Alternators - Multiple Choice Questions

Alternators - Multiple Choice Questions Points : Alternators MCQs, multiple choice questions and answers, electrical objective type question answers, mcqs

1. Majority of the alternators are

   (i)single phase

   (ii)Three phase

   (iii)Two phase

   (iv)none of the above

2. Armature windings of an alternator are generally

   (i)closed windings

   (ii)open windings

   (iii)partly open and partly closed windings

   (iv)none of the above

3. The salient-pole construction for field structure of an alternator is generally used for ______ machine

   (i)2-pole

   (ii)4-pole

   (iii)8-pole

   (iv)none of the above

4. The stator of an alternator is identical to that of a _____________

   (i)d.c. generator

   (ii)3-phase induction motor

   (iii)1-phase induction motor

   (iv)Rosenberg generator

5. In the above question, what is per phase a.c armature resistance?

   (i)0.602 Ω

   (ii)0.452 Ω

   (iii)0.903 Ω

   (iv)1.806 Ω

6. The stator of an alternator rarely uses ______________ slots

   (i)wide open type

   (ii)semi-closed type

   (iii)closed type

   (iv)none of the above

7. If the number of poles of an alternator is increased, the generated e.m.f

   (i)increases

   (ii)decreases

   (iii)remains the same

   (iv)none of the above

8. A two-pole alternator is running at 1500 r.p.m. Its angular velocity is

   (i)118 rad/s

   (ii)192 rad/s

   (iii)157 rad/s

   (iv)212 rad/s

9. The a.c armature winding o f an alternator operates at .................. the field winding.

   (i)the same voltage as

   (ii)much lesser voltage than

   (iii)much higher voltage than

   (iv)none of the above

10. An alternator is sometimes called generator.

    (i)synchronous

    (ii)asynchronous

    (iii)Rosenberg

    (iv)none of the above

11. An alternator supplying power to a load with a leading power factor always has __________ voltage regulation.

    (i)positive

    (ii)negative

    (iii)unity

    (iv)zero

12. A turbo—alternator uses ____________

    (i)salient-pole field structure

    (ii)non-salient-pole field structure

    (iii)rotating a.c armature winding

    (iv)none of the above

13. The non salient-pole field construction is used for alternator.

    (i)low-speed

    (ii)medium-speed

    (iii)high-speed

    (iv)none of the above

14. When the speed of an alternator is reduced by half, the generated e.m.f. will become ___________

    (i)twice

    (ii)one-fourth

    (iii)half

    (iv)four times

15. The alternators driven by __________ do not have a tendency to hunt.

    (i)diesel engines

    (ii)water turbines

    (iii)steam turbines

    (iv)none of the above

16. Low-speed alternators are driven by __________

    (i)hydraulic turbines

    (ii)steam engines

    (iii)steam turbines

    (iv)none of the above

17. If the excitation of an alternator operating in parallel with other alternators is increased above the normal value of excitation, its ___________

    (i)p.f. becomes more lagging

    (ii)p.f. becomes more leading

    (iii)output current decreases

    (iv)output kW decreases.

18. If the excitation of an alternator operating in parallel with other alternators is decreased, its ________________

    (i)p.f becomes more leading

    (ii)p.f becomes more tagging

    (iii)output kW will change

    (iv)none of the above

19. The d.c armature resistance of a Y-connected alternator measured across its two terminals is 0.5 Ω. The per phase resistance is __________

    (i)0.5 Ω

    (ii)1 Ω

    (iii)0.25 Ω

    (iv)1.5 Ω

20. A 3-phase alternator has 3-slots per pole. The distribution factor of the winding is _____________

    (i)0.866

    (ii)0.5

    (iii)1

    (iv)none of the above

21. High-speed alternators are driven by ____________

    (i)diesel engines

    (ii)hydraulic turbines

    (iii)steam turbines

    (iv)none of the above

22. The efficiency of the turbo-alternator ___________ with increase in speed.

    (i)decreases

    (ii)increases

    (iii)remains unchanged

    (iv)none of the above

23. The air-gap in an alternator is ___________ in an induction machine.

    (i)much shorter than

    (ii)much longer than

    (iii)about the same as

    (iv)none of the above

24. The stator of an alternator is wound for _________ on the rotor.

    (i)more number of poles than

    (ii)less number of poles than

    (iii)the same number of poles as

    (iv)none of the above

25. Turbo-alternators have rotors of ________

    (i)small diameter and long axial length

    (ii)large diameter and long axial length

    (iii)large diameter and small axial length

    (iv)same diameter and axial length

26. The rotor of a turbo-alternator is made cylindrical in order to reduce _______

    (i)eddy current loss

    (ii)hysteresis loss

    (iii)windage losses

    (iv)none of the above

27. The number of cycles generated in a 6-pole alternator in one revolution is ___________

    (i)3

    (ii)6

    (iii)50

    (iv)none of the above

28. The speed at which a 6-pole alternator should be driven to generate 50 cycles per second is ________

    (i)1500 r.p.m

    (ii)1000 r.p.m

    (iii)500 r.p.m

    (iv)none of the above

29. The frequency of e.m.f generated in an 8-pole alternator running at 900 r.p.m is ________

    (i)50Hz

    (ii)60Hz

    (iii)120 Hz

    (iv)none of the above

30. in case of a 4-pole machine, 1 mechanical degree corresponds to __________ electrical degrees.

