Outer-rotor ac homopolar motors for flywheel energy storage

Flywheel energy storage technology has been successfully commercialized for applications requiring high power, high cycle-life, and short storage intervals. High idling losses have prevented the use of flywheel technology in applications that require longer storage intervals, such as grid-based, load-following energy storage. This paper proposes the use of an outer-rotor ac homopolar motor to significantly decrease idling losses, increase energy density, and decrease cost. Motor sizing equations, a comparison to the typically-used permanent magnet motor, and 3D finite element analysis of an example design are presented. It is shown that for high-performance flywheel designs, the ac homopolar motor can have a torque density comparable to that of a permanent magnet motor.