Abstract: One aspect of the invention is a rotor core for use with a permanent ring magnet in an electric motor which serves to minimize and evenly distribute thermal stresses in the rotor without undermining the efficiency of the motor by distorting the flux return to the ring magnet. According to one embodiment, the rotor core comprises a plurality of laminated plates. Each of the plates has a rim with a circular outer perimeter, a hub concentric with the outer perimeter of the rim, and first and second sets of curved spokes coupling the hub to the rim. Each spoke of the first set of curved spokes is concave in a clockwise direction and each spoke of the second set of curved spokes is concave in a counter-clockwise direction. According to another embodiment, the rotor core comprises a plurality of laminated structurally-identical perforated plates having obverse and reverse sides such that the obverse side of each plate faces the obverse side of an adjacent plate.