multiple choice questions on induction machines # 7

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objectives on induction motor 7


1) In case of an induction motor the leakage flux is more in comparison to that of a transformer. This is due to
A) revolving rotor
B) air gap between rotor and stator
C) higher flux densities in induction motor
D) none of the above

Ans:B

2) The relationship between rotor frequency f2, slip s and the stator supply frequency f1 is given by
A) f1=sf2
B) f2=sf1
C) f2=f1(1-s)
D) f2=√(SF)

Ans:B

3) In a 3-phase 50 Hz induction motor the voltage between the slip rings at standstill is 50 V. At full load the slip is 0.04. The voltage between slip rings at full load is
A) 50 V
B) 2 V
C) 20 V
D) 5 V

Ans:B

4) For an induction motor under operating condition, the emf induced/phase in rotor circuit is................its standstill rotor induced emf per phase
A) equal to
B) s times
C) (1-s) times
D) 1/s times

Ans:B

5) A 3-phase star connected induction motor is connected to 400 V, 50 Hz ac supply. If the stator to rotor turn ratio is 2, the standstill rotor induced emf per phase is
A) 115.5 V
B) 231 V
C) 346.5 V
D) 200 V

Ans:A

6) In an induction motor the slip will be negative when
A) stator magnetic field and rotor rotate in opposite directions
B) rotor rotates at a speed less than synchronous speed and in the direction of rotation of stator field
C) rotor rotates at a speed more than synchronous speed and in the direction of rotation of stator field
D) none of the above

Ans:C

7) The rotor of a three phase, 5KW , 400V, 50Hz, 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 400 V, 50 Hz supply is
A) 1500 rpm
B) 500 rpm
C) 0 rpm
D) 1000 rpm

Ans:C

8) A 6-pole, 3-phase alternator running at 1000 rpm supplies to an 8-pole, 3-phase induction motor which has a rotor current of frequency 2 Hz. The speed at which the motor operates is
A) 1000 rpm
B) 960 rpm
C) 750 rpm
D) 720 rpm

Ans:D

9) A center zero ammeter connected in the rotor circuit of a 6-pole, 50 Hz induction motor makes 30 oscillations in one minute. The rotor speed is
A) 970 rpm
B) 990 rpm
C) 1010 rpm
D) 1030 rpm

Ans:B

10) The direction of rotation of a 3-phase induction motor is clockwise when it is supplied with 3-phase sinusoidal voltage having phase sequence A-B-C. For counter clock wise 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 and C-A-B

Ans:C

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