How to get 41amps from RPC...

Discussion in 'General Electronics Chat' started by catman81056, May 3, 2012.

  1. shortbus

    shortbus Senior Member

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    @Catman - The control panel will need the contactor heaters changed to the correct ones for the new (220V) amperage. That is the only change there.

    The wires in the motor need to be reconnected for 220V, thats the only change there.

    You do know that a RPC is more than just a idler motor, right? It also needs the correct "balancing capacitors" to correct the out-put of the three phase. This link has all you need to figure out how to select and tune the balance capacitors, but you have to spend some time reading - http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/

    If you spend a little time reading on the PracticalMachinist.com site you will probably find a thread with someone doing a conversion real close to what your doing. Maybe not the same brand of lathe but one with the same horse power motor. http://www.practicalmachinist.com/vb/
  2. Brownout

    Brownout Well-Known Member

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  3. Brownout

    Brownout Well-Known Member

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    BTW, considering the article from my post above, the article said that a 3-phase motor only produces about 2/3 the rated horsepower when operated on a single phase. Does that not imply that the idler motor must be 1.5 times the horsepower of the tool motor before we add the normal "safety" factor. If, for example, we say the over rating of the driving motor should be, say, 1.3 times the rated power for the driven motor to account for losses, etc, then the real rating should be 1.5*1.3=1.95?
  4. gerty

    gerty Well-Known Member

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    These things still puzzle me. I've only worked on a couple of them, and built one from scratch, modeled after on I had seen earlier. The pony motor must be larger than the driven motor, no problem, I understand that. I've seen one
    with 1hp pony and 3/4hp driven, worked fine for years. And another with 3 hp pony and 1hp driven, again worked fine.

    What puzzles me is if you put an amprobe on the incoming single phase power, the reading you get will be lower than the smallest motor alone.for example: the one with the 3 hp pony motor that motor should draw about 17a , and the driven motor, 1hp,about 8amps. The combined circuit drew approx7 amps. I have yet to find anyone that can explain that to me.
  5. crutschow

    crutschow AAC Fanatic!

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    The current draw obviously depends upon the load. So it just depends on what the driven motor was driving at the time of your measurement.
  6. Brownout

    Brownout Well-Known Member

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    Hi gerty,

    Thanks for your input. Not to critisize, but I think the vocabulary needs a little review. The article refers to the "pony" motor as a single-phase motor that spins up the converter, or idler, while power is not connected to the tool. So, the pony motor can be relatively small compared to the converter/idler.

    I just wanted to see that the terminology is consistant so as not to be confused.
  7. Brownout

    Brownout Well-Known Member

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    Good point, Carl. The load is actually at the tool end. The machine will be more loaded while cutting than when just spinning in air.

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