Performance comparisons of radial and axial field, permanent-magnet, brushless machines

The objective of this paper is to provide a comparison between the traditional radial field permanent-magnet brushless machine and four unique configurations of axial field permanent-magnet brushless DC machines. These consist of a single-gap slotted axial field machine, a dual-gap slotted axial field machine, a single-gap slotless axial field machine, and a dual-gap slotless axial field machine. The comparison is done at five power levels ranging from 0.25 to 10 kW. A rated speed of 2000 r/min is chosen for the 0.25-kW designs while 1000 r/min is chosen for the rest of the designs. The trends in performance and sizing for the different power outputs are obtained to get an understanding of the capability of various machine configurations. The comparison consists of required copper, steel, and magnet weights, copper and iron loss, moment of inertia, torque per unit moment of inertia, power per unit active weight, and power per unit active volume for five different power levels. For a given application, the results provide an indication of the machine best suited with respect to performance and size. The basis for the comparison is described with details on the design procedure.