiTION
if S, be the cross-section of the soft iron sheet in square centi-
metres, the flux it will carry, direct from pole to pole, will be
roughly 30,000 S, webers, and this flux has to be provided for
through the magnetic circuit of the field frame in addition to
other leakage and the useful flux through the armature.
181. When the armatare conductors are buried beneath the
surface of the iron, as, for example, when they run in the deep
grooves of toothed-core armatures, practically no eddy currents
are produced in them, for the reason that the space they oc-
cupy is almost free from the flux established by the field. A
toothed-core armature may, therefore, be considered as an
armature in which the eddy currents are confined to the iron
laminiB of the core. This feature constitutes one of the
advantages of toothed-core armatures.
182. Besides the eddy currents set up in the armature, and
in the conducting masses of the metal on the armature, they
also occur in the edges of the pole-pieces of the field magnets,
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fjo ELECTRO-DYNAMIC MACHINERY.
both in the case of dynamos and motors. The strength of
these eddy currents is greater in the pole which is approached
by a generator armature, and in that which is receded from by
a motor armature, as is evidenced by the fact often observed,
that, although both polar edges become warm during the action
of the machine, one edge becomes warmer than the other.
The reason for this difference will be considered later.
183. The tendency to the development of eddy currents in
pole-pieces is incieased when the armature is changed from a
smooth core to one of the toothed-core type. The reasons for
this are twofold; in the first place, in the toothed-core arma-
tare the armature is brought nearer to the pole face, so that
all magnetic disturbances in the armature are more likely to
* set up corresponding disturbances in the poles; in the second
place, because the revolving teeth set up waves of magnetiza-
tion in the polar surfaces, thus giving rise to the development
of eddy currents. Consequently, the change from a smooth-
core to a toothed-core armature suppresses the eddy cur-
rents in the wire on the armature, but creates, or tends to
create, eddy currents in the pole-pieces.
184. In some types of machines the pole-pieces are grooved
or slotted, so as to check the development of eddy currents,
just as the armature is in efEect grooved or slotted by the use
of laminated cores. An example of this is seen in Fig. 14. In
fact, some field magnets are constructed of a frame of cast iron
or cast steel, with receptacles within which are placed the po!e-
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EDDY CURRENTS. 171
pieces formed of a number of iron plates bolted together, the
laminations extending in the same direction as those in the
armature beneath.
It will be evident that there can be no tendency to set ap
edd^ currents in the solid cores of the field magnets excited
by steady, continuous currents. Consequently, no advantage
is derived from a lamination of field magnet cores at distances
beyond the influence of magnetic changes produced by the
teeth or conductors on the revolving armature.
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CHAPTER XVII.
MAGNETIC HYSTERESIS.
185. Besides the losses in the iron masses of a dynamo due
to eddy currents, there are others in the same masses due to
magnetic friction or hysteresii. These latter losses, like the
Others, are dissipated as heat.
The losses due to hysteresis occur in nearly all forms of
dynamo- electric machinery. In continuous-current generators
these losses are practically limited to the armature; in some
forms of alternating-current machines, they exist both in the
armature and field, and are especially present in alternating-
current transformers. It becomes, therefore, a matter of no
little importance to thoroughly undersUnd the nature of this
source of loss.
186. A certain amount of energy has to be expended in
order to magnetize a bar of iron. This energy resides in the
magnetic flux passing through the magnetic circuit of the bar.
The energy is transferred from the magnetizing circuit by the
production of a C. E. M. F. in the magnetizing coil, and
this C. E. M. F. e, (usually very small), multiplied by the
magnetizing current strength /, at that moment, gives as the
product e I, the activity expended in producing the magnetic
field. As soon as the full magnetic flux is established, the
C. E. M. F, ceases, being dependent upon the rate of
change of flux enclosed, .so that no more energy is expended
in the iron, and the current only expends energy as Pr, in heat-
ing the magnetizing coil. When the magnetizing current is
interrupted, say by short circuiting the source of E. M. F. in the
circuit, the magnetism in the bar tends to disappear, and, as
the magnetic flux diminishes, an E. M, F. is set up in the coil,
tending to prolong the action of the waning magnetizing cur-
rent. In other words, the E. M. F. set up in a circuit by the
waning magnetic flux is such as will tend to do work on the
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MAGNBTIC HYSTERESIS. 173
current, with an activity of the type ei watts, and, in this
manner, restore to the circuit the energy expended in the
magnetization. Were all the energy in this case returned to
the circuit, there would be no loss by hysteresis. As a matter
of fact, however, while practically all such energy would be
returned to the circuit, if the coil magnetized air, wood, glass,
etc., yet, when the coil magnetizes iron, although a greater
magnetic flux is obtained, yet some of the energy is not
restored, but is expended in the iron as heat.
187. It is now generally believed that each of the molecules
in a mass of iron is naturally and permanently magnetized, so
that each molecule may, therefore, be regarded as a molecular
compass needle. In the ordinary unmagnetized or neutral
condition of iron, these separate molecular magnets possess
no definite alignment, and, consequently, neutralize one
.another's influence by forming closed loops or chains. When
the iron becomes magnetized by subjection to a magnetizing
force, these loops break up and become polarized or aligned,
all pouring their magnetic flux in the same direction; 1 e.,
parallel to the magnetic axis. When the jnagnetizing force is
removed, the molecular magnets tend to resume their old
positions; and, if they did resume exactly their old positions,
the magnetism in the iron would entirely disappear on the
removal of the magnetizing force, and all the magnetic energy
would be restored to the circuit. In point of fact, however,
they do not exactly resume old positions, but take new inter-
mediate positions, by virtue of which a certain amount of
residual magnetism is left in the bar.
188. When now the magnetizing force is reversed, by
reversing the current through the magnetizing coil, the mole-
cules are forced around, and breaking suddenly from their
positions, fall into new positions, cither with oscillations', or
with a frictional resistance to the motion, that dissipates
energy as heat. The energy thus lost by molecular vibration
or molecular friction cannot be returned to the circuit. Conse-
<)uently, a loss of energy occurs in the circuit supplying the
reversing magnetizing force, at each reversal of magnetism in
the magnetized iron, since the opposing E. M. Fs. developed
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174 ELECTRO-DYNAMIC MACHINERY
in the coil during magnetization and demagnetization are not
equal, and the energy so lost results in an increase in tempera-
ture of the iron. By hysteresis, (his-ter-ce'-sis), is meant that
property of iron, or other magnetic metal, whereby it tends to
resist changes in its magnetization when subjected to changes
in magnetizing force. That is to say, when a mass of iron is
successively magnetized and demagnetized, ot passes through
cycles of magnetization, the magnetic intensity in the mass lags
behind i^^ impressed magnetizing force. The word hysteresis
take its origin from this fact, since it is derived from a Greek
word meaning to lag behind. This phenomenon is called hys-
teresis, and the loss of energy due to this cause is called
hysteretic loss, or loss of energy by hysteresis.
189. When iron undergoes successive magnetic reversals, the
amount of hysteretic loss is found to depend upon the maxipium
magnetic intensity in the iron at each cycle; that is to say,
upon the maximum value of (B. As (B, increases, the amount
of work that has to be expended in reversing the magnetization
increases, and if we double the value of (S, we practically
treble the amount of work that has to be expended. It was-
first pointed out by Steinmetz, as a consequence of this rela-
tion, that the hysteretic loss varied as the i.6th power of <S>, or
as (B '■', the formula for the amount of activity expended in
one cubic centimetre of magnetic metal being P = n r/fSt'''
watts. Since the same loss of energy occurs in a cubic centi*
metre during each cycle, the more rapidly the cycles recur,
the greater will be the wasteful activity, and n, in the above
formula, expresses the number of complete cycles through
which the iron is carried per second. The coefficient t}, is the
hysteresis coefficient for the metal considered, and has to be de-
termined experimentally. It may be regarded as the activity
in watts which would be expended in one cubic centimetre
of the metal when magnetized and demagnetized to a flux
density of one gauss at one complete cycle or double rever-
sal per second. The following table gives the values of this
coefficient, and also the amount of hysteretic loss produced in
a cubic centimetre, and in a pound, of ordinary good com-
mercial sheet iron at various frequencies and intensities.
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MAGNETIC HrSTERESlS. 175
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CtmfUttMat<KtUCjtU ftrSKnd,i»WaiU ftrCmliicCntimttrt.^^llmftr Cuhie
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190. As an example of the hysteretic activity, we may con-
sider a pound of iron subjected to a periodic alternating flux
density of ten kilogausses, with a frequency of 25 cycles-per
second. From the preceding table, it is seen that at 10 kilo-
gausses the hysteretic activity is 0.0365 watts- per- pound, at a
frequency of one cycle per second. At 35 cycles per second
this would be 25X0.0365=0.9125 watt = 0.9125 joule-per-
second = 0-6735 foot-pound per second. Consequently the
hysteretic activity might be represented by lifting the pound at
the rate of 0.6735 ^""^ P^^ second against gravitational force.
If, therefore, all the iron in an armature core be subjected to
an intensity of ten kilogausses, and rotates 25 times per second
in a bipolar field, 12.5 times per second in a quadripolar field.
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I7* ELECTRO-DYNAMIC MACHINERY.
or 6.35 times per second, in an octopolar field, hysteretic
activity is being expended at a raR which is probably repre-
sented by the activity of raising the whole armature core about
eight inches per second.
It is to be observed that the table represents average samples
of good commercial iron, and by no means the best quality of
iron obtainable.
191. As an example of the application of this table, suppose
that it is required to estimate the power expended in hysteresis
during the rotation of the armature of the octopolar generator
represented in Fig. 139, the weight of iron in the armature
being 2,700 lbs.
At the maximum intensity of 9,500 gausses, or 61,390 webers-
per-sq. in., the table shows that the hysteretic activity per
pound at one cycle per second is about 3.4 X lo"* watts. In
each revolution of the armature there would be eight reversals,
or four complete cycles, and at 2.867 revolutions per second,
the frequency of reversal would be ti.468 cycles per second.
The total- hyeto'etic activity is, therefore,
P X 1,700 X 3-4 X IO-* X 11.468 = 1,053 watts.
This would be the hysteretic activity in the armature when
generating 155.7 volts. When generating a lower E.M.F., the
flux intensity in the armature would be reduced, and, therefore,
the hysteretic activity.
192. Hysteresis, therefore, occurs when a mass of iron
undergoes successive magnetizations and demagnetizations,
and this is true whether such are caused by the reversal of the
magnetizing current, with the mass at rest, or by the reversal of
the direction of the mass in a constant magnetic field. Conse-
quently, the revolutions of the armature of a dynamo or motor,
occasioning the successive magnetizations and demagnetiz-
ations of its core, are accompanied by an hysteretic loss of
energy.
The amount of this hysteretic loss increases directly with the
volume V, of iron in the armature in c. c, the number n, of
revolutions of the armature per second, the hysteretic coeffi-
cient >/ of the iron employed, and the i.6th power of the
maximum magnetic intensity in the iron; for, it is evident that
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MAGNETIC HYSTERESIS. 177
in one complete revolution of the armature its direction of
magnetization will have undergmie two reversals, provided that
the field is bipolar. In a multipolar field the number of revers-
als increases with the number of poles/, and the hystcrctic
activity becomes P = ^-^ watts. In the case of a. gen-
erator, this activity must be supplied by the driving power,
and in the case of a motor by the driving current
193. When a generator armature is at rest in an unmagnet-
ized field, the torque; 1. e., the twisting moment of the force
which must be applied to the armature in order to rotate it, is
such as will overcome the friction of the journals and brushes.
When, however, the field is excited, so that the armature
becomes magnetized, the torque which is necessary to rotate
the armature is increased, even when the armature is symmet-
rically placed in regard to the poles. This extra torque is due
to hysteresis. It is sometimes called the hysteretic torque, and
is equal to
T = — ^-^ megadyne-decimetres.
4 w
I()4, The total useless expenditure, therefore, of power in an
armature core is the sum of the hysteretic and eddy current
loss. The former increases as the speed of revolution directly,
but the latter, as already pointed out, increases as the square
of the speed. Consequently, if we have an unwound armature
core, and rotate it on its shaft through a field which is at first
unexcited, we expend an activity which might be measured, and
which would be entirely frictional loss. When the field is ex-
cited, we expend activity against mechanical friction, hysteresis
and eddy currents combined. By varyingthespeedofrotation,
and observing the rate at which the activity given to the rotat-
ing armature increases, it is possible to separate the three
descriptions of losses from each other.
195. Although, as we have seen, the hysteretic loss increases
with the 1. 6th power of the intensity of flux, yet it is stated to
have been found experimentally, that when a mass of iron, such
as an armature, is rotated in a sufiiciently powerful magnetic
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178 ELECTRO-DYNAMIC MACHINERY.
field, the hysteretic loss entirely disappears, owing to the sup-
posed rotation of all the elementary molecular magnets about
their axes during the rotation of the armature without losing
parallelism, and, consequently, without any molecular oscil-
lation and expenditure of magnetic energy as heat So far
as experiments have yet shown, this critical intensity in the
iron is above that which ordinary dynamo or motor armatures
attain, so that under practical conditions, the i.6th power of
the maximum intensity determines the hysteretic loss.
196. From an examination of the formula expressing the
hysteretic activity in the armature, it is evident that the
activity might be decreased by a decrease either in the number
of poles, the speed of revolution, the flux density, or the hys-
teretic coefficient. Since, however, in any machine the first
three factors are practically fixed, it is important that the
remaining factor, or hysteretic coefficient, should be kept as
low as is commercially possible. For this reason, whenever
the hysteretic loss is a considerable item in the total losses of
the generator, care is taken to select the magnetically softest
iron commercially available, in which the hysteretic coefiicient
is a minimum.
197. We have already referred to the increase in tempera-
ture of the edges of the field-magnet poles during the operation
of a dynamo armature, and have ascribed the cause of such
heating to the development of eddy currents locally produced
there. It is to be remarked, however, that some of the heat
in such cases may usually be ascribed to true hysteretic changes
in magnetization.
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CHAPTER XViri.
ARMATURE REACTION AND SPARKING AT COMMUTATORS.
198. In the operation of a dynamo-electric generator, con-
siderable difficulty is frequently experienced from the sparking
which occurs at the commutator, that is to say, instead of the
current being quietly carried off from the armature to the
external circuit, a destructive arc, which produces burning,
occurs between the ends of the brushes and the commutator
segments. The tendency of this sparking, unless promptly
checked, is to grow more and more marked from the mechani-
cal irregularities produced by the pitting and uneven erosion
rtC. 149.^K;KAHME-RlNa ARMATUKE IN BIPOLAR FIELD ON OFEH CIKCUIT.
of the commutator segments. It becomes, therefore, a matter
of considerable practical importance to discuss the causes of
sparking at the commutator, and the means which have been
proposed, and are in use, to overcome the difficulty.
199. When a Gramme-ring armature, such as that shown in
Fig- 149. is rotated on open circuit, in a uniform bipolar field,
the brushes, when placed on the commutator, must be kept at
diametrically opposite points corresponding to the line n n.
If applied to the commutator at any other points, sparking will
probably occur, although the armature is on open circuit.
The reason for this is seen by an examination of the figure,
which represents a pair of coils C, C, about to undergo com-
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i8o ELECTRO-D YNAMIC MACHIWER Y.
mutation ; i. e., about to be transferred by the rotation of the
armature from one side of the brush to the other, and'being
short circuited by the brushes, as they bridge over the adjacent
segments of the commutator to which their ends are connected.
Since the coils C, C, in the position shown, embrace the
maximum amount of flux passing through the armature, there
will be no E. M. F. induced in them, and, consequently, there
will be no current set up during the time of short circuit under
the brushes. In other words, the prime condition for non-
sparking at the commutator is that the coils shall be short
no. ISO. GRAMME-RING ARMATURK WrTK BRUSHES DISPIACED FRDH
circuited only at the time, and in the position, where no
E. M. Fs. are being generated in them.
200. If the brushes be advanced into a position such as that
represented in Fig, 150, so that. the diameter of commutation;
i. e., the diameter of the commutator on which the brushes rest,
is shifted from B, £', to a new position, powerful sparking will,
probably, be set up, for the reason that in this position the
rate of change, in the flux linked with these coils, is consider-
able, and, consequently, there is a considerable E. M. F.
induced in them, so that, when they are short circuited by the
brushes, heavy currents tend to be produced in the circuit of
these coils according to Ohm's law. If, for example, a bipolar
Gramme-ring armature gives passage to a total useful flux of
I megaweber, and there are 1,000 turns on the armature,
and 50 segments in the commutator, then, if the speed of rota-
tion be 10 revolutions per second, the E. M. F. set up between
the brushes will be
100,000,000
volts,
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ARMATURE REACTION. l8l
and, since there are a$ commutator bars on each side of the
diameter of commutation, there will be an average of four
volts per coil of 20 turns. If the resistance of each coil be
o.oi ohm, the current which tends to be set up in a short-
circuited coil having the average E. M. F. is
4
= 400 amperes.
201. It now remains to be explained how the existence
of a powerful current in the short-circuited coil will produce
violent sparking at the commutator. It is well known that
the presence of a spark indicates a higher E. M. F. than the
four volts, which we have assumed in this case is to be gen-
erated in the short-circuited coil. The increase in the voltage
at the moment 0/ sparking is due to what is called the induct-
ance of the coil.
At the moment of short circuiting the coil by the bridging of
the brushes across the two adjacent commutator segments, a
powerful magnetic flux is set up in the coil, owing to its M. M. F.
This flux is so directed through the coil as to set up in it an
E. M. F. which opposes the development of the current. On
the cessation of the current, owingto the breaking of the coil's
circuit at the commutator, the coil is rapidly emptied of flux,
and a powerful E. M. F. is set up in the same direction as the
current, sufficiently powerful to produce sparking between the
brush and the edge of the segment it is leaving. The E. M. F.
so generated during the filling or emptying of the loop by it&
own flux is called the E. M. F. of seif-induetion.
202. Fig. 151 diagrammadcally represents the flux produced
in the short-circuited coils C, C, by the M. M. F. of the current
produced during the short circuit. This flux passes princi-
pally through the air-gap and neighboring pole face, a small
portion passing through the air in the interior of the armature
between the core and the shaft. The greater the flux produced
by the coil, the greater will be the E. M. F. developed in the
coil, when the flux is suddenly withdrawn. The capability
of a conducting loop or turn for producing E. M. F. by self-
induction is called its inductance, and may be measured by
the linkage of flux with the turn per ampere of the current it
carries, that is, by the amount of flux passing through it.
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iSa ELECTRO-DYIVAMIC MACHINERY.
203. We have thus far considered the coils C, C, as being
composed of a single turn. If, however, each of these coils is
composed of two turns, and the same current strength passes
through each of these turns, then the M. M. F. of the coil will
be doubled, and, if the iron in the armature core and pole
face, is far from being saturated, the amount of flux passing
through the two turns will be twice as great as that which pre-
viously passed through one. When this flux is introduced or
removed it will generate E. M. F. in both turns, and, conse-
quently, will induce twice as much E. M. F. in the two turns
together as in a single turn. The inductance of the coil, or its
capacity for developing E. M. F. by self-induction, is thus four
times as great with two turns as with one, because there is
double the amount of flux, and double the number of turns
receiving that flux.
204. It is evident, therefore, that the inductance of a coil
increases rapidly with the number of its turns, and although
not quite proportionally to the square of the number, since,
with a large number of turns, although the M. M. F. is in-
creased in proportion to the number, yet the amount of flux
passing through each of the turns, owing to leakage, is not the
same. The E. M. F. of self-induction generated in each coil
depends:
(i.) Upon the E. M. F. induced in the coil by the revolution
of the armature.
{i.) Upon the resistance of the coil, or its capability for
allowing a large current to flow through it.
(3. ) Upon the inductance of the coil, or Its capability for
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ARMATURE REACTIOlf. 183
permitting that current to induce a powerful E. M. F. when the
circuit is made or broken.
The E. M. F. induced on making the circuit at the commu-
tator is advantageous, since it checks the development of the
current ; the E. M. F. induced on breaking the circuit is
harmful, since it enables a spark to follow the brush.
If, therefore, no sparking is to occur in a dynamo-electric
machine at no load, the brushes must rest on segments, con-
nected with coils in which no E. M. F, is being generated.
205. If the external circuit of the armature be closed
through a resistance, so that current flows through the arma-
ture coils and brushes into the external circuit, the preceding
conditions become considerably modified.
Fig. 152 represents the condition of affairs inwhich a current
PIG. Ija. — DIAGRAUHATIC
is flowing through the armature coils, and the brushes are
resting on the commutator, with the diameter of commutation
at the neutral points, or in a plane at right angles to the polar
axis.
In this figure the direction of the armature rotation is the
same as shown in previous figures; namely, counter-clockwise.
Here the flux produced by the M. M. F. of the armature coils
takes place in the circuits digrammatically indicated by the
curved arrows. The magnetization, therefore, produced by
the current circulating in the armature turns, is a cross mag-
netization, or a magnetization at right angles to the magnetiza-
tion set up by the field flux. The field flux through the poles
and armature is diagrammatic ally indicated in Fig. 153, where
the north pole is assumed to1>e situated at the left-hand side
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i84
ELECTRO-DYNAMIC MACHINERY.
of the figure, and the average direction of the field flux is at
right angles to the average direction of the armature flux. An
inspection of Figs. 153 and 153 will show that at the leading
edges of the pole-piece, L, L', that is, at those edges of the pole-
piece which the armature is approaching, the direction of the
flux produced by the armature is opposite to that of the
FIG. JJ3.— DIAGRAUMATIC KKPRBBENTATION OP VVKLD FLUX PASSING
flux produced by the field, and that, consequently, the effect
of superposing these fluxes is to weaken the flux at the leading"
edge as is shown in Fig. 154. On the contrary, at \\\^ following
edges F and F, of the pole-pieces, the direction of the armature
SUPBKPOSINO ARUATURE FLUX ON FIBLD PLUX.
flux coincides with the direction of the field flux, and the super-
position of these two fluxes will have the effect of intensifying
the flux at the following edges. Consequently, the neutrai lin^ ^
in the armature, or the line symmetrically disposed as regards
the entering and leaving flux, will no longer occupy the posi-
tion N, N, at right angles to the polar axis, but will occupy a
position « n' ; therefore, in order to set the brushes so that
they may rest upon commutator segments connected with coiI»
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ARMATURE REACTION. l8S
having no E. M. F. generated in them, it is necessary to bring
the diameter of commuution into coincidence with the neutral
TSne, or to give the brushes a lead; s. e., a forward motion, or
in the direction in which the armature is rotating.
206. This, however, will not in itself, as a rule, prevent
sparking, for the reason that induced E. M. Fs. are produced
in the coil under commutation at load, even although the coil
being commuted has no resultant E. M. F. set up by rotation.
This induced E. M. F. is due to the inductance of the coil and
COILS DURING COM-
to the load current which it carries. An inspection of Fig. 155
will show that as the coil C, approaches the brush B, the current
in the coil, as shown by the arrows, is directed upward on the
side facing the observer; while on leaving the brush, after
having undergone commutation, the current in the coil will be
flowing in the opposite direction or downward. The sudden
reversal of the current in the coil under commutation produces
a sudden reversal of the magnetic flux linked with the local
magnetic circuit of that coil, and this sudden change in the
magnetic flux through the coil induces in it a powerful E. M. F.,
causing a spark to follow the brush.
In order that no spark shall be produced from this cause, it
is necessary that before the brush leaves the segment the cur-
. rent in the coil shall have become reversed, and will therefore
be flowing in the same direction as that which will pass through
it during its passage before the pole face N. In order to effect
this it is necessary to bring the coil that is being commutated
into a field of sufficient intensity to induce in it, while short
circuited, a current strength equal and opposite to that which
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i86 ELECTRO-DYNAMIC MACHINERY.
passes when it first becomes short circuited by the brush. It
is not, therefore, usually possible to keep the brushes on the
neutral line as shown in Fig. 154, at n n', but their lead must'
be increased, until thd coil under commutation is in a sufficiently
powerful field beneath the pole face to produce, or nearly pro-
duce, this reversal of current The amount of lead necessary
to give to the brushes in order to effect this will depend upon
the inductance of the coils, and also on the strength of the
current in the armature.
207. The lead of the brushes, besides tending to reduce
sparking at the comm.utator', tends to diminish the E. M. F.
generated by the armature, for two distinct reasons : First,
because it connects in series armature windings in which the
E. M. Fs. are in opposition, as will be seen from an examina-
tion of Fig. 156; and second, because the M. M. F, of the
armature coils over which the lead has extended exerts a
C. M. M. F. in the main magnetic circuit of the field coils,
thereby tending to reduce the useful flux passing through the
armature. This effect is called the back-magnetization of the
armature. Cross- magnetization, therefore, exists in every
armature as soon as it generates a current, but back-mag-
netization only exists when a current is generated in the arma-
ture, and the diameter of commutation is shifted from the
neutral points.
208. The conditions which favor marked sparking at the
commutator of a generator are, therefore, as follows; namely,
(i.) A powerful current in the armature; /. e., the sparking
increases with the load.
(2.) A large number of turns in each coil connected to the
commutator; 1, e., the sparking increases with the inductance.
{3.) A great distortion of the neutral line through the
armature, or a powerful armature reaction.
(4.) A high speed of rotation of the armature, since the
current in the coil has less time in which to be reversed during
the period of short circuiting.
(5.) A nearly closed magnetic circuit for each coil; i.e., a
small reluctance in the magnetic circuit of each coil, whereby
the inductance of the coil is increased.
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ARMATURE REACTION. 187
The conditions which favor quiet commutation, or the
absence of sparking, are as follows; namely,
(1.) A small number of turns in each commuted coil, or a
large number of commutator bars.
(2.) Decrease of current strength through the armature.
(3.) A lead of the brushes.
(4.) A powerful field, or a high magnetic intensity in the
entrefer, due to the M. M. F. of the field magnets.
(5.) A large reluctance in the magnetic circuit of each coil.
209. An inspection of Figs. I5a-i54 will render it evident
that the effect of superposition of the armature M. M. F.
upon the M. M. F. of the field magnets, is to weaken the
intensity of the field flux at the leading edges of the pole-
pieces, and to strengthen the intensity at the following edges of
the pole-pieces. At the same time, it is necessary to advance
the brushes; i. e., the diameter of commutation, so as to bring
the commuted coils under the leading edges of the pole-pieces,
in order that they may receive a sufficiently powerful intensity
of field flux to enable the armature current to be reversed in
the coil under the brushes, and sparkless commutation thus
to be effected. If, however, the number of ampere-turns on
the armature; /, ^,, its M, M. F. at a given load, be sufficiently
great, the field flux at the leading edges of the poles will be so
far weakened, that the intensity left there will be insufficient
to effect sparkless commutation, no matter how great the
lead may be. In other words, the flux from the armature will
overpower the field flux, in any position of the brushes. This
will take place when the M. M. F. due to half the turns of
active conductor on the armature, covered by the pole face,
is equal to the drop of magnetic potential in the fteld flux
through the entrefer.
210. The magnetic intensity under the edge of the lead-
ing pole-piece will be zero, when the magnetic difference of
potential between this polar edge and the armature core,
immediately beneath, is zero. The magnetic difference of po-
tential across the gap at this point due to the field flux alone,
will be the magnetic drop in the entrefer, or (firf, where <B, is
the field intensity in the gap with no current in the armature.
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i88 ELECTRO-DYNAMIC MACHINERY.
and d, the length of the entrefer in cms. The total M. M. F.
of the armature, along the arc of one pole, will be — wp,
where wp is the number of turns covered by the pole, and this
will be the total difference of potential in the magnetic circuit
of the armature. Assuming that the armature is not operated
near the intensity of magnetic saturation, almost the entire
reluctance in the armature circuit will be in the entrefer.
Fig, 156 represents dia grammatically the magnetic circuit of
a Gramme-ring armature. The reluctance between be and cd,
in the field pole, also between ef and fa, in the armature, will
be comparatively small, so that the total magnetic difference
of potential developed by the armature will be expended in the
two air-gaps ab and de, half the M. M. F. of the turns beneath
the pole face being expended in each air-gap, Strictly speak-
FIG. 156.— MAGNETIC aUCUlTS OF CRAMME-RINC ARMATURE DUE TO ITS'
ing, the magnetic flux produced by the armature will not be
confined to the paths indicated by the dotted arrows, but will
pass across the air-gap at all points not situated on the neutral
line cf. The above principles may be relied upon, however,
to a first approximation.
211. In order, therefore, that a smooth-core armature
should be capable of sparkless commutation, the M. M. F.
of the turns on its surface, covered by each pole, should be
somewhat less than the drop of magnetic potential in each
air-gap, so as to leave a residual flux from the field in which to
reverse the armature current in the coil under commutation.
For example, if each air-gap or entrefer has a length of 2
cms., and the intensity in the air is 3,000 gausses, the drop of
potential in the air will be 6,000 gilberts. If the number of
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ARMATURE REACTION. 189
Oramme-ring armature turns, covered by one pole-piece, is
200, then a current of 8oamt)cres from the armature will repre-
sent 40 amperes on each Bide, and the M. M. F., produced by
this current will be — X 40 X aoo = 10,056 gilberts, and
half of this amount, or 5,028, being less than the drop of field
flux in the gap, should leave a margin for sparkless commu-
tation.
ai2. Although the preceding rule enables the limit of current
for sparkless commutation, on a smooth-core armature, to be
predicted under the conditions described, yet it by no means
follows that sparkless commutation must necessarily be
obtained if the M. M, F. of the armature lies within this limit.
If, for example, the number of commutator segments is very
small, the inductance of each segment may be considerable,
and a powerful flux intensity may be required to reverse the
current under the brush in the presence of such inductance.
No exact rules have yet been formulated for the determina-
tion of the inductance in a coil with which a given current
strength, speed of rotation, and field intensity, shall render
sparkless commutation possible.
213. The methods in general use for the suppression of
sparking may be classified as follows:
(i.) Those which aim at the reduction of inductance in the
commuted coils.
(a.) Those which aimat the reduction of the current strength
passing through the coil during its short circuit by the brush,
and, therefore, at the reduction of the current strength which
must be reversed before the short circuit is over.
(3.) Those which aim at the reduction of the armature reac-
tion, so as to reduce its influence in weakening the field in-
tensity in which the coil is commuted.
214. There are two methods for reducing the inductance of
the armature coils.
The first is to employ a great number of commutator seg-
ments, thus decreasing the number of turns in each coil under
commutation. It is evident that an indefinitely great number
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igo ELECTRO-DYNAMIC MACHINERY.
of commutator segments would absolutely prevent sparking.
A great number of commutator segments is, however, both
troublesome and expefisive, so that in practice a reasonable
maximum cannot be exceeded.
The second method for lessening the inductance of the arma-
ture coils differs from the preceding only in the method of
connection. It consists in providing two separate winding»
or sets of coils ; or, as it is sometimes called, in double-winding
the armature. The two separate windings are insulated from
each other, but are connected to the commutator at alternate
segments, so that the brush rests coincidently upon segments
that are connected with each winding. Each winding there-
fore, furnishes half the current strength, and the effect of the
inductance in each coil is reduced.
215. When the brushes are not so shifted as to bring the
diameter of commutation into coincidence with, or even in ad-
vance of, the neutral point, the coil under commutation will be
situated in a magnetic flux in the wrong direction; i. e., a mag-
netic flux which tends to increase and not to reverse the cur-
rent strength in the coil, so that when the coil is short circuited
by the brush, the current strength becomes increased in the
wrong direction. When, for any reason, it is impossible to
alter the lead of the brushes during variations of load, as, for
example, when the generator has to run without attendance,
the sparking, which may be produced at the brushes owing to
the resultant flux in which the commuted coils lie, may be
greater than that due to the mere reversal of armature current
in the coil under the influence of its inductance. In such
cases, considerable improvement is effected by the insertion
of a resistance between the coils and the commutator segments
with which they are connected. Thus in Fig. 157, the con-
necting wires ai and ci, are sometimes made of German silver.
