In a power generator unit composed of a generator and a piston internal combustion engine as the drive, the rotor is driven by the crankshaft of the diesel engine, and carries permanent magnets to excite the generator while the stator of the generator is arranged within the rotor and carries the rotor winding of the generator. The stator of the generator is divided, for the purpose of voltage regulation, into an outside stator part that forms an air gap with the rotor, and an inside stator part that forms a control air gap with the outside stator part that surrounds it, and is mounted to rotate, relative to the outside stator part, in such a manner that the geometry of the control air gap changes with the rotational position, and that the rotation takes place as a function of the variations in the terminal voltage of the generator.
