PSC motor capacitor trouble

Discussion in 'The Projects Forum' started by ericaustin, Jul 22, 2008.

  1. ericaustin

    Thread Starter New Member

    Jul 22, 2008
    I'm working with a 1/12 HP GE PSC motor from a surplus shop. It's wired (evidently) as a PSC motor. No centrifugal switch.

    I measured the resistance between all leads and got:

    Yellow-Red ~ 17 ohms
    Yellow-Blue ~ 9 ohms
    Red-Blue ~ 24 ohms

    This basically told me that:

    Yellow-Blue must be the start winding loop. It's got lower resistance and therefore must be shorter.

    Yellow-Red must be the run winding loop. It's got higher resistance and therefore must be longer.

    Red-Blue must be the loop through both windings.

    So I got the motor to run great with a small start capacitor. It was rated at the right voltage and something like 86-109 MFD. The motor requires 98 MFD.

    I ran the motor for a while (5 mins). It ran smoothly and quietly and at full speed. Great! Then I began to let some of the smoke out of the capacitor.:mad:

    A let things cool off...and then a few more times I let some more smoke out the capacitor. Although I have not let all the smoke out of it I think it might be getting close to empty.

    So I researched this and learned that you can use a start cap as a run cap. Otherwise it will boil and smoke and squirt hot oil out of the plastic I had already seen.

    So I bought a 100 MFD metal can RUN capacitor. Unfortunately however, this cap runs the motor but instead of the cap over heating...the motor quickly overheats and shuts off...(it's thermally ptotected).

    Also with the run cap attached, the motor seems to be bogged down a little bit. It doesn't have that same sweet smooth happy gliding ...almost effortless spin as it did with the start cap. Instead it has a slight grumble, and seems to run just a little bit slower.

    I have my own theories about why this is all happening...but what I really need is expert advice.

    Motor is spec'd to run at 115VAC 50 or 60 Hz single phase at ~ 3400 rpm.

  2. SgtWookie


    Jul 17, 2007
    Most helpful would be to get the model number from the motor's plate. That way you could look up the datasheet for the motor, and find out precisely what it requires.
  3. ericaustin

    Thread Starter New Member

    Jul 22, 2008
    I scoured the earth (via google) for a spec sheet. No such luck. This is most likely long since discontinued item, and I haven't found an exact or close match.

    The good thing is though, that all the specs are stamped right on the side of the motor. except the wiring diagram. But based on the motor's very basic construction, it's easy to see it's a PSC motor.

    The big question here is why does the start cap allow the motor to run very smoothl and quietly while the run cap of the same capacitance causes the motor to overheat and run a little slower and noisy.
  4. ericaustin

    Thread Starter New Member

    Jul 22, 2008
    Actually it says it's a 1/12 (one-twelfth) HP motor. Does this make a difference in your reply?

    I received a 10 MFD run cap from McMaster today. I'll wire it in tonight and run it again and post results tomorrow.

    Does anyone need a 370 V 100 MFD run cap? PN 28F1119. $15 and I'll ship it for free. only used once!