- Induction Motor Phenomena: Crawling and cogging are important characteristics to understand in the operation of squirrel cage induction motors.
- Crawling Definition: This is when an induction motor runs at much lower speeds than its designed speed, primarily caused by harmonics like the 5th and 7th producing additional torque.
- Cogging in Induction Motor: Occurs when the motor fails to start because the slots of the stator lock up with the rotor slots, often due to matching slot numbers or harmonic interference.
- Preventing Cogging: Adjusting the number of rotor slots to be different from those in the stator and skewing the rotor slots can effectively prevent cogging.
- Understanding Harmonics: Recognizing how harmonic frequencies interact with the motor’s slot frequencies is crucial for diagnosing and solving motor issues like cogging and crawling.
Squirrel cage induction motors often exhibit two key issues: crawling and cogging. These issues stem from the motor functioning improperly, either by running at an abnormally slow speed or failing to handle the load effectively.
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. The resultant speed is nearly 1/7th of its synchronous speed. Now the question arises why this happens? This action is due to the fact that harmonics fluxes produced in the gap of the stator winding of odd harmonics like 3rd, 5th, 7th etc. These harmonics create additional torque fields in addition to the synchronous torque.
The torque produced by these harmonics rotates in the forward or backward direction at Ns/3, Ns/5, Ns/7 speed respectively. Here we consider only 5th and 7th harmonics and rest are neglected. The torque produced by the 5th harmonic rotates in the backward direction. This torque produced by fifth harmonic which works as a braking action is small in quantity, so it can be neglected. Now the seventh harmonic produces a forward rotating torque at synchronous speed Ns/7. Hence, the net forward torque is equal to the sum of the torque produced by 7th harmonic and fundamental torque. The torque produced by 7th harmonic reaches its maximum positive value just below 1/7 of Ns and at this point slip is high. At this stage motor does not reach up to its normal speed and continue to rotate at a speed which is much lower than its normal speed. This causes crawling of the motor at just below 1/7 synchronous speed and creates the racket. The other speed at which motor crawls is 1/13 of synchronous speed.
Cogging of Induction Motor
Cogging is a characteristic of induction motor that occurs when the motor fails to start, sometimes due to low supply voltage. However, the primary cause is the locking of the stator slots with the rotor slots, preventing the motor from turning.
In induction motors, both the stator and rotor contain a series of slots. When these slots align perfectly due to matching numbers, the magnetic path’s reluctance drops to its minimum, which can prevent the motor from starting.
Cogging in induction motors is defined as the locking of the stator and rotor slots, preventing the motor from starting. Another cause of cogging is torque modulation, which happens when the supply voltage’s harmonic frequencies align with the slot frequencies, enhancing the locking effect. This issue is also referred to as magnetic teeth locking.
Methods to overcome Cogging
This problem can be easily solved by adopting several measures. These solutions are as follows:
- The number of slots in rotor should not be equal to the number of slots in the stator.
- Skewing of the rotor slots, that means the stack of the rotor is arranged in such a way that it angled with the axis of the rotation.
