Hawkins Electrical Guide v. 06 (of 10)
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Title: Hawkins Electrical Guide v. 06 (of 10)
Author: N. Hawkins
Release date: June 21, 2015 [eBook #49252]
Most recently updated: October 24, 2024
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK HAWKINS ELECTRICAL GUIDE V. 06 (OF 10) ***
THE THOUGHT IS IN THE QUESTION THE INFORMATION IS IN THE ANSWER
[Illustration:
HAWKINS
ELECTRICAL GUIDE
NUMBER
SIX
QUESTIONS
ANSWERS
&
ILLUSTRATIONS]
A PROGRESSIVE COURSE OF STUDY
FOR ENGINEERS, ELECTRICIANS, STUDENTS
AND THOSE DESIRING TO ACQUIRE A
WORKING KNOWLEDGE OF
ELECTRICITY AND ITS APPLICATIONS
A PRACTICAL TREATISE
by
HAWKINS AND STAFF
[Illustration: THEO. AUDEL & CO. 72 FIFTH AVE. NEW YORK.]
COPYRIGHTED, 1914,
BY
THEO. AUDEL & CO.,
NEW YORK.
Printed in the United States.
TABLE OF CONTENTS
GUIDE NO. 6.
~ALTERNATING CURRENT MOTORS~ 1,267 to 1,376
Classification--~synchronous motors~--essential
parts--_synchronous motor principles:_ condition for
starting; effective pressure; dead centers; speed; limit
of lag; effect of load changes--effect of altering the
field strength--disadvantages of synchronous motors;
advantages--~the "V" curve~--adaptation--efficiency--~hunting
of synchronous motors;~ mechanical analogy--use as
condenser--surging--characteristics of synchronous motors:
starting; running; stopping; effect upon circuit; power
factor; auxiliary apparatus; adaptation--~induction
(asynchronous) motors~--essential parts--types--_oscillating
magnetic field--rotating magnetic field_--operation of
single phase motor; why not self starting; provision for
starting--operation of polyphase induction motor; why called
asynchronous--speed; classification according to speed--the
terms _primary_ and _secondary_--why polyphase induction
motors are explained before single phase--~polyphase induction
motors~--features--essential parts--_principles_--production
of rotating field--Tesla's rotating field--method of
obtaining resultant flux of Tesla's field--_Arago's
rotations;_ explanation--Faraday's experiment--production
of two phase rotating field; resultant poles--six and
eight pole two phase rotating fields--_physical conception
of two phase rotating field_--production of three phase
rotating field; with ring winding--_physical conception
of three phase rotating field_--three phase six pole
winding--~slip~--_copper cylinder illustrating principle of
operation of induction motor_--calculation of slip--table
of synchronous speeds--variation of slip; why so small;
variation with load; table of variation--sector method of
measuring slip--_evolution of the squirrel cage armature;_
construction--~the field magnets;~ parts; construction--~field
windings for induction motors~--calculation for
revolutions of rotating field; objection to high speed of
field--difficulty with low frequency currents--general
character of field winding--formation of poles--grouping of
coils--_starting of induction motors:_ external resistance,
auto-transformer, internal resistance methods--~internal
resistance induction motors;~ adaptation--how resistance
is cut out--why not desirable for large sizes--~external
resistance or slip ring motors~--operation--armature
connections--~single phase induction motors~--service
suitable for--disadvantage--parts--why not self-starting--how
started--~phase splitting; production of rotating field
from oscillating field~--methods--starting coils--~shading
coils~--character of the starting torque--modification of
armature for starting with heavy load--clutch type of single
phase induction motor; its action in starting--~commutator
motors~--classification--_action of closed coil rotating
in alternating field_--the transformer pressure--generated
pressure--self-induction pressure--local armature currents;
reason for sparking; how reduced--high resistance
connectors--effect of low power factor--effect of
frequency--~series motor~--features--adaptation--~neutralized
series motor~--conductive method--inductive method--~shunt
motors--repulsion motors~--difficulty with early motors--means
employed to stop sparking--~essentials of single repulsion
motors~--the term repulsion induction motor--compensated
repulsion motor--~power factor of induction motors~--its
importance--false ideas in regard to power factor--~speed and
torque of motors~.
~TRANSFORMERS~ 1,377 to 1,456
Their use--essential parts--~basic principles~--the primary
winding--the secondary winding--_magnetic leakage_--~the
induced voltage~--no load current--magnetizing current--action
of transformer with load--classification--~step up
transformers~--use--construction--copper economy--~step
down transformers~--use--construction--~core
transformers~--construction--advantages--~shell
transformers~--comparison of core and shell
types--choice--~combined core and shell transformers~--economy
of construction--~single and polyphase transformers~--features
of each type--choice of types for polyphase
currents--operation of three phase transformer with one phase
damaged--~transformer losses~--~hysteresis~--what governs
the loss--how reduced--~eddy currents~--lamination--thickness
of laminæ--importance of iron losses--how to reduce
iron losses--~copper losses~--how caused--effect
on power factor--effect of resistance--~cooling of
transformers~--cooling mediums employed--heating of
transformers--objection to heating--~dry transformers~--~air
cooled transformers~--natural draught type--forced
draught or air blast type--construction of coils for air
cooling--requirements with respect to air supply--quantity
of air used--~oil cooled transformers~--circulation of
the oil--action of the oil--objection to oil--kind of
oil used--oil requirements--moisture in oil--~water
cooled transformers~--internal coil type--external coil
type--_thermo-circulation_--quantity of circulating
water required--~transformer insulation~--the
~"major"~ and ~"minor" insulation~--mica--outdoor
transformers for irrigation service--oil insulated
transformers--efficiency of transformers--efficiency
curve--~all day efficiency of transformers~--transformer fuse
blocks--auto-transformers--~constant current transformers
for series arc lighting~; elementary diagram illustrating
principles--regulation--transformer connections--single phase
connections--combining transformers--precautions--operating
secondaries in parallel--connections for different
voltages--precautions--two phase connections--~three phase
connections:~ delta, star, delta star, star-delta--comparison
of star and delta connections--three phase
transformers--comparison of air blast, water cooled, and oil
cooled transformers--standard transformer connections--~how
to test transformers~--transformer operation with grounded
secondary--transformer capacity for motors--transformer
connections for motors--arc lamp transformer--transformer
installed on pole--static booster or regulating transformer.
~CONVERTERS~ 1,457 to 1,494
Where used--kinds of converter--A.I.E.E.
classification--~rotary
converters~--operation--speed--principles--relation
between input and output pressures--single and polyphase
types--advantage of polyphase converters--armature
connections of polyphase converter--pressure
relation--voltage variation--advantage of unity power
factor--effect of field too strong--~compounding of
rotary converters~--~ratio of conversion~--~voltage
regulation~--split pole method--regulating pole method--best
location of regulating poles--~reactance method~--~multi-tap
transformer method~--~synchronous booster method~--winding
connections--field connections--adaptation--~motor generator
sets~--classification--standard practice--behavior of
rotary when hunting; comparison with motor generator
sets--racing--~frequency changing sets~--~parallel operation
of frequency changers~--~cascade converter~--speed--action in
motor armature winding--advantages--how started--comparison of
cascade converter with synchronous converter.
~RECTIFIERS~ 1,495 to 1,530
Classification--~mechanical rectifiers~--essential
features--construction--application--~electrolytic
rectifiers~--principles of operation--Mohawk rectifier--the
term "_valve_"--metals for electrodes--electrolyte--Nodon
valve--Audion valve--Buttner valve--Churcher valve--De Faria
valve--Fleming oscillation valve--Grisson valve--Pawlowski
valve--Giles electric valve--Buttner valve--~mercury vapor
rectifiers~--principles--the terms "arc" and "vapor"--three
phase mercury vapor rectifier--construction--auxiliary
apparatus--series mercury arc rectifier--dissipation
of heat from bulb--replacement of bulb--advantages of
rectifier--precautions in installing--~electromagnetic
rectifiers~--construction and operation.
CHAPTER LI
ALTERNATING CURRENT MOTORS
The almost universal adoption of the alternating current system of
distribution of electrical energy for light and power, and the many
inherent advantages of the alternating current motor, have created the
wide field of application now covered by this type of apparatus.
As many central stations furnish only alternating current, it
has become necessary for motor manufacturers to perfect types of
alternating current motor suitable for all classes of industrial
drive and which are adapted for use on the kinds of alternating
circuit employed. This has naturally resulted in a multiplicity of
types and a classification, to be comprehensive, must, as in the
case of alternators, divide the motors into groups as regarded from
several points of view. Accordingly, alternating current motors may be
classified:
1. With respect to their principle of operation, as
_a._ SYNCHRONOUS MOTORS;
_b._ ASYNCHRONOUS MOTORS:
~1. Induction motors;~
{_series;_
~2. Commutator motors~ {_compensated;_
{_shunt;_
{~repulsion~.
2. With respect to the current as
_a. Single phase;
b. Polyphase;_
[Illustration: FIGS. 1,585 to 1,588.--Synchronous motor principles: I.
_A single phase synchronous motor is not self-starting._ The figures
show an elementary alternator and an elementary synchronous motor,
the construction of each being identical as shown. If the alternator
be started, during the first half of a revolution, beginning at the
initial position ABCD, fig. 1,585, current will flow in the direction
indicated by the arrows, passing through the external circuit and
armature of the motor, fig. 1,586, inducing magnetic poles in the
latter as shown by the vertical arrows. These poles are attracted
by unlike poles of the field magnets, which tend to turn the motor
armature in a counter-clockwise direction. Now, ~before~ _the torque
thus set up has time to_ ~overcome the inertia of the motor armature~
_and cause it to rotate, the alternator armature has completed the
half revolution,_ and beginning the second half of the revolution, as
in fig. 1,587, _the current is reversed_ and consequently _the induced
magnetic poles in the motor armature are reversed also_. This tends to
rotate the armature in the reverse direction, as in fig. 1,588. _These
reversals of current occur with such frequency that the force_ ~does
not act long enough~ _in either direction_ ~to overcome the inertia~
_of the armature; consequently it remains at rest_, or to be exact,
~it vibrates~. Hence, a single phase synchronous motor must be started
by some external force and brought up to a speed that gives the same
frequency as the alternator before it will operate. A single phase
synchronous motor, then, is not self-starting, which is one of its
disadvantages; the reason it will operate after being speeded up to
synchronism with the alternator and then connected in the circuit is
explained in figs. 1,589 to 1,592.]
3. With respect to speed, as
_a._ Constant speed;
_b._ Variable speed.
4. With respect to structural features, as
_a._ Enclosed;
_b._ Semi-enclosed;
_c._ Open;
_d._ Pipe ventilated;
_e._ Back geared;
_f._ Skeleton frame;
_g._ Riveted frame;
_h._ Ventilated; etc.
Of the above divisions and sub-divisions some are self-defining and
need little or no explanation; the others, however, will be considered
in detail, with explanations of the principles of operation and
construction.
~Synchronous Motors.~--The term "synchronous" means _in unison_, that
is, _in step_. A so called synchronous motor, then, as generally
defined, _is one which rotates in unison or in step with the phase of
the alternating current which operates it_.
_Strictly speaking, however, it should be noted that this
condition of operation is only approximately realized as will
be later shown._
Any single or polyphase alternator will operate as a synchronous
motor when supplied with current at the same pressure and frequency
as it produces as a generator, the essential condition, in the case
of a single phase machine, being that it be speeded up to so called
synchronism before being put in the circuit.
In construction, synchronous motors are almost identical with the
corresponding alternator, and consist essentially of two elements:
1. An armature,
2. A field.
[Illustration: FIGS. 1,589 to 1,592.--Synchronous motor principles: II.
_The condition necessary for synchronous motor operation is that the
motor be speeded up until it rotates in synchronism, that is, in step
with the alternator._ This means that the motor must be run at the same
frequency as the alternator (not necessarily at the same speed). In the
figures it is assumed that the motor has been brought up to synchronism
with the alternator and connected in the circuit as shown. In figs.
1,589 and 1,590 the arrows indicate the direction of the current for
the armature position shown. The current flowing through the motor
armature induces magnetic poles which are attracted by the field poles,
thus producing a torque in the direction in which the armature is
rotating. After the alternator coil passes the vertical position, the
current reverses as in fig. 1,591, and the current flows through the
motor armature in the opposite direction, thus reversing the induced
poles as in fig. 1,592. _This brings_ ~like~ _poles near each other,
and since the motor coil has rotated beyond the vertical position_ ~the
repelling action~ _of the_ ~like poles~, _and also_ ~the attraction of
unlike poles~, _produces a torque acting in the direction in which the
motor is rotating_. Hence, when the two armatures move synchronously,
the torque produced by the action of the induced poles upon the field
poles is always in the direction in which the motor is running, and
accordingly, tends to keep it in operation.] either of which may
revolve. The field is separately excited with direct current.
[Illustration: FIGS. 1,593 and 1,594.--Synchronous motor principles:
III. _The current which flows through the armature of a synchronous
motor is that due to the_ ~effective~ _pressure_. Since the motor
rotates in a magnetic field, a pressure is induced in its armature in a
direction opposite to that induced in the armature of the alternator,
and called the _reverse pressure_, as distinguished from the pressure
generated by the alternator called the _impressed pressure_. _At any
instant, the pressure available to cause current to flow through
the two armatures, called the_ ~effective pressure~, _is equal to
the difference between the pressure generated by the alternator or_
~impressed pressure~ _and the_ ~reverse pressure~ _induced in the
motor_. Now if the motor be perfectly free to turn, that is, without
load or friction, the reverse pressure will equal the impressed
pressure and no current will flow. This is the case of real synchronous
operation, that is, not only is the frequency of motor and alternator
the same, but the coils rotate without phase difference. In figs. 1,593
and 1,594, the impressed and reverse pressures are represented by the
dotted arrows Pᵢ and P_{_r_}, respectively. Since in this case these
opposing pressures are equal, the resultant or effective pressure is
zero; hence, there is no current. _In actual machines this condition is
impossible_, because even if the motors have no external load, there
is always more or less friction present; hence, in operation there
must be more or less current flowing through the motor armature to
induce magnetic poles so as to produce sufficient torque to carry the
load. The action of the motor in automatically adjusting the effective
pressure to suit the load is explained in figs. 1,595 and 1,596.]
The principles upon which such motors operate may be explained by
considering the action of two elementary alternators connected in
circuit, as illustrated in the accompanying illustrations, one
alternator being used as a generator and the other as a synchronous
motor.
Suppose the motor, as in figs. 1,585 and 1,586, be at rest when it is
connected in circuit with the alternator. The alternating current will
flow through the motor armature and produce a reaction upon the field
tending to rotate the motor armature first in one direction, then in
another.
[Illustration: FIGS. 1,595 and 1,596.--Synchronous motor principles:
IV--_A synchronous motor adjusts itself to changes of load by changing
the phase difference between current and pressure._ If there be no
load and no friction, the motor when speeded up and connected in the
circuit, will run in true synchronism with the alternator, that is, at
any instant, the coils A B C D and A°B°C°D° will be in parallel planes.
When this condition obtains, no current will flow and no torque will
be required (as explained in figs. 1,593 and 1,594). If a load be put
on the motor, the effect will be to cause A°B°C°D° to lag behind the
alternator coil to some position A"B"C"D" and current to flow. The
reverse pressure will lag behind the impressed pressure equally with
the coil, and the current which has now started will ordinarily take an
intermediate phase so that it is ~behind~ _the impressed pressure but_
~in advance~ _of the reverse pressure_. These phase relations may be
represented in the figure by the armature positions shown, viz.: 1, the
synchronous position A°B°C°D° representing the impressed pressure, 2,
the intermediate position A'B'C'D', the current, 3, the actual position
A"B"C"D" (corresponding to mechanical lag), the reverse pressure.
From the figure it will be seen that the current phase represented
by A'B'C'D' is in advance of the reverse pressure phase represented
by A"B"C"D". Hence, by ~armature reaction~, _the current leading
the reverse pressure weakens the motor field and reduces the reverse
pressure_, thus establishing equilibrium between current and load.
As the load is increased, the mechanical lag of the alternator coil
becomes greater and likewise the current lead with respect to the
reverse pressure, which intensifies the armature reaction and allows
more current to flow. In this way equilibrium is maintained for
variations in load within the limits of zero and 90° mechanical lag.
The effect of armature reaction on motors is just the reverse to its
effect on alternators, which results in marked automatic adjustment
between the machines especially when a single motor is operated from
an alternator of about the same size. In other words, the current
which weakens or strengthens the motor field, strengthens or weakens
respectively the alternator field as the load is varied.]
_Because of the very rapid reversals in direction of the torque thus
set up, there is not sufficient time_ ~to overcome the inertia of
the armature~ _before the current reverses and produces a torque in
the opposite direction_, _hence, the armature remains stationary or,
strictly speaking_, ~it vibrates~.
[Illustration: FIGS. 1,597 and 1,598.--Synchronous motor principles:
V. _The effectiveness of armature reaction in weakening the field
is proportional to the_ ~sine of the angle~ _by which the current
lags behind the impressed pressure._ If a motor be without load or
friction, its armature will revolve synchronously (in parallel planes)
with the alternator armature. In the figures let ABCD represent an
instantaneous position of the motor armature when this condition
obtains; it will then represent the phase relationship of impressed
and reverse pressures for the same condition of no load, no friction,
operation. _Now_, ~if a light load be placed on the motor~ for the
same instantaneous position of alternator armature, ~the motor coil
will drop behind to some position as~ A", fig. 1,597 (part of the
coil only being shown). The reverse pressure will also lag an equal
amount and its phase with respect to the impressed pressure will be
represented by A". The armature current will ordinarily take an
intermediate phase, represented by coil position A'B'C'D', inducing a
field strength corresponding to the 9 lines of force OF, O'F', etc.
_The current being_ ~in advance~ _of the phase of the reverse pressure_
A", _the armature reaction_ ~weakens~ _the field_, thus reducing the
reverse pressure and allowing the proper current to flow to balance
the load. The amount by which the field is weakened may be determined
by resolving the induced magnetic lines OF, O'F', O"F", etc., into
components OG, GF, O'G', G'F', O"G", G"F", etc., respectively
parallel and at right angles to the lines of force of the main field.
Of these components, the field is weakened only by OG, O'G', O"G",
etc. Since by construction, angle OFG = AOA', and calling OF unity
length, OG = sine of angle by which the current lags behind the
impressed pressure. The construction is shown better in the enlarged
diagram. For a heavier load the armature coil will drop back further to
some position as A"', fig. 1,598, and the lag of the current increase
to some intermediate phase as A"B"C"D". By similar construction it
is seen that the component OG (fig. 1,597) has increased to OJ (fig.
1,598), this component thus further weakening the main field, by an
amount _proportional to_ ~the sine of the angle~ _by which the current
lags behind the impressed pressure_. The increased current which is
now permitted to flow, causes the induced field to be strengthened
(as indicated by the dotted magnetic lines M, M', M", etc.), thus
increasing the torque to balance the additional load.]
Now if the motor armature be first brought up to a speed corresponding
in frequency to that of the alternator before connecting the motor in
the circuit, the armature will continue revolving at the same frequency
as the alternator.
The armature continues revolving, because, ~at synchronous speed~,
_the field flux and armature current are always in the same relative
position_, producing a torque which always pulls the armature around in
the same direction.
A polyphase synchronous motor is self starting, because, before the
current has died out in the coils of one phase, it is increasing in
those of the other phase or phases, so that there is always some
turning effort exerted on the armature.
The speed of a synchronous motor is that at which it would have to run,
if driven as an alternator, to deliver the number of cycles which is
given by the supply alternator.
[Illustration: FIGS. 1,599 and 1,600.--Synchronous motor principles:
VI. _A single phase synchronous motor has_ "~dead centers~," _just the
same as a one cylinder steam engine_. Two diagrams of the motor are
here shown illustrating the effect of the current in both directions.
When the plane of the coil is perpendicular to the field, the poles
induced in the armature are parallel to field for either direction of
the current; that is to say, the field lines of force and the induced
lines of force acting in parallel or opposite directions, no turning
effect is produced, just as in analogy when an engine is on the dead
center, the piston rod (field line of force) and connecting rod
(induced line of force) being in a straight line, the force exerted by
the steam on the piston produces no torque.]
