A Model Based Design Methodology for Variable Flux PMSMs to Obtain Desired Speed-Torque Characteristics

Variable flux permanent magnet synchronous machines (VFPMSMs) have been designed by using finite element analysis (FEA) to evaluate speed-torque capability considering requirement for magnetization state (MS) manipulation. However, due to its unique characteristic to change the MS, numerous combinations of design parameters need to be evaluated to achieve a final design. To accelerate the design process, this paper presents a method that consists of an equivalent magnetic circuit model and a process to obtain magnet width and thickness that satisfy target maximum torque and power factor (P.F.) capability. This model includes magnet operating point analysis under given magnet width and thickness condition to achieve target MS and avoid demagnetization at full load. This analysis provides desired stator magnetomotive force, magnet and stator induced flux linkage. Therefore, expected torque and P.F. capability is calculated. The model is applied to find a smallest magnet volume that satisfies target maximum torque and P.F. capability for several machine designs with a fixed topology, stator diameter and stacking length, under varying magnet thickness and width. Other dimensions, such as stator-teeth width and depth, air gap radius and back yoke width are varied as a function of the magnet dimensions. The method uses a magnet dimension design space, so the preferred motor dimensions with minimum magnet volume can be found. A design that has preferred dimensions obtained from the method is evaluated with FEA. The calculated torque and P.F. capability with FEA shows reasonable agreement with the predicted torque and P.F. obtained by the method.