Study Unit Industrial Alternating Current Motors · The Repulsion-type Motor 19 POLYPHASE MOTORS 21...

Study Unit

Industrial AlternatingCurrent MotorsBy

Robert L. CecciTechnical Writer

This study unit will cover the construction and operation ofsingle- and three-phase or polyphase alternating current (AC)motors. In the first section, you’ll learn about AC motorbasics through examples of how coils create magnetic fieldswhen they’re supplied with AC electricity. Also, you’ll see howelectromagnetic induction allows the rotor of a motor to havea current flow and, therefore, its own magnetic field. Next,you’ll see how single-phase and split-phase motors operate.This information is then followed by a description of thecapacitor motor and a brief presentation of the repulsion-induction motor. The next two sections cover polyphasemotors and AC control systems.

iii

Previe

wPrevie

wWhen you complete this study unit, you’ll be

able to

• Explain how AC electricity creates a changing magnetic

field in and around a coil

• Discuss the principles of electromagnetic induction

• Explain why a motor needs a system for starting the

rotor and how this is performed with a shaded-pole,

split-phase capacitor, and repulsion-induction motor

• List the possible problems with single-phase motors and

the steps taken to troubleshoot these problems

• Identify the components of a polyphase motor and

describe its operation

• Explain how to troubleshoot polyphase motor systems

• Identify the basic motor starter systems used in single-

phase and three-phase AC motors

BASIC AC MOTOR THEORY 1Alternating Current and Coils 1

Magnetic Induction 3

A Basic AC Motor 4

Getting the Motor to Turn 5

SINGLE-PHASE AC MOTORS 7Uses of Single-phase Motors 7

The Rotor 8

The Shaded-pole Motor 9

The Split-phase Motor 12

The Capacitor Motor 17

The Repulsion-type Motor 19

POLYPHASE MOTORS 21Polyphase Motor Basics 21

The Delta-connected Motor 22

The Wye-connected Motor 24

Multiple-speed Operation 25

Motor Nameplate Information 29

Polyphase Motor Troubleshooting and Repair 32

AC MOTOR RELAY CONTROLS 36The Manual Control Circuit 36

The Basic Electric Control Circuit 38

A Reversing Circuit 40

Two-speed Magnetic Starters 41

SELF-CHECK ANSWERS 47

APPENDIX 49

EXAMINATION 53

v

Contents

Contents

1

BASIC AC MOTOR THEORY

Alternating Current and CoilsAlternating current (AC) motors are one of the most popularforms of motors used in industry. Visit any industrial plantand you’ll most likely find AC motors in sizes from a fractionof a horsepower to 1,000 horsepower or more.

AC motors are very popular because the normal current supplied to industry is in the form of alternating current.Where direct current (DC) motors require special rectifiers orcontrollers, AC motors can be connected directly to the ACline power or controlled by simple contactors. Up until a fewyears ago, when industry needed adjustable or variable-speedmotors, a DC motor was most likely used. However, with theuse of a special control called an inverter, AC motors can noweasily be used for these purposes.

When direct current is applied to a coil of wire, it creates amagnetic field around each wire in the coil. The magneticfields from the coils can run together and get quite strong inmagnetic force or flux. A metal core or pole that’s surroundedby the magnetic field will concentrate that field and exhibit anorth and south magnetic pole.

When alternating current is applied to that same coil of wire,a magnetic field is also formed. This field will be differentfrom the magnetic field that’s created by direct current inthat the field will alternate in two ways.

Industrial Alternating

Current Motors

Industrial Alternating Current Motors2

Figure 1A displays a single cycle of AC.Note that the voltage and current rise in apositive direction until a positive maximumvoltage is reached. The voltage and currentthen reverse and head towards zero. Aftercrossing zero, the voltage and current headfor a negative peak where the voltage andcurrent reverse once again and headtowards zero. This positive and negativecycle occurs sixty times per second (60 Hertz or 60 Hz).

Figure 1B displays the magnetic flux density, or the strength of the magneticforce produced by the coil as the applied AC

goes through one cycle. The field strength isn’t constant butrather follows the applied voltage and current through thecycle.

Figure 2 displays the other property of a coil that’s suppliedby AC. In Figure 2A, the AC signal has created a north pole(N) on the left of the core and a south pole (S) on the right ofthe core. In Figure 2B, the AC signal is now going through itsnegative transition and the poles have reversed with thesouth to the left and the north to the right.

(A)

(B)

FIGURE 1—At each peak in the AC cycle, the

magnetic flux in the motor’s coil or pole also

peaks.

NS

N S

(A)

(B)

FIGURE 2—As this AC

cycle changes from a

positive half cycle to a

negative half cycle, the

poles will switch from

north to south and back

to north in a continuous

cycle.

Industrial Alternating Current Motors 3

So as you can see, AC voltage and current applied to a coilcreate a magnetic field with constantly changing properties:

1. The magnetic field strength constantly varies.

2. The magnetic pole polarities constantly alternate fromthe north to the south pole and back.

Magnetic InductionThere’s one other property of AC and coils that should be discussed. That property is magnetic induction. Figure 3displays two DC-powered circuits. In Figure 3A, the switchhas just been closed. The change in voltage from zero tosome value of DC voltage creates a magnetic field around theprimary coil P. If the primary and secondary coils are veryclose together, this change of magnetic field from zero to ahigher level will magnetically induce a field on the secondarycoil S. For a brief instant, the meter would measure a pulseof voltage. After the pulse arrived, there would be no furthervoltage generated in the secondary coil or viewed at themeter. When the switch is opened as in Figure 3B, the collapsing field will also induce a pulse of voltage in the secondary coil that can be viewed on the meter. Again, afterthis pulse arrives, there will be no further voltage changes inthe secondary.

VCOM

V

SWITCHCLOSED

S RL

DCSOURCE

(A)

S RLP

VCOM

V

SWITCHOPENED

DCSOURCE

(B)

P

FIGURE 3—If DC is

applied to a coil, a mag-

netic field is created

around that coil. The

field is induced on the

secondary coil S when the

switch is closed (A) or

opened (B).


Figure 4 displays the same circuit as Figure 3, except thatthe voltage source is an AC source. The primary coil P will, ofcourse, develop a changing magnetic field that follows thetwo properties listed previously. Its strength and magneticpolarity will change in step with the changing AC cycle. Also,this coil will induce a magnetic field in the secondary coil S.This coil’s magnetic field will follow the same properties of theprimary field. At the secondary coil, both a magnetic field andan AC voltage will be created.

Most rotors of AC motors will operate on magnetic induction.The magnetic fields created by the field windings or statorswill induce a voltage on the conductors of the rotor. Theseinduced voltages will create magnetic fields around the motorconductors that are attracted to or repelled by the statorfields to rotate the rotor.

A Basic AC MotorIt might appear that we have everything needed to build amotor. We have a changing magnetic field in the stator orfield windings and an induced current and voltage in therotor that creates the second magnetic field. There’s oneproblem, however.

Figure 5 displays a simplified view of a motor using two stator poles and a permanent-magnet rotor. The coils aresupplied with a source of AC voltage.

Assume that on the first AC half-cycle, the left pole near thepermanent magnet will become a north pole and the rightpole will become a south pole. The permanent magnet willquickly try to turn until it’s horizontally aligned with the field

VCOM

VS RL

ACPOWERSOURCE P

FIGURE 4—When AC is

supplied to the same

circuit as in Figure 3, the

constantly changing pri-

mary field at P is induced

on the secondary coil S.

