We have a 75HP Seimens VFD unit. The motor is making a high freq ringing sound that is proportional to speed. I measured the freq of the motor legs U V W and found one leg to be operating at 30% less HZ than the other 2 legs. EX: U=1000HZ V=1000HZ and W=700HZ. Could this cause the ringing sound and what could I check as to the cause?

 

My 2 HP unit with a KB VFD also makes a soft but high pitched whine. It is kind of good in the shop, because the tool is so quiet, it helps me to remember it is running. On the other hand, my Hitachi drive is absolutely quiet. All are 3-phase, as I assume yours is too.

I would check with Seimans about it. It may be normal. It may also be related to the motor. Some are inverter rated and others are not. From what little I have read about it, the windings in the not rated motors are a little less rugged and the insulation is different.

The frequency difference you note is a concern. I would put that question to Siemans too. Do you have a oscilloscope to look at it? The frequency measured with a simple meter may simply be an artifact from the same cause as the oscillations you are hearing.

I have no experience above 7.5 HP, however. John

 

I would definitely discontinue use of that motor/drive combination until the cause of the different frequencies is resolved. They should all match.