For instance a 12 pole alternator running at 600 revolutions per
minute will deliver current at a frequency of 60 cycles a second; an
8 pole synchronous motor supplied from that circuit will run at 900
revolutions per minute, which is the speed at which it would have to be
driven as an alternator to give 60 cycles a second--the frequency of
the 12 pole alternator.
[Illustration: FIGS. 1,601 to 1,604.--Synchronous motor principles VII.
_An essential condition for synchronous motor operation is that the
mechanical lag be less than 90°._ Figs. 1,601 and 1,602 represent the
conditions which prevail when the lag of the motor armature A'B'C'D' is
anything less than 90°. As shown, the lag is almost 90°. The direction
of the current and induced poles are indicated by the arrows. _The
inclination of the motor coil is such that_ ~the repulsion of like
poles~ _produces a torque_ ~in the direction of rotation~, thus tending
to keep motor in operation. Now, in figs. 1,603 and 1,604, for the same
position of the alternator coil ABCD, _if the lag be greater than_
90°, _the inclination of the motor coil_ A'B'C'D' _is such that at
this instant_ ~the repulsion of like poles~ _produces a torque in a
direction_ ~opposite~ _to that of_ ~the rotation~, _thus_ ~tending to
stop~ _the motor_. In actual operation this quickly brings the motor
to rest, having the same effect as a strong brake in overcoming the
momentum of a revolving wheel.]
[Illustration: FIGS. 1,605 to 1,608.--Synchronous motor principles:
VIII. _If the torque and current through the motor armature be kept
constant, strengthening the field will increase the mechanical lag, and
the lead of the current with respect to the reverse pressure._ In the
figures, let A be an instantaneous position of the alternator coil,
A°, synchronous position of motor coil, A', position corresponding to
current phase, A", actual position or mechanical lag of motor coil
behind alternator coil necessary to maintain equilibrium. In fig.
1,606, let A' and A" represent respectively the relation of current
phase and mechanical lag _corresponding to a_ ~certain~ _load and field
strength_. For these conditions OG, O'G', O"G", etc., will represent
the components of the induced lines of force in opposition to the
motor field, that is, _they indicate the intensity of the armature
reaction at the instant depicted_. Now, assume the field strength to be
~doubled~, as in fig. 1,608, _the motor_ ~load~ _and_ ~current~ _being
maintained_ ~constant~. Under these conditions, the armature reaction
must be doubled to maintain equilibrium; that is, the components OG,
O'G', etc., fig. 1,608, must be twice the length of OG, O'G', etc.,
fig. 1,605. Also since the current is maintained constant, the induced
magnetic lines OF, O'F' are of same length in both figures. Hence, in
fig. 1,608 the plane of these components is such that their extremities
touch perpendiculars from G, G', etc., giving the other components FG,
F'G', etc. The plane A', normal to OF, O'F', etc., gives the current
phase. By construction, the phase difference between A° and A' is such
that sin A°OA' (fig. 1,608) = 2 × sin A°OA' (fig. 1,606). That is,
doubling the field strength causes an increase of current lag such
that the sine of the angle of this lag is doubled. Since the intensity
of the armature reaction depends on the lead of the current with
respect to the reverse pressure, the mechanical lag of the coil must be
increased to some position as A" (fig. 1,608), such as will give an
armature reaction of an intensity indicated by the components OG, O'G',
etc.]
The following simple formula gives the speed relations between
generators and motors connected to the same circuit and having
different numbers of poles.
P × S
_s_ = -----
_p_
in which
_s_. Revolutions per minute of the motor;
_p_. Number of poles of the motor;
S. Revolutions per minute of the alternator;
P. Number of poles of the alternator.
~Question. If the field strength of a synchronous motor be altered,
what effect does this have on the speed, and why?~
Ans. The speed does not change (save for a momentary variation to
establish the phase relation corresponding to equilibrium), because the
motor has to run at the same frequency as the alternator.
~Ques. How does a synchronous motor adjust itself to changes of load
and field strength?~
Ans. By changing the phase difference between the current and pressure.
If, on connecting a synchronous motor to the mains, the
excitation be too weak, so that the voltage is lower than that
of the supply, this phase difference will appear resulting
in wattless current, since the missing magnetization has,
as it were, to be supplied from an external source. A phase
difference also appears when the magnetization is too strong.
~Ques. State the disadvantages of synchronous motors.~
Ans. A synchronous motor requires an auxiliary power for starting, and
will stop if, for any reason, the synchronism be destroyed; collector
rings and brushes are required. For some purposes synchronous motors
are not desirable, as for driving shafts in small workshops having
no other power available for starting, and in cases where frequent
starting, or a strong torque at starting is necessary. A synchronous
motor has a tendency to _hunt_[1] and requires intelligent attention;
also an exciting current which must be supplied from an external source.
[1] NOTE.--See Hunting of synchronous motors, page 1,280.
~Ques. State the advantage of synchronous motors.~
Ans. The synchronous motor is desirable for large powers where starting
under load is not necessary. Its power factor may be controlled by
varying the field strength. The power factor can be made unity and,
further, the current can be made to lead the pressure.
[Illustration: FIG. 1,609.--Diagram illustrating method of representing
the performance of synchronous motors. The _V shaped curve_ is obtained
by plotting the current taken by motor under different degrees of
excitation, the power developed by the motor remaining constant. The
current may be made to lag or lead while the load remains constant,
by varying the excitation. By varying the excitation, a certain value
may be reached which will give a minimum current in the armature;
this is the condition of unity power factor. If now the excitation be
diminished the current will lag and increase in value to obtain the
same power; if the excitation be increased the current will _lead_
and increase in value to obtain the same power. The results plotted
for several values of the excitation current will give the V curve as
shown. This is an actual curve obtained by Mordey on a 50 kw. machine
running unloaded as a motor. Other curves situated above this one may
be obtained for various loadings of the motor.]
A synchronous motor is frequently connected in a circuit solely
to improve the power factor. In such cases it is often called a
"condenser motor" for the reason that its action is similar to
that of a condenser.
The design of synchronous motors proceeds on the same lines as
that of alternators, and the question of voltage regulation in
the latter becomes a question of power factor regulation in the
former.
~Ques. For what service are they especially suited?~
Ans. For high pressure service.
High voltage current supplied to the armature does not pass
through a commutator or slip rings; the field current which
passes through slip rings being of low pressure does not give
any trouble.
[Illustration: FIG. 1,610.--Westinghouse self-starting synchronous
motor. Motors of this type are suitable for constant speed service
where starting conditions are moderate, such as driving compressors,
pumps, and large blowers. Synchronous motors can be made to operate
not only as motors but as synchronous condensers to improve the power
factor of the circuit. The field is provided with a combined starting
and damper or _amorlisseur_ winding so proportioned that the necessary
starting torque is developed by the minimum current consistent with
satisfactory synchronous running without hunting. The armature slots
are open and the coils form wound, impregnated, and interchangeable.
Malleable iron finger plates at each end of the core support the
teeth. Ventilating finger plates assembled with the laminations form
air ducts. The frames are of cast iron, box section with openings for
ventilation; shoes and slide rails permit adjustment of position. The
brush holders are of the standard sliding shunt type. Two or more
brushes are provided for each ring.]
~Ques. How do synchronous and induction motors compare as to
efficiency?~
Ans. Synchronous motors are usually the more efficient.
[Illustration: FIG. 1,611.--Mechanical analogy illustrating "hunting."
The figure represents two flywheels connected by a spring susceptible
to torsion in either direction of rotation. If the wheels A and B be
rotating at the same speed and a brake be applied, say to B, its speed
will diminish and the spring will coil up, and if fairly flexible, more
than the necessary amount to balance the load imposed by the brake;
because when the position of proper torque is reached, B is still
rotating slightly slower than A, and an additional torque is required
to overcome the inertia of B and bring its speed up to synchronism with
A. Now before the spring stops coiling up the wheels must be rotating
at the same speed. When this occurs the spring has reached a position
of too great torque, and therefore exerting more turning force on B
than is necessary to drive it against the brake. Accordingly B is
accelerated and the spring uncoils. The velocity of B thus oscillates
above and below that of A when a load is put on and taken off. Owing to
friction, the oscillations gradually die out and the second wheel takes
up a steady speed. A similar action takes place in a synchronous motor
when the load is varied.]
~Hunting of Synchronous Motors.~--Since a synchronous motor runs
practically in step with the alternator supplying it with current when
they both have the same number of poles, or some multiple of the ratio
of the number of poles on each machine, it will take an increasing
current from the line as its speed drops behind the alternator, but
will supply current to the line as a generator if for any reason the
speed of the alternator should drop behind that of the motor, or the
current wave lag behind, which produces the same effect, and due to
additional self-induction or inductance produced by starting up or
overloading some other motor or rotary converter in the circuit.
When the motor is first taking current, then giving current back to the
line, and this action is continued periodically, the motor is said to
be _hunting_.
[Illustration: FIG. 1,612.--Diagram illustrating the use of a
synchronous motor as a condenser. If a synchronous motor be
sufficiently excited the current will lead. Hence, if it be connected
across an inductive circuit as in the figure and the field be over
excited it will compensate for the lagging current in the main, thus
increasing the power factor. If the motor be sufficiently over excited
the power factor may be made unity, the minimum current being thus
obtained that will suffice to transmit the power in the main circuit.
A synchronous motor used in this way is called a _rotary condenser_
or _synchronous compensator_. This is especially useful on long lines
containing transformers and induction motors.]
~Ques. What term is applied to describe the behavior of the current
when hunting occurs?~
Ans. The term _surging_ is given to describe the current fluctuations
produced by hunting.
The mechanical analogy of hunting illustrated in fig.
1,611 will help to an understanding of this phenomenon. In
alternating current circuits a precisely similar action takes
place between the alternators and synchronous motors, or even
between the alternators themselves.
~CHARACTERISTICS OF SYNCHRONOUS MOTORS~
~Starting.~--The motor must be brought up to synchronous speed
without load, a _starting compensator_ being used. If provided with
a self-starting device, the latter must be cut out of circuit at the
proper time. The starting torque of motor with self-starting device is
very small.
~Running.~--The motor runs at synchronous speed. The maximum torque is
several times full load torque and occurs at synchronous speed.
~Stopping.~--If the motor receive a sudden overload sufficient to
momentarily reduce its speed, it will stop; this may be brought about
by momentary interruption of the current, sufficient to cause a loss of
synchronism.
~Effect upon Circuit.~--In case of short circuit in the line the motor
acts as a generator and thus increases the intensity of the short
circuit. The motor impresses its own wave form upon the circuit. Over
excitation will give to the circuit the effect of _capacity_, and under
excitation, that of _inductance_.
~Power Factor.~--This depends upon the field current, wave form and
hunting. The power factor may be controlled by varying the field
excitation.
~Necessary Auxiliary Apparatus.~--Power for starting, or if
self-starting, means of reducing the voltage while starting; also,
field exciter, rheostat, friction clutch, main switch and exciter
switch, instruments for indicating when the field current is properly
adjusted.
~Adaptation.~--If induction motors be connected to the same line with
a synchronous motor that has a steady load, then the field of the
synchronous motor can be over excited to produce a leading current,
which will counteract the effect of the lagging currents induced by the
induction motors. Owing to the weak starting torque, skilled attendance
required, and the liability of the motor to stop under abnormal working
conditions, the synchronous motor is not adapted to general power
distribution, but rather to large units which operate under a steady
load and do not require frequent starting and stopping.
[Illustration: FIGS. 1,613 to 1,625.--Disassembled view of Western
Electric three phase squirrel cage skeleton frame induction motor.]
~Induction (Asynchronous) Motors.~--
_An induction motor consists essentially of an armature and a field
magnet, there being, in the simplest and most usual types, no
electrical connection between these two parts_.[2]
[2] NOTE.--The author prefers the terms armature and field magnet,
instead of "primary," "secondary," "stator," "rotor," etc., as used
by other writers, the armature being the part in which currents are
induced and the field magnet (or magnets) that part furnishing the
field in which the induction takes place.
According to the kind of current that an induction motor is designed to
operate on, it may be classified as:
1. Single phase;
2. Polyphase.
The operation of an induction motor depends on the production of a
magnetic field by passing an alternating current through field magnets.
The character of this field is either
1. Oscillating[3], or
2. Rotating,
according as single phase or polyphase current is used.
[3] NOTE.--"The word _oscillating_ is becoming specialized in its
application to those currents and fields whose oscillations are being
damped out, as in electric 'oscillations.' But for this, we should have
spoken of an oscillating field."--_S. P. Thompson_. The author believes
the word _oscillating_, notwithstanding its other usage, best describes
the single phase field, and should be here used.
[Illustration: FIGS. 1,626 to 1,628.--General Electric base
construction for polyphase induction motors. The base is made of cast
iron. Adjusting gear is provided to slide the motor along the base as
shown in the illustrations, the movement being from 6 to 12 inches
according to size. With this design of base, motors are securely held
in position under all conditions and may be run with an upward pull on
the belt. Close fitting guides moving in an accurately machined slot on
the base preserve a correct alignment of the motor when adjustment of
the latter is required. The same base can be used whether the motor be
supported from the wall or ceiling or located on the floor. A single
adjusting screw is placed under the center line of the motor frame,
which produces an even and balanced draw in either direction on all
parts of the motor when the belt tension is altered. This screw can be
located at either end of the base. The base can be omitted when the
motor is direct connected or when provision for belt adjustment is not
required.]
~Ques. Describe briefly the operation of a single phase motor.~
Ans. A single phase current being supplied to the field magnets,
an _oscillating field_ is set up. A single phase motor is not
self-starting; but when the armature has been set in motion _by
external means_, the reaction between the magnetic field and the
induced currents in the armature being no longer zero, a torque is
produced tending to turn the armature.
The current flowing through the armature produces an
alternating polarity such that the attraction between the
unlike armature and field poles is always in one direction,
thus producing the torque.
[Illustration: FIG. 1,629.--Richmond three phase induction motor on
base fitted with screw adjusting gear for shifting the position of the
motor on the base to take up slack of belt.]
~Ques. Why is a single phase induction motor not self-starting?~
Ans. When the armature is at the rest, the currents induced therein are
at a maximum in a plane at right angles to the magnetic field, hence
there is no initial torque to start the motor.
~Ques. What provision is made for starting single phase induction
motors?~
Ans. Apparatus is supplied for "splitting the phase" (later described
in detail) of the single phase current furnished, converting it
temporarily into a two phase current, so as to obtain a _rotating
field_ which is maintained till the motor is brought up to speed. The
phase splitting device is then cut out and the motor operated with the
_oscillating field_ produced by the single phase current.
[Illustration: FIGS. 1,630 to 1,641.--Terminals for General Electric
polyphase induction motors. In order to prevent any mechanical strain
on the leads being transmitted to the motor windings, the terminal
cables are clamped in insulated bushings with a connector for each
cable.]
~Ques. Describe briefly the operation of a polyphase induction motor.~
Ans. Its operation is due to the production of a _rotating magnetic
field_ by the polyphase current furnished. This field "rotating" in
space about the axis of the armature induces currents in the latter.
The reaction between these currents and the rotating field creates a
torque which tends to turn the armature, whether the latter be at rest
or in motion.
[Illustration: FIGS. 1642 and 1643.--Western Electric end flange
rivets and punchings of riveted frame induction motor. The riveted
frame is constructed of two cast iron flanges between which the stator
laminations of sheet steel are securely clamped and riveted under
hydraulic pressure. This construction exposes the laminations directly
to the air and improves the radiation, thus insuring high overload
capacity and low operating temperatures. The field slots are overhung
or partially closed, affording mechanical protection to the coils.]
~Ques. Why are induction motors called "asynchronous"?~
Ans. Because the armature does not turn in synchronism with the
rotating field, or, in the case of a single phase induction motor,
with the oscillating field (considering the latter in the light of a
rotating field).
~Ques. How does the speed vary?~
Ans. It is slower (more or less according to load) than the "field
speed," that is, than "synchronism" or the "synchronous speed."
[Illustration: FIGS. 1,644 to 1,649.--Construction of General Electric
drawn shell fractional horse power motors. The distinguishing feature
of drawn shell motors is the field construction which consists of a
steel shell or cylinder supporting and clamping together the stator
or field punchings. This method avoids the cast frame work outside
the active magnetic material. A disc is first punched or "blanked"
out of soft steel, fig. 1,644, this disc being faced into the shape,
fig. 1,645, with one end closed. The other end of the shell is then
cut out, leaving the small flange as in fig. 1,646. It is now ready to
receive the core punchings. In the next operation a suitable number
of spacing rings, fig. 1,647, are forced into the shell and seated
against the retaining lip, which may be seen in fig. 1,646. The field
punchings or laminæ, fig. 1,648, are now assembled, after which a
second and equal set of spacing rings are put into place to center the
active field iron. The open edge of the shell is then rolled over the
punchings under heavy pressure, thus preparing the field structure for
the machining and fitting of the end heads and base. Fig. 1,649 shows
a section of the completely assembled field structure, the parts being
cut away to indicate the relation between the field punchings, spacing
rings and shell. After the spacing rings at both frame ends have been
turned true and grooved, the bearing heads, fig. 1,649, are ready for
fastening in place by four fillister headed screws. A complete wound
field is shown in fig. 1,858, with flat base casting attached.]
~Ques. What is the difference of speed called?~
Ans. The _slip_.
This is a vital factor in the operation of an induction
motor, since _there must be slip in order that the armature
inductors shall cut magnetic lines_ to ~induce~ (hence the name
"~induction~" motor) currents therein so as to create a driving
torque.
[Illustration: FIG. 1,650.--Ideal fifteen horse power two phase
induction motor. The armature core is supported by a cast iron
frame carried on a base, with sliding ways and screw adjustment for
tightening the belt. The armature core is provided with ventilating
apertures, with metal spacers between each tooth. The revolving field
is a steel casting with radially projecting poles, to which the pole
shoes are bolted. The overhanging pole tips retain the field coils.
All coils of the smaller sizes are wound with insulated copper wire of
square section, and of the larger sizes, with flat copper, wound on
edge, each turn being insulated by sheet insulation. Motors of this
type are adapted for use in small power plants and isolated plants. The
relatively high speed for which they are designed, reduces considerably
the weight and overall dimensions, and likewise the cost. The exciter
is belt driven. The normal kw. capacity of the exciter usually exceeds
the kw. required for the excitation under normal load conditions to
permit of station lighting. All exciters are built as compound wound
dynamos, capable of delivering the exciter current up to 125 volts,
which is sufficient margin in the field to control the alternating
current line voltage on circuits of unusually low power factor.]
~Ques. What is the extent of the slip?~
Ans. It varies from about 2 to 5 per cent. of synchronous speed
depending upon the size.
~Ques. Why are induction motors sometimes called constant speed motors?~
Ans. They are erroneously and ill advisedly, yet conveniently so called
by builders to distinguish them from induction motors fitted with
special devices to obtain widely varying speeds, and which are known as
_variable speed_ induction motors.
The term _adjustable_ would be better.
~Motor, Constant Speed.~--A motor in which the speed is either
constant or does not materially vary; such as synchronous
motors, induction motors with small slip, and ordinary direct
current shunt motors.--Paragraph 46 of 1907 Standardization
Rules of the A.I.E.E.
~Motor, Variable Speed.~--A motor in which provision is
made for varying the speed as desired. The A.I.E.E. has
unfortunately introduced the term _varying speed motor_, to
designate "motors in which the speed varies with the load,
decreasing when the load increases, such as series motors." The
term is objectionable, since by the expression _variable speed
motor_ a much more general meaning is intended.
[Illustration: FIG. 1,651.--Western Electric core construction and
method of winding field of skeleton frame induction motor. The coils
are wound on forms to give them exact shape and dimensions required.
