100 TOP Polyphase Induction Motors Multiple choice Questions and Answers pdf

81. The torque developed by a 3-phase induction motor least depends on
(a)     rotor current
(b)     rotor power factor
(c)    rotor e.m.f.
(d)    shaft diameter
Ans: d

82. In an induction motor if air-gap is increased
(a) the power factor will be low
(b) windage losses will be more
(c) bearing friction will reduce
(d) copper loss will reduce In an induction motor
Ans: a

83. In induction motor, percentage slip depends on
(a)     supply frequency
(b)     supply voltage
(c)    copper losses in motor
(d)    none of the above
Ans: c

84. When /?2 is tne rotor resistance, .X2 the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running condi-tions will be
(a) sR2X2 = 1    (b) sR2 = X2
(c) R2 = sX2    id) R2 = s2X2
Ans: c


85.    In case of a double cage induction motor, the inner cage has
(a)    high inductance arid low resistance
(b)    low inductance and high resistance
(c)    low inductance and low resistance
(d)    high inductance and high resis¬tance
Ans: a

86.    The low power factor of induction motor is due to
(a)     rotor leakage reactance
(b)     stator reactance
(c)    the reactive lagging magnetizing current necessary to generate the magnetic flux
(d)    all of the above
Ans: d

87.    Insertion of reactance in the rotor circuit
(a)     reduces starting torque as well as maximum torque
(b)     increases starting torque as well as maximum torque
(c)    increases starting torque but maxi-mum torque remains unchanged
(d)    increases starting torque but maxi-mum torque decreases
Ans: a

88.    Insertion of resistance in the rotcir of an induction motor to develop a given torque
(a)     decreases the rotor current
(b)     increases the rotor current
(c)    rotor current becomes zero
(d)    rotor current rernains same
Ans: d

89.    For driving high inertia loods best type of induction motor suggested is
(a) slip ring type
(b) squirrel cage type
(c) any of the above
(d) none of the above
Ans: a

90.    Temperature of the stator winding of a three phase induction motor is obtained by
(a)     resistance rise method
(b)     thermometer method
(c)    embedded temperature method
(d)    all above methods
Ans: d

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