COGGING & MAGNETIC LOCK
SQUIRREL CAGE motor sometime s could not be start, the rotor of squirrel cage motor refuse to start at all, particularly when the voltage is low. This happen due to Magnetic Locking between stator and rotor teeth, when number of stator teeth is equal to number of rotor teeth.
For that Kind of reason sometimes this phenomenon is call TEETH LOCKING.
It is found that the reluctance of the magnetic path is minimum when the stator and rotor teeth face to face each other rather than when teeth of one element (rotor or stator) are opposite to the slots on the other. It is in such position of minimum reluctance, that the rotor tends to remain fixed and for that cause serious trouble during starting.
How to overcome from COGGING:
Cogging of squirrel cage motors can easily overcome by making the number of rotor slot prime to the number of stator slots.