An AC voltage will be

measured across RL by

the meter.


poles. However, 1/120 of a second later, thefield poles will switch magnetic polarity andattempt to repel the permanent magnet. At1/120th of a second later, the magnetic polesagain reverse and again attempt to attractthe permanent magnet.

What’s happening here is that the perma-nent magnet rotor will move a certain smallamount at 60 cycles per second creating ahum rather than rotating motion. Thismotor would only work if the rotor wasspun at high speed and then the AC powersource was applied to the motor.

Adding two additional stator poles and coilswouldn’t help. The permanent magnet would vibrate or humbetween two poles or at some other point in its rotation.

Getting the Motor to TurnSomething must be done to create a true rotating magneticfield within the motor, one that the magnetic field in the rotorwill follow.

This rotating magnetic field can be set up in several ways.For example, multiple windings sets can be placed in offsetpositions in the stator. This is done in shaded-pole and split-phase induction motors. A capacitor can also be used to placea set of windings out of phase with the main windings as in acapacitor-start motor. Finally, there’s a motor that starts like a universal motor and runs like an induction motor that’scalled a repulsion-induction motor. More will be seen on thesetypes of motors in the next section.

ROTOR

N

S

STATORPOLE 1

STATORPOLE 2

AC POWERSOURCE

FIGURE 5—This illustration shows a basic

two-pole motor with a permanent magnet

rotor.


Self-Check 1At the end of each section of Industrial Alternating Current Motors, you’ll be asked to

pause and check your understanding of what you’ve just read by completing a “Self-Check”

exercise. Answering these questions will help you review what you’ve studied so far.

Please complete Self-Check 1 now.

1. AC motors are popular in industry because they’re

a. easy to fix.

b. easily connected to line power.

c. quieter than DC motors.

2. When AC is applied to a coil of wire, the magnetic poles will (reverse polarity/get stronger).

3. A coil that is supplied by AC can influence a second nearby coil by means of magnetic

(induction/reluctance).

Check your answers with those on page 47.


SINGLE-PHASE AC MOTORS

Uses of Single-phase MotorsA typical industrial plant can use many different types of single-phase AC motors. These motors power small fans,pumps, chillers, compressors, and other equipment. Homescontain many single-phase motors such as in the air conditioner, refrigerator, washer, dryer, and dehumidifier’scompressor.

Single-phase AC motors are small motors rated from the fractional horsepower sizes to about 10 horsepower (HP). Onsingle phase AC systems the input to the motor circuit willnormally be 120 VAC. This voltage can be supplied by a sim-ple line cord, switch, and fuse, or may be part of a complexcomputer control system. In either case the supplied voltageis often termed AC High, or Hi, and AC Low, or Lo. The ACHigh is the line that carriers the voltage and the AC Low sideis the ground or neutral potential line. Figure 6 displaysthese two types of AC power sources.

120 VACMOTOR120 VAC

(A)

LINE

NEUTRAL

120 VAC

120 VAC

240 VACMOTOR240 VAC

(B)

NEUTRAL

0 B

0 A

FIGURE 6—This figure

shows a standard 120

VAC supply in (A) and a

240 VAC supply in (B).


In Figure 6A, the voltage and current are the common 120VAC. This is the same AC supply that’s delivered to each convenience outlet. Figure 6B shows 240 VAC single-phasepower. This form of energy is developed by using two 120VAC power sources that are 180 electrical degrees apart. Thetotal peak voltage is then 240 VAC (120 VAC plus 120 VAC).This form of energy shouldn’t be confused with 240 VACthree-phase or polyphase power. Three-phase power has threeindividual phases or sources of electricity where split-phasepower has two.

Split-phase power is commonly used to lower the currentdraw of the motor. For example, in a home workshop, thesplit-phase-powered motor can be used to power an air compressor or exhaust fan. At 120 VAC, a three-quarterhorsepower motor (3/4 HP) can draw as much as 15 amps.At 240 VAC, this same motor will draw only 7.5 amps. Thisallows a smaller wire size to be used in 240 VAC-poweredequipment.

The RotorThe key component of most AC motors is the rotor. The rotoris made up of laminations that are made of high-quality steelthat’s notched at equal distances around its outside diameter. These laminations are riveted together and a steelshaft is pressed through this assembly. This is shown inFigure 7A. Copper or aluminum bars are placed, or cast, intothe notches. Two end rings short out these bars at each endof the rotor, as shown in Figure 7B. The end rings often haveshort tabs that are used to move the air inside the motor tohelp cool the bars. In some cases, the lamination slots and,therefore the rotor bars, are skewed instead of placed hori-zontally on the rotor. These skewed rotor bars are used todevelop a greater starting torque and less magnetic hum thanhorizontal bars.

If you were to strip a rotor of its rotor bar and end rings, itwould look like a kind of pet exercise wheel. This type of rotorhas, therefore, been given the name squirrel-cage rotor.


This type of rotor acts through magnetic induction, giving theentire motor the name AC induction motor. When this rotor is placed in the magnetic fields produced by the stator,tremendous currents are created in the rotor bars of themotor. These currents create magnetic fields that are concen-trated in the poles of the laminations. These magnetic fieldsthen interact with the changing magnetic fields in the statorto create the rotation of the rotor.

The Shaded-pole MotorOne of the simplest forms of AC motors is the shaded-polemotor. Shaded-pole motors are widely available in sizes from1/100 to 1/20 HP. The fans on computers, small industrial blow-ers, humidifiers, etc., are all examples of shaded-pole motors.

ROTORBARS

LAMINATIONS

(A)

ENDRING

TABS

ENDRING

(B)

(C)

FIGURE 7—The squirrel-

cage rotor shown here in

(A), (B), and (C) is used

in almost all types of AC

motors.


The shaded-pole motor, like most ACmotors, uses a small squirrel-cage rotor.The key to the operation of this type ofmotor is the field or stator windings.

Figure 8 displays the windings used in atypical shaded-pole motor. The windingsare basically very similar to those used fora four-pole motor of any type, AC or DC.Notice, however, the notch at one side ofeach pole. One turn of fairly heavy wire iswound in this notch. On some motors,you’ll see this winding as one turn of a copper or aluminum band.

Earlier, the motor with the permanent mag-net rotor simply vibrated when electricitywas supplied to the windings. The shadingcoils in this example are present in order toshift the magnetic field enough to start the

motor. Once the motor is turning at near field speed, theshading coils and their magnetic field aren’t required to con-tinue rotation.

When AC is first supplied to the motor, magnetic fields areformed on each of the motor’s four poles. Current is inducedinto the squirrel-cage rotor bars and into the shading coil oneach pole. The induced magnetic field on the shading coil isout of phase with the magnetic fields of the poles. This out-of-phase magnetic field helps pull the rotor in the direction ofthe shading poles. This action gives the motor sufficientstarting torque to start turning the rotor and its load—a fanblade for example.

When the motor reaches near full speed, the voltage and cur-rent that’s induced into the shading winding becomes verysmall. As this occurs, the magnetic field that surrounds theshading pole gets weaker and weaker until it becomes verysmall. This causes the magnetic center of the pole to shift oneach of the four poles towards the center of each pole. This is

MAINWINDINGS

(4)

SHADINGWINDINGS

(4)

FIGURE 8—Shaded-pole motors use small

shading coils in the stator winding of the

motor. These shading coils offset the

magnetic fields to help start the motor.


shown in Figure 9. In Figure 9A, the centerof the magnetic field for the pole is shownas CP and that of the shading pole as CSP.In Figure 9B, where the shading pole’smagnetic field has dropped out, the centerof the physical pole has become the centerof the magnetic pole.

