Small VFD Large Motor 2

[KeithStone]

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.

 

[jraef]

Sure, all you have to do is convince the motor to not try to work harder than 60% of it's rating and convince a local electrical inspector that you think it's fine to use a power device that is not rated for the load and therefore in violation of the NEC clause that says "...suitable for the intended use".

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.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[DickDV]

Actually, jraef is even being a bit generous. You will end up with less than 60% output because some of the ampacity of the 75hp drive is going to be used up just magnetizing the larger motor. Every amp that goes into magnetizing the motor is not available for producing torque in the shaft.

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.