Adjusting an overload properly

There may be a more scientific method, but that's what I have always seen. Even the adjustable breaker on 150 HP pumps are often set by "experience". Set it just high enough for normal operation, and if it ever fails prematurely just bump it up a little.

Makes sense. This is not life support equipment. If the overload pops one day when it should not have, not the end of the world.

What restatarting pressure??? That's what unloader valves are for, it's not supposed to restart under pressure. If it takes any amount of time for the unloaders to unload, I'd examine them closely as I think they should be near instant.

It takes about 1-2 minutes for unloaders to unload enough so that the compressor can restart. Possibly, this is wrong. I do not know.

In practice, this should not be an issue, as going from full tank to restart should take more than that, but still it may indicate that something is wrong with unloaders.

With overload adjusted properly, the worst case outcome would be the overload tripping.

The unloaders on my big compressor at the plant do the job in less than 15 seconds after the compressor stops. Been this way since it was new, about 10 years ago. Does this help?

Paul

Definitely, it helps. I will look into this.

Both my 5 and 10 hp compressors unload in 5 seconds or so.

I'm no expert, but I think I'd do it by looking up the full-load and locked rotor current draw of the motor and setting the overload current somewhere in between.

The adjusting knobs do not have current written on them.

Keep turning it down until you get a nuisance trip then back up a bit. Obviously check running current to make sure you are running inside motor's ability.

Wes

Mine unload the heads in less than 2 seconds. When they start, the unloaders stay open for three to four seconds. Yours looks the same with a diaphragm forcing the valve open until oil pressure shuts off the air to the top side of the diaphragm.

Doesn't your Quincy have pneumatic unloader diaphragms on the intake valves? You apply air pressure to these diaphragms and a pin drops down to keep the intake valves from closing. When the compressor is up to speed, something (electrical, pneumatic or oil pressure) cuts off the air to the unloaders, the pins retract, and the compressor starts pumping. It sounds like you are talking about an unloader in the delivery line to the tank. When it bleeds off pressure, it has to remove all the pressure in the whole compressor, including the high pressure cylinder and intercooler. This is kind of the hard way to unload a 2-stage compressor.

I have a smaller one-stage Quincy, and rigged up an electronic dual-mode controller for it. It unloads until the compressor is up to speed and has oil pressure, then unloads when the tank pressure is at the set point. It keeps the motor running until either more air is needed or a minute has elapsed. When the motor time elapses, it shuts down with the valves unloaded, so the compressor stops smoothly, without a chug-chug-chug. The control also shuts the compressor off if it ever pumps for a full 10 minutes without ever filling the tank, or if it loses oil pressure.

Jon

I have almost no clue as to how unloaders actually operate on the QR-25 head. Do you know anything about it, I will be eternally grateful for any info.

The idea of an unloader is to let the pump start w/o the resistance of the pressure in the tank. Then after you have the pump, motor, and pulleys at full speed, connect the pump output to the tank.

Wes

-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

The top of the diaphragm on top of the valve is supplied air pressure from the tank side. The diaphragm pushes down on three pins in the valve body and prevents the disk valve from seating. There is an air valve on the air tube going to the diaphragm. This valve is operated by oil pressure from the running compressor. Therefore, no oil pressure = air valve open = diaphragm unseating disk valve. If there is no air pressure in on the tank side, the disk valve will seat letting the compressor pump air. If the oil pressure is up, the air valve is closed letting the disk valve seat and the compressor pump.

This is a good set-up, if the oil pressure fails for any reason, the disk valve won't seat and the compressor won't get damaged from no oil. Not fool-proof, but good!

The diaphragm is reinforced rubber and does wear out, be warned.

pressure in the

That's about as far as I actualy knew. Jon's post shed a lot of light on the subject as he described his own Quincy compressor.

Except if you are running it from a VFD (IIRC, a VFD was somewhere in the equation), you really don't want the overloads popping between the VFD and the motor -- and the VFD itself should perform the function of the overloads as well.

As long as you are running it from three phase from a rotary converter, yes you want the overloads.

Enjoy, DoN.

I have more or less changed my mind, and am feeling rather good about running it from a phase converter. It is quiet and reliable. If I would run it from a VFD, which is still a possibility, I would not use that motor starter for any purpose.

Do your unloaders actually drain the tank? If so, then a check valve may be bad. Given the time the unloaders take to work, I suspect something wrong.

Good Luck, DoN.