An apparatus includes a stator with an inner surface and an outer surface, a plurality of rotors magnetically coupled to the stator, wherein a first rotor faces the inner surface of the stator and a second rotor faces the outer surface of the stator, and a first airgap between the inner surface of the stator and the first rotor, and a second airgap between the outer surface of the stator and the second rotor, wherein first conductors, the first airgap, and the first rotor form a first submotor, and second conductors, the second airgap, and the second rotor form a second submotor, and wherein the first submotor and the second submotor are so configured that the first rotor and the second rotor produce mechanical torques in a same direction when currents flow in the plurality of windings in an operation mode.