It is evident, under these circumstances, that the coil under-
going commutation will not only have its own resistance, but
also the resistance of the German silver wires in the local cir-
cuit through the brush, and the current which can be set up
in this circuit by the E. M. F. induced .in the coil, owing to
its motion through the distorted field, is prevented from assum-
ing considerable strength. The value of the German silver
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ARMATURE REACTION. 191
resistances, although great by comparison with the resistance
of a single coil, is small when compared with the resistance of
the entire armature, and, consequently, does not greatly add
to the armature's effective resistance. It is clear that this
method does not obviate the sparking due to the inductance
of the armature coils, but tends rather to obviate that due to
the establishment of unduly powerful currents in the short cir-
cuited-coil in the wrong direction, and which current has sud-
denly to be reversed when the short circuit is broken. The
method is, therefore, often employed with armatures for which
the brushes cannot be shifted.
2l6. The most generally adopted plan for reducing sparking
is to employ a comparatively high resistance in the brush
itself. An examination of Fig. 157, will show that if the resist-
ance in the tip of the brush B, can be made sufficiently great,
the current which enters the commutator from the wires will
be so far reduced, before contact with the brush tip ceases,
that when the rupture does take place, practically all the cur-
rent from the armature will be passing through the coil in the
right direction; i. t., in the same direction as it will have when
the brush has passed to the next coil, and, consequently,
the current strength which has suddenly to be reversed when
the brush leaves the bar is very small.
217. Thus in Fig. 157, suppose the armature is rotating in
the direction of the large curved arrow, and that the commutator
segment 1, is about to move from beneath the brush B. The
coil 2 a j f I, is about to change position, from the left-hand to
the right-hand side of the armature, and the current in the coil
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ipa ELECTRO-DYI^AMIC MACHINERY.
is about to change in direction, as indicated by the small /:Drved
arrows, from'thedirectien-a b (, to' the direction c b a. The
current leaving the armature having recently been flowing to
the brush B, from section i, and the wire c i, is now flowing
'from sections a and i, and from wires a 2 and £ 1. If the resist-
ance in the tip of the brush is considerable, relatively to that
in the whole breadth of the brush, the current through e 1 B,
will be relatively reduced and that through a 2 B, relatively
increased. This will require, however, that the current from
the right-hand side of the armature shall be forced through
the coil b, in the direction ^ b a, and the more nearly this can
be accomphshed, before contact is broken between t and B,
the less is the opportunity that is offered for the inductance of
the coil d ^ 1:, to produce a spark at rupture. With this pur-
PIG. IJS.— DYNAUO BRUSH OF STRIPS OP tNTERLRAVED COPPEK AND
HIGH RESISTIVITY k
pose in view, brushes are made up of strips of German silver,
interleaved with copper or woven gauze; or they may be made
of carbon with a specially high resistivity. Fig. 158 repre-
sents a form of brush in which strips of copper are interleaved
between strips of high resistivity metal. By this means the
brush, as a whole, possesses the requisite conductance for the
current it has to carry, but the tip has sufficient resistance to
assist in the reversal of the current in the coil under commuta-
tion. Fig. 159 represents a block of carbon employed in a
suitable holder or frame as a dynamo brush. In order to
increase the conductance of the brush as a whole, it is usually
thinly copper-plated as shown. Carbon brushes are largely
employed for 120-volt dynamos where the current strength
produced is not great, and almost exclusively employed with
5oo-volt dynamos. The use of such brushes tends to reverse
the current in the armature, during the period of short circuit-
ing, and also aids in checking any undue current in the wrong
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ARMATURE REACTION. 193
direction, caused by distortion of the field flux, owing to arma-
ture reaction.
Artifically compressed graphite is sometimes used fordynamo
brushes. Besides the advantage of high resistivity, it lubricates
the commutator surface.
218. Referring now to the tnird method for suppressing
sparking at the commutator, a variety of plans have been
attempted at different times for bringing about a reversal of
the current in a commuted coil, during the period of short
circuiting, by the action of a specially directed magnetic flux
upon this coil, as, for example, by winding a special magnet
FIG, 159.— CARBON DVNAMO BRUSH.
placed with its pole immediately over the short -circuited coil,
in such a manner that the flux from this magnet, penetrating
the moving coil under commutation, may induce in it an
E. M. F. sufficiently powerful, to set up in the short circuit, a
current strength equal to that which the coil must sustain after
commutation is over, or, in other words, to produce automati-
cally the same effect which the lead of the brushes would be
capable of effecting under the most favorable conditions.
When, however, the current through the armature and its
M. M. F. are powerful, the M. M. F, needed on such control-
ling magnets may require to be very considerable, and, for
this reason, the plan, in this form, has never come into general
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194
ELECTRO-DYNAMIC MACHINERY.
Zllf. In the same direction a method has recently been pro-
posed for obuining sparktess cominutation by introducing into
the magnetic circuit of the machine, a M. M. F. equal in
amount, but opposite in direction, to that of the armature.
This has the effect of practically preventing all armature reac-
tion and distortion of the field flux. It is carried out by wind-
ing around the armature and through the field poles, as shown
in Fig. i6o, a number of turns, between A and B, equal to that
of the armature winding, and in series with the armature, so
that the ampere-turns in the balamtng coil A B, are equal and
opposed to the ampere-turns on the armature. The two
M. M. Fs. thus counterbalance and neutralize each other,
leaving the field flux practically unchanged at all loads of the
machine. By this means all sparking due to distortion of the
field is prevented, and only the sparking due to the inductance
of the commuted coil, and the current reversal in the same, is
left. In order to check this, an additional winding or magnet
over the commuted coil is introduced for the purpose of revers-
ing the E. M. F. in this coil as above described, a process which
is more easy of application when no armature reaction exists
than when armature reaction is unchecked. A quadripolar
machine, wound in this manner with a quadruple set of balanc-
ing coils, is shown in Fig. i6i.
220. While it is claimed for this method that it entirely over-
comes armature reaction, yet it possesses ' the disadvantage
that it requires the use of what is practically an extra armature
winding upon a part of the machine which does not revolve,
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ARMATURE REACTION. 195
thus introducing an additional cost and complexity. It, there-
fore, remains to be determined how far the advantage of
sparkless operation is offset by extra resistance, weight,
material, and cost.
Another method, which has been tried in England for the
purpose of suppressing sparking, adds extra coils on the
armature, one between each commutator segment and its
FIG. 161.— QUAD RIPOLAJt
armature connection. These coils are arranged in such a
manner that the E. M. F. induced in them by their revolution
through the field shall reverse the direction of the current in
the coil under commutation. Fig. 162 represents diagram-
matically the method ofwinding, and Fig. 163 the action of
the various E. M. Fs. In Fig, 162 the inner ring with the
additional coils actually forms part of the armature core and
receives the flux from the field although indicated in the figure
as a separate ring for clearness of description. Fig. 163 shows
a coil being short circuited by the brush, and the direction of
the current in this coil is being reversed by the action' of its
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190 ELECTRO-DYNAMIC MACHINERY.
auxiliary coil which Is still under the trailing pole edge, so
that when the bar B, leaves the brush, no serious sparlc shall
follow.
221. In the dynamo-electric machine represented in Fig. 6,
and which has but three commutator segments, the spark is
Fio. i6a.— 1
prevented from forming by an air blast directed against the
commutator in such a manner as to extinguish the incipient
spark at the breaking of the short circuit This air blast is
'<:;Cr;:>>-
INDICATING ACTION OF DEVtO ILLUSTRATED IN
FIG. 156.
supplied by a small centrifugal pump rotating with the
armature.
222. The number of bars in the commutator of a generator
depends principally upon the sparking limit. If there were no
danger of excessive sparking, the number of commutator bars
in any machine would be very small, except when marked
freedom from pulsation is required in the current strength.
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ARMATURE REACTION. 197
The number of bars will, therefore, depend upon the pressure
and current strength, the armature reaction, and the field flux
intensity. An unduly small number of bars leads to excessive
sparking, and, in the case of high pressure machines, the
sparks may flash completely around the commutator, produc-
ing what is practically a short circuit. Small machines have
been built, however, giving 10,000 volts with only yi commuta-
tor segments.
223, Thus far we have mainly considered smooth-core
armatures. The great majority of dynamos, in construction
at the present time are, however, toothed-core armatures. In
the first production of toothed-core machines, the sparking
which they exhibited was more troublesome and violent than
in smooth-core armatures of equal size, and apparently for
the reason that the inductance of each armature coil was
increased, owing to its being surrounded, ornearly surrounded,
by iron, instead of having an iron base only, as in the smooth-
core type. This difficulty has since been overcome by care-
ful designing, and toothed-core armatures are now con-
structed which give less trouble from sparking than smooth-
core armatures of equal size and output. This is accomplished
by giving such a cross-section to the teeth in the armature
that, at no load, the iron in the teeth is nearly saturated, and
has, therefore, a high reluctivity. The presence of armature
reaction tends to increase the magnetic intensity in the teeth
beneath the trailing pole edges, and to diminish it in the teeth
beneath the leading pole edges, as already observed. This
tendency is opposed by the increasing reluctivity of the
saturated teeth at the trailing pole edges, and, consequently,
the teeth tend to restore an equal distribution of magnetic flux
over the surface of the armature; or, in other words, tend to
check the effect of armature reaction. At the same time, the
high reluctivity of the teeth tends to diminish the inductance
of each coil undergoing commutation, so that, by careful
adjustment, the existence of the teeth is not merely a mechan-
ical advantage but also a considerable electrical advantage.
In practice, the output of a generator is not realty limited
by excessive sparking. As usually designed, the temperature
elevation of the armature, even when thoroughly ventilated.
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19* ELECTRO.DYNAMIC MACHINERY.
fixes the limit to the output before the sparking becomes trou-
blesome. And, in fact, many generators are in use to-day
which never require to have any lead given to their brushes,
and need only occasional attention to their commutators.
224. In the preceding discussion, we have considered
armature reaction from the standpoint of the Gramme-ring
armature only, but the same principles are equally applicable
to disc or drum armatures.
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CHAPTER XIX.
HEATING OF DYNAMOS.
225, The activity expended in any generator invariably
takes the form of heat. These expenditures are:
) /* ^ activity in the field magnets.
) J* R activity in the armature winding.
) /* R activity in eddy currents, in armature and field.
) Hysteretic losses in armature core and field poles.
) Friction in bearings and brushes.
) Friction in air.
226. The number of watts expended in the field magnets
is equal to the product of the pressure in volts at the field
terminals, multiplied by the current in amperes passing
through the field. This activity, although steadily expended
in the form- of heat, is necessary in order to produce the
M. M. F. of the field-coils. In a certain sense, therefore, it
may be said that the /* R activity in the field windings is
■expended in order to magnetize the field, and the /' R activity
in the armature winding is expended in order to magnetize the
armature. In series-wound generators, where the armature
sends its entire current through the field magnets, this
expenditure varies with the load. Thus, in a lo-KW series-
wound generator, designed to supply a maximum current of
200 amperes at 50 volts pressure, if the resistance of the field
magnet coils, when warm, be o.oi ohm, the pressure at the
terminals of the magnets will be 300 x o.oi = 2 volts, and
the activity, 2 x aoo = 400 watts. On light load, however,
of say 20 amperes, the pressure will be o.oi x 20 = o.a volt,
and the activity o. 2 x 20 = 4 watts, so that, in the first case,
the amount of heat generated in the field winding is 100 times
greater than in the second case, and the temperature, which
the field winding would attain in the first case, would be much
higher than in the second.
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20O ELECTRO-DYNAMIC MACHINERY.
227. In a shunt-wound generator, the activity in the field
circuit is nearly constant. For example, a lo-KW generator,
intended to supply iii volts at its terminals, at full load, with
a current strength of 90 amperes in the main circuit, might
supply a current of 3.5 amperes through its field magnets.
Consequently, the activity in the field-magnet circuit would
be in X *-S = 277-5 watts. At light load, the current
strength through the field magnets vould have to be reduced
to say 1.0 amperes, in order to keep the terminal pressure at
III volts, and the activity in the field would be reduced to
III X 3.0 = 232 watts, so that the temperature attained by the
winding on the field magnets would not be much greater at
full load than at no load.
228. It is evident that the /* R activity in the armature
always varies with the load; i. e., with the current strength
/. At no load, this loss must be very small, the current
strength being limited to that required for the excitation of the
field magnets. The temperature elevation of the armature,
due to the armature winding, consequently, increases rapidly
with the load.
229. The activity expended as /* Jt, in eddy currents in the
field poles, or in the armature, is nearly uniform at all loads,
especially in shunt-wound machines, in which the intensity of
magnetic flux is nearly constant, and if this intensity were
absolutely uniform; i. e., if there were no drop in the armature,
requiring a greater M. M. F. and exciting current, and if there
were no armature reaction, the eddy current loss would be
constant at all loads.
230. The activity expended in hysteresis in the armature and
field poles, would, similarly, be constant at all loads if the
magnetic intensity were constant. As the magnetic intensity
is increased by an increase in the M. M. F. of field and
armature at full load, the hysteretic loss increases, approxi-
mately following the i.6th power of the local magnetic intensity
at any point. The heat due to hysteretic loss is developed
principally in the armature.
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HEATING OF DYNAMOS. aol
231. The friction in bearings and brushes produces heat at
those parts. The amount of heat liberated, due to pure
friction, is comparatively small when the lubrication of the
bearings is properly attended to. In large generators, the
heat produced by the friction of the brushes on the com-
mutator is very small compared with the heat developed by
the sparking, and the powerful currents set up in the short
circuited coils undergoing commutation.
The frtctional forces opposing the rotation of an armature
in which there is no appreciable magnetic flux, are due to
gravitation; i. e., to the weight of the revolving parts. When,
however, the field magnets are excited, and magnetic flux
passes through the armature, the frictional forces are due to
gravitation and magnetic attraction combined. If the arma-
ture is situated symmetrically with respect to an external
system of field magnets, if for example, the Gramme-ring
armature of Fig, 129 be revolved concentrically with the polar
bore, the system of magnetic forces all round the machine will
balance, and the friction of the machine will not be increased
by the influence of the magnetic flux. If, however, the
armature were nearer the lower poles, so that the entrefer
was shorter beneath the armature than above it, there would
be a tendency, as we have seen, to produce a greater magnetic
intensity in the lower magnetic circuits than in the upper ones, '
with a corresponding resultant magnetic pull upon the arma-
ture, vertically downward. The armature would consequently
revolve in its bearings as though its weight were increased,
and with an increase in friction and frictional expenditure of
energy. On the other hand if the armature were centred too
high, BO as to develop greater magnetic fluxes in the up|)er
than in the lower magnetic circuits, the effective weight of the
armature in its journals would be reduced, and the frictional
waste of energy in them diminished. This principle has been
employed in the design of some bipolar machines, in which the
resultant magnetic attraction upon the surface of the armature
is upwards, or in opposition to the attraction of gravitation.
232. The friction due to the churning of the air is compara-
tively small in drum armatures, but often constitutes an
appreciable loss in alternators, when a Gramme-ring armature
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of large diameter and rough exterior is revolved at a hi^
speed. In this friction the heat is principally developed in
the surrounding air and not in the mass of the machine. The
air churning, on the contrary, assists in cooling the machine.
233. The magnetic stresses exerted in large electro-dynamic
machines are often of considerable amount. Referring for
example to the machine outlined in Fig. 139, the polar areas
are 1,400 sq. cms., and the useful magnetic Hux passing per-
pendicularly into the armature, 3.534 megawebers: The mean
intensity in the entrefer is therefore 5l££z! = 2,534 gausses.
The attractive force per square centimetre (Par. 7a) is 5— =
— ~ — = 353,400 dynes = 358.4 grammes. The total
stress exerted will be 1,400 X >s8-4 = 361.700 grammes =
797.4 lbs. weight, at each pole.
234. In drum or Gramme-ring armatures with radial field
magnets, the magnetic flux through the armature, can only
alter, witlun certain limits, the vertical forces acting upon the
armature due to gravitation. In machines with parallel field
magnets, as for example, in the dynamo of Fig. 8, the magnetic
stresses exerted upon the armature are side thrusts, or hori-
zontal stresses parallel to the axis of the shaft. If the entrefer
on each side of the armature has the same length, the two
resultant magnetic forces exerted upon the armature will be
equal, but if the armature is nearer one set of poles than the
other, so as to produce a shorter entrefer on one side than on
the other, there will be a tendency to produce a resultant side
thrust toward the side of shorter entrefer. It is important,
therefore, that generators of this type should have their
armatures nearly midway between the polar faces.
235. The expenditure of energy as heat in a generator is
objectionable, first, because it represents loss of power, and,
consequently, reduced efficiency. Ten per cent, of loss in the
generator due to all these causes combined, means approxi-
mately 10 per cent, more coal, 10 per cent, more water, and
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HEATING OF DYNAMOS. ao3
engines and boilers larger by lo percent, to supply a given elec-
tric activity, than would be necessary if it were possible to
avoid these losses entirely; and second, because the heat
developed may raise the temperature of the generator to an
objectionably high degree and ultimately limit its output
236. There are four limitations to the output of a contlnnotis-
current generator; viz.,
(i.) Insufficient mechanical strength to withstand the
mechanical forces brought into play.
(2.) Insufficient efficiency, or insufficient electric pressure
at the brushes, under load.
(3.) Excessive sparking.
(4.) Excessive heating.
The first two cases of limitation can always, by proper
design, be obviated in all but the smallest generators. It ts
the third and fourth considerations Which limit the output in
all practical cases. In modern machinery it is the heating'
which first limits the output.
237. The limiting temperature of the generator armature is
dependent upon a variety of considerations. Ifi the first place,
the hotter the armature winding becomes, the greater its
resistance; for, if r, be the resistance of the armature, in ohms,
at 0° C, its resistance H, at any temperature l" C, will be
approximately, JR = r {i -\- 0.004 ')- In other words, the re-
sistance will rise by 0.4 per cent, per degree centigrade of
temperature elevation above zero. The result is, that at high
temperatures, the wasteful activity, as /*Ji, in the armature,
increases, increasing thereby both the loss in the machine and
the tendency to temperature elevation.
238. The temperature of the armature must not exceed that
at which any of the materials employed in. its construction
would be dcleteriously affected; »'. e., either softened or decom-
posed. In many generator armatures, cotton is the insulator
employed, four thicknesses of cotton (representing each about
— th of an inch, separates adjacent wires, except at specially
protected places, where mica and oil paper are employed.
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a04 ELECTRO-DYNAMIC MACHINERY.
Cotton undergoes slow thermolysis, or decomposition by heat,
at a temperature, approximately, that of the boiling point of
water, or loo" C. Consequently, it is unsafe, in practice, to
maintain cotton covered armatures, even though shellac-var-
nished, at a higher temperature than loo" C. If the tempera-
ture of the room, in which a generator is operated, never
exceeded -^o" C, it would require an elevation of 70" C. in
the armature to reach a dangerously high temperature. As,
however, some engine ruoms attain, in summer, a hi^er
temperature than 30° C, and since a margin has to be left for
accidental overloads, 50" C. is the temperature elevation that
the armature should not exceed at full load, and modern practice
is reducing this to 40° C; so that the temperature of the
armature, as observed after several hours of full toad, is usually
specified not to exceed 40" C. of temperature elevation above
the surrounding air.
United States Navy specifications usually require that the
elevation of temperature shall not exceed 50° F. = 27.8° C,
at any part of the machine. Other things being equal, these
specifications can only be met by increasing the size of machine
for a given output. In other words, with machines of the same
grade, a reduction of the limiting temperature at full load
means a reduction of the load which the machine can carry.
239. Many large generators, however, do not use any insul-
ation for their armature conductors, except mica, and such
generators can safely carry a much higher temperature eleva-
tion without danger.
Here the dangerous temperature, so far as mechanical injury
of the armature is concerned, would be that at which solder
would melt. Electrically, however, the increase in the resist-
ance in the armature would, probably, constitute a limitation
long before this temperature was reached, and if, in fact, the
armature winding were to attain this temperature, the field
coils, and even the bearings of the machine, might be danger-
ously overheated.
240. The activity in the field coils, which will elevate their
external temperature a given number of degrees centigrade,
depends upon their shape, size and arrangement, whether their
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HEATmO OF DYNAMOS. 205
surfaces are freel]^ exposed to the air, or are partly sheltered
from it. Usually, however, the 5urfaj:es of the field coils must
afford 16 square centimetres, or about 3.5 square inches per
watt of activity developed in them as I* R heat. If the field
winding consists of many layers of fine wire, the temperature of
the deep seated layers will be greater than that of the super-
ficial layer ; but if, on the contrary, the layers be few, and the
wire coarse, the difference of temperature in the winding will
be inconsiderable. The elevation of temperature on the field
magnets of a generator is usually not greater than 30° C at
full load.
241. In the case of the armature, the speed at which it
revolves through the air greatly increases its capability for
dissipating heat and reducing its temperature, so that a much
greater surface thermal activity can be permitted in the arma-
ture than in the field coils. The usual allowance for eddy cur-
rents, load currents and hysteretic losses combined, is about
— th watt per square centimetre; /. ?., 1— watts per square
inch of armature surface, including the surface on the sides of
the armature, but excluding its internal core surface ; or, about
three times more activity per unit area than on the field mag-
nets. In some specially ventilated armatures, in which the
core discs are spaced and sepaiated at intervals, to permit the
circulation of air from the interior outward by centrifugal force,
the dissipation of heat can be so far increased that two watts
per square inch of armature surface have been rendered practic-
able. Much depends, however, upon the shape and size of
the armature, as well as upon its peripheral speed, so that no
exact rule can be laid down.
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CHAPTER XX.
REGULATION OF DYNAUOS.
242. As has already been pointed out (Par. 16), all self-excit-
ing continuous-current generators may be wound in one of
three ways ; namely,
(i.) Series-wound.
(3.) Shunt-wound.
(3.) Compound- wound.
243. Fig, 164 represents diagrammatically the connections
between the field and armature of a series-wound generator.
FIO. 164-
It will be observed that the current in the main circuit passes
through the field magnet windings. The M, M. F. of the field
coils, therefore, increases directly with the current strength
through the circuit. So long as the iron in. the magnetic cir-
cuit of the machine is far from being saturated, the flux through
the armature increases with the M. M. F., approximately, in
direct proportion, and the E. M. F. of the armature, conse-
quently, increases nearly in proportion to the current strength.
As soon as the iron in the circuit approaches saturation, the
flux increases more slowly, and finally, the E. M. F. of the
armature is scarcely increased by any increase in the current
strength through the circuit
.244, Fig. 165 represents diagrammatically the connections
between the field and armature of a shunt-wound generator.
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RBCULATION OF DYNAMOS. 207
Here the field magnets are wound with fine wire and the
windings are connected in parallel with the external circuit,
instead of being connected in series with it. Consequently,
if the pressure at the brushes be considered as uniform, the
current strength passing through the magnet coils must, by
Ohm's law, be uniform, independent of the current strength
in the main circuit. Thus, if the pressure at the brushes be
assumed constant, at, say 100 volts, and the resistance of the
magnet coils be 50 ohms, then the current strength through
the magnet coils will be two amperes, independently of the
strength of current supplied to the main circuit.
245. Practically, however, owing to the drop of pressure in
the armature as the load increases, and also on account of the
nc, l6j.— DIACKAM OF
shifting of the brushes that may be necessary with the increase
of load, the pressure at the brushes diminishes, and the cur-
rent strength through the field magnets diminishes in the same
proportion. The tendency in a shunt-wound machine is, there-
fore, to diminish its M. M. F., and its resulting E. M. F., as the
load on the generator increases. In order to maintain a con-
stant pressure at the brushes under all variations of load, it is
necessary to adjust the strength of current passing through
the field magnets, so that the M. M. F. at full load shall be
slightly in excess of the M. M. F. at light load. This is usually
accomplished by the insertion of a rheostat in the field magnet
circuit, so that some or all of this resistance can be cut out by
hand at full load, thereby increasing the current strength
through the magnet coils.
246. If, for example, the full-load activity of the machine
be 10 KW at 100 volts pressure, the full-load current strength
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3o8 ELECTRO-DYNAMIC MACHINERY.
will be loo amperes. Assuming the resistance of the armature
to be 0.05 ohm, the drop of pressure in the armature at full
load will be 100 x 0.05 = 5 volts, and the additional drop of
pressure, owing to the shifting of the brushes in order to
avoid sparking, may be 3 volts more, making a total drop
in pressure of 7 volts. The effect of this drop would be
to reduce the current strength in the lield magnet coils from
3 amperes to— = 1.86 amperes, thus reducing both the.flux
through the armature and the E. M. F., so that a balance
between the £. M. F. and its excitation might be found at,
say, 90 volts, if no means were adopted to regulate the cur-
rent strength through the field coils. In other words, the
no. )66.— DIAGRAM OF COMPOUND WINDIHO.
pressure at the brushes would vary by 10 volts between light
and full load.
247. Fig. 166 represents the connections between the field
and armature of a compound-wound generator. Here the
principal M. M. F. furnished by the magnet coils is that due
to the shunt coil, composed of many turns of fine wire, an
auxiliary series coil, of comparatively few turns of coarse wire,
being also employed in the main circuit. As the load increases,
the M. M. F. generated by the shunt winding tends to diminish
as above described, but the M. M. F. due to the series coil
increases. By suitably proportioning these two oppo^te
influences, the M. M. F. may be automatically so controlled,
that the pressure at the brushes shall remain constant, either
at the brushes of the generator, or at the terminals of the
motor or other translating device, which may be situated at a
considerable distance from the generator. In order to effect
this latter result, the M. M. F. of the series coil must compen-
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REGULA TION OF D YNAMOS. zoy
sate not only for the drop in the armature, but also for the
drop in the conductors leadinj; from the generator to the
motor, so that these external conductors may be regarded,
electrically, as forming an extension of the armature winding,
and, in this sense, the generator delivers a constant pressure
at its final terminals on the motor. Such a machine is said to
be overcompeunded,
24S. Series-wound generators are almost invariably employed
for series-arc lighting, since it would be very difficult to supply
the required M. M. F. for their magnets by a shunt winding,
considering that the pressure at the brushes varies between such
wide limits; and, even if such shunt winding could be supplied,
it would necessarily be formed of a very long and fine wire,
and, consequently, would become troublesome and expensive.
Series arc-lighting generators are sometimes constructed for
as many as 300 hghts, representing about 10,000 volts at the
generator terminals at full load, and a shunt winding for such
a pressure would be very expensive.
249. Shunt-wound generators are usually employed for sup-
plying incandescent lighting from a central station, and
their pressure is varied by hand regulation.
Compound- wound generators are usually employed for sup-
plying motors from central stations, and also for incandescent
lights and motors in isolated plants.
250. In the design and use of generators, it is important to
know how the E. M. F. generated in the armature at a given
speed varies with the current passing through the field magnets.
We have seen that so long as the brushes remained unaltered
in position, the E. M. F. in the armature, in C. G. S. units, is
equal to the product of the number of turns on the armature,
the number of useful wcbers passing through the armature
from each pole, and the number of revolutions per second.
Consequently, the E. M. F. of such an armature, running
at a constant speed, depends directly upoathe flux through
its magnetic circuit or circuits. If we vary the current
strength through the field magnets, and, consequently, the
M. M. F., we can observe the pressure in volts, which the
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aio ELECTRO-DYNAMIC MACHINERY.
mlchine will deliver at its brushes at light load. A series
of such observations, plotted in a curve, gives what is called
the tharacteristk curve of the generator. In the case of
a self-exciting, series-wound generator, it is only possible to
M
V
-
^
70
r
7 /
/ /
(!
fi
/
///
1
//
L-.
i !
\ To
vary the M. M. F. by varying the load, and, consequently, by
including, in the pressure at the brushes, the drop taking place
in the armature. The curve obuined from a series-wound
machine under such circumstances, is called an external ehar-
oiterhtiCy and the internal (haracterUHc may be determined from
it by correcting for the drop in the armature.
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SEGULATION OF DYNAMOS, an
251. Fig. 167 represents the internal and enternal chantc-
teristics of a particular series-wound generator intended to
supply a maximum of 70 amperes at 50 volts terminal pressure
or 3,500 watts.
The pressure at terminals, wlien the load was varied so as to
produce the required variations of current Strength through
the magnets, followed the broken line ABC, which is, there-
fore, the external characteristic of the machine. If we add to
the ordinates of this line from point to point, the drop of pres-
sure in the armature at tl)e corresponding current strength, the
full line o D E F,'\i obtained, which is, therefore, the internal
characteristic of the generator or the curve of its E. M. F. in
relation to the exciting current in its field coils.
The useful E. M. F. developed by the armature may be
expressed by the formula,
E = — ; J volts.
so that, if two observations are secured, the whole internal
characteristic curve may be deduced to a very fair degree of
accuracy. For Example, in Fig. 167, the E. M. F. at so
amperes = 74 volts, and at 70 amperes, 95 volts. From these
observations we may talce the two equations,
20 , 70
74 = and 05 = — ; .
'^ x + aoy ^^ x+ 70^
From these two equations we obtain x = 0.0836 and y =
0.00933, so that the E. M. F. at any current strength through
the field magnets is
E = — „ , / > volts.
0.0836 + 0.00933 /
The dotted curve o If E F, which lies close to the full curve
a D E F, represents the locus of this equation. It will be
observed that the dotted line practically coincides with the
full line representing the observations, except within the first
30 amperes of magnetizing current strength.
252. Fig. 168 represents the characteristic curve of a shunt-
wound generator, of 200 KW capacity. Here the current
strength through the field magnets was not observed, but the
pressure acting on the field coils was noted. Assuming, as
would probably be very nearly true, that the resistance of the
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312 BLBCTRO-DYNAMJC MACHINERY.
field magnet -coils remained constant throughout the observa-
tions, the exciting current strength would be proportional to
the pressure acting on the coils. With 40 volts on the magnets,
the E. M. F. at the brushes with the external circuit broken
was 71 volts, and increased, as shown by the full line ^ ^C, to
■>
,
^
"1
(t
ii'
^
H
„
/
/
/
"
1
/
//
'
/
\
(
!.
/
X
/
/
\
,'
F,
1
V
!
«
L.
L'
h i
185 volts, with 140 volts on the magnets. Here also the
E. M. F., E, may be expressed by the Fr&lich equation,
E = — -r| — -, f being the pressure on the field magnets; taking
the two observations, lao = — i- and 174 = — ; , we
find X =1 0.43 and jr = 0.0023, from which the general ecoation
becomes, .^ ,_ .,_
.Google
REGULATION OF DYNAMOS. 9\Z
The locus of this equation is. represented by the dotted line,
which practically coincides with the full line A B C, oi
observation.
253, When, therefore, two reliable observations have been
made of the E. M. F. generated by an armature, at observed
exciting current strengths, or pressures, situated not too closely
together, it is possible to construct the characteristic curve
throughout to a degree of accuracy sufficient for all practical
purposes.
The Frelich equation, by which this is possible, is a con-
sequence of the fact that the reluctance of the air paths in the
magnetic circuit of a generator is constant, while the reluc-
tivity of the iron in the circuit is everywhere capable of
being expressed by the formula v = a -f- ^ 3C (Par. 59) ; and,
consequently, the total apparent reluctance of the armature
takes the form x -^ y^, and the useful flux passing through
the armature * = 1 tr* ^> f'^ing the magnetomo-
tive force in gilberts, but sy, may be expressed in ampere-
turns, in amperes or in volts applied to the coils.
254. When the characteristic curves of a shunt machine have
been obtained, it is a simple matter to determine what the
series winding must be in order to properly compound it,
either for the drop in the armature, or for the drop in any
given portion of the external circuit as well. Thus, suppose it
be required to determine the series winding for the machine
whose characteristic curve is represented in Fig. 168. If the
£. M. F. required at the terminals of the machine be lao volts
at all loads, and if the drop in the armature, due to its resistance
at full load, as well as the resistance of its series coil, and to
any shifting of the brushes that may be necessary, amounts in
all to 10 volts, then the full-load current must supply the
M. M. F. necessary to carry the E. M. F. from lao to 130
volts, equivalent to raising the pressure by 8 volts from 70
to 78 volts on the shunt winding. The increase in current
strength from the shunt winding represented by these eight
volts multiplied by the number of turns in the shunt winding,
gives the M. M. F. required, and the full-load current must
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214 BLECTRO.DYNAMIC MACHINERY,
pass through a sufficient number of turns to supply th\8
M. M. F. in its series coil.
255. In all commercial circuits, electro-receptive devices
require to be operated either at constant current or at constant
pressure. The majority of such devices are designed for con-
stant pressure; such, for example, are parallel or multiple-
connected incandescent lamps and motors. Some devices,
however, require to be operated by a constant current. Of
these, the arc lamp is, perhaps, the most important. Series*
connected incandescent lamps, and a few forms of motors, also
belong to this class.
