Hybrid excitation of AC and DC machines

The authors demonstrate that the straightforward combination of surface-mounted magnets and a conventional field coil is unlikely to bring any benefit in most small machines where airgaps are dictated by mechanical clearance requirements. In larger machines where armature-reaction demands a larger gap, hybrid excitation can be worthwhile and it is shown that economical use of the expensive magnet material can be achieved with a layer covering only part of the pole surface. In machines of the transverse-flux or homopolar type, a very large improvement in utilisation of material is found when a hybrid excitation arrangement is used, even with cheap ferrite magnets