An electric power generator comprises a rotor and a plurality of stators arranged coaxially and concentrically about a central axis. There is an inner stator provided radially inwardly of the rotor separated by an inner airgap and an outer stator provided radially outwardly of the rotor separated by an outer airgap. The rotor includes a plurality of magnetic pole structures configured to provide or generate a magnetic field having plurality of magnetic poles. The rotor is not of uniform cross-sectional thickness, wherein: an inner surface of the rotor bulges inwardly at the pole structures, the inner airgap being non-uniform as it is radially shorter at the pole structures and longer in between the pole structures; and an outer surface of the rotor bulges outwardly at the pole structures, the outer airgap being non-uniform as it is radially shorter at the pole structures and longer in between the pole structures.