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Cogging in induction motor (definition, reasons and how to reduce)

September 7, 2018
Cogging in induction motor (definition, reasons and how to reduce)

Cogging in induction motor

Cogging in induction motor … Today we will explain important phenomena that occurred in induction motors called cogging (or magnetic locking phenomena or teeth locking phenomena) … before we talking about how to overcome cogging or magnetic locking phenomena in induction motor we should know first what this phenomenon is.

Cogging phenomena in induction motor:

Here I will explain some conditions that lead to magnetic locking phenomena:

  1. Induction motors contain slots or teeth for their rotor and stator (you can revise construction of induction motor here) … when these teeth of the rotor are equal to the teeth of the stator, the motor fails to start. (This because of cogging phenomena)

cogging in induction motor

  1. This phenomena also increased with decreasing applied voltage over stator winding and equality of teeth for both rotor and stator.
  2. Cogging can occur also when supply voltage has some harmonics; when harmonics frequently coincide with slot frequency it causes torque modulation that causes cogging in the induction motor.
  3. In case of permanent magnet motors … cogging results in torque ripples that occur when magnetic reluctance forces acting mainly in the teeth of the stator.

How this phenomenon occurs:

When the conditions of cogging founded at any induction motor (particularly squirrel cage induction motor), the reluctance of the magnetic path is minimum; (this because of rotor and stator teeth comes in front of each other). So, the rotor of the induction motor tends to remain fixed and not started to rotate.

How to Reduce Cogging phenomena:

Because of the above-mentioned drawbacks of cogging … electrical engineers thinking about reducing or eliminating cogging phenomena that I can brief them in some short notes:

  1. We can reduce cogging by changing the number of stator teeth and rotor teeth (should varies).
  2. We can also use the skewed rotor to keep an overlap on all slots that will eliminate this phenomenon.

(Skew the rotor means that we should arrange the stack of rotor laminations so that the rotor slots are “skewed” or angled with respect to the axis of rotation).

skewed rotor

  1. We can minimize cogging by good motor design, but the problem still maintained in sensitive applications

Note that: this phenomenon is less in wound rotor motors this because of the higher starting torque of wound rotor motors.

Finally, I hope to cover all the questions in your head about this important phenomena.