AC MACHINES 5

AC MACHINES

61. A 500 MW 3-phase Y-connected synchronous generator has a rated voltage of 21.5 kV at 0.85 pf. The line current when operating at full load rated conditions will be
a) 13.43 kA
b) 15.79 kA
c) 23.25 kA
d) 27.36 kA
ans. b

62. A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is
a) magnetizing
b) demagnetizing
c) cross-magnetizing
d) ineffective
ans. b

63. A field excitation of 20 A in a certain alternator results in an armature current of 400A in short circuit and a terminal voltage of 200V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200 A is
a) 1 V
b) 10 V
c) 100 V
d) 1000 V
ans. d

64. A 100 kVA ,415V(line) , star-connected synchronous machine generates rated open circuit voltage of 415 V at a field current of 15 A.The short-circuit armature current at a field current of 10 A is equal to the rated armature current. The per unit saturated synchronous reactance is
a) 1.731
b) 1.5
c) 0.666
d) 0.577
ans. a

65. A single-phase ,2000 V alternator has armature resistance and reactance of 0.8 ohms and 4.94 ohms respectively. The voltage regulation of the alternator at 100 A load at 0.8 leading power-factor is
a) 7 %
b) -8.9 %
c) 14 %
d) 0 %
ans. b

66. Which one of the following is not correct with respect of synchronous machines?
a) In salient pole machines , the direct-axis synchronous reactance is greater than the quadrature axis synchronous reactance.
b) The damper bars help the motor to self-start.
c) Short-circuit ratio is the ratio of field current required to produce the rated voltage on the open circuit to the rated armature current.
d) The V-curve of a synchronous motor represents the variation in armature current with field excitation at a given output power.
ans. c

67.Higher synchronous reactance is preferred in the present day alternators because one can have
a) reduced sub transient current
b) reduced harmonic currents
c) reduced transient currents
d) higher voltage regulation with the load.
ans. c

68. A stand-alone engine-driven synchronous generator is feeding a partly inductive load. A capacitor is now connected across the load to completely nullify the inductive current. For this operating condition
a) the field current and fuel input have to be reduced
b) the field current and fuel input have to be increased
c) the field current has to be increased and fuel input left unaltered
d) the field current has to be reduced and fuel input left unaltered.
ans. d

69. A three-phase synchronous motor connected to ac mains is running at full load and unity power factor. If its shaft load is reduced by half, with field current held constant, its new power factor will be
a) unity
b) leading
c) lagging
d) dependent on machine parameters
ans. b

70. A synchronous machine connected to a power system grid bus-bar is operating as a generator. To make the machine operate as a motor, the
a) direction of rotation is to be reversed
b) phase-sequence is to be changed
c) field excitation is to be decreased.
d) mechanical input is to be less than the losses at the shaft.
ans. d