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Thread: motor run or start capacitor

  1. #1
    Join Date
    Aug 2007
    Palm Bay, FL

    motor run or start capacitor

    I haven't had much experience with motors over the years so I need help to identify a couple of motor capacitors that tested bad. How can I tell if it is a run or start capacitor? Is ithe mfd rating what gives it away? I have a a 10 mfd/370 VAC cap and a 10 mfd/330 VAC cap
    *Sometimes I wonder why the frisbee keeps getting bigger...and then it hits me!

  2. #2
    Join Date
    Jun 2003
    NE (9.1 miles @5.07 Degrees from Winged Horses)
    Not always, but usually a start cap is non-metallic and of higher mfd. Runs are metallic. The runs will bulge on the ends as the fail. Starts usually blow the ends out or vent thru a small hole.

    I would hazard a guess that yours are runs.

  3. #3
    Join Date
    Nov 2007
    East of 480sparky
    Two basic types are used in electric motor:
    1) Run capacitors are rated in a range of 3-70 microfarad (mfd). Run capacitors are also rated by voltage classification. The voltage classifications are 370V and 440V. Capacitors with ratings above 70 microfarad (mfd) are starting capacitors. Run capacitors are designed for continuous duty, and are energized the entire time the motor is running. Single phase electric motors need a capacitor to energize a second phase winding. This is why sizing is so critical. If the wrong run capacitor is installed, the motor will not have an even magnetic field. This will cause the rotor to hesitate at those spots that are uneven. This hesitation will cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat.

    2) Starting capacitors are housed in a black plastic case and have a mfd range as opposed to a specific mfd rating on run capacitors. Start capacitors (ratings of 70 microfared or higher) have three voltage classifications: 125V, 250V, and 330V. Examples would be a 35 mfd at 370V run capacitor and an 88-108 mfd at 250V start capacitor. Start capacitors increase motor starting torque and allow a motor to be cycled on and off rapidly. Start capacitors are designed for momentary use. Start capacitors stay energized long enough to rapidly bring the motor to 3/4 of full speed and are then taken out of the circuit.
    from here;
    Why is it, when it comes to the NEC, the obvious is never simple and the simple is never obvious?

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