Influence of Magnetic Iron Teeth in Motors Employing Superconductors for Field Excitation and AC Stator Windings

Synchronous machines are under consideration for applications on electrically propelled planes. Attractive features of such machines include high efficiency, low mass, and volume. To achieve these features, both rotor excitation field and ac stator winding coils employ superconductors. The dc field winding on the rotor experiences little harmonic fields. However, the stator winding, subjected to a full excitation field at rated frequency, experiences significant ac losses. The incorporation of laminated magnetic iron teeth is being considered for diverting the flux away from the superconducting coils to reduce the ac losses in air-core machines. Machines rated 3 MW at 4500 RPM were studied in this article. Two machines, with and without iron teeth within the stator winding, are compared for assessing the impact on the machine's efficiency, mass, and volume. This comparison shows that the inclusion of iron teeth increases total ac losses within the stator winding. This is so because the iron teeth direct the radial field away from the slot but increase the tangential field. Compared to the machine with no iron teeth (air-core), the machine with iron teeth has 10% higher losses, 4% higher volume, and 15% higher mass. The baseline iron teeth machine also creates high-frequency fields (61st harmonic field of 8.9% in the present case), which might present challenges for some electric power drives. The iron teeth in the stator winding also introduce a ground plane requiring thicker coil insulation that reduces available space for the stator winding. The absence of ground plane within the stator winding usually enables higher voltage designs. A machine having iron teeth and iron yoke (back iron) was observed to be high in losses, size, and mass. Based on these findings, it is recommended to forgo the iron teeth.