Unit 19 Motor Controls

9. What is not a typical operating voltage used for relays, contactors, and motor starters?

12 V.

T4. The contactor is designed to control loads that draw more than _____ amps of current.

20

5. What is the difference between a contactor and a starter?

A starter has overload protection devices, while a contactor doesn't.

What would cause a slight voltage drop across a set of closed contacts?

An old set of contacts will have resistance due the the worn contact surfaces.

8. What causes an overload protection device to function?

An overload protection device will disconnect the motor at some current draw value that is slightly more than the FLA value so that the motor can be operated at it's full-load design capacity.

T11. Crankcase heat for compressors is often controlled by using

Auxiliary contacts.

T8. External motor protection

Breaks the circuit to the contactor or motor starter.

T19. How does a magnetic overload device protect the motor?

By shutting off the motor at an accurate amper rating.

T13. Dirty and pitted contacts in a contactor

Cause resistance and heat.

T2. The pilot duty relay is used primarily to switch on and off

Contactors and motor starters that in turn will switch the motor on or off.

4. The two amperages that influence the choice for replacing a motor starter are

FLA and LRA.

15. A motor can be restarted immediately after it has stopped or been overloaded.

False.

6. A contactor can always be converted to a starter.

False.

10. Why is it not a good idea to file or sand the contactor contacts?

Filing or sanding exposes the base metals under the silver plating and speeds deterioration.

T1. The locked rotor amperage for a motor is approximately ______ times that of the full-load amperage.

Five.

13. Describe the difference between inherent and external motor protection.

Inherent motor overload protection is provided by sensing devices within the motor and external overload protection is applied to the current passing device, the relay, the contactor, or the starter.

T3. If the relay does not operate properly

It must be replaced.

T10. A solder pot is an overload-sensing device

Made of a low-melting solder that normally cannot be reset.

T18. Which overload device is not affected by ambient temperature and are extremely accurate?

Magnetic overload devices.

T9. The service factor of an electric motor is determined by the

Motors reserve capacity.

T6. The motor starter has

One or more overload protectors, magnetic coil, and electrical contacts.

3. The two types of relays are

Pilot-duty (switching) and Heavy duty (starting).

T15. Which device is designed to control loads that draw less than 20 amps?

Relays

T5. If a contactor fails, it generally is

Repaired or rebuilt.

1. The recommended repair for a defective relay is to

Replace it (Throw-away device).

T12. The ________ in a contacts keep(s) equal tension on all contact surfaces.

Springs.

T17. Which overload devices are sensitive to temperature?

The bimetal element and solder pot.

12. Under what conditions are motors allowed to operate with slightly higher-than-design loads?

The motor can operate above the FLA and within the service factor without harm, for a limited time.

11. Why is it not a good idea to use circuit protection devices to protect large motors from overload conditions?

The motor may be operating at an overload condition that may not cause the conductor to be overloaded.

2. What components can be changed on a contactor and a starter for rebuilding purposes?

The parts that can be replaced on a contactor are the contacts (both moving and stationary), the holding coil, and the contact holding springs. The components of a starter that may be replaced are the contacts (both moving and stationary), the springs, the coil, and the overload protection devices (heaters and switches).

T16. What are the three types of motor starting and stopping devices?

The relay, the contactor and motor starter

T14. The moving armature in a contactor is actuated by

The solenoid coil.

T7. Inherent motor protection is provided by a

Thermally activated device in the motor windings.

14. What is the purpose of overload protection at the motor?

To prevent the motor from overheating and damaging itself.

7. What are the contact surfaces of relays made of?

fine silver alloy.

Auxiliary contact amperages are rated ________ than main contacts.

lower

What are the contact surfaces of heavy-duty motor switching relays often made of?

silver cadmium oxide.

A relay with one set of contacts that close when the coil is energized is called?

single-pole, single throw, normally open (spst N.O.)