Comparison of Surface-mounted and Switched-flux PM Machines Accounting for Thermal Characteristics and Limits

This paper analyses the electromagnetic performances and thermal characteristics of surface-mounted permanent magnet (PM) and switched-flux (SF) PM machines. Based on the 12 stator slots/10 rotor poles combination and the same space envelope, the SPM and SFPM machines are optimised for maximum torques, with and without restricting the same PM consumption. The thermal characteristics of stator and rotor PM machines are investigated for machines with natural cooling accounting for thermal limits of winding insulation and magnet with different temperature grades. It is shown that when the SFPM machine is designed to produce a torque similar to the SPM machine, its PM consumption will be increased by >50%. Although the higher temperature-grade PM material together with the F-class winding insulation could improve the torque capability, the maximum torques of the SFPM machines are still lower than that of the SPM counterpart due to the thermal limits with natural cooling.