Why Does The Unloader Port On The Compressor Need An Air Line?

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Why does the unloader port on the compressor need a line to take it to the switch/shut-off? All it does is let a little relief out, why not just have it release at the check valve area. Why the need for the line going to switch box, it doesn’t operate anything.

All good questions. Let’s deal with why there is an unloader mechanism on an air compressor first.

For compressors running with 115/120 Volt electric motors a to drive the pump, they are typically plugged into, in North America at least, a standard wall socket. This socket is, in turn, wired to an electric panel in the house.

Pressure switch for air compressor made by Furnas / Hubbell 69JF7LY 95-125 single port w/ unloader & on/off lever

Who knows how many other devices are being powered by that same wire from the panel? All devices running at the same time from the same power line will reduce the flow of current to the downstream plugs. It is possible that the compressor is getting marginal power when it is starting.

An electric compressor motor requires an “inrush” current to get it going. The inrush current is borderline as to whether there is enough current available in a household plug to even start the compressor motor at all, even if there is nothing wrong with it.

Air trapped over the piston in a compressors

When a compressor stops, often there will be air trapped over the piston. This trapped air adds more load to the electric compressor motor when it is trying to start.

Compressor motor already is marginal

A 1.5 HP electric motor when new, and not under load, needs around 10 amps to run. Starting that electric compressor motor taxes the power supply. When that 1.5 HP motor pulls inrush current to start, the demand grows substantially. So much so that the same motor, when under increased load from air trapped over the piston, may draw enough amperage to exceed the supply in household current.

Yes, the start capacitor is there to boost the current on start up. If the load exceeds even that additional capability, then the motor will not start.

The compressor unloader valve

Many small air compressors have a pressure switch with an unloader valve as shown in the photo we added above. That unloader valve is connected to an air line that allows air to flow from over the compressor piston to that valve.

When the compressor stops, the unloader valve is opened by the movement of the springs inside the switch, and that movement moves a lever that opens the unloader valve, allowing it to vent air.

When the air compressor starts to fill the tank with air again, the pressure switch trips to allow flow of power to the motor, and the movement of the pressure switch adjusts the lever, and shuts the air flow from over the piston, so the air generated will go into the the tank.

It is the opening and closing of the unloader valve that is the reason for the line from the pump head over to the unloader valve. Otherwise, air would bleed out all of the time, and less air or none would go into the tank.

By Ashley Pearce

As a passionate manufacturing and mechanical engineer, I've had my fair share of run ins with air compressors and compressed air systems. With over a decade of experience in the industry, I have both a fresh perspective and time-served hands and mind to help you with your compressor problems (along with our able community!)

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