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Thread: capacitor sizing

  1. #1
    Join Date
    Mar 2003
    Location
    Connecticut
    Posts
    81

    capacitor sizing

    How do you determine the proper size capacitor for a motor?
    This this a single phase motor,(frame 6-56-6, code-k) using 208vac@7.5amps and 1725rpm. The motor is dual voltage, low@115vac/15.8amps, uses a single cap for starting and also has a centrifugial switch. There is no old cap to use as a sample.
    Any ideas?
    Thanks Bill

    [ November 26, 2004, 05:06 PM: Message edited by: bill@usps06492 ]

  2. #2
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    17,204

    Re: capacitor sizing

    If you can read all of that information off the nameplate, you should also be able to locate the manufacturer. Have you tried giving them a call? Or is this one of those old motors for which the manufacturer went out of business long ago?
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
    Join Date
    Mar 2003
    Location
    Connecticut
    Posts
    81

    Re: capacitor sizing

    There wasn't any manufactures name on the nameplate, and this is an old motor.

  4. #4
    Join Date
    Feb 2003
    Location
    Portage, Indiana NEC: 2008
    Posts
    9,892

    Re: capacitor sizing

    If you have a motor shop that repairs and rewinds motors they will be able to help you. just take the motor to them and they should be able to tell you the right starting cap for it. I have had to do this a few times. While you have it there have them make sure the motor is in good shape too.

    [ November 28, 2004, 02:00 AM: Message edited by: hurk27 ]
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

  5. #5

    Re: capacitor sizing

    Is there any rule-of-thumb computation for this problem?

    Do you really have to bring the motor to the repair shop?

  6. #6
    Join Date
    Sep 2004
    Posts
    1

    Re: capacitor sizing

    Most motors of that size use a capacitor in the range of 100-200 microfarads.

    I know of one way to find the correct capacitance, but it requires that you already have a capacitor on hand to begin the tests with.

    You need to have access to both capacitor leads as well as the two leads for the start winding. Next lock the rotor in a vice and connect volt meters across the capacitor and start winding. Briefly apply power to the motor and record the voltages. When a correct capacitor is found the voltage across the capacitor should be 10% higher than the voltage across the start winding.

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