Use of the Rotating Rectifier Board as a Capacitive Power Coupler for Brushless Wound Field Synchronous Machines

Wound field synchronous machines (WFSMs) are an attractive alternative to permanent magnet synchronous machines (PMSMs) given their competitive power/torque density and direct field excitation control for easy weakening. Here, a design process is presented for a low-cost and contactless capacitive power coupler (CPC) for WFSM rotor winding excitation using only simple printed circuit boards (PCBs). These PCBs are an extension of the machine's rotating rectifier. Experimental verification of the design shows 675-W power transfer at 90.3% efficiency to the rotor field of a 30/55 kW (continuous/peak) WFSM operating as a generator. A GaN inverter switching at 2 MHz reduces passive component sizes and keeps CPC voltages within safe limits. Parasitic loss minimization design rules for trace layout in the CPC PCB structure are established for high-frequency operation along with a partial capacitance approach for the coupling matrix. Tank inductor loss was reduced by using a cored design, while a buck converter integrated into the rotating rectifier matches the load impedance of the rotor to the CPC system, reducing the complexity of the tank compensation circuit. Finally, the CPC as a platform for position self-sensing is introduced conceptually using two methods. First, a phase-locked loop (PLL) tracks the resonant tank frequency of the CPC, whose saliency is spatially tied to the rotation of the machine. Second, the rotating buck converter effectively injects high-frequency content into the field that can be tracked by the stator drive controls.