Motors and Transformers Final Exam

10. When dealing with frame sizes, a general rule of thumb is that the centerline shaft height above the bottom of the base is the first two digits of the frame number divided by 3. Select one: True False

False

11. The rotor of a squirrel-cage motor is easy to test in the field. True False

False

16. Starter sizes are rated by motor type and horsepower only. True False

False

17. All three basic types of three-phase motors operate on the same principle, and they all use the same basic design for the rotor windings. True False

False

20. The revolving armature type alternator is the more used of the True False

False

29. An example of an intermittent duty motor is the starter motor on an automobile. These motors develop a small amount of horsepower in a small case size. True False

False

33. If a three-phase, wye-connected, dual-voltage motor (240/480) is connected to a 240-V line, the motor windings are connected in series. True False

False

49. One advantage of interpoles is that they restore the field outside their immediate area and are able to overcome all the field distortion. True False

False

1. If a motor nameplate lists code letters that range from A to V, they indicate the type of rotor bars used when the rotor was made. True False

True

12. The amount of counter-EMF produced in the armature is proportional to three factors: the number of turns of wire in the armature, the strength of the magnetic field of the pole pieces, and the speed of the armature. True False

True

18. Although a capacitor-start induction-run motor has a high starting torque, the motor should not be started more than eight times per hour. True False

True

19. As a general rule, the temperature rating of most devices will not exceed 75 degrees C. True False

True

26. In a split-phase motor, synchronous speed is determined by the same two factors that determined the synchronous speed for a three-phase motor: number of stator poles per phase and frequency of the applied voltage. True False

True

27. Generally, when determining the overload size for a motor, the nameplate current rating of the motor is used instead of the current values listed in the NEC tables. True False

True

3. Most industrial motors are 3-phase, which means that the power connected to them is three separate lines with the voltages 120 degrees out-of-phase with each other. True False

True

31. In a capacitor-start capacitor-run motor, a capacitor is connected in series with the winding to provide a continuous leading current in the start winding. True False

True

34. To increase the amount of output voltage, it is common practice to increase the number of turns of wire for each loop. If a loop contains 20 turns of wire, the induced voltage will be 20 times greater than that for a single-loop conductor. True False

True

37. Compound motors can be connected as short shunt or long shunt just as compound generators can. They can also be cumulative-compounded motors or differential-compounded motors. True False

True

43. A shaded-pole induction motor is popular because of its simplicity and long life. True False

True

50. The output frequency of an alternator is determined by two factors, the number of stator poles and the speed of rotation of the rotor. True False

True

6. The locked rotor code letter is determined by the construction of the squirrel cage rotor. It can be used to determine the approximate amount of inrush current when the motor is started. True False

True

7. The amount of output voltage produced by a generator is proportional to three factors, including the number of turns of wire in the armature, the strength of the magnetic field of the pole pieces, and the speed of the cutting action (speed of rotation). True False

True

8. Motors have a rated horsepower that is determined by the amount of torque they can produce at a certain speed. True False

True

28. The thermal capacity of different insulations is rated as ____. The letters indicate the amount of temperature each is designed to handle over a 20,000 hour period operating at full load. a. A, B, F, and H b. F, H, J and P c. A, B, C, and D d. NEMA Design Code B

a. A, B, F, and H

40. ____ is characterized by the fact that the output voltage at full load will be greater than the output voltage at no load. a. Over-compounding b. Flat compounding c. Under-compounding d. Alternate compounding

a. Over-compounding

42. The rotor of a synchronous motor contains a set of squirrel-cage bars similar to those found in a type A rotor. This set of squirrel-cage bars is used to start the motor and is known as the ____ winding. Select one: a. amortisseur b. asynchronous c. field resistor d. electromagnetic

a. amortisseur

38. A(n) ____ compound generator has its fields connected in such a manner that they oppose each other in the production of magnetism. Select one: a. differential b. oppositional c. cumulative d. diametric

a. differential

21. A device that can be used to test an armature is called a ____. a. growler b. de-laminator c. husker d. coiler

a. growler

48. A motor with a NEMA Frame of 404T has a shaft height (centerline shaft height) of ____ inches: a. 8 b. 10 c. 40.4 d. 6.25

