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Thread: Running a single phase motor with a VFD

  1. #1

    Running a single phase motor with a VFD

    Can I run a single phase motor with a VFD?

    I have heard that I cannot run a 1-phase motor with a VFD before. But I came across the following controller. It's specs list 1-phase input for the drive.

    http://shop.actechdrives.com/AC-Tech...v751n01sxb.htm

  2. #2
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    NO.
    VFD requires the extra winding in a 3 phase motor to control speed.

    The VFD is also an inverter. I am running single phase 220 into a Westinghouse fm50 and controlling a 3 phase motor on my drill press.

    The VFD can take the place of a rotary phase converter.

  3. I believe that is for running a 3 phase motor from 1 phase power, probably what they mean by 1 phase input.

  4. #4
    Single-phase motors with capacitors DO NOT play nice when installed on output of a VFD. The magic smoke has been seen to escape.

    If you review the manual for your linked VFD, it diagrams both 1-phase & 3-phase input models, but all models of their VFDs are designed for 3-phase output. So, you can run this VFD in a home shop with 120/240V, 1-phase power supply to the VFD. But you'll need to use a 3-phase motor as the prime mover. The VFD converts the 1-phase input into a close approximation of a 3-phase sine wave.
    Last edited by Malcolm McLeod; 07-26-2016 at 2:40 PM.

  5. #5
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    If you run a single phase motor on a VFD, the start winding will cut in whenever the motor slows very much. Very short motor life will result.

  6. #6
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    The good news is you can find loads of three-phase motors at inexpensive prices if you really want a VFD. Most one phase motors use different pulleys to change speeds.

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    Danny, do you have a particular need or just asking for theory?

  8. #8
    You certainly can run a single phase motor on a VFD - if your VFD will support single phase output. There will be some restrictions, however.

    1. You need to start the motor up at 60 Hz. After all, starting the motor from a VFD at 60 Hz is exactly the same as starting it from the mains at 60Hz and your motor will not know the difference.

    2. Once the motor is up to speed, you can slow it down until just above the RPMs where your centrifugal switch would cut in. Actually, I'm not sure this is really required. The starting coil has a capacitor in series with it, and the impedance of the capacitor increases as the frequency decreases. At the frequency that the centrifugal switch would cut in, the current through the capacitor is likely to be small enough to not cause problems for the starting coil. If you run the motor at the speed where the centrifugal switch just cuts in, keep feeling it to see how hot it's getting.

    3. You need to keep the motor cool. Most motors we use have built in fans and those fans are used to cool the motor. You can put an external fan blowing on the motor and that will probably be enough. Actually, this is true for 3 phase motors run off a VFD, also.

    I think a knowledgeable EE posted that you need to overspec your VFD if you're only going to use one phase of the output. I forget how much. If you search on this topic you'll probably find it. This issue has been discussed a number of times previously.

    Mike

    [I found another thread discussing this topic - see here. In that thread, an EE posted that you need to overspec your VFD by the square root of 3 - which is 1.732. So if you have a 1HP motor, you should get a 2HP VFD.]
    Last edited by Mike Henderson; 07-26-2016 at 11:52 PM.
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  9. #9
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    I would comment that the Run capacitors on a single phase motor are meant to run at a small range of frequency, if you move lower, the motor could lose torque and increase heat .. Its not an ideal way to operate a 1 phase motor.

  10. #10
    Thank you all for the replies.

    I found another controller. That'd work on DC too.

    http://www.clrwtr.com/PDF/LEESON/LEESON-DC-Drives.pdf

  11. #11
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    What are you trying to control? We can help you pick a motor and controller that will work for your application. That controller is only for DC motors. a favorite place to get dc gear is to gut an old treadmill. 3 phase motors are pretty easy to pick up on Craigslist. my new 2hp cost me $80 on CL.

  12. #12
    Mike,
    Your linked thread covers most of the VFD issues, so I won't repeat them here. But I want to advise using caution for anyone who tries the (electrically) unusual. YouTube is populated by an untold number of examples of people who forgot to ask "How can this go wrong?". Some don't even need ER visits.

    My background is in the application of motor drives and control (not theory - to my sometimes embarrassment). I can't in good conscience advise someone to do something I wouldn't attempt myself, especially if I can't at least offer to stand fire-watch when they apply power and hit the 'go handle'.

    There are lots of ways to save a few bucks and modify your equipment to do something not originally designed. If someone knows the hazards and take steps to mitigate them, then get your inner freak on and try it! Please film it too. I'm not sure how many of these Rube Goldberg efforts will stand the test of time, but progress IS sometimes made. 1-ph Motor on 3-ph VFD may be a maintenance hog?!

    Or perhaps consider that many on SMC will spend double the amount for a marginal gain in reliability...??? Or sometimes, even a friendly voice from customer service? For VFD usage, a 3-phase motor will probably give you a considerable nudge in the reliability. Better yet, inverter-duty 3-ph motors, at least those in the 1->10Hp range, generally have better heat dissipation, so no external fans; no need to check if the start cap is melting; no risk of the centrifugal switch contacts welding shut. Fit-for-purpose. Peace of mind. Enjoy!

