Ø QUE-ANS:10-30 words

[1] How can you Reverse the Direction of a Motor?

By swapping any two supply line connections over.

[2] What are the Causes of Single Phasing?

Single Phasing can be caused when one of the three back up fuses blows or if one of the contactor contacts is in open circuit.

[3] What is in brief, the basis of operation of a 3 phase induction motor?

The revolving magnetic field which is produced when a 3-phase stator winding is fed from a 3-phase supply.

[4] What factors determine the direction of rotation of the motor?

The phase sequence of the supply lines and the order in which these lines are connected to the stator winding.

[5] Can a 3-phase motor be run on a single-phase line?

Yes, it can be. But a phase-splitter is essential.

Ø 30-60 words

[1] Enumerate the possible reasons if a 3-phase motor fails to start.

Any one of the following reasons could be responsible:

One or more fuses may be blown.
Voltage may be too low.
The starting load may be too heavy.
Worn bearings due to which the armature may be touching field laminae, thus introducing excessive friction.

[2] What is Single Phasing of a Motor?

Single Phasing is where one of the 3-phase’s supplying the Motor becomes disconnected. The Motor will continue to run if this happens and can result in Motor Burnout. The effect of Single Phasing is to increase the Current in the two remaining Lines and cause the Motor to become very noisy due to uneven Torque produced.

[3] What are the advantages and disadvantages of 3 phase AC motors with wound rotor (slip ring rotors)?

Advantages wound rotor (slip ring rotor):

This has the benefit of reducing the starting current surge while providing a high starting torque. Speed can be regulated, except that rotor external resistance brush is continuously rated.

Disadvantages wound rotor (slip ring rotor):-

It is inefficient because of power wasted in the resistance for speed control purposes.

The external resistance bank may require additional cooling facility.