A comparative analysis of torque production in Halbach and conventional surface-mounted permanent-magnet synchronous motors

The general question of when and why one should use a Halbach rotor in the design of surface-mounted permanent-magnet synchronous motors is addressed. The torque production capabilities of Halbach and conventional magnet arrays under the constraints of fixed magnet volume, and fixed power dissipation in slotless armature windings, are compared and the results presented in a graphical manner along with intuitive models that qualitatively explain these results. The authors conclude that, for permanent-magnet synchronous motors where the application precludes the use of a magnetic back-iron, the Halbach array always produces higher torque than the conventional array, for the same volume of magnets. The use of a magnetic back-iron in both designs increases the achievable torque. However for magnetically-backed rotors, the conventional array, with an optimized pole-arc to pole-pitch ratio, produces higher torque than the Halbach array up to a certain thickness of magnets. Above this thickness, the Halbach array produces a higher torque.