Question

Generally, Crawling in squirrel cage induction motors is the tendency to run stably at speeds as low as

1. One-third of their synchronous speed
2. One-fifth of their synchronous speed
3. One-seventh of their synchronous speed
4. One-ninth of their synchronous speed

Answer 1

Correct Answer - Option 3 : One-seventh of their synchronous speed

Crawling:

• The crawling in the induction motor is caused by harmonics developed in the motor.
• Crawling is the tendency of particularly squirrel-cage rotor to run at speeds as low as one-seventh of their synchronous speed.
• This phenomenon is known as the crawling of an induction motor.
• When the no. of rotor slots is equal to the number of stator slots, the speed of all the harmonics by the stator slotting coincide with the speed of corresponding rotor harmonics.
• Thus harmonics of every order would try to exert synchronous torques at their synchronous speeds and the machine would refuse to start. This is known as cogging or magnetic locking.
• Therefore the no. of stator slots should never equal to the rotor slots.
• Cogging can be easily overcome by making no. of rotor slots prime to no. of stator slots.