Compressor capacitor question

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Compressor capacitor question

Post by Dieselrider » Sat May 16, 2020 9:29 pm

Could use some advise, please. Was running a rotary pneumatic die grinder today with a sanding disc on it. Heard a pop. Thought something had blown off the desk in the shop. Couple minutes later I noticed my air sander was slowing down. Thought I had tripped the reset on the compressor. Opened the door to where the compressor sets and saw the motor on fire. Grabbed the fire extinguisher and took care of that. However, looks like the start capacitor blew up (the pop I heard) and caught on fire. Looks like the wires to the capacitor are toast but nothing burned back into the motor housing. Did notice the compressor oil was low (not out but low) Might be what was overheating the capacitor?

I am assuming it is the start capacitor and not the run capacitor as the wires to this one are bigger than the other one. Also from what I am reading the start capacitor for a 5 horse motor should be somewhere in the 750 micrfarad capacity. Does any of this sound correct? The only other damage that I can see is the reset box on top of the motor got scorched and the corner melted but still pretty much intact.

What do you think, get another capacitor and try the motor or scrap the whiole thing out and put a new motor on the compressor? Am I correct on the size of capacitor?

Any thoughts?


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Re: Compressor capacitor question

Post by PHPaul » Sun May 17, 2020 5:53 am

First - If you have a motor shop in the area, it'd be best to have them check out your motor for issues. Two possibilities come to mind: A shorted or partially shorted winding or a stuck start switch that isn't snapping over to run position. If it's running on the start windings all the time, I could see that being the issue.

However, being the shadetree mechanic sort, yeah - I'd put a new start capacitor on it and give it a whirl. Worst that can happen is it won't work.

Just cut back the wires to a clean, undamaged area and splice in new connections with the same gauge wire. Insulate the connections well using rubber tape and heat shrink.

Red Dave
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Re: Compressor capacitor question

Post by Red Dave » Sun May 17, 2020 9:59 am

What Paul said ^^^^

It is unlikely low oil in the compressor had anything to do with the capacitor failure.
Capacitors can and do fail. It may just have been a capacitor failure and no other issues, it does happen. Make sure none of the windings are grounded.

If you used a dry chemical extinguisher, make sure you get all the chemical cleaned out of the motor before you try to run it again. It can be very corrosive.

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