Supplying a synchronous machine with an excitation current

The invention relates to a regulating device for a brushless excitation system for supplying at least one excitation coil of a rotor of a synchronous machine, in particular a turbogenerator, with an excitation current, having: at least one detection device designed to detect a strength of a magnetic flux, which is generated between a rotor and a stator of a main excitation machine, or at least one parameter connected to said magnetic flux; at least one regulating unit which can be connected between an auxiliary excitation machine and the main excitation machine, which is designed to generate an auxiliary excitation DC current that can be supplied to the main excitation machine by rectifying an auxiliary excitation three-phase current generated by the auxiliary excitation machine, and which is designed to regulate the current strength of the auxiliary excitation DC current such that a magnetic flux generated between the rotor and the stator of the main excitation machine is kept constant independently of the c