How to tell the difference between a start and a run capacitor

  1. cheb Member

    Outer Hebrides
    Specifically on the motor pictured. It came off an air compressor and that's as much as I know about it. The capacitor's wiring looks bad enough that I should probably fit a new one before plugging it in.

    The size of capacitor needed would be helpful too, the internet suggests an order of magnitude between suggested sizes.
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    Memmeddu likes this.
  2. Agroshield Member

    Do you still have the old capacitor? If so, measure its capacitance.

    If it is only dodgy wiring, tidy it up, put it on a wooden bench wearing wellies (you or the bench) and fire it up. A bad cap will not harm the motor, it just will not work very well. It will not start if it is a start cap. but will start and run if you wrap a string around it and jerk it into life, lawnmower style. If it is a run cap., it should start but will not run very well.

    Have a look at:

    That will show you how each is wired: a start cap. will be switched out of circuit once motor reaches synchronous speed. No switching apparatus in the motor probably means it is not a start cap.