An alternator makes electricity by spinning a magnet within a coccoon of windings. There is an electrical potenial that grows inside these windings as the poles of the magnet sweeps through. First polarity in one direction and then the next. The AC electrical potenial backs up against diodes (polarity gates) and the respective voltage passes in both directions (negative and positive) into the battery.
The regulators job is to control the output current by the level that the battery is below a control voltage. It does this by controlling the DC current that is passing through the rotating field. That current called field current is controlled by pulsing the voltage very fast through the rotating field. By changing the duty cycle the output is varied.
To finsh this brief concept, there is one other important point. The power to flow through the alternator as it starts spinning comes from the battery via the ignition switch and the alternator light. This small current flow is necessary to jump start the alternator (these Bosch alternators do not work if the alternator light bulb is blown). As a small AC potential builds the phases each have a smaller diode that shunts part of the output to the same point as the beginning source from the alt. light. The light now has battery voltage on both sides and goes out. The significant current used by the rotating field is now totally provided to the regulator by this diode pack; called the trio diode in three phase alternators.
Bosch Master, ASE Master, L1
33 years MB technician