What Is Crawling and Cogging ?
Induction motors that Crawling and Cogging will not accelerate to full speed. because of 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. 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.
- In 3-phase balance system 3rd harmonic is absent.
- Reverse breaking torque is produced by the 5th harmonic which is negligible.
- 7th harmonic reaches its positive maximum value just before 1/7th of Ns(synchronous speed) beyond which it becomes negative in value.
Resultant torque is less then load torque. Then motor may be not accelerate up to its rated speed. But its rotate at 1/7th of full load speed. This is predominant in squirrel cage motor.
The phenomena of motor run at 1/7th speed of the full load speed due to harmonic is called crawling.
Resultant torque is summation of 7th harmonic torque and fundamental torque.
Harmonic flux wave can be reduced by proper selection of cost pitch and distribution of stator winding.
Cogging is the magnetic locking between the stator slots and rotor slots. When the no. of Rotor slots is equal to the no. of stator slots or integral multiple of rotor slots then there is a possibility of minimum reluctance. When stator teeth and rotor teeth face each other.
In some cases rotor teeth is fixes in front with the stator teeth,that phenomena known as the magnetic locking. Due to this magnetic locking rotor refuse the start is called cogging.
When voltage level is low then this effect can be more.
There is one more reason for cogging. If the harmonic frequencies coincide with the slot frequency due to the harmonics present in the supply voltage then it causes torque modulation. As a result, of it cogging occurs.
- By making rotor slots are skewed, we can eliminate this effect.
- By making position of rotor conductor at small angle of ‘r’ electrical with respect to axis of rotor,skewing can improve.
- By taking different no. of stator slots and rotor slots we can eliminate this effect.
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