1) While delivering an useful power of 20 kW to the full load, a 3 phase, 50 Hz, 6 pole induction motor draws a line current of 50 A. it is connected to supply of 420 V and runs at a speed of 950 rpm. If mechanical losses are 1000 W, then its rotor copper losses and shaft torque are

a. 1105.26 W, 201.03 N-m

b. 1125.26 W, 201.03 N-m

c. 1105.26 W, 208.09 N-m

d. 1125.26 W, 208.09 N-m

ANSWER: 1105.26 W, 201.03 N-m

2) A 4 pole, 25 kW, three phase, 420 V, 50 Hz induction motor operates at an efficiency of 90% with a power factor of 0.8 lagging. The current drawn by the motor from the mains is

a. 42.89 A

b. 45.56 A

c. 47.73 A

d. None of these

ANSWER: 47.73 A

3) In a three phase induction motor, the mechanical load should be such that the equivalent load resistance referred to stator is equal to the

a. Total leakage reactance of the motor referred to stator

b. Total leakage reactance of the motor referred to rotor

c. Total leakage impedance of the motor referred to stator

d. Total leakage impedance of the motor referred to rotor

ANSWER: Total leakage impedance of the motor referred to stator

4) Torque of an induction motor can sometimes be given in synchronous – watt. One synchronous – watt is equal to

a. 60 / (2 Π Ns) N – m

b. (2 Π Ns) / 60 N – m

c. 120 / (2 Π Ns) N – m

d. (2 Π Ns) / 120 N – m

ANSWER: 60 / (2 Π Ns) N – m

5) When an induction motor is working under loaded condition, its induced voltage will

a. Leads the magnetic flux by 90 degree

b. Lags the magnetic flux by 90 degree

c. Lags the magnetic flux by 30 degree

d. In phase with the magnetic flux

ANSWER: Lags the magnetic flux by 90 degree

6) The performance of induction motor is effected by the harmonics in the

a. Space variation of impressed voltage

b. Time variation of impressed voltage

c. Space variation of impressed current

d. Time variation of impressed current

ANSWER: Time variation of impressed voltage

7) The tooth or slot harmonics in an induction motor is caused due to

a. Variation of air gap reluctance

b. Leakage flux

c. Non sinusoidal nature of input voltage

d. None of these

ANSWER: Variation of air gap reluctance

8) The air gap flux of induction motor does not contain

a. 2nd harmonics

b. 3rd harmonics

c. 5th harmonics

d. 7th harmonics

ANSWER: 3rd harmonics

9) A 50 Hz, 3 phase induction motor with 4 pole stator is supplied by mains, which contains a pronounced fifth time harmonic. The speed of the fifth space harmonic will be

a. 100 rpm

b. 300 rpm

c. 600 rpm

d. 7500

ANSWER: 300 rpm

10) Crawling in an induction motor is caused

a. During starting of an induction motor

b. At the fraction of rated speed

c. Due to insulation failure

d. All of these

ANSWER: At the fraction of rated speed

11) Cogging in an induction motor is caused

a. If the number of stator slots are integral multiple of rotor slots

b. If the motor is running at fraction of its rated speed

c. Due to fifth harmonic

d. All of these

ANSWER: If the number of stator slots are integral multiple of rotor slots

12) Crawling is a phenomena mainly associated with

a. 3rd harmonic

b. 5th harmonic

c. 7th harmonic

d. 2nd harmonic

ANSWER: 7th harmonic

13) What happens if the number of stator slots in an induction motor is equal to the number of rotor slots or integral multiple of rotor slots?

a. Motor will run at very high speed

b. Motor will run at very low speed

c. Motor will fail to start

d. Does not get effected

ANSWER: Motor will fail to start

14) The crawling and cogging is not predominant in

a. Slip ring induction motor

b. Squirrel cage induction motor

c. Both (a) and (b)

d. None of these

ANSWER: Slip ring induction motor

15) The Y – axis and X – axis in a circle diagram represents

a. Voltage and current

b. Current and power

c. Power and voltage

d. Current and voltage

ANSWER: Voltage and current

16) The circle diagram can be used to

a. Predict the performance of an induction motor under various load condition

b. Find losses in an induction motor

c. Efficiency of an induction motor

d. All of these

ANSWER: All of these

17) For plotting circle diagram no load power factor is required. The value of no load power factor will be

a. Lesser than 0.5

b. Greater than 0.5

c. Equal to 0.5

d. 1

ANSWER: Lesser than 0.5

18) For conducting no load test on an induction motor two wattmeter’s are required. Readings of the meters

a. Both will be positive

b. One will be positive and other will be negative

c. Both will be negative

d. None of these

ANSWER: One will be positive and other will be negative

19) If Ic is the active component and Im is the magnetizing component of no load current (Io) obtained in the no load test of an induction motor. Then the no load branch resistance and no load branch reactance is

