Quiznetik

Electrical Machine 1 | Set 4

1. The armature core of a D.C. generator is usually made of

Correct : A. silicon steel

2. Open circuited armature coil of a D.C. machine is

Correct : C. both (a) and (b)

3. For the parallel operation of two or more D.C. compound generators, we should ensure that

Correct : D. series fields of all generators should be either on positive side or negative side of the armature

4. D.C. series generator is used

Correct : C. voltage at the toad end of the feeder

5. Following D.C. generator will be in a position to build up without any residual magnetism in the poles

Correct : D. self-excited generator

6. Interpole flux should be sufficient to

Correct : C. neutralise both the armature reaction flux as well as commutating e.m.f. induced in the coil

7. D.C. generator generally preferred for charging automobile batteries is

Correct : C. long shunt compound generator

8. In a D.C. generator the number of mechanical degrees and electrical degrees will be the same when

Correct : D. number of poles is 2

9. Permeance is the reciprocal of

Correct : B. reluctance

10. In D.C. generators the polarity of the interpoles

Correct : A. is the same as that of the main pole ahead

11. The e.m.f. generated in a D.C. generator is directly proportional to

Correct : B. speed of armature

12. In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the coil is neutralised by all of the following except

Correct : B. dummy coils

13. In D.C. generators on no-load, the air gap flux distribution in space is

Correct : D. flat topped

14. A shunt generator running at 1000 r.p.m. has generated e.m.f. as 200 V. If thespeed increases to 1200 r.p.m., the generated e.m.f. will be nearly

Correct : C. 240 V

15. The purpose of providing dummy coils in a generator is

Correct : D. to provide mechanical balance for the rotor

16. If a D.C. generator fails to build up the probable cause could not be

Correct : B. field resistance less than the critical resistance

17. Flashing the field of D.C. generator means

Correct : B. creating residual magnetism by a D.C. source

18. The e.m.f. induced in the armature of a shunt generator is 600 V. The armature resistance is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be

Correct : D. 580V

19. In a D.C. generator the critical resistance refers to the resistance of

Correct : B. field

20. To achieve sparkless commutation brushes of a D.C. generator are rockedm ahead soas to bring them

Correct : A. just ahead of magnetic neutral axis

21. Armature coil is short circuited by brushes when it lies

Correct : A. along neutral axis

22. A cumulatively compounded long shunt generator when operating as a motor would be

Correct : B. differentially compounded long shunt

23. To avoid formation of grooves in the commutator of a D.C. machine

Correct : A. the brushes of opposite polarity should track each other

24. The following constitute short-circuit in the armature winding.

Correct : D. All of the above

25. The rapid wear of brushes takes place due to

Correct : D. all of the above factors

26. Number of tappings for each equilizer ring is equal to

Correct : A. number of pole pairs

27. A D.C. generator can be considered as

Correct : C. rotating amplifier

28. In any rotating machine that part which houses the conductors and in which e.m.f. induced is to be utilised is called

Correct : D. armature

29. In a D.C. machine stray loss is the sum of

Correct : D. iron loss and mechanical loss

30. Lap winding is composed of

Correct : A. any even number of conductors

31. In a D.C. generator in case the resistance of the field winding is increased, then output voltage will

Correct : B. decrease

32. An exciter for a turbo generator is a

Correct : B. shunt generator

33. In case of a flat compounded generator

Correct : C. voltage remains constant irrespec¬tive of the load

34. Which of the following generator will have negligible terminal voltage while running on no-load ?

Correct : A. Series generator

35. Which of the following D.C. generators will be in a position to build up without any residual magnetism in the poles ?

Correct : D. None of the above

36. In over compounded generator, full load terminal voltage is

Correct : C. more than noload terminal voltage

37. In a level compounded D.C. generator, full load terminal voltage is

Correct : B. equal to no-load terminal voltage

38. The terminal voltage of a D.C. shunt generator drops on load because of all of the following reasons except

Correct : D. commut ation

39. In a D.C. generator

Correct : C. external characteristic = magnetisation characteristic - ohmic drop - armature reaction

40. A sinusoidal voltage of 5 Hz is applied to the field of a shunt generator. The armature voltage wave

Correct : B. will be of 5 Hz

41. A 220 V D.C. generator is run at full speed without any excitation. The open circuit voltage will be

Correct : B. about 2 V

42. In a separately excited generator supplying rated load the armature reaction ,

Correct : A. is always present

43. If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speedwill be

Correct : A. zero

44. Armature reaction in a generator results in

Correct : A. demagnetisation of leading pole tip and magnetisation of trailing pole tip

45. Following energized winding of a D.C. machine should not be opened as it would produce high inductive voltage which may be dangerous to personnel and may cause its own insulation failure.

Correct : D. Shunt field

46. Wave winding is composed of

Correct : C. that even number which is exact multiple of poles + 2

47. The critical resistance of the D.C. generator is the resistance of

Correct : A. field

48. When two D.C. series generators are running in parallel, an equilizer bar is used

Correct : C. so that two similar machines will pass approximately equal currents to the load

49. Which of the following generating machine will offer constant voltage on all loads ?

Correct : C. Level compounded generator .

50. Which of the following generators will be preferred if they are required to be run in parallel ?

Correct : A. Shunt generators

51. Two generators are running in parallel. One of the generators may run as motor for which of the following reasons ?

Correct : D. That generator takes large share of loads

52. A D.C. generator works on the principle of

Correct : C. Faraday's law of electromagnetic induction

53. A series generator can self-excite

Correct : B. only if the load current is not zero

54. A shunt generator can self-excite

Correct : A. only if the resistance of the field circuit is less than critical value

55. The terminal voltage of a series generator is 150 V when the load current is 5 A. If the load current is increased to 10 A, the terminal voltage will be

Correct : C. greater than 150 V

56. The open circuit voltage of a compound generator is 250 V. At full load the terminal voltage

Correct : C. may be greater or less than 250 V

57. Two D.C. shunt generators, each with armature resistance of 0.02 ohm and field resistance of 50 ohm run in parallel and supply a total current of 1000 amperes to the load circuit. If their e.m.fs. are 270 V and 265 V, their bus bar voltage will be

Correct : B. 267.5V

58. The essential condition for parallel operation of two D.C. generators is that they have '

Correct : C. the same drooping voltage charac-teristics

59. When two D.C. generators are running in parallel an equilizer bar is used

Correct : D. so that the two identical machines will pass approximately equal currents to the load

60. With a D.C. generator which of the following regulation is preferred ?

Correct : D. 1% regulation

61. Which generator would you prefer for feeding long D.C. transmission lines ?

Correct : C. Over compound generator

62. In a D.C. generator the critical resistance can be increased by

Correct : C. increasing its speed

63. The number of armature parallel paths in a two-pole D.C. generator having duplexlap winding is

Correct : B. 4

64. For both lap and wave windings, there are as many commutator bars as the numberof

Correct : C. winding elements

65. The series field of a short-shunt D.C. generator is excited by

Correct : D. load current

66. As a result of armature reaction, the reduction in the total mutual air gap flux in aD.C. generator is approximately

Correct : D. 5 percent

67. Shunt generators are most suited for stable parallel operation because of their

Correct : C. drooping voltage characteristics

68. The main factor which leads to unstable parallel operation of flat and over compounded generators is

Correct : A. their rising voltage characteristics