Who knows in detail the functioning of a generator also knows the answer to the question, but others who only drive and use the generators maybe wonder what is the purpose of installing a regulating device inside the alternator.
It could be argued that the purpose of the voltage regulator is simply to adjust the output voltage from the alternator terminals.
You can find this answer everywhere, including Wikpedia, but the real question is: why does tension need regulation?
The voltage at the terminals of the alternator, all other conditions being equal, has variations in the presence or absence of load caused by the internal impedance of the alternator.
The voltage regulator uses a reference voltage directly from the alternator and acts on the excitation so as to compensate for the voltage drop by automatically increasing the electromotive force.
It all seems quite simple, but it is only in the case of resistive loads, when instead mainly inductive loads are fed, things change, in fact the out-of-phase current opposes the magnetic field inductor with a consequent demagnetization on the rotor with a reduction of the excitation and relative significant lowering of voltage at the alternator terminals.
The voltage regulator automatically acts by increasing the excitation current which, with a sort of “doping”, stabilizes the voltage value again.
What happens when the load is predominantly capacitive instead?
In the case of predominantly capacitive electrical loads, a magnetization of the inductor occurs which causes an increase in the voltage often also important. In this case, the voltage regulator acts by reducing the excitation current and returning the voltage value within the established limits.