What is a self-exciting, one - two - three wire alternator?

Self-Exciting Alternator:

Is an alternator that uses a special voltage regulator that doesn't need an ignition wire to activate it. This type alternator only requires a battery wire hooked to it. The voltage regulator contains circuitry that uses the residual magnetism in the alternators fields to determine when 
to turn the alternator on.

The regulator does this by sensing the RPM the alternator is turning, when it gets to a certain rpm the voltage regulator "turns on". Typically you start the vehicle, rev the motor slightly then alternator starts charging. This type alternator is commonly used on custom cars & trucks, tractors and other non standard applications when wiring is a factor.

In choosing this type alternator you must consider, do you want to rev your motor slightly to get the alternator to turn on. Also when using the self-exciting alternator on tractors or other slow turning motors does the engine have enough RPM's to start the alternator charging. This can be overcome by using a smaller pulley or by adding an ignition wire. 

One-wire Alternator:

Is another name for the Self-Exciting Alternator mentioned above. You only need connect the battery wire (one-wire).

Two-wire Alternator:

Most standard and all self-exciting regulator alternators will work using the two wire setup. Two wire means that you use the battery wire and an ignition wire to activate the alternator. With this setup the alternator starts charging as soon as the engine is running

Three-wire Alternator:

This setup uses a battery wire, ignition/warning light wire and voltage sensing wire, Three wires.