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Posted by David Sherman on Friday, June 26, 2009 at 00:46:54 :
I know some of you have 3-phase power in your shops. I just bought a building that has "sort of" 3 phase power and I'm not sure how good it will be. One leg consists of 240 volts, center-tapped with the center tap grounded as a neutral. This runs most of the circuits. To supply the other phase, the utility has a "stinger" transformer putting out 240 with one side tied to one of the 120V legs of the main power and the other side producing the third phase in what's known as an "open delta" configuration. Ideally, I'd have a real 3-phase 4-wire wye setup with three transformers on the pole and the midpoint as the neutral, or my second choice would be a 3-pot 3-wire closed delta, but instead I have this goofy low-budget 2-pot 3-phase open delta system in which the neutral is in the middle of one of the phases and the third phase has way less capacity than the main two This means the phase voltage and angles will shift around depending on the load. Presumably the building owners accepted this years ago because all they needed 3-phase for was the air conditioner and maybe a pump or two, while the utility company liked it because it was cheap.
Now I'm thinking eventually this is where I'd like to set up my shop and run my lathe, mill, and generator test set, which I've never hooked up yet, on real 3-phase power rather than off a phase converter. I'm wondering if anyone else has a machine shop with 3-phase tools that's powered by a 2-pot open delta with the midpoint of one of the phases grounded, and if so, does it work okay or do you have trouble with phase voltages getting weird due to loads or when one phase of the utility power gets knocked out by a tree? The lathe and mill are only about 2 hp each, but the generator test set's 10 hp. I don't know what the existing 3-phase loads are yet, but I bet the air conditioner's pretty hefty. I'm also wondering if it would be worthwhile (or necessary) to install my own transformer (I have a 25 kva 3-phase unit kicking around somewhere) to isolate or balance out this setup or convert it to a wye with a neutral in the middle like it ought to be.
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