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RPC amps load question

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  • #16
    Well gosh, I installed the capacitors, and corrected the power factor, can we go back and look at my OP.
    A 3hp 1ph motor is about 18amps
    A 3hp 3ph motor is 10amp + 2.7 amp for RPC. = 12.7 amp total
    Does this really hold true that I can run a 3hp 3ph compressor on my RPC for less amps that straight up 1ph?

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    • #17
      Originally posted by Ringo View Post
      Well gosh, I installed the capacitors, and corrected the power factor, can we go back and look at my OP.
      A 3hp 1ph motor is about 18amps
      A 3hp 3ph motor is 10amp + 2.7 amp for RPC. = 12.7 amp total
      Does this really hold true that I can run a 3hp 3ph compressor on my RPC for less amps that straight up 1ph?
      Those numbers only hold true if you really had 3 phase. The calculation used is the square root of 3 (or 1.73). Think about it for a second. When running a single phase motor on 240 volt AC you have two "hot" wires. When running same motor on 3 phase you have 3 hot wires. This works the same way a 120 volt motor with one hot wire would required twice as many amps as the same motor wired as 240 volts with two hot wires. So back to your 3ph motor... The AMPS will be less on each wire but the apparent power would be the same and power company would bill you the same (excluding power factor etc ). However in your case you need to get that 3ph power from single phase so you would multiply your 3 phase amps by 1.73 to arrive at the single phase amp draw at the converter. This of course would not include the amps required of the converter itself. I hope that makes sense.

      Mike
      Last edited by Ohio Mike; 01-30-2021, 10:10 PM. Reason: Edited for clarity.
      Mike
      Central Ohio, USA

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      • #18
        Ringo , Short answer is NO. The nameplate amperage listed is PER PHASE for 3 phases. If you were to do the math, Ohio Mike has the answer. In simple terms, Horsepower is horsepower and 10.3 amps 3 phase is equal to 18 amps single phase (approximately). So in your case you would need the 18 amps for the 3 HP motor AND some more for the RPC and losses. I am not 100% sure, but I would be very surprised if your 20 amp 220 breaker would hold what you are trying to do....
        Robin

        Happily working on my second million Gave up on the first

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        • #19
          It likely WOULD hold for typical lathe work, for two reasons.


          1) (Channeling Bented here)... Most hobby folks do not push their machines to the max, so the actual full load current on the 3 HP motor would not be reached *

          2) The breaker has a time-current rating. More current, less time, when past the actual rating. But it will "hold" a slight overload (10% or so) for essentially all day, depending on the temperature at the breaker panel. A 20% overload might be "held" for a typical time that a "hobbyist" runs a cut. And any between cuts time even if running idle, allows the breaker to cool and recover.

          * Remember, the motor does not and should not pull FLA at idle. I'd expect maybe 40% even 50% of FLA, but never 100%. Maybe there is a motor design that will, but I have never run into it, and it won't be one of the usual 3 or 4 standard designs.
          2801 3147 6749 8779 4900 4900 4900

          Keep eye on ball.
          Hashim Khan


          It's just a box of rain, I don't know who put it there.

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          • #20
            Originally posted by J Tiers View Post
            It likely WOULD hold for typical lathe work, for two reasons.


            1) (Channeling Bented here)... Most hobby folks do not push their machines to the max, so the actual full load current on the 3 HP motor would not be reached *

            2) The breaker has a time-current rating. More current, less time, when past the actual rating. But it will "hold" a slight overload (10% or so) for essentially all day, depending on the temperature at the breaker panel. A 20% overload might be "held" for a typical time that a "hobbyist" runs a cut. And any between cuts time even if running idle, allows the breaker to cool and recover.

            * Remember, the motor does not and should not pull FLA at idle. I'd expect maybe 40% even 50% of FLA, but never 100%. Maybe there is a motor design that will, but I have never run into it, and it won't be one of the usual 3 or 4 standard designs.

            Jerry is right with what he is saying. Because of my background in industry I NEVER design less than absolute maximum. If I did, some idiot would overload something just to screw with me 😂. You may indeed be OK in a home hobby environment!! Good Luck.
            Robin

            Happily working on my second million Gave up on the first

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            • #21
              Oh, I'd NEVER "design" that way, it is a horrible way to design anything, although it likely has been done.

              And I do not ADVOCATE it.... again, the idea sucks boulders through straws...... I'm only saying that "unfortunately" it would likely actually work... for a while.
              2801 3147 6749 8779 4900 4900 4900

              Keep eye on ball.
              Hashim Khan


              It's just a box of rain, I don't know who put it there.

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