How to power grid suppliers control the voltage and frequency?
Rotor windings control output voltage and armature rotational speed sets the output frequency. The devil is in the detail of course.



The field winding in the above picture is on the rotor - a DC voltage sets the excitation current and effectively produces a rotating magnet when the rotor is spun. This induces voltages in the three stator windings and these voltages, become the three phase supply outputted from the generator.

This is very simplified of course.