a. Vo / Ic , Vo / Im

b. Vo / Im , Vo / Ic

c. Vo / Io , Vo / Ic

d. Vo / Ic , Vo / Io

ANSWER: Vo / Ic , Vo / Im

20) In the blocked rotor tests, slip is equal to

a. 0

b. 1

c. 0.5

d. 0.707

ANSWER: 1

21) In a slip ring induction motor, the rotor resistance is

a. High during starting and low at normal operating speed

b. Low at starting and high at normal operating speed

c. Remains same during starting and normal operating speed

d. None of these

ANSWER: High during starting and low at normal operating speed

22) Deep bar or double cage rotors are used in squirrel cage induction motor to obtain

a. High starting current

b. High starting torque

c. Reduced starting current

d. Reduced rotor resistance

ANSWER: High starting torque

23) Deep bar or double cage rotors make use of skin effect. The skin effect depends upon

a. Nature of material

b. Diameter of wire

c. Shape of wire and frequency

d. All of these

ANSWER: All of these

24) In deep bar induction motor, the rotor consists of deep bars, short circuited by two

end rings one on each side. The leakage inductance of bottom strip

a. Greater than that of top strip

b. Lesser than that of top strip

c. Equal to that of top strip

d. None of these

ANSWER: Greater than that of top strip

25) The skin effect in a deep bar induction motor is maximum when rotor is

a. At standstill

b. Running at its maximum speed

c. Running at no load

d. Running at half load

ANSWER: At standstill

26) In double cage induction motor, the rotor consists of two cages or two layers of bars short circuited by end rings. The upper cage and lower cage respectively has

a. High resistance and low reactance, high resistance and low reactance

b. High resistance and low reactance, low resistance and high reactance

c. Low resistance and high reactance, high resistance and low reactance

d. Low resistance and high reactance, low resistance and high reactance

ANSWER: High resistance and low reactance, low resistance and high reactance

27) When the induction motor runs faster than the synchronous speed, the induction motor runs as

a. Asynchronous generator

b. Induction generator

c. Synchronous motor

d. Such condition is not possible

ANSWER: Induction generator

28) The induction generator is

a. Self exciting

b. Separately exciting

c. Both (a) and (b)

d. None of these

ANSWER: Separately exciting

29) Which of the following is not true about induction generator?

a. Synchronization for induction generator is required

b. Induction generators are more suitable for high speeds

c. With the help of excitation supply and frequency, the voltage and frequency of induction motor are controlled

d. The construction is rugged for rotating parts

ANSWER: Synchronization for induction generator is required

30) While drawing the graph of no load losses versus voltage, the voltage is extrapolated to V = 0. The extrapolated region shows

a. Copper losses

b. Core losses

c. Friction and windage losses

d. None of these

ANSWER: Friction and windage losses

31) Applications of induction generators are

a. In railways for braking purpose

b. Wind mils

c. Drilling machines

d. Only (a) and (b)

e. None of these

ANSWER: Only (a) and (b)

32) While using stator resistance starter with 3 phase induction motor, the resistances of the starter are kept at

a. Maximum

b. Minimum

c. Half of the maximum value

d. None of these

ANSWER: Maximum

33) When stator resistance starter is used, the factor by which stator voltage reduces is say x. If x<1, then due to stator resistance starter, the starting torque

a. Increases by fraction x

b. Reduces by fraction x ∧ 2

c. Reduces by fraction x

d. Increases by fraction x ∧ 2

ANSWER: Reduces by fraction x ∧ 2

34) An autotransformer starter is suitable for

a. Star connected induction motor

b. Delta connected induction motor

c. Both (a) & (b)

d. None of these

ANSWER: Both (a) & (b)

35) The cheapest starter for induction motor is

a. Stator resistance starter

b. Autotransformer starter

c. Star-delta starter

d. Rotor resistance starter

ANSWER: Star-delta starter

36) Windings of star-delta starter while starting and during running are connected in

a. Star, delta

b. Delta, delta

c. VStar, star

d. Delta, star

ANSWER: Star, delta

37) The advantages of star-delta starter over other types of starter is

a. Cheapest of all

b. Maintenance free

c. Both (a) & (b)

d. None of these

ANSWER: Both (a) & (b)

38) When rotor resistance starter is used with induction motor then

a. Only starting current is limited

b. Only starting torque is limited

c. Both starting current and starting torque are limited

d. Neither starting current nor starting torque is limited

ANSWER: Both starting current and starting torque are limited

39) Direct online starter also called D.O.L. starter is used for motors having capacity

a. Less than 5 h.p.

b. Less than 10 h.p.

c. Greater than 10 h.p.

d. For any capacity motor

ANSWER: Less than 5 h.p.

40) The NO contact and NC contact of D.O.L. starter is normally

a. Open, closed

b. Closed, open

c. Open, open

d. Closed, closed

ANSWER: Open, closed