    Sorry if this sounds 'scolding' (not my intent), but IMHO the OP didn't sound like Rube Goldberg.

  13. #13
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    Quote Originally Posted by Malcolm McLeod View Post
    Mike,
    Your linked thread covers most of the VFD issues, so I won't repeat them here. But I want to advise using caution for anyone who tries the (electrically) unusual. YouTube is populated by an untold number of examples of people who forgot to ask "How can this go wrong?". Some don't even need ER visits.

    My background is in the application of motor drives and control (not theory - to my sometimes embarrassment). I can't in good conscience advise someone to do something I wouldn't attempt myself, especially if I can't at least offer to stand fire-watch when they apply power and hit the 'go handle'.

    There are lots of ways to save a few bucks and modify your equipment to do something not originally designed. If someone knows the hazards and take steps to mitigate them, then get your inner freak on and try it! Please film it too. I'm not sure how many of these Rube Goldberg efforts will stand the test of time, but progress IS sometimes made. 1-ph Motor on 3-ph VFD may be a maintenance hog?!

    Or perhaps consider that many on SMC will spend double the amount for a marginal gain in reliability...??? Or sometimes, even a friendly voice from customer service? For VFD usage, a 3-phase motor will probably give you a considerable nudge in the reliability. Better yet, inverter-duty 3-ph motors, at least those in the 1->10Hp range, generally have better heat dissipation, so no external fans; no need to check if the start cap is melting; no risk of the centrifugal switch contacts welding shut. Fit-for-purpose. Peace of mind. Enjoy!

    Sorry if this sounds 'scolding' (not my intent), but IMHO the OP didn't sound like Rube Goldberg.
    What he said!

  14. #14
    Quote Originally Posted by Malcolm McLeod View Post
    Mike,
    Your linked thread covers most of the VFD issues, so I won't repeat them here. But I want to advise using caution for anyone who tries the (electrically) unusual. YouTube is populated by an untold number of examples of people who forgot to ask "How can this go wrong?". Some don't even need ER visits.

    My background is in the application of motor drives and control (not theory - to my sometimes embarrassment). I can't in good conscience advise someone to do something I wouldn't attempt myself, especially if I can't at least offer to stand fire-watch when they apply power and hit the 'go handle'.

    There are lots of ways to save a few bucks and modify your equipment to do something not originally designed. If someone knows the hazards and take steps to mitigate them, then get your inner freak on and try it! Please film it too. I'm not sure how many of these Rube Goldberg efforts will stand the test of time, but progress IS sometimes made. 1-ph Motor on 3-ph VFD may be a maintenance hog?!

    Or perhaps consider that many on SMC will spend double the amount for a marginal gain in reliability...??? Or sometimes, even a friendly voice from customer service? For VFD usage, a 3-phase motor will probably give you a considerable nudge in the reliability. Better yet, inverter-duty 3-ph motors, at least those in the 1->10Hp range, generally have better heat dissipation, so no external fans; no need to check if the start cap is melting; no risk of the centrifugal switch contacts welding shut. Fit-for-purpose. Peace of mind. Enjoy!

    Sorry if this sounds 'scolding' (not my intent), but IMHO the OP didn't sound like Rube Goldberg.
    I'm sorry, Malcolm, but I don't really understand what you're saying in your post. If you know of some specific problems with operating a single phase motor on a VFD, please posts the problems. But your comments are so general that I cannot apply them to this specific situation. And yet, your comments are so general that it could be applied to just about any situation.

    I do not see a problem with operating a single phase motor on a VFD if the VFD will output single phase power (given the limitations I specified). If you have some specific comments regarding such operation, please let us know.

    To me, the biggest limitation is the requirement that the motor must be started a rated RPM. If you had a lathe, for example, that would be a significant problem.

    Mike

    [Okay, let me be specific:

    1. Do you think there's any problem with running a single phase motor on a VFD with single phase output at rated Hz? After all, the output of the VFD should be essentially identical to the mains voltage and current. The motor should not know the difference.

    2. Do you think there's something wrong with then reducing the Hz of the output of the VFD (for this question, let's specify that the speed is above the RPM where the centrifugal switch falls back)? That is, slowing the motor down. We do that with 3 phase motors all the time. If you think there's a problem, why would there be a problem with a single phase motor and not with a 3 phase motor?

    3. If you're okay with #2, do you think that it would be a problem to slow the motor down below the RPM where the centrifugal switch falls back? If so, why?

    You make reference to the to the starting capacitor "melting" or the centrifugal switch contacts "welding" shut. Please explain under what conditions this might occur.

    I can deal with specific issues and questions, but I cannot deal with nebulous doom and gloom.]
    Last edited by Mike Henderson; 07-27-2016 at 6:21 PM.
    Go into the world and do well. But more importantly, go into the world and do good.

  15. #15
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    Mike, I am no expert but would think that raising the Hz on a single phase motor would be okay, it would spin faster and use more current, but with a cap run - cap start motor, wouldn't lowering the RPM cause the voltage to rise and the heat to rise ? Much more than than the reduction of Hz ?

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