Why did these caps fail?

Question

I bought a used air compressor of unknown vintage. I checked everything mechanical, wired it up and it started and ran well. I expected this because I had seen it running before I bought it. Anyway, after several starts over maybe 5 days the motor acted like it was straining to start. Just before I got to the braeker panel the breaker popped. I first checked the pump and it turned over nicely so I took off the capacitor cover on the side of the motor and both caps looked bad. There are two starting caps on this motor connected in parallel. No run caps. One cap failed to the point that one spade lug had fallen off due to heating at the point where it was riveted to the cap. Both caps have vent holes. The cap with the melted off lug had white stuff coming out of the vent. The other cap did too but there was less of it. I know that motor start caps fail over time but I don't know what the fail mechanisms are. This motor did sit idle for several years before I bought it. Do motor start caps need to be "re-formed" like the electrolytics found in electronics? Thanks, Eric

Jon Elson · Accepted Answer

Check that the starting switch is opening when the motor is up to speed. With the start caps disconnected and the belt off (no load) you should be able to start the motor by kicking the pulley and turning the breaker on. Then shut it off, and see if it makes a sharp click-scuff-scuff-scuff when stopping. That will prove the centrifugal mechanism is working, but you probably will have to disassemble the motor to check that the contacts are not welded. You MAY be able to do a continuity check while poking the start switch through a vent hole, depending on the motor construction.

And, of course, it could easily just be bad capacitors, they do go bad eventually.

Jon

Baron · Answer

Jon Elson scribbled thus:

Whilst I agree the centrifugal switch could be sticking or otherwise at fault, those caps have to handle an awful lot of current during start up until the switch opens and disconnects them.

dave · Answer

"Sounds like the goddam Spanish Inquisition!" -Bones Do you have a Grainger catalog or its local equivalent?

etpm · Answer

Greetings Jon, I suppose it could be the starting switch but I doubt it. Every motor I have seen that had a stuck starting switch overheated quickly. This motor has not been running hot. Nevertheless, I'll run the air down until the compressor starts and once the motor is up to speed I'll turn it off and listen for the switch closing. Eric

Why did these caps fail?

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