VVVF drives - Internal speed calculation


Thread Starter


I have an ABB drive which has two speed feedback selections - ENCODER and INTERNAL. How does the drive know the motor speed without encoder?
As per my understanding for drives without encoders the drive calculates the actual speed based on the parameters that are set. Eg full load speed, rated frequency and rated voltage.

The drives without encoder do not give a precise measurement of actual speed.

> For induction motors:
> Drive output freq - slip = motor speed.

How will the drive know what the instantaneous slip is?

Curt Wuollet

An educated guess usually. For most purposes 100% accuracy isn't important and if it is, you use an encoder. From output frequency and load current you can guess within a couple percent or better for known motors. The slip is quite limited once up to speed and before breakdown torque. There are other clues available as well with the information they gather for vector control. They may even be able to count the pole crossings between output and motor reaction. There should be a cyclic signal in there that represents slip.