Double Layer Capacitive Power and Heat Transfer in Rotating Machinery

Doubly fed induction machines, wound field synchronous machines and dc homopolar machines all require a brushed slip ring connection to the rotor conductors. This connection inherently limits the performance and/or lifespan of these machines. Noncontact methods, such as brushless exciters and wireless power transfer, have been used to support some of these machine types, but they can be physically large and/or overly reliant on power electronics. Additionally, these methods do not support the cooling of the rotor conductors since their function is purely electrical. This article introduces a rotating electric double layer capacitor concept, or supercapacitor, to form a noncontact, slip-ring-like, ac electrical connection between a stator and rotor that simultaneously provides a cooling platform. An experimental proof-of-concept consists of concentric stainless-steel sleeves separated by a ceramic bearing to facilitate relative rotary motion but maintain constant facing area and gap distance. Electrolyte floods the space between the sleeves to form double layer capacitances and can also serve as a heat transfer mechanism. The sleeves were tested as bare metal and with an activated carbon coating to enhance capacitance further. Measurements demonstrate coupling capacitances up to Farad levels with three to six orders of magnitude improvement in A/Hz over rotary air capacitors, enabling low frequency operation.