Electrical Revolution

30/01/2018

Crawling

Sometimes the induction motor starts to run at one seventh speed of synchronous speed, it is called as crawling in the induction motor.
When a three phase supply to given to induction motor, it produces rotating magnetic flux which rotates at synchronous speed.
However the stator flux is not pure sinusoidal, it consists of fundamental and harmonics.

Effect of harmonics

The speed of harmonic torque is 1/n th speed of the fundamental torque ( where n is order of harmonic ).
As the third harmonic remain absent under balance load condition, it does not produce any torque.
The fifth harmonic rotates at speed of N_s / 5 and rotates in the anticlockwise direction ( 120 × 5 = 600^o = 600^o – 720^o = – 120^o ). The 5^th harmonic produces small torque in the reverse direction but it is neglected.
The seventh harmonic rotates at speed of N_s / 7 and rotates in the clockwise direction ( 120 × 7 = 840^o = 840^o – 720^o = 120^o ).
If we neglect all higher order harmonic, total torque is sum of fundamental torque and seven harmonic torque.
The seventh harmonic torque reaches its maximum positive value just before 1 / 7^th synchronous speed beyond which it becomes negative.
If the total load torque assume remain constant, the resultant torque may fall below load torque.
The motor in this condition run at normal speed, but runs at 1/7^th synchronous speed.

Cogging

Sometimes the squirrel cage induction motor refuses to start particular at low voltage, it is known as crawling effect.
When the number of stator teeth is equal to number of rotor teeth, there is possible locking tendency of the stator and rotor teeth.
This is happens due to fact that when stator teeth and rotor teeth face to other, the magnetic path becomes minimum.
The rotor tends to remain fixed in this condition and causes serious problem in starting.

Overcome Effect of Cogging

The problem of Cogging is overcome by making the number of rotor slots prime to the number of stator slots.

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