MCQs on Electrical Machines

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01․ Pole changing methods are suitable only for ___________ type induction motor.
wound.
cage.
linear.
none of these.

The methods of speed control by changing the poles are suitable for cage type induction motor because of the cage rotor automatically develops number of poles equal to poles of stator winding.
02․ In a 3 - Φ IM in order to eliminate cogging the number of stator slots must be
unequal to number of rotor slots and skewed in shape.
equal to number of rotor slots.
an integral multiple of number of rotor slots.
either (B) or (C).

In order to reduce or eliminate cogging the number of stator slot are never made equal to or an integral multiple of the number of rotor slots. Cogging can also be reduced by using skewed rotor.
03․ A slip ring Induction motor runs at 290 rpm at full load, when connected to 50 Hz supply. Determine the number of poles and slip.
P = 20, s = 3.33 %.
P = 40, s = 5.79 %.
P = 60, s = 0.96 %.
P = 50, s = 4.43 %.

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04․ _____________ types of induction motor are widely used.
Slip ring IM.
Squirrel cage IM.
Single phase IM.
Three phase IM.

Almost 90 % of induction motors are squirrel cage type IM, because in this type of motor the rotor is of squirrel cage type and this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible.
05․ The rotor of a three phase induction motor has an impedance of 0.4 + j4 ohm per phase at standstill the maximum torque occurs at a slip of _____________.
10 %.
15 %.
25 %.
50 %.

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06․ The speed of a 50 Hz three phase induction motor under full load condition is 725 rpm the number of poles of the motor are ______________.
2.
4.
6.
8.

Full load speed is 725 rpm, Ns near to this value can be taken as 75 rpm where Ns is synchronous speed. \
07․ What will be the maximum power factor of an induction motor for dispersion coefficient of 0.05?
0.05.
0.905.
1.1.
20.

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08․ Value of specific electric loading of Induction motor is in the range of
400 - 4000 ampere conductor / metre.
5000 - 45000 ampere conductor / metre.
30000 - 100000 ampere conductor / metre.
40 - 400 ampere conductor / metre.

Specific electric loading of induction motor is in the range of 5000 - 45000 ampere conductor / metre.
09․ Larger values of air flux density is taken while designing IM for
larger output.
larger bore diameter.
both.
none.

Output equation and bore diameter of IM is proportional to the air flux density.
10․ The stator frame in an induction motor is used to
provide return path for the flux.
protect the whole machine.
hold armature stamping in position.
ventilate the armature.

Under construction.
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