Some alternators have built in circuitry that adjusts the field current as the demand on the regulator changes - or as the RPM of the alternator changes. Others do this with circuitry that is separate from the alternator. If you have the type that has a separate regulator then you'd have to provide current for the field. I could be mistaken but if you were to deliver full power to the field and the current demand was not high, the output voltage could rise to a level that is harmful.
I've seen various regulation schemes in amateur radio publications - amateurs quite often employ and alternator and spare gasoline engine for emergency service.