Cogging and crawling are two phenomena that affect the performance of induction motors. They are caused by the interaction of the harmonics in the stator and rotor magnetic fields.
Let me explain them in more detail.Cogging is when the induction motor refuses to start or gets stuck at a certain position. This happens when the number of stator slots is equal to or a multiple of the number of rotor slots, and they align with each other.
This creates a minimum reluctance path for the magnetic flux, and the motor locks itself in that position. This is also called magnetic locking or teeth locking.
To overcome this problem, the number of stator and rotor slots should be different, and the rotor slots should be skewed or angled slightly¹².
Crawling is when the induction motor runs at a very low speed compared to its normal speed. This happens when the harmonics in the stator flux produce additional torque fields that rotate at a fraction of the synchronous speed, such as 1/3, 1/5, 1/7, etc. These torque fields can sometimes overcome the main torque field and make the motor run at a subharmonic speed.
For example, if the 7th harmonic produces a forward torque field that rotates at 1/7 of the synchronous speed, and the motor slip is high at that speed, then the motor may crawl at that speed instead of accelerating to its normal speed¹³.
To avoid this problem, the stator winding should be distributed uniformly and the air gap should be constant²⁴.I hope this helps you understand cogging and crawling in induction motors better. If you have any more questions, feel free to ask me .