Electrical Engineering ⇒ Topic : Crawling

CRAWLING

It is observed that induction motors, particularly the squirrel-cage type, sometimes exhibit a tendency to run stably at speeds as low as one-seventh of their synchronous speed N_s. This phenomenon is known as crawling of an induction motor.

A 3-phase winding carrying sinusoidal currents produces harmonics of the order n = 6 N± 1, where N is an integer. The movement of the harmonics is with or against the direction of rotation depending upon the sign (+ means with the rotation and means against the rotation). The synchronous speed of nth order harmonic is 1/n th of the synchronous speed of fundamental.

For N = 1, three-phase winding would produce a forward rotating 7th harmonic and backward rotating 5th harmonic. Considering the 7th harmoniò, the interaction between the fictitious stator and rotor 7th harmonic poles will produce a positive torque and if the torque is sufficiently pronounced it may prevent the motor speed exceeding one-seventh of normal. Thus the motor crawls at about 1/7th the normal speed.