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Objective questions and answers or MCQ of Electrical Machines page-10


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161. Which of the following methods is easily applicable to control the speed of the squirrel cage induction motor?

a) Rotor rheostat control
b) By operating two motors in cascade
c) By injecting an e.m.f. in the rotor circuit
d) By changing the number of stator poles

Ans: (d)

162. Rotor rheostat control to control the speed of the induction motor is only applicable to slip-ring induction motors.

a) True
b) False

Ans: (a)

163. The injected e.m.f. in the rotor of induction motor must have

a) The same frequency as the slip frequency
b) The same phase as the rotor e.m.f.
c) High value for the satisfactory speed control
d) Zero frequency

Ans: (a)

164. The additional stator winding is used in the compensated repulsion motor

a) To improve power factor and provide better speed regulation
b) To eliminate armature reaction
c) To prevent hunting in the motor
d) To provide mechanical balance

Ans: (a)

165. Repulsion start induction-run motor can be applicable to commercial refrigerators.

a) True
b) False

Ans: (a)

166. The rotation of the repulsion induction motor can be reversed by the usual brush shifting arrangement.

a) True
b) False

Ans: (a)

167. The power factor improvement of a.c. series motor is only possible by

a) Increasing the magnitude of inductances of field and armature winding
b) Decreasing the magnitude of reactances and armature winding
c) Equalizing the armature resistance to armature reactance
d) None of the above

Ans: (b)

168. If the number of the turns on the field winding in a.c. series motor is decreased, the speed of motor increases but torque decreases.The same torque can be achieved by

a) Increasing the number of turns on armature winding proportionately
b) Decreasing the number of turns on armature winding
c) Increasing armature resistance
d) Equalizing the armature resistance to the armature reactance

Ans: (a)

169. In conductively-compensated a.c. series motors, the compensating winding is

a) Connected in parallel with the armature
b) Short-circuited and has no interconnection with the motor circuit
c) Connected in series with the armature
d) None of the above

Ans: (c)

170. The current in inductively compensated winding of the a.c. series motor is proportional to the armature current and 180 degree out of phase.

a) True
b) False

Ans: (a)

171. The huge voltage, induced in the short-circuited coil (by transformer action) of a.c. series motor when commutation takes place is neutralized by

a) Increasing inductance of the compensating winding
b) Increasing resistance of the compensating winding
c) Decreasing the number of turns in the compensating winding
d) Shunting the winding of each commutating pole with non-inductive resistance

Ans: (d)

172. A universal motor is defined as a motor which may be operated either on d.c. or single phase a.c. supply at

a) Same output but different speeds
b) Same speed and different output
c) Approximately same output and same speed
d) Synchronous speed

Ans: (c)

173. It is preferred to use single turn coil in the armature of the single phase a.c. series motors

a) To facilitate commutation and avoid sparking at the brushes
b) To reduce the resistance of the armature
c) To reduce the resistance of the armature
d) To reduce the weight of the armature

Ans: (a)

174. Which of the following motor is unexcited single phase synchronous motors?

a) A.C. series motor
b) Universal motor
c) Repulsion motor
d) Reluctance motor

Ans: (d)

175. Which of the following motors is most suitable for signalling devices and many kinds of timers?

a) D.C. series motor
b) A.C. series motor
c) Induction motor
d) Reluctance motor

Ans: (d)


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