Rotary Transformer for Wound Rotor Synchronous Motors

In this study, we present a resonant rotary transformer (RT) with an integrated rectifier for brushless excitation of wound rotor synchronous motors in electric vehicle applications. The primary-side of the RT employs an LCC resonant tank with Litz-wire winding and a high-frequency ferrite backplate, while the high-speed rotating secondary integrates the coil and rectifier on a mechanically robust, high-temperature PCB capable of withstanding up to 20,000 rpm. The system was analyzed and optimized for electrical, thermal, and mechanical performance. A laboratory prototype was built and tested up to 20,000 rpm and up to 25 A rotor current. The system achieved a maximum efficiency of 96% at 7 kW output power, confirming both high efficiency and mechanical reliability.