Crawling in Induction Motors

The supply given to an induction motor may have harmonics present in it.  These harmonics will have their own torques in addition to the synchronous torque.  Let us consider a supply with odd harmonics.  The 3rd harmonic will be absent in 3 phase systems.  Hence, we only have to consider the 5th and 7th harmonics.  The other higher order harmonics can be neglected.

The torque produced by the 5th harmonic rotates in the opposite direction.  Thus, the forward torque is given by the sum of torques produced by the primary frequency and the 7th harmonic. 

The rotating field of the 5th will rotate at one fifth of the synchronous frequency (Ns/5).  However, the torque produced by the 5th harmonic rotates in the reverse direction.  Similarly, the 7th harmonic will rotate at one seventh of the synchronous frequency.  The torque produced by the 7th harmonic is maximum at 1/7th of the supply frequency.

When some poorly designed motors are started with load, the motors may not reach the nominal speed.  The motors will get stuck at 1/7th of the nominal speed. 

This phenomenon is known as crawling.  Crawling can be overcome by properly selecting the number of rotor bars in the rotor of the induction motor