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Electrical Engineering Objective Questions { Induction Motors }



358. A 3-phase induction motor requires a stater
A. because there is no back emf induced in the stator winding during starting
B. because the motor does not possess an inherent starting torque
C. because the induced secondary voltage and hence the currents in the windings are abnormally high during starting
D. to increase the starting torque
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359.  For a rotor-fed 3-phase induction motor, the absolute speed of the rotating magnetic field in space is
A. the synchronous speed
B. the difference between the synchronous speed and the rotor speed
C. the rotor speed
D. zero
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360.  A 3-phase induction motor has a full-load slip of 3 per cent at normal voltage. Which one of the following will be the value of the slip of the motor if it develops the same torque theoretically while operating at 110 per cent of its normal voltage?
A.2.48%
B.0.25%
C.0.48%
D.4.83%
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361.  A 6-pole, 3-phase, 60 Hz induction motor runs at 1000 rpm developing maximum torque. Rotor resistance per phase is 1.2 ohms. Neglecting stator impedance, then for developing maximum torque, the external resistance to be connected in series with each rotor phase will be
A. 7.2 ohms
B. 6 ohms
C. 1.44 ohms
D. 1.2 ohms
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362. . The capacitor-start single-phase induction motors develop much larger starting torque in comparison with the "split-phase" motors, because the use of capacitors in the auxiliary winding
A. provision of larger number of turns in the auxiliary winding
B. a larger starting current to be drawn from the supply
C. the torque-slip characteristic in general to get a shape to give a large starting torque
D. the current division between the main the auxiliary windings to be conducive for the development of large starting torque
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363. The rotor of a three-phase induction machine is replaced by a rotor with a commutator type winding having no external connection between the brushes. The stator is supplied from a balanced three-phase source and the rotor is driven at twice the synchronous speed in the direction opposite to the stator mmf. The frequency of the emf induced between consecutive brushes will be (f is the supply frequency)
A. zero
B. f
C. 2f
D. 3 f
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364.  Which one of the following relations between starting torque 'Ts' and maximum torque 'Truax' is correct? ('a' is the ratio of rotor resistance to standstill rotor reactance)Ts
A.Tmax1
B.Ts
C.?1 + a
D. Tmin
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