Stephen L. Herman, Bennie Sparkman. Electricity and Controls for HVAC-R (6th edition)

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170 SECTION 3 Motors

CONTROL TRANSFORMER

THERMOSTAT

A basic control schematic for a wye-delta starter

is shown in Figure 15–22. To understand the opera-

tion of this circuit assume that timer TR has been set

for a delay of 3 seconds.

• When the thermostat contact closes, a current path

exists through coils 1C, TR, and S, Figure 15–23.

• The 1C load contacts close to supply power to the

motor T leads.

• The S contacts change position also. The two S

load contacts close and connect the stator wind-

ing in a wye con guration by shorting T4, T5,

and T6 together. The normally open S contact

connected in series with coil 2C opens to provide

interlocking protection.

• After a delay of 3 seconds, both TR contacts

change position, Figure 15–24.

• The normally closed TR contact connected in

series with S coil opens and deenergizes S contactor

causing all S contacts to return to their normal

position. The normally closed S contact connected

in series with coil 2C recloses to provide a current

path to coil 2C.

• When the normally open TR contact connected

in series with coil 2C closes, coil 2C energizes

causing all 2C contacts to change position. The

2C load contacts close and reconnect the motor

stator windings in a delta con guration. The nor-

mally closed 2C contact connected in series with

S coil opens to provide interlocking protection.

Part Winding Starters

Another method for reducing the starting current

of large three-phase squirrel-cage motors is part

winding starting. Motors intended for use with

part winding starting contains two separate stator

windings, Figure 15–25. Each winding is rated for

Figure 15–22

Basic control schematic for wye-delta starting. (Source: Delmar/Cengage Learning)

UNIT 15 The Squirrel-Cage Induction Motor 171

2C 1C

CONTROL TRANSFORMER

THERMOSTAT

Figure 15–23

The stator windings are connected in wye during starting. (Source: Delmar/Cengage Learning)

172 SECTION 3 Motors

2C 1C

CONTROL TRANSFORMER

THERMOSTAT

Figure 15–24

The stator windings are connected in delta during the run period.

(Source: Delmar/Cengage Learning)

UNIT 15 The Squirrel-Cage Induction Motor 173

the intended line voltage. The impedance of a sin-

gle stator winding is double that of the two wind-

ings connected in parallel. Dual voltage motors

can also be used for part winding starting provided

the line voltage corresponds to the low voltage

rating of the motor. A 480/240-volt motor could

employ part winding starting provided the motor

is connected to 240 volts and not 480 volts. In

this situation T4, T5, and T6 are connected and

power is connected to T1, T2, and T3 during the

starting period. After some period of time, power

is also connected to T7, T8, and T9. This has the

same effect as connecting the stator windings in

parallel.

A basic circuit for part winding starting is shown

in Figure 15–26. To understand the operation of

the circuit, assume that timer TR has been set for a

delay of 3 seconds.

• When the thermostat contact closes power is provided

to coils C1 and TR, Figure 15–27. All 1C contacts

close and connect T1, T2, and T3 to the power line.

• After a delay of 3 seconds, the normally open TR

contact connected in series with 2C closes and

energizes coil 2C, Figure 15–28.

• When the 2C load contacts close, power is con-

nected to T7, T8, and T9. This has the same effect

as connecting the two windings in parallel.

Figure 15–25

A motor intended for part windings starting contains two stator windings. (Source: Delmar/Cengage Learning)

174 SECTION 3 Motors

CONTROL TRANSFORMER

THERMOSTAT

Figure 15–26

Basic part winding starting circuit. (Source: Delmar/Cengage Learning)

UNIT 15 The Squirrel-Cage Induction Motor 175

CONTROL TRANSFORMER

THERMOSTAT

Figure 15–27

During the starting period only one stator winding is energized.

(Source: Delmar/Cengage Learning)

176 SECTION 3 Motors

CONTROL TRANSFORMER

THERMOSTAT

Figure 15–28

During the run period both stator windings are connected.

(Source: Delmar/Cengage Learning)

UNIT 15 The Squirrel-Cage Induction Motor 177

SUMMARY

The squirrel-cage motor receives its name from the fact that the rotor contains a set of bars

connected together at each end, and the entire assembly resembles a squirrel cage.

