The phenomenon of Magnetic Locking between the stator and the rotor teeth is called Cogging or Teeth Locking. Even after applying full voltage to the stator winding, the rotor of a 3 phase induction motor fails to start. This condition arises when the number of stator and rotor slots are either equal or have an integral ratio.
The number of the stator slots equal to or an integral multiple of the rotor slots, strong alignment forces are produced between the stator and the rotor. As a result of these forces an alignment torque greater than the accelerating torque with consequent failure of the motor to start. Thus, a locking is created between the stator and rotor teeth. This condition is known as Cogging or Magnetic locking.
When the stator and the rotor teeth face each other, the reluctance of the magnetic path is minimum. Under this minimum reluctance path condition, magnetic locking takes place between the stator and the rotor teeth.
In order to reduce or eliminate the process of Cogging, the number of stator slots is never made equal to the rotor or has an integral ratio. It can also be reduced by using the skewed rotor. Cogging and Crawling are less eminent in wound rotor motors because of the higher starting torques.