This is known as crawling of induction motor. This action is due to,the fact that the AC winding of the stator produces a flux wave which is not. 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. 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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Home Electrical machines Power system Contact electricaleasy. Crawling and cogging both are particularly related to squirrel cage induction motors.
Sometimes, squirrel cage induction motors exhibits a tendency to run at very slow speeds as low as one-seventh of their synchronous speed. This phenomenon is called as crawling of an induction motor.
This action is due to the fact that, flux wave produced by a stator winding is not purely sine wave. Instead, it is a complex wave consisting a fundamental wave and odd harmonics like 3rd, 5th, 7th etc.
Hence, harmonic torques are also developed in addition with fundamental torque. Hence 3rdd harmonics do not produce rotating field and torque.
Crawling and Cogging in Induction Motors
The total motor torque now consist three components as: If we neglect all the higher harmonics, the resultant torque will be equal to sum of fundamental torque and 7th harmonic torque.
If the mechanical load on the shaft involves constant load torque, the torque developed by the motor may fall below this load torque. This phenomenon is called cohging crawling in induction motors.
Cogging Magnetic locking or teeth locking Sometimes, the rotor of a squirrel cage induction motor refuses to start at all, particularly if the supply voltage is low. This happens especially when number of rotor teeth is equal to number of stator teeth, ans of magnetic locking between the stator teeth and the rotor teeth.
Crawling and Cogging in Induction Motors |
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. This phenomenon is called cogging or magnetic locking of induction motor. Why not share it?
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