I'm probably making some errors here, you'll need to do own research, As I remember from days gone by of working on a dragline, many dc generators were driven by a couple thousand horse sync motor. The stator and the rotor had separate controllable voltages. the more current, excitation, through the stator created more torque, and slower, for a constant rotor voltage. Once spinning, if stator is disconnected, the rotor would run away, going so fast it would fly apart. The variable stator/field is like weaker or strong permanent magnets and the rotor voltage does the same as in a PM motor.