Electrical Machine Design with Combined Field-Coil and Magnet Excitation

Abstract

It is often necessary to vary the field excitation in electrical machines. Hybrid excitation systems may be considered for this purpose, combining permanent magnets and a controllable field coil of reduced capacity. The magnetic circuits of d.c. machine configurations with hybrid excitation are studied to explore the circumstances under which hybrid excitation may be profitably employed. It is found that, in machines where the iron-to-iron separation between stator and rotor is determined by electrical considerations rather than for mechanical clearance, hybrid excitation can produce reduced coil excitation requirements. The quantity of magnet material may be reduced by selection of the magnet distribution and the air-gap profile. An experimental study is reported which illustrates the findings.