Ø QUE-ANS:10-30 words
[1] Can a wound-motor be reversed by transposing any two leads from the slip ring?
No. There is only one way of doing so i.e. By transposing any two line leads.
[2] Why are induction motors called asynchronous?
Because their rotors can never run with the synchronous speed.
[3] How does the slip vary with load?
Greater the load, greater is the slip or slower is the rotor speed.
[4] What is the standard direction of rotation of an induction motor?
Counter clockwise, when looking from the front end i.e. driving end of the motor.
[5] Which is the usual cause of blow-outs in induction motors?
The commonest cause is single-phasing.
[6] How can the motors be protected against single-phasing?
(i) By incorporating a combined overload and single-phasing relay in the control gear.
(ii) By incorporating a phase-failure relay in the control gear. The relay may be voltage or current-operated.
Ø 30-60 words
[1] What are the indications of winding faults in an induction motor?
Some of the indications are as under:
- Excessive and unbalanced starting currents
- Some peculiar noises and (iii) overheating.
Squirrel cage motors sometimes exhibit a tendency stably at speeds as low 1/7th synchronous speed. This is known as crawling.
[2] How do changes in supply voltage and frequency affect the performance of induction motor?
High voltage decreases both power factor and slip, but increases torque. Low voltage has the opposite effect. Increase in frequency increases the power factor, but decreases the torque. The slip remains unchanged. Decrease in frequency has the opposite effect.
[3] Define the significance of the air gap in AC induction motors.
In an 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.
[4] What modifications would be necessary if a motor is required to operate on voltage different from that for which it was originally designed?
The number of conductors per slot will have to be changed in the same ratio as the change in voltage. If the voltage is doubled, the number of conductors per slot will have to be doubled.