Demonstration of 9kW, 97% Efficient Capacitive Power Transfer using Rotary Double Layers

Wound-field synchronous machines are emerging as a rare-earth element free alternative to PM brushless AC machines that are popular in EV powertrains. The primary disadvantage of traditional wound-field machines is reduced lifetime and reliability due to the dependence on brushes and slip rings that provide rotor excitation current. Existing non-contact techniques (e.g. gapped rotary transformers and air capacitors) address wear issues, but sacrifice efficiency while increasing size and cost. To mitigate these issues, wireless power transfer systems have become reliant on higher frequency power electronics (>100 kHz). Previous work with electrolyte-filled capacitive wireless transfer systems shows that the double-layer capacitor, or super capacitor, which forms at the electrode / electrolyte interface can be used to achieve A/Hz ratings that are 3-6 orders of magnitude greater than similarly sized air capacitor designs. In this paper, a new axial flux rotary prototype having increased surface area via sandblasted surfaces is designed and tested. Power transfer and efficiency are reported with respect to variation in voltage, mechanical speed, and surface finish. System operation is demonstrated up to 9 kW with 10 kHz excitation and coupling voltage of less than 1 V, far exceeding power levels achieved with previous rotary capacitive power transfer demonstrations and proving a path towards practical use.