b. 10

36. What is the output frequency of an alternator that contains eight poles per phase and is turning at a speed of 900 RPM? Select one: a. 0.72 Hz b. 60 Hz c. 20,000 Hz d. 50 Hz

b. 60 Hz

44. Most 3 phase dual voltage motors contain ____ leads in the terminal connection box. a. 7 b. 9 c. 12 d. 4

b. 9

9. To determine conductor sizing for a motor, refer to___ a. Locked Rotor Indicating Code Letters b. NEC Tables 430.247 - 430.250 c. nameplate Full-Load Amps (FLA) d. NEC Table 430.52

b. NEC Tables 430.247 - 430.250

14. What is the synchronous speed of a eight-pole motor connected to 60 Hz? a. S = 1500 RPM b. S = 900 RPM c. S = 1.67 RPM d. S = 9.6 RPM

b. S = 900 RPM

35. ____ motors are often referred to as constant speed motors. a. Compound b. Shunt c. Series d. Current

b. Shunt

41. To test a squirrel-cage rotor, it is necessary to access the ____ concealed inside the rotor laminations. Select one: a. fuses b. rotor bars c. slip rings d. copper coils

b. rotor bars

32. A 50-hp three phase squirrel-cage induction motor is connected to a 480 volt line. These are the only current-carrying conductors in a run of conduit to the motor. The motor has a NEMA code of B on the nameplate. The nameplate lists the full-load-current as 44 amps. The termination temperature rating of the devices is not known. Copper conductors with THHN insulation are to be used for this motor connection. What size conductor should be used? Select one: a. #6 b. #3/0 c. #4 d. #2

c. #4

47. The following formula can be used to determine the speed of the rotating magnetic field when the frequency and number of stator poles per phase are known: S = ____. a. (120 - f) / P b. 120 x f x P c. (120 x f) / P d. 120 x (f / P)

c. (120 x f) / P

22. Many split-phase motors are designed for operation on ____. a. 120 V only b. Direct Current (DC) c. 120 or 240 V d. 277 or 480 V

c. 120 or 240 V

2. According to NEC 110.14(C), a conductor is to be selected and coordinated as to not exceed the ____ temperature rating of any connected termination, any connected conductor, or any connected device. a. stated b. highest c. lowest d. average

c. lowest

13. The direction of rotation for a split-phase motor can be changed by: a. reversing the connection of the run winding. b. reversing the connection of the start winding. c. reversing the connection of either the start winding or the run winding, but not both. Correct d. reversing the connection of both the start winding and the run winding.

c. reversing the connection of either the start winding or the run winding, but not both.

23. There are three basic types of direct current generators: the series, ____, and compound. a. simple b. frogleg c. shunt d. open

c. shunt

30. Three factors determine the amount of output voltage of an alternator: length of the armature or stator conductors (number of turns), ____ of the magnetic field of the rotor, and speed of rotation of the rotor. Select one: a. physical size b. depth c. strength d. direction

c. strength

25. Selecting motors with a service factor greater than ____ allows for cooler winding temperatures at rated load, protects against intermittent heat rises, and helps to offset low or unbalanced line voltages. a. 9 b. 100 c. 10 d. 1

d. 1

24. The rotor of a wound rotor motor is constructed by winding three separate coils on the rotor ___ degrees apart. Select one: a. 45 b. 180 c. 90 d. 120

d. 120

15. Most large compound direct current motors have a protective device connected in series with the shunt field called the ____ relay. a. shunt b. armature c. shunt loss d. field loss

d. field loss

46. Periodic maintenance should also include testing the insulation of the ____ and the armature with a megohmmeter. a. coils b. brushes c. commutator bars d. field windings

d. field windings

45. Since the number of stator poles is constant for a particular machine, the ____ frequency is controlled by adjusting the speed of the rotor. a. index b. operational c. input d. output

d. output

5. Two factors that determine the amount of torque produced by a direct current motor are the strength of the magnetic field of the pole pieces and the ____. a. direction of the turning force b. strength of the magnetic lines of flux c. direction of the magnetic field d. strength of the magnetic field of the armature

d. strength of the magnetic field of the armature

4. Most of the AC motors in industry are ____-phase. a. differential b. four c. two d. three

d. three