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Old 09-10-2011, 02:10 PM   #1
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Air Compressor Capacitor Question


Bought a refurb air compressor. 60 gallon, 220volt. The generous 30 day warranty had expired prior to getting a chance to connect it. Wouldn't run. Discovered one of the capacitors was missing. Took the existing capacitor and went to Dayton and bought a matching unit. Compressor starts and runs, but won't run to recharge the tank. It tries to start, but stalls and the motor kicks out. Won't start again until the motor cools and can be reset. Usually, I will run the compressor till it is full, turn it off, and use the air until the pressure is too low to be useful. Then I'll drain the rest of the air out and start over. It is a workable solution, but a PITA.

The capacitors are 20mf, 330v. I think I may have replaced the start capacitor with a run capacitor. I hope if I replace that one with correct unit it may run properly. What size capacitor would you recommend.

Thanks in advance.

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Old 09-10-2011, 03:27 PM   #2
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Air Compressor Capacitor Question


A 20 f rating would be a run capacitor. The start capacitor would be somewhere around 150 - 400 f.

Without the manufacturers engineering data, there's no way to know for sure what the true rating should be. The best you can do is guess.

My guess is 250 f at 220 volts. But this is just a guess, nothing else.

One clue is the size of the 'hump' that it fits in. The physical dimensions of motor start capacitors has changed very little over the years.

When you get a new capacitor, if the motor growls and buzzes when starting, the capacitor is likely too big. If the capacitor is too small, the motor will have less starting torque. If the capacitor blows up after a few starts, the voltage rating was too low.

Usually, you have to experiment a bit to get it right.

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Old 09-11-2011, 08:20 PM   #3
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Air Compressor Capacitor Question


I went out and got all the information off the label on the motor. It is an AOSmith motor. I found it on their web site, and it appears to need a 161MFD/250v capacitor. Checked the Grainger site, and found one for about $12. It is about 30 miles to the nearest Grainger store, so I'll go get one tomorrow and see if that works.

I'll post the results after the new capacitor is in place.
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Old 10-19-2011, 11:04 AM   #4
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Air Compressor Capacitor Question


I bought the 161mf capacitor and installed in instead of the 20mf that was there and it works great now. No hesitation on start up or when it goes to recharge. This was an incredibly easy fix, and I learned more about electric motors. I am now very happy with my compressor.
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