Externally excited electric synchronous machine

An externally excited electric synchronous machine, comprising:

• a machine rotor including a rotor shaft and a machine rotor coil non-rotatably arranged on the rotor shaft, the machine rotor coil providing a magnetic rotor field during operation, and the machine rotor coil including a first rotor coil terminal (106) and a second rotor coil terminal;
• a machine stator including at least one machine stator coil fixed relative to the machine stator, the at least one machine stator coil providing a magnetic stator field during operation, which interacts with the rotor field such that the machine rotor rotates about an axial axis of rotation during operation;
• a transformer secondary coil configured to electrically supply the machine rotor coil, the transformer secondary coil non-rotatably fixed to the machine rotor;
• a rectifier circuit connected between the transformer secondary coil and the machine rotor coil, the rectifier circuit configured to convert a transformer voltage induced in the transformer secondary coil into a DC voltage during operation, and the rectifier circuit including a first rectifier terminal and a second rectifier terminal;
• a protection circuit configured to protect the rectifier circuit from overvoltage, the protection circuit including a first protection terminal connected to the first rectifier terminal and a second protection terminal connected to the second rectifier terminal;
• the first protection terminal connected to the first rotor coil terminal and the second protection terminal connected to the second rotor coil terminal such that the protection circuit is connected in parallel between the machine rotor and the rectifier circuit;
• a switch arranged between the second rectifier terminal and the second protection terminal;
• and
• a trigger circuit connected to the switch and configured to open the switch for demagnetizing the machine rotor coil.