Three factors that determine the amount of voltage induced into the rotor are:

A. The strength of the magnetic  eld of the stator.

B. The number of stator bars contained in the rotor.

C. The difference in speed between the speed of the rotating magnetic  eld and the speed

of the rotor.

The greatest amount of current draw for a squirrel-cage motor is during the starting

period.

Three factors that determine the amount of torque produced by a squirrel-cage motor

are:

A. The strength of the magnetic  eld of the stator.

B. The strength of the magnetic  eld of the rotor.

C. The phase angle difference between stator current and rotor current.

The direction of rotation of a squirrel-cage motor can be changed by reversing any two

stator leads.

The code letter found on the motor nameplate indicates the type of bars used in the con-

struction of the rotor.

The simplest method of starting a squirrel-cage motor is across-the-line starting.

Resistor/reactor starting is accomplished by connecting resistors or inductors in series

with the motor during the starting period.

Autotransformer starting reduces starting current by lowering the applied voltage to the

motor during the starting period.

If the applied voltage is reduced by 50% of normal during the starting period, the starting

torque is reduced to 25% of normal.

Wye-delta starting is accomplished by connecting the stator windings in wye during the

starting period and changing them to a delta connection during the normal run time.

A motor will draw one-third as much current during the starting period with its windings

connected in wye as it will if they are connected in delta.

Two requirements for motors intended for wye-delta starting are:

A. All stator winding leads must be brought out at the terminal connection box.

B. The motor must be designed to run with its stator windings connected in delta.

Motors intended to be used for part winding starting have two separate stator windings.

Dual voltage motors can be used for part winding starting provided the motor is connected

for low voltage operation.

178 SECTION 3 Motors

KEY TERMS

code letter

interlocking

locked rotor current

nameplates

rotor bars

squirrel-cage induction motor

REVIEW QUESTIONS

1. What three factors determine the amount of torque produced by an AC induction

motor?

2. When does an AC induction motor draw more current when starting than it does

when running?

3. Why does the current  ow to the motor increase when load is added to the motor?

4. What does the code letter found on the nameplate of the motor indicate?

5. At what degree angle between the stator current and the rotor current is the maxi-

mum torque developed?

6. What type of squirrel-cage rotor has the highest starting torque?

7. What type of squirrel-cage rotor has the best speed regulation?

8. Why can an induction motor never operate at synchronous speed?

9. What does the locked rotor current of a motor indicate?

10. The nameplate of a squirrel-cage motor indicates that the motor has a full-load speed

of 875 RPM. How many poles per phase does the motor have?

11. What is the simplest of all starting methods for a squirrel-cage motor?

12. Explain interlocking.

13. How does autotransformer starting differ from resistor/reactor starting?

14. A three-phase squirrel-cage induction motor has its stator windings connected in

a delta connection. During the starting period, the motor has a current draw of

360 amperes. If the stator windings were reconnected to form a wye connection dur-

ing the starting period how much starting current would the motor draw?

15. What two conditions must be met before a motor can be used with a wye-delta

starter?

Another type of three-phase induction motor used

for operating large air conditioning units is the

wound rotor induction motor. The stator

winding of this motor is the same as the stator of

a squirrel-cage induction motor. The rotor of the

wound rotor motor, however, does not contain

squirrel-cage bars. The rotor of this motor contains

wound coils of wire as illustrated in Figure 16–1.

The rotor will contain as many poles as there are

stator poles. The motor shown in Figure 16–1

would be for a two-pole stator. Notice that there are

three separate windings on the rotor. The  nish end

of each winding is connected together. This forms

a wye connection for the rotor winding. The start

end of each winding is connected to a separate

slip

ring on the rotor shaft.

The slip rings permit the connection of external

resistance to the rotor windings. Figure 16–2 shows

The Wound

Rotor Induction

Motor

179

UNIT 16

OBJECTIVES

After studying this unit the student should be

able to:

Describe the construction of a wound

rotor induction motor

Discuss the difference in operation

between wound rotor and squirrel-cage

induction motors

Discuss the starting and running

characteristics of a wound rotor induction

motor

Connect a wound rotor induction motor

for operation

Draw the standard schematic symbol for

a wound rotor induction motor

Perform an ohmmeter test on a wound

rotor induction motor