Cogging in Induction Motor

Cogging phenomenon is observed in Squirrel Cage Induction Motor during starting for certain combination of Stator and Rotor Slots. In Cogging motor fails to start at all. For better understanding a view of Stator Core Laminations, Stator Core and Rotor of Squirrel Cage Induction Motor is shown in figure below.

Squirrel Cage Induction Motor Rotor:

Condition for Cogging:

If the number of Stator Slots are either equal to or integral multiple of Rotor Slots then Cogging will definitely be observed. So this is the criteria for Cogging in Squirrel Cage Induction Motor.

Why Cogging Observe?

Let the number Stator Slots = S₁ and that of Rotor Slots = S₂

If the number of Stator Slots S₁ is either equal to or integral multiple of Rotor Slots S₂, then the variation of Reluctance as a function of space will have pronounced effect producing strong forces than the accelerating torque and because of this Squirrel Cage Induction Motor will fail to start.

In other words, If the number of Stator Slots S₁ is either equal to or integral multiple of Rotor Slots S₂, then the Reluctance is very low as teeth of Stator and Rotor face each other and therefore the Rotor will not try to go in high Reluctance region i.e. magnetic locking will take place and therefore Rotor will not rotate. When the rotor teeth and stator teeth face each other, the reluctance of the magnetic path is minimum that is why the rotor tends to remain fixed.

How to Prevent Cogging?

• In order  to  avoid  such  Cogging  a  proper  number  of  rotor  slots  are  to  be  selected  in  relation  to  number  of  stator  slots. 

• In  addition  rotor  slots  will  be  skewed  by  one  slot  pitch  to minimize the tendency of Cogging and noisy operation  while  running. 

• Effect  of  skewing  will  slightly  increase  the  rotor  resistance  and increases  the  starting  torque.  However this will increase the leakage reactance and hence reduces the starting current and power factor.

 

Selection of Number of Rotor Slots:

The  number  of  rotor  slots  may  be  selected  using  the following guidelines. 

To avoid Cogging and Crawling:

• S₁ ≠S₂
• S₁ – S₂  ≠ ±3P

To avoid noisy operation S₁– S₂  ≠ ±1, ±2, (±P ±1), (±P ±2)

Why Cogging and Crawling Not Observed in Slip Ring Induction Motor?

Cogging and Crawling is not predominant in Slip Ring Induction Motor as these Motors are started with higher starting torque with external resistance in Rotor circuit.

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