Replacement capacitor for a motor

kudzu9February 19, 2011

My band saw has a 5 hp motor that runs on 220V. It recently had trouble starting up and I determined that the capacitor that is mounted externally on it was bad (it was cracked and had a hard, foam-like material that was protruding from the crack). The plate on the motor calls for a 40 microfarad capacitor, but I'm having trouble finding one with precisely that spec. I can find ones that are rated at 45-50 microfarads and 33-37 microfarads, but not one whose range covers 40. I've been given conflicting advice: 1) it's ok to go with a slightly higher capacitance, but going lower than the spec could cause problems, and 2) it's ok to go with a slightly lower capacitance, but one that's above spec will possibly damage the motor. Which is it?

While I continue to search for the right capacitor, I'd like to hear from folks who can clarify the issues and potential problems when using a capacitor whose spec doesn't quite match the name plate rating.

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Slightly higher is fine, and it will not damage eh motor.

All the capacitor is doing is providing some phase shift so the start winding can develop torque to start the motor (it actually creates a sec ond phase to allow starting torque).

Larger value capacitors cost more, so manufacturers put in the lowest value they can.

You also need to watch the voltage rating.

The capacitor must withstand the maximum line voltage (120 V times sqrt(2)+ at least 10%) not the nominal RMS value (120 V) and still have margin for long life.

Larger values also may have a larger size.

    Bookmark   February 19, 2011 at 9:12AM
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The actual value of electrolytic caps is often less than the marked value. So when in doubt, go to the next largest value. The alternative would be to try to put two caps in parallel, but I don't think the available values are far enough off that you need to go to that amount of trouble.

For a 220V motor I'd try to find a 400V cap.

    Bookmark   February 19, 2011 at 12:10PM
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Thanks for the input guys. That all makes sense to me. It looks like the capacitor I can get with the slightly higher capacitance has adequate voltage specs. And the physical size won't be a problem because the capacitor is held on to the outside of the motor housing with adjustable clamps.

    Bookmark   February 19, 2011 at 1:12PM
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