256. In order to maintain a constant pressure at the ter<
minals of a motor wiU) a varying load, it is necessary, in
order to compensate for the drop of pressure in supply con-
ductors, that the pressure at the generator terminals either be
kept constant, or slightly raised as the load increases. With
shunt-wound machines this regulation requires to be carried
out by hand, a rheostat being inserted between the field and
the armature, as shown in Fig. 16^.
257. Various forms arc given to rheostats for such purposes.
They consist, however, essentially of coils of wire, usually iron
wire, so arranged as to expose a sufficiently large surface to
the surrounding air, as to enable them to keep within safe
limits of temperature under all conditions of use. The resist-
ance is divided into a number of separate coils and the ter-
minals of these are connected to brass plates usually arranged
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REGULATION OF DYNAMOS. 2iS
in circles, upon the external surface of a plate of slate, wood
or other non-conducting material, so that, by the aid of a
handle, a contact strip can be brought into connection with
any one of them. The coils being arranged in series, the
movement of the handle in one direction adds resistance to the
field circuit, and in the opposite direction, cuts resistance out
FlttS, 170 AND 171.— FORMS OF FIELD RHEOSTAT.
of the circuit. Figs, 170 and 171 show different forms of yfc/rf
rheostats, with wheel controlling handles. In some rheostats
the resistance wire is embedded in an enamel, which is caused
to adhere to a plate of cast iron. This gives a very compact
form of resistance ; for, the intimate contact of the wire with
the iron plate, together with the large free surface of the plate,
enables the heat to be readily dissipated and prevents any
great elevation of temperature from being attained. Two of
such rheostats are shown in Fig. 172.
258. Com pound- wound machines can be made to regulate
automatically, and do not require to have their £. M. F.
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ai6 ■ ELECTRO-DYNAMIC MACHINERY.
adjusted by the aid of a field rheostat. For this reason they
are very extensively used in the operation of electric motors.
Series-wound machines are invariably used for operating arc
lamps in series. Since the load they have to maintain is apt
to be variable, such machines must possess the power of vary-
ing their E. M. F. within wide limits. Two methods are in use
for maintaining constant the strength of current. That in most
general use is to shift the position of the collecting brushes on
the commutator so as to take off a higher or lower E. M. F.
according as the load in the external circuit increases or de-
creases. The effect of this shifting will be evident from an
inspection of Fig. 156 ; for, if the diameter of commutation be
FIO. 171,— ENAl
shifted to the right or left, the E. M. F. in some of the coils
will be opposed to that in the remainder, the difference only
being delivered at the brushes. In practice, the diameter of
commutation would never reach the position of maximum E.
M. F. represented in Fig. 156, and might, on the other hand,
rotate through a sufficiently large angle to produce only a small
fraction of the total E. M. F.
259. In all cases where the brushes are shifted through a
considerable range over the commutator, care has to be taken
to avoid the sparking that is likely to ensue if a certain balance
is not maintained between the M. M. F, of the armature and
the magnetic intensity in the air-gap. The fact that the current
strength through the armature coils is practically constant at
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REGULATION OF DYNAMOS. "7
all loads, enables this balance to be effectually maintained,
when once it has been reached at any load.
260. Series-wound arc-light generators have their armatures
wound in two ways ; namely, cloitd-coil armatures^ and open-coil
armatures. In the former, all the armature coils are constantly
in the circuit, while in the latter, some of the coils are cut out
of the circuit by the commutator, duringa portion of the revo-
lution. The ordinary continuous-current generator for pro-
ducing constant pressure is, therefore, a closed-coil armature.
Fig, 173 represents diagram ma tically a form of open-coil arma-
ture winding. The three colls shown are oosnected to a com-
mon or neutral point o. In the position represented, the coil
A, is disconnected from the circuits, the coils B and C, remain-
ing in the circuit of the brushes b b'.
261. In closed-coil, series-wound, arc-light generators, the
brushes are ^yt.\is. forward lead ; i. e., a lead in the direction of
the rotation of the armature. The amount of this lead controls
the E. M. F. produced between the brushes. It is essential. In
order to prevent violent sparking, that the coil under commuta-
tion should be running through an intensity sufficient to nearly
reverse the current in the commuted coil during the time of its
short circuiting. Since the current strength in the field, and
also In the armature, is maintained constant at all loads, it is
necessary that the intensity of flux, through which the com-
muted coils run, should be uniform, or nearly uniform, at all
loads and of the proper degree to effect current reversal. The
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3i8 ELECTRO-DYNAMIC MACHINERY.
M, M, F. of the field magnet, is constant and the M. M F. of the
armature is also constant, but the flux produced by the M. M.
F. of the armature varies with the position of the brushes and
the number of active turns that exist in that portion of the arma-
ture which is covered by the pole-piece, on each side of the diam-
eter of commutation. The pole-pieces are usually so shaped
that as the number of active turns in the armature covered by
each pole increase ; i. e., as the load and E. M. F. of the
machine increase, the trailing pole corners become more nearly
saturated, and by their increasingreluctance check the tendency
to increase the flux from the armature, so that an approximate
balance between the field flux and the armature flux is main-
PIG. 174.— DIAOKAM OP
COKNBCTtOHS.
tained at all loads. The armature flux always opposes the field
flux at the diameter of commutation. The magnetic circuit,
therefore, has to be so designed that the armature flux shall
never quite neutralize the field flux at this point, but shall
always leave a small residual field flux for the purpose of obtain-
ing sparkless commutation.
362. The other method, which is employed for maintaining
the current strength constant, introduces a variable shunt
around the terminals of the field coil, in such a manner that
when the current through the circuit becomes excessive, the
shunt is lowered in resistance, and diverts a sufficiently large
amount of current from the field magnets to lower their M. M
F. to the required value. In order, however, to avoid the
necessity for making this regulation by hand, it may be effected
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REGULATION OF DYlfAMOS. ai9
automatically as follows : namely, an electromagnet, situated
in the main circuit, is caused by the attraction of its armature,
on an increase in the main current strength, to bring pressure
upon a pile of carbon discs. This pile of discs ofifers a certain
resistance to the passage of a current, the resistance of the pile
diminishing as the pressure upon it increases. The pile is
placed as a shunt around the field magnet, so as to divert from
the magnet a portion of the main current strength. When the
attraction on the armature of the electromagnet increases the
pressure on the pile, the resistance of the shunt path is dimin-
ished, and less current flows through the field magnets, aa
represented in Fig. 174, where S, is the series winding, shunted
by the carbon pile P, and M, is the controlling magnet inserted
in the main circuit.
363. Both the above methods are capable of compensating
not only for variations in the resistance, or C. £. M. F. of the
circuit, but also for variations in the speed of driving. In this
respect the compensation is more nearly complete than that
of constant prcBsure machines; for, com pound -wound gener-
tors can maintain a constant pressure under variations of load,
but not under variations of speed.
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CHAPTER XXI.
COMBINATIONS OF DYNAMOS IN SERIES OR IN PARALLEL.
264. When a system of electric conductors is supplied from
a central station, it is evident, that if the load on the system
was constant, a single large generator unit would be the
simplest and cheapest source of electric supply, except, per-
haps, on the score of reserve, in case of accidental breakdown. ■
In practice, however, the load is never constant,. and, there-
fore, the capacity of the generating unit is always consider-
ably less than the total activity that has to be supplied at the
busiest time. Moreover, engines and generators are neces-
sarily so constructed, that while they may be comparatively
very efficient when working at full load, they are far less effi-
cient when working at a small fraction of their load, so that it
is desirable to maintain such units as are in use, at full load
, under all circumstances. This consideration of wasted power,
in operating large units at light loads, applies with less force
to plants operated by water power, but, even in this case, it is
usually found uneconomical to operate a large generator, for
many hours of a day, when a smaller one would be quite com*
petent to supply the load.
265. The generating units in a central station are, there-
fore, so arranged that they may be individually called upon at
any time to add their activity to the output of the station.
Electrically, these generators must be connected either in
separate circuits, or in series or in parallel in the same circuit.
The method of connecting dynamos in series, so far as con-
tinuous-current circuits are concerned, is only employed for
arc lamps operated in series. When a great number of arc
lamps have to be supplied over a given district, they are usu-
ally arranged in different circuits, each circuit containing ap-
proximately the same number of lamps. Each such circuit is
then connected, as a full load, to a single arc-light generator.
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DYNAMOS IN SERIES OR /A' PARALLEL. *ai
When, however, owing to some failure of continuity in a cir-
cuit, it is fonnd impossible to operate two circuits independ-
ently, it is sometimes desirable to connect the two circuits to-
gether at some point outside the station, and to operate the
increased load of lamps by two or more dynamos connected in
series.
266. Generators are also connected in series when it is de-
sired to employ, on the external circuits, the sum of the pres-
sures of those generators. For example, in cases of the trans-
mission of power to considerable distances, a high pressure in
the conducting circuit is economically necessary. Whenever
this pressure is greater than that which can be readily obtained
from a single continuous-current generator, it is possible to
connect two or more generators in series, so as to obtain the
sum of their pressures. Thus, five generators, each supplying
500 volts pressure, will, when connected in series, supply a
total pressure of 2,500 volts. The plan is rarely followed.
267. As a modification of the above plan, which is rarely
adopted, live-wire, and three-wire systems, employing respec-
tively four and two generators in series, are in use. The five-
wire system, although employed in Europe, has not found
favor in the United States. The three-wire system, however, .
is extensively employed. In this system, two generators of
equal voltage, say 135 volts, are connected in series so as to
supply a total pressure of 250 volts. Such a pressure is cap-
able of operating incandescent lamps in series of two. To
enable single lamps, however, to be operated independently, a
third or neutral wire is carried through the system from the
common connection point of the two generators, and the dis-
tribution of lamps, on the two sides of the system, is so arranged
that the equalizing current, passing through the neutral wire, is
small, and nearly as many lamps are operated at any one time
on the positive, as on the negative side of the system. A pair
of generators connected for three-wire service, therefore, con-
stitutes a generating unit in a three-wire central station.
368. Series-generators are never, in practice, connected in
parallel. Shunt-wound and compound-wound machines are
capable of being connected in parallel, and most central sta-
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aaa ELECTRO-DYNAMIC MACHINERY.
tioQS arrange the generators in such a manner that they may
be connected to, or disconnected from, the mains according to
the requirements of the toad.
269. Central stations, supplying incandescent lamps in par-
allel, usually employ shunt-wound generators, for the reason
that the efficient and economic operation of the lamps requires-
a nearly uniform pressure at all lamp terminals.
Not only does the uniformity in the amount of illumination
from an incandescent lamp depend upon the uniformity of tht
pressure supplied at its terminals, very small variations in the
pressure markedly varying the intensity of light, but also such
variations of pressure materially affect the life of the lamp.
Thus a 50- watt, 16 candle-power, incandescent lamp, intended to
be operated at a pressure of 115 volts, would have its probable
life reduced by about 15 per cent., if operated steadily at 116
votts, and reduced by about 30 per cent,, if operated steadily
at 117 volts pressure. For this reason the pressure in the
street mains supplying the lamps requires constant careful at- .
tention. Since it would be impossible to obtain at the mains
a sufficient uniformity of pressure, under all conditions of load,
by compound winding, and hand regulation would still be re-
quired, there is an advantage in dispensing altogether with
compound winding, and resorting to hand regulation, with
shunt winding, for the entire adjustment.
270. When two or more generators are connected in parallel,,
it becomes necessary that the electromotive forces they supply
shall be equal, within certain limits. If, for example, two
generators are connected in parallel, each working at half load,
then if the drop of pressure in 6ach generator armature at full
load is two per cent, of its total E. M. F., it is evident that it
is only necessary to increase the pressure of one generator two
per cent above that of the other, in order that the pressure at
the brushes of the first shall be equal to the E. M. F. generated
in the armature of the second. Under these circumstances no
current will flow through the armature of the second machine,
and all the load will be thrown on the first machine. If the
E. M. F. of the first machine be still further raised, the pres-
sure at its brushes will be greater than the E. M. F. in the
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DYNAMOS IN SERIES OR IN PARALLEL. JJJ
armature in the second, and a current will pass through the
second armature in a direction opposite to that which it tends
to produce, and, therefore, in a direction tending to rotate the
second generator as a motor. In other words, the control of
pressure between the two machines must be within closer
limits than two per cent. Early in the history of central
station practice, difSculties were experienced in controlling
the pressure of multiple-connected dynamos within limits nec-
essary to avoid this unequalizing action, but at the present
time, the governing of the engines and the control of the field
magnets are so reliable, that this difHcuity has practically dis-
appeared. It is important to remember, however, that the
larger the generator unit employed, and the smaller the drop
in pressure taking place at full load through its armature, the
narrower is the hmit of speed or regulation, in which inde-
pendent units will equalize their load, although as a counter-
acting tendency, the larger will be the amount of power which,
in case of disequalizing, will be thrown upon the leading ma-
chine tending to check its acceleration.
271. Compound-wound generators are almost invariably em-
ployed for supplying electric currents to Street railway sys-
tems. This is principally for the reason that the load in a
street railway system is necessarily liable to sudden and marked
fluctuations, and these fluctuations would be liable to produce
marked variations in the pressure at the generator terminals, if
the machines were merely shunt wound. Such generators are
operated in parallel units. Here, as in the case of shunt-
wound machines, it is necessary that the E, M. F. generated
by each machine should be nearly the same, in order that the
load should be equally distributed ; but instability of control is
greater in the case of compound-wound machines than in the
case of shunt machines, for the reason that when one of a
number of parallel -connected shunt-wound machines acceler-
ates, and thereby rises in £. M. F., so as to assume an undue
share of the load, the drop in the armature thereby increases,
and tends to diminish the irregularity, so that not only does
the greater load tend to retard the engine connected to the
leading machine, but also the drop in its armature aids in
equalizing the distribution.
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214 ELECTRO.DYNAMIC MACHINERY.
In the case of com pound -wound machines in parallel, any
acceleration tends, as before, to increase the E. M. F. of the
generator and, therefore, its share of the load, but the series
coil of the compound winding being excited by the additional
load, tends to increase the output of the machine, and, there-
fore, the governing of the engine has to be entirely depended
on to prevent disequaliration. Of recent years, however, the
plan has been widely adopted of employing an eguaiizing bar
between compound-wound generating units operated in par-
allel. The connections of an equalizing bar are shown in Fig.
175. Here the two compound-wound generators are connected
to the positive and negative omnibm bars, or bus bars, as they
are generally termed, AA and BB, while the series coils are
connected togetlier in parallel by the equalizing bar QQ. It
is evident that the equalizing bar connects all series coils of
the different dynamos in parallel, so that any excess of current,
supplied by the armature of one machine, must necessarily ex-
cite all the generators to the same extent.
272. When a number of compound -wound generators are
running in parallel, and the load increases, so that it is desired
to add another unit to the generating battery of dynamos, the
engine connected with the new unit is brought up to speed,
and the shunt field excited. This brings the E. M. F. of the
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DYNAMOS IN SERIES OR IN PARALLEL. 3t$
machine up to nearly 500 volts. Its series winding is then
connected in parallel with the series winding of the neighbor-
ing machines, by the switch on the equalizing bar, so that its
excitation is then equal to that of all the other machines. The
E. M. F. of the machine is then brought up slightly in excess
of the station pressure by the aid of the field rheostat, and, as
soon as this is accomplished, the main armature switch is closed,
thus connecting the armature with the bus bars. The load of
the machine is finally adjusted by increasing the shunt excita-
tion, with the aid of the rheostat, until the ammeter connected
with the machine shows that its load is approximately equal to
that of the neighboring generators. The same steps are taken
in reverse order to remove a generator from the circuit.
273. Fig. 176 is a diagram of a street-railway switchboard
for two generators. It is customary, both for convenience
and simplicity, to erect switchboards in panels, one for each
generating unit, so that each panel controls a separate unit,
and is in immediate connection with its neighbors. In the
figure, the two panels arc designated by dotted lines, the one
on the left, active, and the one on the right, out of use. On
each panel there are two main switches, P and N, for the posi-
tive and negative armature terminals. A smaller switch, not
shown, is usually located on the right of each panel, and is for
lighting up the station lamps from any panel and its connected
machines, at will. R, is a shunt rheostat, placed at the back of
the panel, with its handle extending through to the front, and
S, isa small switch for opening and closing the shunt circuit of
the field coils through the rheostat, R. A, is the generator
ammeter, brought into use by the switches P and N, and T,
is the automatic circuit-breaker for the panel. This electro-
magnetic circuit- breaker, opens the circuit of the machine
when the current strength, owing to a short circuit or other
abnormal condition, becomes dangerously great, thereby reliev-
ing the generator of the strain. The switch connected to the
equalizing bar E is not placed in this instance, on the panel,
but is mounted close to the generator with the object of
diminishing the amount of copper conductor required. Each
panel is also provided with a voltmeter connection and lightning
arrester, which have been omitted here for the sake of simplicity.
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936
ELECTRO-DYNAMIC MACHINERY.
274. The Operations for introducing a unit into the battery
of generators in this case, is as follows : the generator is brought
up to speed, the equalizing switch is closed, thus connecting
the series coils of the machines in parallel with the machines
in use. The positive main switch P, is next closed, connecting
WOUND GENERATORS.
one dde of the armature to ground and to return troth feedert.
The field switch S, is next closed, and the E. M. F. of the
machine brought up to slightly above station pressure by the
aid of the rheostat R ; finally, the negative main switch N, is
closed, throwing the armature into the battery, and the load is
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DYNAMOS IN SERIES OR IN PARALLEL. "7
adjusted by the rheostat £, in accordance with the indications
of the ammeter A.
275. Another arrangement for railway switchboards consists
in mounting the three switches, in close proximity to each
other and attaching a single handle to the three blades, so that
the three connections may be made or broken by a single
operation.
When the railway mains are connected with the station by
several feeders, it is customary to add another section to the
switchboard where switches and ammeters arc provided for
handling the various feeders.
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CHAPTER XXH.
iS AND SINCLE-FIELD-
276. Before leaving the subject of generators, it may be well
to discuss a few types of generators that do not fall under the
FlC, 177.— DISC- ABM ATUHB CENEKATOK.
types already discussed, and which are occasionally met with
in practice.
These may be described as ;
(i.) Disc-armature machines.
(3.) Single-lield-coil machines.
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FIG. I7B.— DIBC
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ajo ELECTRO-DYHAMIC MACHINERy.
(3.) Unipolar machines, or commutatorless continuous-
current machines.
277. Generators employing disc armatures are frequently
used in Europe, and although they are very seldom employed
in the United States, yet it is proper to describe them as being
types of machines capable of efficient use. In one form of
disc-armature generator, the armature is devoid of iron, and is ,
built of conducting spolces like a wheel, which revolves in a
vertical plane between opposite field-magnet poles. Such a
no. 179.— DIAGRAM OF DISC-ARMATUKE WINDING.
disc-armature machine is shown in Fig. 177. It is to be
observed that the entire machine is practically encased in iron,
and is provided with three windows on the vertical face;
through these windows the brushes, BB, rest on the commu-
tator which is placed on the periphery of the disc, resembling
in this respect the generator in Fig. 103. The armature of this
machine is shown in Fig.. 178 mounted on a suitable support.
The radial spokes are of soft iron, and are connected into loops
by the copper strips leading to the commutator segments on
the periphery. The object of employing iron spokes is to ,
diminish the relucUnce of the air-gap. The field poles face
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DISC ARMATURES. SJi
«acb other, being separated by the disc armature, which
revolves between them. Such an armature is evidently capa-
ble of being operated at an abnormally high temperature
without danger, being constructed of practically fireproof
materials. The electric connections of an octopolar machine
are represented diagrammatically in Fig. 179. The brushes, it
will be observed, are applied at the centres of any adjacent
pair of poles. Another form of the machine is represented in
Fig. 180,
278. An example of a single-field-coil multipolar dynamo 13
shown in Fig. 181. This is a quadripolar generator with four
sets of brushes. The interior of the field frame, with its pro-
jecting pole-pieces and exciting coil, is shown in Fig. iSs.
It will be seen that the field frame is made in halves.
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"^ MACNET. SI NGLB-VI ELD-COIL GEKBRATOK.
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SINGLE-FIELDCOIL MACHmES. 233
between which are enclosed the armature and the single field
magnetizing coil. Four projections N, N, and S, S, form the
pole-pieces of the quadripolar field ; that is to say, the magnetic
PIG. 183. — AKUATURE OF QUADRIPOLAR, SIN
flux produced by the M. M. F. of the single coil C C, passes
through the field frame into the two pole faces N and N, in
parallel through the armature into the adjacent pole faces
S, S, thus completing the circuit through the field frame. The
drum-wound, toothed-core armature, is shown in Fig. 183.
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CHAPTER XXIII.
COMMUTATORLESS CONTINUOUS-CURRENT GENERATORS.
279, Cemmutatorless continuous-current dynamos are sometimes
called unipolar dynamos, although erroneously. It is impossible
to produce a single magnetic pole in a magnet, since all mag-
netic flux is necessarily circuital, and must produce poles, both
where it enters and where it leaves a magnet. The fact that
these machines are capable of furnishing a continuous current
without the aid of a commutator, at one time caused consider-
able study to be given to them in the hope of rendering them
[c. 184.
commercially practicable. The maximum E. M. F. which they
have been constructed to produce, appears, however, to have
been about six volts, and, consequently, they have practically
fallen out of use, although they have been commercially
employed for electroplating.
280. Fig. 184 represents what is known as a Faraday disc.
This was, in fact, the earliest dynamo ever produced, and was
of the so-called unipolar type; for here, a copper disc D,
rotated, by mechanical force, about an axis parallel to the
' direction of the magnetic flux, supplied by a permanent horse-
shoe magnet MM, continuously cuts magnetic ilux in the same
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CONTINUOUS-CURRENT GENERA TORS. ^35
direction, and, consequently, furnishes a continuous E. M. F.
between the terminals 5, S\ without the use of a commutator.
281. The portion of the disc lying between the poles is caused
to rotate in a nearly uniform mag;netic flux, and with a velocity
which depends upon the radius of the disc at the point con-
sidered, as well as on the angular speed of rotation. The di-
rection of the E. M. F. induced will be radially downward from
the axis to the periphery, and, if connection be secured between
the axis as one terminal, and the rotating contact or brush as
the other terminal, an E. M. F. will be continuously produced in
that portion of the disc which lies beneath the poles; or, more
strictly, in that portion of the disc which passes through the
flux between them and around their edges. If| however, as in
Fig. 185, the disc be completely covered by the pole faces, a
PIC. 1S5.— FARADAY DISC.
radial system of E. M. Fs. will be induced outward in the direc-
tions indicated by the arrows, or inward, if the direction of
roUtion be reversed. If no contacts are applied to the disc,
these E. M. Fs. will supply no current, and will do no work.
If brushes are applied at the axis, and at any or all parts of
the periphery, the £. M. F. can be led off to the external circuit
2S2. The value of the E. M. F. will depend upon the angular
speed of rotation, the intensity of the magnetic flux, and the
radius of the disc. The intensity of the magnetic flux can
usually be made much greater by the use of a soft-iron disc
instead of a copper disc, thereby practically reducing the
reluctance of the magnetic circuit between the poles to that
of two clearance films of air, since the reluctance of the iron
disc will be negligibly small.
283. If we consider any small length of radius d I, Fig. 186,
situated at a distance /, from the axis of the disc, the E. M. F.
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3^6 ELECTRO.DYNAMIC MACHINERY.
generated in this element of the disc will be the product of the
intensity, the length of the element, and its velocity across the
fluit. The element will be moving across the magnetic flux of
uniform intensity, 03 gausses, at a velocity / lu centimetres per
second, where w, is the angular velocity of the disc in radians
per second. Consequently, the £. M. F. in this element will be:
flfe = / (w . rfr . B C. G. S. units of E. M. F.
The total E. M. F. will be the sum of the elementary E. M. Fs.
included in the radius taken from / = d, to / = Z, the radius
of the disc, or the integral of de, in the above equation between
the limits I — o, and 1 1= L. This integral is — w iB = e.
The E. M. F. from such a disc, therefore, increases as the
square of the radius of the disc, directly as the speed, and
directly as the uniform intensity of the magnetic flux. The
same result can be obtained in a slightly different expression,
since e»= a n tt, where «, is- the number of revolutions of the
disc in a second, e = — , 2}in<S, = }tL*n<Si = Sn<Si where
3
S, is the active surface of the disc. This will also be true if
the surface S, instead of extending over the entire face of the
disc, extends only from the periphery to some intermediate
radius. From this point of view the E. M. F. of the disc is
equal to the product of the intensity in which it runs, the
number of revolutiohs it makes per second, and its active sur-
face in square centimetres. To reduce this E. M. F. to volts,
we have to divide by 100,000,000.
284. There are two recognized types of commutatorless
continuous-current dynamos; namely, the £s( type and the
tylinder type. The outlines of a particular form of the disc type
are represented in Fig. 187. Here the shaft SS, usually hori-
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CQNTINUOUS.CURRENT GENERATORS. tyi
2ontal, carries a concentric, perpendicular disc of copper or
iron, rotating in a vertical plane, in the ring-stkaped magnetic
frame, in a circular groove, through the flux produced by two
■coils of wire. The general direction of the magnetic flux,
through the field frame and disc, is represented by the curved
arrows. It will be observed that the magnetic flux will be
uniformly distributed so as to pass through the rotating disc
at right angles. Brushes rest on the periphery, and on the
shaft, of the disc. Inasmuch as the E. M. F. in the disc is
radially directed at all points, the brushes for carrying off the
■current may be as numerous as is desired. These brushes are
FIG. 187. DISC TYPB
COHHUTATORLESS DIItECT<^URRBNT CEHERATOK.
marked b, b, in the figure. A and B, are the main terminals
■of the machine, and/, /', the field terminals.
285. If we suppose that the intensity (fi, is 13,000 gausses,
that the radius of the disc is 1 foot, or 30.48 centimetres, that
the active surface on each side of the disc is 3,500 square cen-
timetres, and that the speed of rotation is 1,400 revolutions per
minute, or 40 revolutions per second, then the E, M. F. obtain-
able from the machine will be :
> X 40 X 1
- = 12.0 volts.
In order to produce an £. M. F. of say 140 volts, such as
would be required for continuous-current central-station gen-
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23^ ELBCTRO-DYNAMIC MACHINERY.
erators, it would be necessary either to connect a number of
such machines in series, or to increase the diameter of the disc,
or to increase the speed of rotation. . It would, probably, be
unsafe to run the disc at a peripheral speed exceeding aoo miles
per hour, owing to the dangerously powerful mechanical
stresses that would be developed in it by centrifugal force.
This important mechanical consideration imposes a limit of
speed of rotation and diameter of the disc, taken conjointly.
By increasing, however, the active surface of the disc, and, at
the same time, working at a safe peripheral velocity, it would
be possible to construct large disc generators of this type for
an E. M. F. of loo or 150 volts.
386. It should be borne in mind that although such machines
would be capable of producing continuous currents without the
use of a commutator, yet the necessity of maintaining efficient
rubbing contacts on the periphery of the rapidly-revolving disc
introduces a difficulty and waste of power which has hitherto
prevented the development of this system, and, probably,
accounts for the fact that large machines of this type do not
exist.
287. Irregularities in the distribution of magnetic flux over
the surface of the disc may give rise to strong eddy currents
and waste of power in the same. If the flux be variable along
any radius of the disc B, as represented in Fig. t88, so that
the intensity tB, is not uniform along these lines, this irregu-
larity will not produce eddy currents in the disc unless the dis-
tribution is different along different radii. In other words, if
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CONTINUOUS-CURRENT GENERATORS.
"39
the distribution of magnetic flui and inteosity are symmetrical
about the azis of rotation of the disc, the irregularities which
exist will only alter the intensity of E. M. F. in different
elements of a radius. In Fig. i88, the intensity, instead of
being uniform from centre to edge, as indicated by the straight
line da c, increases toward the edge, following the line e ab.
nc. iSq. — CYUNDEK TVPB OF COMMUTATOR LESS CONTmUOUS-CUKKKNT
The formula for determining the E. M. F. of the disc is in
such case rendered somewhat more complex.
288. If, however, the curve a i, of flux intensity along
different radii is different, so that the distribution of magnetic
intensity is not symmetrical about the axis of rotation, then
eddy currents will tend to form, the amount of power so
wasted depending upon the amount of irregularity, the resis-
tivity of the material in the disc, and the load on the machine.
F.'INDUCBD IN KBVOLVING
289. Fig. 189 represents the outlines of a particular form of
the second, or cylindrical type of commutatorless continuous-
current generator. Here a metallic conducting cylinder eue,
revolves concentrically upon the shaft S S, through the uniform
magnetic flux, produced by the field frame surrounding it.
Here, however, two sets of brushes ib, i'i', have to be applied
to the edges of the cylinder in order to supply the main ter-
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24° ELECTRO-DYNAMIC MACHINERY.
minals A and B. The termiaals of the four drcular coils con*
stituting the field winding are shown aty, f.
290. If the magnetic intensity produced b]' the field is
uniform, the £. M. F. will be generated in lines along the sur.
face of the cylinder parallel to its axis, as represented in Fig.
190. If V, be the peripheral velocity of the cylinder in centi-
metres per second, I, the length of the cylinder in centimetres,
and & the uniform intensity, in gausses, the E. M. F. generated
by the machine will be:
e = • volts.
100,000,000
Machines of the cylindrical type have been constructed and
used for electrolytic apparatus, and give very powerful cur-
rents, as compared with ordinary generators of the same
dimensions employing commutators. Unsatisfactory as these
unipolar machines have so far proved, except in special cases,
they are, nevertheless, the only dynamos which have yet been
successfully constructed for furnishing continuous currents
without the use of a commutator.
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CHAPTER XXIV.
ELECTRO-DYHAlfIC FORCE.
291. In discussing the magnetic flux surrounding an active
conductor, we have observed in Par. 34, that it is distributed
in concentric cylinders around the conductor, as shown in
Figs. 37 and 38. It is evident that if a straight conducting
FIG. 191.— STRAIGHT CONDUCTOR IN UNIFORU HACNETIC FLUX.
wire A B, say / cms. in length, as shown in Fig. 191, be situated
in the uniform magnetic flux represented by the arrows, the
flux will exert no mechanical influence upon the wire. If, how-
ever, the wire carries a uniform current in the direction from
Fia 193.— KACHinc FLUX surroundinc active conductor.
A\Q B, then, as is represented diagrammatically in Fig. 193,
the system of concentric circular flux, indicated by a single
circle of arrows, will be established around the wire, appearing
clockwise to an observer looking from A, along the direction
in which the current flows, and, as has already been pointed
out, this circular magnetic flux will have an intensity propor-
tional to the current strength.
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243 ELECTRO-DYNAMIC MACHINERY.
292* If such a conductor be introduced into a uniform mag-
netic flux, as is represented in Fig. 193, it is evident that above
the wire at C, the direction of the flux produced by the current
is the same as that of the field, while below the wire at D, the
direction of the flux from the current is opposite to that from
the field. Consequently, the flux above the wire is denser.
FIO. 193.— DIAGRAM SHOWING DIRECTION IN ELBCTRO-DYNAMIC POKCE.
and that below the wire is weaker, or less dense, than that of
the rest of the field. The effect of this dissymmetrical .distri-
bution of the flux density in the immediate neighborhood of
the wire, is to produce a mechanical force exerted upon the
substance of the wire, called the electro-dynamic force, tending
to move it from the region of densest flux toward the region
of weakest flux; or, in the case of Fig. 193, vertically doirti-
^*^^^^
FIO. 194. — DIAGRAM SHOWING DIRECTION IN ELECTRO-DVNAHtC FORCE.
ward, as indicated by the large arrow. If, however, the direc-
tion of the current in the wire be reversed, as shown in Fig.
