Help with Air Compressor

RVG8tor · #1 12-27-2007, 12:34 PM

I bought a oiled belt driven compressor a few months back and so far it has worked great. I can rivet for a long time without the compressor cycling on. My question though is when it does come on it start then sounds like it bogs down only to surge up to a high rpm until it has filled the tank. I have never owned an oiled type compressor, and wonder is this surging/bogging down normal. Any one with an oiled compressor have this same issue? I just wonder if something is wrong. Thanks in advance. Oh and I make sure it has the oil required and the level has been good since I filled it up initially.

Cheers
Mike

danielhv · #2 12-27-2007, 01:01 PM

I dont have a clue how these things work... but I can say that mine does NOT bog down on startup... then again its new... maybe its something that just starts happening with age? (not sure how old urs is) Does it do it when its empty and doing a complete fill or is it only when there is already some tank pressure and its building back up to the shutoff point? Reason I ask is it should be easier on the motor to start up on an empty tank than to start up with 100psi already there...

RVadmirer · #3 12-27-2007, 01:06 PM

I bought a 110V powered air compressor that needed a 15A circuit to run. Unfortunately, my hangar is on a single 15A circuit, and is about 500 feet from the breaker box, and the wire is minimum size. The compressor had no problem starting when empty, but when full, and re-starting after use, the motor would turn about two revolutions and stall, and after a few seconds, trip the breaker.

It turns out that this air compressor is a very common design, having an air pump that feeds the tank via a short pipe. At the tank end of the pipe is a check valve, mounted in the tank fitting, that keeps air pressure from the tank from holding pressure in the pipe and air pump valve area when the pump is NOT running. There's also a bleed-down valve that, when the tank reaches full pressure and the motor stops, bleeds the pressure in the pump-to-tank pipe to zero. I guess they do this either to keep pressure off the pump because it's not good for it, or might leak, or (more likely) because having no pressure in the pipe on start-up it makes it easier to get the pump going.

The problem was that when the motor starts, and the tank is already full, the pressure in the pipe builds so quickly that the motor doesn't have a chance to get up to speed before the air pressure builds and the air compressor air pump has to really start working hard. So the motor stalls, unable to turn fast enough to develop enough torque.

The fix: I added a 3/8" Tee fitting at the pump outlet, and added an air reservoir made of a foot-long 1-inch galvanized pipe nipple, with a cap on one end, and a reducer (to 3/8") at the other end. The threads around the bottom cap leak a barely-noticable amount of air, but I didn't bother to seal it too well because it's only under pressure when the motor is running, and it also serves as an automatic water-drain for the reservoir.

Now, the motor can run longer before pressure in the pump outlet pipe builds, so it has time to get up to speed where it can deliver the necessary torque. The compressor now starts just as fast whether it's empty, or pressurized. Hooray!

AltonD · #4 12-27-2007, 01:33 PM

An oil lubricated compressor usually has an unloader valve. As oil pressure comes up, it closes and allows the compressor to load. If you have one of these, maybe the unloader is stuck or otherwise not functioning properly.
If it is splash lubricated and not forced, then it must have another means of unloading on startup.

G-force · #5 12-27-2007, 06:41 PM

Perhaps see if the motor has a bad start capaciter? Its usualy under a cover, a "bump" on the side of the electric motor.

Pallet · #6 12-27-2007, 10:50 PM

Mike,
I had a compressor do this and the belts were a little bit loose and slipping. It kind of sounded like the compressor was slowing down a little sometimes. I tightened them and it has been working fine since.
Todd
Northwest OH

RVG8tor · #7 12-27-2007, 10:56 PM

Todd,

I think this might be my problem. The compressor is new so the belt might have stretched since new, I checked and it seemed loose, and the belt motor hub was warm as if the belt had been slipping. I have not used the air since tightening so I will see how this does. Thanks to all of you for the help, hopefully one or more of the suggestions will get my compressor working normally.

Cheers
Mike