MCQs on Electrical Machines

Page 33 of 114. Go to page
01․ What is the speed of stator rotating magnetic field with respect to rotor rotating magnetic field?
Slip speed
Synchronous speed
None of the above

The stator rotating magnetic field is rotating at Ns. The current carrying rotor conductor also produces rotor magnetic field of speed Ns. Therefore, stator rotating magnetic field and rotor rotating magnetic field are rotating at a same speed Ns in the air gap. They are stationary with respect to each other and make the rotor to rotate at a speed N. This happens at any slip.

02․ An induction motor when started on no load does not accelerate up to full speed but runs at 1/7th of the rated speed. The motor is said to be

Induction motor runs nearer to synchronous speed but this is a peculiar behaviour when the motor runs at a speed near to Ns / 7 falling below the rated torque and unable to accelerate to its full speed. This is called crawling. Crawling is due the presence of slot harmonics mainly due to the dominant presence of 7th harmonic.

03․ The rotor power output of 3-phase induction motor is 15 KW. The rotor copper losses at a slip of 4% will be
700 W
650 W
625 W
600 W

04․ An induction motor having 8 poles runs at 727.5 rpm. If the supply frequency is 50Hz, the emf in the rotor will have a frequency of

05․ The crawling in the induction motor is caused by
improper design of stator laminations
low voltage supply
high loads
harmonics developed in the motor

Induction motor running stably at very low speed (1 / 7th of normal speed) is known as crawling due to asynchronous torque generated due to slot harmonics mainly due to dominant presence of 7th harmonics.

06․ Which one of the following statement is correct ? In an induction motor if the air gap is increased then
its speed will reduce
its speed will increase
its power factor will reduce
its efficiency will improve

If the air gap is increased, the stator will draw more magnetizing current from the supply to maintain the flux in the air gap to the rotor. This will reduce the power factor.

07․ The supply voltage to an induction motor is reduced by 10%. By what percentage, approximately, will the maximum torque decrease?

08․ When applied rated voltage per phase is reduce to one half, the starting torque of three phase squirrel cage induction motor becomes
1/2 of the initial value
1/4 of the intial value
twice the initial value
4 time the initial value

09․ In an induction motor, what is the ratio of rotor input to rotor copper loss?

Rotor copper loss = S × Rotor input power ∴ Rotor input / Rotor copper loss = 1/S.

10․ In an induction motor, air gap power is 10 Kw and mechanically developed power is 8 Kw. What are the rotor ohmic losses?

Rotor ohmic loss = air gap power or rotor input power - mechanical developed power by rotor (∵ Rotor core losses are neglected due to low rotor frequency) Rotor ohmic loss = 10-8 ⇒ 2 kW.