Cogging , Crawling and Skewing


cogging

With the number of stator slots equal to or an integral multiple of rotor slots, the variation of reluctance as a function of space will be quite pronounced resulting in strong alignment forces at the instant of starting. 

These forces may create an aligning torque stronger than the accelerating torque with consequent failure of the motor to start. This phenomenon is known as cogging. Such combination of stator and rotor slots must, therefore, be avoided in machine design.


Crawling

The tendancy of the motor to run at a stable speed as low as 1\7 of normal speed synchronous speed and being unable to pick up its normal speed.
A 3 phase winding carrying sinusoidal currents produces space harmonics of the order
      Belt harmonic            =            h= 6k±1
where k is positive integer 1,2,3,…

when k is 1 a 3 phase winding will produce predominant backward rotating fifth harmonic at a speed of (1\5) of sync. Speed and forward rotating field at (1\7) of sync. Speed. These harmonic will alone will have little effect on operation of motor.
The torque produced by the 5th harmonic rotates in the backward direction. This torque produced by fifth harmonic which works as a braking action is small in quantity, so it can be neglected. Now the seventh harmonic produces a forward rotating torque at synchronous speed Ns/7. Hence, the net forward torque is equal to the sum of the torque produced by 7th harmonic and fundamental torque.

The torque produced by 7thharmonic reaches its maximum positive value just below 1/7 of Ns and at this point slip is high. The seventh harmonic flux
torque has a negative slope. At this stage motor does not reach up to its normal speed and continue to rotate at a speed which is much lower than its normal speed. 
This causes crawling of the motor at a stable speed as low as 1\7 of normal speed synchronous speed and being unable to pick up its normal speed.

Skewing

The rotor teeth are given a slight twist as  a result, each rotor tooth is opposite several stator teeth, thereby reducing the effect of locking of stator and rotor teeth and so eliminating cogging.

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