Novel hybrid permanent magnet interior PM synchronous machines

The rising cost of rare-earth permanent magnets (REPMs) in recent years has provided a challenge in developing high performance PM machines at a competitive price. Therefore, this thesis focuses on novel hybrid PM (HPM) machines with the synergies of high energy product REPM and low-cost ferrite PM (FEPM), with a particular emphasis on enhancement of the ratio of torque to REPM volume.
Six HPM interior PM synchronous machines (IPMSMs) with different topologies using symmetrical and asymmetric rotor structures have been developed and investigated in this thesis. The electromagnetic performance, including open circuit characteristics, output torque, torque and dq-axis inductances with current advancing angle, and efficiency, as well as mechanical robustness, demagnetisation withstand capability, and PM cost, are analysed and compared with those of a REPM-based V-shape baseline using the same stator and specifications of the commercialised Nissan Leaf 2012 IPMSM. The comparison confirms that the proposed HPM machines can produce the same torque at a lower volume of REPM consumption than that of the baseline. It is worth mentioning that the proposed HPM machines have been built and tested in a small size as the proof of concept.
Meanwhile, using the frozen permeability method, the output torques of the proposed HPM machines are divided into three components, including the reluctance torque, and the FEPM and REPM torques. Any improvement in either reluctance torque, FEPM torque or both, along with the torque enhancement caused by the magnetic field shifting effect in asymmetric topologies will result in a lower required REPM torque while maintaining the desired level of output torque. Consequently, a lower volume of REPM would be required which leads to the reduction of total PM cost. As a result, at the same torque and size, the contributions of torque components and volumes of both PM types are compared. It is shown that the HPM utilisation in topologies with a combined spoke arrangement of PM with V- or delta-shape structures can effectively improve the ratio of torque to REPM volume. Meanwhile, in the topologies where spoke FEPM is not used, a combination of asymmetric rotor topology with V- and U-shape structures of PMs can also improve the ratio of torque to REPM volume. Therefore, the proposed HPM machines in this thesis are potential candidates for electric vehicle (EV) applications at a reduced cost.