just a note on wiring up unknown mains alternators.
With AC alternators, the frequency is governed by the engine RPM, this is held reasonably constant by governor action. Any deviation of around +_ 10 cycles is niether here or there, unless you are hoping to run a mains frequency derived electric clock... hardly likely.
The voltage on the other hand is a function of both RPM and load. With reasonably constant RPM already mentioned, the bugbear is loading. Prior to electronic AVR systems, clever ways were developed to hold the voltage at a reasonably constant value, keeping in mind an alternator load fluctuation can be almost 100 percent, ie, no load to full load. One common method was to use what is called a saturating inductor, basically a current transformer placed in series with the output current. As the current increased, the magnetic field would increase as well, the induced secondary voltage would then be added to, or subtracted from the nominal feild current, depending on loading. These work fine, but can cause a voltage overshoot, or spike as when say an appliance such as a refrigerator or air compressor switches off . This is the most common form of regulation in older machines. Machines having a seperate exciter to supply the field current, usually have some form of electronic stabilisation to vary the DC current into the fields. Earlier excited units used tertiary windings on the stator to "bend" or shift the internal magnetic field to vary the efficiency of the main alternator, in effect advancing or retarding the magnetic signature. The more heavily loaded an alternator is, the more constant will be its output voltage. Many alternators are internally wired to supply both 120 or 240 volts, hard wired jumpers are used to do the selection. Field voltage varies considerably between models, I have seen anything from 6 volts to 150 volts used to excite the main alternator, this is always DC, and is identified by having a segmented commutator as opposed to the normal slip rings.... most early exciters are a seperate unit, although they can be integral to the main machine. Exciters depend on residual magnetism to initiate the generation of power, as do all other types. One other method used to regulate voltage is the "phase shift" method, whereby a capacitor is used to create a lag between the field and main windings, here the field is actually fed with pulsating DC. As with a inductive reactor, a capcitance also has reactance, but in the reverse sense, working identically to capacitor start electric motor.
To wire up an alternator from scratch, with no diagram can be a nightmare, but if tackled in a logical manner, all those same coloured wires can be identified quite easily with a simple continuety test. A car or motorbike battery can be used initially to excite the feilds to further identify what winding does what, and with the engine running, the DC exciter connections can also be identified. Take your time, and be safe when working with these, when they bite, they bite hard.