194, and that of the external field remain unchanged, the flux
will be densest beneath the wire and weakest above it, so that
the electro-dynamic force will now be exerted in the opposite
direction, or vertically upward, as shown by the large arrows.
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ELECTRO-D YNAMIC FORCE. 243
293. If the direction both of the current in the wire and the
flux in the external field be reversed, the direction of the
electro-dynamic force will not be changed, as is represented in
Fig, 195, where the direction of the electro-dynamic force is
downward as in Fig. 193, though the direction of the current
and the direction of the magnetic field are both reversed.
294. A convenient rule for remembering the direction of
the motion is known as Fleming's hand rule. It is, in gen-
eral, the same as that already given for dynamos in Par. 8r,
except that in applying it, the left hand must be used instead
of the right. For example, if the hand be held as in the rule
for dynamos, if the/orefinger of the left hand shows the direc-
tion of the/lux, and the middle finger the direction of the cur-
FIG. 195. — DrAGRAU SHOWING DIRECTIUN IN ELECTRO-DYNAMIC FORCE,
rent, then the thumb will show the direction of the motion.
It must be remembered, that in applying Fleming's rule, the
right hand is used for dynamos in determining the direction of
the induced E. M. F.,and the left hand for motors in deter-
mining the direction of motion.
295. We shall now determine the value of the electro-
dynamic force in any given case, on the doctrine of the con-
servation of energy. To do this, we may consider the ideal
apparatus, represented in Fig. 196, where a horizontal con-
ductor E F, moves without friction against two vertical metallic
uprights A B, and CD. This conductor is supported by a
weightless thread, passing over two frictionless pulleys P, P,
and bearing a weight If, If now a current enters the upright
A B, and, passing through the sliding conductor £■/", leaves the
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244 ELECTRO-DYf^AMIC MACHINERY.
Upright C/>, at C, then, in accordance with the preceding
principles, under the influence of the uniform magnetic flux
passing horizontally across the bar in the direction of the
arrows, an electro-dynamic force will act vertically downwards
upon the rod. If this electro-dynamic force is sufficiently
powerful to raise the weight W, it will evidently do work on
such weight, as soon as it causes the bar to move. Let us
suppose that it produces a steady velocity of the bar B F, oiv
cms. per second, in a downward direction. Then if/, be the
■DYNAMIC MOTOR.
electro-dynamic force in dynes exerted on the bar, the activity
exerted will be, v f centimetre-dynes-per-second, or ergs-per-
second. Since 10,000,000 ergs make one joule, this will be an
activity of
"/
- joules-per-second, or watts.
This activity will be expended in raising the weight W,
assuming the absence of friction. As in all cases of work
expended, the requisite activity to perform such work must be
drawn from some source, and in this case the source ts the
electric circuit.
296. When the bar of length / cms. moves with the velocity
of V centimetres-per-second, through the uniform fiuxof den-
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ELECTRO-DYNAMIC FORCE. 345
sity iB, it must generate an E. M. F. as stated in Far. 82, of
< = (ft/i;, C. G. S. units, or
&lv
= volts.
100,000,000
This E. M. F. is always directed against the current in the
wire, and is, therefore, always a C. E. M. F. in the circuit
The current of i amperes passing through the rod will, there-
fore, do work upon this C. E. M. F. with an activity of
This activity must be equal to the activity exerted mechan-
ically by the system, so that we have the equation,
vf _ <S, lvi
10, 000, 000 1 00, 000, 000
From which,
/ = - - dynes.
— will be the number of C. G. S. units of current, since the
C, G. S. unit of current is 10 amperes, so that the funda-
mental expression for the electro-dynamic force exerted on a
straight wire, lying or moving at right angles across a uni-
form Hux, is
f = &l I dynes,
where /, is expressed in C. G. S. units of current. Since the
force of 981 dynes is, approximately, the force exerted by
gravity upon one gramme, we have
/ = — a~°^ — « " grammes weight,
and since 453.6 grammes make one pound,/, expressed in
pounds weight will be
If, for example, the rod shown in Fig, 196 had a length of
one metre, or 100 centimetres, and moved in the earth's flux
whose horizontal component := o.a gauss, then if supplied
with a uniform current of 1,000 amperes, it would exert a
downward force of 0,2 x 100 x — — - = a, 000 dynes; or ap-
proximately, 2 grammes weight.
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246 ELECTRO-DYNAMIC MACHINERY.
297. . We have heretofore considered the wire as Ijring at
right angles to the flux through which it is moved. If, how-
ever, the wire A B, lies obliquely to the flux, at an angle ^, as
is represented in Fig. 197, then the effective length ef the wire,
or the projected length of AB, at right angles to the flux will
be a b. In symbols this will be / sin p, and the electro-
dynamic force will be
f =. (Ri sin p — dynes.
298. Although such a machine as is represented in Fig.
196 is capable of performing mechanical work, and might be,
therefore, regarded as a form of electro -dynamic motor, yet all
practical electro-dynamic motors are operated by means of
conducting loops, capable of rotating about an axis. We
shall, therefore, now consider such forms of conductor.
299. If the rectangular loop a a" a"' a"", Fig. 198, placed in a
horizontal plane, in a uniform magnetic flux, be capable of
rotation about the axis do, then if a current of i amperes be
caused to flow through the loop in the direction a' a" a'" a"",
electro-dynamic forces will be set up, according to the preced-
ing principles, upon the sides a' a", and a'" a"", but there will
be no electro-dynamic' force upon the remaining two sides.
Under the influence of these electro-dynamic forces, the side
a' a", will tend to move upwards, and the side a'" a"", down-
wards. The loop, therefore, if free to move, will rotate, and
will occupy the successive positions b, c and d. At the last
named position, the plane of the loop being vertical, although
the electro-dynamic force will still exist, tending to move the
the side a' a", downwards, and the side a'" a"", upwards, yet
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ELECTRO-DYNAMIC FORCE. 247
these forces can produce no motion, being in opposite direc-
tions and in the same plane as the axis; or, in other words,
the loop considered as a rotatable system is at a dead po<nt
300. It is clear, from what has been already explained, that
if the direction of the current in the loop had been reversed
while the direction of the field flux remained the same ; or, if
the direction of the field flux be reversed with the direction of
currentremaining the same, that the direction of the electro-
dynamic forces would have been changed, tending to move
the side a' a", upwards and the side a'" a"", downwards, so that
the loop would have rotated in the opposite direction until it
reached the vertical plane. Consequently, when a loop, lying
in the plane of the magnetic flux, receives an electric current
it tends to rotate, and, if free, will rotate until it stands at
right angles to the magnetic flux,
301. An inspection of the figure will show that when the
loop is in the plane of magnetic flux, that is to say, when the
rotary electro- dynamic force is a maximum, the loop contains
no magnetic flux passing through it, while when the loop is in
the vertical position, and the rotary power of the electro-
dynamic force is zero, it has the maximum amount of flux
passing through it. The effect of the electro-dynamic force,
therefore, has been to move the conducting loop out of the
position in which no flux passes through it, into the position
in which the maximum possible amount of flux passes through
it, under the given conditions.
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248
ELECTRO-DYNAMIC MACHINERY.
302. When an active conductor is bent in the form of a loop,
such, for example, as is shown in Fig. 199, all the flux pro-
duced by the loop will thread or pass through the loop in the
same direction, and this direction will depend upon the direc-
tion of the current around the loop. If, for example, we con-
sider the loop fl'«'a'a', independently of the magnetic flux
into which it is introduced, and send a current of /' amperes, in
the same direction as before around the loop, the general dis-
tribution of the flux around the sides of the loop is represented
FtG. iq9.-~DIAGRAU SHOWING
FLUX PATU»
I LOOP OF ACTIVE
by the circular arrows, from which it will be seen that all the
flux passes downward through the loop as represented by the
large arrow. If this loop be now introduced into the external
magnetic flux, as shown in Fig. 192, it will tend to rotate, unti[
the external magnetic flux passes through it in the same direc-
tion as the flux produced by its own current. Generally,
therefore, it may be stated that when an active conducting
loop is brought into a magnetic fleld, the electro-dynamic
force tends to move the loop until its flux coincides in direc-
tion with that of the field.
303. During the rotation of the loop as shown in Fig. 19S
from the position a, to the position d, the loop will embrace
a certain amount of flux, say 9 webers, from the externa!
field. In other words, in the position d, the loop holds ^
webers more flux than in the position a. If the current *
amperes, passing through the loop be uniform during the
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ELECTRO-DYNAMIC FORCE. 249
rotation, then it can readily be shown that the amount of
work performed by the loop during this motion is.
but this motion comprises only one quarter of a complete
revolution. At the same rate the work done in one revolu-
tion would be,
4 «' * 4 »' * . ,
ergs = ^ loulcs.
lo 10 X 10,000,000
304. In a bipolar motor with a drum-wound armature on
which there are w wires, counted once completely around the
periphery, or — loops over the surface, there will be — times
as much work performed in one revolution as though a single
loop existed on the surface; the work-per-revolution will,
therefore, be
4/* w . ,
— joules.
If now the motor makes » revolutions per second, the work
performed will be m times this number of joules in a second, or
4i0n w ,, 2 ( * « a/
— — watts. = — ■■ — ■ — — watts.
100,000,000 '2 100,000,000
Then, as will be shown hereafter, the current supplied at the
brushes of the motor will be / = a / amperes, if /, be the cur-
rent through each loop, so that the activity absorbed by the
motor will be.
watts.
100,000,000
We know that the E. M. F. of a rotating armature ts
e = volts (see par. 13a),
100,000,000 ' '
so that we have simply, that the activity absorbed by the
motor armature available for mechanical work \%e I watts, and
this must be true under all conditions, in every motor.
When an E. M. F. of E volts acts in the same direction as a
current / amperes; i. e., drives the current, it does work on
the current with an activity of £ f watts, the activity being
expended by the source of E. M. F. On the other hand,
when an E. M, F. of £ volts acts in the opposite direction to
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*S° ELECTR04>yHAMIC MACHINERY.
a current of / amperes, and therefore opposes it, or is a
C. E. M. F. to the current, the current does work on the
C. E. M. F. with an activity oi E I watts, and this activity
appears at the source of C. E. M. F. If the C. E. M. F. be
merely apparent in a conductor containing a resistance S
ohms, as a drop / Ji volts, the activity E I = f*Ji, and is
expended in the resistance as heat. If the C. E. M. F. be
caused by electro- magnetic induction, as in a revolving motor
armature, the activity £ /, is expended in mechanical work,
including frictions of every kind.
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CHAPTER XXV.
HO TOR TORQUE.
305. We DOW proceed to determine the values of the rotary
effort of a loop at different positions around the axis. This
rotary effort is called the torque. Torque may be defined as
the moment of a force about an axis of rotation. The torque
is measured by the product of a force and the radius at which
it acts. Thus, if in Fig. 200, a weight of P, pounds, be sus-
pended from the pulley Y, and, therefore, acts at a radius /
feet, the torque exerted by the weight about the axis will be
P I pounds-feet If P, be expressed in grammes, and /, in
centimetres, the torque will be expressed in gramme-centi-
metres; and if P, be in dynes and /, in centimetres, the torqut
will be expressed in dyne-centimetres. Thus, at A, Fig. 200,
the torque about the axis of the pulley Y, is 400 pounds-feet
At B, it is 800 pounds-feet At C, it is 400 pounds-feet
As an example of the practical application of torque in
electric motors, let us suppose that the pulley P, is attached
to the armature shaft of a motor, and that the motor succeeds
in raising the weight M, by the cord over the periphery of
the pulley, then the motor will exert a torque at the pulley
of M I pounds-feet Thus, if the pulley be 12 inches in
diameter = 0.5 foot in radius, and the weight be 100 pounds,
then if the thickness of the cord be neglected, the torque
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as* ELECTRO-DYNAMIC MACHINERY.
exerted by the motor will be loo x o-S = 50 pounds-feet,
about the shaft, at the pulley.
306. The work done by the torque which produces rotation
through an angle /5, expressed in radians, is the product of the
torque and the angle. Thus, if the torque r, rptates the sys-
tem through unit angle about an axis, the torque does an
amount of work = r. If the torque be expressed in pounds-
feet, this amount of work will be in foot-pounds. If the torque
be expressed in gm.-cms., the work will be expressed in cm.-
gms., and finally, if the torque be expressed in dyne-cms. the
work will be expressed in cm, -dynes, or ergs. Since there are
a jr radians in one complete revolution, the amount of work done
by a torque r, in one complete revolution will be s n* r units
of work. For example, the motor in the last paragraph, which
produced a torque of 50 pounds-feet, would, in one revolution,
do an amount of work represented by 50 x a »" = 314- 16 foot-
pounds. It is evident, in fact, that since the diameter of
the pulley is one foot, one complete revolution will lift the
weight My through 3. 1416 feet, and the work done in raising
a 100-pound weight through this distance will be 314.16 foot-
pounds. Similarly, if m, expressed in radians per second, be
the angular velocity produced by the torque, then the activity
of this torque will be r qj units of work per second. For
example, a motor making 1,200 revolutions per minute, or ao
revolutions per second, has an aggular velocity of zo x a^r =
135.7 radians per second. If the torque of this motor be 10,000
dyne-cms., the activity of this torque; /. «., of the motor, will
be 10,000 X iaS-7 = i>*S7>*^ ^''S^ P*^ second = 0.1257 watt.
307. A torque must necessarily be independent of the radius
at which it is measured. Thus, if a motor shaft is capable of
lifting a pound weight at a radius of one foot; (', e., of exerting
a torque of one pound-foot, then it will evidently be capable
of supporting half a pound at a radius of two feet, or one third
of a pound at a radius of three feet, etc. In each case the
torque will be the same; i. e., one pound-foot.
308. The torque produced by a loop, situated in a uniform
magnetic flux, varies with the angular position of the loop.
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MOTOR TORQUE. aS3
For example, returning to Fig. 198, the torque of the active
loop is zero in the position d, and is a maximum in the
position a. The electro-dynamic force exerted by the side a' a"
will be (S / — dynes, and, if the radius at which this acts
about the axis — *'. e., half the length of the side a' a"", be a
■Similarly, the torqae exerted in the same direction around the
axis by the side «'" a'", will be also — ■ ■■ dyne-cms., so that
the total torque around the axis will be dyne-cms.
If the loop moves under the influence of this torque through a
very small angle dp, the work done will be r 4P = >— d p,
but a d p =: ds, the small arc moved 'through, as shown in
Fig. aoi, so that the work done will be . . The
amount of flux linked with the loop during this small movement
will \Ki<Sidsl=d9,so that t|ie work donebecomes — d 9,
■or 1 1/ * where I, stands for the current strength in C. G. S.
units of ten amperes each. Consequently, in any small excur-
sion of the loop, the work done will always be the product of
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aS4 ELECTRO-DYNAMIC MACHINERY.
the current strength and the increase of flux therewith
enclosed. It is evident that the amount of flux which is
brought within the loop by 3 given small excursion, varies
with the position of the loop; that is to say, a small excursion
through the arc ds, at the position represented both in plane
and isometric projection, where the plane of the loop coin*
cides with the direction of the flux, in Fig. aoi,' will introduce
an amount of flux = I Sids. But the same small e
NETIC FLUX rEKFENDlCULAK T
the position represented in Fig. aoa — i'. e., where the plane
of the loop is perpendicular to the flux — will introduce practi-
cally no additional flux into the loop. At any intermediate
position, it will be evident that the flux introduced by a small
excursion Of arc ds, will be / ds<Si cos /9, where /3, is the angle
included between the plane of the loop and the direction
of magnetic flux. The torque exerted by the loop, therefore,
varies as the cosine of the angle between the plane of the loop
and the direction of the external flux.
309. Let us now consider the application of the foregoing
principles to the simplest form of electro-magnetic motor. For
this purpose we will consider a smooth-core armature A, Fig.
303, situated in a bipolar field. We will suppose that the total
magnetic flux passing through the loop of the wire in the
position shown, from the north pole J^, to the south pole S, is
d> webers, and that a steady current of / amperes, is maintained
through the loop of wire attached to the armature core. In
the position of the loop as shown in Fig. 203, there will be no
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MOTOR TOKQUE. 355
rotary electro-dynamic force exerted upon the wire, and the
armature will be at a dead point. If, however, the armature
be moved from this position into that shown in Fig. 204, so
that it enters the magnetic flux, assumed to be uniformly dis-
tributed over the surface of the poles and armature core, then
a rotary electro-dynamic force is set up on the wire, and corn-
no, 903. — DKVM AXUATURE WfTK SINGLE TURN OF ACTIVX CONDUCTOSS
AT DEAD POINT.
municated from the wire to the armature core on which it is
/ d 9
secured. The torque being — . -^-5- dyne-cms., where /, is the
cu rre nt strength in amperes, -and — ^ the rate at which flux en-
closed by the loop is altered per unit angle of displacement
If, for example, the total tlux ^ = i megaweber, and the polar
FIG. 104.— ACTIVB CONDUCTOE ENTEXINO FOLAX FLUX.
angle over which we assume that this flux is uniformly dis-
tributed is iao°, or = — radians, then the rate of emptying
flux from the loop during its passage through the polar arc will
1,000,000 1,500,000
be = — webers-per-radian, and if the strength
as- w "^
T
of current in the loop be maintained at so amperes, the torque
exerted by the electro-dynamic forces around the armature shaft
so 1,500,000
will be — X — = 955,000 dyne-cms. Since a torque
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aSfi ELECTRO-DYNAMIC MACHllfERY.
of I pound-foot = 13,550,000 dyne-cms., this torque would be
represented by _Z££! — 0.0705 pound-foot, or 0.0705
pound at one foot radius.
The armature will continue to move under this torque, if
free to do so, until the position of Fig. 305 is reached, where
FIG. 305.— ACTIVE CONDUCTOR
it is evident that a still further displacement will not increase
the amount of flux threaded through the loop.
The amount of work which will have been. performed by the
electro-dynamic forces during this angular displacement of 130°
ergs, or, simply — iP = — x 1,000,000 = 3,000,000 ergs = 0.2
Joule.
310. The armature may continue by its momentum to move
past the position of Fig. 205, to that of Fig. 306. As soon as it
reaches the latter position, a counter electro-dynamic force will
be exerted upon it, tending to arrest and reverse its motion.
Consequently, if the electro-dynamic force is to produce a con-
tinuous rotation, it is necessary that the direction of the cur-
rent through the coil be reversed at this point; f. r, commuted,
or the direction of the field be reversed as soon as this point is
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MOTOR TORQUE. 3$J
reached. As it is not usually practicable to reverse the field,
the direction of current through the coil is reversed by means
of a commutator, so that when the position of Fig. 306 is
reached, the current is passing through the wire in the opposite
direction to that as shown by the arrow. Under these circum-
stances, the electro-dynamic force and torque continue in the
same direction around the axis of the armature and expend
another r>.2 joule upon the armature in Its rotation to the
original position shown in Fig. 203.
It is to be remembered that the representation of. the flux In
Figs. 303-306 is diagrammatic, since the fluk in the entrefer is
rarely uniform, never terminates abruptly at the polar edges,
and is, moreover, affected by the flux produced around the
active conductor.
311. The total amount of work done in one complete revolu-
tion of the armature upon a single turn of active conductor is,
therefore, ergs, or joules.
If the load on the motor be small, so that the momentum of
the armature can be depended upon to carry it past the dead-
points which occur twice in each complete revolution, the
armature will make, say n, revolutions per second, and the
amount of work absorbed by the armature loop in this time
an activity of
The E. M. F, generated by the rotation of this loop through
the magnetic field, by dynamo action, will be
volts, (Par. 133) where w, in this case is 3, since there are two
conductors upon the surface of the armature, counting once
completely around. The C. E. M. F. will, therefore, be
3 4>n
volts, and the activity of the electric current
3 (■ * w
upon this C. E. M. F. will be watts, as above. ,
Hence it appears that in this, as in every case, the torque and
work produced by an electro- magnetic motor depends upon
the C. E. M. F. it can exert as a dynamo.
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as* ELECTRO-DYNAMIC MACHINERY.
312. Fig. 307 represents a Gramme-ring armature, carrying
a single turn of conductor, situated in a bipolar field. If the
total useful flux through the armature is * webers, as before,
half of this amount will pass through the turn, or— webers,
since the flux divides itself into two equal portions, as repre-
sented in the figure. It will be evident, as before, that sUrt-
ing at the position of Fig. 107, there will be no rotary- electro-
' dynamic force exerted upon the loop, until it enters the flux,
. 307.— OKAUUE-KINC ARMATURE
assumed to commence beneath the edge of the pole-piece, and
the torque will then be uniform at the value — •^— s dyne-
centimetres, until the turn emerges from beneath the pole-piece
at Z. The work done in this passage will have been -• ■ —
ergs, and this work will have been taken from the circuit, and,
therefore, from the source of E. M. F. driving the current »,
and will be liberated as mechanical work (including frictions).
If, by the aid of the commutator, the direction of the current
around the loop be reversed, the turn, when caused, either by
momentum or by direct displacement, to enter the field at E,
Fig. ao8, will again receive a rotary electro- dynamic force
whose torque is —
passed, when the work performed will be — ■ -^ ergs, as be-
fore. The total work done upon the armature in one revolu-
tion will, therefore, be a x — X — =
armature make a revolutions per second, the activity expended
upon it will be ergs per second = watts ■ but
10 * "^ 100,000,000
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MOTOR TORQUE. 2S9
considering the rotating armature in this case, as a dynamo
armature, its E. M. F. will average volts, since
° 100,000,000
there is only one turn of the wire upon its surface, and,
consequently, the activity expended on the armature will
u ■ '■*«
313. We have hitherto considered that the armature, whether
of the Gramme-ring or drum type, possessed only a single
no. 308. — OBAMMK'IIING
turn. As a consequence the torque exerted by a constant cur-
rent in the armature will vary between a certain maximum and
zero, that is to say, the motor will possess dead-points. If,
however, a number of turns be uniformly wound upon the arma-
ture, as in the dynamos already considered, it will be evident
that the same number of turns will always be situated in the
magnetic flux beneath the poles-and in the air space beyond
them, in all positions of the armature, and that, consequently,
the torque exerted upon the armature will be constant when
the magnetic flux and the current strength are constant. The
torque exerted by the armature with w wires upon its surface,
counted once completely around, will be * — dyne-cms.,
whether for a Gramme-ring or a drum armature, and this
whether the armature be smooth-core or toothed -co re.
That this is the case will be evident from the following con-
sideration. The work done on a single wire in one complete
1*0
revolution is — ergs, and if there are w wires on the surface
of the armature, the total work done by electro-dynamic forces
But the work done by a
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36o ELECTRO-DYNAMIC MACHINERY.
torque r dyne-cms. exerted through an angle of jS radians is
r yS cm. -dynes or ergs, and since one revolution is a jt radians,
the work done by the torque will be 2 jt t ergs. Therefore,
3 ff T = , or X =:-^* dyne-cms.
For example, if a Gramme-ring armature has 200 turns of
wire, counted once all round the surface, and the current
strength supplied to the armature from the external circuit to
the brushes is 50 amperes, while the total useful flux passing
from one pole through the armature across to the other pole is
5,000,000 webers, or 5 megawebers, then the torque exerted by
the armature under these conditions will be,
50 500,000,000 X 200 „ , 705,800,000
i- X - — ■ ■ = 795,800.000 dyne-cms. = '-^^ '- —
10 ■^ ajr '^^' ' 13,550,000
pounds-feet = 58.73 pounds-feet.
314, The torque produced by multipolar continuous -current
motors is independent of the number of poles, if the armature
winding be of the multiple-connected type; i. e., if there are as
many complete circuits through the armature as there are poles
in the field. In every such case, if *, be the useful flux in
webers passing from one pole into the armature, i, the total
current strength delivered to the armature in amperes, and w,
the number of armature conductors counted once completely
around its surface, the torque will be,
i ^iv ^- . J
centimetre-dynes, or
20 Tt
i $w
pounds-feet
20 B- X 13.550.0
If, however, the armature be series-connected, so that there
are only two circuits through it, and there are/, poles in the
field frame, the torque will be
p i ^v>
2 JO ff X 13,550,000
pound s-feeL
315. In a smooth-core armature, the electro-dynamic force,
and, therefore, the torque, is exerted upon the active con-
ductors, that is to say, the force which routes the armature
acts on the conductors which draw the armature around with
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MOTOR TORQUE. a6i
them. Consequently, a necessity exists in this type of motor
to attach the wires securely to the surface of the core in order
to prevent mechanical displacement.
316. In a toothed-core armature, where the wires are so
deeply embedded in the surface of the core as to be practically
surrounded by iron, the electro-dynamic force or torque is ex-
erted on the mass of the iron itself, and not on the wire. That
is to say, the arnjature current magnetizes the core, and the mag-
netized core is then acted upon by the field flux. As soon as
the iron of the armature core becomes nearly saturated by the
flux passing through it, the electro-dynamic force will be exerted
in a greater degree upon the embedded conductors, but, under
ordinary conditions, the electro -dynamic force which they re-
ceive is comparatively small. A toothed-core armature, there-
fore, not only, serves to protect its conductors from injury,
since they are embedded in its mass, but also prevents their '
receiving severe electro-dynamic stresses. It is not surprising,
therefore, that the tendency of modern dynamo construction
is almost entirely in the direction of toothed-core armatures.
317. It might be supposed that the preceding rule for cal-
culating the value of the torque in a motor, whether running
or at rest, would only hold true where there existed a fairly
uniform distribution of the field flux, such as would be the case
where there was no marked armature reaction. Observation»
appear to show, however, that if we take into consideration
the actual resultant ufteful flux which enters the armature from
any pole, the torque will always be correctly given by the pre-
ceding rule, even when the armature reaction is very marked.
That is to say if ^, be the total useful flux passing through
the armature from one field pole, the torque will be
dyne-centimetres, no matter how much flux may be produced
independently by the M. M. F. of the armature.
318. We have hitherto studied the fundamental rules for
calculating the torque in the case of any continuous-current
motor, whether bipolar or multipolar. It is well to observe
that in practice the torque available from a m^tor at full load
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a«a ■ ELECTRO-DYIfAMIC MACHINERY.
can be determined without reference to either the amount of
useful flux passing through the armature, or to the amount of
full-load current strength. For, if the full-load output of a
motor be P watts, and the speed at which it runs be n revolu-
tions per second, then the worlc done per second will be
10,000,000 P ergs. The angular velocity of the shaft will be
3 nr » radians, and the torque, wilt, therefore be,
10,000,000 P
T = — i dyne -centimetres.
pounds-feet
13,550,000 2 X
P'
T = 0.1174 — pounds-feet.
For example, if a motor gives six horse-power output at full
load, and makes 600 revolutions per minute, required its
torque.
Here the output, P, = 4,476 watts, the speed in revolutions
/>
per second n = 10, — = 447-6, and the torque exerted by the
motor at full load will be,
T = 0.1174 X 4,476 = 53.55 pounds-feet
If the amount of torque which the motor has to exert in order
to start the load connected with it never exceeds the torque
when running at full load, then the current which will be re-
quired to pass through the armature in order to start it will
not exceed the full load current.
319. It is sometimes required to determine what amount of
torque must be developed by a motor armature in order to
operate a machine under given conditions. For example, if a
machine has to be driven with an activity of ten horse- power, at
a speed of 300 revolutions per minute, what will be the torque
exerted by the motor running at 900 revolutions per minute,
suitable countcrshafting being employed between machine and
motor to maintain these speeds? If we employ the formula
in the preceding paragraph, we find for the power P ^ 10 x
746 = 7,460 watts. The speed » = -^--- = 5 revolutions per
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MOTOR TORQUE. »3
second, so that the torque exerted at the shaft of the machine
is r = O.U74 — = 0.1174 X ^^°= 175.1 pounds-feet. The
*> 5
velocity-ratio of motor to machine is — = 3, so that the
300
torque exerted by the motor, neglecting fciction-torque in the
countershafting will be - ^' = 58.37 pounds-feet or 58.37
pounds at i foot radius.
Or, we might consider that the motor would, neglecting
frictional waste of energy in countershafting, be exerting a
power P at 10 X 746 = 7,460 watts at a speed of « = ~- = 15
revolutions per second. Its torque would then be, by the same
4 X :
IS
formula, r = 0.1174 — = " ' ' = 58. 37 pounds-feet.
320. In some cases it is necessary to determine the torque
which must be exerted by a street-car motor at maximum load.
It is not sufficient that the motor shall be able to exert a maxi-
mum activity of say 30 H. P. It is necessary that it shall be
able to exert the given maximum torque at a definite maximum
speed of rotation, and, therefore, the given maximum activity
of 30 H. P. Otherwise, the motor might be of 40 H. P.
capacity, and, yet by failing to exert the required torque,
might be unable to start the car, or, in other words, the motor
would have too high a speed.
For example, required the torque to be exerted by each of two
single-reduction motors in order to start a car with 30" wheels
weighing 6 short tons light, and loaded with 100 passengers,
up a ten per cent grade, the gearing ratio of armature
to car wheel being 3 to 1, Here 100 passengers may be
taken as weighing 15,000 lbs. or 75^ short tons. The total
weight of the car is therefore 27,000 lbs. The frictional pull
required to start a car from rest on level rails, under average
commercial conditions, is about 1.8 per cent, of the weight, or,
in this case, 486 lbs. weight. The pull exerted against gravity is
also 2,700 lbs., making the total pull 3,186 lbs. weight The
radius of the car wheel being -—= 1.35 feet, the torque at the car
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a64 ELECTRO-DYKAMIC MACHINERY.
wheel axle is 3,186 x 1.35 = 3,983 pounds-feet. The torque
at the motor shafts is therefore -■ = 1,328 pounds-feet, and
each motor must therefore exert - — = 664 pounds-feet
If the motors make 600 revolutions per minute or 10 revolu-
tions per second, exerting; this torque, their activity will be
664 X 10 X > 'T X 1-355 = 561530 watts, = 56.53 KW, and
their combined activity 113,1 KW, neglecting gear frictions.
321. Considering the case of a motor armature in rotation,
the speed of its rotation for a given E. M. F. applied to its
armature terminals will depend upon three things : vis.,
(i.) The load imposed upon the armature, or the torque it
has to exert
(2.) The electric resistance of the armature in ohms.
(3,) Its dynamo-power ; i. t., its power of producing C. E.
M. F., or the number of volts it will produce per revolution
per second.
If E, be the E. M. F. in volts applied to the armature termi-
nals, T, the torque, which the motor has to exert, including
the torque of frictions, in megadyne-deci metres (dyne-cms, X
io~') r, the resistance of the motor armature in ohms, and e, the
C. E. M. F, produced in volts per revolution per second of the
E — n e
armature. Then « c, will be the total C. E. M. F. . will
be the current strength received by the armature according
to Ohm's law. The activity of this current expended upon
E — ne
the C. E. M. F. will be their product, or « <r x
watts, and this must be equal to the total rate of working, or
2 It n T, = consequently, « e [ J = :2 t « T and
// = 2 n —c revolutions per second.
For example, if a motor armature, whose resistance is 2
ohms, has a uniformly excited field, which may be either of the
bipolar or multipolar type, and is supplied with 500 volts at
its terminals ; and if the C. E. M. F. it produces by revolution
in the field is 40 volts per-revolution-per-second, then the speed
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- = 13-5 — i.o6 = 11.44 revolutions-
MOTOR TORQUE. ^65
at which the motor will rotate, when exerting a torque, including
all frictions, of 100 pounds-feet (100 X 13,550,000 dyne-centi-
metres, = 135-5 ni egad yne -decimetres) will be
„ = 5^ _ '?r X ax i35-5 _
40 1,600
per- second.
