The air compressor unloader valve regulates the ammount of air that is sucked in by your air compressor. By opening and closing the air intake, the capacity of a rotary screw compressor is regulated.

This valve is also commonly called an inlet valve or an intake valve. There is another type of valve also called 'unloader valve', which is completely different and which can be found on small reciprocating piston compressors.

When the desired pressure is reached, the compressor will go in 'unload' mode: the unloader valve will almost completely close the air intake of the compressor. This way, no more are can be sucked in and compressed, so the compressor is basically just turning around, doing nothing.