Objective Polyphase Induction Motors Questions and Answers pdf

71.    A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6 poles. The slip of the machine will be
(a) 0.01
(b) 0.02
(c) 0.03
(d) 0.04
Ans: d

72.    The complete circle diagram of induetion motor can be drawn with the help of data found from
(a)     noload test
(6)     blocked rotor test
(c)    stator resistance test
(d)    all of the above
Ans: d

73.    In the squirrel-cage induction motor the rotor slots are usually given slight skew
(a)     to reduce the magnetic hum and locking tendency of the rotor
(b)     to increase the tensile strength of the rotor bars
(c)    to ensure easy fabrication
(d)    none of the above
Ans: a

74.    The torque of a rotor in an induction motor under running condition is maximum
(a)    at the unit value of slip
(b)    at the zero value of slip
(c)    at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
(d)    at the value of the slip which makes the rotor reactance half of the rotor
Ans: c

75.    What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ?
(a)     The slip of the motor will be 5%
(b)     The rotor will not run
(c)    The rotor will run at very high speed
(d)    The torque produced will be very large
Ans: b

76. The circle diagram for an induction motor cannot be used to determine
(a) efficiency
(b) power factor
(c) frequency
(d) output
Ans: a

77. Blocked rotor test on induction motors is used to find out
(a)     leakage reactance
(b)     power factor on short circuit
(c)    short-circuit current under rated voltage
(d)    all of the above
Ans: d

78. Lubricant used for ball bearing is usually
(a) graphite
(b) grease
(c) mineral oil
(d) molasses
Ans: b

79. An induction motor can run at synchronous speed when
(a)     it is run on load
(b)     it is run in reverse direction
(c)    it is run on voltage higher than the rated voltage
(d)    e.m.f. is injected in the rotor circuit
Ans: d

80. Which motor is preferred for use in mines where explosive gases exist ?
(a)     Air motor
(b)     Induction motor
(c)    D.C. shunt motor
(d)    Synchronous motor
Ans: a

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