Summary for Units 13-18 -- Induction Motors

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OBJECTIVE:

To give the learner an opportunity to evaluate the knowledge and understanding acquired in the study of the previous six units.

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 of the 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.________________

5. Explain how the direction of rotation of a three-phase, squirrel-cage induction motor is reversed.

6. A two-pole, 60-Hz, three-phase, squirrel-cage induction motor has a full-load speed of 3,475 r/min. 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 over load protection for a three-phase motor rated at more than 1 horsepower would be selected. __________

12. Insert the correct code letter in the following statements.

a. The National Electrical Code requires squirrel-cage induction motors with code marking _____to have starting protection rated at not over 150 percent of full-load current for a non-time-delay fuse.

b. The National Electrical Code requires squirrel-cage induction motors with code letter markings _____________ to have starting protection rated at not over 250 percent of full-load current for a nontime-delay fuse.

c. The National Electrical Code requires squirrel-cage induction motors with _____________ code letter markings to have starting protection rated at not over 300 percent of full-load current for a nontime-delay fuse.

d. The National Electrical Code requires squirrel-cage induction motors with autotransformer starting and code letter markings _____________ to have starting protection rated at not over 200 percent of full-load current for a nontime-delay fuse.

13. Why are starting compensators used with large three-phase, squirrel-cage induction motors?

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

15. Explain how the direction of rotation of a three-phase, wound-rotor induction motor can be reversed.

16. 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 of 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 of the resistance inserted in the rotor circuit's ___________

17. Draw a schematic connection diagram of a wound-rotor induction motor which is started by means of an across-the-line magnetic motor starter controlled from a pushbutton station. Include a wye-connected speed controller in the rotor circuit.

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

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

20. 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._____________

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

22. 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.

23. What is an amortisseur winding?

24. Explain what is meant by the term jogging._

25. Explain what is meant by the term plugging

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

27. A three-phase, squirrel-cage induction motor is rated at 25 hp, 230 volts, 64 amperes per terminal, 40 degrees Celsius, and is classified as code letter F. In the following table, fill in the correct fuse size required for branch-circuit protection and the correct running overload protection for this motor, when used with each of the types of controllers listed.

Type of Controller

Fuse Protection: Nontime-delay

Fuse Protection: Time-delay

Running Overcurrent Protection

a. Resistance starter

 

 

 

b. Automatic autotransformer compensator

 

 

 

c. Across-the-line magnetic motor starting switch with jogging capability

 

 

 

d. Across-the-line magnetic motor starting switch with plugging capability

 

 

 

28. What size of wire and conduit are used for the branch circuit feeding the motor in question 28? (Use Type TW.)

29. 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.

30. How is dynamic braking applied to an induction motor? _____________________

31. How is dynamic braking applied to a synchronous motor?____________________

32. What is a megohmmeter?

33. Draw a schematic diagram of an across-the-line magnetic switch connected to a three-phase, squirrel-cage 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.

34. Draw a schematic diagram of a wye-delta starter, complete with pushbutton station, connected to a three-phase motor.

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

36. How is a megohmmeter used to measure the insulation resistance of the windings of an ac motor?

37. What is a growler?

38. Explain how a growler is used to locate a short-circuit condition in a motor winding.

39. For the sleeve bearings of an ac motor, explain how the old oil is removed and the bearings cleaned and lubricated. _____________

40. Place the correct answers in each of the spaces provided in the following diagram. Refer to the National Electrical Code.

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