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]