    (i)2

    (ii)4

    (iii)8

    (iv)none of the above

31. For the same rating, the size of low-speed alternator is __________ that of high-speed alternator.

    (i)about the same as

    (ii)more than

    (iii)less than

    (iv)none of the above

32. The Synchronous reactance of an alternator is due to

    (i)leakage flux

    (ii)d.c field excitation

    (iii)armature reaction

    (iv)none of the above

33. The synchronous reactance of an alternator is generally _________ armature resistance.

    (i)5 times smaller than

    (ii)5 times greater than

    (iii)10 to too times greater than

    (iv)10 times smaller than

34. The Synchronous reactance of an alternator ___________ as the iron is saturated.

    (i)decreases

    (ii)increases

    (iii)remains unchanged

    (iv)none of the above

35. In an alternator, the effect of armature reaction is minimum at power factor of _____________

    (i)0.866 lagging

    (ii)0.866 leading

    (iii)0.5 lagging

    (iv)unity

36. If the lagging load p.f. of an alternator is decreased, the demagnetizing effect of armature reaction ________

    (i)remains the same

    (ii)is decreased

    (iii)is increased

    (iv)none of the above

37. A 3-phase alternator generates an open-circuit phase voltage of 4000V when exciting current is 50 A ; the short-circuit current for the same excitation being 800 A. The Synchronous reactance per phase is

    (i)80 Ω

    (ii)5 Ω

    (iii)7.5 Ω

    (iv)none of the above

38. A 30 MVA., 15 kV alternator will have a per phase nominal impedance of ____________

    (i)9 Ω

    (ii)15 Ω

    (iii)7.5 Ω

    (iv)none of the above

39. When toad on an alternator is increased, the terminal voltage increases if the load p.f. is ____________

    (i)unity

    (ii)lagging

    (iii)leading

    (iv)zero

40. The full-load efficiency of an alternator with the size of the machine.

    (i)increases

    (ii)decreases

    (iii)remains unchanged

    (iv)none of the above

41. For the same power rating, an alternator is ____________ that of a d.c generator.

    (i)larger in size than

    (ii)smaller in size than

    (iii)of the same size as

    (iv)none of the above

42. In the armature winding of an alternator, the coil span falls short of full-pitch by 60° (electrical). The pitch factor is

    (i)0.866

    (ii)0.5

    (iii)0.25

    (iv)none of the above

43. The disadvantage of a short-pitched coil is that _________________

    (i)harmonics are introduced

    (ii)waveform becomes non-sinusoidal

    (iii)voltage round the coil is reduced

    (iv)none of the above

44. The voltage regulation of an alternator for a p.f of 0.8 lagging is __________at unity

    (i)greater than

    (ii)smaller than

    (iii)the same as

    (iv)none of the above

45. The Voltage regulation of an alternator is larger than that of a d.c generator because of _____________

    (i)large armature resistance

    (ii)large leakage reactance

    (iii)complex effects of armature reaction

    (iv)none of the above

46. The per phase d.c. armature resistance of an alternator is 0.5 Ω. The effective a.c armature resistance would be about

    (i)0.5 Ω

    (ii)0.75 Ω

    (iii)4 Ω

    (iv)0.25 Ω

47. The d.c armature resistance of a delta-connected alternator measured across its two terminals is 1 Ω. The per phase d.c resistance is ________________

    (i)3 Ω

    (ii)1.5 Ω

    (iii)0.33 Ω

    (iv)none of the above

48. An under-êxcit9d alternator supplies _____________.

    (i)lagging VAR

    (ii)leading VAR

    (iii)no reactive power

    (iv)none of the above

49. The distribution of load between two alternators operating in parallel can be changed by changing _____________

    (i)phase sequence

    (ii)field excitation of alternators

    (iii)driving torques of prime movers

    (iv)none of the above

50. When a number of alternators are operating in parallel, the power factor at which each operates is determined by _____________

    (i)power factor of the load

    (ii)driving torque of the prime mover

    (iii)its field excitation

    (iv)none of the above

51. The voltage of the bus-bar to which several alternators are paralleled may be raised by simultaneously __________of all alternators.

