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RPC and Flywheel take two thanks to off topic posts

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  • RPC and Flywheel take two thanks to off topic posts

    I performed a number of experiments on my 4 ton scroll compressor.

    it works great as an RPC, similar in impedance to a 5-6 hp 3 phase motor.
    The run winding is connected across the 120/240v line and the auxiliary winding is connected to the neutral.
    A 50uF capacitor is connected from the other end of the aux winding to the 120v line.

    It self starts with the 50uF capacitor, and open circuit is 240 volts across the auxiliary winding with 4.5 amps flowing through the neutral (due to the 50uf cap). voltage across the capacitor is 270 vac, voltage from the third leg to the other 120v line is 265 when the mains are at 250 volts. so the open circuit 3 phase voltage is 250/265/270 volts.

    The input current is 9.6 amps, and 11.2 amps. (with the capacitor removed these would both be about 16 amps, and the auxiliary coil would generate 225 volts instead of 240)

    When the rpc is loaded with a 16 ohm resistor connected across the auxiliary coil, the voltage drops from 240 to 207 volts*, 12 amps flow into the resistor or about 2500 watts.

    The input current increases from 9.6 to 18.6 amps, and from 11.2 to 16.7 amps. the current flowing through the neutral decreases from 4.5 to 3.9 because the voltage across the capacitor decreases from 270 to ~240 volts.


    I placed a magnet on the shaft and at no load it takes 34 seconds for the rotor to fall behind 1 revolution. so its running at about 3598 rpm.

    When the load is placed on the rpc the rotor takes 13 seconds to lag behind 1 revolution. or about 5 rpm short of 3600. this is a lot less slip than i expected. i can repeat the test again with a heavier load later.

    *(207 is what it should be for 240/240/240/208 3 phase)
    Last edited by johansen; 01-24-2020, 08:31 PM.

  • #2
    I repeated the test again

    10 ohm load drops generated leg from 240 volts down to 190 at 19 amps. (3600 watts dumped into a 1500 watt space heater

    amps on line side increase from 10 to 27, and from 11.7 to 25.

    i couldn't get as good a count this time on the revolutions but, on the order of 7 seconds per revolution, or 8-10 rpm short of 3600.

    There appears to be no discernible voltage drop due to the rpm dropping. there is a bit of a ripple in the first 5 cycles, but after 5 cycles or 80 milliseconds, the voltage is stable and appears to have no fluctuation. so the first two cycles might be slightly higher voltage with the third cycle slightly lower. (need a better than 8 bit oscope to measure this)

    the rpm drop is fairly quick, within 18 line cycles or 1/4th of a second the magnet is 45 degrees behind where it was at initially, which would correspond to one revolution every 2.3 seconds, or 26 rpm. i know it isn't that much slower so i think the current drawn is influencing my coil pickup on the magnet.

    Comment


    • #3
      Maybe.....

      Most two pole motors are rated around 3450 at full load (US). The output on the generated leg would be, at full load amps, 1/3 of full power, so the drop in rpm which would be 150 rpm at full load, might be no more than say 50 rpm at full output on the generated leg.

      Back EMF starts at nearly line voltage at no load, and drops as the motor slows. You can proportion it, but the drop is faster than the speed indicates, since the induced rotor voltage is hit, AND the re-induced stator voltage is hot, and rotor affects the voltage induced on stator. Something close to double the drop expected from speed. On a DC motor, with constant field, it would be straight proportion. With the "field" (rotor) dropping also, it is squared.

      Your voltage drop is largeish, 240 to 190, and is likely to be a combination of impedance and back EMF drops.

      The expected 50 rpm drop, out of 3600, would be only about 3V drop. If it is double, then that would be still only maybe 7V drop. The rest will be impedance.

      That begins to show why flywheels do not have a lot of influence. Not "zero", but clearly not enough compared to other effects.
      CNC machines only go through the motions

      Comment


      • #4
        i know how to measure the rpm accurately, it is that high.

        As for the voltage drop, its similar to using my 5 hp high efficiency induction motor in a traditional rpc configuration. (this is a 4 ton single phase scroll compressor with a 24 amp full load rating, which includes the ~6 amps that would be flowing through the auxiliary winding.) instead, 19 amps is flowing out of the auxiliary winding and 25 amps is flowing into the run winding in the recent test.

        needless to say it heats up real quick at such loads.
        Last edited by johansen; 01-25-2020, 02:00 AM.

        Comment


        • #5
          Originally posted by johansen View Post
          i know how to measure the rpm accurately, it is that high.

          As for the voltage drop, its similar to using my 5 hp high efficiency induction motor in a traditional rpc configuration. (this is a 4 ton single phase scroll compressor with a 24 amp full load rating, which includes the ~6 amps that would be flowing through the auxiliary winding.) instead, 19 amps is flowing out of the auxiliary winding and 25 amps is flowing into the run winding in the recent test.

          needless to say it heats up real quick at such loads.
          Yes, no argument. The maybe was about the influence on the tach reading.

          The load when starting a motor, at 7x normal load, would cause a faster reduction than your several seconds..... and as you point out, the speed reduction is not very much. Even the theoretical max speed reduction at full load is not that much. Tends to reduce the expected benefit of a flywheel......

          And the total drop in voltage you noted far exceeds the amount from any sensible speed reduction.

          It would be interesting to read out the rpm of a regular rpc smaller sized, when starting a tough load, and under typical running loads. A 'scope trace of RPM (or voltage) vs time with load motor current vs time, during a startup of a typical and a heavy start load would be interesting.

          Mine has no external shaft, so it is pretty hard to get an rpm reading.
          CNC machines only go through the motions

          Comment

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