Analytical determination of optimal split ratio for overlapping and non-overlapping winding external rotor PM brushless machines

The split ratio between the rotor and stator outer diameters is an important design parameter. Considerable research has been carried out for the analytical determination of the optimal split ratio in both internal rotor and tubular permanent magnet brushless machines over last decade. In this paper, the optimal split ratio for both non-overlapping and overlapping winding external rotor permanent magnet brushless machines have been derived analytically, with due account for the influence of both fixed and variable rotor back-iron thickness, curvature effect, pole number, slot number, winding factor, tooth-tip, and end-winding length. Moreover, the variation of optimal split ratio with slot and pole combinations, tooth-tips, and maximum flux density in the stator/rotor lamination is also investigated. The theoretical analyses are verified by finite element analyses.