A rotor assembly having a plurality of rotor plates mounted to a shaft, and methods of construction for a rotor assembly are disclosed. Each rotor plate in the assembly may be provided with a central opening extending between the first and second sides through which the shaft extends. In one aspect, the rotor plates are provided with a plurality of lobes extending away from the central opening, wherein each of the lobes has a lobe opening extending through the thickness of the plates. In one embodiment, the rotor plates are rotationally stacked to form a helical rotor.