322. It will be observed from the above formula that if
either the torque be zero, or the resistance of the armature is
second. Or, in other words, that the armature will run at
such a speed that its C. E. M. F. shall just equal the E. M. F.
applied to the armature ; i. e. without drop of pressure in the
armature. If the torque could be made zero, the motor
would do no work and would require no current to be supplied
to it, so that no matter what the resistance of the armature
might be, the drop in the armature would be zero. All
motors necessarily have to exert some torque in order to over-
come various frictions, but on light load their speed approxi-
of the motor is very small, which is approximately true in
the case of a large motor, the second term j— , in the
formula, becomes small, and the diminution in speed due to
load is, therefore.also small. }n other words, the drop which
takes place in the armature due to its resistance is correspond-
ingly reduced, permitting the motor to maintain its speed and
C. E. M. F. of rotation. Fig. aog represents diagrammati-
cally a motor armature revolving in a suitably excited
magnetic field, and supplied from a pair of mains, M, M, with
a steady pressure of 500 volts. The resistance of the arma-
ture is represented as being collected in the coil r, while the
C. E. M, F. of the motor is indicated as opposing the passage
of the current from the mains.
The drop in the resistance is represented as being 40 volts,
while the C, E, M. F. is 500 — 40, or 460 volts.
323. The E. M. F. applied to the terminals of a motor
armature, therefore, has to be met by an equal and opposite or
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266
ELBCTRO-DYNAMIC MACHINERY.
C. E. M. F. in the armature, which is composed of two
parts, that due to rotation in the magnetic flux, or to dynamo-
electric action, and that apparent C. E. M. F. which is
entirely due to drop of pressure in the resistance of the arma-
ture, considered as an equivalent length of wire. The activity
expended against the C. E. M. F. of rotation is activity
expended in producing torque, and, therefore, almost all
available for producing useful work, while the activity expended
against the C. E. M. F, of drop is entirely expended in heating
the wire. As the load on the motor is increased, the current
A KM ATI! RE.
which must be supplied to the motor to overcome the addi-
tional load or torque increases the drop in the armature, and,
therefore, diminishes the C. E. M. F. which has to be made up
by rotation, and the speed falls, or tends to fall, in proportion.
324. When a motor armature is at rest, its C. E. M, F. of
rotation is zero, and the C. E. M. F. which it can produce
under these conditions must be entirely composed of drop of
pressure. In other words, the current which will pass through
it is limited entirely by the ohmic resistance of the circuit.
If/', be the current strength in amperes supplied to a motor
armature at a pressure of E volts, the activity expended in the
armature will be £» watts. The activity expended in produc-
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MOTOR TORQUE. 267
ingf torque will be n ir t watts, so that disregarding mechanical
and electro- magnetic frictions, the efficiency of the motor will
be -^TT- = -^, or simply the ratio of the C. E, M, F, of rota-
tion to the impressed E. M. F. This is a maximum at no
load ; i. e., when the motor does no work, and is zero when
the motor is at rest.
The value of e, the volts-per-revolution-per-second, is in all
cases of multiple-connected armatures equal to * w x ro~',
where 4>, is the number of webers of flux passing usefully into
the armature from any one pole, and tv, is the number of turns
of conductor counted once around its periphery,
325. The speed of a motor, therefore, varies, to the first ap-
proximation, inversely as the useful magnetic flus, and in-
versely as the number of armature conductors. A slow-speed
motor, other things being equal, is a motor of targe flux, or large
number of turns, or both, and, as will afterward be shown, in
order to decrease the speed at which the motor is running, it
is only necessary to increase, by any suitable means, the use-
ful flux passing through its armature.
326, Just as in the case of a generator armature, whose
maximum output is obtained when the drop in its armature is
equal to half its terminal £. M. F. (Par. 9), so in the case of
the motor, the output is a maximum (neglecting frictions),
when the drop in the armature is half the E. M. F. applied at
the armature terminals, or, in symbols, when n e = ■—; the
speed of the motor being then half its theoretical maximum
speed, assuming no friction.
Similarly, just as it is impracticable to operate a generator
of any size at its maximum theoretical output, since the activity
expended within it would be so great as probably to destroy it,
being equal to its external activity, so no motor of any size
can be operated so as to give the maximum theoretical output
of work, since the activity expended in heating the machine,
being equal to its output, would, probably, cause its destruc-
tion.
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CHAPTER XXVI.
EFFICIENCY OF MOTORS.
327, As in the case of generators, the (ommercial epdetuy of
the electric motor is the ratio of the output to the intake:
that is,
-,_ . Output
Since the output must be equal to the intake after subtracting
the loss taking place in the machine, the above may be
expressed as follows:
„_ . Intake — Losses
^'^"""i = — nski —
328. The losses which occur in a motor are of the same
nature as those already pointed out in Par. 224, in connection
with a generator. This is evident from the fact that a motor
is but a generator in reversed action; so that any dynamo is
capable of being operated, either as a generator or as a motor,
according as the driving power is applied to it mechanically or
electrically. There is this difference, however, between the
two cases, that a very small dynamo-electric machine may be
capable of acting as a motor, while it is not capable of acting
as a dynamo, owing to the fact that it is not able, unaided, to
excite its own field magnets, its residual magnetism being
insufficient for this purpose. On this a$:count, motors can be
constructed of much smaller sizes than self-exciting generators.
329, If the losses which occur in a dynamo-electric machine,
acting as a generator, have been determined, we can then
closely estimate what these losses will be when the machine is
operated as a motor, and, consequently, the efficiency of the
machine as a motor can be arrived at.
330. There is this difference between a dynamo and a motor
as regards the output; vh., in the dynamo, the energy lost is
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EFFICIENCY OF MOTORS. a69
derived from the driving source, while in the motor the energy
lost is derived electrically from the circuit; but the output of
a dynaino- electric machine is almost invariably determined by
the electric activity in its armature circuit; that is to say, the
armature is limited to a certain number of amperes received
or delivered at a certain number of volts pressure, so that
^ince this load is the output, when the machine is a generator,
and the intake, when the machine is a motor, it is evident that
after the losses as a motor have been subtracted, the mechani-
cal output will be less than the electrical output which the
.machine produces as a generator.
331, For example, let us suppose that a certain machine,
acting as a series-wound generator, is capable of delivering 10
amperes at a pressure of 100 volts, so that its output is i KW.
Let us also suppose that when acting as a generator, a loss of
350 watts occurs, in friction, hysteresis, eddy currents and
I*R losses, both in the armature and in the field; then the
mechanical intake of the machine will be 1,250 watts, and its
commercial efficiency, — = 0.8, or 80 per cent. When,
however, the machine is operated as a motor, the armature is
limited to the same current strength of 10 amperes, and the
pressure at the machine terminals can only be slightly in
excess of the 100 volts previously delivered. Let us suppose
that this is no volts. Then the intake of the machine will be
1,100 watts. Assuming the same losses as before; namely,
350 watts, the output would be only 850 watts, and the
efficiency, therefore, — ?— = 0.772, or about aj^ per cent less
than in the preceding case. It is clear, therefore, that while
the output of the machine was 1,000 watts when acting as a
generator, it was limited to 85a watts when acting as a motor,
assuming that the same limiting armature temperature and
same liability to sparking were accepted in each case.
332. The difference above pointed out between the output of
a machine acting as a generator and as a motor, diminishes
with an increase in the size of the machine. Thus, while a
i-KW generator is usually only a i-M. P. motor (or has an out-
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Z70 ELECTRO-DYNAMIC MACHINERY.
put of say 750 watts), a generator of 200 KW would, probably,
be a motor of 185 H. P. ; so that in the case of very large
machines, the difference between the outputs in the two cases
would be practically negligible.
333, The curve in the accompanying Fig. 310, approximately
represents the efficiency which may be expected at full load
u
rtt-l'
»s^
fFIOI'
wot
—
/
■
/
"
. 9ia— COMMERCIAL
KILOWim OUTPUT
V CURVE O
AT FULL LOAD.
from motors of varying capacity up to zoo KW. This curve
has been plotted from a number of actual observations with
machines constructed in the United States.
334. It is to be remembered, however, that the full load
efficiency of a motor is not always the criterion upon which its
suitability for economically performing a given service is to be
determined. It not infrequently happens that the character of
the work which a motor has to perform is necessarily exceed-
ingly variable, so that the average load might not be half
the full load of the machine. Under such conditions, the
average efficUmy is of more importance than the fuil-loai
efficiency. Were the efficiency curve of all motors in relation
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EFFICIENCY OF MOTORS. 271
to their load of the same general outline, the average efficiency
would be, approximately, the same in all motors having the
same full-load efficiency. As a matter of fact, however, the
efficiency curves of different machines may be very different.
Thus one machine may have its maximum efficiency at half
load, and behave at full load, in regard to its efficiency, as
though it were actually overloaded, while another machine,
with the same full-load efficiency, may show a lower efficiency
at half load. Obviously the first machine would be preferred
for variable work, other things being equal.
335. Similar considerations apply to electric generators.
The full-load efficiency is not in every case the ultimate
criterion of economical delivery of work, but it generally
happens that generators are installed in such a manner, and
under such conditions, that a nearer approach to their full load
is attained, so that ordinarily the shape of the effi(;iency curve
of a generator is not of such great importance as that of a
motor.
Fig. an represents the efficiency curves of two motors, each
having a full-load efficiency of 78 per cent. One of these
machines has an' efficiency, at about two-thirds load, of 84 per
cent., but at overloads is inefficient, while the other becomes
more efficient at slight overloads.
336. In order to produce a motor of given full-load efficiency
with comparatively small loss at moderate loads, and, there-
fore, a comparatively heavy loss at heavy loads, we may em-
ploy a slow-speed motor, or a motor which shall generate the
necessary C. E. M. F. at a comparatively low speed. Such a
machine will probably have a small loss in mechanical friction,
because of its lower speed of revolution. It will, similarly,
have, probably, a small loss in hysteresis and eddy currents
for the same reason, but a slow speed motor will probably
have a greater number of armature turns in order to com-
pensate for the smaller rate of revolution, and the I*R loss in
the armature is, therefore, likely to be greater at full load. In
such a machine, the loss at full load is principally due to PH;
and, since this loss decreases rapidly with /, it will evidently
have a small loss at moderate toads.
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a?*
ELECTRO-DYNAMtC MACHINERY.
337. The Speed at which a motor will run in performing a
given amount of work varies considerably with different types
of motors. For example, of two motors of lo KW capacity,
one may run at 400 rcvolutions-per-minute, and the other at
1,000 revolutions-per-minute. It is evident that the first
machine will have two and a half times the full-load torque of
the second. The lower speed is, however, generally speaking,
only to be obtained at the expenee of additional copper and
iron ; that is to say, the cost of material in a slow-speed
machine will, probably, be greater than the cost of material
— ,
/
/
--
'•"-
/.
/
/
/
//
/
)
— EFflCIENCY C
FULL-LOAD
in a high-speed machine of the same output and relative excel-
lence of design. It becomes,, therefore, a question as to the
relative commercial advantage of slow speed versus high speed
in a motor.
338. Motors are generally installed to drive machinery either
by belts or gears, and the belt speed or the gear speed of
machinery is, in practice, a comparatively fixed quantity. If,
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EFFICIENCY OF MOTORS. '73
therefore, the speed of the motor be greater than the speed of
the main driving wheel of the machines with which the motor
is connected, intermediate reducing gear or countershafttng has
to be installed. This adds to the expense of installation, not
only in first cost, but also in maintenance, lubrication, and the
continuous loss of power it introduces through friction. The
result is, that up to a certain point, stow-speed motors are
economically preferable, and the tendency of recent years has
been toward the production of slower speed dynamo machinery.
In comparing, therefore, the prices of two motors of equal
output, the speed at which they run has to be taken into
account, as well as the efficiency at which they will operate.
It is to be remembered that any means in the design which
will enable a motor to supply its output at a slower speed, are
equivalent to means which will enable a motor of the higher
speed to supply a greater output.
339. The weight of a motor is a matter of considerable im-
portance in cases of locomotors ; i. e., of travelling motors, as in
the case of electric locomotives, street-car motors or launch
motors, but in the case of stationary motors, their weight is of
less consequence, since, after freight has been once paid for
their shipment, no extra expense is entailed by reason of their
increased mass when in operation. Indeed, weight is often a
desirable quality for a motor to possess in order to ensure
steadiness of driving, although undue weight in the armature
is apt to produce frictional loss, and diminished efficiency.
340. In comparing the relative weights of motors, two cri-
teria may be established; namely,
(i) In regard to torque, and (2) in regard to activity. In
some cases, the work required from the motor is such that
the pull or torque which must be given in reference' to its
weight is the main consideration, while in other cases it is not
the torque, but the output per-pound of weight, which must be
considered.
341. The torqne-per-pound, in the case of street-car motors,
where lightness is an important factor, has been increased to
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274 ELECTRO-DYNAMIC MACHINBkY.
133,000 centiraetre-dynes per-ampere, per-kilogramme of
weight; or, 0.0045 pound-foot per-ampere per-pound of total
motor weight, exclusive of gears, so that a 500-volt street-car
motor, weighing 223 pounds, and supplied with one ampere of
current, would exert a torque of one pound-foot. In stationary
motors, the torque is usually only o.ooi to 0.0015 pound-foot
per-ampere per-pound of weight, or about four times less than
with street-car motors. This is owing to the fact that cast
iron is more largely employed in stationary motors, owing to
its lesser cost.
The output per-pound of weight in motors varies from 5
watts per pound to 15 watts per pound, according to the size
and speed of the motor.
342, We may now allude to the theoretical conditions
which must be complied with in order to obtain the maximum
amount of torque in a motor for a given mass of material. It
must be carefully remembered, however, that these theoretical
conditions require both modification and amplification, when
applied to practice, so that the practical problem is the theo-
retical problem combined with the problem of mechanical
construction.
dynes, we require to make this expression a maximum for a
given mass of copper wire in the armature and in the field
magnets, neglecting at present all considerations of structural
strength.
The torque-per-arap'ere will be cm, -dynes.
In order to make this a maximum, both ^ and w, should be
as great as possible.
344, It is evident that if we simply desired a motor of power-
ful torque-pe'r-ampere, regardless of its weight, we should
employ as much useful iron as possible, so as to obtain as
great a useful magnetic flux 9, through the armature, as
possible, and we should employ as many turns of wire upon
the surface of the armature as could be obtmned without mak-
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EFFICIENCY OF MOTORS. 275
ing the armature reaction excessive, or without introducing
too high a resistance, and too much expenditure of energy in
the armature winding. Such a motor would essentially be a
heavy motor, so that the requirements of a motor with power-
ful torque-per-ampere would simply be met by a motor of great
useful weight, and this, indeed, would be obvious without any
arithmetical reasoning.
345. When, however, the torque-per-ampere per-pound-of-
weight has to be a maximum, the best means of attacking the
problem is to consider a given total weight of copper and iron
in the armature, and examine by what means this total weight
can be most effectually employed for producing dynamo-power;
/. e., volts-per-revolution-per-second, and torque-per-ampere.
346. It wilt, in the first place, be obvious that a long mag-
netic circuit will not be consistent with these requirements,
since, as we shorten the magnetic circuit, retaining the same
mass of material, we make it wider, or of greater section, and
so increase the total flux ^. In the second place, the material
of which the magnetic circuit is formed should have as small a
reluctivity, and as powerful a flux density as possible, since
this will increase the toUl flux without adding to the weight.
For this reason soft cast steel is much to be preferred to cast
iron.
347. Again, it will be evident that as we increase the number
of turns on the armature, having determined upon a certain
total mass of armature copper, or armature winding space, we
increase, according to the formula, the torque-per-ampere.
But, in occupying the given winding space with many turns
instead of with few turns, we increase, for a given speed, the
voltage of the armature. Thus, if a motor armature be intended
to rotate at a speed of 10 revolutions per second, its £. M. F.,
other things being equal, will be 10 times as great, when we
use 10 times as many wires upon its surface, and its torque-
per-ampere will be also increased 10 times. A high E. M. F.
motor is, therefore, necessarily a motor of high torque-per-
ampere. A soo-volt armature would, therefore, in accordance
with preceding principles, necessarily be a motor of greater
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»76 ELECTRO-DYNAMIC MACHINERY.
torque -per-ampere than the same armature wound for loo volts,
although the torque at full load might be the same in each
case, since the low-pressure armature might make up by in-
crease of current what it lacked in torque-per-ampere.
348. Having selected a field frame with as short a magnetic
circuit as is consistent with not excessive magnetic leakage,
and with room for magnetizing coils, and having placed a large
number of turns upon . the armature surface, there remain
several important detail considerations which should be taken
into account to enable a high torque-per-ampere to be obtained.
349. In the first place, the reluctance in the magnetic circuit
should be as small as possible in order to diminish the M. M. F.
and the mass of magnetizing copper. With smooth-core
armatures this would represent a small entrefer and a small
winding space, whereas, to obtain many turns, we require a
large entrefer and large winding space, so that with a smooth-
core armature, a compromise is necessary at some point of
maximum effect, depending upon a great variety of details.
With toothed-core armatures, however, a large number of
turns may be disposed upon the armature surface, yet the
reluctance in the entrefer may be comparatively sm^II. This
consideration affords an additional argument in favor of
tootbed-core armatures for high torque.
350. In the second place, the number of poles in the field
frame should be as great as possible. If we double the number
of poles in the Held frame, retaining the same armature, and
make suitable changes in the connection of the armature turns,
we double the E. M. F. of the armature (Par. 148). Thus, if we
have an armature with a given number of turns on its surface
and a given speed of rotation, in a bipolar field, and the E. M. F.
obtained from the armature is 100 volts, then, if we change
the field to a quadripolar frame, and suitably change the con-
nection of the armature turns, the E. M, F. of the armature
will be aoo volts. If, instead of changing the armature con-
nections, we simply change the number of brushes from two to
four, and suitably connect these brushes, we obtain only 100
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EFJ-'ICIhXCV OF MOrOJiS. 277
volts as before, but as there are now four complete electric
circuits through the armature, we have doubled the load which
the armature can sustain without overlieating, and, therefore,
practically doubled the output of the armature, so that when we
double the number of poles covering the armature, assuming
the useful flux through each pole to be the same as before, we
either double the torque-per-ampere directly, if the armature
be series-connected, or we retain the torque-per-ampere with
a multiple-connected armature and, by changing the winding,
obtain a greater output from the motor.
351. There will, of course, be a limit to the number of poles
which can be employed with any armature without increasing
its diameter, since there will only be sufficient room for a cer-
tain number of poles carrying a given maximum flux, and also,
since the difficulty of magnetizing a greater number of poles
will be insuperable, either for want of space, or owing to
increased magnetic leakage. The principle, however, is
important.
352. The number of turns which can be utilized upon the
surface of an armature is itself limited; first, by the resistance
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378 ELECTRO-DYNAMIC MACHINERY.
of the armature and consequent excessive heating under load;
second, by excessive armature reaction and consequent spark-
ing ; and, third, in rarer cases, by the E. M, F. of the circuit,
FIG. 213. — QUADRtPOLAR CAR MOTOR WITH FOUR FIELD COILS.
and, consequently, the unduly slow speed at which a powerful
armature will run on such circuit.
353- The best embodiment of the foregoing principles in ex-
isting practice is found in a modern street-car motor. Here a
powerful torque-per-ampere, with minimum weight, is desired
in order to start a loaded car from rest up a steep gradient.
Two forms of such motors are shown in Figs. 212 and J13.
354. Fig. an shows a cast-steel quadripolar field frame with
two magnetizing coils M, M. These produce not pnly poles
at the opposite sides of the armature, in the cores over which
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EFFICIENCY OF MOTORS. 279
they are wound, but also poles at the cylindrical projections
P, P, which lie above and below the armature so that there
are four complete magnetic circuits through the field frame
and armature, two circuits through each magnetizing coil.
The brushes B, B, are set 90 degrees apart on the commutator
C. The armature A, is of the toothed-core type.
355. In Fig. 213 the same results are obtained with various
detailed differences in mechanical construction. There are
four poles around the armature, two of which, P, P, are seen
in the raised cover, and two others are similarly contained in
the lower half of the frame. Each of these poles is, in this
case, surrounded by a magnetizing coil, M. B, B, are the
brushes, set 90" apart from the commutator. The armature,
A, is of the toothed-core type.
In both of these cases the magnetic circuits are as short as
is practically possible, and the useful magnetic flux is as great
as possible.
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CHAPTER XXVII.
REGULATION OF MOTORS.
356. The requirements of a motor depend upon the nature
and use of the apparatus which the motor is designed to drive.
All these requirements, in relation to driving machinery, may
be embraced under three heads; viz.,
(1.) Control of starting and stopping.
(z.) Control of speed, both as to constancy and as to vari-
ability.
{$.) Control of torque, both as to constancy and as to
variability.
The above requirements are by no means met to an equal
degree by the electric motor.
For example, the requirement of constant speed is much
more readily dealt with than the requirement of variable speed.
357. The conditions under which motors have to operate
may be divided into four classes; namely,
(i.) Constant torque and constant speed.
(2.) Variable torque and constant speed.
(3.) Constant torque and variable speed.
(4. ) Variable torque and variable speed.
358. The first two conditions are readily secured, the third
and fourth are only secured with difhculty. For example, a
rotary pump belongs to the first class. Here the load is con-
stant and the speed is presumably constant
The second class comprises the greater number of machine
tools, where the speed is constant but the activity is variable.
The third class embraces most elevators and hoisting ma-
chines.
The fourth class is well represented by street-car motors.
359. Any continuous-current electric motor will supply a
constant torque at a constant speed when operated at a constant
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REGULATION OF MOTORS.
28i
pressure. Thus, whether the motor be self-excited or sep-
arately-excited, and whether it be shunt-wound, series-wound
or compound- wound, it will, if supplied with a constant pres-
sure at its terminals, and assuming constant frictions in the
machine, deliver a constant torque at a constant speed, and
taking from the mains supplying it, a constant current strength,
and, therefore, constant activity. The condition of constant
torque and constant speed is one which is, therefore, readily
dealt with by electric motors.
The above statement, however, is true only of single motors;
for, if two motors, of any continuous-current type, be con-
SEItras BETWEEN
nected in series across a pair of constant- potential mains, they
will be in unstable equilibrium as to speed under a given load.
If the torque on each of the two machines in Fig. 214 were
maintained absolutely equal; then, by symmetry, the two series
motors represented would run at equal speeds, and absorb
equal activities. But should the load on one accidentally
increase, even to a small extent, above that of the other, the
tendency would be to slow down the over-loaded motor and
accelerate the other, so that it would be possible to have one
motor at rest exerting a constant torque, and the other motor
exerting the same torque at double its former speed. If, how-
ever, the two motors are rigidly coupled together to a coun-
tershaft, so that their speeds must be alike, then they will
behave as a single motor. Consequently, a continuous-current
motor employed for pumping or driving a fan, and which, there-
by GoOglc
aSa ELECTRO-DYNAMIC MACHINERY.
fore, has a constant torque to supply, will run at constant speed
when supplied with constant pressure, whatever the type of
motor may be.
360. The important requirement of constant speed under
variable load is nearly met by a shunt-wound motor. It may
be almost perfectly met by the compound- wound motor. It
is not met, without the aid of special mechanism, by the
series- wound motor.
361. Considering first the case of a shunt-wound motor,
represented in Fig. 165, the speed at which the armature will
run is — revolutions-per-second(Par.32i),whenat noload,pro-
vided that the friction of the machine is so small that we may
safely neglect the drop of pressure in the armature running light
When the full-load current / amperes, passes through the
armature, the speed will be reduced to revolutions-per-
second, r, being the armature resistance in ohms.
Thus a particular shunt-wound, iio-volt motor has an arma-
ture resistance (hot) of 0.075 o'"". ^^^ '^^ full-load output is
9 H. P. What will be its fall in speed between no load and full
load, its no-load speed being 1,395 revolutions-per-minute or
23,25 revolutions-per-second?
Here, neglecting the armature torque and drop in pressure
at no load, e, the dynamo power, or volts-per-revolution-per-
second = . — — = 4.73. Its output at full load being 9 x 746
= 6,714 watts, and its armature efficiency, say, 0.84, the arma-
ture intake will be -^-^ = 7,994 watts = 72.68 amperesx no
o. »4
■volts. The full-load armature drop will, therefore, be 7^.68 X
0.075 = S-4S volts, and the full-load speed .1 '° ~S-^' = 22.1
revolutions-per-second, approximately, or 1,326 revolutions-
per-minute.
The drop in speed of this motor between no load and full load
is, therefore, 69 revoIutions-per-minutc ; or, approximately
5 per cent.
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REGULATION OF MOTORS. 2%%
362. If the variation of speed due to the drop in the armature
with the full-load current is greater than that which the con-
ditions of driving will permit, then means may be adopted
to reduce the value of t, at full load in the above formula, so
as to increase the speed in compensation for the necessary
drop. This is frequently accomplished by inserting resistance
in the circuit of the Held magnet so as to reduce its M. M. F.,
and, consequently, the useful flux which it sends through the
armature. A rheostat in the shunt-field circuit, therefore,
enables such regulation to be made by hand, as will maintain
the speed of a shunt motor constant under all torques within
its full load. For most commercial purposes the automatic
regulation of the shunt motor is sufficiently close, the rheo-
stat only being employed on special occasions. The larger
the shunt motor the less the drop in speed which is brought
about by the full-load current. Thus a i-H. P. shunt motor
will usually drop only 10 per cent, in speed at full load, a 10-
H. P. motor 5 per cent, and a 100-H. P. motor, 5 per cent
363. When a series motor is operated on a series circuit, as
for example, on a series-arc circuit, some device is necessary
which will regulate the speed of the motor. If no such device
were provided, if the starting torque of the motor due to the
constant current passing through it, exceeded the torque due
to load and frictions combined, the motor would accelerate
indefinitely in its endeavor to oppose by C. E. M. F. the
passage of the current If the load were of such a nature that
the torque increased with the speed, as in the case of a {an,
the speed might be automatically controlled, but, since, in
driving machinery, the torque is nearly independent of the
speed, a controlling mechanism becomes essential. One
method by which this is accomplished is by rotating the
rocker arm and brushes into such a position about the com-
mutator, that the useful flux from the constantly excited series-
wound field coils, passing through the armature coils, is virtu-
ally reduced by passing both into and out of the armature
coils when the diameter of commutation is shifted, thereby
neutralizing the" electro-dynamic force on the windings.
The- method corresponds to that adopted for varying the
E, M. F, of arc dynamos, in order to keep the current
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a84 ELECTRO-DYNAMIC MACHItfERY.
Strength constant in the circuit, despite variations of load.
(Par. 261.)
Fig. 215, represents a small series-wound ^-H. P. motor for
use on series-arc circuits and provided with a hand regulator
to control the speed. The rocker arm, which supports the
brush-holders, has a projection P, to which an insulating
115. ONB-5IXTH H
handle or treadle is atuched. Under ordinary conditions,
the spiral spring S, pulls the rocker arm, into the position
shown, so that the brushes b, b, rest upon the commutator at
a diameter at right angles to the diameter of neutral commu-
tation in an ordinary bipolar motor, so that the torque of the
motor will be reduced to zero. By rotating the rocker arm
with handle or treadle against the tension of the spring S, so
that the projection P, occupies the position P', the brushes
are brought forward to the position b', of maximum torque,
so that the speed of the motor may be controlled.
In the motor represented in Fig. 116, this rotation of the
rocker arm is effected automatically by the aid of a centrifugal
governor G, mounted at one end of the armature shaft.
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REGULATION OF MOTORS. 385
"When the motor is started, by throwing it into the scries cir-
cuit by a switch, th? brushes are at the diameter of neutral
commutation or maximum torque. If the load torque is not
too great for the armature to overcome, the motor will
accelerate until the governor G, has lifted its wings to such
-a distance by centrifugal force against the tension of its
FIG. ai6.— ONE-H. p. lUlC MOTOR W
spring, that the lever Z, following the motion of the governor,
has pulled round the rocker arm and brushes to a diameter at
which the torque of the armature is equal to that of the load,
364. In the ordinary motor the speed increases until the
current strength / amperes passing the armature at the ter-
minal pressure E volts, limits the intake, E I watts, to the load
activity and energy losses combined. In this motor the sp^ed
increases until the governor moves the brushes into such a
position that the C. E. M. ?.,£" volts, limits the activity of the
constant current / amperes to the amount E f watts, equal to
the load activity and en«rgy- losses. The speed will, therefore,
vary with the load by a small amount depending upon the
sensibility of the governor.
Motors for series-arc circuits are not usually employed
above 3 H. P. Owing to the high pressure which may exist
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386 ELECTRO-DYNAMIC MACHINERY.
upon their circuits, they may be dangerous to handle unless
precautions are taken.
365. When a series-wound motor is employed across con-
stant-potential mains, in the manner indicated in Fig. 164, the
value of e, the dynamo power, or E. M. F. pc r- re volution- pcr-
second, being equal to 4> w, varies with the torque or load,
since anychange in the current strength through the armature,
changes the M. M. F. of the field magnets, and, therefore, the
flux d>. The tendency of a series motor is, therefore, tO'
reduce its speed, as the torque imposed upon the motor is
increased, and such a motor would run, theoretically, at an
infinite speed on light load, if there were no frictions in the
armature to be overcome. A shunt-wound motor, therefore,
tends to drop in speed with load to an extent proportional to
the drop of pressure in the armature. A series-wound motor
falls off in speed with load, not only owing to the drop of
pressure in the armature, but also owing to the increase in
M. M. F. and flux.
366. A com pound- wound motor will, however, maintain its
speed practically constant under all loads, if the series winding
on the field coils be so adjusted that the increase in current
strength through these coils and the armature shall diminish
the M. M. F. of the field magnets to the degree necessary to
compensate for the drop of pressure in the armature winding.
The connections of such a compound-wound motor are the
same as for the compound-wound dynamo shown in Fig, r66.
367. Although a series-wound motor is unfitted for maintain-
ing a constant speed on constant-potential mains with vari^ible
torque, yet it is possible to connect two series-wound machines
of the same type and character together, one acting as a gener-
ator and the other as a motor, and to obtain a nearly constant
speed of the motor by compensatory changes in the £. M. F.
of the generator automatically brought about by the variations
of load. This case, however, can only apply to a single motor
driven by a single generator, and is, therefore, inapplicable to
a system of motors driven by a single generating source.
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REGULATION OF MOTORS. 287
368. Figs. 317 and 218 are diagrams taken from actual tests
of two small 500-volt, )^-H. P. motors, of good construction and
well-known manufacture, one being a series-wound motor and
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the other a shunt-wound motor. The armatures of the two
machines and also their field frames were practically identical,
the only essential difference between the two being in the field
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aSi ELECTRO-DYNAMIC MACHINERY.
winding. The weight of the machines was 105 lbs. each, that
of the armature nearly 21 lbs. The resistance of the armatures
was 40 ohms each, and the resistance of the fields 3,680 ohms
for the shunt-wound, and 37.5 ohms for the series-wound,
machine.
In these diagrams, the ordinates represent the expenditure
of activity in the field windings, armature windings, frictions
(including hysteresis, eddy currents, and mechanical frictions),
and output at the shaft. The abscissas represent the intake
in watts. Thus, referring to Fig. 117 for the shunt-wound
machine, it will be seen that when delivering full load, or 373
watts, the machine absorbed 690 watts, expending 90 in the
field magnets, as P R, 67 watts in the armature as P H, and
160 watts in total frictions. The commercial efficiency of the
machine at full load, was, therefore, |^ or 54 per cent. The
speed of the machine falls from 99.3 to 35 revel utions-per-
second, or from 1,753 to 1,500 revolutions per minute, a drop
of 14.4 per cent, and this drop is closely proportional to the
output The highest commercial efficiency reached was 55 per
cent, at 340 watts output.
Taking now the series-wound machine referred to in Fig. 318,
it will be observed that the field loss is much smaller, particu-
larly at light loads, owing to the fact that it increases with the
current strength, and practically disappears when the current
strength is very small. Owing to this fact it will be observed
that the commercial efficiency of this machine is greater
throughout than that of the shunt machine. At a delivery of
340 watts, the intake was 600 watts, expended as follows : 57
watts in the magnets, 63 in the armature, and 140 watts in
frictions. , It will be seen, however, that the speed falls from
38.5 to 31-5 revolutions-per- second, or from 3,310 to 1,390
revolutions-per-minute, a drop of 44.2 per cent. It is clear,
therefore, that a series-wound machme'is; in smaltsiKes, cheaper -
to construct than a shunt-wound machine, since it employs only
a few turns of coarse wire instead of many turns of fine wire
in its field coils. It also has a slightly higher efficiency. It
also dispenses with the use of a starting rheostat in the arma-
ture, but has the disadvantage of possessing a much greater
variation in speed under variations of load.
