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Motor start capacitor?

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Old 10-17-2008, 04:38:58 PM
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krawlr krawlr is offline
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Default Motor start capacitor?

I just rebuilt my friends oil-less air compressor pump. The "rings" were completely shot so there wasn't much load on start-up anymore. When I got it back together it couldn't start with more than 20psi in the tank.

With some help from Google I got familiar with the parts of the system. I tested the cap with the ohmeter method and it passed. So then I pulled the rear housing and cleaned and inspected the switch mechanism finding no problems. One last check after it was back together gave the same results.

I understand the ohmeter method of checking the cap isn't the best. Is it possible the cap is still bad yet passes the ohmeter test?

I really hate the thought of getting a motor for this thing. I would have scrapped it if I had known the motor had issues too.

Oh, and the relief valve in the control is working correctly.
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Old 10-17-2008, 06:16:27 PM
Tom Kilgore Tom Kilgore is offline
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Arrow Re: Motor start capacitor ?????

you might try a unloader switch valve which would let the motor come to speed before pumping air
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Old 10-17-2008, 07:22:38 PM
Dave R Dave R is offline
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Default Re: Motor start capacitor ?????

The ohm meter method is mostly good for finding capacitors that are very very bad.There is such a thing as a capacitor tester but they are not common. If I were pretty shure everything else was ok then i would try another capacitor. If I dont have one on hand I have been known to borrow a known good one from some other motor for a test. W W Graingers has them.
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Old 10-17-2008, 07:52:18 PM
Jack Hottel Jack Hottel is offline
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Default Re: Motor start capacitor ?????

Substitution is the best way to check for a capacitor that has lost capacity. I worked on a compressor for a friend. It would start with the tank empty but would stall before any pressure showed on the gauge. Substituted another run capacitor and it pumped up and turned off ~130 PSI. It will start and recharge as necessary so I concluded that the run capacitor (30MF, 330VAC) is at fault. Now the problem is to find one in the needed form factor to fit the housing. All the replacements seem to be too fat, or are not cylindrical.
In your case substitute for the start cap and see what happens. Good luck.
Jack Hottel
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Old 10-17-2008, 08:40:02 PM
JHFoster JHFoster is offline
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Default Re: Motor start capacitor?

I test them with a meter that will read MF. +/-10% is considered good.
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Old 10-17-2008, 09:09:46 PM
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Default Re: Motor start capacitor?

Thanks for the replys! I'm going to go ahead and order one from McMaster-Carr. It's under $12 and I have a few other things to get from them anyway.

I didn't see or smell anything to indicate the windings had ever been too hot and the switch contacts for the cap looked fine. So I was pretty sure it was the cap. Just wasn't sure about the possible ways it could fail.

It does have the unloader valve, I just called it the wrong thing.

Thanks again!
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Old 10-18-2008, 01:42:48 AM
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ListerDiesel ListerDiesel is offline
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Default Re: Motor start capacitor?

Capacitors dry out over time and don't conduct power in the same way as when new.

Replacing is the easiest cure, get the same working voltage and capacitance as the original, make sure you don't get a polarised electrolytic, it should be non-polarised.

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