Design considerations of a direct drive brushless PM machine with concentrated windings

The paper considers the optimal design of an open slot, external rotor torque motor. Combinations of pole and slot numbers are presented for which the winding factor is maximal and torque ripple is minimal. A detailed optimisation of the magnetic circuit design of the six most promising pole-slot configurations has been undertaken using the combination of a genetic algorithm (GA) and a parametric finite element model. These candidate motor versions are characterised by the same total slot area available for the winding and by equal volumes of permanent magnet. An alternative two-segment Halbach arrangement of the rotor permanent magnets is also proposed to further increase the volumetric torque density