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slowing down 1-phase motor

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  • slowing down 1-phase motor

    i quite often put a vfd on a 1-phase motor, e.g. a a grinder. that usually works fine, except on the drill press. i can only run it down to 35 hz (from 50) until the torque drops off dramatically (presumably due to oversaturation?). how much would it help to change the capacitor? is it worth it?

  • #2
    It may be poor configurations. Check what the vfd is actually delivering to the motor.
    If it is a capacitor run motor, the aux winding loses torque production because the capacitor must incease in uF as the frequency drops.

    If you increase the frequency, you could burn out the motor as well,

    as for properly driving a split capacitor motor, you can use transformers to phase shift the 3p output to properly drive a 2phase motor,x get rid of the capacitor.

    sometimes you only need a 120:60v transformer, or a 120:120v tx to do the job. Othertimes you need those transformers and a few 208:24 or 208:12v to adjust the voltage delivered to the "start" winding.


    • #3
      You presumably are only doing this with "PSC" motors (capacitor motors with no start switch), which are typical on many grinders, and single phase VFDs, which will work with PSC and shaded pole motors, but not with "start switch motors", unless the VFD is drastically over-sized..

      On drill presses (in the US, at least) it is not typical to find a PSC motor, most have "split phase" or capacitor start type motors, both of which have start switches and are not suitable for VFDs. In Europe, more machines use PSC motors.

      The PSC motor loses torque because at low frequencies the capacitor passes less current. Current makes torque in a motor. So, yes, changing the capacitor when at lower frequency (slower speed) would allow the torque to increase. It would be, as mentioned, necessary to have the capacitor increase at low frequency, but reduce again at higher frequency, to avoid overheating the motor.

      Yes, a custom VFD with the correct phase shift would also work well.

      And yes, a transformer system set up for a "Scott connection" (transformation from 3 phase to 2phase) would also do the job when combined with a standard VFD, although it would be somewhat complex. It would, despite being more complex, likely be easier than the other alternatives, and would require nothing in the way of switching of capacitors or the like.

      Both PSC and shaded pole motors will work (with reduced torque) when slowed using plain resistors, variacs, or triac motor controls when they have a speed-variable load, such as a fan. That will not work with most machine tools.
      CNC machines only go through the motions


      • #4
        so is it worth it? if i double the size of the cap, how much more torque could i get at 25 hz? (i would leave the motor in this configuration, the machine has a variable drive.)

        its a two cap mototr, btw.


        • #5
          Two capacitor? Then NOT a "PSC" but has a centrifugal "start switch" with start and run capacitors? Not usually a candidate for a VFD.

          Well, ignoring that issue, you would need to make the run capacitor inversely proportional to frequency, and doing that would allow you to maintain the torque, theoretically. Problem with single phase motors and speed control is that it does not quite work that way.

          Standard single phase motors rely on inertia to "coast through" the times of low/zero torque. Two capacitor types do better, with the run capacitor supplying an added "phase", approximating a two-phase motor, so the torque does at least not go to zero, and may have a "high minimum". (PSC motors also have that)

          However, if your motor does have a start switch, that is your minimum speed, since the switch will cut in at perhaps 70-80% of synchronous speed. That is largely why start switch motors are not generally used with VFDs.
          CNC machines only go through the motions


          • #6
            It might be possible to remove the capacitors and simply connect the third phase from the VFD to the run winding. You should first measure the current in that winding when running normally on single phase, and make sure the current using 3 phase is similar. Otherwise it might need a step-down transformer. The start winding and associated capacitors might not be needed if the starting load is very light.
            Paul , P S Technology, Inc. and MrTibbs
            USA Maryland 21030


            • #7
              lets keep it simple, disregarding centrifugal switch or looking at an one-cap motor. if i double the impendance at 25 hz, will i get: a) double torque, b) considerably less than double torque, c) depends on the motor, d) motor might burn?


              • #8
                You would want to have the impedance in reverse proportion to frequency, leading to same current at 25 Hz as the original has at 50 Hz. That gives same torque. That consideration would mean 2x capacitance at 25 Hz.

                But, since there is also a volts per Hz related change, the capacitor must be chosen to get the proper current at the lower voltage as well. So maybe 4x capacitance. However, that may not give a good phase shift. You will need to compromise for best torque.

                The compromise may not give same torque.

                Basically answer "B & C", but with footnotes!

                Torque may also not be as smooth at lower frequency, because the capacitor is not a perfect phase shifter, and the motor resistance starts to play a part. A VFD that inherently produced the correct phase shift would allow removing the capacitor, and should give the best performance
                CNC machines only go through the motions