I have a 220V contactor with a 120V coil in it which I use to start my air compressor. The coil works fine, but I am curious about what the two poles numbered 2 and 3 are used for. These are located above the poles that receive the 120V and neutral wires as in the picture. Can anyone advise me on these?
The starter contacts labeled 2 and 3 are used as a latch in conjunction with start-stop buttons. These contacts are wired in parallel with the start button. Both the contacts and the start button are normally open (off). When the start button is pressed, the coil is energized, pulling the contacts closed, thus simulating holding the start button in. The stop button is normally closed (on), and is wired in series with the control power. When it is pressed, the coil is de-energized, and drops out. It stays de-energized because the 2-3 contacts are now open, waiting for the start button to be pressed again.
A pressure switch control doesn't need a latch, because it stays closed (on) until cut-off pressure is reached, then it opens (turns off). This is called a maintained contact, the start button is a momentary contact. Think of the pressure switch as a light switch, and the coil is the 'light'. Switch turns on, light (coil) turns on; switch turns off, light (coil) turns off.