Motor Performance and VFD

This may help you. Read the operation section.

No. Power factor with the VFD, as seen by the line, will remain relatively constant regardless of speed, because the motor itself is now essentially isolated from the source. Think of it this way; When you connect a motor to a VFD, you are now recreating an entirely new power source for the motor, The VFD uses the line power now simply as "raw material" for this new motor power source. So only the power (kW) itself is seen by the line, and the PF is based solely on the PF of the VFD itself, not the motor PF. PF on a VFD is however a little more complicated because now you have displacement PF and distortion PF. The displacement PF will remain constant at about .95. The distortion PF varies with speed a little, but because speed also relates to load, when the distortion PF is at it's worst, the load is at its lowest, so the net effect is much less as well. In addition, most PF meters will not be able to record distortion PF anyway, so the issue is somewhat moot.

Efficiency is similar but does vary by speed more than PF does in a VFD system. This is mainly because your switching losses percentage in the VFD itself increase as speed decreases. But again, power use is decreasing at the same time, so although losses increase by percentage of total load, the amount of losses is still lower at lower speeds. Throughput efficiency varies from 97-98% at full speed down to 94-95% at 1/2 speed and 85-87% at 10% speed in the worst case.