Re: O.T. 3-phase power question


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Posted by David Sherman on Friday, June 26, 2009 at 02:50:13 :

In Reply to: Re: O.T. 3-phase power question posted by Franz© on Friday, June 26, 2009 at 01:26:01 :

I've got a lot of 120 (it's an office building), but it looks like a 600 amp service and presumably it's been okay the way it is, so balancing out my phases with single-phase loads isn't important. It's that wild phase that bothers me. Worst case, if the regulation and the phase angle are poor, all it would do is act like a starting winding and the motors would draw most of their power single-phase from the "strong" pair. This means not only less power out of the motors, but torque fluctuations (might be significant in a machine you want running smoothly) that you wouldn't get in a balanced 3-phase system. Also, I've already experienced what happens when you lose one leg of an open delta -- 60 volts on a 120 circuit in my case, which is hell on refrigeration compressors, and presumably somebody else in town got a comparable over-voltage at the same time which is even worse.

Not much I can do about it, but I'm just wondering if anybody else has had trouble with this kludge in the real world. The lineman tells me that if the transformers blow, they'll replace them with a 3-pot bank, closed delta, but they'll still leave the center-tap grounded leg because that's what everybody needs. What they ought to do is run another set of low-voltage wires down the alley hooked up as a 4-wire wye and make everybody use them for new work.

But hey, I guess this is more modern than the first electrical system in town. In that one, you had a choice of evening-only or all-night lights, and paid by the light bulb, paying more for all-night power, which was mostly just the bars. They ran two sets of wire from the powerhouse, and the guy at the power house turned the evening-only juice off at bedtime. The powerhouse is still there, but the Pelton wheel and generator are long gone.



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