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High-speed axial-flux generators (HSAFGs) with coreless stators are regarded as high-power-density, small-size, high-efficiency machines for use in distributed power generation systems (as microturbines). This paper presents a modelling procedure for such generators with details. The ratio of inner to outer diameter (lambda) in axial-flux machines is considered in the same way that the ratio of length to diameter is considered in radial-flux (conventional) machines. The optimized lambda is determined for an HSAFG with three rotors and two stators by careful consideration of the output voltage and efficiency variations versus lambda. It is concluded that lambda plays an important role in the HSAFG and requires careful attention when the machine is designed.