Q2Electrical Machine Design
Question
Q.2. What do you understand by cogging?
Answer
Cogging is a phenomenon where an induction motor refuses to start (or starts with great difficulty) due to magnetic locking between stator and rotor teeth when the number of stator and rotor slots are equal or in certain unfavorable ratios.
Cogging (also called magnetic locking or teeth locking) is a starting problem in induction motors where the rotor fails to start, or starts only with a strong jerk, because the stator and rotor teeth face each other at every instant in such a way that the reluctance of the magnetic path is minimized in that aligned position, creating a strong magnetic 'locking' force that opposes any rotation away from alignment. This occurs specifically when the number of stator slots S1 equals the number of rotor slots S2 (or in other closely related unfavorable combinations), and is avoided in practice by ensuring S1 ≠ S2 and by selecting the slot combination to avoid specific harmonic-locking relationships identified by established design rules.