Electric motors with heteropolar permanent magnets and homopolar windings: computational study of performance limits

There has been considerable interest in electric motors of the general form in which a stator winding produces an mmf distribution with one pole pattern, which is then modulated by a toothed steel structure so as to produce an airgap flux component with a different pole pattern, which in turn interacts with a heteropolar arrangement of permanent magnets so as to produce useful output torque. Various configurations have been explored that employ a circular homopolar phase winding and an arrangement of laminated steel C-cores, forming the stator, together with a heteropolar PM rotor, usually employing rare-earth magnets for maximum output. This generic type is central to the present paper. Whilst the prototype motor certainly produces a high level of torque, its performance appears to be adversely affected by magnetic saturation at a lower winding mmf than simple calculations might have suggested. That has led to a detailed study of the magnetic circuit by finite element field computation, to discover in the first place the causes and location of saturation effects. More generally, the studies have been extended, to establish optimum design parameters in terms of core and magnet proportions, taking account of second-order effects beyond the capability of the simplified analytical methods that were employed in the original design