CRAWLING

It is observed that induction motors, particularly the squirrel-cage type, some-times exhibits a tendency to run stably at speeds as low as one-seventh of their synchronous speed. This phenomenon is called the crawling.
A-3 phase winding carrying sinusoidal currents produces harmonics of the order n = 6N + 1 where N is an integer (+ means with the rotation and – means against the rotation). The synchronous speed of nth order of harmonic is 1/n th of the synchronous speed of the fundamental. For N=1, a 3-phase winding produces a forward rotating 7th harmonic and backward rotating 5th harmonic. Considering 7th harmonic, 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 to exceed one-seventh of normal speed. Thus the motor crawls at about one-seventh the normal speed.