Copied from the beginner's section. 3 ph vs 1 ph????

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H-M Supporter - Gold Member ($25)
Feb 13, 2017
This is a duplicate post from an answer to a beginner's question. Any serious thoughts will be greatly appreciated.

This will be a "theoretical" or hypothetical thought. It just struck me, not very hard, but I have zero experience in the matter. Just the theoretical knowledge of a retired engineer........ Now, it seems that every one is all wound up with converting to 3 phase motors and using the electronicly synthesized variable frequency from the VFDs.

It would seem to me that while single phase motors aren't quite as stable rotationally as a three phase, they do get their speed regulation from frequency, same as a 3 phase motor. By that thought, what would be the problem of acquiring a 3 phase VFD and running a single phase motor from it until such time as the motor conversion could take place? It should run well enough from 30 to 90 hertz, in either case. With a 4 pole motor, synchrounous speed of 1800 (less slip) at 60 hz, the yield would be from 900 to 2700 rpm. I doubt many people would need to excede either speed. Just change the pulley there.

There would be some limitations, I'm sure. A three phase motor would have a usable output somewhat slower than a single phase. And I assume it would be necessary to disable the phase loss for one leg. But what I see in my mind, a single phase motor should be able to run on a three phase device. Making the voltage match-up, of course. A good many of my machines will work on 120 or 240 volt. Depending on hook-up, of course. 3 phase motors will run on 240 or 480. So, 240 is the common link. For short runs, wire size shouldn't be that expensive.

It was just a thought that came up as I was thinking about the author's question.

Bill Hudson​


Apr 30, 2015
Seems like there would be an issue with the 120 degree overlap of any of the two phases that would cause a little loss in the motor?
Beats me


H-M Supporter - Silver Member
H-M Supporter - Silver Member ($10)
Jun 12, 2017
I suspect that VFDs will prevent operation with only one phase connected. There are also the capacitors in the single phase motor that might be a problem for the VFD.

I seem to remember reading that the capacitors are used to phase shift the AC for some of the motor coils and that static phase converters do much the same thing.

The motors aren't particularly expensive and can often be found in local classifieds cheaply as people don't know how to use them or just don't want to. VFDs are petty cheap as well. Though not cheap enough that I would want to risk frying one driving a single phase motor.


Mar 12, 2018
It won't work period. If you want to vary speed on a 1p motor it can be done with various stuff but not a vfd. The vfd would sense a problem and shut down.


Global Moderator
Staff member
H-M Platinum Supporter ($50)
Feb 8, 2014
It won't work period. If you want to vary speed on a 1p motor it can be done with various stuff but not a vfd. The vfd would sense a problem and shut down.
I have to agree, most VFDs would fault out on an open phase and I don't think there is any way to override that function. Then there is the problem of the start/run windings in the single phase motor, maybe some rewiring, but 3 phase motors are cheap. :)


Active User
H-M Supporter - Gold Member ($25)
Jun 12, 2014
There are single phase VFDs, but very limited application of motors that they will work on and with loss of Hp, use is usually limited to fans and pumps. Capacitor start and centrifugal or relay start will not work. They are also expensive and cannot be used with 3 phase motors. Hard to say what the single phase motor capacitor load and output phase loss would do to a VFD's protection circuity, nor could it be disabled. I would assume that the principle of the single phase VFD is similar to 3 phase except the the phase difference is 180 degrees. There are of some generic single phase output VFDs in addition to the Invertek drives, but more practical to go with a 3 phase motor and be done with it.

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