    (i)increasing field excitation

    (ii)decreasing field excitation

    (iii)increasing input to prime movers

    (iv)decreasing input to prime movers

52. The frequency of the system with which several alternators are paralleled can be increased by simultaneously ______________of all generators.

    (i)increasing the field excitation

    (ii)decreasing the field excitation

    (iii)increasing the speed of prime movers

    (iv)decreasing the speed of prime movers

53. Damper windings are used in alternators to ____________

    (i)prevent hunting

    (ii)reduce windage losses

    (iii)achieve synchronism

    (iv)none of the above

54. The rating of an alternator is expressed in _____________

    (i)kW

    (ii)H.P.

    (iii)kVA

    (iv)kVAR

55. A 3-phase, 16-pole alternator has a star-connected winding with 144 slots and 10 conductors per slot. The flux per pole is 30 mWb sinusoidally distributed and the speed is 375 r.p.m. The frequency of generated e.m.f. is ____________

    (i)25 Hz

    (ii)50 Hz

    (iii)100 Hz

    (iv)200 Hz

56. In the above question, the line e.m.f. is ____________

    (i)1120 V

    (ii)3750 V

    (iii)1875 V

    (iv)2770 V

57. The magnitude of e.m.f. generated by an alternator depends on ____________

    (i)number of poles

    (ii)rotor speed

    (iii)flux per pole

    (iv)all of the above

58. Voltage drop in an alternator when under load is due to __________

    (i)armature resistance

    (ii)armature reactance

    (iii)armature reaction

    (iv)all of the above

59. The a.c armature winding of an alternator is ______________

    (i)always star-connected

    (ii)generally delta-connected

    (iii)star-delta connected

    (iv)none of the above

60. For producing an e.m.f of given frequency, an alternator will run at greatest speed for ____________ poles.

    (i)2

    (ii)4

    (iii)8

    (iv)12

61. A 5000 kVA, 1100 V, 50 Hz Y-connected 3-ɸ alternator has armature resistance of 0.1 Ω / phase and synchronous reactance/phase of 1.5 Ω, find the generated e.m.f per phase

    (i)769.2 V

    (ii)832.6 V

    (iii)692.4 V

    (iv)935.3 V

62. In the above question, what is the voltage regulation?

    (i)15.6 %

    (ii)29 %

    (iii)9.8 %

    (iv)21.1 %

63. The field winding of an alternator is _____________ excited.

    (i)d.c

    (ii)a.c

    (iii)both d.c and a.c

    (iv)none of the above

64. A Y-connected 3-ɸ alternator is adjusted to its rated line-to-line voltage of 230 V while under its rated load and at 80% lagging power factor. The same field excitation current results in a no-load line-to-line voltage of 328.6 V. The voltage regulation is

    (i)32.7%

    (ii)21.4%

    (iii)13.9%

    (iv)42.8%

65. A Y-connected three-phase alternator is delivering power to a three-phase line. The line-to-line voltage is 460 V. The line currents are 7.73A and the total wattage is 5.12 kW. The load power factor is

    (i)72%

    (ii)83.1%

    (iii)70.7%

    (iv)92%

66. If the line-to-line voltage in the above question rises to 618 V at no load with the same excitation, then voltage regulation of the alternator is

    (i)34.3%

    (ii)22.8%

    (iii)17.4%

    (iv)41.2%

67. A voltage regulation test is to be performed on a 3-phase Y-connected alternator. Its line-to-line voltage drop is taken with a d.c supply to find its armature circuit resistance. The readings are : d.c voltage drop = 11.15V and line current 18.5A. What is the per phase d.c resistance?

    (i)0.602 Ω

    (ii)0.301 Ω

    (iii)0.451 Ω

    (iv)1.22 Ω

68. The losses in an alternator are the same as in

    (i)a d.c machine

    (ii)a transformer

    (iii)a rectifier

    (iv)none of the above

69. For a given load, the efficiency of an alternator is maximum at

    (i)0.8 p.f

    (ii)0.707 p.f

    (iii)1 p.f

    (iv)none of the above

70. Majority of alternators in the use have _____________

    (i)revolving a.c. armature winding

    (ii)stationary field type construction

    (iii)revolving field type construction

    (iv)none of the above.

71. Iron and friction losses of an alternator can be measured by a

    (i)transformer

    (ii)d.c. motor

    (iii)rectifier

    (iv)none of the above

72. Copper losses of an alternator can be measured by a

    (i)rectifier

    (ii)transformer

    (iii)d.c. motor

    (iv)none of the above

73. The voltage regulation of an alternator depends on

    (i)load current only

    (ii)p.f. only

    (iii)both load current and p.f

    (iv)none of the above