Often the main field windings of a shaded-pole motor are protected by a thermal fuse.If the stator windings get too hot, this fusewill melt and open the motor circuit. Thisfuse is often inside a glass capsule and,once opened, can’t be repaired, but can bereplaced.

Usually a shaded-pole motor will fail due toan overload condition. This condition canbe caused by dirty or damaged fan bladesthat cause excessive drag on the motor’srotor. Also, the bearings, usually an oil-impregnated copper-sleeve bearing, willwear and allow the rotor to strike or rubagainst the stator poles, stalling the rotor.

Shaded-pole motors aren’t normally reversible. The motor willstart turning and will continue to turn in the direction fromthe main pole toward the shaded pole. To reverse such amotor, it’s required that you disassemble the motor, pressthe stator from the case or housing, turn the stator end-for-end, and press the stator back into the motor. A fewreversible shaded-pole motors have been manufactured.These motors use a shading pole on each side of the mainpole and use an external switch to select which poles areconnected and, therefore, which direction the motor willrotate.

CP

CSP

(A)

CP

(B)

FIGURE 9—As the shaded-pole motor comes

up to speed, the magnetic centers change

from their composit in (A) to a single center

on each pole as shown in (B).


The Split-phase MotorThe limitation of the shaded-pole motor is the amount ofhorsepower that can be economically obtained from thedesign. At 1/20 horsepower, it’s more economical to build anduse a split-phase motor.

The split-phase motor also uses a squirrel-cage rotor. Thesplit-phase motor also uses a wound stator designed withtwo sets of windings. These are shown in Figure 10.

As shown in Figure 10, there are four run windings labeled Rand four start windings labeled S. The run windings, like themain windings, will be energized at all times the motor isturning and will maintain the rotation when the motor is atfull speed.

START WINDINGS (S)(4)

RUN WINDINGS (R)(4)

STATOR

BASE

FIGURE 10—The stator of

a split-phase motor has

two sets of windings, the

run windings R and the

start or auxiliary wind-

ings S.


The start windings are similar to the shaded-pole windings.However, these windings are made of many turns of lighter-gauge wire than shaded-pole windings. The start windingswill be wound across the same stator poles used for the runwindings and will be 90 electrical degrees from the run wind-ings. Also, the start windings are electrically disconnectedfrom the power source after the motor has obtained a certainspeed.

When a split-phase motor starts, the shaft begins to turn andthen, when it’s almost up to speed, a definite click can beheard coming from inside the motor. As the motor de-energizes, a second click signals the motor slowing. Thisclicking noise is caused by a centrifugal switch assembly thatconnects or disconnects the AC power source to or from thestart windings.

The centrifugal switch assembly is almost always mounted inthe rear section of the motor opposite the output shaft. Itconsists of two parts, the centrifugal mechanism and theswitch assembly.

The centrifugal mechanism (Figure 11) is mounted to therotor shaft. The weights are close to the shaft, pulled togetherby a soft spring. When the shaft reaches full speed, theweights will move out under centrifugal force. This motioncauses a free-moving disc on the rotor’s shaft to move back-wards toward the rear of the motor. This disc, in turn, makescontact with the centrifugal switch assembly. When enough

SWITCH SIDE

MOVABLEDISC

WEIGHT

SPRING(2)

MOTORSIDE

FIGURE 11—Most split-

phase or capacitor-start

motors use a centrifugal

mechanism on the rear

end of the squirrel-cage

rotor.


pressure is placed on the centrifugalswitch, as shown in Figure 12, the contactsof the switch will open, disconnecting thestart windings from the AC power source.This action occurs at about 70 percent ofthe motor’s full speed.

The opening and closing of the switch canbe noted by the definite click heard as themotor reaches speed or slows down whende-energized. If the switch were to fail, themotor wouldn’t start, causing instead aloud hum. With safety in mind, you canadd power to see if the motor rotates. If itdoes rotate, the run windings are in goodcondition, but the centrifugal switch or thestart windings have failed. Often you’ll seethe start windings called the auxiliary windings by some motor manufacturers.

Two methods of showing a split-phase motor are given inFigure 13. The centrifugal switch is the device marked C.S. inthe illustration.

Split-phase motors are easily reversed. Simply open themotor’s junction box and reverse the run or start windings inreference to the other winding. The example in Figure 13 isdrawn from looking into the motor’s shaft. The motor inFigure 13A will turn clockwise. The motor in Figure 13B willturn counterclockwise using the same reference point.

Some split-phase motors are set up to run on either 120 or240 VAC. This is performed by having two run windings anda single start winding. If the run windings are connected inseries, the motor will operate at the higher voltage. If the runwindings are connected in parallel, the motor will operate atthe lower voltage. These connections are shown in Figure 14.Follow the connections in the illustration for each connectiondiagram and see how these series and parallel arrangementsof the run windings are formed. Note that the start windingsare always connected to a source of 120 VAC. To reverse thismotor, simply switch the locations of T5 and T8 in either con-nection diagram.

SWITCH

MOTORTERMINAL

FIGURE 12—A centrifugal switch mechanism

is triggered by a free-moving disc on the rotor

that connects or disconnects the AC power

source to the start windings.


Split-phase motors normally fail due to either centrifugalswitch or start winding failure, bearing failure, and run winding failure. If the motor is cycled ON and OFF at regularintervals, the centrifugal switch, followed by a start windingfailure, are the two most common types of failures. Due tothe motor’s small size, a centrifugal switch may be replacedbut the start winding is rarely rewound. Instead the motor issimply replaced. Centrifugal switch and start run windingfailures are usually noted as a motor humming loudlyinstead of rotating. In some rare cases, the centrifugalswitch’s contacts can stick closed due to contact welding.This problem is identified by a motor that quickly heats up,trips the overloads, or blows the fuse shortly after it starts.

RUN

C.S.

START

AC

AC

(A)

RUN

C.S.

AC AC

(B)

START

HIGH

LOW

DIRECTION

DIRECTION

HIGH LOWA

B

B

A

FIGURE 13—Shown here

are two methods of

denoting a split-phase

motor.


Bearing problems are usually noted by excessive shaft rota-tion resistance or excessive noise. Bearing noise changesaudio pitch as the bearing degrades. At first, a faulty bearingwill have a high-pitched click or whine. Then the pitch of thesound will lower until the bearing completely decays.

Run winding problems can show up as excessive motor heator as a lack of motor torque. Usually, run winding problemscan be found with a meter set to measure resistance, whichis then compared to a known good motor. You can also disassemble the motor, supply the run windings with asource of DC current, and use a metal bar or compass neareach winding to check magnetic field strength. If a run wind-ing is shorted to the motor’s case, the motor will normallyopen the fuse or circuit breaker in its power circuit.

T8

T1

T2

T3

T4

T5

T8T1 T3T2 T4 T5

ACLOW

ACHIGH

LOW VOLTAGE(120 VAC)

T1T4 T5 T3T2 T8

ACHIGH

ACHIGH

HIGH VOLTAGE(240 VAC)

FIGURE 14—The leads of

an industrial split-phase

motor are terminated

with wires labeled with

T numbers such as those

shown here. Also shown

are the connections to

operate the motor on

120 or 240 VAC.


The Capacitor MotorA version of a split-phase motor is the capacitor motor. Ifyou’ve studied motors for any length of time, you’ve probablyseen a small motor that has a small half-tubular-shapedmetal enclosure on its case. The case houses a capacitor.

