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Start Capacitor Questions

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  • Start Capacitor Questions

    My dust collector has decided to stop working. Instead of starting up, it just hums then trips the breaker. I'm starting under the assumption that it is the start capacitor. It is a 400mfd 125V cap.

    My questions:

    How should I discharge the capacitor? Is it safe to just use a screwdriver across the terminals? For some reason that just seems like something NOT to do.

    Do start capacitors have polarity?

    Is this an item that I would likely find around town?


  • #2
    Re: Start Capacitor Questions

    I use a piece of wire to jumper across the terminals of the capacitor,hold them on the terminals for 10 seconds or so to make sure it gets drained from charge.
    You should be able to find that capacitor at any motor repair shop or hvac supply house.Maybe Johnstone supply or grainger also.



    • #3
      Re: Start Capacitor Questions

      You should be able to find one most anywhere motors are sold or repaired.
      Yes, you can use a screw driver to discharge it. Are you certain its the cap thats bad?
      Can you help turn the shaft by hand (safely) to get the motor spinning? If its the start cap it will run normally (usually) with some help getting it spinning.
      Licensed Electrician


      • #4
        Re: Start Capacitor Questions

        When using the screw driver, short the terminals several times, the first time doesn't always get all the stored energy.

        There is also a set of start points and a centrifugal switch in that motor. Chances are, the points have something lodged between them. This will keep the motor from starting just like a bad cap. When someone brings me a motor that is used on a table saw or similar, it's usually the points.

        If you have an analog ohm meter, I can tell you how to check the cap.


        • #5
          Re: Start Capacitor Questions

          Replaced the cap and have been up and running. Thanks