Synchronous Machine Rotor Excitation via Rotary Transformer with an Integrated Capacitive Resolver

Wound rotor synchronous machines require a brushless excitation system to reach higher speeds reliably and avoid maintenance issues. This paper presents the design, assembly, and testing of a capacitive resolver integrated into the rectifier of a gapped rotary transformer. Incorporating the shaft position sensor into the rotor excitation mechanism reduces the drive component footprint. An axial-flux rotary transformer using a ferrite core and litz wire windings serves as the baseline system for resolver integration, but the proposed integration is compatible with any brushless system. A capacitive resolver architecture was chosen because electrode surfaces are naturally accommodated by the rotating rectifier printed circuit board (PCB) with little manufacturing effort. The operation is analogous to a magnetic resolver and is compatible with existing resolver software demodulation algorithms. The rotary transformer with integrated resolver was tested at the full 7 kW power requirement, reaching a peak transformer efficiency of 97%, and the resolver achieving a maximum angle error of 0.3° at 2800 RPM.