MCQs on Electrical Machines

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01․ When the rotor of a 3 phase induction motor is blocked then the slip is

When the motor is blocked i.e., rotor is at zero speed. \.

02․ In induction motor the 7th harmonic torque may make the motor rotate at (1 / 7)th synchronous speed. This is called

It is found that induction motor particularly the squirrel cage type sometimes exhibit a tendency to runs stably at a speeds as low as ( 1 / 7 )th of synchronous speed. This phenomenon is called crawling of induction motor.

03․ In induction motor especially the cage rotors slots are nearly parallel to shaft axis. This is called

In squirrel cage rotors consists of cylindrical laminated core with slots nearly parallel to shaft axis this process is called skewing.

04․ Skewing of cage rotors offers the following advantages
all of below.
less noise.
locking tendency is reduced.
uniform torque.

Skewing offers advantages - 1. More uniform torque is produced. 2. Less noise during operation. 3. It reduces or even entirely eliminates cogging.

05․ If the number of rotor slots is equal to number of stator slots, then motor refuses to start. Hence it is known as
air gap.
relative speed.

During locking tendency of rotor and stator teeth attract each other due to magnetic attraction. During this condition motor refuses to start. This is known as cogging. In case of an induction motor when the number of stator slots is equal to or an integral multiple of rotor slots staying parallel to each other strong alignment forces may be produced at the instant of starting due to variation of reluctance which may create an aligning torque stronger than the accelerating torque with consequent failure to start. This phenomenon of refusing to start sometimes observed is known as cogging.

06․ In IM the rotor lamination can be ________________ than stator laminations.
rotor can not have any laminations.

Normally the lamination thickness depends up on frequency. In IM rotor is having lesser frequency than stator frequency because of slip i.e., fr = sfs. Hence rotor lamination can be thicker than stator lamination because rotor frequency is less.

07․ In induction motor wound rotor motor is mainly is used in application where
less costly motor is not required.
low starting torque.
high starting torque.
power factor is less.

In IM the wound rotor winding are connected in star. The open end of the star is connected to slip rings. The slip rings mounted on the shaft with the brushes resting on them. The purpose of slip rings and brushes is to provide a means for connecting external resistance in the rotor circuit. The resistors enable the variation of each rotor. The purpose of using resistors at ends 1. To increase the starting torque and decease a starting current from supply. 2. To control the speed.

08․ The slip of an induction motor normally does not depend on
core loss component.
rotor speed.
synchronous speed.
shaft torque.

\ So slip is depends upon synchronous speed, rotor speed and hence depends on shaft torque also.

09․ Commonly used starters of cage motor is
star delta.
both A and B.
none of the above.

DOL or direct online starter and star delta starter is used for starting of cage induction motor.

10․ A 3 - Φ, 4 pole, 50 Hz induction motor 4 % slip, calculate frequency of the rotor emf?
3 Hz.
2 Hz.
10 Hz.
50 Hz.