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.

 

[wawoodwa]

Although it looks good, could that be the problem? I thought capacitors were mf rated.

Most likely. It is mf rated. That cap is rated from 136 to 163 microfarads (MFD). Start caps are always much higher. You can see the bleed down resistor on the top, and the circuit goes through the points (white/red wire). I think the sears tech is saying it doesn't have a "user maintainable start capacitor." As it is inside the motor in this case.

 

[flyerdan]

The first step is to ascertain whether the problem lies in the motor or the air head; did you separate the two and determine that the motor alone won't start?

 

[wawoodwa]

The first step is to ascertain whether the problem lies in the motor or the air head; did you separate the two and determine that the motor alone won't start?

Good point. If the air head has increased in load from low use, the motor won't come up to speed, drawing excessive current and tripping the breaker. Also, make sure you aren't running it through an extension cord. I've seen the same symptoms that you are having in that scenario.

 

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Most likely. It is mf rated. That cap is rated from 136 to 163 microfarads (MFD). Start caps are always much higher. You can see the bleed down resistor on the top, and the circuit goes through the points (white/red wire). I think the sears tech is saying it doesn't have a "user maintainable start capacitor." As it is inside the motor in this case.

Do I have to solder on the resistor? Nothing I've looked at as a replacement seems to carry the resistor on it. The tech is saying nothing is replaceable as the motor is no longer available. But he distinctly stated that this motor doesn't have a separate start cap. Perhaps he did not see two casings on top of the motor as opposed to it being inside.

 

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The first step is to ascertain whether the problem lies in the motor or the air head; did you separate the two and determine that the motor alone won't start?

If I disconnected all air lines, would that be a valid separation? Piston and fan are still all attached. Same result when I broke air lines. Never had an extension on it. Piston moves freely and easily by hand.

 

[wawoodwa]

Do I have to solder on the resistor? Nothing I've looked at as a replacement seems to carry the resistor on it. The tech is saying nothing is replaceable as the motor is no longer available. But he distinctly stated that this motor doesn't have a separate start cap. Perhaps he did not see two casings on top of the motor as opposed to it being inside.

You will have to place the resistor back on the new cap. In reading the label on the motor, it seems to say that internally, there aren't any consumer replaceable parts. That is why the tech is making that statement. Almost all AC motors have a start cap. Induction run motors won't have a run cap.

However, as Flyerdan is pointing out, the symptoms you are identifying are high current problems and making sure the load hasn't increased for some other reason, like a mechanical problem with the pump, needs to be ruled out. If you are confident that is OK, then you can keep looking at the motor.

Your motor has 2 windings. You should check the resistance of each to make sure you don't have an internal winding short or open. They should be around 12-16 ohms or thereabouts.

Lastly, and you probably already know this, and more for anyone else who may read this thread, capacitors store energy. If the caps have failed, they are usually safe to handle (shorted internally). If they are good, they can store line voltage. They should always be drained with a drain down resistor prior to handling. That start cap should be safe with the resistor between the two terminals, if still intact.

 

[wawoodwa]

If I disconnected all air lines, would that be a valid separation? Piston and fan are still all attached. Same result when I broke air lines. Never had an extension on it. Piston moves freely and easily by hand.

I believe flyerdan was looking more along the line of removing the belt if belt driven compressor.

 

[flyerdan]

Disconnecting the air lines assures that it is not starting under load of having pressure (which should be sorted via an unloading valve anyway) but there could be a problem in the air head, like a broken reed valve that won't allow it to draw in air. It would indicate fine while turning by hand but at 3450 rpm it would put up quite a bit of resistance.

 

[TheGoose]

Replace the cap. You don't really need the resistor. It will work without it.

 

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I believe flyerdan was looking more along the line of removing the belt if belt driven compressor.

Direct drive. No belt.