Electric Motor Maintenance and Troubleshooting

19. A two-pole motor haselectrical degrees (page 8).

20. The strength of an electromagnet is controlled by the number of amperes that flow through its coils, and the number of turns (page 9).

21. The magnetic strength of a pole with 10 turns, and 10 amps flowing through it, is the same as a pole with 20 turns, and 5 amps flowing through it (page 9). T_ F

22. A complete magnetic circuit is vital to the operation of a motor or generator (pages 9-10). T_ F

23. All motor and generator poles resemble an electromagnet (page 10).

24. Magnetic saturation is reached when the iron in a motor or generator is fully magnetized (page 11). T_ F

25. Magnetic balance requires that all poles have the same strength, and that they are equally spaced in a machine (page 12). T_ F

26. The neutral spot in all stators is located equal distance from adjacent pole centers (page 13). T_ F

27. When the armature poles align with the stator poles, the brush setting is correct (page 13). T_ F

28. Name the three basic circuits of the DC machine (pages 13-14).

29. The armature createsin a generator and in a motor (page 14).

30. The voltage output of a generator is controlled by the number of conductors in its armature circuit, the number of lines of force, and the speed at which the armature conductors cut the lines of force (page 14).

19. Draw the connection schematic of a shunt generator (page 15).

20. Amperes determine the size of wire used in an armature winding

(page 16). T_ F

21. The length of the wire in the coils of the shunt field controls the amperes (page 16). T_ F

22. What is the purpose of residual magnetism (pages 17-18)?

23. Lowering the amperes in the shunt field raises the voltage output

(page 19). T_ F

Chapter 1 The DC Machine 53

24. The series field coils are made of heavy wire because all of the ampere output goes through this circuit (page 19). T_ F

25. A series generator with no load will produce full voltage (page 19).

 

26. Draw the NEMA standard connection for a compound generator (page 20).

27. The purpose of the series field in a compound generator is to stabilize the voltage output (page 22). T_ F

28. The power output of an over-compound generator is controlled downward by a control (page 22). T_ F

29. Diverting some series field amperes will reduce the series field’s ability to increase the generator’s output (page 22). T_ F

30. The cumulative compound connection in the compound generator reduces the voltage output when the load increases (page 22). T_ F 43. The differential compound connection in the compound generator reduces the voltage output when the load increases (page 23). T_ F

44. The series field is connected the same in a motor as in a generator

(page 24). T_ F

45. An arc welder is connected differential (page 25). T_ F

46. The shunt field of a welder is separately excited (page 26). T_ F

47. Selecting a tap that adds more turns to the series field will increase the amperes (pages 26-27). T_ F

48. A reactor in the welder’s circuit bucks sudden changes and makes welding easier (page 27). T_ F

44. Interpoles are connected in the armature circuit (Al and A2) (page 27).

45. The polarity of interpoles in a generator will be the same as the field poles behind them (in the direction of rotation) (page 28). T_ F

46. The polarity of interpoles in a motor will be the same as the field poles ahead of them (in the direction of rotation) (page 28). T_ F

47. Interpoles create a canceling voltage that decreases brush arcing

(pages 29-30). T_ F

48. What happens when the interpoles are connected wrong (page 30)?

54 Electric Motor Maintenance and Troubleshooting

54. If interpoles are removed from a stator, it is vital that they (and their shims) be reassembled exactly as they were originally (page 30).

55. The compensating winding is necessary in machines that have large bores, and that have sudden load swings (page 30). T_ F

56. What controls the amperes in a motor that is running at full speed

(page 34)?

57. Why is there no ampere flow between batteries that have equal voltage (page 35)?

58. Counter-voltage controls amperes in the armature circuit of a motor

(page 36). T_ F

59. If a motor’s armature is rotated fast enough, it becomes a generator

(page 35). T_ F

60. Define base speed (page 36).

61. Below base speed is achieved by reducing the armature amperes, and applying full voltage to the shunt field (page 36). T_ F

62. Above base speed is achieved by applying full voltage to the armature, and reducing the strength of the shunt field (page 36). T_ F

63. A motor’s torque is reduced when the shunt field is weakened (page 37).

54. A motor’s horsepower remains the same when the shunt field is weakened and the RPM increase (page 37). T_

55. RPM of a shunt motor are higher with no load than it is with full load

(page 38). T_ F

56. Large DC motors will suffer commutator damage if started across the line (page 39). T_ F

57. An open shunt field will cause a motor (with no load) to stop (page 39).

58. The speed of the permanent magnet motor will increase if the magnets become weak (page 39). T_ F

59. The permanent magnet’s strength is acceptable if the motor runs 20% higher than its nameplate RPM (page 39). T_ F

60. A permanent magnet’s strength can be restored almost instantly with a re-gaussing unit (page 40). T_ F

Chapter 1 The DC Machine 55

61. A series motor’s field can have full voltage applied to it (like a shunt field) (page 41). T_ F

62. The series motor has low starting torque (page 41). T_ F

63. The series motor’s torque decreases as its RPM increases (page 41).

64. What is the series motor’s top speed (page 41)?

65. Draw the NEMA standard connection for a compound DC motor (page 42).

66. The no-load speed of the compound motor is determined by the shunt field (page 43). T_ F

67. The stabilized shunt motor is connected differential to make the motor react quickly to load changes (page 43). T_ F

68. The stabilized shunt makes a motor more efficient (page 43). T_ F

69. Starting a motor and below base speed both require limiting the current in the armature circuit (page 44). T_ F

54. If the shunt field amperes are limited when the motor starts, the amperes of the armature become excessively high, and the torque is lowered (page 44). T_ F

55. Connecting a cumulative compound connected motor differential can cause erratic speed swings when it’s loaded (pages 45-46). T_ F

56. The commutator is one side of a sliding switch (that acts as a diode)

(page 47). T_ F

57. Commutator film should be cleaned off at regular intervals to lengthen the life of the commutator (page 48). T_ F

58. Armature windings carry most, and sometimes all, of a DC machine’s amperes (page 49). T_ F

59. Equalizer shunts connect commutator segments that are located under like pole brushes (page 50). T_ F

60. Laminated iron reduces eddy current (page 50). T_ F

This page intentionally left blank