Using a lower voltage motor on a higher voltage VFD

[lncabin]

Our European office uses a 0.5HP drum motor, from a small, specialized manufacturer, on a conveyor application which has a nominal nameplate voltage 181V and with a 230V 1HP VFD.

My office wants to replace a 1HP induction motor that is side mounted on a different kind of conveyor application with the previously mentioned drum motor. Mainly because of the special environmental and IP rating offered by this motor. This conveyor application uses 1HP VFDs but they are 480V supply rated.

They ordered some of these motors to do this replacement test on some conveyor equipment, that is already built here. Just to see if it preforms well enough or if they need to put back in the side mounted induction motors, before it gets shipped out.

The problem, I think, is that they think the current 1HP VFD is sufficiently power rated to use a 0.5HP motor and thus we don't need to swap out VFDs. We just need to change the VFD parameters to correspond to the new drum motor.

Well...the minimum nominal output voltage I can set for these 480V 1HP VFDs is 200V, which is higher than that 181V I mentioned in the first sentence.

Not really sure how big of a deal a 10% higher voltage supplied to the motor will be but I wouldn't be surprised. Not really concerned about speed or torque performance, we don't necessarily need anything accurate, but worried about just frying the motor.

And we don't have any 230V 1HP VFDs on hand either.

 

[Jraef]

Higher voltage increases the torque correspondingly, so it might invalidate your “test” in that when the higher torque is no longer there with the proper voltage, the drum may no longer turn. It will also result in the motor running significantly hotter, but if it’s a short duration, that may not matter.

Also along the lines of the long term operation at the higher voltage issue is that when you program a 480V drive to run a 200V motor, the DC pulses that comprise the PWM waveform going to the motor will have a 650+VDC peak, as opposed to a 330VDC if it were a 240V feed. That can be problematic for sone motors if the winding insulation was not designed to handle that. Again, for short duration tests I wouldn’t worry too much, but running it for a few weeks like that may damage the winding insulation.

As an aside, I happen to have a 480V Rockwell PowerFlex 525 drive in front of me, I can program the motor nameplate voltage down to 20V if I want to. But looking at manuals I have for different brands, I see that is the exception, not the norm.

 

[winnie]

The output voltage of the VFD had better be able to go below 200V, otherwise you wouldn't be able to operate your 480V 60 Hz motors at less than 25 Hz. So I am guessing that the number you can't set below 200V is the nominal base operating voltage of the motor; the voltage needed at the rated operating frequency.

So you might be able to fake things out by upping the base operating frequency.

You need to be careful that you don't inadvertently let the system drive the motor to higher than its maximum operating speed.

A 480V 1Hp VFD has about half the operating current rating of a 240V 1Hp VFD; be careful that you aren't hitting your current limits.

The DC pulses issue that @Jraef mentions is real and important; you might see reduced motor life.

Finally, it is worth checking that if these 181V motors could be re-connected for a higher voltage by a simple change in the terminal box.

-Jon