A rotor assembly for use in an electric motor or generator where the mass
of the rotor assembly is reduced with respect to conventional rotor
assemblies. In addition, the rotor assembly is configured to be scalable
to different sized electric motors. Within the rotor assembly, the rotor
hub, the shaft, and the permanent magnets can independently or
collectively be modified to have a reduced mass. In one aspect, a portion
of the rotor hub adjacent to the shaft is configured with passages and
spokes. In another aspect, an intermediate hub with lightening holes is
provided between the shaft and the rotor hub. In yet another aspect, a
large diameter hollow shaft replaces a portion of the rotor hub. In yet
another aspect, the permanent magnets are configured to have an
arc-shape, which permits the thickness of the magnets to be reduced
without reducing the efficiency of the magnets.