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Rotary Phase Converter Question

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  • Rotary Phase Converter Question

    I just put a clamp-on ammeter on each of the 3 wires coming from the RPC to the lathe to see how much current was flowing. Two of the wires read 6 amps and the third wire read 2.2 amps. The 2HP motor nameplate says 6.2 amps at 230 volts. I assume that 6.2 number would be at maximum load, but the lathe was just running and I wasn't doing any cutting. What's the story here? I would think that all 3 wires would be carrying the same amount of current, and it would be lower than the maximum of 6.2. Do I have some kind of problem here or is this the nature of a RPC? By the way, I tested the ammeter on a 75 watt light bulb and it showed 0.6 amps and the calculated value was 0.64 amps so it seems that the ammeter works.


  • #2
    I presume the low value is on the generated leg. You need to add or remove some capacitors to that leg to alter the phase angle to get a better balance. It cannot be balanced perfectly and the phase balance will change with load but you sould be able to do better than that. About 50 mfd per HP IIRC. What does it do under load?
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    • #3
      The clamp-on ammeter can't measure the phase angle between the voltage and current. The motor is an inductive load so voltage, current and power are not a simple relationship like for a light bulb.

      When the lathe motor is just spinning not doing anything the current is at its highest during the part of the AC cycle when the voltage is lower so less power is being consumed than the simple measurement would suggest. Your clamp-on can't measure the real power but your utility electric meter can.

      The simple answer is don't worry about it.

      If this is a homebrew RPC, yes, balancing the output with run capacitors will help, and it may make the lathe motor run quieter and cooler, but it's not a problem to worry about. Evan, I recall a norm of 15 uF per HP per leg for run capacitors, 70 uF per HP for start capacitors.



      • #4
        Add some capacitors to the other two legs, from the generated phase like he says. I think I ended up with about 70 or so per leg on a 5 HP idler motor.
        Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation CNC 4x4 Router


        • #5
          Thanks for the replies. It is a commercial RPC, but I can't remember the brand name.

          The max reading on this leg was 3 amps when cutting, and that was at the point of dragging the motor speed down by taking a very heavy cut.

          I'm not going to fool with it, would like to replace it with a VFD.

          On the subject of VFD's, most of the ones I've seen are constant torque. As the speed goes down (below 60 HZ) the torque remains constant, but the HP drops. I've seen references to constant HP VFD's but have not seen any graphs as to what happens with the torque in this instance. I don't know much about motors and electricity, so I'm trying to get an education. One paper I read said a constant HP VFD should be used on a lathe, but from what I've seen on this forum and others, most people are using the constant torque type. Any help would be appreciated.