They are pressed into hot moulds to remove any irregularities and then
the coils are impregnated with hot cement, to bind the layers together
in their permanent shape. The portion of the coil which fits into the
slot is wrapped with varnished cloth and a layer of dry tape is wound
over the entire coil. The coils are then impregnated with an insulating
compound and baked, the process being repeated six times. Coils for
1,100 and 2,200 volt motors have an extra covering of insulation
and double the amount of impregnating and baking. The coils may be
furnished with special insulation and treatment for exceptionally
severe service conditions, such as exposure to excessive moisture,
extreme heat, acid or alkaline fumes, etc. The coils are accessible and
for the final finish are sprayed with black varnish.]
~Ques. Why do some writers call the field magnets and armature the
_primary_ and _secondary_, respectively?~
Ans. Because, in one sense, the induction motor is a species of
transformer, that is, it acts in many respects like a transformer, the
primary winding of which is on the field and the secondary winding on
the armature.
In the motor the function of the secondary circuit is to
furnish energy to produce a torque, instead of producing light
and heat as in the case of the transformer. Such comparisons
are ill advised when made for the purpose of supplying names
for motor parts. There can be no confusion by employing the
simple terms armature and field magnets, remembering that the
latter is _that part that produces the oscillating or rotating
field_ (according as the motor is single or polyphase), and the
former, _that part in which currents are induced_.
[Illustration: FIG. 1,652.--Armature of Allis-Chalmers squirrel cage
induction motor. The frame casting is of the box type and has large
cored openings for ventilation. Lugs are cast on the interior surface
of the frame to support the core, leaving a large air space between.]
~Ques. Why are polyphase induction motors usually presented in text
books before single phase motors?~
Ans. Because the latter must start with a rotating field and come up to
speed before the oscillating field can be employed.
A knowledge then of the production of a rotating field is
necessary to understand the action of the single phase motor at
starting.
~Polyphase Induction Motors.~--As many central stations put out only
alternating current circuits, it has become necessary for motor
builders to perfect types of alternating current motor suitable for
all classes of industrial drive and which are adapted for use on these
commercial circuits. Three phase induction motors are slightly more
efficient at all loads than two phase motors of corresponding size, due
to the superior distribution of the field windings. The power factor is
higher, especially at light loads, and the starting torque with full
load current is also greater. Furthermore, for given requirements of
load and voltage, the amount of copper required in the distributing
system is less; consequently, wherever service conditions will permit,
three phase motors are preferable to two phase.
[Illustration: FIG. 1,653.--Sectional view showing parts of Reliance
polyphase induction motor. A special feature of the squirrel cage
armature construction is the multiplicity of short circuiting rings.
The holes in the rings are bored slightly smaller than the diameter
of the copper rods, and the force fit gives good contact. The rings
having been forced in place are dip soldered in an alloy of tin of
high melting point. The motor parts are: 1, end yoke; 2, shaft;
3, armature short circuiting rings; 4, oil ring; 5, self-aligning
bearing bushing; 6, spider; 7, armature bars; 8, field coils; 9, field
lamination end plate; 10, field laminations; 11, eye bolt; 12, stator
locking key; 13, armature laminations; 14, armature lamination end
plate; 15, armature locking key; 16, dust cap; 17, oil well cover; 18,
oil throws; 19, field frame; 20, squirrel cage armature.]
[Illustration: FIG. 1,654.--Tesla's rotating magnetic field. The figure
is from one of Tesla's papers as given in The Electrician, illustrating
how a rotating magnetic field may be produced with stationary magnets
and polyphase currents. The illustration shows a laminated iron
ring overwound with four separate coils, AA, and BB, each occupying
about 90° of the periphery. The opposite pairs of coils AA and BB
respectively are connected in series and joined to the leads from a two
phase alternator, the pair of coils AA being on one circuit and the
coils BB on the other. The resultant flux may be obtained by combining
the two fluxes due to coils AA and BB, taking account of the phase
difference of the two phase current, as in fig. 1,655.]
The construction of an induction motor is very simple, and since there
are no sliding contacts as with commutator motors, there can be no
sparks during operation--a feature which adapts the motor for use in
places where fire hazards are prominent.
The motor consists, as already mentioned, simply of two parts: _an
armature and field magnets, without any electrical connection between
these parts_. Its operation depends upon:
1. _The production of a rotating field;_
2. _Induction of current in the armature;_
3. _Reaction between the revolving field and the induced currents._
[Illustration: FIG. 1,655.--Method of obtaining resultant flux of
Tesla's rotating magnetic field. The eight small diagrams here seen
show the two components and resultant for eight equivalent successive
instants of time during one cycle. At 1, the vertical flux is at +
maximum and the horizontal is zero. At 2, the vertical flux is still +
but decreasing, and the horizontal is + and increasing, the resultant
is the thick line sloping at 45° upwards to the right. At 3, the
vertical flux is zero, and the horizontal is at its + maximum, and
similarly for the other diagrams. Thus at 8, the vertical flux is +
and increasing, while the horizontal is-and decreasing, the resultant
is the thick line sloping at 45° upwards to the left. At points 2, 4,
6, and 8 the increasing fluxes are denoted by full and the decreasing
by dotted lines. The laminated iron of the ring is indicated by the
circles, and the result is that at the instants chosen the flux across
the plane of the ring is directed inwards from the points 1, 2, 3, 4,
etc., on the inner periphery of the iron. There will, therefore, appear
successively at these points effective north poles, the corresponding
south poles being simultaneously developed at the points diametrically
opposite. These poles travel continuously from one position to the
next, and thus the magnetic flux across the plane of the ring swings
round and round, completing a revolution without change of intensity
during the cycle time of the current.]
~Production of a Rotating Field.~--It should at once be understood that
the term "rotating field" does not signify that part of the apparatus
revolves, the expression merely refers to the magnetic lines of force
set up by the field magnets without regard to whether the latter be the
stationary or rotating member.
A rotating field then may be defined as _the resultant magnetic field
produced by a system of coils symmetrically placed and supplied with
polyphase currents_.
A rotating magnetic field can, of course, be produced by
spinning a horse shoe magnet around its longitudinal axis, but
with polyphase currents, as will be later shown, the rotation
of the field can be produced Without any movement of the
mechanical parts of the electro magnets.
[Illustration: FIG. 1,656.--Arago's rotations. The apparatus necessary
to make the experiment consists of a copper disc M, arranged to rotate
around a vertical axis and operated by belt drive, as shown. By
turning the large pulley by hand, the disc M may be rotated with great
rapidity. Above the disc is a glass plate on which is a small pivot
supporting a magnetic needle N. If the disc now be rotated with a slow
and uniform velocity, the needle is deflected in the direction of the
motion, and stops at an angle of from 20° to 30° with the direction of
the magnetic meridian, according to the velocity of the rotation of the
disc. If the velocity increase, the needle is ultimately deflected more
than 90° and then continues to follow the motion of the disc.]
The original rotating magnetic field dates back to 1823, when Francois
Jean Arago, an assistant in Davy's laboratory, discovered that if a
magnet be rotated before a metal disc, the latter had a tendency to
follow the motion of the magnet, as shown in fig. 290, page 270 and
also in fig. 1,656. This experiment led up to the discovery which
was made by Arago in 1824, when he observed that the _number of
oscillations which a magnetized needle makes in a given time, under
the influence of the earth's magnetism, is very much lessened by the
proximity of certain metallic masses_, and especially of copper, which,
may reduce the number in a given time from 300 to 4.
[Illustration: FIG. 1,657.--Explanation of Arago's rotations. Part of
fig. 1,656 is here reproduced in plan. Faraday was the first to give an
explanation of the phenomena of magnetism by rotation in attributing
it to the induction of currents which by their electro-dynamic action,
oppose the motion producing them; the action is mechanically analogous
to friction. In the figure, let AB be a needle oscillating over a
copper disc, and suppose that in one of its oscillations it goes in the
direction of the arrow from M to S. In approaching the point S, for
instance, it develops there a current in the opposite direction, and
which therefore repels it; in moving away from M it produces currents
which are of the same kind, and which therefore attract, and both these
actions concur in bringing it to rest. Again, suppose the metallic
mass turn from M towards S, and that the magnet be fixed; the magnet
will repel by induction points such as M which are approaching A, and
will attract S which is moving away; hence the motion of the metal
stops, as in Faraday's experiment. If in Arago's experiment the disc be
moving from M to S, M approaches A and repels it, while S, moving away,
attracts it; hence the needle moves in the same direction as the disc.
If this explanation be true, all circumstances which favor induction
will increase the dynamic action; and those which diminish the former
will also lessen the latter.]
The explanation of Arago's rotations is that _the magnetic field
cutting the disc produces eddy currents therein and the reaction
between the latter and the field causes the disc to follow the
rotations of the field_.
The induction motor is a logical development of the experiment of
Arago, which so interested Faraday while an assistant in Davy's
laboratory and which led him to the discovery of the laws of
electromagnetic induction, which are given in Chapter X.
[4]In 1885, Professor ~Ferraris~, of Turin ~discovered~ that _a
rotating field could be produced from stationary coils by means of
polyphase currents_.
[4] NOTE.--Walmsley attributes the first production of rotating fields
to Walter Bailey in 1879, who exhibited a model at a meeting of the
Physical Society of London, but very little was done, it is stated,
until Ferraris took up the subject.
[Illustration: FIG. 1,658.--Experiment made by Faraday being the
reverse of Arago's first observation. Faraday assumed that since the
presence of a metal at rest stops the oscillations of a magnetic
needle, the neighborhood of a magnet at rest ought to stop the motion
of a rotating mass of metal. He suspended a cube of copper by a twisted
thread, which was placed between the poles of a powerful electromagnet.
When the thread was left to itself, it began to spin round with great
velocity, but stopped the moment a powerful current was passed through
the electromagnet.]
[5]This discovery was commercially applied a few years later by Tesla,
Brown, and Dobrowolsky.
[5] NOTE.--The Tesla patents were acquired in the U.S. by the
Westinghouse Co. in 1888, and polyphase induction motors, as they were
called, were soon on the market. Brown of the Oerlikon Machine Works
developed the single phase system and operated a transmission plant
over five miles in length at Kassel, Germany, which operated at 2,000
volts.
_The principles of polyphase motors_ can be best understood by means
of elementary diagrams illustrating the action of polyphase currents
in producing a rotating magnetic field, as explained in the paragraphs
following.
~Production of a Rotating Magnetic Field by Two Phase Currents.~--Fig.
1,659 represents an iron ring wound with coils of insulated wire, which
are supplied with a two phase current at the four points A, B, C, D,
the points A and B, and C and D, being electrically connected.
[Illustration: FIG. 1,659.--Production of a rotating magnetic field
by two phase currents. The figure represents an iron ring, wound with
coils of insulated wire, and supplied with two phase currents at the
four points A, B, C, and D. The action of the two phase current on
the ring in producing a rotating magnetic field is explained in the
accompanying text.]
According to the principles of electromagnetic induction, if only one
current _a_ entered the ring at A, and the direction of the winding be
suitable, a negative pole (-) will be produced at A and a positive pole
(+) at B, so that a magnetic needle pivoted in the center of the ring
would tend to point vertically upward towards A. Now suppose that at
this instant, corresponding to the beginning of an alternating current
cycle, a second current _b_, differing in phase from the first by 90
degrees, is allowed to enter the ring at C. As shown in fig. 1,659,
when the pressure of the current _a_ is at its maximum, that of the
current _b_ is at its minimum; therefore, even a two phase current, at
the beginning of the cycle, the needle will point toward A.
[Illustration: FIG. 1,660.--Production of rotating magnetic field in
a two pole two phase motor. The poles are numbered from 1 to 4 in a
clockwise direction. Phase A winding is around poles 1 and 3, and phase
B winding, around poles 2 and 4. In each case the poles are wound
alternately, that is, if 1 be wound clockwise, 3 will be wound counter
clockwise, thus producing unlike polarity in opposite poles. Now during
one cycle of the two phase current, the following changes take place,
starting with pole 1 of N polarity and 3, of S polarity:
+--------------------------------------------------------------+
| | One Cycle |
|----------+-----------+-------------+------------+------------|
| Degrees | 0° to 90° | 90° to 180° |180° to 270°|270° to 360°|
+----------+-----------+-------------+------------+------------+
| Polarity | 1N - 3S | 2N - 4S | 3N - 1S | 4N - 2S |
+--------------------------------------------------------------+]
[Illustration: FIG. 1,661.--Diagram showing resultant poles due to two
phase current.]
[Illustration: FIG. 1,662.--Diagram of two phase, six pole field
winding. There are six coils in each phase, as shown. The coils of each
phase are connected in series, adjacent coils being joined in opposite
senses, thus, for each phase, first one coil is wound clockwise, and
the next counter clockwise.]
As the cycle continues, however, the strength of _a_ will diminish and
that of _b_ increase, thus shifting the induced pole toward C, until
_b_ attains its maximum and _a_ falls to its minimum at 90° or the end
of the first quarter of the cycle, when the needle will point toward
C. At 90°, the phase _a_ current reverses in direction and produces a
negative pole at B, and as its strength increases from 90° to the 180°
point of the cycle, and that of phase _b_ diminishes, the resultant
negative pole is shifted past C toward B, until _a_ attains its maximum
and _b_ falls to its minimum at 180°, and the needle points in the
direction of B.
[Illustration: FIG. 1,663.--Diagram of two phase, eight pole field
winding. The winding is divided into 16 groups (equal to the product
of the number of poles multiplied by the number of phases). Each group
such as at A comprises a number of coils in series, each coil being
located in a separate pair of slots, the end of one being connected to
the beginning of the next. When the currents are in the same direction,
the currents circulate in the same direction in two adjacent groups, a
pole then with this arrangement being formed by two groups, both phases
contributing to the formation of the pole. After ½ cycle when the
current in each phase reverses, the pole advances the angular distance,
covered by two groups; hence the field completes one revolution in
eight alternations of current.]
[Illustration: FIGS. 1,664 to 1,683.--Sine curves of two phase current
and diagrams showing the physical conception of a ~two phase rotating
magnetic field~. The alternating magnetizing current is assumed to be
of such strength that, at its maximum strength, the field produced may
be represented by 10 lines of force as indicated by the parallel lines.
At the beginning of the rotation, fig. 1,664, phase A magnetization,
according to sine curve is zero, indicated by the solid black poles,
while phase B is of strength 10 with]
[Illustration: current in the direction to produce a south pole at
B. Similarly, in fig. 1,665, the strength of A is 4 lines, and of
B, 9 lines, the resultant magnetization having rotated 22½°. The
direction of the resultant magnetization is indicated by the arrow in
each figure. It should be noted in fig. 1,669, that the polarity of
B is reversed, the current curve now being above the zero line. By
following the arrow through the successive positions the rotation of
the resultant magnetization is clearly seen.]
At the 180° point of the cycle, _b_ reverses in direction and produces
a negative pole at D, and as the fluctuation of the pressure of the
two currents during the second half of the cycle, from 180° to 360°,
bear the same relation to each other as during the first half, the
resultant poles of the rotating magnetic field thus produced carry the
needle around in continuous rotation so long as the two phase current
traverses the windings of the ring.
[Illustration: FIG. 1,684.--Moving picture method of showing motion
of a rotary magnetic field. A number of sheets of paper are prepared,
each containing a drawing of the motor frame and a magnetic needle
in successively advancing angular positions, indicating resultant
directions of the magnetism. The sheets are bound together so that the
axis of the needle on each sheet coincides. When passing the sheets in
one way the revolving field will be seen to rotate in one direction,
while, when moving the sheets backward, the rotation of the magnetic
field is in the opposite direction, showing that the reversal of the
order of the coils has the effect of reversing the rotation of the
magnetic field.]
~Production of Rotating Magnetic Field by Three Phase Current.~--A
rotating magnetic field is produced by the action of a three phase
current in a manner quite similar to the action of a two phase current.
Fig. 1,685 shows a ring suitably wound and supplied with a three phase
current at three points A, B, C, 120° of a cycle apart.
[Illustration: FIG. 1,685.--Production of a rotating magnetic field
by three phase current. A ring wound as shown is tapped at points
A, B, and C, 120° apart, and connected with leads to a three phase
alternator. As described on page 1,304, a rotating magnetic field is
produced in a manner similar to the two phase method.]
[Illustration: FIG. 1,686.--Diagram of three phase, four pole ~Y~
connected field winding.]
At the instant when the current _a_, flowing in at A, is at its
maximum, two currents _b_ and _c_, each one-half the value of _a_, will
flow out B and C, thus producing a negative pole at A and a positive
pole at B and at C. The resultant of the latter will be a positive pole
at E, and consequently, the magnetic needle will point towards A.
[Illustration: FIG. 1,687.--Production of a rotating magnetic field in
a two pole three phase motor. In order to obtain a uniformly rotating
magnetic field, it is necessary to arrange the phase windings in the
direction of rotation, in the sequence ACB, not ABC as indicated on the
magnets. Thus poles 1 and 4 are connected in series to phase A, 2 and 5
in series to phase C, and 3 and 6 in series to phase B. The different
phase windings are differently lined, and it should be noted that they
have a common return wire, though this is not absolutely necessary.
Since the phases of the three currents differ from each other by
one-third of a period or cycle, each of the phase windings will
therefore set up a field between its poles, which at any instant will
differ, both in direction and magnitude, from the fields set up by the
other phase windings. Hence, the three phase windings acting together
will produce a resultant field, and if plotted out, the directions of
this field for various fractions of the period is such that in one
complete period the resultant field will make one complete round of
the poles in a clockwise direction, as indicated by the curved arrow.
The positions of the resultant field during one complete period may be
tabulated as follows:
+--------------------------------------------------------------------+
| | One Cycle |
| |---------+---------+---------+---------+---------+---------|
| |0° to 60°| 60° to | 120° to | 180° to | 240° to | 300° to |
| | | 120° | 180° | 240° | 300° | 360° |
| |---------+---------+---------+---------+---------+---------|
|Polarity| 1N - 4S | 2N - 5S | 3N - 6S | 4N - 1S | 5N - 2S | 6N - 3S |
|--------+---------+---------+---------+---------+---------+---------|]
As the cycle advances, however, the mutual relations of the
fluctuations of the pressures of the three currents, and the time of
their reversals of direction will be such, that when a maximum current
is flowing at any one of the points A, B, and C, two currents each of
one-half the value of the entering current will flow out of the other
two points, and when two currents are entering at any two points, a
current of maximum value will flow out of the other point. This action
will produce one complete rotation of the magnetic field during each
cycle of the current.
[Illustration: FIG. 1,688.--Production of three phase rotating magnetic
field with winding on laminated iron ring. The winding is divided into
twelve sections, which are connected in three groups, A, B, and C, of
four sections each, the sections in each group being evenly placed
round the ring with the sections of the two other groups between
them. One end of each group is to be connected to the line wire and
the other end to the common junction J, from which it follows that
the winding given is an example of "star" winding. With three phase
currents the winding will give at every instant four N poles and four S
poles round the ring, and in actual working these poles will be on the
inner periphery because of the presence of an inner ring or cylinder
of good magnetic iron placed, with the requisite clearance to allow
of rotation, as close as is mechanically possible to the outer ring.
Each one of these eight poles will make a complete revolution round the
ring in four times the periodic time of the currents supplied. Thus, if
the supply current has a frequency of 50, a complete revolution of the
field will take place in .08 (=⁴/₅₀) of a second, which corresponds
to an angular velocity of 750 revolutions per minute in place of 3,000
revolutions per minute, which would be the angular velocity with a
bipolar field at this periodicity. Similarly a continuously wound
Gramme ring tapped at twelve points, joined in three groups of four
each to the supply mains, would give an eight pole rotary field. In
this case the grouping would be a "mesh" grouping, with each side of
the mesh formed of four coils in parallel.]
[Illustration: FIGS. 1,689 to 1,708.--Sine curves of three phase
current and diagrams showing the physical conception of a ~three phase
rotating magnetic field~. The diagrams are constructed in the same
manner as explained in figs. 1,664 to 1,683. It should be noted that
the phase windings are arranged in the direction of rotation in the
sequence ACB, phase C being wound in opposite]
[Illustration: sense to A and B, as indicated by the curves, in that
north poles are produced at A and B when the respective curves are
above the zero line, a south pole being produced at C when its curve
is above the zero line. The rotation of the resultant magnetization is
clearly seen by following the arrow through its successive positions.]