Capacitors serve many different purposes.Capacitors can store electric energy andcan pass AC while blocking DC. However,there’s another property of capacitors thatcan be used to great advantage when start-ing a motor.

Figure 15A displays a simple circuit with anAC current source, a load resistor R1, and aseries circuit of C1 and R2. In Figure 15B,you see two sine waves. The solid sine waveis taken between points A and D or acrossthe load resistor R1. This waveform is inphase with the source. The dashed line inFigure 15B reflects the waveform acrosspoints B and D. Here the capacitor hasdelayed the applied voltage by 90 degrees.

Delayed applied voltage can easily create arotating magnetic field within a split-phase-style motor. The advantages in usinga capacitor to start a motor are that themotor will draw less current on starting andthat the motor will provide more starting torque.

Figure 16 displays the connection of a typical capacitor-startmotor. Note that this is the same connection system as thesplit-phase motor, with the exception that there’s a capacitorin series with the centrifugal switch and start or auxiliarywinding. As with the split-phase motor, the motor’s directioncan be reversed by reversing the start or the running wind-ing’s connections with respect to the opposite winding. Also,the capacitor-start motor comes in dual-voltage models tooperate on 120 or 240 VAC.

On most capacitor motors, the capacitor is used to start themotor and is then disconnected by means of a centrifugalswitch or relay contact. On some small motors (under 1/4 HP),

R1 R2

C1A B

(A)

ES ER2

(B)

D

FIGURE 15—This illustration displays how a

capacitor can cause the voltage to lag the cur-

rent in an AC circuit.


the capacitor can remain in the motor’selectric circuit. This type of motor is oftentermed a permanent split-capacitor motor.

The capacitor itself is normally encased in ametal or plastic shell and will have eitherpush-on or screw-type terminals. This is anon-polarized type of capacitor with a valueranging from a few microfarads to about250 microfarads (mFd, �Fd, or �F). The rating of capacitance is important in replac-ing a failed unit, although you can test amotor’s capacitor by replacing it with aknown good unit of a value close to theoriginal rated value (within 20%). There are

two conditions, however, that are very important when work-ing on a motor’s capacitor.

1. A capacitor holds a potentially lethal charge of electricity.This charge should be bled off twice with a jumper wireor screwdriver across the capacitor’s terminals. Normallythe start or auxiliary winding bleeds off the capacitorwhen the motor stops. However, if the centrifugal switchor winding has failed, the capacitor can and will remaincharged for a few days.

2. Capacitors have a second rating called working voltage.This working voltage is the AC voltage across the capaci-tor that’s safe for the capacitor, often a value of 250WVAC. This means that the maximum voltage that canappear across the capacitor is 250 VAC.

The same types of problems found with the split-phase motorare common in the capacitor motor. The centrifugal switchand start winding are the most common causes of motor failure, followed by the bearings and the stator’s run wind-ings. The capacitor can also fail. Capacitors and centrifugalswitches can be changed in small motors, but stators are seldom rewound. Two to 10 HP motors will often be rewoundfor economical reasons.

C.S.

RUN START

CAP.ACIN

ACIN

FIGURE 16—The internal connections for a

capacitor-start motor are shown in this

diagram.


The Repulsion-type MotorRepulsion-type motors are a form of motor that’s a crossbetween a wound-rotor motor and a standard inductionmotor. The repulsion motor has a stator with the typical single-phase run windings. However, instead of start or auxil-iary windings, the rotor contains a wound set of coils and acommutator that’s very similar to those on a DC motor.

This type of repulsion motor will typically short-circuit thebrushes that ride in the commutator. The magnetic axis created by the rotor’s windings is offset by the position of thebrushes, allowing the motor to start and run as a form ofinduction motor. The speed of the motor can be varied byrepositioning the brushes.

One different kind of repulsion motor is the repulsion-induction motor. This motor has an armature that looks muchlike a DC motor’s armature. In addition, another section ofthe same armature has squirrel-cage rotor bars or windings.This motor will start as a repulsion motor and then, at aspecified speed, the brushes are disconnected and the squirrel-cage windings operate as in a typical split-phasemotor.


Self-Check 21. The special switch that’s used to start a split-phase motor is the (centrifugal/pressure) switch.

2. The simplest of the small AC motor types uses (shaded/wire wound) coils.

3. The type of motor that has both a wound and a squirrel-cage rotor is the (split-phase/

repulsion) motor.



POLYPHASE MOTORS

Polyphase Motor BasicsSo far, the motors we’ve looked at use single-phase or split-phase AC power. These motors typically range from smallfractional HP to about 10 HP. These motors run fans, pumps,air compressors, and other forms of small equipment.

Three-phase motors are available in fractional HP to thou-sands of HP, at voltages from 120 VAC to thousands of volts.Most of the motors seen on the floor of industrial plants willbe three-phase motors from 1/2 to 25 HP. Larger motorspower exhaust fans, circulating pumps, conveyors, and com-pressors. These motors can be rated at 1,000 HP or more andoperate at a voltage of 4,400 VAC or higher.

First, let’s look at the power that’s suppliedto a three-phase motor. Figure 17 displaysthree-phase power. Phase A power is shownas a solid line, phase B is shown as a longdashed line, and phase C is shown as shortdashed lines. These phases are separatedfrom each other by 120 degrees, makingthem an ideal power source for creating therotating magnetic fields needed inside amotor.

Figure 18 displays the arrangement of coilsin the stator of a four-pole three-phasemotor. It might seem that these coils wouldbe connected to a phase and then to aground to get the rotating magnetic fielddeveloped inside the motor. However, these stator windingsare connected across the phases in one of two methods. Theelectrical neutral is not used on a typical three-phase motor,and the ground is used to ground the motor’s metal parts forsafety purposes.

A B C

120° 120° 120°

FIGURE 17—Three-phase or polyphase AC

voltage waveforms are shown here. Notice

the phase relationships of the voltage peaks.


The Delta-connected MotorThe delta connection is very common in electrical work. Manytransformer systems are connected in a delta configuration.The coils of a three-phase motor can also be connected inthis configuration.

Figure 19 displays a typical delta connec-tion for a three-phase motor. Note how thephases are connected so that each phase isa source of power for the other two phases.

The actual wiring for a delta-connecteddual-voltage three-phase motor is shown inFigure 20. In this high-voltage connection,the coils are connected in series. The wind-ing terminal markings are shown below theinternal coil diagram. Note that the threeinput AC phases are listed as L1, L2, and L3instead of A, B, and C.

A

A

B

B

AA

C

C

C

C B

B

FIGURE 18—The three-phase coils are

normally connected phase-to-phase and not

phase-to-neutral or ground.

L1 OR A

L2 OR B

L3 OR C

FIGURE 19—A basic symbol for a delta con-

nection is shown here.


Figure 21 displays the low-voltage connections used for adelta-connected motor. The coil connections look very compli-cated. However, this is simply placing two coils of the motorin parallel across each set of phases. The terminal board orwire connections for this motor are shown below the coil connection. Often these connections are made in the motor’sjunction box on terminals with studs, jumpers, and nuts.The motor’s wires can be connected together with wire nuts,eyelet terminals and screws or bolts and nuts, or other suit-able electrical connections. The most common shop floormotors will operate at 220 or 440 VAC. Larger motors willoften have a single higher-voltage range.

