Big electric motors and little VFD's

Mar 20, 2010 9 Replies

I got a good deal on a couple of little VFD's. They take 120 volt single phase and put out 240 volt 3 phase and are not very big, but they are only rated for 1/4 hp. And me without any 1/4 hp three phase motors.



But at the scrap yard, I ran across a 1/2 hp three phase motor that had a key taped in place on the shaft. May not have ever been used. There were no wire nuts on the power leads and it did not look like they had ever had a wire nut on them.



So can you run an unloaded motor off a VFD that is only rated at half the motors hp? Well the answer is that you can not at least with the VFD that I was trying. After a second or so, the vfd would stop and show an error code. But wait, the motor can be connected for either



240 or 480 volts. Meaning it has two sets of windings. Connect is parallel for 240 volts and in series for 480 volts.

Or you can connect just one set of windings, and voila a 1/4 hp motor. Works with the VFD just fine.



Dan


I had the same result trying a 5 hp. VFD on a 7.5 hp motor.

Karl

I have run motors on smaller VFDs unloaded once or twice and it worked but I don't remember how far out I was

Do the VFD's have any provison to sync two together? If so, you could use one on each winding set....

I will have to look at the manual again, but I do not think there is any way to sync the VFD's.

=20 Dan

Under-powered VFDs cannot cope with the starting surge demands of a "too" large motor.

I had the same result trying a 5 hp. VFD on a 7.5 hp motor.

Karl

It is not the starting surge, it is the line current. The line current of most motors doesn't change much from no-load to full load. What changes is the angle between voltage and current. So, an undersize VFD can't supply the line current the motor draws, even at no-load.

Jon

NEVER, EVER, try this, unless you like BIG sparks! There are modular VFDs, usually in hundred HP sizes, that can be paralleled and synched for larger motors. otherwise, they will not tolerate this AT ALL!

Jon

Most VFDs incorporate over current protection for the output transistors. A large motor will be seen as a overload and shutdown the output. The inrush current looks like a short circuit to the vfd and it shuts down.

Best Regards Tom.

Even when they say so? Note I was asking.,,,

(I used to work on low megawatt UPS's, built out of 10KW power modules.)

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