Unit 17 Types of Electric Motors

1. The two popular operating voltages for residences are

115V (120V) and 230V (240V).

2. When an open motor gets up to speed, the _____ switch takes the start windind out of the circuit.

Centrifugal switch. (Electronic start switch)

3. Is the resistance in the start winding greater or less than the resistance in the run winding?

Greater.

4. What device may be wired into the starting circuit of a motor to improve the starting torque?

A start capacitor.

5. What device may be wired into the running circuit of a motor to improve the running efficiency?

A run capacitor.

6. Define BEMF.

The large metal mass of the motors rotor turning at high speeds with motor winding's in close proximity has a voltage-generating effect, more across the start winding because it's wound with a longer thinner wire than the run winding. This generated voltage is often referred to as induced voltage, or Back Electromotive Force.

7. Why is the hermetic compressor motor manufactured from special materials?

A hermetic compressor motor is manufactured from special materials because it is designed to operate in a refrigerant, usually vapor, atmosphere. The materials must be compatible with the refrigerants and oils circulating in the system.

8. The two types of motors used for hermetic compressors are

Single phase hermetic motors and three phase compressor motors.

9. How does power pass through the compressor shell to the motor inside?

Power passes through the compressor shell in insulated terminal boxes. (Usually made of a ceramic material)

10. The two types or relays used to start hermetic compressors are

Potential relays, and current relays.

11. A PTC device is used

To start a hermetic compressor.

12. Three-phase motors have low stating torque.

False.

13. All large motors are three phase.

True.

14. Why is it desirable to have a two-speed compressor?

It is desirable to have a two-speed compressor motor to control the capacity of small compressors.

15. All electric motors must be cooled or they will overheat.

True.

16. An SCR is an electronic switch.

True.

17. When a transistor is used as a switch

It does not create an arc.

18. Capacitors are used after the converter to

To smooth out the DC current.

19. The converter

Changes AC to DC.

20. The inverter

Adjusts the frequency.

21. Briefly describe the operation of the three electronic sections of a variable frequency drive (VFD) using figure 17.60 for assistance.

Line one enters the VFD and the voltage is rectified via a diode bridge. After the voltage turns from AC to DC, the DC voltage needs to be filtered and smoothed out, via capacitors. Then the voltage gets switched back to AC current. This happens via transistors or inverters, the frequency is also perfected so that you can control motor speed.

T1. When an alternating current is applied to the running windings of a split-phase electric motor, an alternating current

With reverse polarity is induced.

T2. A two-pole split-phase motor operates at a speed just under

3600 RPM.

T3. Start windings are placed _____ the run windings.

Between.

T4. The start winding has

More turns than the run winding.

T5. A four-pole split-phase motor runs _____ a two-pole split-phase motor.

Slower than.

T6. When a typical split-phase motor reaches 75% of its operating speed, the start winding circuit is removed from the circuit by a

Centrifugal switch.

T7. A three-phase power supply can be found in most

Factories.

T8. The electronic relay is used with some electric motors to

Open the circuit to the start windings after the motor has started.

T9. The amount by which the current leads or lags the voltage in an alternating current circuit is known as the

Phase angle.

T10. In an alternating current circuit in which there is a capacitor, the current ______ the voltage.

Leads.

T11. Capacitors wired in series with the start windings provide

A greater starting torque.

T12. The shaded-pole motor

Has little starting torque.

T13. Three-phase motors have

No start windings or start capacitors.

T14. The term "slip" is used to indicate

The difference between the full operational rpm rating of the motor and the actual speed under a load.

T15. The hermetic motor must run in a

Refrigerant vapor atmosphere.

T16. The start winding circuit in a hermetic motor is interrupted by a potential relay or a current relay because ______ mechanical switch.

Of the electric arc produced by a.

T17. The positive-temperature-coefficient start device is a type of

Thermistor.

T18. A two-speed compressor motor may operate as a

Two-pole or four-pole motor.

T19. Three-phase motors have a

High starting torque.

T20. Most small hermetic motors are cooled by

Refrigerant gas.