Electromagnetic performance of high-torque and low-speed PM brushless machines

This paper presents investigation of electromagnetic performances such as open-circuit flux-density, back-EMF and output torque on two fractional-slot PM brushless machines. Their ratios of slot-number to pole number are similar respectively, i.e. one with slot-number to pole number is differ by one and the other differs by two. Theirs main parameters such as split-ratio, axial length as well as rated torque and rated speed are fixed for the purpose of electromagnetic comparison. The investigation is performed by using 2D finite-element analysis in which saturation condition is considered. It is shown that optimum magnet thickness and machine split-ratio are desirable for a given electromagnetic torque. On the other hand, winding factor and winding topology have direct influence on the profile of machine back-EMF, which may affect the performance of electromagnetic torque. From the investigation, the machine designs are adequately reliable for robotic application as they are calculated for high torque and low speed performance.