They get their supply power on the A lead and also sense the voltage on it. I sends increasingly higher current on the F lead in order to make the alternator output enough juice to match the voltage set point configured in the regulator. At a constant field current, as the demand increases on the Alt, it's volts will drop and vice versa. Alts with a powered field will regulated it;s output with the field current. As the alternator struggles to keep up with demand, its voltage will drop and the regulator will keep upping the field current until the voltage comes up to the set point of the regulator or the Alt will not take anymore current (reached it's output capacity).
Alt output also varies with RPM and the regulator deals with this in the same way.
Decades ago, these reg's used a set of points to open and close the field using the buss power source with unlimited current (poor mans PWM) and modern ones are solid state that regulate the current on the F lead with a chip (think LM317)
Regulators for PM Alt's behave very differently. They can't control the Alt, but instead massage the output to reach a desired voltage.
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 01-26-2021 at 11:59 AM.