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RPC Amperage Draw with Second Idler

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  • RPC Amperage Draw with Second Idler

    Simple question likely not so simple answer.

    Is the total amperage draw of an RPC with a second idler the sum of the two idlers and the load motor? If this is previously answered I missed it.

    Bob W
    Bob, 71193, Central Arkansas

  • #2
    Short answer is Yes, but you also have losses and inefficiencies to add in. You also have to account for the single to three phase conversion.
    Robin

    Happily working on my second million Gave up on the first

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    • #3
      Generally , YES. The RPC current INPUT is going to be powering all of them.

      Some factors may cancel out, and reduce current a bit.

      And, the idlers are supplying power, so they will draw a bit more than a simple "idle current".

      They will NOT draw as much as their full load current. You have to measure their actual currents to know for sure. An estimate would be that the idlers will draw around 60% to 70% of full load current WHEN driving a maximum size motor that is itself at full load current.

      The "three phase conversion" does not entirely apply.... The idlers do no "conversion", they only supply the 3rd phase, the other two wires are a pass-through. So the idlers draw around 1/3 of their max current to generate the added leg, plus their idle current.
      Last edited by J Tiers; 07-12-2021, 08:57 PM.
      CNC machines only go through the motions

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      • #4
        rdfeil, J Tiers:

        Thanks, that is about what I thought. But, my knowledge of electrical things is limited, Ben Franklin is likely more knowledgeable. A clamp on ampmeter should tell all when I get things wired up.

        Bob W
        Bob, 71193, Central Arkansas

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        • #5
          RPCs generate 2 additional phases,
          but we covered this before.

          -D
          DZER

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          • #6
            Originally posted by Doozer View Post
            RPCs generate 2 additional phases,
            but we covered this before.

            -D
            Yes, you need to see which method of description is in use.

            "Phases" may be viewed "wye", in which case the wire is the "phase", or "delta", in which case the difference voltage is the "phase".

            Power distribution is commonly described "wye", so the "A" phase refers to the wire, and in some cases the pole will even have the wire location labeled "A" or "B" etc.
            CNC machines only go through the motions

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            • #7
              If an RPC is fed single phase
              and 3 phase comes out
              simple math gives me my answer
              as to how many phases a RPC makes.

              -D
              DZER

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              • #8
                Did the RPC with a 10 hp 3 ph motor years ago. Ran 4 knee mills , lathe, surface grinder and EDM machine. In 1980 that was the best way of doing it. Now with the VFD, I would not go that route.

                The 3rd phase on the idle motor is about 60% effecent at best. How ever, run say 2 mills while starting a lathe will back feed power into the 3rd phase. You dont get that power for free, it comes from the single phase power. If you are calculating wire size with the power question , go heavy on the gage. Lots of inefecenties with RPC. Mr. J T. Knows his stuff !!

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