>y Google
REGULATION OF MOTORS.
369. As already mentioned, the condition of constant torque
and variable speed is one which it is much more difficult for
the electric motor to meet. If it were possible to vary the
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useful magfnetic flux through the armature within wide limits,
the method of varying the M. M. F. of the field magnets
would effect the result desired. While, however, it is possible
to produce a variation of speed in the ratio of 3 to i,
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290 ELECTRO-DYNAMIC MACHINERY.
by varying the M. M. F. ; that is to say, while motors have
been constructed, under special conditions, which will run, say
at from a maitimum of 900, to a minimum of 300 revolutions-
per-minute, merely owing to variation in the M. M. F. of their
fields, yet such a range is only obtained with great difficulty,
owing to the fact that magnetic saturation is reached at
maximum M. M. Fs. in the iron constituting the magnetic
circuit, and that when the field Aux is greatly reduced, the
armature reaction at full load is liable to be excessive, with
heavy sparking at the commutator. The maximum range of
FtG. 3IQ. — DIAURAU
BXCITATION IN A STREET-CAK MOTOR.
speed in an ordinary shunt motor, brought about by field
regulation, is only about 35 percent., so that a motor whose '
maximum safe speed is 1,000 re volutionsi per-minute, can be
reduced to minimum' of about 750 revolutions.
370. The M. M. F. of a motor field may be varied electric-
ally in two ways; namely, by altering the current strength
through the field coils as a whole, by inserting a varied resist-
ance in their circuit; and second, by altering the action of
certain portions of the field coils relatively to other portions,
as, for example, by changing them from series to parallel, or
the reverse. In shunt- wound motors, the regulation is
usually effected by the introduction of a field rheostat. In
series-wound motors it is usually effected by varying the
number or arrangement of the field coils. Thus the arrange-
ment for connecting the field coils of a particular form of
street-car motor is represented in Fig. 219. It will be seen
that there are three coils on each limb of the field, but each
>y Google
REGULATION^ OF MOTORS.
391
pair is permanently connected as shown, so that electrically
there are only three coils, A, B and C. By the action of the
controlling switch, these coils may be connected as shown in
the diagram.
In Position i, all three coils are in series, making the rela-
tive M. M. F. 3 and the relative resistance 3.
In Position a, one coil is short circuited, making the rela-
tive M. M, F. 2 and the relative resistance 2,
In Position 3, two coils are connected in parallel, making
the relative M. M, F, a and the relative resistance 1.5.
In Position 4, two coils only are connected in parallel, mak-
ing the relative M. M. F. i and the relative resistance 0.5.
In Position 5, all three coils are connected in parallel, mak-
ing the relative M, M, F. i and the resistance 0.333.
Fig. 330 represents the characteristic curve of a particular
motor of this character, with the flux in megawebers, passing
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through the armature with different excitations of the field
magnets, expressed in ampere-turns. With the aid of this
curve it is possible to estimate the range of speed which can
be obtained by connecting the coils in different arrangements.
For example, at half load of yj^ H. P., or say 5,600 watts out-
put, and an efficiency of say 0.8, the activity absorbed would
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29" ELECTRO-DYNAMIC MACHINERY.
be 7,000 watts, or 14 amperes at 500 volts pressure. There
are, approximately, 3,100 turns in the field coils, or 70010
each pair, so that with all in series, the total M. M. F. would
be 14 X 3,100 =: 29,400, which might produce a flux of 2.9
megawebers through the armature. With all the coils in
parallel, the M. M. F. would be three times less or 9,800, and
the flux 3.13 megawebers. The ratio of speed, therefore,
would be — ^ = 1-368, so far as regards the effect of change
in magnetic flux through the armature. In practice, the
speed would vary in a somewhat greater ratio, owing to the
influence of grtater drop in the field magnets when connected
in series than when connected in parallel, We may consider,
therefore, that at light loads the influence on the speed of
varying the field coil connections is considerable, but at heavy
loads the influence is relatively small.
371. We have seen how the speed of a motor can be con-
trolled within certain limits by varying the magnetic flux use-
fully passing through its armature. The same results can be
effected by introducing resisUnce into the armature circuit.
372. If the consUnt torque imposed upon the motor is such as
requires a current of / amperes to pass through its armature,
while a given constant magnetic flux is produced by the field,
and if E, be the pressure in volts across the main leads, and r,
the resistance of the armature in ohms, the drop in the armature
will be Ir volts, and the armature of the motor must develop
that speed which will produce a C. E. M. F. of (^ — / r) volts.
If it be required to reduce this speed to say, one half, then
the total resistance of the armature circuit must be increased
to R ohms, in such a manner that E — I R := , so
a
that R =. — i-j — , While this plan is theoretically effective,
it is practically objectionable, because, in the first place, it
wastes energy by the introduction of the additional resistance
{R — r) ohms, the amount of activity wastefully expended in
such resistance being 7' {R ~- r) watts. In the second place,
a comparatively small accidental variation in the torque, which
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REGULATION QF MQTOKS.
^95
we have hitherto supposed constant, would effect a large
variation in the speed, owing to the varying drop in the added
resistance. Again, a powerful motor requires a powerful cur-
rent strength to be supplied to it, and a large expenditure of
energy is necessary in order to greatly reduce its speed in this
t CONSTANT Ton Q UK.
manner, requiring the use of bulky and expensive resistances,
to dissipate the heat developed. For these reasons this
method of maintaining the speed constant is seldom employed.
373. It has been found so difficult in practice to vary the
speed of a motor at constant torque between full speed and
rest, without loss of efficiency, that in cases where complete
control is imperative, as in some rolling mills, where the
machinery hag to run occasionally at a definite very low speed,
and at other times at full speed, a method, which is repre-
sented in Fig. 331, has been invented and applied. Here M,
is a shunt-wound motor, connected across a pair of supply
mains, A A, B B, and, therefore, running at practically con-
stant speed under all conditions of use. The armature of this
motor is connected directly, either by a belt or by a rigid
coupling, to the armature of the generator (?, whose field
magnets are excited through a rheostat R. The generator
armature consequently runs at a practically constant speed
under all conditions of service. The E. M. F., which thii
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"94 ELECTRO-DYNAMIC MACHINERY.
generator armature develops, depends, however, upon the
excitation of its field magnets, which is regulated by the
rheostat R, so that, when no current passes through the
generator field coils, the E. M. F. of its armature is nearly
zero, while, when full current strength passes through the
field coils, the E, M. F. of the generator is at its maximum.
The brushes of the generator are directly connected with the
brushes of the working motor m, whose field magnet is con-
stantly excited, and the speed of the armature m, will be con-
trolled directly by the E. M. F. of the generator Q. If the
generator is fully excited, the E. M. F. at the terminals of the
motor m, will be a maximum, and the speed of the motor to
meet this E. M. F. with a corresponding C. E. M. F. will also be
a maximum, while if the generator has its excitation removed,
the armature of the motor m may come almost or quite to a
standstill. If necessary, the connecting wires between the
armatures of G and m, can then be reversed so that the direc-
tion of m's rotation can be reversed.
374. The fact that this combination of machines operates
satisfactorily without excessive sparking at the commutator of
the generator, often occasions some surprise to those who are
accustomed to varying the field excitation of generators and
motors, under ordinary conditions, since it is known that, in
general, when a generator, and particularly a motor, has its
field magnets considerably weakened, a violent sparking is apt
to be produced at the commutator. It is to be remembered,
however, in this case, that the armature of the weakened
generator G, is never permitted to send more than the full-
load current strength, which is required to overcome the full-
load torque, while on the contrary, if this machine were
employed across constant- potential mains as a motor and
the magnetic flux through the armature was considerably
weakened, the current strength which would pass through the
armature would be, probably, much in excess of the full-load
current, with a corresponding tendency to produce excessive
armature reaction and sparking.
375. Although the preceding combination of apparatus
effects the desired result of varying or reversing the speed of
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REGULATION OF MOTORS. 29S
the motor at will, under constant or even under variable torque,
within the limits of full load, yet it has the double disadvan-
tage of requiring the installation of three times the amount of
machinery which would otherwise be necessary, and of hav-
ing a considerably reduced efficiency of operation. If, for
example, the motor ^, has to be a lo-KW machine, then the
generator G, must at least have a capacity of 10 KW, and at
least an equal capacity will have to be given to the prime
motor M ; so that 30 KW of machinery are installed where but
10 are directly brought into use. Again, if the commercial
efficiency of each machine were 83 per cent, at full load, the
commercial efficiency of the combination, under full load,
would be, approximately, 0.83 X 0.83 X 0.83 = 0.57a, so that
the combination would have a full-load efficiency of 57.2 per
cent. At light loads the combination efficiency would be
still lower; for example, if at half load the efficiency of each
machine were 73 per cent., the combination efficiency would
be 41.2 per cent. On the other hand, however, the introduc-
tion of resistance into the armature circuit of a motor, in
order to reduce its speed, would probably effect as low or even
a lower efficiency. It is evident, therefore, that in this direc-
tion the electric motor shows its weakest side.
376. The fourth condition of working; namely, under vari-
able torque and variable speed, differs from the last only in
the variability of the torque. This being, as we have seen,
the condition of working with street-car motors, it is probably
one of the most important conditions to be met. It is met
within the limits of practical requirements in street-car motors,
partly by controlling the field magnets, and partly by the
introduction of resistance into the armature circuits. This
resistance may be added either through the series windings of
the field coils, or by the direct insertion of external resistance.
The problem, however, of controlling within full range the
speed of a single continuous-current motor, under varying
torque, with high efficiency, is, strictly speaking, yet unsolved.
377. In some cases two motors are rigidly coupled together
so that they may have their armatures connected in series or
in parallel. In the first case they divide the pressure of the
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WJ* ELECTRO-DYN^AMK MACUmERY.
C. E. M. F. b«tween them, so that their speed will be a mini-
mum under that condition. In the second case they each take
the full pressure, and so yield the maximum speed. At slow
speed, however, when connected in series, it is evident that
the activity of the combination will be .£/ watts, since each
machine can now take / amperes, E, being the pressure. be-
tween the mains, in volts. At full speed, since each armature
can take / amperes, the available activity will be a .£ / watts.
The combined torque, for the full-load current through each
armature, will be the same whether they are in parallel or in
scries.
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CHAPTER XXVI H.
STARTING AND REVERSING OF MOTORS.
378, If a series motor be at rest, and be connected directly
across the mains, then if the resistance of the armature and
magnet coils together be ^ ohms, the current strength passing
through the motor tends to become -r= amperes, £, being the
E, M. F. in volts at the supply mains. Thus, if a i-H. P. series-
wound motor has a resistance in the armature of 0.5 ohm, and
a resistance in the field coil of 0.5 ohm, the total resistance in
the machine will be i ohm, so that the first tendency is to
produce a current strength of — = 110 amperes, as soon as
the machine is connected with the circuit, assuming the mains
to have a constant pressure of 110 volts, whereas the fuli-toad
current strength of the machine will be about 10 amperes. As
soon as the armature has become able to develop its full speed,
the motor will generate such a C. E. M. F. as will limit the
current through it to that required to expend the energy it
wastes and delivers. The rapidity with which the armature
will reach its full speed depends upon the load connected with
it, upon the inertia of the armature and of its load, as well as
upon the current strength entering the armature. Moreover,
owing to the self induction, or inductance, of the field-magnet
coils, it is impossible to develop the full current strength
immediately in them, even assuming that the armature were
to remain at rest. As soon as the current excites the field
magnets, the flux they produce, passing through the magnetic
circuit, develops in the field coils a temporary C. E. M. P.,
which has a powerful influence in checking the first inrush of
current into the armature during the first half second or second
of time. For this reason, a series-wound machine is much
more safely started from rest to full speed than a shunt-
wound machine, in which the armature has to be connected
directly across the mains.
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298 ELECTRO-DYh'AMIC MACHIf^ERY.
379. In all except the smallest machines of the shunl-wound
type, it is necessary to insert some resistance in the armature
circuit when starting from a state of rest, so that the drop
produced in such resistance by the starting current may limit
the amount of current passing through the armature. For this
purpose special rheostats, called starting rheostats, are inserted
in the armature circuit. Since they are only intended to carry
the current during the time that the motor is coming up to
speed, they are not usually designed to carry the full current
strength of the motor indefinitely, and, therefore, a starting
rheostat should never be maintained constantly in circuit.
Fig. 222 represents a form of starting rheostat employed with
shunt-wound motors. Here a number of coils or spirals of
galvanized iron wire, arc mounted in a fire-proof frame under a
cover of slate or composition, on which a number of contacts
are arranged in a circle. Fig. 223 represents the manner in
which such a rheostat is connected in the armature circuit.
380. If it becomes necessary, as we have shown, to insert
resistance into the circuit of a shunt-wound ipotor armature,
in order to start it from rest, it is still more necessary to insert
resistance into the armature circuit, in order suddenly to
reverse its direction of motion. When the armature terminals
of a shunt-wound motor are suddenly reversed, relatively to the
mains, while the field magnet coils remain permanently excited.
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STARTING AND REVERSING OF MOTORS.
199
the E. M. F. of the armature due to its speed, which was,
before the reversal, a C. E. M. F., tending to check the passage
of current strength through its windings, becomes now a driv-
ing E. M. F., tending to increase the current strength passing
through it from the mains. The effect of a sudden reversal in
a shunt-wound motor armature is, therefore, practically equiva-
lent to suddenly throwing the armature across a pair of mains
having double the pressure of those actually employed, and
WITH SHUNT MOTOR.
with the attending consequences of an enormous overload of
current strength, which first checks, and then reverses, the
direction of armature rotation.
381. Various devices are employed for preventing a motor
armature from being injured by the sudden reversal of its
terminals with the mains. At the time when armatures were
almost all of the smooth-core type, damage was frequently done
by shearing the wires off thearmature core-under the very heavy
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ELECTRO-DYNAMIC MACHINERY.
electro-magnetic stresses thus brought to bear upon them dur-
ing rotation. When toothed-core armatures became generally
used this danger practically disappeared, but the danger of
damaging either the insulation of the wires, or the mechanical
framework of the armature, or of burningout some of the con-
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STARTING AND REVERSING OF MOTORS. 301
ductors, still remains. A starting ceil is frequently employed
vith street-car motors which consists of a coil of strip-iron
'Conductor, having a hollow interior, so that it contains a large
magnetic flux when excited. The C. E. M. F. suddenly
developed from such a coil, on being magnetized, is sufficiently
great, to check, for the moment, the first rush of current, and
such a coil may be called an inductance coil.
382. Fig. 124, represents.the form, and Fig. 225, the diagram-
matic connections of a particular automatic switch and starting
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joa ELECTRO-DYNAMIC MACHINEk'Y.
rheostat sometimes employed with Urge motors. The larger
the motor the more expensive does any accident become which
may happen to its armature, and the more economical it
becomes to take precautions against suchacctdents. Referring
to the figures, it will be seen that the mains or line wires are
connected directly to two circular contact segments S, S,
through the coils of a relay magnet R. When the handle H,
is in such a position that the two contact bars B^ B, rest in the
intermediate position, they lie out of contact with the seg-
ments, and the current is then entirely cut olf the motor. A
powerful spring, wound about the axis on which the handle H,
moves, tends to bring the handle and the bars B, B, back to
this zero or "off" position. If the handle is pressed forward
in the clockwise direction against the pressure of its spring,
the line wires are connected with the armature through the
resistance coils r, r, r, which are wound upon spools of insulat-
ing and non-inflammable material within the box, and also
through the field coils of the motor. When the handle is
pushed completely around to the " on " position, the extra re-
sistances are cut out of the afmature circuit and the armature
thus becomes enabled to run at full speed. In this position
the handle is prevented from returning to zero and is kept in
place by the detent magnet Z>, excited by the current passings
through the field coils. If the circuit of the field colls should
accidentally become broken, the magnet I}, will release its
armature, which will release the detent, which will allow the
handle If, with its conuct bars B, B, to return to the " off "
position, under the action of the spiral spring; or, should the
armature current become excessively strong, thereby endanger-
ing the armature, the relay magnet will attract its armature,
which will thereby short-circuit the detent magnet, and the
same result will follow. The armature will, therefore, be
stopped by any overload, and will be cut out of circuit by any
accidental cessation of the current in the field. By means of a
push-button circuit, the armature can be brought to rest, by
pressing a push button placed at any distance from the
machine.
383. All the phenomena of armature reaction which we have
traced In conweetitm-with dynamos in Pars. 198 to 123 are pre-
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STARTING AND REVERSING OF MOTORS. iOi
seated by motors, with the exception that the direction of the
M. M. F. of the armature, relatively to the field magnets, is
reversed; that is to say, a motor runs so that the magnetic
flus produced by its armature tends to pass through the pole
which the armature approaches; i. e., \h& leading pole, instead
of the trailing pole, or that from which it is forced in the
dynamo. With this exception alt the effects of sparking and
cross- magnetization present themselves equally in motors as in
dynamos. The diameter of commutation in a generator has to
be advanced in order to obtain a sparkless position; in other
words, a lead has to be given to the brushes, while in a motor
the diameter of commutation has to be retrograded to arrive at
the same result; in other words, a lag has to be given to the
brushes.
384. In order to reverse the direction of rotation of a motor,
a single rule has to be borne in mind; namely, the M. M. F.
either of the field or of the armature must be reversed. If
the M. M. F. of both field and armature be simultaneously
reversed, the direction of rotation of the motors remains
unaltered.
385. Fig. 316 is a complete diagram showing the relations
which exist between the direction of rotation and the direction
of current in the field and armature of different machines.
The horizontal row on the top represents separately-excited
machines; the next lower row, shunt-wound machines, and the
lowest horizontal row, series-wound machines. The first
vertical column, No. I, on the right, represents generators.
Column II, next in order to the left, represents the action of
these machines as motors, when mounted in connection with
the mains, but not supplied with sufficient driving power to
maintain the machines as generators. Column III represents
the effect of reversing the connection of the armature when
the machine is acting as a motor. Column IV represents the
effect of reversing the field connections instead of the con-
nections of the armature. Column V represents the effect of
reversing both field and armature connections, which is equiv-
alent to reversing the entire machine relatively to the mains.
The large arrow on the field coil represents the direction of
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304
ELECTRO-DYNAMIC MACHINERY.
the M. M. F., or of flux through the field. The large arrow
on the armature represents the direction of the M. M. F. in
the armature, due to the curfent. The small arrow in the
centre of the armature represents the direction of the arma-
' rr
lI'^^^T
1.
^
^1 1,
tf,
ttire E, M. F., relatively to the circuit, and the curved arrow,
outside the armature, represents the direction of rotation of
the armature.
386. Referring to the line or row of separately-excited
machines, in Column I, each machine appears as a generator,
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STARTING AND REVERSING OF MOTORS. 305
rotated by the driving belt in the direction of the curved
arrow. The E. M. F, of the armature is in the direction of
the current throug;h the armature, and the mains are supplied
with current from the brushes, as shown. If the driving belt
be suddenly thrown off the armature pulley, the machine will
run for a few moments by its inertia, still supplying current to
the mains, until the power so expended has absorbed the sur-
plus energy of motion of the armature, when the speed and
E. M, F. of the armature will diminish, until the E. M. F, is
exactly equal to that between the mains, which are assumed to
be maintained at a constant difference of potential by another
source of supply. At this moment there will be no current
through the armature. If there were no friction in the arma-
ture, this condition might be retained indefinitely, but since
«very machine must expend energy against frictions, the speed
of the armature continues to slacken, and the E. M. F. in the
armature falls below that in the mains. Current will then pass
back from the mains through the armature, as shown in Column
II, reversing the M. M. F. of the armature, but maintaining
the same direction of rotation. The machine is now rotated
as a motor, absorbing energy from the mains, and the E. M. F.
of the armature is now a C. E. M. F., as shown by the opposi-
tion between the directions of the small arrow in the centre of
the armature, and the arrows representing the direction of
current through the armature. Consequently, a separately-
excited machine runs in the same direction as generator or
motor, if no change is made in the armature or field connec-
tions. If the armature connections be reversed, as represented
in Column III, or if the field connections be reversed, as rep-
resented in Column IV, the direction of rotation of the arma-
ture is reversed; but, if both field and armature connections be
reversed, as in Column V, the original direction of rotation is
retained.
387. In the shunt-wound machines, represented in the second
row, practically the same conditions are observed to follow;
namely, if the driving belt be thrown off the pulley of the
machine acting as a generator, when connected to constant-
potential mains, current will pass through the armature in the
opposite direction to that which passes when the machine is a
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3o6 ELECTRO-DYNAMIC MACHINERY.
generator, thus reversing the M. M. F. of the armature, but
maintaining the direction of rotation. Reversing either the
field or the armature, reverses the direction of rotation, but
reversing the entire machine; /. e., both field and armature,
has no effect upon the direction of rotation.
588. Thethirdrow; viz., that of series-wound niachines, dif-
fers, however, essentially from the foregoing. Here, it will be
observed, that if the belt be thrown off the generator, as soon as
the £. M. F. of the armature is brought down to that existing
between the mains, no current passes through the mains and
the field magnets lose their excitation. It will follow from
this that the E. M. F. of the armature will very rapidly dis-
appear, and a large rush of current will pass through the arma-
ture from the mains, reversing the direction, not only of the
armature M. M. F., but also of the field M. M. F., so that the
machine is first brought to a standstill, and then rotated in the
opposite direction. It is clear, therefore, from this considera-
tion, why series-wound machines are never employed as inde-
pendent units, in parallel, for supplying a system of mains; for,
if by anyacccident the engine driving a series-wound generator
failed to maintain the E. M. F. of its armature above that of
the mains, the machine would become a short circuit upon the
mains, and an enormous rush of current, with a correspond-
ingly violent mechanical effort, would be brought to bear upon
the machine, tending to reverse its motion and drive the
engine backward.
389. If the series-wound machine be considered as running
in the direction represented in Column 11, and the armature
connections are then reversed, or the field magnet connections
reversed, as in Columns III and IV, the direction of rotation
of the armature will be reversed, or' restored to the direction
of rotation as a generator ; while, if both field and armature
be reversed, as shown in Column V, the direction of rotation
will be the same as in Column II.
390. It is evident, therefore, from an inspection of the
diagram, that it is only necessary either to reverse the direc<
tion of the M. M. F. in the armature or in the field, to reverse
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STARTING AND REVERSING OF AfOTORS. 307
the direction of rotation of the motor, and that the relative
direction of the M. M. F. in- field and armature is opposite in
a motor to what it is in the same machine as a generator.
For this reason the leading pole-pieces of a machine, when
operating as a generator, and the following pole-pieces when
operating as a motor are weakened by armature reaction.
391. In practice, it is always the connections of the armature
of a machine which are reversed, in order suddenly to reverse
the direction of its rotation, for the reason that the inductance
of the armature being usually much less than that of the field,
the change is more readily effected, and with less danger of
injuring the machine by an excessive rise of pressure. On
the other hand, if the machine be brought to rest and dis-
connected from the circuit, it may be just as convenient to
reverse the field magnet connections as the armature connec-
tions, in order to effect a reversal of rotary direction when
the machine is next started.
392. In all cases it has to be remembered that it is dangerous
to break the circuit of the field magnets of a motor when in
operation, not only because by so doing the M. M. F. of the
field is almost entirely removed, and thereby the armature is
unable to develop a C. E. M. F., becoming practically a short
circuit on the mains; but also, because the powerful E. M. F.
generated in the field coils by self-induction, when their circuit
is interrupted, may ffnd a discharge through the armature
insulation, in such a manner as to pierce the same and per-
manently injure the armature. The same remarks apply to
the operation of machines as generators. The field magnet
connections should always be the first to be completed, and the
last to be interrupted, when the machine is operated in either
capacity.
393. In some cases, it is possible for the M. M, F. of the
armature to overcome that of the field magnets, and actually
to reverse the direction of magnetic flux through the mag- '
netic circuit of the machine. For example, if a shunt-wound
machine be operating alone, and supplying a system of mains,
it is possible for a very powerful current passing through the
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JoS ELECTRO-DYNAMIC MACHINERY.
armature to produce such an armature reaction as shall effect
a large C. M. M. F. in the magnetic circuit of the machine, and
so reverse the magnetic flux in the circuit. As soon as this is
effected, the E. M. F. of the armature will be extinguished and
the machine will cease to send a current. This effect is
distinct from the tendency of shunt-wound generators to lower
their E. M. F. under heavy loads, by reason of the drop in the
armature, and its effect upon the excitation of the field mag-
nets. It can only happen when the brushes of the machine
are given a considerable lead; for, if the brushes be maintained
at the neutral point midway between the poles, it will be
impossible for the armature reaction to produce a dangerously
large C. M. M. F. in the main magnetic circuit. Such acci-
dents have, however, taken place in central stations with types
of generator in which the armature reaction and lead of the
brushes at full load is considerable. For this reason it is
preferable to excite the field magnets of targe central station
generators from independent machines, when possible.
394. Iii motors, which are required to have their direction
reversed, it is necessary that the brushes shall rest upon the
commutator in such a position as shall permit of this reversal
of direction without danger. Carbon brushes are employed
with practically all soo-volt generators and motors, and with
such machines for lower pressures as will permit of the passage
of the full-load current through the carbon brushes without
dangerously overheating them. Their advantage is that they
wear evenly, lubricate the surface of the commutator, and are
readily replaced. Their only disadvantage is their high
resistivity, and the noise they are apt to make if the commuta-
tor surface is not perfectly uniform.
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CHAPTER XXIX.
HETER-HOTORS.
395. It sometimes becomes necessary to design a motor,
whose speed shall be proportional to the current strength
passing through it. This problem arises in devising motor-
meters for determining the quantity of electricity supplied to-
a customer from a pair of constant-potential mains, as in elec-
tric lighting. The motors employed for this purpose are of
very small sizes. We propose to consider the conditions
under which the speed of the motor shall be proportional to
the driving current strength.
396. Fig. 227 represents a pair of constant-potentiil mains,
marked -|- and — , with a small motor M, designed to measure
the current strength supplied to the incandescent lamps, L L,
with which it is connected in series. It is evident that the
current which passes through the motor armature will vary
directly with the number of lamps which are turned on. The
connections of the motor field magnets are not shown. These
magnets may be constantly excited from the mains, thus virtu-
ally constituting a separately-excited field; or, a permanent
magnet field may be employed for this purpose. In either
case the strength of the field flux may be considered as inde-
pendent of the load.
397. We know that (Par. 313) if t, be the current strength
passing through the armature in amperes, 0, the field flux, in
webers, usefully passing through the armature, and w, the
number of turns on the armature, counted once completely
around; the torque-per-ampere, which will be exerted about
the armature shaft will be
r = cm. -dynes per ampere.
If no load except friction were imposed upon the armature,
that is to say, if it were free to run without retarding torque^
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3IO
BLECTRO-DYNAMIC MACHiNERY.
beyond a frictional torque of / cm.-dyncs, due to mechanical
and electric frictions, then the speed which the motor would
attain, as soon as the first lamp was turned on, would be very
great, assuming that the torque /' r, was sufficient to start the
motor; for, the friction /, would be practically constant at all
speeds, and if i r, be greater than /, the accelerating force
being greater than the retarding forces, will continually
increase the speed of the motor until the C. E. M. F. of the
armature reduces the current strength to that which is needed
to exactly neutralize the retarding torque. Such a small
motor, therefore, if unloaded, would tend to run at a very
INCANDBSCBKT LAMPS.
high speed and to reduce the pressure at the terminals of the
lamp.
398. It is also evident that the resistance of the armature
must be sufficiently small, in order that the drop and C. £. M. F.
in the armature, produced by the full-load current, shall not
be greater than say one per cent, of the total pressure at the
mains. Let us assume that we are able to impose a load or
torque upon the motor proportional to its speed. If m, be the
number of revolutions-per-second made by the motor, r, the
load torque in cm. -dynes will then be r = a «, where a, is a
constant quantity. Under these conditions, the speed which
the motor will attain will be determined by the equality of the
driving and resisting torques or i r = « « + /■ From which
n =. ' — — — revolutions per second = — .
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ME TEH-MO TORS.
3"
399. For example, suppose a small motor to be connected
as shown in Fig. 337, in circuit with 20 incandescent tamps,
each taking one half ampere from a pair of mains supplied with
no volts pressure. The full-load current will be, 10 amperes,
and, if the resistance of the armature be 0.1 ohm, the drop
of pressure in the armature at full load will be 1 volt. If
the torque t, of the motor be 200 centimetre-grammes, or,
approximately, 100,000 centimetre-dynes per ampere of cur-
rent, also if the torque due to frictions be 75 centimetre-
grammes, or, approximately, 75,000 centimetre-dynes, and the
torque due to load be no centimetre-grammes, or, approxi-
mately, I Jo,ooo centimetre-dynes-per-revolution-per-second,
then, if one lamp were turned on, the current through the
armature would be 0.5 ampere. The starting torque would be
100 centimetre-grammes, the resisting torque of friction, 75
centimetre-grammes, and the motor would therefore start
under a resultant torque of 35 centimetre -grammes. It would
accelerate until a speed of 0.208 revolution-per-second was
attained, when the resisting load torque would be 0.108 x
120 = 25 centimetre-grammes. Proceeding in this way, we
can determine what the speed of the motor would be with any
current strength as follows:
St»d,/
SfHd
30 lao
Here a ■=
Boo 75 735 6.04 0.7SS
i.ooo 75 93s 7.71 0.771
1,200 75 i.ias 9.37s 0.781
J,40O 7S 1,335 ii.Q« 0.789
1.600 75 . 1,535 13.71 0.794
1,800 75 i,7as 14-375 0.799
3,ooo 75 1,935 16.04 0.S03
120,900 T = 200,000/ = 75,000, SO that with
400. It will be observed that, after the first two lamps
have been lighted, the speed of the motor is nearly pro*
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31* ELECTRO-DYNAMIC .WACHINERY.
portional to the number of lamps, and, therefore, the total
number of revolutions of a motor armature in a given time,
will be an approximate measure of the total quantity of elec-
tricity supplied through the meter in coulombs, or in ampere-
hours.
In order that the error, introduced into the indications of
the meter, by constant friction of the armature, shall be as
small as possible, it is important that the constant torque-per-
revolution-per-second shall be as great as possible, relatively
to the friction, or that --- shall be a small fraction.
401. In practice it would be very difficult to arrange a motor
of this kind, having its armature placed directly in the main
circuit of the lamps, for the reason that if the brushes were
sufficiently fine to permit the friction of the armature to
become negligibly small, any accidental short- circuit, occurring
between the lamp-leads, would probably destroy the brushes,
or the armature, or both. The problem has, however, been
successfully met in practice by making the armature in this
case the fixed element of the motor, and the field magnet the
moving element.
Fig. 2j8 represents a well-known type of meter, in which the
current to be measured passes through the stationary element
of the field coils F, F, while the moving element, or armature
Af, is permanently magnetized by a feeble current passing
through a comparatively high resistance, wound on a frame at
the back of the instrument and kept in circuit with the mains.
The armature M, receives its current through the delicate
brushes d, which rest on opposite sides of a small silver com-
mutator c. No iron is employed in either the field or armature
of the apparatus. The vertical shaft of the armature M, is
geared directly with a dial-recording mechanism similar to that
of a gas meter. In order to apply a load torque proportioned
to the speed, a disc of copper D, is mounted horizontally upon
the vertical armature axis, so as to rotate between the poles
of the three permanent magnets P, P, P, as shown. When
the disc is at rest there is no retarding torque other than a
small mechanical friction due to the brushes resting 00 the
commutator and the weight of the armature in its bearings.
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METER-MOTORS. 313
As soon as the disc is set in motion by the rotation of the
armature, eddy currents are produced in its substance by the
(lynamo action of the permanent magnets upon it, and a re-
tarding torque Is set up between the disc and these magnets.
At all ordinary speeds this torque is proportional to the rate
of rotation, thus complying with the requirements of the motor
as a meter.
402. The armature of the motor represented in Fig. 127 is
only capable of acting as a coulomb meter, or ampere-hour meter,
but the apparatus shown in Fig. zz8, while acting as an ampere-
hour meter on constant potential mains, also operates as a
watt-meter, in cases where the pressure between the mains is
not constant; for, all variations in the pressure will also
increase in direct proportion the useful flux 0, linked with
field and armature, and so the speed of the armature will be
accelerated and retarded in proportion to the pressure, as well
as in proportion to the current strength.
