# Electrical Machines Interview Questions and Answers

Electrical Machines Interview Questions and Answers

[1] Why is the efficiency of a three-phase Induction Motor is less than that of a three-phase Transformer? [Anna Univ. May-11]

• Three phase Induction motor is nothing but a rotating transformer.
• Due to rotating nature, there are friction and windage losses associated with it.
• On the other hand, the three-phase transformer is a static device, thus there are no rotational losses.
• Thus the efficiency of three-phase induction motor is less than that of a three-phase transformer.

[2] Why the air gap between the pole pieces and the armature is kept very small? [Anna Univ. May-06]

• The air gap between the pole pieces and the armature is kept very small to keep the field strength at its highest value.
• This is due to the reason the flux lines can pass through iron very easily than the air gap.
• The small air gap generates high voltage.
• Larger the air gap means stronger must be the magnetising force to produce the required voltage hence the air gap is kept very small.

[3] Why does curving the pole faces in a DC machines contributes to a smoother DC output voltage from it? [Anna Univ. Dec-10]

• For having smoother output voltage from DC machines its commutation must be as ideal as possible.
• In order to have good commutation the flux density in the air gap must decrease gradually from maximum value under the centre of the pole to zero on the interpolar axis.
• The flux distribution which drops abruptly from maximum to zero leads to commutation difficulties and magnetic noise.
• Thus to achieve good air gap flux distribution the pole faces are curved which improves the commutation and contributes to a smoother DC output voltage.

[4] The series field winding has low resistance while the stunt field winding has high resistance. Why? [Anna Univ. May-07]

• The series filed winding is always connected in series with the armature and thus has to carry the armature current which directly gets decided by the load.
• Thus the current passing through series field winding is of high level.
• The voltage drop across series field winding gets added to the voltage drop across armature winding while deciding the back emf. This voltage drop must be very small.
• Hence as the current through series field winding is high, in order to keep voltage drop across it to a small value, its resistance is very low.
• On the other hand, the shunt field winding is directly connected across the rated supply voltage hence to limit current through it, resistance is very high.

[5] What are the conditions to be satisfied for parallel operation of DC generators?[Anna Univ. Dec-08]

1. The polarities of the generators must be same or the connections must be interchanged till they become same.
2. The voltages of both the generators must be equal.
3. The change of voltage with change of load should be of same character.
4. The prime movers driving the generators should have stable and similar rotational speed characteristics.
5. The positive and negative terminals of generators must be connected properly to the positive and negative of busbar.

[6] What are the major parts of the DC generators? [Anna Univ. May-10]

1. Yoke Poles
2. Field winding
3. Armature
4. Armature winding
5. Commutator brushes

[7] What is a commutator? [Anna Univ. May-09]
The commutator is a device which converts alternating induced emf in a generator to DC emf. In case of motor it produces unidirectional torque.

[8] What is meant by reactance voltage? [ Anna Univ. May-12]

• An armature coil has certain self inductance.
• When coil undergoes commutation, there is change in current (di) in that coil and the commutation takes certain time(dt).
• Thus there is self-induced emf in a coil undergoing commutation given by L(di/dt).
• This EMF induced in the coil undergoing commutation is called reactance voltage.