~Slip.~--Instead of the magnetic needle as was used in the preceding
figures, a copper cylinder may be placed in a rotating magnetic field
and it will be urged also to turn in the same direction as the rotation
of the field.
[Illustration: FIG. 1,709.--Diagram of three phase, six pole field
winding. There are 18 groups, and the sequence of phases is ABC in a
counter clockwise direction. For a ~Y~ connection, the middle phase is
reversed, so that a pole will be formed by the three consecutive phases
when the current is in the same direction in A and C, and opposite in
B. The beginning of the middle coil C, and not the end, as with the
other two, is connected to the common point O. In this case the pole
shifts a distance equal to three groups for each alternation, so that
one revolution of the field requires three cycles.]
_The torque tending to turn the cylinder is due to the induction of
currents of opposite polarity in the cylinder._
For simplicity, the rotating magnetic field may be supposed to be
produced by a pair of magnetic poles placed at opposite sides of the
cylinder and _revolved around it_ as in fig. 1,710.
Now, for instance in starting, the cylinder being at rest any element
or section of the surface as the shaded area AB, will, as it comes into
the magnetic field of the rotating magnet, cut
[Illustration: FIG. 1,710.--Copper cylinder and rotating magnet
_illustrating the principle of operation of an induction motor_. The
"rotating magnetic field" which is necessary for induction motor
operation is for simplicity here produced by rotating a magnet as
shown. In starting, the cylinder being at rest, any element as AB, as
it is swept by the field will cut magnetic lines, which will induce a
current upward in direction as determined by applying Fleming's rule
(fig. 132, page 133). The inductive action is strongest at the center
of the field hence as AB passes the center the induced pressure along
AB is greater than along elements more or less remote on either side.
Accordingly a pair of eddy currents will result as shown (see fig.
291, page 271). Applying the right hand rule for polarity of these
eddy currents (see fig. 119, page 117) it will be seen that a S pole
is induced by the eddy on the side of the cylinder receding from the
magnet, and a N pole by the eddy on the side toward which the magnet
is approaching. The cylinder, then, is _attracted_ in the direction of
rotation of the magnet by the induced pole on the receding side, and
_repelled_ in the same direction by the induced pole on the approaching
side. Accordingly, the cylinder begins to rotate. The velocity with
which it turns depends upon the load; it must always turn _slower_
than the magnet, in order that its elements may cut magnetic lines and
induce poles to produce the necessary torque to balance the load. The
difference in speed of the magnet and cylinder is called the _slip_.
Evidently the greater the load, the greater is the slip required to
induce poles of sufficient strength to maintain equilibrium. The
figure is drawn somewhat distorted, so that both eddies are visible.]
magnetic lines of force inducing a current therein, whose direction is
easily determined by applying Fleming's rule.[6]
[6] NOTE.--In order to avoid confusion in applying Fleming's rule, it
may be well to regard the pole as being stationary and the cylinder as
in motion; for, since motion is "purely a relative matter" (see fig.
1,393), the inductive action will be the same as if the pole stood
still while the cylinder revolved from left to right, that is, counter
clockwise, looking down on it. Regarding it thus (pole stationary and
cylinder revolving counter clockwise) Fleming's rule (see fig. 132,
page 133) is easily applied to ascertain the direction of the induced
current, which is found to flow upward in the shaded area as shown.
Since the field is not uniform, but gradually weakens, as shown, on
either side of the shaded area (which is just passing the center), the
pressure induced on either side will be less than that induced in the
shaded area, giving rise to eddy currents (as illustrated in fig. 291,
page 271). These eddy currents induce poles as indicated at the centers
of the whorls, the polarity being determined by applying the right hand
rule (fig. 119, page 117).
[Illustration: FIGS. 1,711 to 1,718.--Parts of Allis-Chalmers polyphase
induction motor with squirrel cage armature.]
By inspection of fig. 1710, it is seen that _the induced pole toward
which the magnet is moving is of the same polarity as the magnet;
therefore it is_ ~repelled~, _while the induced pole from which the
magnet is receding, being of opposite polarity, is_ ~attracted~. _A
torque is thus produced tending to rotate the cylinder._
It must be evident that this torque is greatest when the cylinder is
at rest, because the magnetic lines are cut by any element on the
cylindrical surface at the maximum rate.
Moreover, as cylinder is set in motion and brought up to speed, the
torque is gradually reduced, because the rate with which the magnetic
lines are cut is gradually reduced.
~Ques. What is the essential condition for the operation of an
induction motor?~
Ans. The armature, or part in which currents are induced, must rotate
at a speed slower than that of the rotating magnetic field.
In the elementary induction motor, fig. 1,710, the cylinder is
the armature, and the rotating magnets are the equivalent of a
rotating magnetic field.
~Ques. What is the difference of speed called?~
Ans. _The slip._
~Ques. Why is slip necessary in the operation of an induction motor?~
Ans. If the armature had no weight and there was no friction offered by
the bearings and air, it would revolve in synchronism with the rotating
magnetic field, that is, the slip would be zero; but since weight and
friction are always present and constitute a small load, its speed
of rotation will be a little less than that of the rotating magnetic
field, so that induction will take place, in amount sufficient to
produce a torque that will balance the load.
[Illustration: FIG. 1,719.--General Electric vertical type induction
motor; sectional view showing oiling system. It is provided with ball
thrust bearings and top and bottom guide bearings, and a continuous
flow of oil is maintained through all the bearings by means of a pump
which is made integral with the motor. The ball thrust bearings are
designed to support the weight of the armature only. In cases where
the armature is direct connected a flexible coupling should be used to
prevent additional weight coming on the thrust bearings. In operation,
when the motor starts, the oil, revolving with the pan, flows against
the stationary nozzle and is forced by its velocity at a high pressure
through the oil pipe into the reservoir on top. It then flows down
through the ball bearing and upper guide bearing, through a slot in the
armature spider into the lower guide bearing and thence into the oil
pan. Thus a continuous stream of oil is delivered through all bearings.]
~Ques. How is slip expressed?~
Ans. In terms of synchronism, that is, as a percentage of the speed of
the rotating magnetic field.
The slip is obtained from the following formula:
Slip (rev. per sec.) = S_f_ - Sₐ
or, expressed as a percentage of synchronism, that is, of the
synchronous speed,
(S_f_ - Sₐ) × 100
Slip (%) = -----------------
S_f_
where
S_f_ = Synchronous speed, or R.P.M. of the rotatory magnetic field;
Sₐ = Speed of the armature.
The synchronous speed is determined the same as for synchronous motor
by use of the following formula:
2_f_
S_f_ = ----
P
where
S_f_ = Synchronous speed or R.P.M. of the rotating magnetic field;
P = Number of poles;
_f_ = frequency.
[Illustration: FIG. 1720.--Triumph back geared polyphase induction
motor. A great many applications, especially for direct attachment,
require the use of either a very slow or special speed motor. As these
are quite costly, the preferable arrangement, and one equally as
satisfactory, is the use of a standard speed motor combined with a back
geared attachment. Rawhide pinions are furnished whenever possible,
insuring smooth running with a minimum of noise.]
[Illustration: FIGS. 1,721 to 1,735.--Parts of General Electric small
polyphase induction motors. A, armature; B, key for armature shaft; C,
oil ring; D, bearing lining; E, bearing head, pulley end; F, cap screw
for bearing heads; G, field, complete with winding, terminal plate
and leads; H, motor leads; I, terminal connector for motor leads; J,
soft rubber bushing for motor leads; K, terminal plate; L, screw for
terminal board; M, field coils; N, wooden top sticks for field coils;
O, oil filler; P, bearing head opposite pulley end; Q, screw for oil
well cover; R, oil well cover; S, socket pipe plug for bearing head; U,
motor base; V, yoke for motor base; W, motor base adjusting screw; X,
bolt for motor base and frame (short); Y, cap screw for bearing head;
Z, internal directive fan; Aa, pulley.]
The following table gives the synchronous speed for various frequencies
and different numbers of poles:
~Table of Synchronous Speeds~
+------------------------------------------------------+
| | R.P.M. of the rotating magnetic field, |
| | when number of poles is |
|Frequency|--------------------------------------------|
| | 2 | 6 | 10 | 16 | 20 | 24 |
|---------+-------+-------+-------+------+------+------|
| 25 | 1,500 | 500 | 300 | 188 | 150 | 125 |
| 60 | 3,600 | 1,200 | 720 | 450 | 360 | 300 |
| 80 | 4,800 | 1,600 | 960 | 600 | 480 | 400 |
| 100 | 6,000 | 2,000 | 1,200 | 750 | 600 | 500 |
| 120 | 7,200 | 2,400 | 1,440 | 900 | 720 | 600 |
| 125 | 7,500 | 2,500 | 1,500 | 938 | 750 | 625 |
+------------------------------------------------------+
~Ques. How does the slip vary?~
Ans. It varies from about 1 per cent. in a motor designed for very
close regulation to 40 per cent. in one badly designed, or designed for
some special purpose.
~Ques. Why is the slip ordinarily so small?~
Ans. Because of the very low resistance of the armature, very little
pressure is required to produce currents therein, of sufficient
strength to give the required torque. Hence, the necessary rate of
cutting the magnetic lines to induce this pressure in the armature
is reached with very little difference between the field speed and
armature speed, that is, with very little slip.
~Ques. How does the slip vary with the load?~
Ans. The greater the load the greater the slip.
In other words, if the load increase, the motor will run
slower, and the slip will increase. With the increased slip,
the induced currents and the driving force will further
increase. If the motor be well designed so that the field
strength is constant and the lag of the armature currents
is small, the driving force developed or _torque_ will be
proportional to the slip, that is the slip will increase
automatically as the load is increased, so that the _torque_
will be proportional to the load.
According to Weiner, the slip varies according to the following table:
~SLIP OF INDUCTION MOTORS~
+----------+------------------------+----------+------------------------+
| | Slip at full load | | Slip at full load |
| Capacity | per cent. | Capacity | per cent. |
| of motor |------------------------| of motor |------------------------|
| H. P. | Usual limits | Average | H. P. | Usual limits | Average |
+----------+--------------+---------+----------+--------------+---------+
| ⅛ | 20 to 40 | 30 | 15 | 5 to 11 | 8 |
| ¼ | 10 " 30 | 20 | 20 | 4 " 10 | 7 |
| ½ | 10 " 20 | 15 | 30 | 3 " 9 | 6 |
| 1 | 8 " 20 | 14 | 50 | 2 " 8 | 5 |
| 2 | 8 " 18 | 13 | 75 | 1 " 7 | 4 |
| 3 | 8 " 16 | 12 | 100 | 1 " 6 | 3.5 |
| 5 | 7 " 15 | 11 | 150 | 1 " 5 | 3 |
| 7½ | 6 " 14 | 10 | 200 | 1 " 4 | 2.5 |
| 10 | 7 " 12 | 9 | 300 | 1 " 3 | 2 |
+----------+--------------+---------+----------+--------------+---------+
[Illustration: FIG. 1,736.--Sector method of measuring the slip of
induction motors. A black disc having a number of white sectors
(generally the same as the number of poles of the induction motor) is
fastened with wax to shaft of the induction motor, and is observed
through another disc having an equal number of sector shaped slits
(that is a similar disc with the white sectors cut out) and attached to
the shaft of a small self-starting synchronous motor, which is fitted
with a revolution counter that can be thrown in or out of gear at
will; then the slip (in terms of N_{_r_}) = N ÷ (N_{_s_} ÷ N_{_r_}),
in which: N = number of passages of the sectors; N_{_s_} = number of
sectors; N_{_r_} = number of revolutions recorded by the counter during
the interval of observation. For large values of slip, the observations
may be simplified by using only one sector (N_{_s_} = 1), then N will
equal the slip in revolutions.]
~Ques. Describe one way of measuring the slip.~
Ans. A simple though rough way is to observe simultaneously the speed
of the armature and the frequency, calculating the slip from the data
thus obtained, as on page 1,315.
This method is not accurate, as, even with the most careful
readings, large errors cannot be avoided. A better way is shown
in fig. 1,736.
[Illustration: FIG. 1,737.--Detail of Westinghouse squirrel cage
armature for induction motor. This is an example of _cast on_
construction similar to that of Morse-Fairbanks (see figs. 1,752, 1,753
and 1,915). The inductors are embedded in a special cement.]
~Evolution of the Squirrel Cage Armature.~--In the early experiments
with rotating magnetic fields, copper discs were used; in fact, it was
then discovered that _a mass of copper or any conducting metal, if
placed in a rotating magnetic field, will be urged in the direction of
rotation of the field_.
Ferraris used a copper cylinder as in figs. 1,710 and 1,738, which was
the first step in the evolution of the squirrel cage armature. The
trouble with an armature of this kind is that there is no definite path
provided for the induced currents.
[Illustration: SO CALLED SQUIRREL CAGE]
[Illustration: FIGS. 1,738 to 1,744.--Evolution of the squirrel cage
armature. The early experiments of Arago, Herschel, Babbage and
Baily demonstrated that a mass of copper or any conducting metal, if
placed in a revolving magnetic field, will be urged to revolve in the
direction of the revolving field. They used discs, but Ferraris used
a copper cylinder as shown in figs. 1,710 and 1,738; this was the
first squirrel cage armature. Figs. 1,739 to 1,744 show the gradual
development of the primitive device shown in fig. 1,738; fig. 1,739,
Ferraris' cylinder with slots restricting the path of induced currents;
fig. 1,740, Dobrowolsky's so called squirrel cage which he embedded in
a solid iron core, as in fig. 1,741; fig. 1,742, design with insulated
bars and laminated core to prevent eddy currents in the core; fig.
1,743, laminated core with ventilating ducts; fig. 1,744, modern
squirrel cage armature representing the latest practice as built by
Mechanical Appliance Co. The core is built up of discs punched from No.
29 gauge electrical sheet, insulated from each other and firmly clamped
between end plates locked on the shaft. The slots in the discs are of
the same general form as those in the core. Heavy fibre end pieces,
punched to match the discs are placed at each end of the core, to
prevent the bars coming in contact with the sharp edges of the teeth.
The winding is made up of rectangular copper bars, passing through
slots in the core, and short circuited on each other by means of copper
end rings of special design. The bars are pressed into holes punched
in the end rings, and the contact is then protected from corrosion by
being dipped in a solder bath. The bars are insulated from the iron of
the core by fibre cell projecting beyond the end of the slot. To secure
ventilation the short circuiting rings are set some distance from the
end of the core. In this way the bars between the core and the ring
act as the vanes of a pressure blower, forcing a large volume of air
through the field coils and ventilating openings.]
[Illustration: FIG. 1,745.--Mechanical Appliance Co. solid core discs
as used on small and medium size induction motors.]
[Illustration: FIG. 1,746.--Allis-Chalmers squirrel cage armature
construction. The core laminæ are mounted on a cast iron spider having
arms shaped to act as fan blades for forcing air through the motor. The
spider is pressed on to the shaft. In the smallest sizes the punchings
are mounted directly on the shaft, which is properly machined to hold
them firmly. Copper bars are used as inductors in the larger sizes,
and copper rods in the smaller sizes. The ends of the inductors are
turned down somewhat smaller than the body and fit in holes in the end
rings. The shoulder thus formed fits firmly against the end rings.
Good electrical contact is obtained by expanding the inductors in the
end ring holes. In large armatures both bars and end rings are of
rectangular cross section, the bars and rings being fastened by machine
steel cap screws.]
Obviously, a better result is obtained if, in fig. 1,738, the downward
returning currents of the eddies are led into some path where they will
return across a field of opposite polarity from that across which
they ascended, as in such case, the turning effect will be doubled.
Accordingly the design of fig. 1,738 was modified by cutting a number
of parallel slits which extended nearly to the ends, leaving at each
end an uninterrupted "ring" of metal. This may be called the first
squirrel cage armature, and in the later development Dobrowolsky was
the first to employ a built-up construction, using a number of bars
joined together by a ring at each end, as in fig. 1,740, and embedded
in a solid mass of iron, as in fig. 1,741; he regarding the bars merely
as veins of copper lying buried in the iron.
[Illustration: FIG. 1,747.--Triumph squirrel cage armature. In
construction thin sheet steel laminations, japanned, are built up to
form the core, and are rigidly clamped together by heavy malleable iron
end plates. Semi-enclosed slots are punched in the outer periphery to
receive the windings, so that none of the centrifugal force is carried
by the inductors. These inductors are set edge on, and are riveted and
soldered into resistance rings. These rings are punched to receive the
inductors in such a manner that there is an unbroken strip of metal
completely surrounding them. Moreover, the short circuiting rings are
set some distance from the end of the core, so that the inductors
between the core and ring act as vanes to force air through the coils
for ventilation.]
[Illustration: FIG. 1,748.--General Electric soldered form of end
ring construction on squirrel cage armatures. The armature inductors
or copper bars laid in the core slots are short circuited by these
end rings, which are also made of copper. For the smaller sizes the
rings are thin, but of considerable radial depth and are held apart by
spacing washers. They have rectangular holes punched near their outer
peripheries through which the bars pass. Lips are formed on the rings,
as shown, to which the bars are soldered.]
[Illustration: FIG. 1,749.--General Electric welded form of end ring
construction on squirrel cage armatures. Space limitations make it
difficult to provide multiple soldered rings of sufficient area for
large motors; hence, on such machines welding is resorted to, as shown.
The ring in welded construction is placed beneath the bars at each end
of the armature. Short radial bars are welded to the edges of these
rings and to the inductors or squirrel cage bars, thereby making good
electrical contact.]
A solid cylinder of iron will of course serve as an armature,
as it is magnetically excellent; but the high specific
resistance of iron prevents the flow of induced currents taking
place sufficiently copiously; hence a solid cylinder of iron
is improved by surrounding it with a mantle of copper, or by a
squirrel cage of copper bars (like fig. 1,740), or by embedding
rods of copper (short circuited together at their ends with
rings) in holes just beneath its surface. However, since all
eddy currents that circle round, as those sketched in fig.
1,738, are not so efficient in their mechanical effect as
currents confined to proper paths, and as they consume power
and spend it in heating effects, the core was then constructed
with laminations lightly insulated from each other, and further
the squirrel cage copper bar inductors were fully insulated
from contact with the core. Tunnel slots were later replaced by
designs with open tops.
[Illustration: Figs. 1,750 and 1,751.--Built up core construction
with discs punched in one piece. The spider proper consists of a hub
provided with four radial arms, which fit the inner diameter of the
disc. The hub is bored out so that it fits very tightly on the shaft,
and a key is provided to avoid any chance of turning. The core disks
are clamped firmly in place by two heavy cast iron end plates which are
pressed up and held by the bolts. These bolts pass under the discs, so
that there is no danger of their giving rise to eddy currents. The key
not only prevents the discs turning on the spider but also ensures the
alinement of discs, which is necessary to make the teeth form smooth
slots when the core is assembled.]
Fig. 1,744 shows a modern squirrel cage armature conforming to the
latest practice, other designs being illustrated in the numerous
accompanying cuts.
In the smaller sizes, the core laminæ are of the solid type as shown
in fig. 1,745, but for larger motors the core consists of a spider and
segmental discs as shown in figs. 1,750 and 1,751.
Fig. 1,748 shows a soldered form of end ring construction, and figs.
1,752 and 1,753 the method of welding the end ring to the inductors.
~The Field Magnets.~--The construction of the field magnets, which,
when energized with alternating current produce the rotating magnetic
field, is in many respects identical with the armature construction of
revolving field alternators.
[Illustration: FIG. 1,752.--Fairbanks-Morse squirrel cage armature with
cast-on rings showing inspection grooves. The method consists in fusing
the ends of the inductors into an end ring of a special composition,
thereby producing a perfect electrical and mechanically strong joint.
In this process the armature with its bars in place is put into a mould
and the molten metal poured around the inductors, melting their ends
and effectually fusing them into the body of the ring. The ring is then
turned down to finished size and polished. An inspection groove is cut
as shown to indicate that the fusion is complete and the joint perfect.]
