DEVELOPMENT OF A CAPACITIVE POWER TRANSFER SYSTEM FOR ROTOR FIELD CURRENTS USING GaN SEMICONDUCTORS

Abstract

In this thesis, the theory and practical realization of Capacitive Power Transfer Systems (CPTS) are investigated, with particular focus on their feasibility for delivering rotor field currents in synchronous machines. An analytical framework is developed to determine the optimal inductance and capacitance values at each stage of the system, maximizing power transfer efficiency while satisfying specific application constraints. Electromagnetic simulations are conducted using LTSpice, FEMM, and Octave to validate the theoretical models and optimize the electrical parameters. Subsequently, the design of the printed circuit boards (PCBs) is carried out using Autodesk Eagle, ensuring compactness and manufacturability. On the mechanical side, a detailed 3D model of the assembly is created using CATIA V5, using primarily existing components where possible. For custom and application-specific parts, in-house manufacturing was performed at the German Aerospace Center (DLR). A working prototype is then built and experimentally tested. The prototype demonstrates the ability to transfer up to 300 W of power at an operating frequency of 440 kHz, achieving an efficiency exceeding 80% when delivering energy to the rotor of a synchronous machine. The proposed capacitive coupling system occupies a compact axial volume, with a total thickness of 10 cm and a coupling interface diameter of 22 cm. While Inductive Power Transfer Systems (IPTS) can be engineered for high power density, they often require bulky magnetic cores and heavy copper windings that scale poorly in large-diameter applications. By utilizing GaN-based semiconductors to operate at high switching frequencies, the displacement current can be maximized across a relatively small gap. This allows the system to leverage the existing mechanical structure of the rotor, resulting in a significant reduction in active weight and material cost compared to traditional inductive solutions. This advantage is particularly pronounced in large-diameter synchronous machines, where the high-aspect-ratio geometry favors the planar nature of multilayer capacitive plates over the voluminous nature of magnetic couplers. This novel CPTS approach enables high-efficiency, maintenance-free power delivery to the rotor of synchronous machines, eliminating the need for slip rings, brushes, or permanent magnets, and offering a promising alternative for future electric drive systems.