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Thread: Run Capacitor - What are effects of different capacitance?

  1. Back To Top    #1

    Join Date
    May 2010
    Atlanta, GA

    Run Capacitor - What are effects of different capacitance?

    Question for the electricians:

    I have a single speed pump with a run capacitor rated at 30 uF, 370 vac.

    The capacitor burned out last week and the only local replacement I could find on short notice was 35 uF, 370 vac. Since I was leaving out of town for a week, I installed it. Pump runs fine but got me to wondering what the long term effects are; different current draw, different power output?

    In my case, I'm guessing a 5 uF difference is within tolerence but I am wondering what motor properties affect the capacitance the mfg chooses to use. In other words, what would be the effect of changing to a significantly higher or lower capacitance? I am not considering doing this, just curious.
    24000 gal vinyl IG; CPSC36 SWG, TA-60 Sand Filter

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: Run Capacitor - What are effects of different capacitan

    The run capacitor is used to shift the phase of the current so it can be used to power a second set of windings. If the capacitor is the wrong size the phase shift will be larger or smaller than intended and the second phase windings will be energized at the wrong time relative to the rotational phase. For small errors this isn't a big deal, but for larger errors it causes noise, hesitation, increased energy usage, overheating, and/or failure to start.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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