Induction motors that cog or crawl will not accelerate to full speed. Cogging motors do not accelerate at all, and crawling induction motors stop accelerating at part speed. Acceleration can also be limited by the torque output of the motor relative to the load torque at that speed. Induction motors have a series of slots in the stator and in the rotor. 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. The slots will align like a stepper motor.
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The phenomenon of Magnetic Locking between the stator and the rotor teeth is called Cogging or Teeth Locking.
Even after applying full voltage to the stator winding, the rotor of a 3 phase induction motor fails to start. This condition arises when the number of stator and rotor slots are either equal or have an integral ratio. The number of the stator slots equal to or an integral multiple of the rotor slots, strong alignment forces is produced between the stator and the rotor.
As a result of these forces an alignment torque greater than the accelerating torque with consequent failure of the motor to start. Thus, a locking is created between the stator and rotor teeth. This condition is known as Cogging or Magnetic locking. When the stator and the rotor teeth faces each other, the reluctance of the magnetic path is minimum.
Under this minimum reluctance path condition, magnetic locking takes place between the stator and the rotor teeth. 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 torques. Under this minimum reluctance path condition, magnetic locking takes place between the stator and the rotor teeth Why????
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Induction Motor Cogging and Crawling
Induction motors that Crawling and Cogging will not accelerate to full speed. Acceleration can also be limited by the torque output of the motor relative to the load torque at that speed. Crawling and Cogging is explain for Induction Motor below. The wave produces by the AC winding in induction motor is flux wave which is not pure sine wave. But it is a complex wave which is consisting of a fundamental wave. And odd harmonics like 3rd,5th,7th. Thus in addition to the fundamental torque, harmonic torque are produce.
What Is Crawling and Cogging ?
Post a Comment. Motor Power System Power Quality. What is Cogging and Crawling? The cogging phenomenon is also called the magnetic locking. If the load torque is less the motor may keep on operating at lower speed. The adverse effects of crawling and cogging of induction motor can be eradicated or minimized by taking care while designing the motor and also by taking care when the motor is operated through variable frequency drive.
Cogging and Crawling of Induction Motor
Cogging in Induction Motor