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Three Phase Motor takes too much Current

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  • #16
    You might try altering the phase connections ( A-B, B-C, C-A ), to see if the high current still occurs on the same motor lead. In that case, the motor is OK and the RPC is at fault. You might try fiddling the compensation capacitors at normal load.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

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    • #17
      RPC is fine.

      High current is on two of the legs and they are equal. The third is low as the voltage is low, so it's nothing to be suprised at there.
      21" Royersford Excelsior CamelBack Drillpress Restoration
      1943 Sidney 16x54 Lathe Restoration

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      • #18
        Originally posted by The Metal Butcher View Post
        Interesting. I did not know that the current could be spiked over nameplate. Well I got some free brake rotors, I will turn them to dust and see if I can't stall it out. Results... possibly later today. Stay tuned.
        ‚Äč
        The way that works is:

        The motor resistance is far less than what would draw nameplate current. The motor, when running, develops a reverse voltage ("back EMF") which opposes the input voltage. This happens because the stator current induces current in the rotor, and the rotor current produces a magnetic field. Spinning that magnetic field generates the back EMF.

        So, that back EMF is dependent on RPM. Slower means less of it. When running with light load, the back EMF comes up to nearly mains voltage, with the difference being just enough to allow the idle current to flow through the motor resistance.

        If you apply a sudden load, the motor slows down, and the back EMF drops. That allows more current to flow, which produces more torque. The added torque allows the rotor to speed up, and the voltage will balance at a new current that provides just enough torque to spin the new load.

        If the "load" is just inertial, requiring to speed up added mass that has little added friction etc, then the motor will draw more current and speed up until the speed returns to near the previous speed, and will hold at whatever speed allows just enough added current/torque to run the new load. But while it is accelerating the inertial load, it will draw quite a bit more current as it needs a lot of torque to speed up the added mass.
        4357 2773 5647 3671 3645 0087 1276

        CNC machines only go through the motions

        "There's no pleasing these serpents"......Lewis Carroll

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        • #19
          Originally posted by The Metal Butcher View Post
          RPC is fine.

          High current is on two of the legs and they are equal. The third is low as the voltage is low, so it's nothing to be suprised at there.
          How are you measuring voltage? It should be phase-to-phase, or A-B, B-C, and C-A. Assuming the manufactured leg is B, and that current is low, and voltage is low on B-C, that means that the manufactured leg B is collapsing toward the C leg, and its phase with respect to A-C will be far from the 60o that it should be. This is where a proper three phase power analysis meter is useful, being able to read all three phase currents, voltages, power, and phase angle.
          http://pauleschoen.com/pix/PM08_P76_P54.png
          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030

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          • #20
            I have had similar experiences with running 5hp motors on a 7.5hp rpc. The 5hp Baldor 2 pole motor on the table saw runs great with good phase to phase balance. The Reliance 5hp 6 pole motor on the Sidney lathe has low voltage (200v) on one phase and vibrates somewhat ( I attribute that to the electrical imbalance ). 3hp (both older GE Triclad) motors on a mill and a radial drill press run well balanced and reverse instantly. I have found that most commercial rpc manfacturers recommend a minimum 10hp converter to run a 5hp load. I am looking for a 10hp motor to build a new rpc.

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            • #21
              The case of the excessive pixies has been cracked by Detective Tiers and others!

              You are correct. My aggressive use of the clutch was slowing the motor more than I thought. I attempted to bog it down to get some new current readings, a feat more more difficult than I thought. I couldn't read the current at the lathe (safety) while operating it, so I measured current at the RPC, but isolating the lathe alone this time. It was hard enough to get the clamp on one wire, so I only tested one of the real legs.
              1. Idle current was 10.5 amps. Not running the spindle. I had measured 15 before at the lathe. I attribute the drop to 3 things: Digital meter that is probably more accurate, upgrade to 12 gauge wire from 14, measured at the RPC side instead of at the lathe so voltage hasn't had a chance to drop yet.
              2. Spindle running: 11.3
              3. Light cut: 11.3
              4. Medium cut: 15.5
              5. Heavy cut: 17.6
              So, eh, that's within reason. I'm sure the generated leg was an amp or two lower than the reals. My machining calculator I made says that the last cut was a 5.06HP cut assuming the machine is 85% efficient. The details, should anyone want to calculate it themselves: Cast iron brake rotor, ~9" OD (eyeball, forgot to measure), facing cut, 0.1" stepover, 0.01" radial feed, 238 RPM. K factor used is 1.56.

              Tl;dr, other than being a noisy POC, low efficiency motor, it's fine, and it was just me, and undersized wire thinking it was not.

              Thanks all!
              21" Royersford Excelsior CamelBack Drillpress Restoration
              1943 Sidney 16x54 Lathe Restoration

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              • #22
                Originally posted by The Metal Butcher View Post
                The case of the excessive pixies has been cracked by Detective Tiers and others!

                Tl;dr, other than being a noisy POC, low efficiency motor, it's fine, and it was just me, and undersized wire thinking it was not.

                Thanks all!
                When in doubt, over-engineer the >bleeep< out of it I used 10/3 wire on my big Black & Decker grinder....
                (my old boss was noted for his over-engineering.... it actually made sense after a few yrs)
                Glad you got it sorted!
                Last edited by nickel-city-fab; 03-18-2021, 11:35 PM.
                25 miles north of Buffalo NY, USA

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                • #23
                  Good that you got that figured out!

                  4357 2773 5647 3671 3645 0087 1276

                  CNC machines only go through the motions

                  "There's no pleasing these serpents"......Lewis Carroll

                  Comment

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