Cogging And Crawling of Induction Motor

Induction Motor
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Crawling

 It has been found that induction motor practically this squirrel cage type ,sometimes exhibit,a tendency to run stable at speeds as low as ¹/₇th of their synchronous speed .This is known as crawling of induction motor. This action is due to ,the fact that the AC winding of the stator produces a flux wave which is not pure sine wave .It is a complex wave consisting of a fundamental wave which revolves synchronously and odd harmonics like 3^rd,5^th,7^th etc. which rotate either in the forward or backward direction at ^Ns/₃, ^Ns/₅, ^Ns/₇ speed respectively. As a result in addition to the fundamental torque ,harmonics torques also developed whose synchronous speed for fundamental torque. For Example Ns/n, where N is the order of harmonics torque. Since the 3^rd harmonics current are absent in a balanced three phase system ,they produce  no torque .Hence total motor torque has there components.

1. The fundamental torque rotating with synchronous speed 
2. Fifth harmonics torque rotating at ^Ns/₅ 
3. Seventh harmonics torque having a speed of ^Ns/₇

If we neglect all the higher  harmonics, the resultant torque can be taken as equal to the sum of the fundamental torque and the seventh harmonics torque. When this happen the motor will not accelerate  up-to its normal speed  but will remain running at a speed which is nearly equal to ¹/₇th of its full speed. This is referred as a crawling and  motor starts running with unwanted sound .

Cogging

The rotor of particularly squirrel cage  induction motor sometimes refuse to start at all particularly a  when the voltage is low. This happens of stator teeth is equal to the  number of  rotor teeth ,and therefore  due to the magnetic locking or cogging .It is found that the reluctance of magnetic paths  is minimum  when the stator and rotor teeth comes in front of each other, it is in such position of maximum reluctance that the stator tends to remain fixed thus causes serious trouble during starting.

This can be easily overcome by making number of rotor slots more than the number of stator slots and by giving slightly skew to the rotor slots (skewed).What is meant is to arrange the stack of rotor laminations so that the rotor slots are "skewed" or angled with respect to the axis of rotation. Constructing the rotor with skewed slots and providing more (or fewer) rotor slots than stator slots is the remedy for both cogging and crawling.