Objective questions and answers or MCQ of Electrical Machines page-7

116. The rotor slots are usually given slight skew in the squirrel case indication motor

a) To increase the tensile strength of the rotor bars and hence strength
b) To reduce the magnetic hum and locking tendency of the rotor
c) To see the copper used
d) Because of easy in fabrication

Ans: (b)

117. The starting torque of the slip ring induction motor can be increased by

a) Adding external resistance to the rotor
b) Adding the external inductance to the rotor
c) Adding both resistance and inductance to the rotor
d) Adding external capacitance to the rotor

Ans: (a)

118. What will happen if the relation speed between the rotating flux of stator and rotor of the induction motor is zero?

a) The rotor will not run
b) The rotor will run at very high speed
c) The slip of the motor will be zero
d) The torque produced will be very large

Ans: (a)

119. When the rotor starts rotation the frequency of the rotor of induction motor will depend on relative speed of the stator and the rotor.

a) true
b) false

Ans: (a)

120. A 400 KW, 3 phase, 440 volts, 50 Hz AC induction motor has a speed of 950 rpm on full load. The machine has 6 poles. The slip of machine will be

a) 0.06
b)0.10
c) 0.04
d) 0.05

Ans: (d)

121. If the rotor of the induction motor is assumed non-inductive, the torque acting on each conductor will be positive or unidirectional.

a) true
b) false

Ans: (a)

122. Which of the following statement is most appropriate if T is the starting torque developed in the rotor and V is the supply voltage to the stator.

a) T is proportional to V²
b) T is proportional to V
c) T is proportional to √V
d) T is proportional to V^1/4

Ans: (a)

123. A change of 5% in supply voltage to an induction motor will produce a change of approximately

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

Ans: (c)

124. The torque of a rotor in an induction motor under running condition is maximum

a) At the value of the slip which makes rotor reactance per phase equal to the resistance per phase
b) At the value of the slip which makes the rotor reactance half of the rotor resistance
c) At the unit value of the slip
d) At the zero value of the slip

Ans: (a)

125. By varying the rotor resistance in the slip ring induction motor the maximum torque can be achieved at any desired slip or speed of.

a) true
b) false

Ans: (a)

126. The maximum torque of an induction motor varies directly as applied voltage.

a) true
b) false

Ans: (a)

127. The shape of the torque/slip curve of induction motor is

a) hyperbola
b)parabola
c) straight line
d) rectangular parabola

Ans: (d)

128. Which of the following statement is correct when referred to induction motor?

a) The maximum torque will depend on rotor resistance
b) Although the maximum torque does not depend on rotor resistance yet the exact location of maximum torque is depend on it
c) The maximum torque will depend on standstill reactance of rotor
d) The slip of the induction motor decreases as the torque increases

Ans: (b)

129. If an induction motor has a slip of 2% at normal voltage, what will be the approximate slip when developing the same torque at 10% above normal voltage?

a) 1.6%
b) 2%
c) 1.65%
d) 1.1%

Ans: (c)

130. The slip of the induction motor can be measured by comparing the rotor and stator supply frequencies.

a) true
b) false

Ans: (a)

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Objective questions and answers or MCQ of Electrical Machines page-7

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