T3

T6

T9

L3

T2

T8

T5

L2

T1

T7

T4

L1

T8

L1

L2L3

T2

T5

T4

T7

T3

T9

T6

T1

FIGURE 20—A high-voltage, or 440 VAC, delta

connection is shown here. The terminal or

wire connections are shown below the wiring

diagram.


The Wye-connected MotorA very common type of three-phase motor is the wye-connected motor. The basic symbol for such a motor is shownin Figure 22.

As with the delta-connected motor, all ninecoil leads are brought out to the motorjunction box. These leads are labeled T1through T9.

The high-voltage connections for a wye-connected motor are shown in Figure 23. Inthis illustration, notice that the coils areplaced in series. This causes part of theincoming AC voltage to be dropped acrosseach coil.

T3

T6

T9

L3

T2

T8

T5

L2

T1

T7

T4

L1

T8

L1

L2L3T2

T5

T4

T7

T3

T9

T6

T1

T8

L1

L2L3T2T5

T4

T7

T3

T9

T6

T1

FIGURE 21—Two low-voltage delta, or 220 VAC, connection diagrams and the terminal block or wiring

diagrams are given here. The two upper connection diagrams are identical but are drawn differently

to help you identify the connections.

L1 OR A

L2 OR B

L3 OR C

FIGURE 22—An elementary diagram for a

wye- or star-connected motor is given here.


The low-voltage arrangement for a wye-connected motor isshown in Figure 24. Here, the coil sets are placed in parallelacross a lower incoming line voltage. The terminal connec-tions are also shown below the coil sets.

Multiple-speed OperationMany types of three-phase motors are wound in a specialarrangement that allows the motor to operate at two differentspeeds with the motor performing at constant torque at eachspeed.

L1

L2

L3 T3

T9

T6

T2

T5

T8

T1

T4

T7

T3

T9

T6

L3

T2

T5

T8

L2

T1

T4

T7

L1

HIGH VOLTAGECONNECTION

FIGURE 23—This illustra-

tion displays how to

connect a wye-connection

motor for 440 VAC.


The AC three-phase or polyphase motor is a synchronous-speed motor. This means that the squirrel-cage-wound rotorwill attempt to lock in exactly at a speed that’s a function ofline frequency and the number of poles inside the motor. Theoperating speed of a motor is found by using the followingequation:

Where S = speed in rpm

F = frequency of the applied AC signal (in Hz)

P = number of poles (always an even number)

Table 1 displays these values for a 60 Hz input line frequency.

T3

T6

T9

T2

T8

T5

T1

T7

T4

L3L2L1

LOW VOLTAGECONNECTION

L1

L2

L3T3

T9

T6

T2

T5

T8

T1

T4

T7

FIGURE 24—This illustra-

tion shows how to

connect a wye-connected

motor to a source of

220 VAC.


As mentioned, the motor will attempt to lock into thesespeeds. A tachometer placed on the motor’s shaft wouldmeasure these speeds minus a small deviation called slip. Forexample, a motor may measure 1,782 RPM instead of 1,800RPM because of slip. Slip will increase as motor load isincreased until it reaches a condition where the motor stopsturning. At this time, the motor is drawing maximum currentand this current value is often identified as locked rotor current.

In normal operation, the motor will have a small amount ofslip and will be rotating at a value similar to one of thoseseen in the tables. A two-speed motor must, therefore,change its pole number to change its speed. This motor will,however, be locked into only one voltage for which its internalcoils are wound.

A typical variable-speed wye-connected motor is shown inFigure 25. The connection diagram displays how the motorshould be connected for low- and high-speed operations. Forlow speed, the three input phases L1, L2, and L3 connect toT1, T2, and T3 respectively. Terminals T4, T5, and T6 are leftseparated from each other and insulated from the incomingpower lines.

Table 1

Motor Poles Speed RPM

2 3600

4 1800

6 1200

8 900

10 720

12 600

14 514.3

16 450

18 400

20 360


For high speed, T1, T2, and T3 are shorted together. Now L1,L2, and L3 are connected to T4, T5, and T6. If you were todraw this diagram as a grouping of internal motor coils atlow speed, you would have a delta. At high speed, the coilswould be connected as a parallel wye or double-wye connec-tion. These two arrangements will give a constant torque two-speed motor.

A second type of two-speed motor uses the opposite arrange-ment of a two-parallel wye connection for low speed and aseries delta for high speed. Such a motor is shown in Figure 26. This type of arrangement will offer a constanthorsepower (HP) at either the lower or higher speeds.

T2

T1 T3

T4

T5 T6

SPEED

LOW

HIGH

L1 L2 L3

T1 T2 T3

T6 T4 T5

T4 , T5 , T6

T1 , T2 , T3

OPEN CONNECT

FIGURE 25—This type of

motor offers two speeds

when it’s connected as

illustrated in the lower

chart.


Actually, there are various types of multispeed motors, fromwye- to delta-connected, to one-winding two-speed throughtwo-winding three-speed motors. The label on the motor’scase will identify the type of motor and display its connectiondiagram for various speeds.

AC single-phase motors are typically controlled with a rathersimple switch assembly. Three-phase or polyphase motorsuse more complex relays called motor starters or contactors.Multispeed motors will also often use motor starters or con-tactors to control the motor’s speeds. More will be seen onthese devices in the next section.

Motor Nameplate InformationA nameplate is attached to every motor to identify the motorand the motor manufacturer. A typical motor nameplate isshown in Figure 27.

T1

T6

T2

T5

T3

T4

SPEED

LOW

HIGH

L1 L2 L3

T1 T2 T3

T6 T4 T5

T4 , T5 , T6

T1 , T2 , T3

OPEN CONNECT

FIGURE 26—This motor

also offers two speeds

when connected as shown

in the chart.


This nameplate is useful in identifying themotor for replacement purposes. Look atthis label closely.

The frame type is listed in the upper leftcorner. This number or number/letter combination describes the physical characteristics of the motor, such as thedimensions of the mounting holes, height of the shaft above the baseplate, and soforth. A list of frame styles is given in theAppendix section of this study unit.

The model number is the identificationnumber given the motor by the manufac-turer. This number is helpful when replacing the motor if an exact replacementis desired.

The next two columns define the motor’s HPand RPM with the next lower two valuessignifying that this is a three-phase motorthat operates at 60 Hz.

The next line identifies the voltages and the currents at theapplied voltages. This motor operates at 220 VAC at 12.2amps or 440 VAC at 6.1 amps. Notice how this motor drawsone-half of the current at the higher voltage than it does atthe lower voltage.

The rating CONT. stands for continuous. This means that atthe rated voltage and frequency, the motor will develop andmaintain at least the horsepower listed on the nameplate.You may also see the terms inching, plugging, or jog duty forthose motors that operate for only short periods of time.

Celsius rise and insulation code ratings go hand-in-hand. Thecelsius rise rating denotes the motor’s temperature rise aboveambient (surrounding) temperature. The insulation code rat-ings reveal the temperature rating of the insulation used onthe windings of the motor. Table 2 displays a list of the morecommon temperature ratings.

HIGH LOW

VOLTAGE

L1 L2 L3 L1 L2 L3

1718 - 1A5

H

1.15

12.26.1

220

440

60

17505.03

215

THREE = PHASE INDUCTION MOTOR

FRAME

HP

PHASE

VOLTS

VOLTS

INSULATION CODE

SERVICE FACTOR

A

ARATING

CYCLES

RPM

MODEL

55C

CONT.

FIGURE 27—A nameplate for a three-phase

AC motor is shown here.


