Question Set 4: Induction Motor

41) A 3-phase, 50 Hz, 4 pole induction motor at a speed of 1400 rpm. The slip of the motor is

a. 0.023
b. 0.034
c. 0.066
d. 0.072

42) The rotor of an induction motor carries

a. No conductors
b. Open circuited conductors
c. Short circuited conductors
d. A solid iron core

43) A 4 pole, 50 Hz star connected induction motor is supplied with square wave voltage from an inverter. The motor rotates at a speed of 1425 rpm. The slip of the motor with respect to 5th harmonic field is

a. 1
b. 1.05
c. 1.13
d. 1.19

44) The slip of the motor in Q43 with respect to 7th harmonic field is

a. 0.75
b. 0.86
c. 0.95
d. 1

45) The average torque due to harmonic voltages is

a. quite significant and it adds to the fundamental torque
b. quite significant and it subtracts from the fundamental torque
c. quite insignificant
d. pulsating in nature

46) An unbalance is created in the rotor circuit of an induction motor which is started from rest. The motor speed will settle down to

a. a value close to half the synchronous speed
b. a value close to the synchronous speed
c. a value close to one fourth of the synchronous speed
d. a value close to zero

47) A 400 V. 50 Hz, 4 pole, 1440 rpm induction motor has following parameters:

\(R_s = 1.5 \Omega\) , \(R'_r = 0.4 \Omega\) , \(X_s = X'_r = 1.2 \Omega\) , \(Xm = 50 \Omega\)
The peak value of the motoring torque, \(T
{max}\), of the motor is
a.142.4 N.m
b. 165.3 N.m
c. 172.5 N.m
d. 192.3 N.m

48) What is the effect of unbalanced voltage on the operation of induction motor?

a. It results in reduced torque and reduced power
b. It results in reduced torque but rated power
c. It results in rated torque but reduced power
d. It results in zero torque and zero power

49) A three phase induction motor was running in steady state with a slip of s=0.04. One of the motor phases got open circuited. Assuming the speed change to be insignificant, the slip of induction motor with respect to the reverse rotating field is

a. 0.04
b. 0.96
c. 1.04
d. 1.96

50) For regenerative braking of induction motor

a. Synchronous speed should be a little higher than the rotor speed
b. Synchronous speed should be a little lower than the rotor speed
c. Synchronous speed should be doubled
d. Synchronous speed should be increased by a factor of 1.5

51) A star connected induction motor is fed from a square wave inverter. The rms values of various harmonic components of the stator current are the following:

$I_s^{1st} = 10 A $ , $ I_s^{5th} = 3 A $ , $ I_s^{7th} = 1.5 A $
Harmonics higher than 7th harmonic can be neglected. The rms value of the total stator current is
a. 10A
b. 10.31 A
c. 10.54 A
d. 14.5 A

52) Plugging of induction motor involves

a. Increase of synchronous speed
b. Decrease of synchronous speed
c. Reversal of the phase sequence of the stator
d. Increase of the stator voltage magnitude

53) A delta connected three phase induction motor is supplied from a square wave 3-phase inverter. Which of the frequency components of current will flow in the stator phases?

a. 1,2,3,4,5,..
b. 1,5,7,11,13,..
c. 1,3,5,7,9,..
d. 2,4,6,8,10...