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Anyone interested in a RPC build?

Alan DuBoff said:
You have some terminology wrong, possibly, as I don't believe there is such thing as a 2-phase, it's really 1-phase. A single phase (1 phase) 220v circuit has 2 x 120v hot lines, and to make it 3 phase, you produce the 3rd 120v line. A 3 phase circuit will at any time produce 240v by using any 2 of the 3 hot lines, hence 120v+120v give you the 2 needed hots for 240v.
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I don't think thats quite correct, you'd get 240v peak to peak on any of the three legs of a three phase circuit, though I don't know how you'd measure it... mabe with an oscilloscope (and 120v peak to ground, easy to measure with a meter). You should get 208v measuring between two legs of a three phase circuit -- 240v * sine (120) = 207.85v

I think split phase (that Jason mentions at the bottom) is some way of tapping a 3 phase circuit to get 170v, but don't quote me on that. Looking now at the wikipedia link Jeff posted, it seems that they're using split phase as synonomous with single phase, so I'll go along with that. Some nutty old electrician was telling me about the 170v thing maybe 10 years ago, so I probably warped it a bit. I do remember he said it was common for old factories to have 170v on the lighting circuits for some reason or another.
 
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John Downey said:
I don't think thats quite correct, you'd get 240v peak to peak on any of the three legs of a three phase circuit, though I don't know how you'd measure it... mabe with an oscilloscope (and 120v peak to ground, easy to measure with a meter). You should get 208v measuring between two legs of a three phase circuit -- 240v * sin(60) = 207.85v

I think split phase (that Jason mentions at the bottom) is some way of tapping a 3 phase circuit to get 170v, but don't quote me on that.
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John, I'm no electrician and learn enough to not burn my house down, but get help when I need it. I had a friend help me wire my garage for me, and we are supposed to add some outlets which will be nice. When he helped me run the 240v for my RPC, we ran from the box to the RPC location in conduit, and made sure it was all grounded properly.

My understanding could be incorrect, and I probably don't know the proper terminology myself, but a 3 phase motor will spin and change between the 2 active so that at any time it would give 240v, or 208v (but 240v is the sanest to think, since it is 120 that calculate from, I think, although we hear 115v as well, and 110v. It's no wonder I am not crazy about electricity...:doh:

It's also not clear to me exactly what differences people like Anderson put in their RPC enclosure vs. the static, but they do add some stuff for the start I believe. I have a toggle switch on mine, and just flip it on, I think it does as Jeff suggests and turns it off after. However, phase-a-matic shows using just a toggle and a 3 phase motor to build an RPC using their static, at this link. (PDF file)

I have one of these, from Anderson which I bought from a guy locally, it has an A.O.Smith 5HP motor that was reconditioned to be an RPC idler and has the arbor cut off specific. Mine is in a compact book box the galoot made out of ply, it works well and let's met stuff it in under the bench. Anderson sells them for $500, I bought it for $300.

Parts were probably cheaper if you get the same stuff, I don't know exactly what's inside as I have never drilled it open (it's riveted shut). There is certainly differences between Anderson's static converter and the RPC panel, but I'm not certain what.

I think it's worth considering getting the panel (at thebottom of that page) from someone like Anderson and getting a 3 phase motor to make your own, elliminating some of the guesswork that Jeff is presented with. Notice that you can get a full 7.5HP panel for $200. I have a PAM-600HD (3-5HP phase-a-matic static) which is brand new and came with my table saw. I'm gonna toss it together some day with a 5HP motor and have another RPC that will most likely start my machines as well. Either sell it or keep it as a spare. I don't expect the RPC to be a high failure though. They seem to last a long time, most of those motors are rated for continuous duty, mine are.
 
Alan DuBoff said:
My understanding could be incorrect, and I probably don't know the proper terminology myself, but a 3 phase motor will spin and change between the 2 active so that at any time it would give 240v, or 208v (but 240v is the sanest to think, since it is 120 that calculate from, I think, although we hear 115v as well, and 110v. It's no wonder I am not crazy about electricity...:doh:

It's also not clear to me exactly what differences people like Anderson put in their RPC enclosure vs. the static, but they do add some stuff for the start I believe. I have a toggle switch on mine, and just flip it on, I think it does as Jeff suggests and turns it off after. However, phase-a-matic shows using just a toggle and a 3 phase motor to build an RPC using their static, at this link. (PDF file)
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In a nutshell -- which is where I usually run into trouble :D -- the three phases are 120 degrees out of phase on a time vs voltage plot. The plot is basically just three sine waves equally out of phase with one another. A static converter uses continous duty capacitors to shift one or the other hot leg of single phase 240 to imitate the third phase, and uses other capacitors to shift the other leg a bit less to imitate the second phase. The most basic RPC just uses a booster motor (single phase) to get the idler up to speed, then the two hot legs of the single phase 240v are used to run two sets of windings in the idler, and the third set generates the third phase. Getting a bit more sophisticated, start capacitors are used to in place of the booster motor to get the idler running. The trouble with both these methods is that the voltages between the third phase and the 240v line (single phase) are not balanced, since they are not all 120 degrees out of phase. This is where the run capacitors are used to shift one leg of the single phase 240v and get a better balanced system.

On another note, I've always assumed that the whole 220v - 240v and 110v - 115v - 120v thing had to do with the power company's tolerances for how far you are from the transformer. If you're as far from it as they allow, and there are also a lot of taps between you and it, you'll see a lower line voltage at your panel. But, the key word is "assumed".

I should go back and fix the math in my last post - it'd be clearer if I used sine 120 rather than 60.
 
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