Ducati Monster Forum

I scored an older Campbell Hausfield 20 gallon compressor at an auction for $40 and was pretty happy about that.

It works, sort of.

From empty it will charge to 90psi and shut off. When pressure drops below the cut off is the problem.

The motor will either not turn and hum (trips the breaker if it goes long enough) or it does turn, but slow (struggling)

I'm almost certain the unloader valve works correctly. When it hits cut off pressure the motor stops and a little finger actuates the small valve at the end of the unloader line which vents pressure over the piston to atmosphere.

Both the start and run capacitors on the motor "look" OK but I haven't actually tested anything.

When the motor will not start and just sits there humming, if I kill power and rotate things with my fingers on the drive belt, even just a little, the motor will run but slow as mentioned above.

Any ideas?

From a very basic view, it sounds like something is binding in the motor/piston driveline.  Something is causing enough resistance to draw enough power to trip the breaker.  It could be anything from the electric motor winding coming undone to something bent somewhere between there and the piston.

JM

There's a check valve between the compressor output and the tank, usually mounted directly to the tank.

Likely that's leaking, backfilling the cylinder, and the motor cant get past that load from a dead stop.

I think Nick nailed it.

If that were the case, pressure should vent from the unloader line when I actuate the relief valve with a screwdriver, but that line doesn't recharge over time.

When the motor won't turn under power it takes relatively little effort to roll things over by moving the belt.

Should I still look at the check valve?

Well, that complicates things.

Does the motor turn even a little bit when it restarts?

Will it start turning if you hold the unloader valve open and power it back up??

No and no.

It just hums. If I roll the belt a bit it will turn under power but barely a rev and its slow and struggling.

As a test I just emptied the tank and it still just hums with no turn.

Then, rolling the belt a couple inches... motor struggles for just an instant then runs normal rpm and the tank is charging as it should.

Also, and thisay be new, I'm hearing a scraping sound that sounds cyclic in its timing  but it doesn't match motor or compressor rpm.

Could be unrelated bearing noise maybe.

Check the start windings.

Capacitor? 

Or actual windings in the motor?  Motor functionality is not something I know well.

Yeah...the capacitor.

You'll recognize it Kevin-it's full of flux.

[laugh]

Would the start capacitor be the larger one?

There are two

Check the centrifugal switch, the start winding, and the start capacitor.
Something is lacking in the start circuit.
http://www.capacitorguide.com/motor-starting-capacitor/ (http://www.capacitorguide.com/motor-starting-capacitor/)

[thumbsup]

Just following this.