Low power actuator and valve-actuator combination

An actuator comprising a thrust member moveable between two end of travel positions, a rotor having magnetic poles, a magnetisable stator having at least one winding and poles with which the rotor poles are aligned in each of a plurality of rotational positions of the rotor relative to the stator, in which the rotor poles remain 20 attracted to a set of stator poles until a current is caused to flow in the at least one winding, which alters the magnetic field acting between the rotor and stator poles sufficiently to cause the former to be repelled from the stator poles with which they are currently aligned and to be attracted to and aligned with another set of stator poles, first spring means which acts on the thrust member and is compressed or stretched 25 from its relaxed length as the rotor rotates, and a drive connection between the rotor and the thrust member which prevents linear movement of the latter under the action of the spring means except when the rotor rotates and is adapted to convert rotational movement of the rotor into linear movement of the thrust member such that, while the spring means is at least partly compressed or stretched, the spring force acting on the 30 thrust member combines with the thrust exerted thereon due to the rotation of the rotor through the drive connection to enhance the actuating force exerted by the thrust member, wherein the actuator is arranged so as to define a flowpath through it, which extends through the rotor and stator.