Objective Questions on Alternators

46. The windings for an alternator are

I. 36 slots, four poles, span 1 to 8
II. 72 slots, six poles, span 1 to 10
III. 96 slots, six poles, span 1 to 12.

The windings having pitch factors of more than 0.9 are

(A) I and II only
(B) II and III only
(C) I and II only
(D) I, II and III.

Questions 47 to 48 refer to the data given below:

A 500 kVA ,2300 volt three phase star connected alternator has a full load armature-resistance drop per phase of 50 volts and a combined armature reactance plus armature-reaction drop of 500 volts per phase

47. The percent regulation of the alternator at unity power factor is

(A) 1.05
(B) 10.5
(C) 21.5
(D) 27.5.

48. The percent regulation of the alternator at 0.866 power factor lagging is

(A) 26.3
(B) 20.1
(C) 16.6
(D) 10.5.

49. The percent regulation of the alternator at 0.8 power factor leading is

(A) 13.2
(B) 26.4
(C) – 26.4
(D) – 13.2.

50. The imaginary or fictitious part of synchronous reactance takes care of

(A) armature reaction
(B) voltage regulation
(C) inductive reactance
(D) none of the above.

51. In an alternator, the use of short pitch coils of 160° will indicate the absence of

(A) third harmonic
(B) fifth harmonic
(C) seventh harmonic
(D) ninth harmonic.

52. When a generator designed for operation at 60 Hz is operated at 50 Hz

(A) operating voltage must be derated to (50/60) of its original value
(B) operating voltage must be derated to (50/60)2 of its original value
(C) kVA rating can be upgraded to (60/50) of the rated value
(D) the generator will not take any load.

53. Overheating of generator’s winding

(A) reduces generated voltage
(B) reduces power factor
(C) reduces life of the machine
(D) does not have any significant effect.

54. Rotor shaft of 500 MW alternator is supported in

(A) ball bearings
(B) roller bearings
(C) needle bearings
(D) journal bearings.

55. The voltage of field system for an alternator is usually

(A) less than 200 V
(B) between 200 V and 440 V
(C) 400 V
(D) more than 1 kV.

56. When the number of slots / pole / phase is fractional, then this winding is called

(A) short pitch winding.
(B) full pitch winding.
(C) fractional slot winding.
(D) integral slot winding.

57. The electrical angle between the field axis and axis of armature reaction of a loaded synchronous generator with armature current lagging behind the excitation emf by ψ is

(A) ψ – 90
(B) ψ + 90
(C) 90 – ψ
(D) ψ + 180.

58. Two synchronous generators G1and G2 are equally sharing the KVAR of the load while operating in parallel. Keeping the terminal voltage fixed in order to shift part of the KVAR load from G2 to G1

(A) The field current of G1 is lowered
(B) The field current of G2 is raised
(C) The field current of G1 is raised and of G2 lowered
(D) The field current of G1 is lowered and of G2 is raised.

59. A synchronous generator is operating with excitation adjusted for unity power factor current at constant load. When on increasing the excitation the power factor

(A) will lag
(B) will lead
(C) will become zero
(D) none of the above.