Objective questions and answers or MCQ of Electrical Machines page-9
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)
a) True
b) False
Ans: (a)
a) Star connected only
b) Delta connected only
c) (a) and (b) both
d) None of the above
Ans: (c)
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)
a) True
b) False
Ans: (b)
a) True
b) False
Ans: (a)
a) Improper design of the machine
b) Low voltage supply
c) High Loads
d) Harmonics developed in the motor
Ans: (d)
a) True
b) False
Ans: (b)
a) True
b) False
Ans: (b)
a) True
b) False
Ans: (a)
a) In parallel
b) In series parallel
c) In series
d) In parallel with stator
Ans: (a)
a) True
b) False
Ans: (b)
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