Answer / shankar

Have you ever heard about cogging in induction motors. When the number of stator slots is equal to the number of rotor slots, forces due to reluctance are produced which dominates the accelerating starting torque of induction motor and hence reduces it. So the motor wont rotate when the no of stator slots is equal to number of rotor slots. This phenomenon in induction motor is called Cogging.

This does not occur in slip ring induction motors. you know why?? because of high starting torque.

Answer / raul

yes the exact term is COGGING....which means if the stator
poles and rotor poles are equal or in integral ratio, then
there is a strong alignment b/w the stator n rotor poles
and hence the alignment torque is greater than the starting
torque.....so magnetic locking takes place b/w the stator n
rotor teeth........

Answer / souhadri

To avoid the cogging which makes the rotor not to start.

Answer / gundra chakrababu

if the stator and rotor have same umber of poles,the harmonics produced in them are also of the same order.as these harmonics slip past each other, pulsating torques are produced,if the speed of these harmonics coincide,the stator & rotor poles would lock each other.this condition is called "cogging".under this condition,the motor would crawl at a subsynchronous speed and develops a synchronous torque.

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