Can You Run a Servo Motor with a VFD Not so Fast

Can you run a servo motor with a VFD?  First, lets look at how a servo amplifier works with a servo motor.

A servo amplifier actually fires DC voltage into the three phases of a servo motor.  Each time a phase is fired, it is completely dependent upon rotor positioning reference to the stator windings to be fired.  In other words, you need commutation to run a servo motor.  Most servo motors will have an encoder that is coupled to the rotor and aligned to give rotor positioning reference to stator windings.

We have actually ran a Yaskawa servo motor open-loop on a variable frequency drive (VFD)   The results were not good.  You see, no matter how we tuned the drive, the end result was a very inefficient motor that drew high amps and had no torque.