Rob,

I feel your doubt. In fact, in spite of a background in physics and math, it took me some time to come to the realization that it is possible to get real three phase from a pair of Voltages that are 180 degrees out of phase with each other. It seems to be contradictory, I mean just how do two Voltages that are 180 degrees out of phase suddenly become 120 degrees out of phase? In most phase converters those original two phases are just passed through with no change. But when you do the vector math, it really does work. In fact, this is exactly what is happening when, here in the US, we take a 220V feed (call it single phase or two phase as you like, that's just terminology) and, using a rotary phase converter to convert it to three phase.

The "magic" lies in the vector math which shows a change in the ground reference. When a rotary phase converter is used to generate a third leg from the two existing phases of the power feed, although it does not actually generate an actual, new ground point, the third phase is generated in a manner so that the theoretical ground point is shifted in both Voltage and phase from the original ground point that is half way between the original two phases. And those original two phases are not changed in any way, but from the viewpoint of this new, theoretical ground point they REALLY are 120 degrees out of phase. This is at the same time that those original two legs are still 180 degrees out of phase when viewed from or referenced to the original, earth ground point. It sounds contradictory and I had to work out all the math before I was ready to believe it, but it does really work.

So, if you first convert the real single phase current which has one side grounded and the other at 220V to the 110V-G-110V configuration used in the US and then run it through a rotary phase converter, then you do get actual, true three phase. In fact, if you are using a rotary phase converter, you do not even have to use the transformers to convert it to 110V-G-110V, you can just use the 220V power from the wall outlet directly to the rotary phase converter and it will still work and produce true three phase.

This will also work for any other kind of phase converter that produces three phase from single/two phase. I talked about using the transformers because I do not have a copy of the plans being sold and I do not know if they require the center ground of the 110V-G-110V system or not. There are many schemes for producing three phase from 110V-G-110V and some of them may very well need that center ground.



Quote Originally Posted by old kodger View Post
Hi Paul,
I had been thinking about a 230-0-230 volt transformer arrangement such as is used in electronic circuit supplies (usually 25-0-25 for medium output amplifiers), however, as you point out, the phases are in fact 180 degrees out of phase not 120 and I did wonder how that would effect the performance of the motor, in that it would "confuse" the third phase orientation.

Regards,

Rob.