I've got more questions than answers

Dec 02, 2005 5 Replies
G

I'm trying to understand the application of VFDs in industry. I understand the basics,ie; by changing the frequency of AC current, you can change the rpm of an AC induction motor. This is a good thing. I know that things like fans, pumps and conveyors are great applications for VFDs because you can change the speed of the motor to match the load. I also think that VFDs will work with machine tools because the VDF allows the spindles to be run at an infinite number of speeds, negating the need for some belts, pulleys or transmissions.



Looking at VDFs available on Ebay, it appears that the input on some of them are single phase and some are 3 phase, but all VDFs have a 3 phase output.It appears to me that one could use (a properly rated) VDF instead of a phase converter to convert 230v single phase to 3phase at



230v or 460v. Am I correct?

The VDFs listed on Ebay are for fractional horsepower all the way up to



50 hp and higher. Does the VDF rating have to match the HP rating of the motor, or could one use a VDF rated at 3 hp to drive a smaller motor like a 1 hp motor? I know that it would not be a good thing to use a VDF rated at 3hp to run a 25 hp motor.

Greg


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R

I asked an electrical engineer at work and he says that yes, this will work, but you should size the VFD by amperage, not by HP (though the amperage/hp sizing will be close to the same). You are also correct in your assumption that you can use a larger VFD to run a smaller motor, but you shouldn't go the other way around.

Relz

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G

Almost. You can change the rpm of a three phase AC induction motor, but not a single phase one.

Yes, you can run them single phase input - three phase output.

You can certainly use e.g. a 3hp VFD to run a 1hp motor, don't know if it would be wise to use a 25hp VFD to run a 1hp motor. Can't go the other way, of course.

GWE

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C

Well unless your manual mentions that single phasing is a-ok w.o derating, I'm thinking that you might want to derate it by 1.7 or so. Just incase the full wave rectifier is sized for 3 ph.

To some degree on most there are parameters you can set to protect motor and drive. I'd stay away from more than a 2 or 3x mismatch where the load is the small guy in circuit. JMHO.

Wes S

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D

According to Greg Postma :

[ ... ]

I'm using a 30A (7.5 HP max when running from single phase -- limited by the 30A circuit breaker) to run a 1HP motor on my Nichols horizontal milling machine. They have no trouble running motors smaller than their rating, unlike a static converter, which must be fairly closely matched to the motor's HP rating so the start capacitor is properly sized.

Enjoy, DoN.

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