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

SYNCHRONOUS MACHINES

SYNCHRONOUS MACHINES

Q.The damper winding in the synchronous moor is often used to

a) prevent hunting only
b) prevent hunting & provide the starting toque
c) maintain synchronism
d) provide the starting torque only.

Ans. b

Q.Which of the following statements is true?
Short-pitched coils in the alternators are used to

a) reduce the size of the machine
b) reduce the harmonics or to eliminate the harmonics from the generated emf
c) provide accurate phase difference of 120 degrees between phases
d) reduce copper losses.

Ans. b

Q.The armature flux of an alternator at unity power factor load will be
a) demagnetizing
b) of square waveform
c) cross-magnetizing

Ans. c

Q.The armature reaction of an alternator will be completely magnetizing when load power factor is

a) unity
b) zero lagging
c) zero leading
d) 0.8 lagging

Ans. c

Q.If two alternators are running in parallel and he voltage of one machine is suddenly increased

a) the machine winding will burn
b) both machines will stop
c) synchronizing torque will be produced to restore synchronism

Ans. c

6If the input to the prime mover of an alternator ( in a system of three alternators) is kept constant, and the excitation is changed, then

(Q) the reactive component of the output is changed
(Q.) the active component of the output is changed
(Q.) the power factor of the load remains the same
(Q.) (a) and (b) occur simultaneously.

Ans. a

Q.The coupling angle or load angle of a synchronous motor is defined as the angle between the

a) rotor and the stator poles of same polarity
b) rotor and the stator poles of opposite polarity
c) rotor and stator teeth

Ans. b

DC MACHINES

DC MACHINES

Q.The speed of a d. c series motor at no load is

a) Zero
b) Infinity
c) Q00 r. p. m

Ans. b

Q.A sinusoidal voltage of frequency 1 Hz is applied to the field of a d.c generator. The armature voltage will be

a) 1 Hz square wave
b) 1 Hz sin e wave
c) d. c voltage

Ans. b

Q.The function of the commutator in a d. c machine is

a) to change ac to dc
b) to change alternating voltage to direct voltage
c) for easy control of voltage

Ans. b

Q.The direction of rotation of a dc series motor can be reversed

a) by interchanging the supply terminals
b) by interchanging the field terminals
c) either a) or b) .

Ans. b

Q.A dc shunt generator driven at normal speed in the normal direction fails to build up armature voltage because

a) the armature resistance is high
b) the field current is not sufficiently high
c) there is no residual magnetism.

Ans. c

Q.If a dc motor is connected across an ac supply , the dc motor will

a) burn as the eddy currents in the field produce heat
b) run at its normal speed
c) run at a lower speed

Ans. a

Q.What would happen if the field of dc shunt motors is opened?

a) the speed of the motor will be reduced
b) it will run at its normal speed
c) the speed will be enormously high , and the motor will destroy itself.

Ans. c

Q.Which one of the following motors is used in a locomotive?

a) Induction motor
b) dc series motor
c) Synchronous motor

Ans. b

Q.The efficiency of a dc shunt generator has a maximum value when

a) magnetic losses are equal to mechanical losses
b) armature copper loss is equal to the constant loss
c) field copper loss is equal to the constant loss.

Ans. b

Q.The dummy coil in the dc machine is used to

a) eliminate armature reaction
b) bring about mechanical balance of armature
c) eliminate reactance voltage

Ans. b

Q. The sparking at the brushes in a dc generator is attributed to

a) quick reversal of current in the coil under commutation
b) armature reaction
c) reactance voltage

Ans. . c

Q.Interpoles in the armature of a dc generator are used to

a) neutralize the reactance voltage
b) neutralize the reactance voltage and the cross-magnetizing effect of the armature reaction
c) neutralize the cross-magnetizing effect of the armature reaction

Ans. b

Q.The mechanical power develop by the dc motor is maximum when

a) back emf = applied voltage
b) back emf = 0
c) back emf = 0.Q times the applied voltage.

Ans. c

Q.In series –parallel control method when two dc series motors are connected in series , the speed of the set is
a) half of the speed of he motors when connected in parallel
b) ¼ th of the speed of the motors when connected in parallel
c) same as in parallel.

Ans. b

Q.The armature torque of the dc shunt motor is proportional to

a) armature current only
b) field flux only
c) armature current and flux both

Ans. c

Q.The reversal of speed of motors used as electric braking is known as

a) regenerative braking
b) plugging
c) dynamic braking.

Ans. b

Q.The method used for braking in rolling mills is

a) plugging
b) regenerative braking
c) dynamic braking

Ans. a