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Objective questions and answers or MCQ of Electrical Machines page-8


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131. When the frequency of the rotor of induction motor is small, it can be measured by

a) Galvanometer
b) D.c. moving coil millivoltmeter
c) D.c. moving coil ammeter
d) A.c. voltmeter

Ans: (b)

132. The rotor efficiency of induction motor is called

a) I.H.P.
b) F.H.P.
c) B.H.P.
d) none of the above

Ans: (c)

133. The rotor efficiency of induction motor is defined as the ratio of actual speed of rotor to synchronous speed of rotor.

a) true
c) false

Ans: (a)

134. The synchronous watt can be defined as follows :

a) This is the unit of rating of synchronous machine
b) This is the unit of torque defined at radius unit
c) This is the torque, which at the synchronous speed of the machine under consideration, would develop a power of 1 watt
d) This is the unit of the power when the power factor in the power equation omitted

Ans: (c)

135. The synchronous wattage of an induction motor equals the power transferred across the air gap to the rotor in the induction motor.

a) true
c) false

Ans: (a)

136. When it is said that an induction motor is developing a torque of 900 synchronous watts, it means that the rotor input is 900 watts and that torque is such that the power developed would be 900 watts provided the rotor was running synchronously and developing the same torque.

a) true
c) false

Ans: (a)

137. The power output of induction motor will be maximum when

a) The equivalent load resistance is equal to the standstill reactance of the motor
b) The equivalent load resistance is equal to the resistance of the rotor
c) The equivalent resistance is equal to the standstill leakage impedance of the motor
d) The slip is zero

Ans: (c)

138. The complete circle diagram of induction motor can be drawn with the help of data found from

a) no load test
b) blocked rotor test
c) (a) and (b) both
d) (a), (b) and stator resistance test

Ans: (d)

139. The transformation ratio of the induction motor cannot be defined in terms of stator and rotor currents.

a) true
b) false

Ans: (b)

140. The maximum torque of induction motor varies directly as standstill reactance of rotor.

a) true
b) false

Ans: (b)

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

a) Short circuit test only
b) Open circuit test only
c) Slip test
d) Stator resistance test

Ans: (a)

142. The power scale of circle diagram of an induction motor can be found from

a) No load test only
b) Short circuit test only
c) Stator resistance test
d) None of the above

Ans: (b)

143. The starting torque of induction motor cannot be determined from circle diagram

a) true
b) false

Ans: (a)

144. It is advisable to avoid line-starting of induction motor and use starter because

a) Starting torque is very high
b) Motor takes five to seven times its full load current
c) It will pick-up very high speed and may go out of step
d) It will run in reverse direction

Ans: (b)

145. The induced e.m.f. in the rotor of an induction motor is proportional to

a) Supply voltage only
b) Relative velocity between flux and rotor conductors
c) (a) and (b) both
d) Slip

Ans: (c)


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