MCQs on Electrical Machines

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01․ Slip exists in
induction motor.
synchronous motor.
in both machines.
none of above.

Because in induction motor relative speed between stator magnetic field and rotor is exists i.e., slip will exists but not in synchronous motor.

02․ Example of double excited machine is
DC Shunt Motor
Synchronous motor.
Both are correct.
None of the above.

The synchronous motor is double excited machine i.e., giving AC at stator side and DC at rotor side.

03․ The principal of starting of 3 - Φ IM is similar to
capacitor run IM.
synchronous motor.
repulsion motor type starting.
transformer with short-circuited secondary.

At standstill condition when supply is given to the stator winding of 3 phase IM, then 3 phase rotating flux of the stator induces emf in the rotor winding as in the case of a transformer with short circuited secondary.

04․ 3rd harmonics are eliminated in induction motor by using
V connection.
delta connection.
star connection.
zig - zag connection.

Because in star connection phases or winding are 120° apart so that are they eliminated but it is not possible in delta connection.

05․ If the air gap of IM is increased, ____________ will increase.
air gap flux.
magnetizing current.
power factor.

When a constant voltage is applied to the stator of the IM, the mutual flux remains constant. Now reluctance, offered to the mutual flux, increases with air gap and therefore the same mutual flux [ = (mmf / reluctance)] will require more magnetizing mmf and hence an increased amount of magnetizing current will be required.

06․ An IM cannot run at synchronous speed because
rotor torque will not be produced.
air friction will prevent it.
induction motor will become synchronous motor.
none of above.

If rotor runs at synchronous speed then there will be no flux cutting between stator and rotor, and hence no emf will be produced and as a result there will be no current flowing through the rotor, and consequently rotor torque will be zero.

07․ How can the synchronous speed of a 3 - Φ IM be varied?
By varying the supply frequency.
By varying number of poles.
Both (A) and (B).
By varying supply voltage.

\ So Ns can be varied by varying both frequency and number of poles.

08․ In a 3 - φ induction motor running at full load which of these parameters is stationary with respect to the stator mmf wave?
Stator winding.
Rotor winding.
Both rotor and stator winding.
Rotor mmf wave.

Stator MMF wave rotates at synchronous speed whereas stator is fixed. Rotor winding rotates at a speed less than synchronous speed and depends on the slip. Both rotor and stator MMF waves rotate in the air gap in the same direction at synchronous speed when balanced 3 - φ supply is given.

09․ In a 3 - Φ IM with line frequency f1 and winding factor kw1 and kw2 for stator and rotor respectively the ratio of per phase stator winding emf E1 to per phase rotor winding emf E2 at standstill is given by
E1 / E2 = N1kw1 / N2kw2.
E1 / E2 = 1.
E1 / E2 = f1kw1 / f2kw2.
E1 / E2 = N1 / N2.


10․ A 3 - Φ, 6 pole, 50 Hz IM has a rotor speed of 987 rpm find rotor frequency?
60 Hz.
0.65 Hz.
0.75 Hz.
0.70 Hz.