Slip-ring skin currents as observed in the Canberra homopolar generator

The Canberra homopolar generator, a kinetic-energy storage device with electric input and output, operates with solid brushes running on a mild-steel brush track on each rotor. In particular, the output brushes operate at a high current density (2000A/cm2) linear velocity (up to 150m/s). During initial testing, significant potential differences were observed along the brush track in the direction of motion. The observations are explained in terms of skin currents in the brush track. Computations are aided by the application of a theorem relating to the electric and magnetic properties of circuits containing rotating elements. The significance of this theorem as an aid to understanding similar phenomena is discussed.