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Permanent magnet machines are great candidates to be chosen as the preferred option in many low-speed applications. The improvement in the magnetic material's characteristics and the reduction in their cost are the key factors for this trend. The level of irreversibility achieved by a permanent magnet machine, i.e., the losses for a given power output, may be reduced to almost half of that got by an electrically excited synchronous machine, being the stator copper loss virtually the only source. The efficiency is, therefore, directly related with the ratio between the stator copper loss and the torque developed through the machine's air gap, being the minimization of this ratio the way to get a better machine design, once some other design constraints are fixed. In this paper, the authors provide a methodology devoted to the selection of the main dimensions of a permanent magnet machine in order to achieve an optimal machine's performance in low-speed applications, reaching a set of conclusions that may be applied to any machine of this kind.