Three-Phase Resonant Capacitive Power Transfer for Rotary Applications

This article proposes a three-phase resonant capacitive power transfer (RCPT) system for rotating applications requiring loose coupling. Existing plate designs for rotational CPT, such as the stacked four-plate and concentric structures, are found to exhibit imbalances that give rise to ground return or common-mode currents. These are particularly harmful at the high switching frequencies employed by compact and low-cost RCPT systems. Inherently balanced multiphase electrode designs are proposed to address this at the expense of permitting some coupling variation over rotation. Simulations show that three or more phases are required to avoid coupling nulls. While four or more phases may be preferred for applications requiring very stable coupling, three-phase is found to be the most practical solution in the context of cost, complexity, and inductor size. The proposed electrode structure offers high balance, a competitive average coupling coefficient, and a low per-plate resonating inductance. A highly integrated 27.12-MHz RCPT system was designed and implemented to validate these findings while accounting for practical limitations, such as manufacturability and cost. Using primarily commercial-off-the-shelf components, the system transferred 50 W at 25 mm under full rotation with an end-to-end efficiency of 73%.