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The split ratio, i.e. the ratio of the stator bore diameter to the stator outer diameter, is one of the most important design parameters for cylindrical permanent magnet (PM) brushless machines. The optimal split ratio is investigated analytically for both brushless AC and DC motors having either overlapping or non-overlapping windings, and accounting for the influence of the airgap flux distribution, the stator tooth-tips and the end-windings. It is shown that the split ratio can significantly influence the torque capability and efficiency of a PM brushless motor.