From: http://alumni.media.mit.edu/~nathan/nepal/ghatta/alternator.html

The voltage coming out of the alternator depends on two variables: the amount of current flowing through 
the field coil (i.e. the strength of the magnetic field) and the speed at which the alternator’s field is 
rotating. The alternator has a regulator that tries to keep the voltage across the battery at a steady 
14.4V (the optimal voltage to recharge 12V car batteries). It does this by regulating the amount of current 
flowing to the field coil. Once the alternator is self-sustaining, the only current flowing to the field 
originates from the alternator itself. If the output voltage is too high, the regulator lowers the current 
flowing to the field coil. If the output voltage is too low, the regulator increases the current flowing to 
the field coil. Simply put, as long as the alternator can maintain at least 14.4V across the battery, making 
the pulley spin faster or slower will have absolutely no effect on the power output. Power output in such a 
case will depend only on the load attached to the alternator.