Re: 3 Phase - 3 Phases

Randy Gross wrote: 
Chris wrote:


Randy Gross wrote:

In lieu of a 3 phase mains feed, can 3 independent single phase
feeds be applied to a 3 phase transformer?

In this scenerio, there would be no power overlap as in
conventional 3 phase. All three phases would rise and fall, in
unison, in phase with each other. The unit is a rated 575 volt 3
phase welder with plasma cutter.

My question is, can the single phase feeds be applied in this manner
without dire effect.


Hi, Randy.  Not a very good idea, for many reasons.  Not safe.  Don't
go there.

Chris 

cfoley1064@xxxxxxxxx

I was asked this question by a curious owner. I don't know but, to
reduce temptation, I think a solid answer is needed.

rg
There are parts of the core that bridge between the sections (legs) that have coils on them that carry flux from more than 1 coil. When the coils are excited with 3 phase, the coils, in effect, take turns using that bridging part of the core.

If two coils are energized at the same time, there are two possibilities. Either their fields around the loop through those two legs adds, so that that loop has twice the rated peak flux, or they buck each other, putting twice the normal flux over through the third leg. In either case, at least you get the same volts per turn and (with voltage derating for the saturation taking place at least somewhere at half the rated volts per turn) it still acts like a transformer. This assumes that you can get to all the coil ends and wire then either in series or parallel, not delta or Y like they were connected for 3 phase.

With 3 coils energized, simultaneously, either they all create field in the same direction so that there is no return path for all that flux, or two fields are in one direction and the third is in the other direction, so that the odd leg still has twice the flux of the other two. Since volts per turn are proportional to the peak to peak flux (for a given frequency) you either get twice the volts per turn on the odd leg, the same volts per turn on each coil, but no closed path, so very low inductance and very high magnetizing current and lots of fringing fields. I'll leave the first case as an exercise how you series parallel connect the coils to get some kind of transformer. The second case is just a low impedance reactor across the line.

Is this what you were trying to picture?
.

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