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Wayne
12-30-2002 23:47:11
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Re: Re: Re: 3 phase vs. single phase in reply to Wardner, 12-30-2002 19:04:54
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Your right about the power company charging for 3 phase service nowdays, at least in NC. We checked several years ago about having a service pulled into our shop, and the price we got back from them was in the neighborhood of $8000. Thing is that's for a pull of maybe 1 1/2 blocks from the lines feeding the VFD up the road which has the closest 3ph service. We had used a small static converter for a while, but wanted to step up the size of our compressor, but a static type will not handle the starting loads of a compressor. So we chose instead a 20 HP, running load rated, Roto-phase converter. We had to size based on it's capability to handle a 7.5Hp compressor under the starting load because a compressor has a long, high amp draw on startup (many single phase motors you'll see are rated as "compressor duty" for this very reason, that's why most are capacitor start, to make up for their inherent weakness, as one of the other posts stated). I believe it's max single motor starting capability is 10 Hp, but with a converter this large you also have a minumum single load of about 3HP for the converter to put out full voltage on the manufactured leg. So we just set the compressor up for continuous run and it acts as an idler for the smaller motors connected to the circuit. We've been running our machine shop this way for the past 4 years with a 5HP lathe, 1 1/2Hp drill press, 1/2 Hp tool grinder, 2HP Bridgport mill, a 1HP hydraulic power unit powering an old lathe we've converted to a buildup machine for redoing worn shafts, etc, plus various other things we've had plugged into it over the years. Through it all we've never had any problems. I wired the system in using a 3 pole contactor interlocked with the motor starter for the converter that way it prevents single phasing any of the equipment (ie- there is NO power to the equipment at all if the converter isn't running). The only problem is running an industrial 3-phase welder on a converter is another story. Acording the the folks at Roto-phase, it can be done, but then you still get back into the whole minimum and maximum amp draw thing. If you get a machine with a large draw at high output, then you need a large converter to handle it, but when your welding at a lower output, you would then have to have some sort of idler motor/load to keep full output on the manufactured leg. Well good luck guys. Sorry for such a long post but this is just my 2 cents based on personal experience, hope it helps some of you out.
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