Design considerations of a brushless open-slot radial-flux PM hub motor

Initial sizing of a brushless radial-flux permanent magnet (PM) machine is typically based on assumptions regarding the electric- and magnetic- loading together with the motor´s active volume. Additional design assumptions include the winding current density and magnetic flux density within the core pack. These initial design decisions are crucial as they largely define the resultant electromagnetic and thermal behaviour of the motor. The general rules regarding the choice of the initial design parameters are well known. However, the effects of the initial sizing on the final motor performance are not widely reported. This paper presents an analysis of design considerations for a brushless open-slot radial-flux PM hub-motor. A number of alternative motor designs are compared to demonstrate the effect of the initial design decisions on final motor performance. Both the electromagnetic and thermal aspects of the motor design are considered. The design variants are characterised by the same electric- and magnetic- loading together with active volume while the winding rated current density and no-load/open-circuit magnetic flux density within the core pack are varied. The employed sizing methodology combines the classical approach with a non-linear magneto-static finite element solver and an optimisation routine. A prototype hub-motor has been manufactured to validate the theoretical findings from the design process. The experimental data shows good agreement with the theoretical findings.