Abstract: A rotor assembly is disclosed, including a rotatable rotor shaft defining a central axis, the rotor shaft coupled to a rotor core, the rotor core including a plurality of rotor pole pieces circumferentially located about the central axis, each pair of adjacent pole pieces being separated by a space, said rotor core comprising a plurality of core magnets located in the spaces separating the adjacent pole pieces; at least one magnet retainer plate positioned adjacent the rotor core and connected to the rotatable rotor shaft, the at least one magnet retainer plate including a plurality of circumferentially spaced magnet slots and balancing slots located between adjacent magnet slots, the balancing slots configured to receive a balancing weight; and, compression springs located in the balancing slots, each compression spring located adjacent a respective core magnet when the at least one magnet retainer plate is located adjacent the rotor core.