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Thread: 3 phase motors

  1. #1
    Associate Engineer
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    3 phase motors

    ipt>
    can be run on single phase 230v, using 2 of the 3 windings. Is there anyway to use all 3 windings? Is there a formula to figure the aprox HP of a 3 ph mtr powered by 230v single phase?

  2. #2
    Technical Fellow
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    Quote Originally Posted by paulmars View Post
    Is there anyway to use all 3 windings?
    Yes: I have a lot of three phase machinery here with the biggest having a 5hp motor. I use a converter that I built using a timed-drop-relay, a bunch of caps and a 10HP three phase motor. It produces perfect three phase power from 1Ph, 230v. The converter motor needs to be about double the HP of the largest motor you need to drive. It does not consume very much power even though it is rated at 10HP, but it is under virtually no load.

    I bought the motor and caps from a surplus place for 35-bucks. New bearings cost $20 and I machined the shaft off at the case end while I had it in pieces. A couple of boxes to house the caps and I have solid reliable three phase. I use a remote push button for start and stop before starting the three-phase motor I need to drive.

    It took a little trial an error to balance the phases by adjusting cap-values but they are now all within 5v. There are many circuits and instructions available online. There is also non-motor-based phase converter boxes available for under $100, but I burnt up three of them before I finally built this motor-based one and have never had a problem since then.

    I am not sure running just two of the three phases is a wise idea and I am sure HP would suffer.
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    Last edited by PinkertonD; 08-27-2011 at 10:03 AM.

  3. #3
    Associate Engineer Nashty's Avatar
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    Sounds cool!

    Quote Originally Posted by PinkertonD View Post
    Yes: I have a lot of three phase machinery here with the biggest having a 5hp motor. I use a converter that I built using a timed-drop-relay, a bunch of caps and a 10HP three phase motor. It produces perfect three phase power from 1Ph, 230v. The converter motor needs to be about double the HP of the largest motor you need to drive. It does not consume very much power even though it is rated at 10HP, but it is under virtually no load.
    I had a 3 phase air compressor from my old shop that I considered running on single phase power. Is your converter more cost effective (including the time to do this) than just replacing the motor? Also, what is the loss in efficiency?

    Really curious about this.

  4. #4
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    Hi Nashty,
    If you have just one motor to replace then that would probably be the best idea. It takes some messing around with cap-values to get the three legs balanced with the windings of the idler motor. I have five 3-ph machines here so it made sense to build the converter. Since you have to buy a motor to make the converter, you may as well buy a 220v 1-ph for the compressor.

    Two of the other main assets of the 3-ph motors are almost instant stop with momentary reversal, and simple speed control. Since you are using it on a compressor, there is no advantage in building the converter, but a fun project none the less. Especially if you mismatch the caps badly at the start (always guess) as the fireworks can be interesting.

    There is no loss in efficiency once the caps are balanced well, as the output is true 3-ph. The trade off is a little more power usage running the idler, but at a guess that is below 1hp for the 10hp-rated idler.
    Last edited by PinkertonD; 04-23-2012 at 12:02 PM. Reason: efficiency comment

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