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Help with Rotary Phase Converter

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  • Help with Rotary Phase Converter

    I want to build a Rotary Phase Converter, i have a 5 HP Motor and have some drawings from the Internet on how to build them, but i need some help from some members. I need to talk to members who have built Rotary Phase Converts so i can get the right parts and how to put it together right. I went to Grainger to get the Run and Start Capacitors, Potential Relay and other things and thay told me that you need this and that and " i don't think it will work" one man in there told me to go and buy one for about $2,000.00. Ok Members i need help here.
    The whole is greater then the sum of the parts.

  • #2
    I typed a long reply and the stupid timeout killed me so here goes again. It was probably better the first time--

    I wouldn't expect expertise in building an RPC from anyone at Grainger. Buy parts there as needed, but know what you are after before you go.

    I bought 15uf run caps for balancing for like $10 for a dozen off ebay. Capacitances add in parallel, so you can parrallel them up with solid copper wire into "banks" and then just nip a wire to remove one during the balancing phase.

    One of the things you should do to keep things simple is make yourself a block diagram and a wiring diagram of *your* phase converter before you start. I used internet info from several sources, and none had a diagram that incorporated the use of run caps for balancing between both inbound lines and the generated phase, the use of a potential relay to disconnect the starting cap, and the use of a magnetic starter to start the whole affair. You do not need the mag starter. I chose to go that route since in the event that supply power is interrupted to the RPC, nothing will restart until I disconnect the load motors and go restart the RPC....a safety thing. I wired mine initially with just a standard 220v disconnect to disconnect supply power, and left that in place so I could disconnect all power to my RPC panel when working on it, even after adding the mag starter.

    In my case, a simple block diagram would show that I have the following
    An idler motor with balancing run capacitors connected from each inbound power leg to the third generated phase, a starting capacitor between one inbound leg and a generated leg that is disconnected by a potential relay after starting, the magnetic starter circuit to disconnect inbound power, and a fuse block on the outputs of it all.

    I got the run caps off ebay, the enclosure, mag starter, and idler motor from a local industrial surplus place, and the start cap, potential relay, and fuse block from Grainger.

    I bought a cheap clamp-type (inductive pickup) multimeter from Harbor freight to do the voltage and current balancing after I was hooked up. The initial guestimates on run cap values I got from internet sources were so good that once I figured out I should use caps from *both* inbound legs to the generated phase, I got it within 4-5 volts on each leg on the first try. Some guys use an idler with *no* balancing caps and the corresponding poor voltage balance and get away with it, so it is not rocket science. Balancing should produce a bit more power and less heat in the load motors, however. You want to do your balancing with your most "typical" load motor connected, but check output voltages with nothing attached first to make sure you wired things correctly before burning up an expensive motor on a lathe or mill or something.

    Here are some good sources for info. The last one includes a post from bnelson that should give you good starting values for each of the involved capacitors. The start cap value for the third URL seemed higher than the others as I recall, and the value I started with from one of the other sources would not bring the motor up to speed.

    Let me know off line if a picture of my RPC control panel would help you out in some way.

    Paul Carpenter
    Mapleton, IL