Test2

Question 2: The locked rotor current in a 3-phase, star connected 15 kW, 4 poles, 230 V, 50 Hz induction motor at rated conditions is 50 A. Neglecting losses and magnetizing current, the approximate locked rotor line current drawn when the motor is connected to a 236 V, 57 Hz supply is
a) 58.5 A.
b) 45.0 A.
c) 42.7 A.
d) 55.6 A.

Question 3: A 400 V, 50 Hz, 22.1 kW, the three-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 are 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.

Question 4: A three-phase 440 V, 6 poles, 50 Hz, the squirrel-cage induction motor is running at a slip of 5%. The speed of stator magnetic field to rotor magnetic field and speed of rotor with respect of stator magnetic field are:
a) zero, -5 rpm.
b) zero, 955 rpm.
c) 1000 rpm, -5 rpm.
d) 1000 rpm, 955 rpm.

Question 5: A 400 V, 50 Hz, 4-pole, 1400 rpm, star-connected squirrel-cage induction motor has the following parameters referred to the stator:
R’r = 1.0 Ω ,Xs = X’r = 1.5 Ω. Neglect stator resistance and core and rotational losses of the motor. The motor is controlled from a 3-phase voltage source inverter with constant V/f control. The stator line-to-line voltage(RMS) and frequency to obtain the maximum torque at starting will be :
a) 20.6 V, 2.7 Hz.
b) 133.3 V, 16.7 Hz.
c) 266.6 V, 33.3 Hz.
d) 323.3 V, 40.3 Hz.

Question 6: The speed of a 4-pole induction motor is controlled by varying the supply frequency while maintaining the ratio of the supply voltage to supply frequency (V/f) constant. At a rated frequency of 50 Hz and rated voltage of 400 V, its speed is 1440 rpm. Find the speed at 30 Hz, if the load torque is constant.
a) 882 rpm.
b) 864 rpm.
c) 840 rpm.
d) 828 rpm.

Question 7: A 3-phase, 10 kW, 400 V, 4-pole, 50Hz, star connected induction motor draws 20 A on full load. Its no load and blocked rotor test data are given below.
No Load Test : 400 V – 6 A – 1002 W
Blocked Rotor Test: 90 V – 15 A 762 W
Neglecting copper loss in no-load test and core loss in blocked rotor test, estimate motor’s full load efficiency
a) 76%.
b) 81%.
c) 82.4%.
d) 85%.

Question 8: Under no-load condition, if the applied voltage to an induction motor is reduced from the rated voltage to half the rated value,
a) the speed decreases and the stator current increases.
b) both the speed and the stator current decreases.
c) the speed and the stator current remains practically constant.
d) there is negligible change in the speed but the stator current decreases.

Question 9: The direction of rotation of a 3-phase induction motor is clockwise when it is supplied with a 3-phase sinusoidal voltage having phase sequence A-B-C. For counterclockwise rotation of the motor, the phase sequence of the power supply should be:
a) B-C-A.
b) C-A-B.
c) A-C-B.
d) B-C-A or C-A-B.

Question 10: A 3-phase induction motor is driving a constant torque load at rated voltage and frequency. If both voltage and frequency are halved, following statements relate to the new condition if stator resistance, leakage reactance, and core loss are ignored:
1. The difference between synchronous speed and actual speed remains same
2. The air gap flux remains same
3. The stator current remains same
4. The p.u. slip remains same
Among the above, correct statements are

a) All.
b) 1, 2 and 3.
c) 2, 3 and 4.
d) 1 and 4.