The only job of an unloader in air Compressor!

Air compressor unloader has got only one job to do. which is to unload the air compressor.

Pic: The unloading of the air compressor or simply put, compressor unloader.!

It's a small valve attached to an air compressor which saves our effort.
this valve is automatically operated

for

1. Starting (wich most of the people knows about)
2. Stopping 
3. For intermediate discharging of moisture (say every 20 to 30 minutes)

Most of the air compressors have one automatic unloader and manual unloading valve (solenoid drain and hand drain)
(manual Unloader is provided if the automatic unloader doesn't operate)


Q1) Why unloader at the starting time?

For the beginners let me give it to you in the simplest. shortest way

The motor should pick up some initial momentum for compressing air or else if induction motor if started with a full load it draws high current and it also requires heavy torque.

Q2) Why at Unloader at the Stopping time?

For a smooth stop, or else the compressor will oscillate due to the high-pressure air which is trapped inside the cylinder. And proper hydrodynamic lubrication won't take place.

At the same time, a non-return-valve prevents the whole air bottle from draining.
and unloader of an air compressor is activated at regular intervals for the draining of moisture.

This is the sssssssshhhh sound that you can hear when the compressor stops.