[Illustration: FIG. 1,753.--Section of Fairbanks-Morse "cast-on" joint
showing union of end ring and inductor. The view shows the V-shape
inspection groove as described in fig. 1,752.] Broadly, the field
magnets of induction motors consists of:
1. Yoke or frame;
2. Laminæ, or core stampings;
3. Winding.
[Illustration: FIG. 1,754.--Richmond field construction for polyphase
induction motors, showing style of winding for use with squirrel cage
and wound armature types.]
~Ques. What is the construction of the yoke and laminæ?~
Ans. They are in every way similar to the armature frame and core
construction of revolving field alternators.
[Illustration: FIG. 1,755.--Western Electric squirrel cage armature
of high speed induction motor for centrifugal pump service. This
armature is an example of heavy duty construction. The inductors are
welded to the short circuiting end rings, the latter being located
beneath the inductors, as shown. Fan vanes are provided at one end
for ventilation. In the field construction, the core laminations are
assembled in a closed box frame, and clamped by heavy rings while under
hydraulic pressure. The stator coils are form wound and subjected to a
special insulating process, which renders them especially impervious
to moisture, and capable of operating without breakdown in locations
which are too damp for ordinary motors. The bearing brackets are of
rigid mechanical construction, and the pulley end bracket and bearings
of all sizes are split to facilitate removal of the rotor and complete
inspection. These machines range in size from 50 to 200 horse power,
the rugged construction adapting them to heavy and severe service, such
as is met with in mining, the construction of dams, canals, aqueducts,
tunnels, etc.]
[Illustration: FIG. 1,756.--Wagner squirrel cage armature for polyphase
induction motor, as employed on motors of from 5 to 25 horse power.
The features of construction as seen in the illustration are bar
inductors, ventilating passages through the core laminæ, riveted
connection between inductors and end rings ventilating vanes on end
plate, extra large end rings. The object of making the rings unusually
large is to make the resistance of the rings lower than is desirable
for some classes of service, in order to obtain motors having minimum
slip, increased efficiency, and maximum overload capacity under normal
operation. When the torque required by some very unusual and entirely
abnormal installation exceeds that of the average conditions, it is
an easy matter to reduce the section of the end rings, by turning
them down in a lathe, thereby increasing the resistance and starting
torque.]
~Field Windings for Induction Motors.~--The field windings of induction
motors are almost always made to produce more than two poles in order
that the speed may not be unreasonably high. This will be seen from the
following:
If P be the number of _pairs_ of poles per phase, _f_, the
frequency, and N, the number of revolutions of the rotating
field per minute, then
60 × _f_
N = --------
P
Thus for a frequency of 100 and one pair of poles, N = 60 × 100
÷ 1 = 6,000. By increasing the number of pairs of poles to 10,
the frequency remaining the same, N = 60 × 100 ÷ 10 = 600.
Hence, in design, by increasing the number of pairs of poles
the speed of the motor is reduced.
[Illustration: FIG. 1,757.--Richmond squirrel cage armature. The copper
bars are double riveted at either end to the resistance rings, then
dipped into a solder bath.]
~Ques. State an objection to very high speed of the rotating field.~
Ans. The more rapid the rotation of the field, the greater is the
starting difficulty.
~Ques. Besides employing a multiplicity of poles, what other means is
used to reduce the speed?~
Ans. Reducing the frequency.
~Ques. What difficulty is encountered with low frequency currents?~
Ans. If the frequency be very low, the current would not be suitable
for incandescent lamp lighting, because at low frequency the rise and
fall of the current in the lamps is perceptible.
[Illustration: FIG. 1,758.--Field construction of Crocker-Wheeler
induction motor with _magnetic bridge_. Steel bridges are inserted in
the grooves where the coils are placed, to protect them from dirt and
mechanical injury and at the same time provide a path for the magnetic
flux which has a more uniform reluctance, thereby insuring a better
distribution of the flux in the air gap and at the same time retaining
open slot construction from which the coils can be readily removed.]
~Ques. What is the general character of the field winding?~
Ans. The field core slots contain a distributed winding of
substantially the same character as the armature winding of a revolving
field polyphase alternator.
~Ques. Are the poles formed in the usual way?~
Ans. They are produced by properly connecting the groups of coils and
not by windings concentrated at certain points on salient or separately
projecting masses of iron, as in direct current machines.
~Ques. How are the coils grouped?~
Ans. Three phase windings are usually ~Y~ connected.
[Illustration: FIG. 1,759.--Western Electric squirrel cage armature.
The inductors consist of solid copper bars embedded in the slots of a
laminated core, with their projecting ends securely fitted and soldered
to heavy copper rings.]
~Ques. What other arrangement is sometimes used?~
Ans. In some cases ~Y~ grouping is used for starting and Δ grouping for
running.
* * * * *
~Starting of Induction Motors.~--It must be evident that if the field
winding of an induction motor whose armature is at rest, be connected
directly in the circuit without using any starting device, the machine
is placed in the same condition as a transformer with the secondary
short circuited and the primary connected to the supply circuit. Owing
to the very low resistance of the armature, the machine, unless it be
of very small size, would probably be destroyed by the heat generated
before it could come up to speed. Accordingly some form of starting
device is necessary. There are several methods of starting, as with:
1. Resistances in the field;
2. Auto-transformer or compensator;
3. Resistance in armature.
[Illustration: FIG. 1,760.--Holzer Cabot combination polyphase
induction motor set, consisting of wound frame and three rotors: 1,
squirrel cage armature, 2, wound armature, 3, rotating field. The set
is intended for school demonstration of induction motor phenomena.
The motor operating with the squirrel cage armature has an inherent
constant speed characteristic and on brake tests will show its
exceptionally strong starting torque and ability to take excessive
overloads. This motor can be used as a generator also, in the sense
that if connected to the line and driven above synchronous speed by
some external means, it will act as an asynchronous generator and
return power to the line. For variable speed service, an armature
having a winding upon it similar to that on the frame must be used.
External resistances inserted in the armature circuit may be used to
produce, first, a reduction of starting current, second, an increase
of starting torque, or third, a variation of speed. Thus an extensive
list of experiments can be performed with this phase wound armature
directly along the line of present engineering practice. The phase
wound armature can be used as an alternator in the same sense as
mentioned above for the squirrel cage machine. For synchronous motor
and three phase operation the revolving field with projecting poles and
slip rings would be used, the field being excited from a direct current
supply.]
~Ques. Explain the method of inserting resistances in the field.~
Ans. Variable resistances are inserted in the circuits leading to the
field magnets and mechanically arranged so that the resistances are
varied simultaneously for each phase in equal amounts. These starting
resistances are enclosed in a box similar to a direct current motor
rheostat.
~Ques. Is this a good method?~
Ans. It is more economical to insert a variable inductance in the
circuit, by using an auto-transformer.
[Illustration: FIG. 1,761.--Westinghouse auto-starter. Polyphase
induction motors may be started by connecting them directly to the
circuit with an ordinary switch, and the smaller motors are started
in this way in practice. In the larger motors, however, the starting
torque at normal voltage is several times its full load torque;
therefore, they are started on a reduced voltage, and the full
pressure of the circuit is not applied until they have practically
reached their operating speed. The figure shows connections with a
two phase alternating current circuit. The auto-starter consists of
two auto-transformers T and T', each having only a single winding for
both primary and secondary, which are tapped at certain points by
switches, thus dividing the winding into a number of loops, so that
one of several voltages may be applied for starting, and the starting
torque thus adjusted to the work that has to be performed. At the
highest points tapped by the switches S, and S', the full pressure,
and at the lowest points, the lowest pressure, is applied to the motor
by the operation of the main switch M. This switch has four blades and
three positions. When thrown to the left as indicated, it connects the
auto-transformers T and T', across the circuits A and B respectively,
so that the pressure across the transformer coils, as determined by the
position of the switches S and S', is applied to the motor circuits
A and B. The intermediate position of the switch M interrupts both
circuits. To start the motor, the switch M is thrown to the left and
a reduced pressure applied; after the motor has started and come up
to speed the switch M is thrown to the right, thus cutting out the
transformer and connecting the motor directly to the circuit. The
starting device can be located at a point remote from the motor, thus
eliminating danger from fire due to possible sparks, in case where it
is necessary to install the motors in grain elevators, woolen mills,
or in any place exposed to inflammable gases, or floating particles of
combustible matter. This feature is also valuable in cases where motors
are suspended from the ceiling, or installed in places not easily
accessible.]
~Ques. What is the auto-transformer or compensator method of starting?~
Ans. It consists of reducing the pressure at the field terminals by
interposing an impedance coil across the supply circuit and feeding the
motor from variable points on its windings.
[Illustration: FIG. 1,762.--Auto-transformer or compensator connections
for three phase induction motor. In operation when the double throw
switch is thrown over to starting position, the current for each phase
of the motor flows through an auto-transformer, which consists of a
choking coil for each phase, arranged so that the current may be made
to pass through any portion of it (as 1, 2, 3) to reduce the voltage
to the proper amount for starting. After the motor has come up to
speed on the reduced voltage, the switch is thrown over to running
position, thus supplying the full line voltage to the motor. [7]In
actual construction fuses are usually connected, so that they will be
in circuit in the running position, but not in the starting position,
where they might be blown by the large starting current.]
[7] NOTE.--The construction of starting devices for induction motors
is fully explained later, the accompanying cuts serving merely to
illustrate the principles involved.
~Internal Resistance Induction Motors.~--The armature of this type of
induction motor differs from the squirrel cage variety in that the
winding is not short circuited through copper rings, but, in starting,
is short circuited through a resistance mounted directly on the shaft
in the interior of the armature.
When the motor is thrown in circuit, a very low starting
current is drawn from the line due to the added resistance in
the armature. As the motor comes up to speed, this resistance
is gradually cut out, and at full speed the motor operates as a
squirrel cage motor, with short circuited winding. #/
~Ques. How is the resistance gradually cut out in internal resistance
motors?~
Ans. By operating a lever which engages a collar free to slide
horizontally on the shaft. The collar moves over the internal
resistance grids (located within the armature spider), thus gradually
reducing their value until they are cut out.
[Illustration: FIG. 1,763.--View of armature interior of Wagner
polyphase induction motor with wound armature, showing the centrifugal
device which at the proper speed short circuits all the coils,
transforming the motor to the squirrel cage type. The winding is
connected with a vertical "commutator" so called. Inside the armature
are two governor weights, which are thrown outwards by the centrifugal
force when the machine reaches the proper speed, thus pushing a solid
copper ring (which encircles the shaft) into contact with the inner
ends of the "commutator" bars, thus completely short circuiting the
armature winding.]
~Ques. For what size motors is the internal resistance method suited?~
Ans. Small motors.
~Ques. Why is it not desirable for large motors?~
Ans. The excessive I²R loss in the resistances, if confined within
the armature spider, would produce considerable heating, and on this
account it is best placed external to the motor.
~Ques. On what class of circuit are internal resistance motors
desirable?~
Ans. On circuits devoted to lighting service as well as power service,
where a high degree of voltage regulation is essential.
The initial rush of current when a squirrel cage motor is
thrown on the line is more or less objectionable and there are
central stations which allow only resistance type of induction
motor to be used on their lines.
[Illustration: FIG. 1,764.--Western Electric wound armature for
internal resistance induction motor. In starting the inductors are
short circuited through a resistance which is gradually cut out as the
motor comes up to speed.]
[Illustration: FIGS. 1,765 TO 1769.--Western Electric wound armature
for external resistance, or slip ring induction motor, showing brush
rigging, slip rings and bar winding.]
~External Resistance or Slip Ring Motors.~--In large machines, and
those which must run at variable speed, such as is required in the
operations of cranes, hoists, dredges, etc., it is advisable that the
regulating resistances be placed externally to the motor. Motors having
this feature are commercially known as ~slip ring motors~, because
_connections are made between the external resistances and the armature
inductors by means of slip rings_.
[Illustration: FIG. 1,770.--Richmond slip ring motor.]
[Illustration: FIG. 1,771.--Richmond slip ring armature as used on
motor in fig. 1,770.]
[Illustration: FIG. 1,772.--Western Electric riveted frame slip
ring induction motor for variable speed service; adapted either to
continuous or intermittent operation.]
As with the internal resistance motor the armature winding of a slip
ring motor is not short circuited through copper rings in starting, but
through a resistance, which in this case is located externally.
~Ques. How is the armature winding connected?~
Ans. It is connected in ~Y~ grouping and the free ends connected to
the slip rings, leads going from the brushes to the variable external
resistances, these being illustrated in fig. 1,779.
[Illustration: FIGS. 1,773 to 1,778.--Sprague skeleton type motor
frame with various types of armature. Fig. 1,777, plain squirrel
cage armature; fig. 1,778, internal resistance armature; fig. 1,773
slip ring armature. In the construction of the plain squirrel cage
armature, fig. 1,777, copper bars are inserted in the slots of the
core, and are insulated from the core by enclosing tubes which project
about one-half inch beyond the iron at each side. The bars are short
circuited at their ends by copper rings. These rings are thin, but of
considerable radial depth and are held apart by spacing washers. They
have rectangular holes punched near their outward periphery, through
which the armature bars pass, and to which they are soldered. The
internal resistance armature, fig. 1,778, is provided with a phase
winding, starting (internal) resistance, and switch located on the
shaft. The starting resistance is designed to give approximately full
load torque with full load current at starting. A greater torque than
full load torque can be obtained for starting, if required, by cutting
out resistance. The resistance consists of cast iron grids enclosed
in a triangular cover which is bolted to the end plates holding the
armature laminæ together, and is short circuited by sliding laminated
spring metal brushes along the inside surface of the grids. The
brushes are supported by a metal sleeve sliding on the shaft which
is operated by a lever secured to the bearing bracket and located
just above the bearing. A rod passing through the end of the shaft
operates the short circuiting arrangement in sizes up to about 25 horse
power. The external resistance or slip ring armature, fig. 1,773, is
similar in construction to fig. 1,778, with the exception that slip
rings are provided because of the external location of the resistance.
These rings connect the inductor through brushes to a controlling and
external resistance, two or more carbon brushes being provided for each
ring, as in fig. 1,776.]
~Single Phase Induction Motors.~--The general utility of single phase
motors, particularly the smaller sizes, is constantly being enlarged
by the growing practice of central stations generating polyphase
current, of supplying their lighting service through single phase
distribution, and permitting the use of single phase motors of moderate
capacity on the lighting circuit.
[Illustration: FIG. 1,779.--External resistance or slip ring induction
motor connections. The squirrel cage armature winding is not short
circuited by copper end rings, but connected in ~Y~ grouping and the
three free ends connected to three slip rings, leads going from the
brushes to three external resistances, arranged as triplex rheostat
having three arms rigidly connected as shown, so that the three
resistances may be varied simultaneously and in equal amounts.]
[Illustration: FIG. 1,800.--Allis-Chalmers phase wound external
resistance type or slip ring armature construction. The winding is for
three phases and the terminals are brought out to three slip rings. The
front bracket is slightly modified to make room for these rings on the
inside. For starting duty sufficient resistance is supplied to reduce
the starting current taken by the motor to 1¼ times the normal full
load current. In the running position the resistance is all cut out of
the circuit. For speed regulation sufficient resistance is supplied to
reduce the speed 50% on normal full load torque.]
[Illustration: FIGS. 1,801 to 1,828.--Disassembled view of Western
Electric three phase external resistance or slip ring mill type
induction motor. It is adapted to severe working conditions, such as
are met with in steel mills, crane and hoist service, etc. Designed for
220 or 440 volt, 25 cycle circuits. The frame is divided horizontally
into an upper and a lower steel casting, both of which are bolted
together at the corners by four heavy bolts. The lower casting is
provided with four feet for bolting the motor to its foundation.
The end of the upper frame which covers the slip rings is equipped
with malleable iron covers held in place by lock bolts. The field
and armature are of the usual construction. One end of the armature
winding is protected against mechanical injury by the slip rings which
are of heavy construction and of practically the same diameter as
the armature, and the other end by a detachable flange of the same
diameter as the outside of the winding. The slip rings are mounted
on the same spider as the armature, so that the shaft can be removed
without disturbing any of the connections. The brushes are equipped
with riveted pigtails, and held in brass brush boxes machined to gauge.
Heavy coiled clock springs are used to maintain an even pressure of the
brushes on the slip rings. The armature leads are brought out through
holes in the upper half of the frame, and the field leads are brought
through a block, which fits in an opening in the upper edge of the
lower half.]
The simplicity of single phase systems in comparison with polyphase
systems, makes them more desirable for small alternating current plants.
The disadvantage of single phase motors is that they are not
self-starting.
A single phase motor consists essentially of an _armature and
field magnet having a single phase winding and also some phase
splitting arrangement for starting_.
[Illustration: FIG. 1829.--General Electric single phase induction
motor. It is suitable for constant speed service where full load torque
at starting does not exceed 140 per cent., and in general is adapted to
drive all geared and belted machinery requiring _constant speed with
light or moderate_ starting torque.]
[Illustration: FIG. 1,830.--Simplified diagram showing the principle of
phase splitting for starting single phase induction motors. By the use
of an auxiliary set of coils connected in parallel with the main coils
and having in series a resistance or condenser as shown, the single
phase current delivered by the alternator is "split" into two phases,
which are employed to produce a rotating field on which the motor is
started.]
[Illustration: FIGS. 1,831 to 1,850.--Parts of Sprague single phase
clutch type induction motor. The armature is of the high resistance
smooth core squirrel cage type, the core laminæ being assembled upon
a steel sleeve. On starting the armature revolves _freely_ around the
shaft on roller bearings until it accelerates to about 75% of its
rated speed, when a centrifugal clutch engages with an outer shell
keyed directly on the shaft, thus throwing on the load. This type of
motor is adapted to drive all belted, geared, or direct connected
machinery requiring constant speed with moderate starting torque, such
as generators, blowers, line shafting in machine shops and factories,
drill presses, laundry machinery, baking machinery, and the like. When
greater torque is required at the moment of starting type RI motors
should be used, or clutch couplings may be installed between the motor
and the machine it is to drive. The parts are as follows; A, field
frame; B, field coils; C, terminal block; D, terminal block screws; E,
connectors; F, bearing head pulley end; G, bearing head opposite pulley
end; H, motor clamping bolts; I, oil well cover; J, oil well plug; K,
drain plug; L, oil filter; M, cap bolts; N, bearing lining; O, oil
ring; P, belt tightener screw; Q, armature core; R, latch; S, driving
shell; T, driving shell set screw; U, clutch ring; V, clutch ring
spring; W, spring adjusting screw; X, nut for belt tightener screw;
Y, shaft; Z, driving shell key; Aa, armature bearing; Ba, pulley; Ca,
pulley set screw; Da, pulley key; Ea, sliding base; Fa, yoke.]
~Ques. Why is a single phase motor not self-starting?~
Ans. Because the nature of the field produced by a single phase current
is oscillating and not rotating.
~Ques. How is a single phase motor started?~
Ans. By splitting the phase, a field is set up normal to the axis of
the armature, and nearly 90° displaced in phase from the field in that
axis. This cross field produces the useful torque.
[Illustration: FIG. 1,851.--General Electric high resistance clutch
type smooth core squirrel cage armature of single phase induction
motor. The core laminæ are slotted near the circumference to retain the
bar inductors, which extend beyond the core at either end where they
are permanently connected to heavy short circuiting rings.]
[Illustration: FIGS. 1,852 to 1,855.--Parts of General Electric
centrifugal clutch pulley as used on clutch type, single phase
induction motor. A, clutch; B, friction band; C, adjusting spring; D,
outer clutch shell with pulley sleeve; E, solid removable pulley; F,
internal mechanism comprising parts A, B, and C; G, outer shell and
pulley comprising parts D and E.]
[Illustration: FIGS. 1,856 and 1,857.--Partly assembled clutch pulley.
F, internal mechanism comprising parts A, B, C, of fig. 1,852. G, outer
shell and pulley, comprising parts D and E of fig. 1,852.]
