VFD Output Voltage

eggslime; usually the voltage rises as the frequency rises up to a limit perry

Dear all,

Your are correct, many VFD's are V/f constant. In lower frequency, voltage to be reduced to avoid magnetic saturation in a motor.

Regards

More specifically, when you program a VFD for a particular motor voltage, the VFD sets up and maintains a specific Volts per Hertz ratio. As long as that V/Hz ratio is maintained, the motor will put out full torque. So as the Hz is reduced, so must the voltage be reduced to maintain a constant ratio. So if you had set up your VFD for a 440V 60Hz motor, the V/Hz ratio was 440/60 or 7.33:1. When you turn the frequency down to 50Hz, the voltage becomes 50*7.333 = 367V.

If your VFD is set up for a Variable Torque (Quadratic) load such as a centrifugal pump or fan, the reduction in the V/Hz ratio is no longer linear because the load would require less torque as speed decreases. That might explain why your output voltage is a little less than expected. If that is the case, hopefully you are not using it on a load that requires constant torque, such as a conveyor or machine tool.