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Thread: Motor capacitor size?

  1. #1
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    Default Motor capacitor size?

    My mill has a single-phase motor in the column that powers the table feed. Last weekend it stopped working, or rather it would only hum or start randomly going either way. I stripped it out and found the capacitor has failed. Tried it with another capacitor, but one that is half the size, and it works just fine on the bench.

    I've ordered another 16uf capacitor for the motor but will I do any harm to the motor by running it with a 7.5uf one for now? There's very little load on the motor running the table feed.

    Pete.
    Peter - novice home machinist, modern motorcycle enthusiast.

    Denford Viceroy 280 Synchro (11 x 24)
    Herbert 0V adapted to R8 by 'Sir John'.
    Monarch 10EE 1942

  2. #2
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    Default

    A smaller value capacitor will reduce current through the starting winding, ie, it will be fine, as long as its still able to start up reliably, and you don't let it sit in a stalled state if it fails to start up.

  3. #3
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    Ok thanks for that. This motor has no centrifugal switch, so the start winding is always energised. Is it still ok?
    Peter - novice home machinist, modern motorcycle enthusiast.

    Denford Viceroy 280 Synchro (11 x 24)
    Herbert 0V adapted to R8 by 'Sir John'.
    Monarch 10EE 1942

  4. #4
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    Default

    Quite true a smaller value cap will reduce the current in the start windings, however its main function is to create a phase shift, so the motor effectively becomes 2 phase.
    Running with a smaller cap as moony says will be ok so long as the motor doesnt sit stalled.
    The effect with a smaller cap is that the motor will have less starting torque.
    I bunged a fluorescent light power factor correction cap (any old thing) into my mates lawnmower a couple of years back, and thats still going.
    Last edited by dr pepper; 04-25-2010 at 05:30 AM.
    Build it, bodge it, but dont buy it.

  5. #5
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    Great - thanks for the info guys. The motor does start quite smarlty with the smaller cap so I'll put it in and change it for the one I've ordered next time I have the thing apart.

    Cheers!
    Peter - novice home machinist, modern motorcycle enthusiast.

    Denford Viceroy 280 Synchro (11 x 24)
    Herbert 0V adapted to R8 by 'Sir John'.
    Monarch 10EE 1942

  6. #6
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    Default

    That is a "PSC" motor, and a small PSC may have a cap as low as 3 or 4 microfarads. I have a small Baldor gearmotor that uses 3.3 uF , IIRC. It would be about the power of a feed drive.

    A client has PSC motors on ventilating fans, and their 3/4 HP motor uses only a 10 uF capacitor.

    if you want to know what the *right* value is, it is the value that results in the LEAST motor current when running under load.

  7. #7
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    Thanks Jamie. The plate actually says 16uf, just that 7.5 was the only spare I had. I should have another which I take I could fit in parallel but I'm buggered if I could find it. I've ordered another 16uf now at less than a fiver.
    Peter - novice home machinist, modern motorcycle enthusiast.

    Denford Viceroy 280 Synchro (11 x 24)
    Herbert 0V adapted to R8 by 'Sir John'.
    Monarch 10EE 1942

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