Electrical Machines- Induction Machines

Induction machines - gate previous year questions ......

1.Skew is used in induction motors in order to reduce torque due to[GATE-1994]
a. Time Harmonics
b. Space Harmonics
c. Slot Harmonics
d. reverse rotating fields

2. An inverter capable of supplying a balanced three-phase variable voltage variable frequency output is feeding a three-phase induction motor rated for 50 Hz and 440V. The stator winding resistances of the motor are negligible small. During starting, the current inrush can be avoided without sacrificing the starting torque by suitably applying: GATE-1995]
a. low voltage at rated frequency
b. low voltage keeping the V/f ratio constant
c. Rated voltage at low frequency
d. Rated  voltage at rated frequency

3. When the supply voltage to an induction motor is reduced by 10%, the maximum torque will decrease by approximately [GATE-1996]
a. 5%   b. 10%   c. 20%   d. 40%

4. If an induction machine is run at above synchronous speed, it acts as [GATE-1997]
a. Synchronous motor
b. Induction motor
c. Induction generator
d. None of the above

5. A 3 phase squirrel cage induction motor has a full load efficiency of 0.8 and a maximum efficiency of 0.9. It is operated at a slip of 0.6 by applying a reduced voltage. The efficiency of the motor at this operating point is: [GATE-1998]
a. less than 0.4
b. Greater than 0.6
c. in the range of 0.8(+or-)0
d. None of this

6. The following starting method for an induction motor is inferior view of the poor starting torque per ampere of the line current drawn:  [GATE-1999]
a. Direct lining method
b. Auto transformer method of starting
c. Star-delta method of starting
d. series-induction method of starting

7. The power input to a 415V, 50 Hz, 6 pole, 3-phase induction motor running at 975 rpm is 40 kW. The stator losses are 1kW and friction and windage losses total 2 kW. The efficiency of the motor is
[GATE-2000]
a. 92.5%  b. 90%  c.91%  d. 88%

8. The rotor of a three phase, 5 kW, 400V, 50 Hz, slip ring induction motor is wound for 6 poles while its stator is wound for 4 poles. The approximate average no load steady state speed when this motor is connected to 400V, 50 Hz supply is [GATE-2002]
a.1500rpm     b.500rpm. c.0. d.1000rpm

9. A 3-phase Induction Motor is driving a constant torque load at rated voltage andfrequency . If both voltage and frequency are halved ,following statementsrelate to the new condition if stator resistance, leakage reactance and coreloss are ignored [GATE-2003]
1.The differencebetween synchronous speed and actual speed remain same
2.their air gap flux remains same
3. Thyristor current remains same
4. Thep.u slip remains same
Choose correct among above:

a. All
b. 1,2 & 3
c. 2,3 & 4
d. 1 & 4

10. The synchronous speed for the seventh space harmonic mmf wave of a 3-phase ,8-pole ,50 Hz induction machine [GATE-2004]
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

11. Under no load condition, if the applied voltage to an induction motor is reduced from the rated voltage to half the rated value, [GATE-2005]
a. the speed decreases and the stator current increases
b. both the speed and the stator current decrease
c. the speed and the stator current remain practically constant.
d. there is negligible change in the speed but the stator current decreases

12. A 3-phase ,4-pole ,400 V ,50 Hz ,star connected induction motor has the following circuit parameters
r1=1ohm, r′2=0.5 ohm, x1=x′2=1.2 ohm, Xm=35 ohm.The starting torque when the motor is started direct-on-line is (use approximate equivalent circuit model)
[GATE-2006]
a. 63.6 Nm          b.74.3 Nm
c. 190.8 Nm        d.222.9 Nm

13.  A three phase squirrel cage induction motor has a starting current of seven times the full load current and full load slip of 5%
If an auto transformer is used for reduced voltage starting to provide 1.5 per unit starting torque ,the auto transformer ratio(%) should be
[GATE-2007]
a. 57.77%                b. 72.56%
c. 78.25%                d. 81.33%

14. A 400 V, 50 Hz, 30 hp, 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 [GATE-2008]
a. 23.06kw        b.24.11kw
c. 25.01kw        d. 26.21kw

15. A three –phase 440V, 6 pole, 50Hz, 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 to stator magnetic field are [GATE-2011]
a. 0,50 rpm       c. 1000, -5 rpm
b. 0,950 rpm     d. 1000,950 rpm

16. The locked rotor current in a 3-phase star connected 15 kW 4-pole 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 [GATE-2012]
a. 58.5   b. 45.0    c. 45.7    d. 55.6

17. A 4-pole induction motor, supplied by a slightly unbalanced three-phase 50Hz source, is rotating at 1440 rpm. The electrical frequency in Hz of the induced negative sequence current in the rotor is [GATE-2013]
a. 100     b. 98      c. 52     d. 48