Air-gap winding impregnated with magnetic material in a binder

An air-gap winding, in which the magnetic flux is caused to pass through a magnetic material (40) rather than through the strands of the winding which are typically made from non-magnetic material such as copper, comprises mixture of discrete particles of the magnetic material (40) and a binder (41) impregnated into the winding under pressure, particularly in gaps (43) between individual wires (50) of the winding and in gaps between individual strands (42) in the wires (50). To avoid unsuitable deformation of the winding, the winding is non-circular in section, so that the mixture (40,41) is forced inwardly and outwardly through cavities formed between a stator (27) and the winding, and between the winding and a cylindrical inner member (32). Litz wire is used as for the individual wires 50.