Abstract: A compact energy storage system includes a high speed rotating flywheel and an integral motor/generator unit. The rotating components are contained within a vacuum enclosure to minimize windage losses. The flywheel rotor has a unique axial profile to both maximize the energy density of the flywheel and to maximize the volumetric efficiency of the entire system. The rotor is configured with hollowed-out regions at each axial end to accommodate magnetic bearing assemblies. The integral motor/generator is disposed on a tail shaft of the flywheel rotor, outboard of the magnetic bearing assembly. The motor/generator stator is mounted on a translation carriage for axial movement. During normal operation, the stator is in operative alignment with a rotor on the flywheel shaft. However, when neither motor nor generator operation is required, the stator is extended to an axial position where it is effectively decoupled from the rotor.