ECN Forum Forums General Topics of the Trade General Discussion Area 3 phase motor powered by single phase

Has anyone ever come across a 3 phase motor powered by single phase? I came across this at a tire shop today. The motor is 3 phase on the name plate but it has a cord with just 3 wires in it. 2 to the motor and 1 grounded to the frame. Open up the pecker head and the taps don't match the high or low voltage diagrams. My boss said I was crazy. That you couldn't do that without a phase converter which there was not. But I've heard an old hillbilly tell me before it was possible.

Does it also have a large capacitor (or2) on it?

I've found 3Ø motors on single phase Euro machinery that manufactured their own 3rd Ø.

It is possible, with the use of the appropriate capacitors, etc., to "create" a third phase. It's won't be true three phase, but it will get things spinning the right way.

I'm not about to try to explain the 'how' or the 'why' .... but, since you mentioned that the tap hook-ups were unusual, it is possible that something like that was done.

The other thing you said that caught my attention was "auto shop." I have seen more screwed-up compressor arrangements in auto shops that in all other locations combined. I would almost say "if you saw it in an auto shop, there is no way what you saw was correct."

I suspect that, if you checked, that the motor is performing very poorly. That is, running hot, gobbling power, and not performing anywhere near its' capacity.

They do have 2 capacitors..... Very interesting.

I got free 3Ø 3/4 HP motor one, and run it on 240V with capacitor from an A/C compressor. It is on my power washer.

Good explanation of the circuit on this site: http://www.gwm4-3phase.com/uands/static1.htm

This way of wiring is quite known in Europe as "Steinmetz wiring". It is limiting the motor to about 70% of its nominal power on 3 phases. So it has to be oversized. In some applications it may overheat due to unregular wave form etc.

On the other hand there is a certain number of applications where smaller 3 phase motors can be wired in Steinmetz alternatively.

Last I saw was a fan in a big blower housing where all necessary data was directly given on the name plate.

I have seen the big rotophase converters and recently I had a look at a VFD that used a 240 volt single phase input with a variable frequency 3 phase output. It was on a milling machine and the VFD gave him even more speed control than through the gearbox alone.
It was under $1000 CDN for up to 5 horse power.

There is an other thread regarding this topic too on ECN with schematics.

It works well. I do the same to convert single phase to three phase in my workshop and then start 3 Ø motors of the running main (3Ø running on 1Ø) motor.

Most motors under 3 HP start directly with no problems from the RST main terminals.

Check out this thread.

I found it interesting.

Hillbilly Sawmill