Torque at zero speed

By Clarence Klassen

At a recent AIMCAL Converting School class in Charlotte, I was asked how a drive can achieve torque even when the motor is at zero speed. The problem is that if the drive sent out power with zero frequency (direct current), nothing would get to the motor to create torque.

Drives create speed by controlling the frequency sent to the induction motor. Induction motors slip. It is the slip that creates torque when magnetic flux lines are cut. The drive does not send out a zero or dc signal, instead, it sends out a very low frequency, perhaps 0.5 Hz. The motor slips to the point that it does not rotate (zero speed). However, flux lines are still being cut so torque is produced.