A permanent-magnet electrical machine is disclosed in which the rotor has a fixed back iron and movable back iron segments. When the movable back iron segments are in a first position, such as in contact with the fixed back iron, the field strength is high. When the movable back iron segments are in a second position in which the movable back iron segments are displaced away from the fixed back iron, the field strength is low. The ability to weaken the field strength causes the constant-power, speed ratio to be increased and thereby increases the utility of the motor for applications in which a wide speed range is desired. The disclosure applies to both permanent-magnet motors and generators. In an alternative embodiment, the stator ring is provided with a fixed portion and at least one movable stator segment.