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RPC question (amp ratings)

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  • RPC question (amp ratings)

    I have a Dyna-Phase RPC I bought used some time ago. I was looking at the label the other day and I noticed that it says:

    Max 1-phase input: 36 A
    Converter input: 20 A
    Max 3-phase single motor: 10 hp
    Max 3-phase system rating: 30 hp, 80 A

    So.. how can this thing be rated to output 80 A with less than 40 A input? I understand the difference between the single phase input and converter input, but if this thing is wired to a 40A breaker (as is implied by the max input), shouldn't the max output be somewhat less than 40A as well (like ~23A, or 40/1.73)?

    Thanks all!

  • #2
    I am not familiar with a Dyna-Phase RPC, but I have built numerous static and rotary units.
    I have to assume that the key word they use is " system".

    We know that once we have one motor running , it is easier to start other motors in the same "system" (circuit)
    because the motors share the starting loads which makes it easier for the RPC. In fact they become slave RPC's
    So I see the max size motor you can start is 10 HP and for multiple motors , you can have up to 30 HP total running.
    Obviously ( to me ) you cannot have all the motors run at 100 % load as it would far exceed the 36 amp rating.
    As long as the total load is 36 amps or less , the system should be OK
    I have no idea why they put a 80 amp max on the name plate unless they are trying to show a system start max.
    Certainly no running !
    Some nameplates are suspect to say the least
    This is as good as looking at those foreign made air compressors that say 5 HP and yet the label says 1400 Watts ?????

    One HP = 700 Watts ~
    700 Watts/220 Volts= 3.2 Amps ( at 220 v)
    10 HP = 10 x 3.2= 32 amps



    • #3
      Keep in mind that the total 'system' single phase input current includes the current which goes directly to two leads of the 3 phase motors. Their max system rating would indeed require a breaker suitable for the 80A current but only a portion of that actually goes to the RPC. The RPC only supplies current for the 3rd lead and when other motors are running they too contribute to the 3rd phase.
      Don Young


      • #4
        The unit rating is 10HP load.... no problem there.

        10 HP is 28A per the NEC ratings, at 230V 3 phase. That's a per-wire rating....the current you would measure with an amp-clamp on the wire. There is no double talk about adding up the wires or other forms of BS.

        With 36A input, and 28A output, 230V, there should be no issues. The RPC only supplies the 3rd wire, the other two wires are supplied directly.

        Another way to look at it is that a 3HP single phase motor draws 34A per NEC.... actually most motors draw a bit less than the NEC assumptions. With the somewhat less required draw, and the fact that 36A is more than 34A, it isn't hard to figure that 3.3HP might be supplied with that 36A. And that is shaft power output....

        So, if that is 1/3 of the total power, then the total must be 3 x 3.3 = 10 HP. So it adds up correctly as a 10 HP converter, which is what they state.

        The unstated assumption of the 30 HP is that you can have up to 30 HP of motors running, so long as no more than 10 HP worth of them are actually doing work at any time. The others serve as added idlers, basically don't draw any significant power, and can supply extra 3rd wire current if needed.

        The reason for 30 HP total, is that the 10HP RPC can supply the 3rd phase for 10 HP of load, plus 20 HP of idling motors. The 80A deal is a bit "technical", but seems to relate to what the RPC plus 20 HP of idlers can supply, if needed, to the 10HP load. If one can supply 28A/10HP, then 3 total presumably can supply 84A/30 HP.

        Whatever the reasoning used by the manufacturer, it's a relatively common way to rate RPC units. Arco "Roto-Phase" are rated that way, apparently "Dyna-Phase" units are, and I have seen others that I cannot recall also rated that way.

        Keep eye on ball.
        Hashim Khan


        • #5
          Thanks for the help, all. I need to run a VMC with a FLA rating of 55 A and a 12 HP spindle motor.. I was hoping this might run it as a stop gap but it sounds like I might be better off just building a 25 HP converter and doing it right from the beginning.