Bucking 3-phase -- why only buck two of the legs?

Finegrain · Aug 18, 2016

Hi guys,

I need to buck the 3-phase going into my new Brother VMC. The machine's rated input power is 10 kVA. My Phase Perfect 355 makes lovely 240V, but the Brother wants 208. I'm looking at McMaster-Carr and see that their docs say to buy two, and buck two of the three legs and simply pass the 3rd leg through. Wouldn't that leave one of the legs higher than the others?

BTW, I don't want to buck the 1-phase going into my PP for a couple of reasons.

Thanks, and regards.

Mike

Modelman · Aug 18, 2016

Each phase shares wires with one of the other two... So, all of the electrons will have to pass through one of the two transformers.

Dennis

Monarchist · Aug 18, 2016

What Dennis said.

Or you could "think of it" as the two with transformers pushing or pulling their voltages closer to or further FROM the unaltered third as if it were a sort of baseline reference value.

Anyway.. it Just F(raternally) Works.

Mark Rand · Aug 18, 2016

It'll work for three wire/delta circuits. if the requirement is 4 wire/star/wye with a neutral, then you ought to buck all three phases.

9100 · Aug 18, 2016

That connection is called "open delta" and is common. In some cases you can't avoid it, as in connecting Variac units on a three phase line. Normally they use two units in open delta. If you try to use three Variac sections you have to use them in wye or have transformers following them.

Bill

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Bucking 3-phase -- why only buck two of the legs?

Finegrain

Diamond

Modelman

Titanium

Monarchist

Diamond

Mark Rand

Diamond

9100

Diamond

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