Small VFD Large Motor
Small VFD Large Motor
(OP)
I have been told I can use a 75HP rated VFD to run a 125HP motor of a centrifugal water pump. I understand I can ramp up the motor.
I also understand I can only deliver the max amp rating of the VFD which is about 95 amps.
I also understand I can only deliver the max amp rating of the VFD which is about 95 amps.





RE: Small VFD Large Motor
This sort of thing is done all the time for testing, but you cannot legally install it permanently (assuming you are in North America because you used HP).
All that said, it is technically possible. When you use a VFD on a motor, lowering the speed also lowers the effective HP proportionally. So how that can apply here is that technically, since your VFD is 60% of the motor size, if you NEVER try to run it at more than 60% speed, not technically a problem. BUT (and this is a big BUT), most centrifugal pumps stop pumping at around 40% speed, so you are only going to have a VERY narrow operating range.
You also have to factor in Murphy's Law and the corollary that says just because YOU knew what you were doing does not mean the NEXT GUY will understand it later when you aren't around.
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RE: Small VFD Large Motor
Just to put some rough numbers on it, let's say the drive is rated 86amps. The 75hp motor has an FLA of 86 and the 125hp motor has an FLA of 150. The motors will likely have magnetizing amps around 25% so that would be 22amps and 38amps respectively.
So, after the magnetizing amps are subtracted out, the drive has 86-22=64amps for torque on the smaller motor and only 86-38=48amps for the larger motor.
It's actually a bit more complex than that because there are vectors and the motor magnetization changes somewhat with frequency, but, in general, that's the process.