These values listed are the maximum values of temperaturesthat can appear on the windings before the insulation willbegin to break down and fail.

Returning to Figure 27, the service factor of the motor servesas a numerical multiplier. The rated horsepower of the motorcan be multiplied by the service factor number to give thetrue maximum horsepower of the motor for the rated voltageand frequency.

Another group of numbers commonly found on the name-plate signifies the bearings used in the motor. For example,Front Bearing 62062RS means that the bearing at the shaftend of the motor is a 6206 bearing with two rubber seals.You may also see an AFBMA number. This abbreviationstands for the Antifriction Bearing Manufacturer’sAssociation numbering system, which requires that youdecode the number to find the bearing inside and outsidediameters.

You may also see a CODE listing followed by a letter. This letter signifies the locked rotor current in kilovolt amperes or KVA. This value of current is used to provide the propercircuit protection values.

One final number sometimes seen on a motor nameplate is aPower Factor or PF number. A motor is an inductive device.This means the current will lag the voltage in the motor’s circuit. The power factor is simply a number from 0 to 1 that reflects the cosine of the angle of the phase differencebetween the applied current and the motor’s circuit current.A PF of 1.0 would be ideal; however, a PF of 0.85 to 0.95 is

Table 2

Class Temperature C

A 105°

B 130°

F 155°

H 180°


more common. The problem with low PF motors is that theymay require the use of power-factor-correction capacitors tobring the PF closer to 1.

All of the ratings on a nameplate are important. To replace amotor, you should use an exact replacement or a motor withhigher ratings. However, in most cases, you can’t increasethe horsepower rating of the new motor without increasingthe size of the circuit protection devices, components, andconductors.

Polyphase Motor Troubleshooting andRepairA polyphase motor, like a transformer, is nearly an idealmachine. Almost all of the applied electrical energy is con-verted to mechanical energy. The lost energy is mostly in theform of heat.

Some motors are open motors where the end bells have openslots. A fan at the end of the motor moves air through themotor to cool the rotor and the stator. A typical maintenancetask is to clean these slots to provide for proper air flow. Ifthe motor is operated in a dusty location, the motor will oftenfill up with dust until it begins to bind. Obviously, a goodcleaning with compressed air or disassembly and cleaning isin order.

Some motors are enclosed to prevent dirt from entering themotor. These motors are termed totally enclosed. Thesemotors may or may not have a fan on the rear shaft of themotor to help cool the case.

There are also drip-proof motors that are specially sealed tohelp prevent moisture from entering the motor. Finally, thereare hermetically sealed or explosion proof motors for use inareas where there are flammable gases, dusts, or liquids.

Any type of motor will fail when the service life of the bearings is exceeded or when the bearings are overloaded.Bearing failures account for most of the problems withpolyphase motors.


Most bearing failures will result in an audible high-pitchedwhine or a rapid ringing noise at first and then degrade to alow-pitched rumble as the bearing progressively fails. Often,if there are grease fittings, you can apply grease with a greasegun to the bearing to see if it gets quieter. Bearing failuresrequire that the motor be disassembled and the bearingsreplaced.

Winding problems normally are revealed as an unbalancedload. A good clamp-type ammeter can be used to measure thecurrent in each phase (L1, L2, and L3). If one phase is about10 percent higher or lower than the other two phases, thatphase is suspect. Shorted windings will usually result in ahot-running, vibrating motor, with a loss of torque or horse-power. If the winding shorts to the motor’s case, the motor’soverload protection device will normally open the circuit.

Rotor bar conductor failures are more common in largemotors (100 HP and up). These rotor bar failures usuallyshow up as a lack of horsepower or torque and especially asexcessive vibration. However, a condition known as rotoreccentricity can mimic rotor bar failures. This condition isnormally caused by someone bolting the motor down to anuneven plate or mount. As the motor’s bolts are torqueddown, the motor’s case deforms until the rotor is no longercentered within the stator. This is also called a soft foot condition.

Two rather new technologies are being used to detect theseand other motor problems. These technologies are vibrationanalysis and electrical signature analysis. Vibration analysiselectronically measures motor vibration and identifies thecause of each frequency within a computer-stored spectrum.By careful analysis of these spectrums, the causes of excessive vibration (such as soft foot, faulty rotor or machinebalance, misalignment, and so forth) can be identified andcorrected. Electrical signature analysis also stores a spec-trum, but this kind is an electrical frequency spectrum delivered by a transducer or clamp-type ammeter. Problemssuch as stator winding failures or rotor bar failures are easilyidentified using this technology.


The key pieces of motor test equipment are simply your senses of smell and touch, a good meter for measuring volt-age and resistance, and an accessory clamp-type ammeter formeasuring current. You can hear a motor’s bearings and feelexcessive temperature, even inches or feet away from amotor. You can also take voltage measurements to make surethe proper voltages are reaching the motor, and take resist-ance readings phase-to-phase and phase-to-ground to checkfor shorted or open stator windings. You can also use theclamp-type ammeter to check for phase-to-phase balance incurrent draw.

However, don’t always condemn the motor as faulty at thefirst sign of trouble. Remember that the motor is moving ordriving something and that a jammed machine or a failedbearing in the load device won’t be corrected by changing themotor. Insufficiently low supply voltage is a common cause ofhot-running AC motors. This condition, which often resultsin the motor’s thermal cutout switch tripping, is often causedby improperly sized power-supply-circuit wiring. Small wiresincrease the voltage drop in the supply circuit and, therefore,reduce the voltage at the motor. Whenever in doubt andwhenever possible, uncouple the motor from the load andcheck each component individually.

One final test conducted on motors is often part of a preven-tative maintenance (PM) program. A clamp-type ammeter isused to measure motor-starting current or inrush current oneach phase as the motor is started from a standstill. Thesevalues can be four or more times the normal running currentvalues. The values should also be within 10 percent of eachother and can be recorded for trending purposes. If theinrush current has suddenly increased, the motor or the loaddevice is beginning to have a problem that should be identi-fied and repaired.


Self-Check 31. The number that can be multiplied by the horsepower to get a maximum horsepower rating

for the motor is the (service/power) factor rating.

2. The dimensions of the mounting holes of the motor are identified by the (case/frame) type.

3. If you are connecting a constant-torque delta-wound motor for low speed, you should short

out (T1, T2, and T3/T4, T5, and T6 )



AC MOTOR RELAY CONTROLSAC motor controls can be divided into many categories. Forour purposes, we will look at two sections of AC controls, thecontrol circuit and the power circuit. The control circuit contains the manual operators, switches, or sensors used toenergize and close the contacts or energize the coil of themotor starter or contactor. The power circuit contains thelarge contacts and thermal overload protection devices thatconnect the motor to the source of AC electricity.

The Manual Control CircuitThe simplest of all control circuits is the manual control circuit. This circuit is shown in Figure 28. This illustrationshows how a manually operated motor controller operates. InFigure 28A, a single-phase AC motor is powered by a singleAC line feed. This feed is controlled by a toggle switch-typedevice that, from the outside, looks like a typical wall lightswitch. However, the body of the switch is much heavier, asare its internal contacts. Also incorporated into the switch’shousing is a location for mounting one or two thermal overload elements. These elements are used to monitor thecurrent flow to the motor.

MOTOR

ACHIGH

ACHIGH

(B)

MOTOR

SWITCH

ENCLOSURE

OVERLOAD

ACHIGH

ACLOW

(A)

FIGURE 28—A simple

manual starter circuit for

a 120 or 220 VAC motor

is shown here.


