I am building a trick battery charger from an old electric motor and an old auto alternator. On a typical alternator (external regulated) there are 3 wires plus a ground. 12v positive that goes to the battery, Field that gets the input voltage (4-12 volts) that the voltage regulaters uses to charge the field, plus another termal that is normaly there. What does this last termal do?

Andy

 

Im just guessing here, but the third wire may be the return (or common or ground) for the voltage regulator signal. Having a seprarate return for the regulator signal would allow the regulator to keep a more constant signal to the field, regardles of the the current fluxuations on the armature.