Strange capacitor failure.

Discussion in 'General Electronics Chat' started by JimAlex, Dec 10, 2011.

  1. JimAlex

    Thread Starter New Member

    Dec 10, 2011
    I was recently asked to troubleshoot a cap start/cap run motor. It won't start, does not kick overload protection. I didn't have the saw it was from, but the operator said it turned freely but wasn't sure if it was humming when he tried to start it.

    I didn't have a hi-pot (megger) tester, but i did read the continuity of the windings, test the capacitor and the windings of the motor.

    LCR multimeter measurements:
    200 uF cap tested at 212 uF.
    60 uF cap (the run cap) tested at 2,000 uF.

    Single phase 120 VAC motor windings :
    1.20 ohms DC resistance from L to N.
    several Kohms resistance L/N to ground case.

    I didn't take the motor apart, but I assume (since there was no motor data plate on it) it was around a 1 to 1.5 hp single phase motor with a centrifugal switch or potential relay. I know the hi-pot resistance tester may have revealed a shorted winding the multimeter can't see. I would have thought that an open capacitor wouldn't have stopped the motor from starting.

    The run cap tested much higher than it should have, but i've never seen this kind of failure before. There appeared to be nothing wrong with it from a visual inspection; not swollen or bulging. usually, they fail and get hot, boil a little, the dielectric is compromised and the capacitance drops.

    To get to the point, how does the amount of capacitance in the run circuit effect the motor's operation?

  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Welcome to AAC...

    Replace the running cap.
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    If the capacitance had truly increased by 33 times(!) from 60μF to 2000μF, it would be almost as good as a short-circuit, and would likely kill off the torque as it would fail to produce much of a phase shift at all.

    A capacitor expert may be able to explain a mechanism for this really happening (I cannot), but I wonder if the capacitor may actually have some other problem. Depending on exactly what method the meter uses, the indication could for instance be affected by excessive leakage.

    In any case, the capacitor has to be junked.
  4. jimkeith

    Active Member

    Oct 26, 2011
    I would question your capacitance measuring device--it may not read defective (leaky or shorted) capacitors correctly.

    The motor must not have any measurable resistance to the frame--leakage in the K ohms indicates a shorted motor.