Cogging and Crawling of Induction Motor

 Cogging and Crawling of Induction Motor

Crawling of induction motor Sometimes, squirrel cage induction motors exhibits a tendency to run at very slow speeds (a low as one-seventh of their synchronous speed). 

This phenomenon is called as crawling of a induction motor.

This action is due to the fact that, flux wave produced by a stator winding is not purely sine wave. Instead, it is a complex wave consisting a fundamental wave and odd harmonics like 3rd. 5th, 7th etc. 

The fundamental wave revolves synchronously at synchronous speed Ns whereas 3rd, 5th, 7th harmonics may rotate in forward or backward direction at Ns/3, N5/5, Ns/7 speeds respectively. 

Hence, harmonic torques are also developed in addition with fundamental torque 3rd harmonics are absent in a balanced 3-phase system. Hence 3rdd harmonics do not produce rotating field and torque. 

The total motor torque now consist three components us: 

(1) the fundamental torque with synchronous speed Ns, 

(ii) 5th harmonic torque with synchronous speed.

Cogging (Magnetic Locking or Teeth Locking) of induction motor

Sometimes, the rotor of a squirrel cage induction motor refuses to start at all, particularly if the supply voltage is low. 

This happens especially when number of rotor teeth is equal to number of stator teeth, because of magnetic locking between the stator teeth and the rotor teeth. 

When the rotor teeth and stator teeth face each other, the reluctance of the magnetic path is minimum that is why the rotor tends to remain fixed. This phenomenon is called cogging or magnetic locking of induction motor.