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3'4
ELECTRO-DYNAMIC MACHINERY.
403. No law of retarding torque, other than a torque pro-
portional to the speed, can give a rate of revolution in the
armature proportional to the current strength passing through
it, when the Reld flux is constant. If, however, the field
magnets be in series with the armature, so that # increases
with the load, it is possible for an instrument of this character
to register fairly accurately, even although the load torque is
not proportional to the speed. In such cases, however, the
results can only be approximate, since the hysteresis in the
magnetic circuit of the field will bring about a complicated
relation between load and flux.
404. Another problem which sometimes arises, is to design
a motor whose speed shall be proportional to the pressure in
FIG. 329,— MOTOE
volts at its terminals. This problem presents itself in motor-
meters having an armature which, instead of being inserted
directly in the lamp circuit, is shunted by a constant small
resistance r. A motor-meter of this type is shown in Fig. 229.
Here the danger of burning out the armature by an accidental
overload is not nearly so great, since the pressure at the arma-
ture terminals can never exceed that of the drop in the shunt
resistance r. If i, be the total current strength in amperes
passing through the lamps, and e, the dynamo power of
the armature, in volts-per-revolution-per-second, the current
strength passing through the armature will be
,, = ^-^ amperes. = -^-^^
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METER-MOTORS.
315
where R is the resistance of the motor armature in ohms, and
the driving torque will be 1, r cm. -dynes.
If the frictional torque /, centimetre-dynes, be assumed
constant, the speed of the motor will be determined by the
relation i r = /or
(* + ') ■'■
from which n ■= ■ — ■ — — revoli
e e T
From this it will be seen that the motor '
Cions-per-second.
ill develop a speed
Rovol-
ihro'ar.
Rvolu.
j
Lampper
"T"
vohi.
vol 11.
volll.
■«.,■,
Mcoad.
1
O.J
oj)5
0.4J
0.04s
oooj
0.040
oe.6
<>.««
OflS
«,
""^
9.«
'-Wi
0.00s
°*"°
.6,S
proportional to the main current /', if the frictional torque /,
be constant, and sufficiently small to make"^-'^ — — — ^ small
compared with — -, The following case will illustrate this result.
Let R = 0.1 ohm, r = o.i ohm, /= 50 cm.-gms.,T = r,ooo
cm.-gms.-per-ampere, e = 0.06 volt per revolution per second.
Then « = i.667< — 0.1667. The preceding table shows the
results which follow for various currents up to 10 amperes,
either directly from the formula or by independent reasoning.
Such a motor will usually operate at a comparatively high
speed at full load, since it depends upon the influence of its
C E. M. F. in reducing the current strength through the
armature to that required in order just to balance the resisting
torque/
405. If, however, a load torque be imposed on the armature,
proportional to the speed, represented by r, =■ a n, then our
relation becomes
', = '■/+««
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3'6
ELECTRO-DYNAMIC MACHINERY.
- f ^ an, from which n = -
r r + a (JP + r)
revolutions-per-second.
If, for example, in the last case, the motor develops a re-
tarding torque of 6ocm.-gms. per-revolution-per-second (« =
60 cm.-gms. or 60,000 cm. -dynes approximately), we obtain
either from the formula, or by direct analysis, the following
results ;
c..„„.
; To.qU<. 1
Ditof IN MoTtm
s,....
1
■5
7-.
^"1 f" It-"
"[3 'Hi
1
s
1
1.
II
i
1
1
1
s
0.
1
it
i
ill 1
i
o.og,
0.108
^g?6
3
-."3. 1 0.04.1
I'l
=■633
406. It is, therefore, evident that a motor armature, with
constant field excitation, can develop a speed closely propor-
tional to the pressure at its terminals, and, therefore, serve as
a motor-meter, if the retarding torque be small and constant,
or, if it be partly small and constant, and partly proportional
to the speed.
407. One of the most important recent Spplications of
motors is their distributed application to machine tools in
large factories. Instead of employing long lines of counter-
shafting, which must necessarily be constantly driven during
working hours, a separate electric motor is applied directly to
each machine, so that each machine is started and stopped
according to its own requirements. Moreover, the range of
regulation of speed, which is ohtainable from a common coun-
tershafting, is necessarily more limited in degree than that
which can be effected by the use of independent motors.
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METER.MOTORS. 317
408. By the use of indimdual electric motors, not only is each-
tool capable of operation at its best speeds, and under com-
plete control, but also the friction of long lines of counter-
shafting is eliminated. The economy is greatest where the
nature of the work in the machine shop is such that the average
power supplied to the tools is much less than the maximum
power, or the ratio of average to maximum power; (. e., the
load factor is small, since the motors, when completely dis-
connected from a circuit, take no power, whereas, the
countershafting consumes, practically, the same amount of
power friction, whether the tools be active or idle.
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CHAPTER XXX.
MOTOR DYNAMOS.
409. The consideration of dynamos and motors naturally
leads to that of a third class of apparatus, which partakes of
the nature of each; namely, motor-dynamos, or, as they are
sometimes called, dyna-motors. It is evident that if a motor
be rigidly connected to a dynamo, either by a belt or by a
coupling, that we obtain a means wherebjt electric power can
be transformed through the intermediary of mechanical power.
Thus, the motor may be operated from a high-tension circuit,
while the dynamo operates a low-tension circuit, or vice versa ;
but, neglecting losses taking place in the two mactiines, the
amount of electric energy absorbed and delivered in the re-
spective circuits will be the same, the combination being
utilized for the purpose of transforming the pressure and cur-
rent strength. For this reason a motor-dynamo is commonly
called a rotary transformer, in order to distinguish it from an
ordinary alternating-current transformer, which always remains
at rest.
410. Instead of rigidly connecting together two separate
machines; i. e., two armatures in two separate fields, the plan
has been adopted of placing the two armatures in a field com-
mon to both; as, for example, by placing them in a common
field of double length. Or, a still closer union can be effected
by winding both the armature and motor coils on a common
armature core, care being taken to insulate the two sets of
windings from each other. Under these circumstances, since
the intake of the motor winding is practically equal to the out-
put of the dynamo winding, the space occupied by each wind-
ing will be practically the same, so that where both are asso-
ciated on a common core, half the winding space is appropriated
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MOTOR DYNAMOS. 319
for each. The result will be that if the motor winding or
dynamo winding be such as would appertain to, say, a lo-KW
capacity, the armature in which the two are associated will be
a machine having, approximately, the size and weight corre-
sponding to a 20-KW capacity. There is, however, an econ-
omy in constructing one machine of double capacity, instead
of two machines of single capacity, both in first coat and in
efficiency.
411. Rotary transformers, like all transformers, maybe either
of the step-up or step-down type. Fig. 230 represents a step-
up rotary transformer of i.5-K\V capacity, transforming from
I zo volts and n.5 amperes, to 5,000 volts and 0.3 ampere.
The motor winding of the armature is connected with the com-
mutator on the left, while the generator winding of the arma-
ture is connected with the commutator on the right. The
magnet coils are excited from the low-tension mains. The
two armature windings, in such cases, may he either placed one
below the other, or ihey may be interspersed. The left hand
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5'0 ELECTRO-DYNAMIC MACHINERY.
brushes receive the 120-voU pressure, and the right hand
brushes deliver the 5,000-volt-pressure. The function of such
a machine is to test high-tension insulation under practical
conditions of pressure.
412. Fig. 231 represents a step-down rotary transformer for
transforming from 500 to 120 volts. In this case the smaller
brushes are connected to the 500-volt mains, as is also the field
winding, and the lowtr pressure is delivered at the heavy
brushes.
413, It is important to observe that in a motor dynamo of
the preceding types there is no appreciable armature reaction.
The reason for this is as follows: The M. M. F. of the motor
armature winding is, as we know, opposite in direction of that
of the generator winding; and, since these M. M. Fs. are
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MOTOR DYNAMOS. 3"
nearly equal, and are produced on the same core, they will
nearly neutralize each other. Consequently, the brushes of
such a machine never require to be shifted during variations
of load, and the commutators are characterized by quiet and
sparkless operation.
414. Under ordinary circumstances it is necessary to excite
the field magnet of a motor dynamo from the primary circuit,
since, otherwise, the motor side could not be operated. It is
often possible, however, to place a series winding on the motor
side, and a shunt winding on the secondary or dynamo side.
Thus, if it be required to transform from 1,000 to 50 volts, a
shunt-field winding for 1,000 volts would be more expensive
than one for 50 volts. In such a case it becomes possible to
excite the fields by a few turns of series winding, carefully in-
sulated, in the primary circuit, in order to start the machine
from rest, and to supply the balance of the field excitation by
a shunt winding on the secondary side, which commences to
be actuated as soon as the motor starts.
415. It will be evident that any variation in the strength of
the field magnets, whether these be shunt- or series-wound, will
not vary the ratio of transformation; for, although by varying
the field excitation the motor can be made to change speed,
yet this speed will not produce any appreciable effect upon the
generated E. M. F., since the field is proportionally weakened.
In other words, the C. E. M. F. in the motor being always
equal to the E. M. F. at the brushes, after deducting the drop
in the armature, the generated E. M. F. , which is always
some fixed fraction of the motor C. E. M. F., must be constant
within the same limits. If the number of turns in the motor
winding, counted once all round the armature, be w„, and the
number of turns in the generator winding, counted in the same
manner, be u>^ then the ratio -- is called the ratio of transfer'
mation. If, then, the primary E. M. F. be £, volts, the primary
current /, amperes, and the resistance in the primary winding
/-, ohms, while the corresponding quantities in the secondary
circuit are E^, /„ and r„ respectively, the C. E. M. F. in the
primary winding will be « ^. = £, — /, /•„ where «, is the speed
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32» ELECTRO-DYNAMIC MACHINERY.
of revolution in turns-per- second, and «„ the dynamo power, or
^■wm X ">"'. The generated secondary E, M. F. will ben ^ay
X io~* volts = {£, — /, r,) — ^.
The pressure at the secondary terminals will be further re-
duced by the drop in the secondary winding; or
£. = <£ - /, r-S -^ - /, r,.
If the weight of copper in the two windings is equal, 7, r„ will
practically be equal to — — ' — -, so that
£, = £, !^- - 1/ r.
The machine, therefore, acts as though it were a dynamo of
E. M, F. ''- E., with an internal resistance of ir^, or twice that
■Wm
of the secondary winding.
416. In all motor dynamos, having a field magnet common
to both armatures, the ratio of transformation, neglecting ar-
mature drop, is constant, no matter how the field excitation is
varied. Motor-generators are often employed for raising pr
lowering the pressure of continuous-current circuits. Thus
electroplating E. M. Fs. of, say 6 volts, are obtainable in this
manner from circuits of no, aao or 500 volts pressure. Simi-
larly, pressure of 150 volts are obtainable from a few storage
latteries by such apparatus.
417. In central stations for low-pressure distribution, say at
2ZO volts, by a three-wire system, some of the feeders have to
be maintained at a higher pressure than others, in order that
all the feeding points, or points of connection between feeders
and the mains, should have the same pressure. This is ac-
complished either by employing separate dynamos, operated
at slightly different pressures, or by introducing at the central
station motor-dynamos having their dynamos in circuit with the
feeders. Such motor-dynamos are frequently called boosters.
The motor-dynamo for this purpose requires that means should
be provided for regulating the E. M. F. which is to be added
to the feeder circuit. This can only be done by employing
■separate field magnets for the motor and generator armatures.
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MOTOR DYNAMOS. 3^3
Fig. 232 represents a practical form of booster employed in
a Ihree-wire central station. The middle machine is a motor
operated at central-station pressure of, perhaps, 350 volts; the
others are generators, having their armatures coupled to the
same shaft as that of the motor armature. One dynamo is
connected in circuit with the positive conductor of the feeder
whose pressure is to be raised, and the other is connected in
the circuit of the negative conductor. Since these feeders
carry heavy currents and require to be of very low resistance,
the necessity for the massive copper brushes and connections
of the dynamos will be evident. The amount of E. M. F. which
will be generated in these armatures will be determined by the
excitation of their field magnets.
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.Google
INDEX.
Active Conductor, Magnetic Flux
of. 37
Aero-Ferric Magnetic Circuits,
68-73
Air-Gap. Magnetic, 57
Air- Path, Altenaative Magnetic, 43
— Aligned M. M. P., 56
Alternating-Current Dynamos, 17
Alternative Magnetic Air-Path, 52
Alteniators, 17
— Multiphase, 25
— Uniphase, 26
Ampere, [>elinition of, 49
Ampere-Hour Meter. 313
Amptere-Tuni, DefinitioD of. 40
Anomalous Magnet, 47
Arc- Light Dynamos, 26
Armature, Back Magnetization of,
186
— Cores, Cross-Sections of, 126
— , Core Discs for, 152
— Core, Lamination of, loj
— , Cylinder or Drum, 13
— Disc, 33
— , Double Winding of, 190
— , Grtunme-Ring, 23
— , I» R, Loss in, 200
— , Iron-Clad, Definition of, 24
— , Journal Bearings, 15^163
— of Machine, 9
— , Neutral Line of, 184
— , Pole. iicf-ii6
— , Radial, 110
— Reaction and Sparking at Com-
mntators, 179-198
— Ring, 33
— , Smooth-Core, 23, 152
Definition of, 34
— Toothed-Core, 153
, Definition of, 14
1 23
— Turns, Effect of, on E. M. P., 3
— Winding. Closed-Coil, no
, Disc, 230
, Dissymmetry of, 12s
. Inter-Connected, 145
Space, 375
— Wire, Effective Length of, 246
Armatures, Closed-Coil, 217
— , Gramme-Ring. 117-127
— , Lap Winding for, 155
— , Open -Coil, 217
— , Wave-Winding for, 155
Attractions and Repulsions. Laws
of Magnetic. 33
Automatic Regulation of Dyna-
Average Efficiency of Motor, 279
Back Magnetization of Armature,
186
Balancing Coil of Armature, 194
Bar, Equalizing, 224
Bars. Bus, 234
— , Omnibus, 224
Bearings. Self-Oiling. 161
Belt-Driven Dynamos, iB, 135
Bipolar Dynamo, 16
Boosters, 322, 323
Box, Field-Regulating, for Dy-
namo. 14
Brush, Dynamo, 124
Brushes, Vorward Lead of, 217
— , Lead of, 185
— of Dynamo, 9
— of Motor, Lag of, 303
Bus Bars, 234
Calculation of Gramme-Ring Dy-
namo Windings, 12S-134
Capability. Electric, of Dynamo.
116
— , Electric, of Dynamo-Electric
Machine, 4
Car Motor, 277
Characteristic Curve of Dynamos,
— External, of Seri«B-Wonnd Dy-
— Internal, of Series-Wonnd Dy-
namo, 210
— of Shunt- Wound DynaDto, 213
Circuit, Magnetic, 48
Return, tor Track Peedera, 126
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3^6 IN
Circait, Transmission, Definitior
— , Magnetic Flux, Assumed Di-
rection of, 39
Closed Circular Solenoid, jo
— Coil Armature Winding, no
Armatures, 317
Coefficient, Hysteretic, 174
Coil, Balancing, of Armature, 194
— , Inductance, 301
— , —of. tSi
— , Starting, 301
Combinations of Dynamos in Se-
ries or in Parallel, lao-^s?
Commercial Efficiency of Dyna-
mo, s
of Dynamos, Circumstances
Affecting, 7
-of Motor, a68
Commutation, Definition of, 180
— , Diameter of, 180
— , Quiet. Circimistances Favor-
ing. 187
— , S[)arkless, Circumstances Fa-
voring. i8b
Commutator, Circumstances Fa-
voring Sparking at, 186
— . Forrns of, 133
— of Dynamo, 9
Commutatorless, Continuous- Car-
rent Dynamo, Disc Type of, 336
Dynamos. ^34
Generators, 334-340
Commutators, Sparking at, 179-
Compounf Wound Dynamos, 14
, Uses for, aog
Conductor, Active. Magnetic Flux
of, 37
Consequent Poles of Dynamo, 33
Constant-Current Dynamos. 10
Constant-Potential Dynamos. 10
Constants, Reluctivity, Table of.
6S
Continuous-Current Commutator-
less Dynamos. 18. 334
. Cylinder TyP^ "'• *3^
Dynamo, lo
Generators, 334-140
Generator, Limitations to
Output of, 303
Convention as to Direction of Cir-
cular Magnetic Flux, 39
Converging Magnetic Flux, 35
Core Discs for Ar
Core, Effect of Lamination on
Eddy Currents. 166
Coulomb Meter, 313
Counter Electro- Dynamic Force,
— , Eddy. Definition of, 16^
— . — , Effect of Lamination of
Core on. 166
— , — . OrigiQ of, 165
Curves, Coaracteristic of Dyaa-
— of Reluctivity in Relation to
Ftux Density, 66
Cutting Process vs. Enclosing of
Magnetic Flux, 83
Cycles of Magnetization, (74
Cylinder or Drum Armature, 33
— Type of Commutatorless Con-
tinuous-Cuirent Dynamos, 236
Decipolar Dynamos, 17
Density, Flux, 34
— , Prime Flux, 54
Devices, Receptive, Deflnitioa
of. I
Diameter of Commutation, 180
Diffusion, Magnetic, 53. 53
Diphase Dynamo, 27
Direct-Urivea Dynamos, 135
Disc Armature. 33
— Armature Windiiig, 330
— Armatures and Single Field-
coil Machines, 338-333
tinuous-Current Dynamos, 336
Dissymmetry, Magnetic, 134
— ot Armature Winding, 135
Distribution of Magnetic Field, 41
- of Magfnetic Flux of Conductor,
Double Winding of Armature, 190
Drum Armatures. 153
— or Cylinder Armatures, 33
Dynamo Armatures. Electro-Dy-
namic Induction in, go-i03
— . Bipolar, 16
— Brush, 134
— Brushes of, 9
— . Commercial Efficiency of, 5
— Commutator, 9
— , Consequent Poles of, 33
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Dynamo, Continuous-Current, so
— , Diphase, a?
— , Double-Circuit, Bipolar, 16
— , Electric Capability of, ia6
— , — Efficiency of, 5
Dynamo-Electric Generator, 1
Machine, Electric Capability
of, 5
Dynamo Field-Regulating Box, 14
— , iDtake, 5
— , Load of, IS
— , Magneto-Electric, 11
— , Output of, s
— , Platins, a6
Dynamo-Power of Motor, 366
Dynamo Relation betweenOu^nt
end ReiuBtance, 6
— , Self-Encited, is
— . — . Compound-Woxrad, 13
— , Separately Excited, la
— , Single-Circuit, Bipolar, 16
— , Telegraphic, 36
Dynamos. Attemating-Current, 17
— , Arc-Light, 26
— , Automatic Reg^ulation of, 31S
— , Belt-Driven, 18
— . Characteristic Curves of, aio
— , Circumstances Influencing*
Electric and Commercial Effi-
ciency of, 7
— , Combination of, in Series or
Parallel, 330-337
— , Commutatorless Continuous-
Current, 38, 334
— , Compound- Wound, 14
— , — , Uses for, 309
— , Constant- Current, 10
— , Constant-Potential, 10
— , Decipolar, 17
— , Direct-Driven, 135
— , Heating of, 199-305
— , Incandescent Light, 36
— , Inductor, 35
— , Multipolar, t6
— , Multipolar, Gramme-Ring, 13s
— , Octopolar, 17
— , Over-Compounded, 309
— , Qnadripolar. 17
— . Regulation of, 3o6-3ig
— , Self-Excited, Series-Wound, 13
— , Series-Wound, Uses for, 309
— . Sextipolar, 17
— , Shunt-Wound, Uses for, 309
— . Simple Magnetic Circuits, z3
— , Single-Field-Coi], Multipolar.
Dynamos. Triphase. 27
— , Two-Phase, 37
— , Unipolar, 18
Dynamotors, 317
Dyne, Definition of,'_69
E. M. P., Effect of Number of
Armature Turns on, 3
— , Effect of Speed of Revolution
on, 3
— , Induced by Magneto Genera-
tors, 103^109
— , Induced m Loop, Rule for
Direction of, 94
—, of Electro-Dynamic Induction,
Value of, 75-82
— , of Self-induction, iBi
— , of Self-induction, Circum-
stances Affecting Value of, iSl
— , Produced by Cutting Earth's
Flux, 90
Earth's Flux, E. M. F. Produced
by Cutting, 90
Eddy Currents, 164-171
, Definition of, 164
, Effect of Lamination of
Core on, 166
, PormatiOD of, in Pole-pieces,
169
, Origin of, 16s
E^es, Leading, of Pole-pieces, 1S4
Efficiency, Average, of Motor, 370
— . Full Load of Motor, 370
— of Motors, 368-379
Electric Capability of Dynamo,
136
— — of Dynamo- Electric Ma-
— Efficiency of Dynamos, Circum-
stances Affecting, 7
— Flux, Unit of. 49
Electro-Dynamic Force, 341-349
— Induction, 75-8a
in Dynamo Armature, 90-103
, Laws of, 74-89
— Machinery, i
— Machinery, Classiflcation of, i
Enameled Rheostats, 316
Entrefer, 105
Eaualising Bar. 334
Etner, Assumed Properties of, 39
Ether Path of Reluctivity, 60
External Characteristic of Series-
Wound Dynamo, 310
Factor, Leakage, 133
— , Load. 317
Faraday's Disc, J34
Feeders for Return Track, 336
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FeediD ; Points, 33a
Feme Magnetic Circuits, ss-67
— Pftth of Metallic Retoctivitjr,
60
Field Magnet of Machine, g
— , Magnetic, 33
— Magnets, 1' R Losses in, 199
— Poles, Eddy-Current Losses in.
— Regulating Box for Dynamo, 14
— Rheostats, 215
Fleming's Hand Rule for Dyna-
Motors, 243
Flux, Circular Magnetic, Conven-
tion as to Direction of, 39
— , Converging Magnetic, 35
— Density. 34
— . Diverging Mag^netic, 35
— , Magnetic, Unit of, 49
— Density, Prime, 54
— . Prime, 56
— , Magnetic, 39
— , — , Distribution of, 31
— , — . Irregular, 35
— , — , Vanations of, 33
— Paths, Magnetic, a
Following Edges of Pcde-Pieces,
184
Force, M, M.. Induced, 56
— , Electro-Dynamic, 241-249
— , Lines of Magnetic, 34
— , Magnetic. Tubes. 35
— . Magnetiiing, S3
— , Magnetomotive. 31
-.-.l-rime. 56
Forces, Electromotive, Methods
for Increasing. 3
French Measures. Table of, 8
Friction Losses in Bearings and
Gap, Magnetic Air. 57
Gauss, Definition of, 35
Generator Armature, Limiting
Temperature of, 203
— , Dynarao-Electric, a
Generators, Commutatorless Con-
tinuous-Current, 234-240
— . Definition of, 1
Gilbert, Definition of, 40
Gramme-Ring Armatnre. 23
— Armatures, 117-137
— Dynamos, Multipolar, 135-151
Hand Rule. Fleming's, for Dyna-
Heating of Dynamos, 109-aos
Hysteretic Activity. Tai>l« <rf, 175
— Losses in Armature and Field
Poles, aoo
— Coefficient. 174
Hysteresis. Magnetic. I7»-178
— , — , E)efinition of, 172
Incandescent Light Dynamos, 26
Individual Electric Motors, 317
Idle Wire on Armature, 100
Inductance Coil, 301
— of Coil, 181
Induction, Electro- Dynamic. 75-S2
— , — , Laws of, 74-89
— in Dynamo Armature, 90-103
— , Self. E. M. F. of, 181
Inductor Dynamos, 25
Intake of Dynamo, Definition of, 5
Inter- Connected Armature Wind-
ing. 145
Internal Characteristics of Series-
Wound Dynamo, 210
Iron-Clad Armature, 34
Irregular Magnetic Flux, 35
Joint Reluctivity, 60
oumal Bearings for Armatures,
159-163
Lamo, Pilot, Definition of, 13
Lap Winding for Armatures. 155
Laws of Electro- Dynamic Induc-
tion. 74-89
— — Magnetic Attractions and
Repulsions, 33
Lead, Forward, of Dynamo
Brushes, 217
— of Brushes, 195
Leading Edges of Pole-pieces, 1S4
— Pole of Motor, 303
Leakage Factor, 132
— , Magnetic, 52, 53
Length, Effective, of Armature
Wre. 246
Limitation to Output of Continu-
ous-Current Generator. 303
Limiting Temperature of Genera-
tor Armature, 203
Line, Neutral, of Armature, 194
Lines of Magnetic Force, 34
— , Stream, 30
Load Factor. 317
— of Dynamo, 15
Locomotors. 273
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Loss by Eddy Currents in Arma-
ture and'Field Poles, aoo
— . Hysteretic, 174
— , — , in Arinature and Field
Poles, aoo
Losses, I' R. in Field Magnets. 199
— in Armature, 1' R, 100
— Produced by Air-Churning, aoi
Friction in Bearings and
Brushes, aoi
M. M. F., Aligned, s6
— . Induced, $t
— , Uethods of Producing, 38
— , Prime, 56
— , Structural, 56
— , Unit of, 40
Machine, Armature of, g
— Circumstances Influencing
Electric Efficiency of Dyna-
— , Magnetic
"--■linery, E
-, Classification
Machines, EHsc Armature and
Single Field-Coil, 238-233
Magnet, Anomalous, 47
— . Mechanical Analogue of, 30
— . North-Seeking Pole of, tq
Magnets, Componnd, los
— , Holecnlar, 5b
Magnetic Air-Gap, 57
— Air Path, Alternative, 52
— Attractions and Repulsions,
Laws of, 33
— Circuit, 4S
— Circuit, Application of Ohm's
— Circuits. Aero-Ferric, 68-73
— Di£Fu3ioa, ja, 53
— Field, 33
— Dissymmetry, 134
— Field, Dlstribiitioa of. 41-47 ,
, Method of Mapping, 33
, Negatives of, 3a
, Photographic Positives of, 33
, Converging. 35
, Cutting Process, Enclos-
Density, 34
. Diverging, 35
, Effect of, on C. E. M. P., 58
, Irregular, 35
of Dynamo, 9
, Uniform. 35
, Unit of. 49
, Unit of Intensity of, 35
£X. 319
Magnetic Flux, Variations of, 33
— Force, Tubes of, 35
— Friction, 174
— Hysteresis, 172-178
1 Definition of, 17a
— Intensity, 34
— Leakage, 53, 53
— Permeability, 55
, Definition of. 3
— Potential, Fall of, 53
— Reluctance, 48
Magnetism, Definition of, 39
— , Molecular, 56
— , Residual, js, 173
— , Streaming-Ether Theory of,
Magnetiiation, Back, of Arma-
— , Cross, 183
— . Cycles of. 174
Magnetiaing Poree, 53
in Relation to Reluctivity,
S9
Magneto-Electric Dynamo, 11
Magneto Generators, E. M. P.
Induced by, 103-109
Magnetomotive Force. 31
Mapping of Magnetic Field, 33
Mechanical Analog^ne of M^-
net. 30
Meter Motors, 309-317
Methods for Suppressing Spark-
ing, 189
Molecular Magnetism, 56
— Magnets, 56
Motor, Average Efficiency of, 370
— , Commercial Efficiency of, 368
— , Dynamo-Power of, a&t
— Dynamos. 318-323
— , Definition of. 318
— , Full-Load Efficiency of, 870
— , Leading Pole of, 303
— Torcjue. 251-267
— , Trailing Pole of, 303
Motors, Efficiency of, 268-279
— , Fleming's Hand Rule for. 343
— tor Street Car, 277
— , Individual Electric. 117
— , Regulation of, 380-296
— , Slow Speed, 371
— , Starting and Reversing of, 391
-308
— , Stationary, 273
— , Traveling, 273
Multiphase Alternators. >6
Multipolar Dynamos, 16
, Single-Field-Coil, 28
— Gramme-Ring Dynamos, 135-
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33° /JV^fl
Negatives of Magnetic Fields, 31
Neutral Line of Armature, 184
— Wire of Three-Wire SyBtem, an
Non-Feiric Magnetic Circuits,
48-54
North-Seeking Pole of Uagnet, ig
Octc^lar Dynamos. 17
Oersted. Definition of, 49
Ohm. Definition of, 49
Ohm's Law, 49
Applied ■ to Magnetic Cir-
cuit, 49
Oilers, Sight-Feeding, 160
Omnibus Bars, 324
Open-Coil Armatures, 317
Over-Compounded Dynamos, 209
Output ana Dimenaions of Dyna-
mos. Relation Between, 136
— of Dynamo, Definition of, S
, Relation Between and Re-
sistance, 6
Permeability. Magnetic, 55
— , — , Definition of, 3
Photo^aphic Positives of Mag-
netic Fields, 33
Pilot-Lamp, Definition of, la
Plating Dynamo, a6
Points, Feeding, 33a
Pole Armature, as
— Armatures. 110-116
— , Leading, of Motor, 303
— , North-Seeking of Magnet, ag
— , Formation of Eddy Cnrrents
— , Leading Edges of, 184
Poles, Consequent, of Dynamo, aa
Potential, Magnetic, F^l of, 53
Prime Flun, ^6
— PIux Density, 54
— M.M. F.,56
Properties, Assumed, of Ether, 29
enadripolar Dynamos, 17
uiet Commutation, Circumstan-
ces Favoring, 187
Radial Armature, no
Ratio of Transformation, 321
Receptive Devices, Definition of, i
Regulation of Dynamos, aofr-aig
— of Motors, 280-296
Reluctance, 48
— , Maenetic, 48
Reluctance, Unit of, 49
Reluctivity, 48
— , Constants, Table of, 65
— Curves in Relation to Flux
Density, 66
— . Ether Path of, 60
— in Relation to Magnetizing
Force, 59
-, Joint, 60
— , Metallic, Ferric Path of, 60
Residual Magnetism, 55, 173
Resistivity, 48
Return Track Feeders, 336
Reversing and Starting of Motors,
agi-308
Rheostats, Enameled, 316
— , Field, aij
-, Starting, 398
117-
Self- Excited Compound- Wound
Dynamo, 13
— Dynamo, 13
— Series- Wound Dynamos, 13
Self-induction, E. M. P., of, 181
— E. M. F., of, Circumstances Af-
fecting Value of, 183
Self-Oiling Bearings, t6i
Separately-Excited Dynamo, 13
Series or Parallel Combinations of
Dynamos, 220-227.
— Winding of Dynamos, 306
Series-Wound Dynamo, External
Characteristic of, zio
— D^amo, Internal Character-
istic of, 2ia
Sextipolar Dynamo, 17
Shunt Winding of Dynamos, 307
Shunt-Wound Dynamo, Charac-
teristic of, at3
— Dynamos, Uses for, ao9
Sight-Feeding Oilers, 160
Simple Magnetic Circuit Dyna-
Single-Circuit Bipolar Dynamo,
16
Single Field-Coil Multipolar Dy-
namos. 28
Single-Phase Dynamos, 37
Slow Speed Motor, 271
Smooth -Core Armature, 33
— Armatures, 152
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Smootii-core Armature, Definition
of, 14
Solenoid, Closed Circnlar. 50
SoDTccs. Electromotive, 3
Sonth-Seeking Pole, m
Space for Armature Winding, 375
Sparking and Armature Reaction,
179-198
— at Commntator, CircumBtancea
— , Methods for Suppressing, 1S9
Sparkleas Commutation, Circum-
stances Favoring, 1S6
Specific Resistance. 48
Speed of Revolution, Effect of, on
E.M.F., 3
Starting and Reversing of Motors,
391-308
— Coil. 301
— Rheostats. agS
Stationary Motors, 373
Step-Down Transformers, 319
Step-Up Transformers, 319
Stream Lines, 30
Streaming-Ether Theory of Mag-
netism, 3Q
Structural M. H. P., 56
System. Three- Wire, z3i
Table of French Measures, 8
— of Hysteretic Activity, 175
— of Reluctivity Constants, 65
Telegraphic Dynamo, 36
Thennal Losses, 904
Three-phase Dynamos, 37
Three Phasers, 37
Three-Wire System, ssi
, Neutral Wire of, 331
Toothed-Core Armature, 33
, Definition of, 34
— Armatures, 153
Torque, Definition of, 351
— , Motor, 351-367
Transformation, Ratio of, 331
Transformers, Rotary, 318
— , Step-Down, 319
— , Step;Up,3i9
TranamisBion Circuits, Definition
of, I
Travelling Motors. 373
Triphase Dynamos, 37
Tripbasers, 37
Tubes of Magnetic Force, 35
Tuns, Armatare, Effect of, on
E. M. F., 3
Two-Phase Dynamos, 37
Two Phasers, 37
Uniform Magnetic Flux. 3;
Uniphase Alternators, 16
Unipolar Dynamos, 38, 334
Unit of Electric Flux, 49
Force, in C. G. S. System, 68
M. M. P., 40
Magnetic Flux, 49
Intenstty, 35
Reluctance, 49
Variations of Magnetic Flux, 33
Volt, Definition of, 49
Voltaic Analogue of Aero-Perric
Circuit, 69
— , Compound, of Dynamos, 30B
— , Disc Armature. 330
— for Armature, Inter-Connected,
145
Armatures, Lap, 155
Armature, Wave. 155
— of Gramme-Ring Dynamo, Cal-
culations of, 138-134
— , Shunt, of Dynamos, 307
— , Space, for Armature, 375
Wire, Armature, Effective Length
— , Idle, on Armature, 100
— , Neutral, ofThree-wire System,
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Elementary Electro-Technical Series.
