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Thread: 220V single phase to single phase VFD

  1. #1
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    220V single phase to single phase VFD

    One advantage I've seen with my single phase 220V to three phase 220V VFD is variable speed. I'd like the same convenience with a single phase 220V machine, but the maker of the VFD I've been using has told me they do not support single phase output with their products. Does such a VFD exist that will not convert phase, pass through 220V and permit altering the motor speed by altering the frequency?

  2. #2
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    running variable frequency on a single phase motor requires a motor without capacitor start, start switch or start winding. IE, a single phase motor built only for running on a VFD. found pretty much only in specialized industrial applications.

  3. #3
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    I've done some extensive searching myself for this & it just doesn't seem to exist. The only single phase motors that you can run with a VFD are, as Adam said, ones that don't use a start winding. Motors that meet this criteria are small & don't get larger than fractional HP.

  4. #4
    As Adam pointed out, there are special considerations for running a standard single phase induction motor on a single phase VFD - and, yes, I have seen single phase output VFDs but I can't point you to one right now.

    The problem with running a standard single phase motor on a VFD has to do with the starting circuit and the centrifugal switch. So the first consideration is that you'd have to start the motor at 60hz. The starting capacitor is sized to pass sufficient current for starting the motor at 60hz. If you try to start significantly below 60hz the motor may not start. You could test and see how low in hertz you could go before the motor wouldn't start. It will be load dependant.

    Then, as you slow the RPMs down, you could go below the RPM where the centrifugal switch cuts in and the starting circuit would be engaged. That may or may not cause problems. The starting coil is in series with the starting capacitor and at the lower hertz it might not pass enough current to cause a problem (excessive current and heat).

    There's a way to solve that problem and that's to put a switch in series with the starting circuit. The starting capacitor is usually on the outside of the motor so you could insert a switch in the path there. The switch should be momentary closed, normally open.

    So the starting sequence would be as follows. Set the VFD to 60hz and depress your momentary closed starting switch. Engage power - this will start your motor and bring it up to rated RPM. Release your starting switch and begin to reduce your hertz to slow the motor down. If you go below the engagement RPM for the centrifugal switch it will not energize the starting circuit because your switch is now open. You can adjust and change the RPMs of the motor without any problems.

    One other problem, common to all VFD motor operation, is cooling the motor. As the motor slows down, the fan in the motor will move less air. If heat becomes a problem, you can put an external fan blowing on the motor.

    Mike

    [The RPM of single phase induction motors is dependant on the input hertz, just like three phase motors. If anyone doubts this, look at the rated RPM for a 60hz motor and the rated RPM for a 50hz motor (single phase or three phase). For two pole motors, the 60 hz motor will be a bit under 3600 RPM and the 50hz motor will be a bit less than 3000 RPM.]

    [There was a discussion of VFDs for single phase motors in the "General Woodworking" forum on Sawmill Creek some time back and it had a pointer to at least one VFD with a single phase output. Finding that thread could be a problem, however.]
    Last edited by Mike Henderson; 07-03-2019 at 7:50 PM.
    Go into the world and do well. But more importantly, go into the world and do good.

  5. #5
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    Thank you all for your responses; especially Mike with all the detail. I'll keep investigating.

  6. #6
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    Cheaper to run them with a SCR speed control or light dimmer. That is what I did with my whole house fan. Max top speed is maybe 70% of original. I lost the two speed option as well. I never ran it at the higher speed anyway, too loud. This fan motor, like most, has no capacitors and no start windings. maybe split phase or shaded pole type?
    Bill D.

  7. #7
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    Might be easiest to swap that 240V single-phase motor for a 3-phase motor of a similar HP.
    Jason

    "Don't get stuck on stupid." --Lt. Gen. Russel Honore


  8. #8
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    I'm actually considering just that.

  9. #9
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    Quote Originally Posted by Jason Roehl View Post
    Might be easiest to swap that 240V single-phase motor for a 3-phase motor of a similar HP.
    I agree 100 percent!!

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