What is Crawling of Induction Motor:
As you know fifth harmonic (5*fundamental frequency) has negative sequence current hence it creates negative field and which opposite to the rotor rotation, the torque produced by the fifth harmonic oppose the fundamental harmonic. It is also called as breaking Torque on motor. Refer the detailed article about fifth harmonic.
At the same time the seventh harmonic which has frequency of seven times of fundamental component. It creates positive sequence current in the motor. It rotates the rotor in direction of the fundamentals component. The torque produced by the seventh harmonic is addition to the fundamental torque.
With certain combination of S1 (stator poles at one area) and S2 (stator poles in another area), there can be space harmonic of the mmf wave. i.e 5th and 7th harmonics corresponding to poles five and seven time to that of fundamental respectively. Since space phase difference of the fundamental is (0, 120, 240) and that of seventh 0 120, 240, therefore fifth harmonics poles rotates backward at Ns/5 and seventh harmonic poles rotates forward at Ns/7 respectively. These harmonic waves also reduce their own induction torque as shown, hence, the motor runs unstably at 1/7 of the normal speed. This phenomenon is called Crawling.
In above diagram You can see that, the 5th harmonic characteristics, 7th harmonic characteristics, fundamental harmonic characteristics, load characteristics (dotted line), motor normal characteristics and all combination of 5th, 7th and fundamental (resultant).
In below mentioned motor characteristics, You can see that the 5th harmonic creates breaking torque and 7th harmonic creates positive Torque. But at one instant the drive torque is less than the load torque, at that instant the motor runs the 1/7th of the original speed. This is called crawling of induction motor.
Key Points of Induction Motor crawling:
- Motor Runs 1/7th speed of the full speed.
- It occurs due to 5th harmonic and 7th harmonic
- 5th harmonic rotates the motor in opposite direction and 7th harmonic rotate the motor in forward direction.
- These phenomena are predominant in slip ring induction motor.
- Crawling causes Noise and vibration.