EDWIN J. HOUSTON, Ph.D., .
A. E. KENNELLY, ScD.
AherutiaE Electric Cunvnts. Electric locandeMeot Ughtiag^
Electric HeaOat. Electric Motor.
Electrwn^pietUm. Electric Street itallways.
Electrldty In Electro-Therapeutics. Electric Telephony.
Electric Arc Uchtlns. Electric' Telegraphy.
Cloth. Price per volume, $1.00.
The poblication of (his series of eleinenta.ry electro- technical treatises
ea applied electricitjr has been undertaken to meet a demand which is
believed to exist on the part of the public and others for reliable informa-
tion regarding such matters in electricity as cannot be readily understood
bj those not specially trained in electro-technics. The general public,
students of elementary electricity and the many interested in the subject
from a financial or other indirect connection, as well as electricians desiring;
information in other branches than their own. will find In these works
precise and authoritative statements concerning the several branches ol
applied electrical science of which the separate volumes treat. The repu-
tation of the authors and their recognized abilities as writers, are a
sufficient guarantee for the accuracy and reliability of the statements con-
taiaed. The entire issue, though published in a series of ten volumes, ts
nevertheless so prepared that each book is complete In itself and can be
understood Independently of the others. The volumes are profusely illus-
trated, printed on a superior quality of paper, and handsomely bound in
covers of a special design.
Csfii a/tkit trmnj Mktr lUctrn
The W. J. Johnston Company, Publishers,
aS3 BROADWAY, NEW YORK.
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Etectricity and Magnetism.
A Series of Advanced Primers.
By EDWIN J. HOUSTON, Ph.D. (Princeton),
*'A dictionary of Electrical Words, Terms, and Phrases"
etc., etc., etc.
Ooth, 306 panes, 1X6 lUwilritUons. Price, $1.00.
During the Philadelphia Electrical Exhibition of 1884, Prof. Homton i»ued
a set Of elementary electrical primer* for the benefit of the vlsitora to the exhi-
iton, which attained a irkle popularity. Daring the last ten years, however,
the wlvancet in the applications of electricity have been so great and so wide-
spread that the public would no longer be satisfied with instruction in regard to
only the most obvious and simple points, and nccordingljr Che author has pre-
pared a set of new primers of a more advanced cbaracler as regards matter and
extent. The treatment, nevertheless, remains such that they can be easily un-
derstood by any one without a previous knowledjfe of electricity. Electricians
will find these primers of marked interest from their lucid explanations of prin-
dples, and the general public will find in them an easily read and agreeable
introduction to a fascinating subject. The first volume, as will be seen from
(he contents, deals with the theory and general aspects of the subject. As no
mathematics is used and the explanations are couched In the simplest terma,
this volume is an ideal first book from which to obtain ibe prelimiiuiry ideas
accessary for the proper understanding of more advanced works.
CONTENTS.
T. Effects of Electric Chargc.—II, laiuUlora and CoDdncton.— III. ESect* of u Electric
OischuKe— IV. Electric Souma.— V. Eleciro-reopclTc DcTiccB.—Vl. Blectiic CDrrcoE.—
VII. Bleetrk UnlU.-VIII. Elecirie Work and Power.-IX. Varietiei of Electric CinnlU.--
X. KacaetStm.— XI. Migoeiic InducttoD.— XII. Tlieoria of Migactiioi XIII. PtwBOBiena
of the Eanti'i UngDetiim— XIV. ElKlro-nuKneti.— XV. EleccmrUtlc InductloD.— XVI.
Frfctlodll ud Influent* M»chine».— XVII. AtmoipLcric Hlectriclty.— XVTII. Voltaic Celts—
XIX. Review : Primer of PrlmerB.
Cefit'ei eftldi <»■ any ollar electrical iooi publislad will U satt by maS, POSraOS
nilPAID, to any address in t^ ux/rld, on receipt a/price.
The W. J. Johnston Company, Publishers,
as3 BROADWAY, NEW YORK.
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THIRD EDITION. GREA TL Y ENLARGED.
A DICTIONARY OF
Electrical Words, Terms,
and Phrases.
By EDWIN J. HOUSTON, Ph.D. (Princeton),
Advanced Primers of Electricity ; Electricity One Htindred
Years Age and To-day, etc., etc., etc.
<Xolb. 007 large octavo pagea, S82 lUvatrations.
r^Hce, $5.00.
Some idea of the scope of (his important nork and of the Innnense amoont
of labor involved in il, may be formed when it is stated that it contains defini-
tions of about 6000 distinct words, terms, or phrases. The dictionary is not a
mere word-book ; the words, terms, and phrases arc invariably followed by a
short, concise definition, giving the sense in which they are correctly employed,
and a general statement of the principles o( electrical science on which the defi-
nition is founded. Each o( the great classes or divisions of electrical investiga-
tion or utilization comes under careful and exhaustive treatment ; and while
close attention is given to the more settled and hackneyed phraseology of the
older branches of work. Ihe newer words and the novel departments Ihey tielong
to are noi less thoroughly handled. Every source of informalion has been re-
ferred to. and while libraries have been ransacked, the note-book of the labora-
tory and the catalogue of the nareroom have not been forgoticn or neglected.
So far has the work been carried in respect to Ihe policy of inclusion that the
book has been brought down to date by means of an appendix, in which are
placed the very newest words, as well as many whose rareness of use had con-
signed them to obscurity and oblivion. As one feature, an elaborate system of
cross-references has been adopted, so that it is as easy to find Ihe definitions as
the words, and aliaits are readily delected and traced. The lypc^raphy is ex-
cellent, being large and bold, and so arranged that each word catches the eye at
a glance by standing out in sharp relief from the page.
Cvfaes 0/ t»is or any olhcr iltclrical bonk p«blisktd OiU ie smt if maU, FOST-
At>E PREPAID, to any addrtss in tht taorld, on rtctift i^prie*.
The W. J. Johnston Company, Publisliere,
1«3 BIHMOWAY, NBW YORK.
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The Measurement of Electrical Currents
and Other Advanced Primers
of Electricity.
By EDWIN J. HOUSTON, Ph.D. (Princeton),
"A Dictionary of Electrical Words. Terms, and Phrases,"
etc., etc., etc.
do£A. ^29 pages, 169 XUii^raUona. Frice, $1.00,
This volume is (he second of Prof. Houston's admirable series o[ AdvaHcrd
Primers of Eleitrieiiy. and is devoted to the measurenient and practical applica-
tions of the electric current. The difterent sources of eiectrlcily are taken up
in turn, the apparatus described nilh reference to commercial forms, and the
different systems of distribution explained. The sections on alternating cur-
rents will be found a useful iniioduction to a branch which is daily assuming
larger proportions, and which is here treated without (he use of mathematics.
An excellent feature of this series of primers is the care ot statement and logical
ireatmeni of the subjects. In this respect there is a marked contrast to most
given, to the exclusion or subordination of more Important ones. The ab-
stracts from standard electrical authors at the end of each primer have in gen-
eral reference and furnish an extension to some imporiant point in the primer,
and at the same time give the reader an iniroduction to electrical literature.
The abstracts have been chosen with care from authoritative professional sonrces
or from treatises of educational value in the various braoches.
CONTENTS.
I. The Manncment of Electric Correnti.— II. The Meaiuremmt of P.lectromotlTe Force.
—III. The Meaauremcnt of Electric RcBlsUaces.—IV. Voluic Cells.— V. Thermo- raeclric
Cell> aad Oll«r Electric Sources.— VI. The Diuribuiian ot Bleciricity by Coniianl Currenia.
VII. Arc^iBhting— VIII. iDcandescent Electiic4i([hlInK.— IX. Aliernitlng Current).- X.
Altenutlns-Caireal Dl»ributk».>-Xt. Rlectric Cnrrenu of High FrequcDcy.- XII. Eleciro-
Dynamic Inductian.- XIII. Induction Coila and Truuformers.—XIV. Dynsoio-Eiectric Ma-
chine..— XV Electro-Dynamia—XVL The Klectro-Motor.-XVII. The Electric Tranmis-
•ion of Power.-XVIII. Review: Primer of Primers.
Copies c/ this or any ether
P09TA0B PREPAID, to any address in the a
The W. J. Johnston Company, Publishers,
253 BROADWAY NEW YORK.
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THE
Electrical Transmission of Intelligence
AND OTHER ADVANCED PRIMERS
OP ELECTRICITY.
By EDWIN J. HOUSTON, Ph.D. (Princeton),
"A Dictionary of Electrical Words, Terms, and Pkrases^
etc., etc., etc.
Cloth, 330 pages, 88 lUvstrattoHa. Friee, $1.00.
The IhLrd and concluding Tolame of Prof. Hcmston's Series of Advanced
Primen of Electricity is devoted to the telegraph, telephone, and miscettancous
■ipplicalions of the electric current. In this volume (he difficult subjects of
Diulliple and cable telegraphy and electrolysis, as nellas the telephone, storage
battery, etc., are treated in a manner thai enables the beginner to easily grasp
the principles, and yet with no sacrifice in completeness of presentation. The
electric apparatus for use in houses, such as electric- bells, annunciators, Iher-
■uostals, electric locks, gas-lighting systems, etc., are explained and itlusiraied.
The primer on electro-therapeutics describes (he medical coil and gives insiruc-
bn man body. The interesting primers on cable telegraphy and on telephony
will be appreciated by those who wish to obtain a clear idea of the theory of
these attractive branches of electrical science and a knowledge of the details of
the Bppara[us. Attention is called to the fact that each of the primers in this
series is, as far as possible, complete in itself, and that there is ao necessary
connection beineen the several volumes.
CONTENTS.
I. The Eleitric TrutmilukRi of Intelligence.— II. The Electric Telegraph.— III. Hultipl*
Telegraphy.-IV. Cable Telegraphr.— V. Eleeuic Annuociatora and Alarms.— VI. Time
Tilegnipliy.-VII. Tbe Telephone. -VI 1 1. ElecIrolysii-IX. Eircwo-metallurgy.-X. Slor.
age or SecoDdsr7 Ballnia.- Xt. Electricftf in Warfare ; Electric Welding.— XII. Some
Oiber Application* o( Electricity.— XIII. Electro-tberapeutica.— XIV, Review! Primen of
Ca/iies of this or any alMer lUcMcal hooh pubtishtd will be tml by mail, POSib
AOE PREPAID, to any addrea in Ikt world, on rictipt pf frin.
The W. J. Johnston Company, Publishers,
au BROADWAY. NBW YORK.
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ELECTRICITY
ONE HUNDRED YEARS AGO AND TO-DAY.
By EDWIN J. HOUSTON, Ph.D. (Princeton),
A Dktionary of EUtirieal Words, Terms, and Phrases,
eU., etc., eh
dcth. 179 page*, ittustrated. Prtee, $lMO.
In tnciiiK the blstoir : f electrical tcience from pmctiMllr Its birtli to tb«
present day, the aaihor bai, vherever poMJble, consolud original (oarcct of
information. Ai a result of these researches, several revisloai aa to tbe dale of
discoverf of som? important principles Jo electrical science are made necessai;.
Wbile ibe compass of the book does not permit of anj other than a genera]
treatment of the subject, yet numerous references are Kiveu in foui-notes, which
also in many cases quote tbc woida -n which a discorcry was first announced
to the world, or give mote specific information in regard to the subjects men-
tioned in the main portion of the book. This feature is one of interest and
value, for often a. clearer idea may be obtained from the words of a discoverer
of a phenomenon or principle iban is possible through other sources. The
work is not a mere catalogue of subjects and dates, nor is it couched in tech-
nical language that only appeals to a few. On the contrary, one of lis most
admirable features is tbe agreeable siyle in which the work is written, its philo-
sophical discussion as lo the cause and effect of various discoveries, and iu
personal references to great names in electrical science. Much information as
to electrical phenomena may also be obtained from the book, aa the author
b not satisfied to merely give the history of a discovery, but also adds a concise
and clear explanlion of it,
Copits of Ihis or any elhtr iltclrical boat publiihid will bt itnl by mail, POST-
AOE PRBFAID, lo any addrta in tht aorti, oh rtctipt a/ prtix.
The W. J. Johnston Company, Publishers,
as3 BROADWAY, NEW YORK.
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THIRD EDITION. EXTENSIVELY REVISED ASD BS'LARGED.
THE ELECTRIC RAILWAY
IN THEORY AND PRACTICE.
By O. T. CROSBY and Dr. LOUIS BELL.
Large Octavo. ProfHuely lUuatrated. Price, $2.30.
Few techaicat books have met with so wide an appreciation as " The
Electric Railway," which has had the distinction of being accepted throiifi;h-
out the world as the standard authority on the subject of which it treats.
The advances in electric traction made since the second edition of the worl;
have been so notable, that the authors, in undertalting the preparation of a
new edition, found it necessary to practically rewrite the book, so that the
present edition conforms to the very latest knowledge on the subject, both
in the domain of theory and of practice. The original purpose oC the book
has, however, been strictly adhered to — namely, to place before those in-
terested in electric railways, whether in a technical, financial, or general
way, the principles upon which electric traction is founded and the standard
methods employed in their application. In view of the probable applica-
tion in the near future of alternating currents to electric traction, the
present edition includes their consideration in this relation, thus largely
extending the value of the treatise. The recent developments in electric
locomotives and high-speed electric traction, and the application of elec-
tricity to elevated railways and to passenger traffic on steam roads, are in
this work considered for the lirst time connectedly with reference to their
engineering and commercial aspects. In the first section of the book
are developed the fundamental principles of electricity upon which the
apparatus and methods of operation are based. The following section is
devoted toprime movers, steam, hydraulic, and gas — the modern gas-engine
here receiving the full treatment which its growing importance calls for.
The remainder of the work is devoted to the engineering, practical, and
commercial features of electric traction, all of the factors that enter being
considered from the standpoint of Che best practice, and the more impor-
tant ones elaborated in detail. The plan of the book, in fact, includes the
consideration of everything relating to the electrical and mechanical prin-
ciples and details which enter into electric railway design, construction,
and operation, the whole being treated from the engineering and com-
mercial standpoint, and without the use of mathematics or resort to purely
scientific theory.
Cpfit, 0/ l/iil nr mny elktr lltclrlcal hx-k pmMUkid n'ill i, sifil iy mnll. POITACI
The W. J. Johnston Company, Publishers,
253 BROADWAY, NEW VCHtK.
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Electrical Power Transmission.
By LOUIS BELL, Ph.D.
Uniform in size uHth " The Electric Railway in Theory
ana Practice. " Price, $2.50.
The plan of the work is similar to that of " The Electric Railway in
Theory and Practice," the irealmenl of the subject being non-mathematical
and not involving on the part of the reader a knowledge of the purely
scienlilic theories relating to electrical currents. The buok is essentially
practical in its character, and while primarily an engineering treatise, is
also intended fur the information of those interested in electrical trans-
mission of power, financially or in a general way. The author has a
practical acquaintance with the problems met with 'in the electrical trans-
mission of energy from his connection with many of the most important
installations yel made in America, and in these pages the subject is devel-
oped for the first lime with respect to its practical aspects as met with ia
actual work. The fir^l two chapters review the fundamenial principles
relating to the generation and distribution of electrical energy, and in the
three succeeding ones their methods of application with both continuous
and alternating currents are described. The sixth chapter gives a general
discussion of the methods of transformation, the various considerations
applying to converters and rotary transformers being developed and these
apparatus described. In the chapter en prime movers various forms of
water-wheels, gas and steam engines are discussed with respect lo their
applicability to the purpose in view, and in the chapter on hydraulic
development the limitations that decide (he commercial availability' of
water power for electrical transmission of power are pointed out in de-
tail. The five succeeding chapters deal with practical design and with
construction work— the potfer-house, line, and centres of distribution being
taken up in turn. The chapter on the latter subject will be found of par-
ticular value, as it treats for the first time in a thorough and practical
manner one of the most difficult points in electrical transmission. The
chapter on commercial data contains the first information given as to costs,
and will, therefore, be much appreciated by engineers and others in decid-
ing as to the commercial practicability of proposed transmission projects.
This is the first work covering the entire ground of the electrical trans-
mission of power that has been written by an engineer of wide practical
experience in all of the details included in the subject, and thus forms a
valuable and much-needed addition to electrical engineerl
KEHAID, r. a*/ adaria ra Ike mirld. Dn rtciifl if frUl.
The W. J. Johnston Company, Publishers,
353 BttOADWAY, NEW YORK.
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The Theory and Calculation of
Alter nating^Cur rent Phenomena.
CHARLES PROTEUS STEINMETZ.
Cloth, rrice, $2.iiO,
This is Ihe first work yet written in any language dealing ii
and logical manner with all of Ihe phenomena of allernaling i
Iheir calculation in the design of alternaling-curreni machinery. In the first
six chapters the various primary conceptions and methods of irealmenl arc
developed, the use of compleit quantities being explained in a remarltably clear
and eftccliwe manner. The various alternating-cur rem phenomena are then
taken up in turn and the more complex parts of ihc subject approached so
gradually and with such a logical preparation that but liiile if any difficulty will
be me; in their understanding. The remaining chapters of the book, forming
half of its contents, are devoted to the methods ol calculation of transformers,
simple alternating and polyphase generators and motors, all of the various
reactions involved being thoroughly analyied and discussed. The work con-
tains Ihe very latest knowledge relaiing to alternating-current phenomena,
much of which is original with the author, and here appears for the first time
in book form. The high authority of the author on the questions of which he
trcals, and the original methods which he pursues in Iheir exposition, give ihis
work a character which will assign it 10 a high place in electrical literature.
in which it promises to rank as a classic.
Cofiii of Ikit ar anj- ilher tirclrical iivi fmblillnd villi it ItKl itr mail, po»TAG«
The W. J. Johnston Company, Publishers,
353 BROADWAY. NEW YORK.
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Lessons in Electricity and Magnetism.
Prof. ERIC GERARD.
L'Jnstitut Ehctrotechnique Monte fiore, Univenily of Liege, Belgium.
TRANSLATED UNUER TUB IHRECTION OK
LOUIS DUNCAN, Ph.D.,
>*« lUfbin. UHi^r.,-!,.
with American Additions as follows : A Clupter on the Rotary Field,
by Dr. Louis Duncan ; A Chapter on Hysteresis, by Charles
Proteus Stelnmetz; A Chapter on Impedance, by
A. E. Kennelly ; A Chapter on Units, by Dr.
Cary T. ilutchlnson.
t^Mt,. Price, $2.50.
As a beaulifuU)' clear treatise for students on the theory of electricity and
magnelism, as well as a rtsumfi (or engineers of electrical theories that have a
practical bearing, the work of Professor Gerard has been vrithout a rival in any
language. As a text-book of reference it has been largely used in American
colle;;es, Ihe logical methods of the author and his faculty of lucid expression
and illustration simplifying to students in a remarkable manner the understand-
ing of the various subjects treated. The scope of the present translation has
been timtlcd to those parts of the original work trealing of theory alone, as the
practical portions would not have the same value for American students as for
those lo whom the book was originally addressed. In order to make it a
(realise comprehensive of all electrical theory having a beating on practical
work, and lo bring the subject-matter up to date, several chapters written by
American authors are added. As will be seen above, the authors of these chap-
ters are authorities on the several subjects with which they deal, and the work
as thus extended forma the most complete treatise yet published relating par-
ticularly to electrical theory as it enters into the domain of the engineer.
Copin of Ikil or any ttktr tltclrical intk ftiblilktd mill it umI if mail. rosTACl
The W. J. Johnston Company, Publishers,
a53 BROADWAY, NEW YORK.
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PRACTICAL CALCULATION
Dynamo=Electric Machines.
A MANUAL FOR ELECTRICAL AND MECHANICAL EN-
GINEERS, AND A TEXT-BOOK FOR STUDENTS
OF ELECTRO-TECHNICS.
ALFRED E. WIENER, E.E., M.E..
Member cf Ibe American Inititulc of Electrical Enpmeers.
Cloth, niustrated. Price, $2.S0.
Based upon the practical data and tests of nearly two hundred of the
best modern dynamos, including the machines used at the recent World's
Fair and those in the largest and most modern central -stations, a complete
and entirely practical method of dynamo-calculations is developed. Differ-
ing from the tlsual text-book methods, in which the application of the vari-
ous formulas requires more or less experience in dynamo-design, the present
treatise gives such practical information In the farm of original tables and
(ortnulas derived from the result of practical machines of American as well
fts European make, comprising all the usual types of field-magnets and
•rtnstures, and ranging through all commercial siies. The book contains
nearly a hundred of such tables, giving the values of the various constants,
etc., which enter into the formulas of dynamo-design, and for all capacities,
from one-tenth to jooo kilowatts, for high and slow speed, for bipolar and
multipolar fields, and for smooth and toothed drum and ring armatures.
Although intended as a text-book tor students and a manual for practical
dynamo-designers, any one possessing but a fundamental knowledge of
algebra will be able to apply the information contained in the book to the
calculation and design of any kind of a continuous-current dynamo.
Copies of tkii or any ether electrical book published leiU bi sent fiyn
FREFAID, to any addresi in the viorld on receipt of the price.
The W. J. Johnston Company, Publlsbers,
953 BROADWAY, NEW YORK.
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THIRD EDITION.
ALTERNATING CURRENTS.
An Aoalytkal and Oraphlul Treatment for 5tiideiits
aod Enjclaeers.
By FREDERICK BEDELL, Ph.D^
AND
A. C CREHORE, Ph.D.
0,0^. 32S pagea, 112 lUwArationn. Prioe, $9.BO.
The present woik \a the first book thai treats the subject of alternating cur-
rents in a connected, logical, and complete manner. The principles are gradn-
alt; and logically developed from the eletneniaiy experiments upon which they
are based, and in a manner so clear and simple as to make the book easily read
by any one having even a limited knowledge oi the mathematics involved. Hy
this method the student becomes faTnillar with every step of the process of
development, and the mysteries usually associated with the theory of alternat-
ing currents are found to be rather the restilt of unsatisfactory treatment than
due to any inherent difficulty. The first fourteen chapters contain the analytical
development, commencing with circuits containing resistance and self-induc-
tion only, resistance and capacity only, and proceeding to more complex cir-
cuits containing resistance, self-induction, and capacity, and resistance and dis-
tributed capacity. Afeature is the numerical caiculationit given as illustrations.
The remaining chapters are devoted to the graphical conEideration of the same
subjects, enabling a reader niih tittle maSiematical knowledge to follow the
authors, and with extensions to cases that are belter treated by the graphical
than by the analytical method.
CONTEP^TS.
Part I. Aoaljilkal Tmtmmt.— Chapwr I. lalrodactory to ClrcolU CantalalnB Re-
(iaiance and Self-indnction.-Chapler II. On Hinnoaic PuacUoot.-CbBpter III- CirruiU
Containing Reiiitance and Self-induction.— Chapter IV, toifaductDry to Circuits ContaininE
RcRiitance and Capncilf.— Chapter V. Cirenlu ConUlnlnR Reilitaace and Capacity.— Chap-
Icrs VI, VII, VIII, IX, X, XI. Circuits CoalalnlnK ResfaUnce, Self-induction, and Capacity.
-Chaplera XII, XIII. Circuits Contaiolng Dl«trlbut«l Capacilir and Self-induction.
Part II. ar^hleal TrMtmcnt.- Chapten XIV, XV, XVI. XVII. ClrcuiU Contalnins
Resisunce and Se)[-indiictlon.—C hapten XVIII. XIX. Circuits ContalnlnK RslKance and
Capacity -Chapter XX. Circuiia ConUininj Resisunce, Self-lnducilon, and Capicily.
Aptwndix A. Relation between Practical and C. G. S. Limita.— Appendix B. Some He-
ll Aoaloslet.— Appendix C System of Koutfcw Adopted.
iai7,
The W. J. Johnston Company, Publishers,
253 BROADWAY, NBW YORK,
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Continuous Current
Dynamos and Motors:
THEIR THEORY, DE5I0N, AND TESTING.
WltIiSK:tk«
AN ELEMENTARY TREATISE FOR STUDENTS.
By FRANK P. COX, B.S.
CMh. 271 pagtM, 83 lUuatroHons, Price, $2.00.
Intended for siudenls, this work gives Lhe Lheory and design of conlinuoua
current dynamos and motors as understood and praciisrd in the designing-
room, and the methods ol testing described are those of the factory tcsling-shop.
The practical side of the various questions treated is alnays kept in view, dis-
cussions having liitle bearing in Ibis direction being excluded, and the higher
branches ol mathematics avoided. The application to the design of armatures,
field-magnets and motors of the principles developed is explained by refer,
ence 10 numerical problems, thus thoroughly impressing them on the student's
mind. Methods of testing a complete tnachine and of investigating its char-
acteristics are given, with discussions on the effect of various changes in
design and operalion. The steam-engine being so closely allied to the testing
and operation of dynamos and motors, sections on indicator diagrams, steam-
power calculations, and belting are included. Almost all of the numerous
curves in the book are derived from actual commercial tests.
CONTENTS.
Chapter I. The AtMolute STnem of Meuurenunt. —Chapter II. Slectro - magnette
Inducllan. -Chapter III. CIuMllicadaa of Michin« and General Frinclpla of the Magnetk:
CircuiL-Chapier IV. The DyoaiBo ■■ ■ Moior.-Chapter V, CalcuUtions Pertaining ta
Ilie Magnetic Circuit.— Chapter VI. Tbeory of Windings, Lona, elc.-Cbapier VII. The
DynamoCoDSideredBiaMotor.— CbuplerVIII. Design ol ArmatUTcs.— Chapter IX. Design
o( Field-magneta, — Chapter X. Design ol Motors. — Thapler XI, Dynamo and Motor
Tealing.— Chapter XII. Efficiency Tnti.— Chaptei XIII. Indicator Diigratni.- Chapter
ilV. Sieam-enKlDe CalcnUlioni. — Appendbc f. Tests of Iron.— Append ii II, Ampera
Turntables.— Appendix III. Determioailon <if Sliea of Wire for Armatures and Field Coils.
—Appendix IV. Belling.
Capits ef tkii or any Bthtr eltciricat booh publiskti taill be stnt by mail, POSTAOB
pltEPAlD, (a any addrtsi in tit aor/d en riceift 0/ tJu prict.
The W. J. Johnston Company, Publishers,
253 BROADWAY, NEW YORIf.
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ALTERNATING CURRENTS
OF ELECTRICITY:
Hielr Oenerstion, Measurement, DiBtribntlon, ud AppUcatloB.
Bv GISBERT KAPP, M.I.C.E., M.I.E.E.
iAulkBrlitJ Amtriean Edilitu.)
With on Introduction by WlUlam Stanley, Jr.
Clotli. 104 poge», 37 lUustrations, and 2 plates. $1.00.
The rapid development of alLernating currents and the great part the/ are
destined to play in the transmission of power have caused an increased intereu
in the subject, but unfortunately it has heretofore been presented in such a
manner as to be beyond Ibe reach of readers without a mathematical education.
In Ibe present work, the principles are developed in a simple manner that can
be followed by any reader, and the various applications ate sketched in a broad
and instructive way that renders their understanding an easy task. Tbe few
mat be mat ica I formulas in the book are confined to appendices. Several chap-
ters treat nf various forms of allernating motors, especial attention being paid
to the explanation and discussion of multiphase motors. This difficult subject
is treated so lucidly that the reader is enabled to form as clear an idea of these
new forms o( motors as of the simpler continuous current machines. The
treatment throughout is thoroughly practical, and the data and discussion on
the design and construction o! apparatus are invaluable to tbe electrician and
designer. To the student and the general public this work will be e patltctilftr
boon, bringing within their grasp a subject of the greatest imponance and
CONTENTS.
iDlrodDction by WIIKatn Stanle)', Jr.— Chap, I. Introductory. — Chap. II. Measnremeni at
PrcHure, Curreat.aad Power.-Cbap. III. CoaditioM o( Maximum Power.— Clwp. IV. Alter,
nalins-current Machinu.- Chap. V. Mechanical CDniiniction of AlcernaiorB.- Chap. VI.
Deseriplion of Some Alteraalom— Chap. VII. Tran^ormen.— Chnp. VIII. Central Suiions
and Dittribulion of Power.— Chap. IX. Eximplei o( Ceoiral Slatlons.-Chap. X. Parallel
Coupling of Alternilari.— Chap, XI. AltetnatinK-current M atari.— Cbap. XXl. Self^tartioi
Hoion— Chap. ZIII. Hulllphaie Currents.
Ctfiia of this er any ethtr electrical book published will be sent by mail, POSiAas
PHEPAID, fo any adireis in tki world, oh receipt of price.
The W. J. Johnston Company, Publishers.
9ii BROADWAY, NEW YORK.
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Dynamo and Motor Building
KOR AMATEURS.
WITH WORKINQ DRAWINGS.
By Lieut. C. D. PARKHURST, U.S.A.
Goth. 163 pageB, 71 lUiutrtUiona. Price, 91.00.
One of the most faicinaling RcMi for the amateur ia Ihat afforded by elec-
trical science, and the simplicitr of construction of small dynamos and motors,
in particular, enables him not onljr Co gratify his tastes, but at the same time to
construct apparslui thai can be directly applied lo useful purposes. In Parlc-
hursi's Dynamo and Motor Building for Amateurs clear and concise instruc-
tions. Bccompan^ed by norking drawings, are given for Che Construction of such
forms and types of dynamos and motors as are simply made and yec nill
produce fairly efficient results. While primarily intended for amateurs and
xiudents, the detailed information, particularly in the chapters on armature
windings, connections, and currents, and on the design of a lifLy-light dynamo,
win be of value to every electrician. In the latter chapter the subject of the
proper proportioning of the armature and armature wire, the calculation of the
magnetic circuit and field-windings, etc., are gone into at length, and in the
light of the most recent knowledge and practice. The large and clear drawings
showing how to wind armatures are supplemented by tables, so ihat ihe
beginner will have n.i difficulty whatever in carrying out the insiructiona.
Every part of the machines, even the most simple, Is illustrated and marked
CONTENTS.
Chapter 1. A Small Electric Motor (or Amateurs.— Chapter II. A "Home-maiJe"
Electric Hotor.—CbapCer III. A Sewing.iniicbioe Motor lor Amateurs.— Chapter IV. Atniii. '
lure Wiodlngl, ConnectioDi, and Currents. — Chapter V. A Pifly-light IncandcKeDl
Dynamo.^Appendix. Giving Data of Modern High-class Motors and Dynamos of Standard
Pirm> and Makers.
Cepiti ef this er of any tltetrieal boot published tcill bt stnt by mail, postaok
PREPAID, to any addrtu in Ikf mortd, an rtcdpl o/fnie.
The W. J. Johnston Company^ Publishersi
353 BROADWAY, NEW YOIUC ^
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