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Previous work has shown that it is possible to design surface permanent magnet (SPM) synchronous machines, using fractional-slot concentrated windings to achieve wide speed ranges of constant-power operation. A closed-form analytical technique is now available to rapidly analyze candidate SPM machine designs using concentrated windings and tune them to meet the critical condition for optimal flux weakening. This analytical tool is used to demonstrate that optimal flux weakening can be achieved in SPM machines that are varied over wide ranges in several key dimensions including machine pole numbers, diameter-to-length aspect ratios, and output power ratings. Performance requirements for a 6-kW direct-drive automotive starter/alternator machine are used as the starting point for the scalability investigation. Finite element analysis results are presented to confirm the validity of key results in this investigation.