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slowing down single phase motor

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  • #16
    Originally posted by gvasale View Post
    Question then: What's different about a single phase motor that runs at 1125 rpm? Most are 1725 or 3450...
    rpm = 120 * hz / poles so you've got a six pole motor there.

    You could wind a 10 pole or 12 pole motor if you wanted. Of course at some point it becomes simpler or at least more economical to use a gearbox if you want lower RPMs.

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    • #17
      Thanks for the explanation.
      gvasale

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      • #18
        For smaller motors there is also a trade-off of power versus torque versus size. I have three motors of approximately the same physical size and weight, but the two pole is 2 HP, the four pole is 1.5 HP, and the eight pole is 1 HP. I have not found exactly why this is so, but I think it has to do with winding efficiency and overlap of adjacent poles. But another possibility is that higher pole count motors have higher torque and thus the frame and shaft need to be stronger. I have rewound several motors for the highest possible pole count, and for a 24 slot stator it is four poles and for a 36 slot stator it is six poles. I also found that when I reversed one of the phase windings the motor ran at half speed, but with very little torque. I think this was due to the "salient pole" phenomenon, or due to the fact that each pole of the stator has a phase angle and amplitude determined by the vector sum of the phase currents flowing in each adjacent slot.

        BTW, a three phase VFD might be able to run a small single phase motor on just two of its phases, although it might kick out due to sensing phase imbalance. So a very small motor on a large VFD might be OK, especially (perhaps) if you added some load to the remaining phases. Or possibly put a 240/120 autotransformer across A and C, and connect the motor from the center tap to phase B.

        But another possibility might be remove the PSC and run the third phase to that winding. I think the capacitor does not provide a full 90 degree phase shift because of the resistive series element of the winding to which it is connected. It is probably about 80 degrees at maximum torque (at which point the winding voltage will be low), and then closer to 60 degrees, or less, when the load is very light.
        http://pauleschoen.com/pix/PM08_P76_P54.png
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030

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        • #19
          Originally posted by dian View Post
          of course if its a futile attempt, i would like to be told. as well about the torque issue. if its only going to slow down the motor with constant torque, its useless. also the drill press is in a location, where i cant easyly connect it to 3 phase power.
          If, as it sounds, you are looking for torque to increase along with a slower speed, you won't get that by just changing motor speed. At best you could hope for the same torque at the lower speed.

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          • #20
            Originally posted by PStechPaul View Post
            For smaller motors there is also a trade-off of power versus torque versus size. I have three motors of approximately the same physical size and weight, but the two pole is 2 HP, the four pole is 1.5 HP, and the eight pole is 1 HP. I have not found exactly why this is so, but I think it has to do with winding efficiency and overlap of adjacent poles.
            ..............
            But another possibility might be remove the PSC and run the third phase to that winding. I think the capacitor does not provide a full 90 degree phase shift because of the resistive series element of the winding to which it is connected. It is probably about 80 degrees at maximum torque (at which point the winding voltage will be low), and then closer to 60 degrees, or less, when the load is very light.
            The diameter of rotor, combined with the possible current in the pole windings will limit torque.... you might get more torque, but would overheat the windings. Larger diameter makes same pull produce more torque....

            The motor will have it's "physical phase" (winding position) aligned with the electrical phase. Giving it a different electrical phase may not work well
            CNC machines only go through the motions.

            Ideas expressed may be mine, or from anyone else in the universe.
            Not responsible for clerical errors. Or those made by lay people either.
            Number formats and units may be chosen at random depending on what day it is.
            I reserve the right to use a number system with any integer base without prior notice.
            Generalizations are understood to be "often" true, but not true in every case.

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            • #21
              A two pole motor develops torque according to the tangential force exerted by the misalignment (or slip) of the rotor with the rotating field of the stator. This is a vector sum of all the magnetic fields along the gap, and since there is essentially one electromagnet with a N and S pole, the force will be greatest at the center and then diminish to nearly zero at perhaps +/- 45 physical degrees. So you have one effective torque-producing sector that covers perhaps 25% of the circumference (2 * 45 / 360).

              A four pole motor has two pole pairs, each of which have perhaps a +/- 30 physical degrees of magnetic field. But there are two pairs, so with the same current in each pair, the torque may be greater, perhaps 4 * 30 / 360 or 33%. An 8 pole motor might get 8 * 20 / 360 or 44%. The power depends on the speed, so with 50% more torque but half the speed, the power is about 75%, which explains why my 4 pole motor is 1.5 HP compared to 2 HP for the 2 pole.

              But the benefit of a higher pole count is that the drive frequency can be increased, and more power obtained at the same speed. You can double the power of a two pole motor by doubling the current for more torque, but resistive losses are quadrupled, so a 90% efficient (10% losses) motor will have 40% copper losses and 60% efficiency, requiring a 25% duty cycle. But magnetic losses are more like a square root function of frequency, so 10% losses at 60 Hz and 1800 RPM might be 14% at 120 Hz and 3600 RPM.
              http://pauleschoen.com/pix/PM08_P76_P54.png
              Paul , P S Technology, Inc. and MrTibbs
              USA Maryland 21030

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              • #22
                huh:

                "But the benefit of a higher pole count is that the drive frequency can be increased, and more power obtained at the same speed."

                i probably should go to bed.

                if the frequency increases, the speed increases, i thought. but how can that deliver more power (torque) at the same speed?

                or are you saying you can overdrive the motor more as the pole count increases?

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                • #23
                  Huh and huh. All the guy wants is a practical way to vary the motor speed on his drill press/milling machine whatever. Take the single phase motor off, replace with a 3 phase motor, used ones are cheap. Put on a VFD that can take a single phase input and output the 3 phase he needs. Job done. Been there and done that.
                  Retired - Journeyman Refrigeration Pipefitter - Master Electrician

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                  • #24
                    If you run a given motor at double speed, with double voltage, you get double power, IF the motor is good for the volts. In US, a 230/460 motor would be OK for 460 if set for 230 and run 460V/120Hz. Same volts/Hz.

                    If you had a 4 pole motor and did that, it would run at the speed of a 60 Hz 2 pole and have double power for same size..... but that isn't the OP's question.


                    But this does not vary speed, and if you could, you'd have a VFD, so what's the point of the comment about poles and so forth?????
                    CNC machines only go through the motions.

                    Ideas expressed may be mine, or from anyone else in the universe.
                    Not responsible for clerical errors. Or those made by lay people either.
                    Number formats and units may be chosen at random depending on what day it is.
                    I reserve the right to use a number system with any integer base without prior notice.
                    Generalizations are understood to be "often" true, but not true in every case.

                    Comment

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