Crawling in induction motor
Crawling in induction motor … previously we talk about cogging in induction motor and know that the induction motor fails to start rotation … in this article I will explain the crawling phenomenon and how to overcome this phenomenon.
Firstly, I want to explain some definitions that we will need to understand our topic well.
When we apply AC current to terminals of the stator winding in induction motors, the stator winding produces a rotating magnetic field; assume the rotation speed of this field is synchronous speed NS.
As the rotor cuts this magnetic field lines will rotate also but with specified speed (Nr) which is fewer than the synchronous speed (NS) … in ideal case this speed (Nr) is equal to the synchronous speed.
The difference between the two speeds called (Slip).
S = Syn. Speed (NS) – Rotor speed (Nr)
Now I will start my topic with a question … I want you to start to think and analyze with me.
What is crawling in the induction motor?
Simply … Crawling is which induction motor (mainly squirrel cage) fails to reach its full speed (rotate with very low speed near 1/7 of synchronous speed!!!!).
Now we will start to analyze the reasons that this phenomenon occurs in some short notes:
- As we know that the supply source for induction motor (AC source) has some harmonics.
- Also, the flux wave that stator winding producing is not pure sine wave (has some odd harmonics).
- These harmonics divided into some odd harmonics 3rd harmonic, 5th harmonic and 7thharmonic … 13thharmonic… Etc.
- Assume that the main wave rotates at synchronous speed.
- In 3 phase system, 3rd harmonic is absent so, 5th and 7th harmonics are more effective (other harmonics can be neglected).
- The rotating field of 5th harmonic will rotate at (1/5) from synchronous frequency.
- Similarly the 7th harmonic will rotate at (1/7) from synchronous frequency.
- Now we have 3 kinds of torques and speeds … main (fundamental) torque (rotating with synchronous speed NS), 5th harmonic torque (rotating with NS/5 speed) and 7th harmonic torque (rotating with NS/7 speed).
- The torque produced by 5th and 7th harmonics generates reverse torque opposing main torque.
- The net developed torque will be fewer than required so, the motor will rotate at (1/7) of its synchronous speed.
How to overcome crawling phenomenon?
We can overcome this phenomenon by:
- Using soft starting method for producing a pure sine wave.
- Good design for the stater winding to reduce harmonics.