~Phase Splitting; Production of Rotating Field from Oscillating
Field.~--As previously stated, an oscillating field, that is, one
due to a single phase current, does not furnish any starting torque.
It is therefore necessary to provide a rotating field for a single
phase induction motor to start on, which, after the motor has come
up to speed, may be cut out and the motor will then operate with the
oscillating field.
A rotating field may be obtained from single phase current by what is
known as _splitting the phase_.
[Illustration: FIG. 1,858.--Switch end view of General Electric drawn
shell type fractional horse power single phase motor.]
~Ques. Describe one method of splitting the phase.~
Ans. The field of the motor is provided, in addition to the main single
phase winding, with an auxiliary single phase winding, and the two
windings are connected in parallel to the single phase supply mains
with a resistance or a condenser placed in series with the single
phase winding, as shown in diagram fig. 1,830, the two windings being
displaced from each other on the armature about 90 magnetic degrees,
just as in the ordinary two phase motor.
~Ques. What is the construction of the two windings?~
Ans. The main coils are of more turns than the auxiliary, being spread
over more surface, and are heavier because they are for constant use;
whereas the auxiliary coils are used only while starting.
[Illustration: FIGS. 1,859 to 1,862.--Detail construction of clutch
parts of General Electric drawn shell type fractional horse power
single phase motor. The starting switch, which is assembled within the
motor frame, consists essentially of three parts: a rotating member
mounted on the armature and provided with two spring controlled pivoted
levers in contact with an insulated collector ring.]
~Ques. What are the auxiliary coils sometimes called?~
Ans. _Starting coils._
~Ques. What are "shading" coils?~
Ans. Auxiliary coils as placed on fan motors in the manner shown in
fig. 1,863.
~Ques. How can single phase motors be started without the use of
external phase splitting devices?~
Ans. Such apparatus may be avoided by having the auxiliary winding of
larger self-inductance than the main winding.
~Ques. What is the character of the starting torque produced by
splitting the phase?~
Ans. It does not give strong starting torque.
[Illustration: FIG. 1,863.--Single phase fan motor with _shading
coils_ for starting. In addition to the main field coils, one tip of
each pole piece is surrounded by a short circuited coil of wire or
frame of copper, as indicated in the figure. This coil, or copper
frame, is called a _shading coil_ and it causes a phase difference
between the pulsating flux that emanates from the main portion of each
polar projection and the pulsating flux which emanates from the pole
tip, thus introducing a two phase action on the armature which is
sufficiently pronounced to start the motor.]
~Ques. How is the plain squirrel cage armature modified to enable the
motor to start with a heavier load?~
Ans. An automatic clutch is provided which allows the armature to turn
free on the shaft until it accelerates almost to running speed.
This type motor is known as the _clutch type_ of single phase
induction motor. In operation when the circuit is closed, the
armature starts to revolve upon the shaft; when it reaches a
premeditated speed, a centrifugal clutch expands and engages
the clutch disc, which is fastened to the shaft.
[Illustration: FIG. 1,864.--Diagram showing action of shading coil in
alternating current motor. The extremities of these pole pieces are
divided into two branches, one of which a copper ring called a _shading
coil_ is placed as shown, while the other is left _unshaded_. The
action of the shading coils is as follows: Consider the field poles
to be energized by single phase current, and assume the current to be
flowing in a direction to make a north pole at the top. Consider the
poles to be just at the point of forming. Lines of force will tend to
pass downward through the shading coil and the remainder of the pole.
Any change of lines within the shading coil generates an e.m.f., which
causes to flow through the coil a current of a value depending on the
e.m.f. and always in a direction to oppose the change of lines. The
field flux is, therefore, partly shifted to the free portion of the
pole, while the accumulation of lines through the shading coil is
retarded.]
[Illustration: FIGS. 1,865 and 1,866.--Fort Wayne split phase factional
horse power induction motor with stationary armature. The object of
placing the squirrel cage armature winding on the stationary part or
frame is to decrease the radial depth of the latter more than would
be possible with the usual arrangement where the armature forms the
rotating part. The small radial depth of the stationary armature
makes possible a revolving field of maximum diameter giving in turn
an exceptionally large air gap area, which reduces the magnetizing
current, hence improves the power factor of the motor.]
~Ques. Explain in detail the action of the clutch type of motor in
starting.~
Ans. It can start a load which requires much more than full load torque
at starting, because the motor being nearly up to full speed, has
available not only its maximum overload capacity, but also the momentum
of the armature to overcome the inertia of the driven apparatus. In
this it is assisted by a certain amount of slippage in the clutch,
which is the case when the armature speed is pulled down to such a
point as to reduce the grip of the centrifugal clutch.
[Illustration: FIGS. 1,867 and 1,868.--General Electric disassembled
clutch as used on clutch type, single phase (KS) induction motor. In
starting, the armature revolves freely on the shaft until approximately
75 per cent. of normal rated speed is reached. The load is then picked
up by the automatic action of a centrifugal clutch, which rigidly
engages an outer shell, keyed directly to the shaft. The brass friction
band of the clutch is permanently keyed to the pulley end of the
armature.]
~Commutator Motors.~--Machines of this class are similar in general
construction to direct current motors. They have a closed coil winding,
which is connected to a commutator.
There are several types of commutator motor, namely:
1. Series;
2. Shunt;
3. Compensated;
4. Repulsion.
Since, as stated, commutator motors are similar to direct current
motors, the question may be asked: Is it possible to run a direct
current motor with alternating current? If the mains leading to a
direct current motor be reversed, the direction or rotation remains the
same, because the currents through both the field magnets and armature
are reversed. It must follow then that an alternating current applied
to a direct current motor would cause rotation of the armature.
[Illustration: FIG. 1,869.--Wagner single phase variable speed
commutator motor. The commutator is of the regular horizontal type
and the brushes remain in contact all the time. As the torque of
alternating motors varies directly as the square of the applied
pressure, wide speed variation may be obtained by varying the voltage
applied at the motor terminals.]
~Action of Closed Coil Rotating in Alternating Field.~--When a closed
coil rotates in an alternating field, there are several different
pressures set up and in order to carefully distinguish between them,
they may be called:
[Illustration: FIGS. 1,870 and 1,871.--Diagrams illustrating
construction and operation of Wagner "unity power factor" single
phase motor. In the field construction, fig. 1,870, two windings are
used. The main winding 1 produces the initial field magnetization
as heretofore; the auxiliary winding 2 controls the power factor or
"compensates" the motor. The main structural departure is in the
armature, the construction of which is more clearly indicated in fig.
1,871. Here again two windings are employed. The main or principal
winding 4 is of the usual well known squirrel cage type and occupies
the bottom of the armature slots. The second or auxiliary winding 3
is of the usual commuted type, is connected to a standard form of
horizontal commutator and occupies the upper portion of the armature
slots. Between the two is placed a magnetic separator in the form of
a rolled steel bar. Two sets of brushes are provided, as indicated
in the diagram of connections shown in fig. 1,870. The main pair of
brushes 5-6 is placed in the axis of the main field winding 1 and is
short circuited. The auxiliary pair of brushes 7-8 is placed at right
angles to the axis of the main field winding and is connected in series
with it at starting. The auxiliary field winding 2 is permanently
connected to one auxiliary brush 7, and is adapted to be connected to
the other auxiliary brush 8 by means of the switch 9. The purpose of
the peculiar armature construction illustrated in fig. 1,871 and of the
brush arrangement and connections shown in fig. 1,870 is to accentuate,
at starting, the effect of the squirrel cage along the axis 5-6 of the
main field winding 1, while suppressing it as far as possible along
the axis 7-8 at right angles to main winding. The magnetic separator
placed above the squirrel cage winding 4 tends to suppress the effect
of that winding along all axes, by making it less responsive to outside
inductive effects. But the influence of the separator is nullified
along the axis of the main field winding by the presence of the short
circuited brushes 5-6, while no means are provided for nullifying
its effects along the axis at right angles to that of the main field
winding. Thus the main field winding 1 will be able to induce heavy
currents in both armature windings because of the short circuited
brushes in the axis 5-6, and in spite of the magnetic separator; while
the armature winding 3, connected in series with 1, will not be able to
produce heavy currents in the squirrel cage winding 4 along the axis
7-8 because of the magnetic separator between 3 and 4, which shunts
or side tracks the inducing magnetic flux. In operation, at starting,
switch 9 of fig. 1,870 is open, the commuted winding 3 along the axis
7-8 being connected in series with the main field winding 1 and across
the mains. The winding 1 induces a large current in the armature
windings 3 and 4 along the axis 5-6, and the winding 3 produces a
large flux along the axis 7-8. The armature currents in the main axis
co-acting with the flux threading the armature along the auxiliary axis
yield the greater part of the starting torque. As the motor speeds
up, the squirrel cage gradually assumes those functions which it
performs in the ordinary single phase, squirrel cage motor, developing
a magnetic field of its own along the axis 7-8 and a correspondingly
powerful torque, which increases very rapidly as synchronism is
approached, but falls suddenly to zero at or near actual synchronism.
It is known that the magnetizing currents circulating in the bars of
the squirrel cage of a single phase motor have, at synchronism, double
the frequency of the stator currents; the fluxes they produce must
therefore also be of double frequency. Now, the magnetic separator is
made of solid steel, and, while this separator forms a sufficiently
effective shunt for the fluxes of line frequency induced from the
field, it is quite ineffective as a shunt for the double frequency
fluxes produced by the armature. With respect to the squirrel cage, the
effect of this magnetic separator diminishes with increasing speed, and
at synchronism the machine operates practically in the same manner as
if the magnetic separator did not exist.]
1. The transformer pressure;
2. The generated pressure;
3. The self-induction pressure.
These pressures may be defined as follows:
~The transformer pressure~ _is that pressure induced in the armature by
the alternating flux from the field magnets._
[Illustration: FIG. 1,872.--Diagram of ring armature in alternating
field illustrating the principles of commutator motors.]
For instance, assuming in fig. 1,872 the armature to be at
rest, as the alternating current which energizes the magnets
rises and falls in value, the variations of flux which threads
through the coils of the ring winding, induce pressure in them
in just the same way that pressure is induced in the secondary
of a transformer.
A ring winding is used for simplicity; the same conditions
obtain in a drum winding.
~The generated pressure~ is that pressure _induced in the armature by
the cutting of the flux when the armature rotates_.
~The self-induction pressure~ _is that pressure induced in both the
field and armature by self-induction._
~Nature of the Generated Pressure.~--In fig. 1,872, the
generated pressure induced by the rotation of the armature is
minimum at the neutral plane C D and maximum at A B. It tends
to cause current to flow up each half of the armature from D to
C, producing poles at these points.
[Illustration: FIG. 1,873.--Wagner single phase repulsion induction
commutator motor. Its working principle is _repulsion start_ and
_induction operation_. Starting with the machine at rest, brushes in
pairs cross connected through a low resistance conductor, bear upon
the commutator, temporarily short circuiting the armature winding then
developing a strong starting torque on the repulsion principle. On
attaining full load speed the individual segments of the commutator
are all positively connected together by the operation of an automatic
centrifugal governor, thereby transforming the armature winding to the
squirrel cage form, the motor then continuing as an induction motor.
The governor at the same time removes the brushes from contact with
the commutator to save wear. If the power service should fail for any
reason, the motor returns to the starting condition, and picks up its
load when the power comes on again without attention of the operator.]
~Nature of the Transformer Pressure.~--This is caused by
variations of the flux passing through each coil of the
armature winding. Evidently this variation is least at the
plane A B because at this point the coils are inclined very
acutely to the flux, and greatest at the plane C D where
the coils are perpendicular to the flux. Accordingly, the
transformer pressure induced in the armature winding is least
at A B and greatest at C D.
The transformer pressure acts in the same direction as the
generated pressure as indicated by the long arrows and gives
rise to what may be called _local armature currents_.
[Illustration: FIGS. 1,874 and 1,875.--Armature of Wagner single phase
repulsion-induction commutator motor as seen from the commutator
and rear ends, showing the vertical commutator and type of governor
employed on the smaller sizes. The operation is explained in fig.
1,873.]
~Nature of the Self-induction Pressure.~--The self-induction
pressure, being opposite in direction to the impressed
pressure, it must be evident that in the operation of an
alternating current commutator motor, the impressed pressure
must overcome not only the generated
[Illustration: FIG. 1,876.--General Electric single phase compensated
repulsion motor. The frame is of the riveted form and the field winding
consists of distributed concentric coils, each being separately
insulated and taped up to each core slot. The compensating winding
(depending usually on the size of frame), forms either the center
portion of the main winding or a separate winding concentric therewith.
The polar groupings are arranged for a frequency of 25 and 60. There
are four terminal leads permitting interchangeability of operation
on 110 or 220 volt circuits. By connecting adjacent pairs of these
terminals in multiple, motors of this type are made adaptable for 110
volt service; for double this pressure the four leading in wires are
connected in series. The motor will operate satisfactorily where the
arithmetical sum of voltage and frequency variation does not exceed
10 per cent.; that is, the voltage may be 10 per cent. high if the
frequency remain at normal, or the frequency may be 10 per cent.
high assuming no variation in voltage. A decrease of 5 per cent. in
frequency accompanied by a similar increase in voltage is permissible
or, as above stated, any similar combination whose arithmetical sum is
within 10 per cent. of normal. The armature winding is of the series
drum type connected to a commutator carrying two sets of brushes,
each set being displaced electrically from the other by 90 degrees.
The first set, known as the ~energy brushes~, is permanently short
circuited and disposed at an angle to the lines of field or primary
magnetization, as in an ordinary repulsion motor. The second set, or
~compensating brushes~, is connected to a small portion of the primary
winding included in the field circuit, so as to impress upon the
armature an electromotive force, which serves both to raise the power
factor and at the same time maintain approximately synchronous speed at
all loads. The armature laminations are built up on a cast iron sleeve
having the same inside bore as the commutator. In case the shaft become
damaged or worn, it can be readily pressed out and replaced without
disturbing the commutator or windings. The motor is connected to run
counter clockwise. Clockwise rotation is obtained by interchanging
the leads to the compensating brushes and slightly shifting the brush
holder yoke. This type motor may be thrown on the line without the use
of a rheostat, and is suitable for operating refrigerating machines,
air compressors, house pumps or similar apparatus where a float switch
or pressure regulator is used to close or open the supply circuit.] /#
pressure but also the self-induction pressure. Hence, as compared to an
equivalent direct current motor, the applied voltage must be greater
than in the direct current machine, to produce an equal current.
[Illustration: FIG. 1,877.--Armature of General Electric single phase
compensated repulsion motor, assembled ready for dip and banding.]
[Illustration: FIG. 1,878.--Cast brush rigging of General Electric
single phase compensated repulsion motor as used for the 3 and 5 horse
power motors.]
~The Local Armature Currents.~--These currents produced by the
transformer pressure occur in those coils undergoing commutation. They
are large, because the maximum transformer action occurs in them, that
is, in the coils short circuited by the brushes.
~Ques. Why do the local armature currents cause sparking?~
Ans. Because of the sudden interruption of the large volume of current,
and also because the flux set up by the local currents being in
opposition to the field flux, tends to weaken the field just when and
where its greatest strength is required for commutation.
[Illustration: FIG. 1,879.--Field of Sprague single phase compensated
repulsion motor. The frame is of the skeleton form which exposes the
core, giving effective heat radiation. The single phase field winding
is of the distributed concentric type. To facilitate connection to
circuits of either 110 or 220 volts, four plainly tagged leads are
brought out to the back of the removable terminal board.]
~Ques. What is the strength of the local current?~
Ans. They may be from 5 to 15 times the strength of the normal armature
current.
~Ques. Upon what does the local armature current depend?~
Ans. Upon the number of turns of the short circuited coils, their
resistance, and the frequency.
~Ques. How can the local currents be reduced to avoid heavy sparking?~
Ans. 1. By reducing the number of turns of the short circuited coils,
that is, providing a greater number of commutator bars; 2, reducing the
frequency; and 3, increasing the resistance of the short circuited coil
circuit: _a_, by means of high resistance connectors; or _b_, by using
brushes of higher resistance.
[Illustration: _Figs._ 1,880 to 1,884.--Assembly and disassembled view
of short circuiting device as used on Bell single phase repulsion
induction motor. The armature, which is wound in a similar manner to
those used in direct current motors, has a commutator, and brushes,
which being short circuited on themselves, allow great starting
torque, with small starting current. The motor starts by the repulsion
principle, and on reaching nearly full speed, a centrifugal governor
pushes the copper ring against the commutator segments, thereby
short circuiting them, and the motor then operates on the induction
principle.]
~Ques. What are high resistance connectors?~
Ans. The connectors between the armature winding and the commutator
bars, as shown in fig. 1,885.
~Ques. Does the added resistance of preventive leads, or high
resistance brushes, materially reduce the efficiency of the machine?~
Ans. Not to any great extent, because it is very small in comparison
with the resistance of the whole armature winding.
[Illustration: FIG. 1,885.--Section of ring armature of commutator
motor showing local current set up by transformer action of the
alternating flux.]
~Ques. What is the objection to reducing the number of turns of the
short circuited coils to diminish the tendency to sparking?~
Ans. The cost of the additional number of commutator bars and
connectors as well as the added mechanism.
~Ques. What effect has the inductance of the field and armature on the
power factor?~
Ans. It produces phase difference between the current and impressed
pressure resulting in a low power factor.
~Ques. What is the effect of this low power factor?~
Ans. The regulation and efficiency of the system is impaired.
The frequency, the field flux and the number of turns in the
winding have influence on the power factor.
~Ques. How does the frequency affect the power factor?~
Ans. Lowering the frequency tends to improve the power factor.
The use of very low frequencies has the disadvantage of
departing from standard frequencies, and the probability that
the greater cost of transformers and alternators would offset
the gain.
[Illustration: FIG. 1,886.--General Electric 5 H.P., 6 pole adjustable
speed single phase compensated repulsion motor. This type is suitable
for service requirements demanding the use of a motor whose speed can
be adjusted over a considerable range, this speed at a fixed controller
setting remaining practically unaffected by any load within the
motor's rated capacity. With the controller on the high speed points,
the motor possesses an inherent speed regulation between no load and
full load of approximately 6 per cent. At the low speed points, under
similar load conditions, the speed variation will be approximately 20
per cent. To secure adjustable speed control, the armature circuits
employ transformers, whose primaries are excited by the line circuit.
The secondaries of these transformers are divided into two sections;
the first or "regulating" circuit is placed across the _energy_
brushes; the other section, since it is connected in series with
the compensating winding, maintains the high power factor and speed
regulation obtained in the constant speed type. The speed range is 2:1,
approximately one-half of this range being below and one-half above
synchronous speed.]
~Series Motors.~--This class of commutator motor is about the simplest
of the several types belonging to this division. In general design the
series motor is identical with the series direct current motor, but all
the iron of the magnetic circuit must be laminated and a _neutralizing
winding_ is often employed.
It will be readily understood that the torque is produced in the same
way as in the direct current machine, when it is remembered that the
direction of rotation of the direct current series motor is independent
of the direction of the voltage applied.
At any moment the torque will be proportional to the product of the
current and the flux which it is at that moment producing in the
magnetic system, and the average torque will be the product of the
average current and the average flux it produces, so that if the iron
parts be unsaturated, as they must be if the iron losses are not to be
too high, _the torque will be proportional simply to the square of the
current_, there being no question of power factor entering into the
consideration.
[Illustration: FIG. 1,887.--Diagram of single phase series commutator
motor. It is practically the same as the series direct current motor,
with the exception that all the metal of the magnetic circuit must be
laminated.]
~Ques. What are the characteristics of the series motor?~
Ans. They are similar to the direct current series motor, the torque
being a maximum at starting and decreasing as the speed increases.
~Ques. For what service is the series motor especially suited?~
Ans. On account of its powerful starting torque it is particularly
desirable for traction service.
~Neutralized Series Motor.~--A chief defect of the series motor is the
excessive self-induction of the armature, hence in almost every modern
single phase series motor a neutralizing coil is employed _to diminish
the armature self-induction_.
