Novel external rotor switched flux memory motor with hybrid magnets

This paper proposes a novel external rotor switched flux permanent magnet memory machine (SFPMM) with both normal NdFeB magnet and low coercive force magnet with focus on the influence of stator/rotor pole (N_s/N_r) combinations. The operation and torque enhancing principles for external rotor SFPMM are introduced firstly, while the possible N_s/N_r combinations are analysed with the help of winding factors and conditions for symmetrical back EMF. Then the design parameters which influence the torque capability are discussed. Further, three models with N_s=6 and N_r=kN_s+1, k=1, 2, 3 respectively, are optimized under the same copper loss and magnet volume, which are consequently used for comparison of machine performances, such as open circuit field and back EMF, dq-axis inductances, cogging torque, torque capability, torque-speed characteristics and efficiency maps. The results show that the machine with larger N_r will have higher maximum torque but poorer flux weakening performance due to higher back EMF and smaller inductances, thus the machine having N_r=2N_s+1 is the optimal choice for external rotor SFPMM with hybrid magnets.