The typical thermal overload element is mounted on a ratch-et-type assembly that protrudes from the switch. If too muchcurrent flows though the overload element, the element heatsup to a point where the ratchet is released. This opens thecircuit to the motor in a way similar to that used with a circuit breaker. In this situation, the switch lever will moveaway from the ON position to a neutral position between ON and OFF. To reset the switch/overload protection device,simply move the switch to the OFF position and then turnthe switch back to ON.

Figure 28B displays how a manual switch should be connected for a 240 VAC single-phase or split-phase system.Note that both of the phases of the source are placed throughthe switch’s contacts and through their own thermal overloadelements. If either or both phases experience an overcurrent,then both circuits will be opened to the motor. It’s neitherwise nor legal, according to the NationalElectrical Code, to disconnect only a singlephase of a 240 VAC system. The systemmust also be grounded in areas such as themetal switch enclosure and the motor’scase.

A manual three-phase starter is shown in Figure 29. All three phases of the incom-ing AC lines are controlled by the push-button-type switch. A square ON button ispressed to close these contacts and asquare OFF button is pressed to open thesecontacts. All three phases are shown withoverload protection. Often in older manualstarters, only two phases are protected byoverload protection, while the third phase isallowed to pass from the switch’s load sidecontact directly to the motor. As with thesingle-phase system, if an overcurrent con-dition exists in any phase, all phases will beopened.

MOTOR3 0

MANUALSTARTER

L1 L2 L3

FIGURE 29—A manual three-phase starter will

disconnect or connect all three motor wires to

the three incoming phases.


The Basic Electric Control CircuitThe most basic form of three-phase AC motor control isshown in Figure 30. The circuit in Figure 30A is the controlcircuit for the across-the-line motor starter in Figure 30B.

In Figure 30A, the control circuit is shown in ladder-logic for-mat. The two side sections or rails denote the power supplyto the circuit. This power supply is normally a low voltage,such as 24 or 48 volts. If the low voltage source is an ACvoltage, this voltage is supplied by a step-down or controltransformer. The input to the transformer may be 120, 240,or 480 VAC while the output of the transformer is 12, 24, or

MOTOR3 0

L1 L2 L3

M

COIL

OVERLOADCONTACTS

AUXILIARYCONTACTS

MSTOP

STARTERCOIL

COM24 V

M OVERLOADCONTACTS

START

AUXILIARYCONTACTS

(A)

(B)

FIGURE 30—This circuit

is often called a seal cir-

cuit since the relay’s

auxiliary contacts seal

the circuit around the

START push button.


48 VAC. If the source of the control voltage is a DC voltage,then the source is typically a stand-alone power supply that has an output voltage of 12 or 24 VDC. Sometimes acomputer control system’s internal power supply is also usedto power the field devices. However, electrical noise from thewires, relays, and solenoids can play havoc with sensitivecomputer circuits making the stand-alone power supply thebetter choice. The lower voltages in the control circuits areused to prevent an electric shock hazard where operatorpushbuttons and selector switches are used. The horizontalsections or rungs contain the circuit elements. In this circuit,the rung contains a STOP and START push button. TheSTOP push button is normally closed, allowing for a closedcircuit to this point. The START push button is normallyopen and must be pressed to close the circuit to the coil ofthe motor starter labeled M. To the right of coil M, there’s aset of contacts off the thermal overload section of the motorstarter. As long as the thermal overload section hasn’ttripped due to overcurrent, these contacts will be closed.

When the START button is pressed, the circuit will be com-pleted, energizing the coil of the motor starter M. An auxiliarycontact of its motor starter parallels the START push button.When the coil is energized, the mechanical motion of themotor starter will close this contact along with the three largecontacts for the motor. The auxiliary contact will then closeacross the START push button, keeping the coil of the motorstarter energized. This allows the START push button to bereleased and the motor to stay energized until either theSTOP push button is pressed or the overloads trip.

Figure 30B shows the coil and the three contacts that connect the motor to the three-phase AC lines. Below thesecontacts are the thermal overloads with their auxiliary con-tact that’s placed in the right side of the coil’s circuit in thisexample.


A Reversing CircuitOften a motor must be reversed during normal machine oper-ations, such as in loading or unloading the machine or inclearing its jaws. Figure 31 gives us an example of such a circuit.

The top rung of this figure is very similar to our STOP/STARTcircuit given earlier. The STOP push button is normallyclosed, allowing power to flow through to the two control cir-cuits. When the MOTOR FORWARD push button is pressed,the motor forward coil will be energized. However, if themotor’s reverse coil is already energized, its contacts MR onthe top rung will be open, keeping the forward motor coilfrom energizing. This is a safety interlock contact that allowsonly one coil to be energized at one time. The motor reversesection operates in a similar manner.

Figure 32 displays the power control section of the reversingcircuit. To reverse the direction of any AC three-phase motor,you simply swap any two phases to the motor. As seen inthis figure, when the forward starter has its coil energized,the three left contacts will close. This places L1 on T1, L2 onT2, and L3 on T3. Let’s assume the motor now turns clockwiseor to the right. If the forward coil MF is de-energized and thereverse coil is energized, a different connection path is taken.Now, the three contacts to the right of the illustration areclosed and those on the left are open. Now L1 connects to T3,

MF

STOPCOIL

COM24 V

MR

MF

MR

COIL

MOTORFORWARD

MOTORREVERSE

MF

MR

OL

FIGURE 31—This is the

control circuit for a

reversing motor starter

for a three-phase motor.


L2 to T2, and L3 to T1. Because two phasesare swapped, this motor will turn in anopposite direction or counterclockwise inthis example.

A careful study of this illustration showsthe importance of interlocking the coils ofthe contactors to prevent both from turningon at the same time. If this were to occur, itwould be like shorting phases L1 and L3together. This action will almost alwaysdraw such a great current that the mainfuses will blow or the main circuit breakerwill trip. The current draw is in fact so greatthat fuses can literally explode!

Some reversing motor starters havemechanical interlocks, a kind of teeter-tot-ter bar between the contact carriers of eachsection of the motor starter. This bar physi-cally prevents both carriers from rising intocontact with the fixed contacts at the sametime.

Two-speed Magnetic StartersThe control circuit for a LOW/HIGH speed AC motor controlsystem is very similar to the one used for FORWARD/REVERSE control. This circuit is shown in Figure 33.

As with the reversing starter, the SLOW/FAST starter isinterlocked to prevent both coils from energizing simultane-ously. For example, if the motor’s SLOW push button ispressed, the MS coil will be energized. This coil will seal itselfon with its contacts in parallel with the MOTOR SLOW pushbutton. A second set of contacts from the MS starter willopen the circuit to the FAST, or MF, coil, preventing it frombeing energized until the motor SLOW coil is de-energized.

MOTOR3 0

L1 L2 L3

MRMF

T1 T2 T3

FIGURE 32—In order to reverse an AC motor,

two of the three incoming phases are

swapped before they reach the motor.


A second version of the SLOW/FAST control circuit is shownin Figure 34. This circuit is modified to eliminate the need topress the STOP push button when you desire to changemotor speeds.

This circuit starts out with the typical STOP push button inits normally closed state. Next, there’s a normally closed contact of the FAST push button and a jumper wire to thenormally closed contact of the SLOW push button. Beneatheach of these contacts are normally open contacts that willclose when the associated push button is pressed.

