Abstract: An energy storage device comprises a flywheel rotor in the form of a spokeless ring of filamentary, composite material rotating around a ring-shaped stator. Along the inner rim of the rotor, there is a layer of magnetically soft iron that forms homopolar, salient pole pairs. An ironless armature winding mounted on the stator extends into an air gap formed between the pole pairs. To bring the rotor up to speed, alternating current is supplied to the armature winding using electronic commutation. The armature current interacts with flux in the air gap to create a force acting to rotate the rotor. During coast down, current may be tapped from the rotor using commutation diodes. The rotor and stator are maintained in axial alignment to each other by magnetic flux produced by a permanent magnet in the stator. An electromagnet incorporated in the stator modulates permanent magnet flux to compensate for any eccentricity between the stator and the rotor that may develop during rotation.