The neutralizing coil is wound upon the frame 90 magnetic degrees
or half a pole pitch from the field winding and arranged to carry a
current equal in magnetic pressure and opposite in phase to the current
in the armature.
[Illustration: FIG. 1,888.--Diagram of neutralized series motor;
~conductive method~. In the simple series motor, there will be
a distortion of the flux as in the direct current motor. As the
distorting magnetic pressure is in phase with that of the magnets, the
distortion of the flux will be a fixed effect. If the poles be definite
as in direct current machines, this distortion may not seriously affect
the running of the motor, but with a magnetizing system like that
universally adopted in induction motors the flux will be shifted as a
whole in the direction of the distortion, which will produce the same
effect as if in the former case the brushes had been shifted forward,
whereas for good commutation they should have been shifted backward.
As in direct current machines, this distortion is undesirable since it
is not conducive to sparkless working, and also reduces to a more or
less extent the torque exerted by the motor. The simplest remedy is
to provide _neutralizing coils_ displaced 90 magnetic degrees to the
main field coils as shown. The neutralizing current is obtained by the
method of connecting the _neutralizing coils_ in series in the main
circuit.]
The current through the neutralizing winding may be obtained, either
1. Conductively; or
2. Inductively.
In the conductive method, fig. 1,888, the winding is connected
in series as shown.
In the inductive method, fig. 1,889, the winding is short
circuited upon itself and the current obtained inductively,
the neutralizing winding being virtually the secondary of a
transformer, of which the armature is the primary.
~Ques. When is the conductive method to be preferred?~
Ans. When the motor is to be used on mixed circuits.
[Illustration: FIG. 1,889.--Diagram of neutralized series motor;
~inductive method~. Although the conductive method of neutralization
is employed in nearly all machines, it is possible merely to short
circuit the neutralizing winding upon itself, instead of connecting
it in series with the armature circuit. In this case the flux due to
the armature circuit cannot be eliminated altogether, as sufficient
flux must always remain to produce enough pressure to balance that
due to the residual impedance of the neutralizing coil. It would be
a mistake to infer, however, that on this account this method of
neutralization is less effective than the conductive one, since the
residual flux simply serves to transfer to the armature circuit a drop
in pressure precisely equivalent to that due to the resistance and
local self-induction of the neutralizing coil in the conductive method.]
~Shunt Motors.~--The simple shunt motor has inherently many properties
which render it unsuitable for practical use, and accordingly is of
little importance. Owing to the many turns of the field winding there
is large inductance in the shunt field circuit.
[Illustration: FIG. 1,890.--Diagram of simple shunt commutator motor.
Owing to its many inherent defects it is of little importance.]
[Illustration: FIG. 1,891.--Compensated shunt induction single phase
motor. The transformer shown in the arrangement is capable of being
replaced by a coil placed on the frame having the same axis as the
field winding, so that the flux produced by the field winding induces
in the coil a pressure in phase with the supply pressure. Such a coil
will now be at right angles to the circuit to which it is connected. In
a similar manner a coil at right angles to the armature circuit, that
is, the circuit parallel to the stator axis, if connected in series
with that circuit, will also serve to compensate the motor.]
The inductance of the armature is small as compared with that of the
field; accordingly, the two currents differ considerably in phase.
The phase difference between the field and armature currents and the
corresponding relation between the respective fluxes results in a weak
torque.
[Illustration: FIG. 1,892.--Fynn's shunt conductive single phase motor.
In order to supply along the stator axis a constant field, suitable for
producing the cross flux to which the torque is due by its action on
the circuit perpendicular to the stator axis, the "armature circuit,"
as it may be called, has a neutralizing coil in series with it, so that
the armature circuit and neutralizing coil together produce no flux.
In addition to this, there is a magnetizing coil along the same axis,
which is connected across the mains and so produces the same flux as
the primary coil in a shunt induction machine. Fynn has proposed a
number of methods of varying the speed and compensating this machine.
It is, however, complicated in itself, and is only suited for very
low voltages, so that on ordinary circuits it would need a separate
transformer.]
_It is necessary to use laminated construction in the field circuit to
avoid eddy currents_, which otherwise would be excessive. Fig. 1,890 is
a diagram of a simple shunt commutator motor.
~Repulsion Motors.~--In the course of his observations on the effects
of alternating currents, in 1886-7, Elihu Thomson observed that a
copper ring placed in an alternating magnetic field tends either to
move out of the field, that is, it is _repelled_ by the field (hence
the name ~repulsion motor~), or to return so as to set itself edgeways
to the magnetic lines.
The explanation of the repulsion phenomenon is as follows:
When a closed coil is suspended in an alternating field so that lines
of force pass through it, as in fig. 1,893, an alternating pressure
will be induced in the coil which will be 90° later in phase than
the inducing flux, and since every coil contains some inductance the
resulting current will lag more or less with respect to the pressure
induced in the coil.
[Illustration: FIG. 1,893.--Effect of alternating field on copper ring.
_If a copper ring be suspended in an alternating field so that the
plane of the ring is oblique to the lines of force, it will turn until
its plane is parallel to the lines of force_, that is, to the position
in which it does not encircle any lines of force. The turning moment
acting upon the ring is proportional to the current in it, to the
strength of the field, and to the cosine of the angle ß.
Hence it is proportional to the product sin ß cos ß. The tendency to
turn is zero both at 0° and at 90°; in the former case because there
is no current, in the latter because the current has no leverage. It
is a maximum when ß = 45°. Even in this position there would be no
torque if there were no lag of the currents in the ring, for the phase
of the induced pressure is in quadrature with the phase state of the
field. When the field is of maximum strength there is no pressure, and
when the pressure reaches its maximum there is no field. If there be
self-induction in the ring causing the current to lag, there will be a
net turning moment tending to diminish ß. The largest torque will be
obtained when the lag of the current in the ring is 45°.]
The cosine of this phase relation becomes a negative quantity which
means that the coil is ~repelled~ by the field.
_It is only when the ring is in an oblique position that it tends to
turn._ If it be placed with its plane directly at right angles to
the direction of the magnetic lines, it will not turn; if ever so
little displaced to the right or left, it will turn until its plane is
parallel to the lines.
[Illustration: FIGS. 1,894 to 1,908.--Parts of General Electric
single phase compensated repulsion motor. The field frame employs the
riveted form of construction, so that the ends of the laminations are
exposed directly to the air, insuring low operating temperatures and
high overload capacity. The field winding consists of a main winding
of the distributed concentric type and a compensating winding. The
series type of winding is employed, and the completed rotor is treated
with a special insulating compound, which renders the coils moisture
proof under ordinary conditions. On motors of more than 2 horse power
capacity a ventilating fan is attached to the rotor which provides a
continuous supply of cool air while the motor is in operation. Two
types of brush holder yoke are used. The smaller motors use a moulded
yoke of insulting compound, reinforced by a cast iron L section ring
embedded in the moulded structure. Cast iron yokes are used on larger
motors. The brushes are of carbon with copper pigtails, which carry all
the current. The brushes in this machine remain permanently in contact
with the commutator. The parts are: A, field; B, field winding; C,
line terminal; D, tube terminal; E, compensating terminal; F, terminal
board; G, brush yoke; H, brush holder; I, carbon brush; J, brush stud;
K, short circuit connection; L, armature; M, commutator, N, shaft; O,
fan; P, commutator end shield; Q, pulley end shield; R, oil well cover;
S, oil plugs; T, oil gauge; U, bearing lining; V, oil ring; W, pulley;
X, pulley set screw; Y, commutator end shield holding bolts; Z, pulley
end shield holding bolts; AA, base; BB, float bolts; CC, belt tightener
screw.]
The production of torque may be explained by saying that the current
induced in the ring produces a cross field which being out of phase
with, and inclined to the field impressed by the primary alternating
current, causes a rotary field, and this in turn, reacting on the
conductor, a turning moment results.
[Illustration: FIG. 1,909.--Fynn's compensated shunt induction motor.
This is a combination of the compensated shunt induction motor with the
ordinary squirrel cage form. In one form, in addition to the ordinary
drum winding on the armature, there is another three phase winding
into the "star," of which the drum winding is connected. This second
winding is connected to three slip rings which are short circuited when
the machine is up to speed. Upon the commutator are placed a pair of
brushes connected to an auxiliary winding placed on the frame in such a
position that the flux from the primary coil induces in it a pressure
of suitable phase to produce compensation. The same pair of brushes is
also used for starting.]
Elihu Thompson took an ordinary direct current armature, placed
it in an alternating field, and having short circuited the
brushes, placed them in an oblique position with respect to the
direction of the field. The effect was to cause the armature to
rotate with a considerable torque.
The inductors of the armature acted just as an obliquely
placed ring, but with this difference, that the obliquity
was continuously preserved by the brushes and commutator,
notwithstanding that the armature turned, and thus the rotation
was continuous. This tendency of a conductor to turn from
an oblique position was thus utilized by him to get over
the difficulty of starting a single phase motor. With this
object in view he then constructed motors in which the use of
commutator and brushes was restricted to the work of merely
starting the armature, which when so started was then entirely
short circuited on itself, though disconnected from the rest of
the circuit, the operation then being solely on the induction
principle.
[Illustration: Fig. 1,910.--Diagram of connection of Sprague single
phase compensated repulsion motor. To reverse direction of rotation
interchange leads C₁ and C₂ and slightly shift the brush holder yoke.
Brushes E₁ and E₂ are permanently short circuited. This diagram of
connections applies also to fig. 1,911.]
~Ques. What difficulty was experienced with Thomson's motor?~
Ans. Since an open coil armature was used, the torque developed was due
to only one coil at a time, which involved a necessarily high current
in the short circuited coil resulting in heavy sparking.
~Ques. How was this remedied?~
Ans. By the use of closed coil armatures in later construction.
~Ques. Did this effectually stop sparking?~
Ans. No.
~Ques. What other means is employed in modern designs to reduce
sparking?~
Ans. Compensation and the use of a distributed field winding, high
resistance connectors, high resistance brushes, etc.
~Ques. What are the names of the two classes of repulsion motor?~
Ans. The simple and the compensated types.
~Ques. Describe a simple repulsion motor.~
Ans. It consists essentially of an armature, commutator and field
magnets. The armature is wound exactly like a direct current armature,
and the windings are connected to a commutator. The carbon brushes
which rest on this commutator are not connected to the outside line,
however, but are all connected together through heavy short circuiting
connectors. The brushes are placed about 60° or 70° from the neutral
axis. The field is wound exactly like that of the usual induction motor.
~Ques. What is the action of this type of motor?~
Ans. If nothing be done to prevent, the motor will increase in speed at
no load until the armature bursts, just as it will in a series direct
current motor.
~Ques. What provision is made to avoid this danger?~
Ans. A governor is usually mounted on the armature which short circuits
the windings, after the motor has been started. The motor then runs as
a squirrel cage induction motor. As a rule the brushes are lifted off
the commutator when the armature is short circuited, so as to prolong
their life.
This is a very successful motor, but it is of course more
costly than the simple squirrel cage motor used on two and
three-phase circuits.
[Illustration: FIG. 1,911.--Diagram of connections of Sprague variable
speed single phase compensated repulsion motor and controller. The
controller is designed to give speed reduction and speed increase as
resistance or reactance is inserted in the energy and compensating
circuits. With the exception of the leads brought out from these
circuits, the constant speed and variable speed motors are identical.
The standard controller gives approximately 2:1 speed variation.]
~Ques. What name may appropriately be applied to the motor?~
Ans. It may be called the _repulsion induction motor,_ because it is
constructed for repulsion start and induction running.
~Ques. Describe a compensated repulsion motor.~
Ans. In its simplest form it consists of a simple repulsion motor in
which there are two independent sets of brushes, one set being short
circuited, while the other set is in series with the field magnet
winding, as in the series alternating current motor.
~Ques. What names are given to the two sets of brushes on a compensated
repulsion motor?~
Ans. The _energy_ or main short circuiting brushes, and the
_compensating_ brushes.
[Illustration: Fig. 1,912.--Diagram of connections of Sprague reversing
type of single phase compensated repulsion motor. As shown, there is a
special reverse field winding having terminals for connection to a four
pole double throw switch.]
~Ques. What is the behavior of the armature of a compensated repulsion
motor at starting?~
Ans. It possesses at starting most of the apparent reactance of the
motor, and the effect of speed is to decrease such apparent reactance,
the latter becoming zero at either positive or negative synchronism,
and negative at higher speeds in either direction.
~Ques. What is the nature of the field circuit of the compensated
repulsion motor at starting?~
Ans. At starting it is practically non-inductive, the effect of speed
being to introduce a spurious resistance which increases directly with
the speed, and becomes negative when the speed is reversed.
~Ques. For what use is the compensated repulsion motor especially
adapted?~
Ans. For light railroad service.
~Ques. When employed thus what is the method of control?~
Ans. A series transformer is used in the field circuit.
~Ques. What frequencies are employed with this motor?~
Ans. 25 to 60, the preferred frequency being 40.
~Ques. To what important use is the repulsion principle put?~
Ans. It is sometimes employed for starting on single phase induction
motors.
In this method, after bringing the motor up to speed, the
winding is then short circuited upon itself, and the motor then
operates on the induction principle.
~Ques. What name is given to this type of motor?~
Ans. It is called the repulsion induction motor.
~Power Factor of Induction Motors.~--In the case of a direct current
motor, the energy supplied is found by multiplying the current
strength by the voltage, but in all induction motors the effect of
self-induction causes the current to lag behind the pressure, thereby
increasing the amount of current taken by the motor. Accordingly, as
the increased current is not utilized by the motor in developing power,
the value obtained by multiplying the current by the voltage represents
an _apparent energy_ which is greater than the real energy supplied to
the motor.
[Illustration: Fig. 1,913.--Fairbanks-Morse squirrel cage armature,
showing ball bearings.]
It is evident, that if it were possible to eliminate the lag entirely,
the real and apparent watts would be equal, and the power factor would
be unity.
The importance of power factor and its effect upon both alternator
capacity and voltage regulation is deserving of the most careful
consideration with all electrical apparatus, in which an inherent phase
difference exists between the pressure and the current, as for instance
in static transformers and induction motors.
While the belief is current that any decrease in power factor from
unity value does not demand any increase of mechanical output, this
is not true, since all internal alternator and line losses manifest
themselves as heat, the wasted energy to produce this heat being
supplied by the prime mover.
Apart from the poor voltage regulation of alternating current
generators requiring abnormal field excitation to compensate for
low power factor, some of the station's rated output is rendered
unavailable and consequently produces no revenue. The poor steam
economy of underloaded engines is also a serious source of fuel wastage.
[Illustration: Fig. 1,914.--Fairbanks-Morse 20 horse power squirrel
cage induction motor connected to a 20 inch self-feed rip and
chamfering saw. The absence of commutator and brushes on the squirrel
cage armature eliminates sparking and therefore renders this type of
motor particularly adapted for use in places where sparking would be
dangerous, such as in wood working plants, textile mills, etc.]
Careful investigations have shown that the power factor of
industrial plants using induction motor drive with units
of various sizes will average between 60 and 80 per cent.
With plants supplying current to underloaded motors having
inherently high lagging current values, a combined factor as
low as 50 per cent. may be expected. Since standard alternators
are seldom designed to carry their rated kilowatt load at less
than 80 per cent. power factor, the net available output is,
therefore, considerably increased.
[Illustration: Fig. 1,915.--Method of casting end rings on squirrel
cage armatures of Fairbanks-Morse induction motors. The metal being
fused to the bars at a temperature in excess of 1,832 degrees Fahr., it
is readily seen that the destructive effect of any subsequent heating
is eliminated. While giving the most intimate contact at the joints, a
multiplicity of joints is avoided as well as solder.]
~Speed and Torque of Motors.~--The speed of an induction motor depends
chiefly on the frequency of the circuit and runs within 5 per cent. of
its rated speed; it will produce full torque if the line voltage do not
vary more than 5 to 10 per cent.
At low voltage the speed will not be greatly reduced as in a direct
current motor, but as the torque is low the motor is easily stopped
when a light load is thrown on.
The current taken by an induction motor from a constant pressure
line varies with the speed as in a direct current motor. When a
load is thrown on, the speed is reduced correspondingly and as the
self-induction or reactance is diminished, more current circulates in
the squirrel cage winding, which in turn reacts on the field coils in
a similar manner and more current flows in them from the line. In this
manner the motor automatically takes current from the line proportional
to the load and maintains a nearly constant speed.
The so-called constant speed motors require slight variations in speed
to automatically take current from the line when the load varies.
Induction motors vary in speed from 5 to 10 per cent., while
synchronous motors vary but a fraction of one per cent.
Single phase motors to render efficient service must be able,
where requisite, to develop sufficient turning moment or torque
to accelerate, from standstill, loads possessing large inertia or
excessive static friction; for example, meat choppers and grinders,
sugar or laundry centrifugals; heavy punch presses; group driven
machines running from countershafts with possibly over taut belting,
poor alignment, lubrication, etc.
CHAPTER LII
TRANSFORMERS
The developments in the field of electrical engineering which have
rendered feasible the transmission of high pressure currents over long
distances, together with the reliability and efficiency of modern
generating units, have resulted in notable economies in the generation
and distribution of electric current.
This has been accomplished largely by the use of distant water power or
the centralization of the generating plants of a large territory in a
single power station.
The transformer is one of the essential factors in effecting the
economical distribution of electric energy, and may be defined as _an
apparatus used for changing the voltage and current of an alternating
circuit_. A transformer consists essentially of:
1. A primary winding;
2. A secondary winding;
3. An iron core.
~Basic Principles.~--If a current be passed through a coil of wire
encircling a bar of soft iron the iron will become a magnet; when the
current is discontinued the bar loses its magnetization.
_Conversely:_ If a bar of iron carrying a coil of wire be magnetized
in a direction at right angles to the plane of the coil a momentary
electric pressure will be induced in the wire; if the current be
reversed, another momentary pressure will be induced in the opposite
direction in the coil.
These actions are fully explained in chaps. X and XI, and as they are
perfectly familiar phenomena, a detailed explanation of the principles
upon which they depend is not necessary here.
From the first two statements given above it is evident that
if a bar of iron be provided with two coils of wire, one of
which is supplied from a source of alternating current, as
shown diagrammatically by fig. 1,916, at each impulse of the
exciting current a pressure will be induced in the secondary
coil, the direction of these impulses alternating like that of
the exciting current.
~Ques. What name is given to the coil through which current from the
source flows?~
Ans. _The primary winding._
[Illustration: Fig. 1,916.--Diagram of elementary transformer with
non-continuous core and connection with single phase alternator. The
three essential parts are: primary winding, secondary winding, and an
iron core.]
~Ques. What name is given to the coil in which a current is induced?~
Ans. _The secondary winding._
Similarly, the current from the source (alternator) is called
the _primary current_ and the induced current, the _secondary
current_.
~Ques. What is the objection to the elementary transformer shown in
fig. 1,916?~
Ans. The non-continuous core. With this type core, the flux emanating
from the north pole of the bar has to return to the south pole through
the surrounding air; and as the reluctance of air is much greater than
that of iron, the magnetism will be weak.
~Ques. How is this overcome?~
Ans. By the use of a continuous core as shown in fig. 1,917.
~Ques. Is this the best arrangement, and why?~
Ans. No. If the windings were put on as in fig. 1,917, the leakage of
magnetic lines of force would be excessive, as indicated by the dotted
lines. In such a case the lines which leak through air have no effect
upon the secondary winding, and are therefore wasted.
[Illustration: Fig. 1,917.--Diagram of elementary transformer with
continuous core and connections with alternator. The dotted lines show
the leakage of magnetic lines. To remedy this the arrangement shown in
fig. 1,918 is used.]
~Ques. How is the magnetic leakage reduced to a minimum in commercial
transformers?~
Ans. In these, and even in ordinary induction coils (the operating
principle of which is the same as that of transformers) the magnetic
leakage is reduced to the lowest possible amount by arranging the coils
one within the other, as shown in cross section in fig. 1,918.
