Crawling and Cogging of Induction Motor

An induction motor is an AC electric motor; it is also named as an asynchronous motor. The rotor in this motor has an electric current that generates torque from an electromagnetic induction through the rotary magnetic field in the winding of the stator. The rotor type in the induction motor is a wound type of squirrel cage. The main characteristics of this motor are crawling and cogging. These are mainly occurred because of the motor’s improper functioning which means motor runs with less speed otherwise it doesn’t use the load. This article discusses an overview of Crawling and Cogging of Induction Motor

Crawling and Cogging of Induction Motor

The crawling and cogging of an induction motor will not go faster because the cogging characteristic of the motor will not go faster at all & the crawling characteristic of this motor will stop accelerating at a particular speed. This acceleration can be controlled through the torque in the motor.


Crawling and Cogging of Induction Motor
Crawling and Cogging of Induction Motor

Cogging of Induction Motor

The stator and rotor of the induction motor include a set of slots. The number of these slots should not be the same because there is a chance of the motor to stop running because of the cogging characteristic. Due to this reason, the number of slots within the rotor & rotor is not equal, however, there is a change for the slot frequencies which correspond through harmonic frequencies to cause torque modulations. The slots are twisted to maintain an overlap over all the slots to decrease this problem.

What are the Methods to Overcome Cogging?

There are different methods that can be adopted to solve the cogging problem.

  • The number of slots within the stator as well as the rotor should not equal.
  • The arrangement of rotor slots skewing can be done in such a way that it can be angled through the rotation axis.

Crawling of Induction Motor

The second characteristic of an induction motor is crawling. The gap in the motor can be developed with harmonic fluxes because of the magnetic field of the induction motor. The harmonic fluxes in the motor will create an extra torque field.
A common trouble that can be occurred due to the seventh harmonic will create a forward rotary torque magnetic field at 1/7th speed of the synchronous. The maximum torque of the motor is just under 1/7 Ns. If it is high sufficient, then the net torque is higher as compared with the torque because of the line frequency wherever the slip at 1/7 Ns is high. When the synchronous speed of the motor is under 1/7 then crawling can occur.

Reasons for Crawling in Induction Motor

Crawling in an induction motor can be caused due to some reasons which include the following.

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  • It can be caused due to the presence of space harmonics within the air gap flux.
  • Air gap harmonics can be occurred because of the uneven sharing of the Stator winding.
  • The difference in air gap reluctance can be occurred due to the slots in Stator & Rotor.

How to Avoid Crawling in Induction Motor?

In an induction motor, crawling is less because of its high starting torque. So crawling and cogging in cage rotor motor can be avoided through suitable selection of coil span & bending the teeth of the rotor slightly. So crawling can be avoided or minimized by selecting the right combination of the slots of stator & rotor

Thus, this is all about an overview of crawling and cogging of the induction motor. The crawling of this motor can occur at the portion of rated speed whereas the cogging of this motor can occur if the no. of stator slots are important several of rotor slots. Here is a question for you, in which motor, the crawling & cogging are not predominant?

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