MCQs on Electrical Machines

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01․ For larger units high torque it is preferred to use
DC shunt motor.
Slip induction motor.
Universal motor.
Partial motor.

For larger units high torque squirrel cage induction motor, slip ring induction motor are preferred.

02․ Belt conveyors offer _______________ starting torque.

Belt conveyors require high starting torque. Hence for belt conveyors slip ring IM will employ.

03․ The application that need frequent starting and stopping is
paper mills.
lifts and hoists.
jaw crushers.

Lifts and hoists are requires frequent starting and stopping because load is not constant.

04․ Fifth harmonic component of the induced voltage in three phase AC generator can be eliminated by using a winding pitch of
2 / 3.
4 / 5.
5 / 6.
6 / 7.

Coil span factor nth harmonic, Kc = cos( 0.5 X n X θ). Where θ is an angle by which the coil is short of the full pitch. \ Those fifth harmonic component gets eliminated by using a coil pitch of ( 4 / 5 ).

05․ In escalator which motor is used?
1 - φ IM.
DC shunt motor.
3 - φ IM.
Universal motor.

In escalator 3 - φ IM is used because it has higher starting torque.

06․ The speed of rotation of 5th harmonic component in the stator of an induction motor is
0.5 times the synchronous speed.
(1 / 5) times the synchronous speed.
(1 / 15) times the synchronous speed.
5 times the synchronous speed.

The speed of rotation of 5th harmonic component in stator of an induction motor is (1 / 5) times of the speed of synchronous motor.

07․ In an induction motor, the no-load slip is generally equal to
1.5 %.
4 %.
less than 1 %.

For no-load the reactance becomes almost negligible and the rotor impedance now depends on only resistance. Further, if the rotor resistance is small the rotor current is large, so that motor works with a large torque (since torque ∝ current) which brings the speed near to synchronous speed. The slip is thus reduced to 1 % or less.

08․ Star - delta method of starting of motor is not possible in case of
high speed motor.
low horse power motor.
single phase induction motor.
high torque motor.

Star - delta winding is three phase winding, and is only applicable in case of 3 - φ motors. ∴ A single phase induction motor can not be started by this method.

09․ If s is the slip and f is the supply frequency of an induction motor, the frequency of rotor current is given by
sf / 2.
f / s.


10․ In case of IM the torque is
inversely proportional to √slip.
directly proportional to slip.
directly proportional to slip2.
inversely proportional to slip.