HomeExperienceOversized VFD

Oversized VFD

Having a higher rated Variable Frequency Drive (VFD) will not damage a motor as long as it is within the control range of the VFD. The VFD is rated in amperes and nor motor horsepower.

The current technology force vector VFDs will train to the motor if the motor ratings are input and it is within the parameters of the VFD. If the VFD is grossly oversized it will not auto tune, however.

Please note that different rpm synchronous speeds will usually have a rating that is different than the NEMA ratings. This will be especially true of motors that are very low speed. The hp rating at a lower speed requires higher torque and perhaps as much as twice the current of a two or four pole motor, depending on the design.

Operating the motor at low HZ may require auxiliary cooling for the motor since the fan will be less effective. Basically the fan effectiveness will increase on the square of the rotor speed so high loads at low speeds will increase the heating.

Remember the HP is equal to the torque * RPM / ~5280 (OK, I know that number is the number of feet in a mile, but it is close and I can remember it).
The motor VFD can be specified as either constant torque or basically a pump curve. The constant torque will require a VFD with a higher rating.

I recommend erring on the side of more than necessary, since it will cost more to replace the VFD with a larger one than buying the larger VFD to start with. Plus you don't look quite so silly explaining why you specified an undersized VFD in the first place. Plus the VFD will run cooler. Ask the VFD rep for advice of how to choose the VFD.

In the old days we used to oversize the VFD by a factor of two to be sure we didn't end up with egg on our faces.
It is also possible that to have enough starting torque for your process you may have to install a larger motor. At least with the larger motor the excess power factor penalty will be reduced by the VFD.

You may like:

Buy VFD on
Buy Variable Frequency Drives (VFD) online, direct sale by manufacturer with wholesale price.


AC Motor Control