All the parameters of the PMA that define the voltage are set with the alternator design. Only the rpm is left to set the PMA voltage during normal operation.
The basics of a PMA (Permanent Magnet Alternator) are as a voltage generator. What this implies is that the current is limited by the complete “connected circuit.” The “connected circuit” includes the unavoidable internal resistance and inductance of the alternator coil itself.
Ultimately, the voltage regulator (typically incorporated with a rectifier) constrain the charge current to a connected load. Unlike the regulator for a field controlled alternator, which modifies the field strength to set an output voltage, the PMA regulator simply (electronically) disconnects the PMA from the charge circuit (in the case of the series controlled regulator) or shorts (shunts) the output of the PMA to limit charge current to the connected load (battery).
The ability for a PMA to charge a battery is ideally designed to provide a small charge at idle speeds. In order to achieve charging at idle speeds, the PMA must overcome the intial charge voltage threshold of the battery. For a flooded, lead acid battery, this is 13.8 volts. A typical voltage constant for a PMA is in the order of 20 volts / 1000 rpm so at 900 rpm the peak voltage would be 18 volts ( a few volts above the 13.8 charge threshold). At 3000 rpm the PMA generates 60 volts, a voltage that is substantially above the battery threshold. One might think this would be damaging to the battery and electronics but a lead acid battery has a fantastic ability to absorb the cyclical voltage surges from the alternator. As the battery continues to gains charge current, the abilty to supress the voltage surges from the alternator is limited as the battery charge voltage rises.
Eventually the battery voltage reaches a target level (14.7 volts, for example) the PMA regulator saves the battery from continued assault from the PMA by separating the charge current path between the PMA and the battery. For the Revmaster charge system, the regulator opens the negative path between the alternator and the battery and the voltage at the PMA is allowed to rise to “open circuit voltage” levels but there is no completed circuit path and the PMA supplies no current to the battery.