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The VFD Answer Man, Volume 1: VFD's over 60 Hz

December 20, 2015 |By: Terry M. Smith

The question has arisen about ABB VFD’s operating motors above 60 Hz. All VFD’s are inherently capable of running above 60 Hz.

The limitations are as follows:

1. Motor base speed. If the motor base speed is 3600 RPM, the top limit for speed will partially be determined by motor bearings and balance. Any motor operating above 120 Hz will likely need high performance bearings and a precision rotor balance to function properly.

2. Motor full load amps. The limitation for all motors is the nameplate full load amps. In a variable torque system it is relatively simple to calculate the maximum speed based on the amp draw at 60 Hz. Take the ratio of the full load speed required to the 60 Hz speed, then square that number. Multiply that by the full load amps observed at 60 Hz. If that resultant is less than the motor full load amps the motor can be successfully operated at the required speed. If it is greater than that, the motor will be limited to a lower speed, which is easily calculated based on the above.

3. Motor manufacturer’s recommendation for maximum speed. Ordinarily they will advise you to limit the motor to the full load nameplate amps. There may be some rare instances where the motor has a limitation, so it is always best to consult the manufacturer directly.  

I have successfully programmed dozens of drives to operate motors anywhere from 61 Hz to as high as 330 Hz (those were very special).

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