Interview Questions (Electrical Machines)

Why the performance of single-phase induction motor is poorer as compared to a three-phase induction motor? 
In a three-phase induction motor, the maximum torque is independent of the rotor resistance, while the slip at which it occurs increases with the rotor resistance. No such neat result is possible in a single phase induction motor as the backward rotating field thereby reducing the forward torque. As a result, the maximum torque in a single phase induction motor reduces as the motor resistance increases.

What is the main function of a starter in a three-phase induction motor?
The main function of the starter is to limit the high starting current to a reasonable value and to provide overload and no-voltage protection.

Why squirrel cage induction motor is preferred to the slip ring induction motor?
The rotor circuit of a squirrel cage induction motor cannot be tampered with and the machine has a low starting torque, while it has excellent running performance.

Which method gives a more accurate result for the determination of voltage regulation of an alternator?
Synchronous impedance method i.e. emf method is pessimistic method i.e. VR by this method is much higher than the accurate value given by zpf method or Potier triangle method. The VR by mmf method is less than the accurate value; therefore this method is termed as an optimistic method. American standard association method is also accurate method but is slightly less accurate than the Potier triangle method.

What is the shape of armature mmf waveform of a dc machine?

In which type of dc machine, wave winding is employed?
Wave winding is employed in a dc machine of low current and high voltage rating.

A dc motor operated from a type A chopper is switched to type B chopper. How does the motor operate when type A and B are in operation respectively?
In chopper-A voltage and current both are positive so motor operates motoring and in chopper-B, it will be reverse direction due to negative current and motor operating as regenerative braking.

Comment upon the torque produced if a d.c series motor is accidentally connected to single phase a.c supply?
In d.c series motor, the ac currents through the field and armature windings will always be in the same direction. So torque will be unidirectional but pulsating due to ac.

How the speed of a d.c shunt motor can be varied?
Speed of a dc shunt motor may be varied by field flux control, armature voltage control and armature resistance control.

What do you mean by the terms air-gap power, internal mechanical power developed and shaft power and how these terms are correlated to each other?
Air-gap power (Pg) - The power that is transformed from the stator to the rotor via the air-gap magnetic field, is known as the air gap power.
Subtracting the rotor copper loss from air-gap power gives the internal mechanical power developed (Pm). 
Shaft power (Psh) is the power available at shaft after subtracting mechanical losses from internal mechanical power developed.


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