[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.