Design And Research On Variable-flux Permanent Magnet Synchronous Machine Used On Electric Vehicle

Permanent magnet synchronous machines(PMSMs),especially interior permanent magnet synchronous machines(IPMSMs), have been applied in many industrial applications, such as electric vehicles(EVs), due to their excellent features of high power density, good mechanical performance and wide speed region. However, in conventional IPMSMs, there always exists current component weakening the flux produced by permanent magnets(PMs), which leads to reduced efficiency and low utilization of PMs. Moreover, large flux-weakening current is needed when rotation speed rises up, leading to increased copper loss and reduced efficiency. In addition, due to the limit of the inverter capacity, the flux-weakening process is limited, resulting in a limited constant power region. Therefore, the variable-flux permanent magnet synchronous machine(VF-PMSM) which has the feature of variable flux controllability has been attracted many attentions in.Firstly, this paper proposes the principle of the VF-PMSM based on the problem of flux-weakening in conventional IPMSMs, including the “memory” function of the Alnico material, types of variable-flux permanent magnet synchronous machine and the principle to adjust speed.To obtain high torque density and decrease the magnetization current, the design and optimization process of the VF-PMSM are studied in this paper as an important part. First is design of the PMs including selection of magnetization direction, selection of PM material and the hysteresis model of Alnico. Flux barriers in the q-axis magnetic circuit are designed to obtain reversed saliency ratio. The effect of uneven air-gap on torque ripple is studied. Three rotor structures are designed and compared to find out the one with low leakage flux and high torque performance. Position of the PMs is adjusted to reduce the magnetization current. Through the design and optimization process discussed above, an optimized topology which has high torque density and low magnetization current is obtained. Then the inductance and mechanical performance are calculated and analyzed. Finally, the performance of the optimized model is analyzed by FEA, including the magnetization characteristic and the output performances at different magnetization ratios. By analyzing, it can be found that speed range can be extended and high efficiency at wide speed range can be obtained by adjusting the magnetization ratio along with the operation condition.To intentify the performance of the VF-PMSM desgined in this paper, the prototype of the optimized model is designed and tested by using vector control method(VCM), including the magnetization and demagnetization characteristic, output performances and inductance at different magnetization ratios. By analyzing the test results, it could be concluded that the speed region can be adjusted by controlling the flux. However, the test results have some difference with those of FEA. Moreove, the output performances of the prototype should be improved furtherly.