MS

STOP

MOTORSLOWCOIL

COM+ V

MF

MS

MF

MOTORFASTCOIL

MOTORSLOW

MOTORFAST

MS

MF

FIGURE 33—This is a typ-

ical control circuit for a

two-speed motor. In this

circuit, the STOP push

button must be pressed

to change speeds.

MS

COMMOTORSLOWCOILMF

MF

MS

MOTORFASTCOIL

+ V

STOP

MOTORFAST

MOTORSLOW

MF MF

FIGURE 34—This is also a

circuit for a reversing

system. In this circuit,

the STOP push button

doesn’t need to be

pressed to change

speeds.


Let’s assume that you want slow speed. By pressing theSLOW push button, you open the top set of contacts on thatpush button and close the lower set. This supplies power tothe motor SLOW coil MS through the normally closed auxil-iary interlocking contacts of the MF coil. The MS coil will sealitself around the normally open contacts of the motor SLOWpush button. If high speed is desired, simply push the FASTpush button. The normally closed contact set will open, drop-ping out or de-energizing the MS coil. At the same time, theMF coil will energize as soon as the interlocking contacts ofthe MS coil close as the coil de-energizes. The FAST coil willbe energized and that starter will be enabled.

The power circuit for a two-speed motor is shown in Figure 35. In slow speed, the contactor acts like a simpleacross-the-line motor starter. The three-phase lines L1, L2,and L3 are connected to T1, T2, and T3 through the three con-tacts on the right side of the diagram.

L1 L2 L3

MF

T1T2T3

T4

T5

T6

MS

FIGURE 35—This is the

power circuit for the two-

speed motor starter.


However, when the FAST starter is chosen,its coil is energized, and all six contactsclose. The left three of these supply powerto T4, T5, and T6. The right three short outT1, T2, and T3.

AC motor starters are often controlled by asingle output from a programmable con-troller PC or PLC. In regular hard-wired circuits, however, the AC motor starter coilscan have quite a few input devices, such asselector switches, pressure switches, lightoutputs, and so forth. These devices are inseries with the coil. Before condemning amotor starter as faulty, make sure the coilis receiving the proper amount of voltage bymeasuring across the coil’s terminals with ameter. These terminals are where the wiresattach to the small terminals in the motorstarters, as shown in Figure 36.

This starter can also be tested for faultycontacts by measuring with a meter set toAC volts and placing the leads of the meteras follows:

L1 to T1L2 to T2L3 to T3

In each of these readings, little, if any, voltage should bemeasured. A reading of about 2.0 VAC or more indicates afaulty contact. A reading of 10 V or more indicates a burntcontact that will soon fail. Full line voltage across a measur-ing point means that the contact is open.

In the same starter, there are no thermal overload elementsin place. They would be mounted on the lower screws wherethe square white overload reset push button is located.

FIGURE 36—A magnetic across-the-line three-

phase motor starter has the AC line inputs at

the top and the motor output at the bottom.

(Photo courtesy of Allen-Bradely, a RockwellInternational Company)


Self-Check 41. In a 240 VAC single-phase manual motor starter, how many AC power lines should be discon-

nected by switches or contacts?

a. 1b. 2

c. 3

2. On some older three-phase manual starters, how many phases of the starter have overload

protection?

a. 1

b. 2

c. 3

3. When an auxiliary contact of one motor starter is placed in series with the coil of a second

motor starter, the contact is called an (interchangeable/interlocking) contact.



NOTES

Self-Check 11. b

2. Reverse polarity

3. induction

Self-Check 21. centrifugal

2. shaded

3. repulsion

Self-Check 31. service

2. frame

3. T4, T5, and T6

Self-Check 41. b

2. b

3. interlocking

47

Answers

Answers

NOTES

Self-Check Answers48

APPENDIX A

49

Appendix

Appendix

FULL LOAD CURRENT OF SINGLE-PHASE AC MOTORS AT 120 AND 240 VAC

HP 120 V 240 V

1/61/41/31/23/4

4.45.87.29.813.8

2.22.93.64.96.9

111/2

23

16 20 24 34

8 10 12 17

5710

56 80 100

28 40 50

APPENDIX B

Appendix50

FULL LOAD CURRENT FOR TYPICAL 240 V AND 480 VTHREE-PHASE MOTORS

HP 240 V 480 V

1/23/4

1

22.83.6

11.41.8

11/2

23

5.26.89.6

2.63.44.8

571/2

10

15.22228

7.61114

152025

425468

212734

304050

80104130

405265

6075100

154192248

7796124

125150200

312260480

156180240

APPENDIX C

Appendix 51

D

E E

NEMA FRAME DIMENSIONSFOR AC MOTORS

V

U

F F

M + N

Appendix52

MotorFrame

NEMA Frame Dimension — Inches

D E F U V M+N

42485666

143T

25/8

331/2

41/8

31/2

13/4

21/8

27/16

215/16

23/4

27/32

13/8

11/2

21/2

2

3/81/25/83/47/8

––––2

41/32

53/8

61/8

77/8

61/2

145T182182T184184T

31/2

41/2

41/2

41/2

41/2

23/4

33/4

33/4

33/4

33/4

21/2

21/4

21/4

23/4

21/4

7/87/8

11/87/8

11/8

22

21/2

221/2

771/4

73/4

73/4

81/4

213213T215215T254T

51/4

51/4

51/4

51/4

61/4

41/4

41/4

41/4

41/4

5

23/4

23/4

31/2

31/2

41/8

11/8

13/8

113/8

15/8

23/4

31/8

23/4

31/8

33/4

91/4

95/8

10103/8

123/8

254U256T256U284T284Ts

61/4

61/4

61/4

77

555

51/2

51/2

41/8

55

43/4

41/4

13/8

15/8

13/8

17/8

15/8

31/2

33/4

31/2

43/8

3

121/8

131/4

13141/8

131/2

284U286T286U324T324U

77788

51/2

51/2

51/2

61/4

61/4

43/4

51/2

51/2

51/4

51/4

15/8

17/8

15/8

21/8

17/8

45/8

43/8

45/8

553/8

141/8

147/8

151/8

153/4

161/8

326T326TS326U364T364U

88899

61/4

61/4

61/4

77

666

55/8

55/8

21/8

17/8

17/8

23/8

21/8

531/2

53/8

55/8

61/8

161/2

15167/8

173/8

177/8

365T365U404T404U405T

99101010

77888

61/8

61/8

61/8

61/8

67/8

23/8

21/8

27/8

23/8

27/8

55/8

61/8

767/8

7

177/8

13/8

20197/8

203/4

405U444T444U445T445U

1111111111

89999

67/8

71/4

71/4

81/4

81/4

23/8

33/8

27/8

33/8

27/8

67/8

81/4

83/8

81/4

83/8

205/8

231/4

233/8

241/4

243/8

Examination56

22. Which one of the following types of AC motors uses brushes and a commutator?

A. Shaded-pole C. Split-phase

B. Repulsion-induction D. Capacitor start

23. The deviation in a motor’s speed between the nameplate value and the actual value is

called

A. stagger. C. lag.

B. slip. D. decline.

24. The most common failure item with a three-phase induction motor system is the

A. bearings. C. rotor bars.

B. stator windings. D. controller or starter.

25. Three current readings are taken on a motor and are 7.7A, 8.1A, and 8.4A. What’s the

condition of this motor?

A. The motor is in good working condition.

B. Phase A is too low.

C. Phase C is too high.

D. Phase B should be 8.3A.

Date post:	24-Jun-2020
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Study Unit Industrial Alternating Current Motors · The Repulsion-type Motor 19 POLYPHASE MOTORS 21...

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