AC MACHINES
1.The power input to a 415 V, 50 Hz, 6 pole, 3-phase induction motor running at 975 rpm is 49 kW. The stator losses are 1 kW and friction and windage losses total 2 kW. the efficiency of the motor is
a) 92.5%
b) 92%
c) 90%
d) 88%
ans. c
2. A 400 V, 15 kW, 4-pole, 50 Hz, Y-connected induction motor has a full-load slip of 4%. The output torque of the machine at full load is
a) 1.66 Nm
b) 95.5 N
c) 99.47 Nm
d) 624.73 Nm
ans. c
3. A 400 V,50 Hz, 30 hp, a 3-phase induction motor is drawing 50 A current at 0.8 power factor lagging. The stator and rotor copper losses are 1.5 kW and 900 W respectively. The friction and windage losses of 1050 W and the core losses are 1200 W. The air-gap power of the motor will be
a) 23.06 kW
b) 24.11 kW
c) 25.01 kW
d) 26.21 kW
ans. c
4. No-load test on a 3-phase induction motor was conducted at different supply voltages and a plot of input power versus voltage was drawn. This curve was extrapolated to intersect the y-axis. This intersection point yields
a) core loss
b) stator copper loss
c) stray load loss
d) friction and windage loss
ans. d
5.What is the shunt resistance component in the equivalent circuit obtained by a no-load test of an induction motor representative of?
a) windage and frictional losses only
b) core losses only
c) core, windage, and frictional losses
d) copper losses
ans. b
6. The synchronous speed for the seventh space harmonic MMF wave of a 3-phase,8-pole,50 Hz induction machine is
a) 107.14 rpm in forward direction
b) 107.14 rpm in reverse direction
c) 5250 rpm in forward direction
d) 5250 rpm in reverse direction
ans.a
7. In an induction motor, if the air-gap is increased
a) speed will reduce
b) efficiency will improve
c) power factor will be lowered
d) breakdown torque will reduce
ans. c
8. Unbalanced supply voltage is given to a 3-phase, delta-connected induction motor will cause
a) zero sequence currents
b) less heating of the motor
c) negative sequence component current
d) all the above
ans. c
9. An induction motor is fed from a balanced three-phase supply at rated voltage and frequency through a bank of three single-phase transformers connected in delta-delta. One unit of the bank develops a fault and is removed. Then
a) single phasing will occur and the machine fails to start
b) single phasing will not occur but the motor terminal voltages will become unbalanced and the machine can be loaded to the extent of 57.7% of its rating
c) the machine can be loaded to the extent of 57.7% of its rating with balanced supply at its terminals
d) the machine can be loaded to the extent of 66 2/3% of its rating with balanced supply at its terminals
ans. a
10. An induction motor having full-load torque of 60 Nm when delta connected develops a starting torque of 120 Nm. For the same supply voltage, if the motor is changed to star-connection, the starting torque developed will be
a) 40 Nm
b) 60 Nm
c) 90 Nm
d) 120 Nm
ans. a
a) 92.5%
b) 92%
c) 90%
d) 88%
ans. c
2. A 400 V, 15 kW, 4-pole, 50 Hz, Y-connected induction motor has a full-load slip of 4%. The output torque of the machine at full load is
a) 1.66 Nm
b) 95.5 N
c) 99.47 Nm
d) 624.73 Nm
ans. c
3. A 400 V,50 Hz, 30 hp, a 3-phase induction motor is drawing 50 A current at 0.8 power factor lagging. The stator and rotor copper losses are 1.5 kW and 900 W respectively. The friction and windage losses of 1050 W and the core losses are 1200 W. The air-gap power of the motor will be
a) 23.06 kW
b) 24.11 kW
c) 25.01 kW
d) 26.21 kW
ans. c
4. No-load test on a 3-phase induction motor was conducted at different supply voltages and a plot of input power versus voltage was drawn. This curve was extrapolated to intersect the y-axis. This intersection point yields
a) core loss
b) stator copper loss
c) stray load loss
d) friction and windage loss
ans. d
5.What is the shunt resistance component in the equivalent circuit obtained by a no-load test of an induction motor representative of?
a) windage and frictional losses only
b) core losses only
c) core, windage, and frictional losses
d) copper losses
ans. b
6. The synchronous speed for the seventh space harmonic MMF wave of a 3-phase,8-pole,50 Hz induction machine is
a) 107.14 rpm in forward direction
b) 107.14 rpm in reverse direction
c) 5250 rpm in forward direction
d) 5250 rpm in reverse direction
ans.a
7. In an induction motor, if the air-gap is increased
a) speed will reduce
b) efficiency will improve
c) power factor will be lowered
d) breakdown torque will reduce
ans. c
8. Unbalanced supply voltage is given to a 3-phase, delta-connected induction motor will cause
a) zero sequence currents
b) less heating of the motor
c) negative sequence component current
d) all the above
ans. c
9. An induction motor is fed from a balanced three-phase supply at rated voltage and frequency through a bank of three single-phase transformers connected in delta-delta. One unit of the bank develops a fault and is removed. Then
a) single phasing will occur and the machine fails to start
b) single phasing will not occur but the motor terminal voltages will become unbalanced and the machine can be loaded to the extent of 57.7% of its rating
c) the machine can be loaded to the extent of 57.7% of its rating with balanced supply at its terminals
d) the machine can be loaded to the extent of 66 2/3% of its rating with balanced supply at its terminals
ans. a
10. An induction motor having full-load torque of 60 Nm when delta connected develops a starting torque of 120 Nm. For the same supply voltage, if the motor is changed to star-connection, the starting torque developed will be
a) 40 Nm
b) 60 Nm
c) 90 Nm
d) 120 Nm
ans. a
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