The air compressor is really a pump driven by an electric motor, or perhaps with a motor driven by other fuel types like gasoline or diesel.
The part that actually compresses the air is often referred to as the pump, or compressor pump.
This is an air compressor pump, though it’s missing the pump head cover in this photo.
In the photo above, part of what’s missing is the pump head, which will contain an intake port. This pump is also missing the valves or valve plates which open and close depending on whether the pump is in the intake or compression cycle.
On all air compressors, the pump pulls in free air from the atmosphere through an intake valve. That intake port on the pump typically has a filter on it to keep dust out of the pump. The pump is then supposed to drive the air it takes in into the tank, and in so doing increases air pressure in the compressor tank.
Different styles of air compressors, reciprocating versus rotary screw for example, accomplish this with different methods, yet they pretty much all do the same thing, suck in air from around the room or through an outside air intake and drive that air into a tank to build up pressure.
When a compressor pump is driven by a properly working motor, and the pump is cycling, why won’t the pressure in the tank build? Next comes the most likely reasons.