I could be wrong but I am under the assumption that any automotive alternator needs 12 volts supplied to it to initialize the electromagnets to start creating power.
Yes, you are correct, by design, Many auto alternator systems need a power source to create the initial magnatism, as a they are designed without conventional, permament magnets. (note auto style alternators cannot self excite) You can start a 12v alternator with a 9v battery temporarily applied, + to the field terminal and - to ground in a pinch.
That is why with a totaly dead battery, even with a point ign system you cannot push or tow start a car. There has to be some power left to energize the field to create that initial magnatism. Output is controlled by varing the field voltage, which directly varies the field magnets strength, and a lesser magnetic field will induce less output. Hence a 200 amp alternator is not always producing 200 amps, but varies acording to the load, which is regulated by, well, somethign called a regulator.
Alternator output is AC, rectified to DC. This "rectified" is built into the alternator in the form of the diode trio - three negative and three positive in the case of many auto alternators. In a nutshell, half of the AC wave is shunted to ground, and the result is DC left, simplified explamation, and there are some other designs, but basicly you are using half of the AC wave.
True genertors by definition have residual magnatism due to large field magnets. These are self exciting systems. Dead battery, no problem, as soon as the generator starts to turn, voltage is produced. The output is DC, directly usable by our DC tractor and automotive systems.
There are some hybrid alt/gens out there that do not need a battery to start charging. A common use of these is on outboard motors. The stator is normally fixed, and the rotor has magnents. The output is AC voltage, again rectified to DC. BUT there is one important difference to note. The output is constant for a paticular RPM. Yes, FIXED as the field magnatism cannot be alterered as the rotor has permenment magnets at a fixed strength. This system "self" excites anytime the rotor is turning.
Where does the extra constant output go? It cannot just evaporate or disappear, it has to go somewhere, so see the next paragraph...
If you have a fixed, constant output alternator type charging system, without a dispersal system that turns the excess power generation to something, IE if you need 20 amps, and the system is putting out 50 amps, that power HAS to go somewhere!! Most commonly it is turned to heat (resistance grid, either air or water cooled) Or, electrical things, normally the rectifier first, will began to cook themselves.
Again, a battery also works as a capacitor to absorb voltage spikes, which electronics do not tolerate well, of course how well depends upon the duty for which they were built for, as well as a power supply source. This is the case withany charging systems, be them AC, DC, alternator or generator.
Even the largest power producer, Hoover dam, if the load is suddenly removed from a system, voltage spikes and shocks can do serious damage.
Some people pull there positive lead off the battery to check and see if the alternator is charging..... WRONG! If you were to put a volt meter in the system, and if the alternator was working, you might see voltage spikes of up to 33-35+v. While most of the time people get away with this, it shortenes the electronic components life as well as any energized lamp filiments.
David
