- An inductive energy transfer system (IETS) for the energy supply of the excitation coil of an externally excited synchronous machine (EESM) has been developed. Due to the removal of the slip rings, it is impossible to measure the excitation current and the rotor resistance on the rotating rotor. Therefore, a nonlinear observer for the EESM with IETS is presented, which estimates the excitation current and the rotor resistance. This observer is designed with the direct method of Lyapunov. Keywords—externally excited synchronous machine; electric mobility; drive control; inductive energy transfer; direct method of Lyapunov; nonlinear observer