An electric motor assembly includes a rotor hub, and a rotor supported on the rotor hub. The rotor hub has first and second hub portions, and the rotor has first and second annular rotor segments supported for rotation on the respective first and second hub portions. Each of the rotor segments has a respective set of magnets spaced circumferentially therearound. The rotor hub and the rotor segments are configured so that at least one of the rotor segments moves about the axis of rotation relative to the other of the rotor segments as the rotor hub rotates. The movement of the at least one of the rotor segments is an amount that increases as the speed of the rotor hub increases to reduce back electromotive force. The movement may be due to centrifugal force, which increases as speed increases, without requiring a control system to effect the movement.