Design of a Low-Leakage In-Shaft-Integrated Wireless Excitation Coupler for Electrically Excited Machines

Wireless excitation technology effectively addresses the issues of poor reliability and limited rotational speed caused by brushes and slip rings in electrically excited synchronous machines (EESMs). However, existing out-of-shaft wireless excitation couplers introduce additional components at the motor end, occupying substantial end space and significantly reducing the power density of EESMs. To address this limitation, this article proposes a novel in-shaft-integrated wireless excitation coupler. By integrating the coupler into the rotor hollow shaft, end space occupation is eliminated, thereby enhancing the overall power density of EESMs. Additionally, the electromagnetic interaction mechanisms between the in-shaft coupler and surrounding metal components are revealed, and a leakage flux suppression scheme is proposed to substantially improve the transmission performance of the coupler. Based on this, the design guidelines and design processes for the in-shaft-integrated wireless excitation coupler are proposed, enabling electromagnetic compatibility (EMC) optimization and structural-parameter design. Finally, a coupler prototype with an output power of 226 W and an efficiency of 98.62% is constructed and integrated into an axial-flux motor for experimental validation. The results demonstrate that, compared with directly integrating the coupler into the rotor hollow shaft, the proposed leakage flux suppression scheme increases output power by 13.06% and transmission efficiency by 2.88%, validating the feasibility and effectiveness of the proposed approach.