Getting 120v Single Phase from 3 Phase

Only in a special circumstance, which is a 4-wire, center-tap grounded delta service. This has one 240 V L-L circuit with a center tap, just like everybody's home service. Then, it has another transformer that develops the 3rd phase. In this system, there are only TWO of the three hot wires that will give 120 V to neutral. (The third will give ~207 V, so you will definitely be able to tell which is which with a meter.)

This service is pretty rare, at least around where I've lived. Corner-grounded delta is more common, but you can't get 120 V directly from that. You need a 240 - 120 step-down transformer. Corner-grounded delta is most obvious because 2-pole circuit breakers and disconnects are used. The 3-phase wires are hot, hot and neutral, and you can wire a 3-phase motor up to those 3 wires. (These are also labeled hot (A), neutral (B), and hot (C) phases, and therefore sometimes called grounded B phase.)

If you have 208 V WYE service (sometimes called star) you have three

120 V circuits, from any line to neutral. But, in this system, you can't get 240 V, without a transformer. Like, the building I work in, has 208 V Wye for the office section, and we have little autotransformers to step 208 up to 240 for the window air conditioners.

If you try this on a true 240 V Wye system, which has a neutral, you will get a rather high voltage of about 138 V. But, 240 V Wye is pretty rare. If you try this on a real delta system, you might kill all the lights in the building, as it may trip the ground fault protection. But, then, a true delta system doesn't have a NEUTRAL, although sometimes telling the difference between a neutral and a safety ground can be difficult. Delta transformers have a balancing transformer in them that makes it look like they are referenced to a neutral, but if you draw any current from line to neutral that unbalances it, the transformer should shut down. This normally wouldn't apply to open delta and corner or center-tap grounded deltas, as they are expected to feed unbalanced loads.

Is that more than you wanted to know?

Jon

No, but it might educate some of the self-proclaimed "engineers" who claim that the two opposite poles of the 240 center-tapped are "180 degrees out of phase." They are not "out of phase" - they are simply opposite polarity! It's a significant difference, in the realm of phasors and imaginary power and stuff.

sorry Rich, but the two ends of a center-tapped winding *are* 180 degrees out of phase. sin(wt+pi) = -sin(wt).

Cheers Terry

OK, what would a pair of sine waves look like if they *were* 180 degrees out of phase? How would they be different from a pair of sine waves that were "simply opposite polarity"? [1]

John [2]

[1] there's a relevant trig identity somewhere, I think [2] self-proclaimed "engineer" [3] [3] except for the BSEE, which makes me an other-proclaimed "engineer"

Sounds like a lot of blithering crap from one of those 1940's vintage BS Audel books- absolutely no sense to made of it and zero credibility. And it's a very rare circumstance to find dedicated 230V or 208V compressors for A/C- vast majority are dual 208/230.

"It's a significant difference, in the realm of phasors and imaginary power and stuff. Really, do tell? I think mostly "stuff." Bob

Well. They are not "in phase". they are "antiphase" with reference to the center tap.

One is (162 sin(wt)) The other is (162 sin(wt+PI))

That's out of phase enough for me :-)