SUMMARY REVIEW OF Sections 12-17 [AC/DC Motors, Controls, and Maintenance]

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GOAL:

• To provide the student with an opportunity to evaluate the knowledge and understanding acquired in the study of the previous six sections.

1. List three types of three-phase AC motors.

a. _____

b. _____

c. _____

2. Insert the word or phrase to complete each of the following statements.

a. The speed of a three-phase induction motor falls slightly from a no-load condition to a full load. This is true of a three-phase induction motor with a _ rotor.

b. A speed controller is used only with a three-phase induction motor of the type.

c. When all resistance of the speed controller is inserted in the secondary circuit of a three-phase induction motor, the starting torque is very good.

d. A three-phase motor is operated with an overexcited DC field to obtain a leading power factor.

e. The speed of a three-phase motor remains constant from a no load condition to full load if the operating frequency remains constant.

3. State two advantages of using a squirrel-cage induction motor.

4. State one disadvantage of using a squirrel-cage induction motor.

Explain how the direction of rotation of a three-phase AC induction motor is reversed.

6. A two-pole, 60 Hz, three-phase AC induction motor has a full-load speed of 3475 RPM. Determine the synchronous speed of this motor.

7. Determine the percent slip of the motor in question 6.

8. What is the purpose of starting protection for a three-phase motor?

9. What is the purpose of running protection for a three-phase motor?

10. Show the connection diagram for the nine terminal leads of a wye-connected, three phase motor rated at 230/460 volts for three-phase operation on 230 volts.

11. Explain how the running overload protection for a three-phase motor rated at more than 1 hp would be selected.

12. Why are electronic drives used with large three-phase AC induction motors?

13. Why is a wound-rotor induction motor used in place of a squirrel-cage induction motor for some industrial applications?

14. Explain how the direction of rotation of a wound-rotor induction motor can be reversed.

15. Insert the correct word or phrase to complete each of the following statements.

a. The speed of a wound-rotor induction motor is by inserting resistance in the rotor circuit through a speed controller.

b. The starting surge of current of a wound-rotor induction motor is minimized by.

c. The rotation of a wound-rotor induction motor is by changing any two of the three leads feeding from the rotor slip rings to the speed controller.

d. The of a wound-rotor induction motor is very good if all the resistance of the speed controller is inserted in the rotor circuit.

e. The efficiency of a wound-rotor induction motor operating at rated load with all the resistance inserted in the rotor circuit is ___.

16. Draw a schematic connection diagram of a wound-rotor induction motor that is started by means of an ATL magnetic motor starter controlled from a pushbutton station.

17. Explain how the direction of rotation of a three-phase synchronous motor is reversed.

18. List two important applications for three-phase synchronous motors.

19. A three-phase synchronous motor with four stator poles and four rotor poles is operated from a three-phase, 60 Hz line of the correct voltage rating. Determine the speed of the motor. ___

20. Explain the correct procedure for starting a three-phase synchronous motor.

21. Insert the correct word or phrase to complete each of the following statements.

a. The speed of a synchronous motor is from no load to full load.

b. A synchronous motor with an underexcited DC field has a power factor.

c. A three-phase motor must be started as an induction motor.

22. Explain what is meant by the term jogging. _____

23. Explain what is meant by the term plugging. ____

24. What identifying information should appear on a motor controller to comply with the requirements of the National Electrical Code?

25. What is meant by a PWM AC drive?

26. Insert the correct word or phrase to complete each of the following statements.

a. A controller with may be used to stop a motor quickly.

b. When fuses are used as protection for a three-phase, three-wire, ungrounded branch motor circuit, the fuses must be installed in line leads.

c. Motors operating on a three-phase, three-wire ungrounded system require thermal overload units for running overcurrent protection.

27. How is dynamic braking applied to an induction motor?

28. How is dynamic braking applied to a synchronous motor?

29. Draw a schematic diagram of an ATL magnetic switch connected to a three-phase AC induction motor. The magnetic switch has jogging capability. Include in the connection diagram the main relay coil; the pushbutton station with start, jog, and stop pushbuttons; and the maintaining contact.

30. Draw a schematic diagram of a wye-delta, open-transition starter, complete with push button station, connected to a three-phase motor.

31. On periodic tests, a motor winding suddenly drops to a low resistance value. Testing with a determines this condition.

32. For the sleeve bearings of an AC motor, explain how the old oil is removed and the bearings cleaned and lubricated.

33. Place the correct answers in each of the spaces provided in Figure 1. Refer to the National Electrical Code.


FIG. 1 Fill in the blanks for question 33.

 

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