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Objective questions and answers or MCQ of Electrical Machines page-9


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146. The relation between maximum torque and full load torque (Tmax, Tf respectively) when referred to induction motor is given by

a) Tf/Tmax =2a/(a2 + s2)
b) Tf/Tmax =2/(a2 + s2)
c) Tf/Tmax =2as/(a2 + s2)
d) Tf/Tmax =a/(a2 + s2)

Ans: (c)

Where s is the slip and a = R2/X2 = (Resistance of the rotor)/(Resistance of the rotor)

147.In the above question the relation between standstill torque (Ts) and maximum torque (Tmax) is given by

a) Ts/Tmax = 2a/(1 + a2 )
b) Ts/Tmax = 2a/(S2 + a2 )
c) Ts/Tmax = 2/(1 + s2 )
d) Ts/Tmax = 2a/(S2 + 1)

Ans: (a)

148. The slip of the induction motor can be calculated if the rotor copper losses and rotor input are known.

a) True
b) False

Ans: (a)

149. The auto-starters (using three auto-transformers) can be used to start cage induction motor of the following type :

a) Star connected only
b) Delta connected only
c) (a) and (b) both
d) None of the above

Ans: (c)

150 The torque developed in the cage induction motor with auto-starter is

a) K × torque with direct switching
b) K/ torque with direct switching
c) K2 × torque with direct switching
d) K2/torque with direct switching

Ans: (c)

Where tapping of transformation ratio K is used in case of auto-transformer.

151. Percentage tapping required of an auto-transformer for a cage motor to start the motor against 1/4 of full load will be

a) 70%
b) 71%
c) 71.5%
d) 72.2%

Ans: (d)

When the short circuit current on normal voltage is 4 times the full load and full load slip is 30%.

152. The start-delta switch is equivalent to auto-transformer of ratio (when applied to delta connected cage induction motor)

a) 57%
b) 56.5%
c) 86.6%
d) 58% approximately

Ans: (d)

153. The rotor current can be reduced by introducing star connected resistance starter in the rotor circuit of the cage motor and slip ring motor as well.

a) True
b) False

Ans: (b)

154. The slip ring motor can be started under load conditions.

a) True
b) False

Ans: (a)

155. The crawling in the induction motor is caused by

a) Improper design of the machine
b) Low voltage supply
c) High Loads
d) Harmonics developed in the motor

Ans: (d)

156. Cogging in the induction motor usually occurs at high voltage.

a) True
b) False

Ans: (b)

157. The outer cage in the double squirrel cage motor has low resistance copper bars.

a) True
b) False

Ans: (b)

158. The torque/speed characteristics of double squirrel cage induction motor may be taken to be sum of two motors, one having a high resistance rotor and other a low resistance one.

a) True
b) False

Ans: (a)

159. When the equivalent circuit diagram of double squirrel cage induction motor is constructed the two rotor cages can be considered

a) In parallel
b) In series parallel
c) In series
d) In parallel with stator

Ans: (a)

160. The speed of the cage induction motor cannot be controlled from the rotor side.

a) True
b) False

Ans: (b)


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