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3-phase question

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  • 3-phase question

    I've owned a Emco super 11 for a few years now and while it has always done what I have needed, I've noticed a fairly significant RPM drop when I am in the high-range (1100,2200 rpm) and taking moderate cuts (.040 or so) in aluminum. Maybe this is normal but I was hoping for better.
    The 2-speed drive motor is approx. 1 1/2 hp 3-phase and is connected to a phasematic rotary converter which was pre-wired when I bought the lathe. Could incorrect wiring cause this problem? Do rotary converters get weak? Or am I expecting too much?

  • #2
    using a phase converter to run a 3phase motor will usually only give you around 70% of the rating of the motor anyway.With that said you could have a bad capacitor in your converter making it even worse.


    • #3
      John and Scatter Cat, rotary phase converters will do much better then 70% more like 95% I f I recall correctly on the one I have, 70-75% for static converters is about right. First thing that comes to my mind is to get out the voltage meter and check voltage on all three lines under load as well.

      Edit: Also check that appropriate wire size is being used.

      Those sure are nice looking little lathes, but I cannot help on what to expect from them.

      Last edited by Ken_Shea; 01-08-2009, 08:00 PM.


      • #4
        Thanks for the replies.
        Ken, The voltage across the lines are 250, 250, 235 with lathe motor off.
        With motor on: 240, 240, 225.
        Checking each leg to ground I get: 121, 121, 225.
        The wires from the converter are 10 gauge
        I did find an article that mentioned voltage tolerances and I seem to be right at the limit but it was mainly addressing heat/longevity issues.
        I'll try to locate someone with the same lathe, the RPM loss just may be the nature of the beast.
        Thanks again.
        Last edited by johnc; 01-08-2009, 11:26 PM.


        • #5
          Slowing of an induction motor generally is due to heavy loading.... the "slip" increases.

          heavy can be relative to the voltage. If input voltage drops, current must increase to provide constant power. If current increases drop voltage more, the motor slows more than expected. It looks like yo do have some drop. And it is fairly proportional on all wires, about 10 V. That is approximately 5%, which does affect power noticeably, but not drastically.

          The numbers suggest that the converter is not the basic problem. If it were, the generated leg should drop more than the voltage between the others. The voltage between the supplied legs would be the same, because they are passed through from the incoming power line, unaffected by any converter drops.

          Since the incoming power is also dropping, you probably can't blame the converter for much of the drop.

          It may be the incoming power, or partly the nature of your machine, or your machine may have problems that show up most when in high speed. High speed will load the motor more to begin with. Any abnormal extra loading should presumably show up most in high speed, because the most motor torque is demanded in high speed.

          Keep eye on ball.
          Hashim Khan


          • #6
            It could be the nature John, that motor looks small for a 1 1/2HP.
            My lathe is a 1 1/2 3 phase and it is three times that big.

            You can compensate by using lower rpm's, lighter cuts and faster feeds for roughing and then the higher rpm, slower feed and light cuts for the finish, also very sharp tools, you probably know that already anyway. All machines are the same, they all have limits and we have to learn to use them with in their limits.



            • #7
              John, I lied to you

              My lathe is 3 HP and 3X as large it is the mill that is 1 1/2 hp and it is 2X larger.