air compressor


Old 09-26-01, 02:42 PM
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Join Date: May 2001
Location: North Alabama, USA
Posts: 487
Puma 2 hp induction motor on compressor. Two capacitors. One start capcitor and one that says MPP. CAP 250V 40uF. When I try to ohm the latter, I get no reading. Have had this compressor a few years and it would never run on an extension. I just thought the cord may not have been heavy enough and was losing amperage. I have always ran it on 110 voltage. Noe whwen I plug it up, it blows the circuit. What is the purpose of the MPP. cap with the red and blue wires? A run capacitor? Hard start? Included in case I wanted to wire it 220? Since I get no reading from it, I assume it is bad, but wondering if the windings have gone bad and caused it to go bad.

P.S. the start cap. has a red and a blue wire coming from one side with the blue going to the other cap. and the red going to the windings, I think.. Then it has a grey wire going to the centrifical switch/clutch. The red on the second cap. goes to the centrifical clutch switch ( I think). Can anyone tell me how to ohm this motor to see what is actually wrong with it?
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Old 09-28-01, 03:00 AM
Join Date: Jul 2001
Posts: 186
There are oodles of different induction motor types, but I think what you have is a capacitor run motor. This type motor has an oil filled capacitor connected to an auxiliary winding. In sizes above 1/2 hp or so there is frequently a second, larger electrolytic capacitor in parallel with it for starting. The starting capacitor is disconnected from the motor by the centrifugal switch after reaching running speed. The running capacitor is normally an oil filled type (mostly oval) and has lower capacitance than the starting capacitor (mostly round). Unless you have a lot of experience at it you probably can't tell much about a capacitor with just an ohmeter. You need to be able to read the capacitance. But in general the sizes you are talking about should read a fairly low resistance that gradually increases to a fairly high value. It sounds like you tried ohmming the running capacitor and didn't get the low reading you would expect. The electrolytic start capacitors have a tendency to dry out with time, so it's a likely candidate for replacement. Replacing both capacitors should get you back in business and shouldn't cost more than 20 or 30 bucks. If you run the compressor on an extension cord make sure it's a large gauge wire, no smaller than no. 12, and stay away from the longer cords. I would say 25 feet or less. Air hose is cheaper by the way. The NEC table shows about 24 amps for a 2 hp 110 volt motor, but I'm sure that's not what your motor nameplate says. But it's still a lot for a 120 volt circuit.
Old 10-01-01, 06:34 AM
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I agree. Air hose is cheaper. Don't run a compressor on an extension cord if you don't have to.

100' hoses are pretty cheap. You can pick them up for under 20 bucks. The motor of the compressor will last longer without an extension cord.

When I wire up my compressor in the garage, I will have two outlets. One in place by the compressor and one by the edge of the garage. This way I can move the compressor if I'm working outside on a nice day .


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