Air Compressor puzzle

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Craftsman 30 gal. oil less. Attempted to start it one day recently. Motor ran weakly for about 3 seconds and then it tripped a breaker. Tried it on different amp plugs and still always blew the breaker. No air in it at start up. Turns easily by hand. Tried a new capacitor (cheap enough at $9) of the same 30 mf -370 v. Did the same thing. Tried it with no capacitor and it did the same thing. Tested both capacitors and they both react the same on the volt meter with a quick swing and back so I guess it was never the capacitor. It does not have two capacitors, just the one. Why does it do the exact same thing with no capacitor and what clue is this giving me?

 

[Raspy]

That motor may have a starting circuit and a running circuit, I think these are called capacitor start/induction run, look behind the rear cover plate on the back of the motor for a set of points that centrifugally open after about 1 second as the motor starts. This is how swimming pool pump motors work too. See if the points and their mechanism looks OK. They should be closed and contacting at rest and fly open with RPM.

 

[Stimw]

That motor may have a starting circuit and a running circuit, I think these are called capacitor start/induction run, look behind the rear cover plate on the back of the motor for a set of points that centrifugally open after about 1 second as the motor starts. This is how swimming pool pump motors work too. See if the points and their mechanism looks OK. They should be closed and contacting at rest and fly open with RPM.

What he said. If it has the points usually you can clean them with sand paper or points file.

 

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That motor may have a starting circuit and a running circuit, I think these are called capacitor start/induction run, look behind the rear cover plate on the back of the motor for a set of points that centrifugally open after about 1 second as the motor starts. This is how swimming pool pump motors work too. See if the points and their mechanism looks OK. They should be closed and contacting at rest and fly open with RPM.

Thanks Raspy. Did as you suggested to no avail. Points were together when I found them and there was a little oxidation that I filed off. Everything else in there looked new. I spun the motor slowly by hand at that point but saw no difference in point separation if that was supposed to happen. I then attempted to start it and again saw the points pulse back and forth but did so as a unit and still no separation. Starter switch checks out and even if it didn't, I'd be reluctant in buying a $132 non returnable part on a $300 compressor before I bought another compressor. This thing is only 13 years old of relatively light use. Perhaps that's oiless compressors are supposed to last motor wise. Am at a loss as to what to try next but I have a nagging feeling that there is something real simple causing this.

This motor is no longer available and neither is the spring centrifugal point set as it comes as a unit according to the Sears tech.

 

[KYErik]

The points do not open and close with each revolution like a gas engine. They are supposed to stay closed until the motor spins up to half speed or so and then open up and stay open while the motor is spinning at the rated rpm. Does it sound like the motor ever gets close to its normal speed? Or is it just humming and barely spinning when it throws the breaker?

 

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The points do not open and close with each revolution like a gas engine. They are supposed to stay closed until the motor spins up to half speed or so and then open up and stay open while the motor is spinning at the rated rpm. Does it sound like the motor ever gets close to its normal speed? Or is it just humming and barely spinning when it throws the breaker?

No, doesn't sound like it's approaching half speed. It rotates with a bit of a hum but only for a few seconds like 3 or 4 before the breaker snaps. I'd say about a quarter speed of it's potential.

 

[JRobyn]

IIRC, motors of this design have a separate set of START windings that get energized thru the centrifugal switch. If the switch is functioning and the capacitor is good, IMHO there's nothing left except expired start windings.

 

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I was told by a Sears tech that this motor does not have a separate capacitor for start/run functions. The switch turns the thing on ok but it just isn't getting to speed. What's this thing I found in there?

 

[wawoodwa]

That's a start capacitor.

 

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That's a start capacitor.

Although it looks good, perhaps it's the problem? Gottta love Sears techs.