Saturday, October 17, 2009

INDUCTION MOTOR

INDUCTION MOTOR

Q.The speed at which the rotating magnetic field produced by stator currents rotates in an induction motor is:

a) Synchronous speed
b) Rotor speed
c) Greater than synchronous speed

Ans. a

Q.A 3-phase 50 Hz induction motor runs at speed of 940 RPM; the speed of the rotating magnetic field will be:

a) 940 RPM
b) 1000RPM
c) 1050 RPM
d) 1100 RPM

Ans. b

Q.The no-load speed of an induction motor is of the order of

a) 8%
b) 4%
c) 2%
d) 6%

Ans. c

Q.Iron losses in the rotor of a 3-phase induction motor are negligible, because

a) FreQ.uency of rotor emf is too low
b) Flux linking the rotor is of constant magnitude
c) Flux density in the rotor parts is too low

Ans: a

Q.Full-load copper losses in a 3-phase 50 Hz 4-pole induction motor running at 1455 rpm are 300 W. The rotor input is

a) 5 kW
b) 10 kW
c) 20 kW
d) 50 kW

Ans. b

Q.The no-load current of a 3-phase induction motor in terms of its full-load current is of the order of

a) 5%
b) 10%
c) 25%
d) 50%

Ans. c

Q.The power factor of a 3-phase induction motor at no load is approximately

a) 0.2
b) 0.7
c) 0.85
d) 1

Ans: a

Q.The maximum torque in a 3-phase induction motor occurs at a slip

a) R2/ X2
b) X2/ R2
c) R22/ X2
d) R22/ X22

Ans: a

Q.For high starting torque, the most 3-phase induction motor is

a) SQ.uirrel-cage type
b) Slip-ring type
c) Deep bar squirrel-cage type
d) Double-cage induction motor

Ans. b

Q.The starting torque in a single-phase induction motor is

a) High
b) Low
c) Zero
d) Very low

Ans. c

Q.The efficiency of a single-phase induction motor as compared to a 3-phase induction motor rating is

a) Lower
b) Higher
c) Equal

Ans: a

Q.The starting torque of a 3-phase induction motor can be increased by increasing

a) the rotor reactance
b) the rotor resistance
c) the stator resistance

Ans. b

Q.The maximum starting torque in the induction motor is developed when the rotor reactance is

a) half of the rotor resistance
b) twice of the rotor resistance
c) equal to the rotor resistance

Ans. c

Q.A change of 55 in the supply voltage of an induction motor will produce a change of approximately

a) 5%
b) 7%
c) 10%
d) 25%
in the rotor torque.

Ans. d

Q.The transformation ratio of an induction motor can be found by

a) blocked rotor test
b) open circuit test
c) slip test
d) stator resistance test

Ans. a

Q.The crawling in the induction motor is caused by

a) improper design of the machine
b) low supply voltage
c) high loads
d) harmonics developed in the motor.

Ans. d

Q.Which one of the following motors is most suitable for signalling devices and timers?

a) dc series motor
b) ac series motor
c) induction motor
d) reluctance motor

Ans. d

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