Compressor unloader?

First. Compressor tank explosions have killed quite a few people, and working on a compressor which won't shut off without understanding it is a good way to try for a darwin award.

The unloader is something different. There is a one-way valve into the tank, from the compressor output. Between the compressor output and the valve is a small controlled leak. When the compressor stops, the valve shuts, and the leak drains the small bit of pipe, so that the compressor can start easily, not have to start against a load. (which it may not)

There will be a pressure switch, which should turn off the motor. However, if you can't find this, you should really consider either having it serviced, or buying one.

Alternatively, if you'd mentioned the make, people might be able to help more, with hints on where you might get bits or manuals.

There are some compressors that unload the pump instead of shutting the motor off. (I have one that was originally set up that way, a Quincy.) It had a totally pneumatic unloader that held the intake valves open when more air was not needed. If you have somthing that looks like little pneumatic diaphragms on top of the compressor head, with an air line connecting to it, that is an unloader. In this case, you need a pneumatic control to switch the unloaders on and off.

If the power cord goes straight to the motor without going through a little 3" cube box first, then there appears to be no electrical pressure switch. Depending on the setup of the pump to tank connection, an unloading-type pressure switch may or may not be needed to convert it to an electrical pressure control.

But, most important of all, you want to make sure there is a pressure relief valve, and that it works properly at safe pressure, generally something like 10 PSI above your normal cutoff pressure. If in doubt, get a new one from an air compressor dealer. That's what I did when I refitted my Quincy.

Jon

Thanks, Jon. The relief valve works fine, as when it pops, I know to unplug the unit.

I'll take it apart tomorrow and see what's inside the thing........Maybe I can just add/replace the pressure switch ro something.....

Thanks, Ian, but the unit is not handy right now. I'll go get it tomorrow and list the specs.....

The idea behind this in an industrial setting is sound - if you use a lot of air and have a VERY large compressor, you don't want to get hit with the extra power charges for the motor starting surges several times an hour. (Turn the motor off when everyone goes home.)

With a commercial "Demand Meter" panel, the utility not only measures how much power you use, but how much you use at any one moment. The higher the peak demand, the higher the power bill.

The head unloaders act like pushing in the clutch on your car. It's cheaper to unload the compressor and let the motor and compressor spin unloaded for a few minutes until there is another call for air. The current draw of one compressor motor at idle is far less than increasing the demand charges on the entire month's power bill for the start surges bumping up your demand meter.