Cogging
The phenomenon of magnetic locking between the rotor and stator teeth in an induction motor(mainly in squirrel cage) is called cogging. Due to the cogging motor is unable to start. This happens due to the presence of magnetic interaction between the stator and rotor slots(teeth), so also called teeth locking. If the ratio between stator and rotor slots is an integer ratio this causes strong cogging. for example the stator slots=30 and rotor slots=30 then the ratio is an integer. to reduce the cogging phenomenon the ratio between stator and rotor slots is kept as fractional. this reduces the magnetic interaction between the stator and the rotor. also, the rotor conductors in an induction motor are placed skewed to reduce the alignment between stator and rotor slots.
Crawling
Crawling is a phenomenon in squirrel cage induction motor due to which motor operates at low speed.this happens due to the presence of space harmonics in air-gap flux. crawling happens when the flux produced is not a pure sine wave. to types of harmonics are produced. one who rotates in the direction of fundamental flux A=(6A-1), where A=1,2,3 and other from the opposite of flux A=(6A+1), where A=1,2,3. The main harmonics which cause the crawling are the 5th and 7th harmonics, so the other harmonics can be neglected. as you can see from the above equations 7th is aids, but 5th rotates in the opposite direction of flux. so thus breaking the torque of the motor and reducing the speed of the motor.
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