Define the significance of the air gap in AC induction motors

(a)        Define the significance of the air gap in AC induction motors.                     (3m)

(b)        State why the air gap needs to be checked occasionally.                               (4m)

(c)                Suggest with reasons the problem that may arise if air gap change is not

corrected.                                                                                                        (3m)

Answer :-

(a)                In a assembled squirrel cage induction motor, the air gap is made as small as possible to ensure that the strongest possible electromagnetic induction will takes place.

The size of the air gap affects the power factor of the motor because if the air gap width increase, the magnetizing current will be increased and the power factor is lowered.

(b)               Air gap checked with feeler gauge. It’s need to be checked periodically so as to keep a record of the air gap taken approximately 90° apart. This to ensure that a worn out bearing (especially on the power take off side) does not permit rotor to rub against the stator.

Grinding the rotor lamination , filing the stator teeth, increasing in magnetising current can definitely lower the power factor and air gap measurement is to keep a check so that it is not excessive.

(c)                If air gap measurement takes at 90° apart indicates that it is due to worn out bearing, then there is a possibility that the rotor will rub against a stator. A very slight rub will generate sufficient heat causing overheating which can destroy the coil insulation.

If the air gap are uneven, it will reduce the life of bearing due to unbalanced torque and vibration occur as a result of uneven magnetic flux. Excessive heat generate to the bearing may cause seizure, resulting noise sound in the motor.

Directly, it will result in damaging the moving part of the pump which can reduce the efficiency of the pump performance.