Systematic design study into the influence of rotational speed on the torque density of surface-mounted permanent magnet machines

A series of systematic studies are carried out to investigate the influence on torque density and power density on machine speed rating. Several series of permanent magnet machine designs are established using an array of different electrical and thermal design constraints. The paper demonstrates that torque density in permanent magnet machines tends to decrease with speed rating due to complex interplay between electromagnetic and mechanical considerations, e.g. fixed winding current density, fixed split ratio etc. Interesting trends are observed, including cases in which there is an optimum rotational speed in terms of power density, beyond which the power density starts to decrease with increased speed rating.