16 Oct It can also be reduced by using the skewed rotor. Cogging and Crawling are less eminent in wound rotor motors because of the higher starting. 30 Jan Sometimes the induction motor starts to run at one seventh speed of synchronous speed, it is called as crawling in the induction motor. When a. Induction motors will not accelerate to full speed if they cog or crawl. Cogging motors do not accelerate at all, and crawling induction motors stop accelerating.

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Crawling and Cogging of Induction Motor

These slots should not be equal in number because if they are, there is a good chance that the motor will not start at all due to a characteristic known as cogging. In the same way that the torque just below crawl speed is a maximum, the torque just above crawl speed is a minimum.

There are harmonic fluxes developed in the gap due to the magnetics of the motor. What is meant is to arrange the stack of rotor laminations so that the rotor slots are “skewed” or angled with respect to the axis of rotation. The phenomenon of Magnetic Locking between the stator and the rotor teeth is called Cogging or Teeth Locking. Crawling of Induction Motor It has been observed that squirrel cage type induction motor has a tendency to run at very low speed compared to its synchronous speed, this phenomenon is known as crawling.

Where the SCRs are triggered relative to the voltage wave form, there will be a modulation of the conduction angle of the SCRs as observed.

Crawling and Cogging in Induction Motors

Thus, a locking is created between the stator and rotor teeth. The seventh harmonic is the best known and documented crawl speed, but others can exist due to the total interaction of all harmonic sources.

These harmonics create additional torque fields in addition to the synchronous torque. But generally, three constants R, L and C are considered and the Recieve free updates Via Email!


For this reason, there are an unequal number of slots in the rotor and in the stator, but there can still be situations where the slot frequencies coincide with harmonic frequencies and this can cause torque modulations.

This is happens due to fact that when stator teeth and rotor teeth face to other, the magnetic path becomes minimum.

Crawling and cogging both are particularly related to squirrel cage induction motors. Apart from this, there is one more reason for cogging. When a three phase supply to given to induction motor, it produces rotating magnetic flux which rotates at synchronous speed.

These harmonics create additional torque fields.

Cogging And Crawling of Induction Motor

Hence, the net forward torque is equal to the sum of the torque produced by 7 th harmonic and fundamental torque. Crawping using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Please clarify your specific problem or add additional details to highlight exactly what you need. A reduced supply impedance will cause a reduced conduction angle for the same current and this will increase the harmonic content and result in an increased crawl torque.

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Induction Motor Cogging and Crawling Induction motors that cog or crawl will not accelerate to full speed. Crawling Sometime the squirrel cage induction motor refuses to start particular crawlign low voltage, it is known as crawling effect.

Instead, it is a complex wave consisting a fundamental wave and odd harmonics like 3rd, 5th, 7th etc. If we neglect all higher order harmonic, total torque is sum of fundamental torque and seven harmonic torque.

This happens especially when number of rotor teeth is equal to number of fo teeth, because of magnetic locking between the stator teeth and the rotor teeth. Under this minimum reluctance path condition, magnetic locking takes place between the stator and the rotor teeth.

Manoj are you sure you are not trying to increase traffic to your blog! Sometimes the induction motor starts to run at one seventh speed of synchronous speedit is called as crawling in the induction motor. Note that the acoustical noise related to this phenomena can also be problematic.


Thank you Charles Cowie. Group Drives Photo Credit – http: This phenomenon is called cogging or magnetic locking of induction motor. This torque produced by fifth harmonic which works as a braking action is small in quantity, so it coging be neglected.

It is also helpful to design the motor so that the torque vs.

Read articles related to various Electrical Engineering topics. The total motor torque now consist three components as: Here is a bit more detail on the reason that distorted flux waves cause problematic torque variations. Even after applying full voltage to the stator winding, the rotor of a 3 phase induction motor fails to start.

If the motor is well designed, it should not experience crawling with any reasonably acceptable commercial power waveform or PWM VFD waveform.

If the mechanical load on the shaft involves constant load torque, the torque developed by the motor may fall below this load torque. This action is due to the fact that, flux wave produced by a stator winding is not purely sine wave. This crxwling cause a modulation in the current flow in the motor and this in turn can modulate the commutation angle of the SCRs.

These characteristics are the result of improper functioning of the motor that means either motor is running at very slow speed or it is not taking the load. There must be sufficient synchronous inductiln at the minimum to accelerate the load to full speed. Induction motors that cog or crawl will not accelerate to full speed.