~The Induced Voltage.~--The pressure induced in the secondary winding
will depend on the _ratio_ between the number of turns in the two
windings. For example, a transformer with 500 turns of wire in its
primary winding and 50 turns in its secondary winding would have a
transformation ratio of 10 to 1, and if it were supplied with primary
current at 1,000 volts, the secondary pressure at no load would be 100
volts.
[Illustration: Fig. 1,918.--Cross section showing commercial
arrangement of primary and secondary windings on core. One is
superposed on the other. This arrangement compels practically all of
the magnetic lines created by the primary winding to pass through the
secondary winding.]
EXAMPLE.--If ten amperes flow in the primary winding and the
transformation ratio be 10, then 10 × 10 = 100 amperes will
flow through the secondary winding.
Thus, a direct proportion exists between the pressures and
turns in the two windings and an inverse proportion between the
amperes and turns, that is:
_primary voltage: secondary voltage = primary turns: secondary turns_
_primary current: secondary current = secondary turns: primary turns_
From the above equations it is seen that the watts of the primary
circuit equal the watts of the secondary circuit.
~Ques. Are the above relations strictly true, and why?~
Ans. No, they are only approximate, because of transformer losses.
In the above example, the total wattage in the primary circuit
is 1,000 × 10 = 10 kw., and that in the secondary circuit
is 100 × 100 = 10 kw. Hence, while both volts and amperes
are widely different in the two circuits, the watts for each
are the same in the ideal case, that is, assuming perfect
transformer action or 100% efficiency. Now, the usual loss in
commercial transformers is about 10%, so that the actual watts
delivered in the secondary circuit is (100 × 100) × 90% = 9 kw.
[Illustration: FIG. 1,919.--Wagner transformer coil formed, ready for
taping. These are known as "pan cake" coils. They are wound with flat
cotton covered copper strip. In heavy coils, several strips in parallel
are used per turn in order to facilitate the winding and produce a more
compact coil.]
~The No Load Current.~--When the secondary winding of a transformer is
open or disconnected from the secondary circuit no current will flow in
the winding, but a very small current called the _no load current_ will
flow in the primary circuit.
The reason for this is as follows: The current flowing in
the primary winding causes repeated reversals of magnetic
flux through the iron core. These variations of flux induce
pressures in both coils; that induced in the primary called the
_reverse pressure_ is opposite in direction and very nearly
equal to the impressed pressure, that is, to the pressure
applied to the primary winding. Accordingly the only force
available to cause current to flow through the primary winding
is the difference between the impressed pressure and reverse
pressure, the _effective pressure_.
[Illustration: FIG. 1,920.--Wagner coils with insulation ready for core
assembly. The flat coils, sometimes called pancake coils are wound
of flat, cotton covered, copper strip with ample insulation between
layers. In heavy coils several flat strips in multiple are used per
turn in order to facilitate the winding and produce a more compact
coil. In many cases normal current flow per high tension coil is very
low and could be carried with a very small cross sectional area of
copper; however, flat strip is almost always used on account of the
increased mechanical stability thus obtained.]
~The Magnetizing Current.~--The magnetizing current of a transformer
is sometimes spoken of as that current which the primary winding takes
from the mains when working at normal pressure. The _true magnetizing
current_ is only that component of this total no load current which is
in quadrature with the supply pressure. The remaining component has
to overcome the various iron losses, and is therefore an "in phase"
component. The relation between these two components determines the
power factor of the so called "magnetizing current."
[Illustration: FIGS. 1,921 and 1,922.--Assembled coils of Westinghouse
10 and 15 kva. transformers; views showing ventilating ducts.]
The true magnetizing component is small if the transformer be well
designed, and be worked at low flux density.
~Action of Transformer with Load.~--If the secondary winding of a
transformer be connected to the secondary circuit by closing a switch
so that current flows through the secondary winding, the transformer is
said to be _loaded_.
The action of this secondary current is to oppose the
magnetizing action of the slight current already flowing in the
primary winding, thus decreasing the maximum value reached by
the alternating magnetic flux in the core, thereby decreasing
the induced pressure in each winding.
The amount of this decrease, however, is _very small_,
inasmuch as a very small decrease of the induced pressure in
the primary coil greatly increases the difference between the
pressure applied to the primary coil and the opposing pressure
induced in the primary coil, so that the primary current is
greatly increased. In fact, _the increase of primary current
due to the loading of the transformer is just great enough
(or very nearly) to exactly balance the magnetizing action of
the current in the secondary coil;_ that is, the flux in the
core must be maintained approximately constant by the primary
current whatever value the secondary current may have.
When the load on a transformer is increased, the primary of the
transformer automatically takes additional current and power
from the supply mains in direct proportion to the load on the
secondary.
When the load on the secondary is reduced, for example by
turning off lamps, the power taken from the supply mains by
the primary coil is automatically reduced in proportion to the
decrease in the load. This automatic action of the transformer
is due to the balanced magnetizing action of the primary and
secondary currents.
[Illustration: FIG. 1,923.--Rear view of Fort Wayne distributing
transformer, showing hanger irons for attaching to pole cross arm.]
~Classification of Transformers.~--As in the case of motors, the great
variety of transformer makes it necessary that a classification, to be
comprehensive, must be made from several points of view, as:
1. With respect to the transformation, as
_a._ Step up transformers;
_b._ Step down transformers.
2. With respect to the arrangement of the coils and magnetic circuit, as
_a._ Core transformers;
_b._ Shell transformers;
_c._ Combined core and shell transformers.
3. With respect to the kind of circuit they are to be used on, as
_a._ Single phase transformers;
_b._ Polyphase transformers.
4. With respect to the method employed in cooling, as
_a._ Dry transformers;
_b._ Air cooled transformers {natural draught;
{forced draught, or air blast;
_c._ Oil cooled transformers;
_d._ Water cooled transformers.
5. With respect to the nature of their output, as
_a._ Constant pressure transformers;
_b._ Constant current transformers;
_c._ Current transformers;
_d._ Auto-transformers.
6. With respect to the kind of service, as
_a._ Distributing;
_b._ Power.
7. With respect to the circuit connection that the transformer
is constructed for, as
_a._ Series transformers;
_b._ Shunt transformers.
~Step Up Transformers.~--This form of transformer is used to transform
a low voltage current into a high voltage current. Such transformers
are employed at the generating end of a transmission line to raise the
voltage of the alternators to such value as will enable the electric
power to be economically transmitted to a distant point.
[Illustration: FIG. 1,924.--Diagram of elementary _step up_
transformer. As shown the primary winding has two turns and secondary
10 turns, giving a ratio of voltage transformation of 10 ÷ 2 = 5.
Since only ⅕ as much current flows in the secondary winding as in the
primary, the latter requires heavier wire than the former.]
~Copper Economy with Step Up Transformers.~--To comprehend
fully the bearing of the matter, it must be remembered that the
energy supplied per second is the product of two factors, the
current and the pressure at which that current is supplied;
the magnitudes of the two factors may vary, but the value of
the power supplied depends only on the product of the two; for
example, the energy furnished per second by a current of 10
amperes supplied at a pressure of 2,000 volts is exactly the
same in amount as that furnished per second by a current of 400
amperes supplied at a pressure of 50 volts; in each case, the
product is 20,000 watts.
Now the loss of energy that occurs in transmission through a
well insulated wire depends also on two factors, the current
and the resistance of the wire, and in a given wire is
proportional to the square of the current. In the above example
the current of 400 amperes, if transmitted through the same
wire as the 10 amperes current, would, because it is forty
times as great, waste sixteen hundred times as much energy in
heating the wire. It follows that, for the same loss of energy,
the 10 ampere current at 2,000 volts may be carried by a wire
having only ¹/₁₆₀₀th of the sectional area of the wire used
for the 400 ampere current at 50 volts.
The cost of copper conductors for the distributing lines is
therefore very greatly economized by employing high pressures
for distribution of small currents.
[Illustration: FIG. 1,925.--Diagram of elementary step down
transformer. As shown the primary winding has 10 turns and the
secondary 2, giving a ratio of voltage transformation of 2 ÷ 10 = .2.
The current in the secondary being 5 times greater than in the primary
will require a proportionately heavier wire.]
~Step Down Transformers.~--When current is supplied to consumers
for lighting purposes, and for the operation of motors, etc.,
considerations of safety as well as those of suitability, require the
delivery of the current at comparatively low pressures ranging from 100
to 250 volts for lamps, and from 100 to 600 volts for motors.
This involves that the high pressure current in the transmission
lines must be transformed to low pressure current at the receiving
or distributing points by _step down transformers_, an elementary
transformer being shown in fig. 1,925.
[Illustration: FIGS. 1,926 and 1,927.--Core type transformer. It
consists of a central core of laminated iron, around which the coils
are wound. A usual form of core type transformer consists of a
rectangular core, around the two long limbs of which the primary and
secondary coils are wound, the low tension coil being placed next the
core.]
Transformers of this type have a large number of turns in the primary
winding and a small number in the secondary, in ratio depending on the
amount of pressure reduction required.
~Core Transformers.~--This type of transformer may be defined as one
having an iron core, upon which the wire is wound in such a manner that
the iron is enveloped within the coils, the outer surface of the coils
being exposed to the air as shown in figs. 1,926 and 1,927.
~Shell Transformers.~--In the shell type of transformer, as shown in
fig. 1,928, the core is in the form of a shell, being built around and
through the coils. A shell transformer has, as a rule, fewer turns and
a higher voltage per turn than the core type.
~Ques. What is the comparison between core and shell transformers?~
Ans. The relative advantages of the two types has been the subject of
considerable discussion among manufacturers; the companies who formerly
built only shell type transformers, now build core types, while with
other builders the opposite practice obtains.
[Illustration: FIG. 1,928.--Shell type transformer. In construction
the laminated core is built around and through the coils as shown. For
large ratings this type has some advantages with respect to insulation,
while for small ratings the core type is to be preferred in this
respect. The shell arrangement of the core gives better cooling; with
this arrangement minimum magnetic leakage is easily obtained.]
~Ques. Upon what does the choice between the two types chiefly depend?~
Ans. Upon manufacturing convenience rather than operating
characteristics.
The major insulation in a core type transformer consists of
several large pieces of great mechanical strength, while in
the shell type, there are required an extremely large number
of relatively small pieces of insulating material, which
necessitates careful workmanship to prevent defects in the
finished transformer, when thin or fragile material is used.
Both core and shell transformers are built for all ratings; for
small ratings the core type possesses certain advantages with
reference to insulation, while for large ratings, the shell
type possesses better cooling properties, and has less magnetic
leakage than the core type.
[Illustration: FIG. 1,929.--View illustrating the construction of cores
and coils of Maloney transformers.]
[Illustration: FIG. 1,930.--Maloney mica shield between primary and
secondary coils, showing lapping feature which prevents the wrinkling
and cracking of the mica.]
~Combined Core and Shell Transformers.~--An improved type of
transformer has been introduced which can be considered either as two
superposed shell transformers with coils in common, or as a single core
type transformer with divided magnetic circuit and having coils on only
one leg. It is best considered however, as a combined core and shell
transformer, and for small sizes it possesses most of the advantages
of both types. It can be constructed at less cost than can either a
core or a shell transformer having the same operating characteristics
and temperature limits.
[Illustration: FIGS. 1,931 and 1,932.--The Berry combined core and
shell transformer. It consists of a number of inner and outer vertical
and radial laminated iron blocks built up of the usual thin sheet iron,
with the coils between. The magnetic circuit is completed at the top
and bottom by other laminated blocks placed horizontally, and the whole
is held together between top and bottom cast iron frame plates by a
bolt passing right down the center. Fig. 1,931 gives a general view,
W being the winding, and B, B, B, etc., the outer laminated blocks.
The construction will be better understood from fig. 1,932, where it
may be supposed that the top cap and laminated cross pieces have been
removed. Here I, I, I and O, O, O are respectively the inner and outer
radial vertical blocks, P the primary, and S, S the secondary; the
latter being in two sections with the primary sandwiched between, as an
extra precaution against shock. It will be evident that this form of
transformer possesses excellent ventilation; and this is still further
enhanced by opening out the winding at intervals to leave ventilating
apertures, as at A, A, A. Fig. 1,932 shows only 6 sets of radial
blocks, but the usual plan is to provide 24 or 36, according to the
size of the transformer.]
Fig. 1,932 shows a cross section of the first transformer of this type
to be developed commercially, and known as an "iron clad" transformer;
this construction has been used in England for some time. Fig. 1,933
shows the American practice.
[Illustration: FIG. 1,933.--Plan of core of General Electric combined
core and shell transformer. The core used contains four magnetic
circuits of equal reluctance, in multiple; each circuit consisting of a
separate core. In this construction one leg of each circuit is built up
of two different widths of punchings forming such a cross section that
when the four circuits are assembled together they interlock to form
a central leg, upon which the winding is placed. The four remaining
legs consist of punchings of equal width. These occupy a position
surrounding the coil at equal distances from the center, on the four
sides; forming a channel between each leg and coil, thereby presenting
large surfaces to the oil and allowing its free access to all parts
of the winding. The punchings of each size transformer are all of the
same length, assembled alternately, and forming two lap joints equally
distributed in the four corners of the core, thereby giving a magnetic
circuit of low reluctance.]
~Ques. How is economy of construction obtained in designing combined
core and shell transformers?~
Ans. The cross section of iron in the central leg of the core is made
somewhat less than that external to the coils, in order to reduce the
amount of copper used in the coils.
~Single and Polyphase Transformers.~--A single phase transformer may be
defined as _one having only one set of primary and secondary terminals,
and in which the fluxes in the one or more magnetic circuits are all in
phase_, as distinguished from a polyphase transformer, or combination
in one unit of several one phase transformers with separate electric
circuits but having certain magnetic circuits in common. In polyphase
transformers there are two or more magnetic circuits through the core,
and the fluxes in the various circuits are displaced in phase.
~Ques. Is it necessary to use a polyphase transformer to transform a
polyphase current?~
Ans. No, a separate single phase transformer may be used for each phase.
[Illustration: FIGS. 1,934 and 1,935.--Top view showing core and
coils in place, and view of coils of Westinghouse distributing
transformer. The coils are wound from round wire in the smaller sizes
of transformers and from strap copper in the larger sizes. Strap wound
coils allow a greater current carrying conductor section than coils
wound from large round wire, as there is little waste space between the
different turns of the conductor. The coils are arranged concentrically
with the high tension winding between the two low tension coils, this
arrangement giving the fine regulation found in these transformers. The
low tension coils are wound in layers which extend across the whole
length of the coil opening in the iron, while the high tension coils
are wound in two parts and placed end to end. This construction reduces
the normal voltage strains to a value which will not give trouble
under any condition of service. The magnetic circuit is built up of
laminated, alloy steel punchings, each layer of laminæ being reversed
with reference to the preceding layer and all joints butted. This gives
a continuous magnetic circuit of low reluctance, low iron loss and low
exciting current. When assembled, the magnetic circuit consists of four
separate parallel circuits encircling the coils and protecting the
windings from mechanical injury. Separate high and low tension terminal
blocks of glazed porcelain are mounted upon extensions of the upper
end frames. All danger of confusing the leads or inadvertently making
an electrical connection between the high and low tension sides of the
transformer is thus averted. The high tension winding has four leads
brought to the studs in the terminal block. Adjustable brass connectors
or links between the studs provide for series or multiple connections
between two points of the high tension winding. The position of the
studs and the length of the links are so proportioned that wrong
connections on the block are impossible. Barriers on the porcelain
block separate the studs and prevent danger of arcing. Leads with means
of preventing creeping of oil by capillary action are attached to these
studs and brought out of the core through porcelain bushings.]
~Ques. Is there any choice between a polyphase transformer and separate
single phase transformers for transforming a polyphase current?~
Ans. Yes, the polyphase transformer is preferable, because less iron
is required than would be with the several single phase transformers.
The polyphase transformer therefore is somewhat lighter and also more
efficient.
[Illustration: FIGS. 1,936 and 1,937.--Core and shell types of three
phase transformer. In the core type, fig. 1,936, there are three cores
A, B, and C, joined by the yokes D and D'. This forms a three phase
magnetic circuit, since the instantaneous sum of the fluxes is zero.
Each core is wound with a primary coil P, and a secondary coil S. As
shown, the primary winding of each phase is divided into three coils
to ensure better insulation. The primaries and secondaries may be
connected _star_ or _mesh_. The core B has a shorter return path than
A and C, which causes the magnetizing current in that phase to be less
than in the A and C phases. This has sometimes been obviated by placing
the three cores at the corners of an equilateral triangle (as in figs.
1,939 and 1,940), but the extra trouble involved is not justified, as
the unbalancing is a no load condition, and practically disappears
when the transformer is loaded. The shell type, fig. 1,937, consists
practically of three separate transformers in one unit. The flux paths
are here separate, each pair of coils being threaded by its own flux,
which does not, as in the core type, return through the other coils.
This gives the shell type an advantage over the core type, for should
one phase burn out, the other two may still be used, especially if the
faulty coils be short circuited. The effect of such short circuiting is
to prevent all but a very small flux from threading the faulty coil.]
~Ques. Name two varieties of polyphase transformer?~
Ans. The core, and the shell types as shown in figs. 1,936 and 1,937.
~Ques. How should a three phase transformer be operated with one phase
damaged?~
Ans. The damaged windings should be separated electrically from the
other coils.
The pressure winding of the damaged phase should be short
circuited upon itself and the corresponding low pressure
winding should also be short circuited upon itself. The winding
thus short circuited will choke down the flux passing through
the portion of the core surrounded by them without producing
in any portion of the winding a current greater than a small
fraction of the current which would normally exist in such
portion at full load.
~Transformer Losses.~--As previously mentioned, the ratio between
the applied primary voltage and the secondary terminal voltage of a
transformer is not always equal to the ratio of primary to secondary
turns of wire around the core.
The commercial transformer is not a perfect converter of energy, that
is, the ~input~, or watts applied to the primary circuit is always more
than the ~output~ or watts delivered from the secondary winding.
[Illustration: FIG. 1,938.--Interior of General Electric oil cooled 500
kva. 33,000 volt outdoor transformer showing lifting arrangement.]
This is due to the various losses which take place, and the difference
between the input and the output is equal to the sum of these losses.
They are divided into two classes:
1. The ~iron~ or ~core~ losses;
2. The ~copper~ losses.
The iron or core losses are due to
1. Hysteresis;
2. Eddy currents;
3. Magnetic leakage (negligibly small).
[Illustration: FIGS. 1,939 and 1,940.--Triangular arrangements of cores
of three phase transformer. Fig. 1,939, form with three cornered yokes
at bottom and top of cores; fig. 1,940, form with circular yokes. While
these designs give perfect symmetry for the three phases, there is some
trouble in the mechanical arrangement of the yokes. If these be stamped
out triangularly and inserted horizontally between the three cores, it
is necessary to interpose a layer of insulation, otherwise there would
be objectionable eddy currents formed in the stampings.]
Those which are classed as copper losses are due to
1. Heating the conductors (the I²R loss);
2. Eddy currents in conductors.
~Hysteresis.~--In the operation of a transformer the alternating
current causes the core to undergo rapid reversals of magnetism. This
requires an expenditure of energy which is converted into heat.
[Illustration: FIG. 1,941.--View showing mechanical construction of
coil and core of Moloney pole type ½ to 50 kw. transformer. Moloney
standard transformers of these sizes are regularly wound for 1,100 to
2,200 primary volts. For 1,100 volts the primary coils are connected
in parallel by means of connecting links; for 2,200 volts, they are
connected in series. The porcelain primary terminal board is provided
with two connecting links so that connections can be made for either
1,100 or 2,200 volts.]
This loss of energy as before explained is due to the work required
to change the position of the molecules of the iron, in reversing the
magnetization. Extra power then must be taken from the line to make up
for this loss, thus reducing the efficiency of the transformer.
~Ques. Upon what does the hysteresis loss depend?~
Ans. Upon the quality of the iron in the core, the magnetic density at
which it is worked and the frequency.
~Ques. With a given quality of iron how does the hysteresis loss vary?~
Ans. It varies as the 1.6 power of the voltage with constant frequency.
~Ques. In construction, what is done to obtain minimum hysteresis loss?~
Ans. The softes
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