Static torque production in saturated doubly-salient machines

A new theory is presented for the static reluctance torque produced in a doubly-salient machine when themagnetic circuit is strongly saturated, as occurs, for example, in the majority of stepping motors of modernmanufacture. It is shown that, despite saturation, it is still possible to use linear airgap-permeance data to derive expressions for the optimum proportions of tooth, slot and gap, and to establish a limiting value of specific torque. From the point of view of maximising static torque, the smallest possible airgap is the best, and the optimum tooth-width/tooth-pitch ratio is rather less than 0.42. The theoretical conclusions are in line with modern design practice (empirically based), and are well supported by experimental results.