Design and Analysis of a Novel Series-Parallel Variable Flux Machine with Segmented Hybrid Permanent Magnets

Abstract

I. Introduction.With the merits of broad speed range and high efficiency at high-speed region, variable flux machine (VFM) is gaining more and more attentions in the wide-speed applications, such as electric vehicle, railway traction and numerical control machine tool. With the high coercive-force (HCF) and low coercive-force (LCF) permanent magnets (PMs) both utilized, VFMs with hybrid PMs are widely investigated. According to the combination way of hybrid PMs, VFM can be divided into series scheme and parallel scheme. Parallel VFMs are proven with relatively small demagnetization current and wide speed-adjusting range. But the HCF PM of parallel VFM has demagnetization effect on LCF PM, which will reduce the stability of LCF PM. While the LCF PM of series VFM is relatively stable, but the demagnetization current of series scheme is always large.This digest proposes a novel series-parallel VFM, which improves the stability of LCF PM by applying segmented PMs. The structure of the machine is introduced firstly, and then the magnetization-regulation principle is analyzed. The influences of V-shape pole parameters on electromagnetic torque and speed adjusting range of the machine are investigated.II. Machine Structure and Magnetization-Regulation PrincipleThe structure of the proposed series-parallel VFM with segmented PMs is shown in Fig.1, which is evolved from traditional V-shape PM synchronous machine (PMSM). The magnetic pole of the machine is composed of NdFeB and AlNiCo, and each side of the V-shape PM is divided into three parallel segments along the long-side direction of PM slot. The inner and outer PM segments are composed of series NdFeB and AlNiCo, and the middle PM segment is pure AlNiCo.Compared with the traditional parallel VFM, the proposed VFM has higher stability at load state, because of the NdFeB PM placed at the outer part of V-shape pole, where is more likely to be demagnetized by q-axis armature reaction. In addition, the series segments composed of NdFeB and AlNiCo has weaker demagnetization effect on middle AlNiCo than pure NdFeB, which can also improve the stability of the magnetic pole. The no-load flux distributions of the proposed VFM at forward and reverse magnetization state are shown in Fig. 1. It is illustrated that the flux provided by inner and outer PM segments are shorted by middle AlNiCo at reverse magnetization state, and the fundamental amplitude of no-load back electromotive force (EMF) is decreased from 73.00V to 5.94V, which proves the wide speed-adjusting range of the proposed VFM.III. Research on Electromagnetic Torque and Speed-Adjusting RangeElectromagnetic torque and speed-adjusting range are important characteristics of VFM, and thus influence laws of V-shape pole parameters on them are investigated. Since the change of PM dimensions will influence the magnetization result of middle AlNiCo when the same forward magnetization current is applied, the magnetization states of middle AlNiCo in VFMs with different PM dimensions at fully forward magnetization state are different. Therefore, the influences of V-shape pole parameters are different from traditional interior PMSM.According to the relationship between NdFeB thickness and the average flux density of series segments at no-load state, it can come to the conclusion that the equivalent remanence of the series segment can be adjusted by changing NdFeB thickness with the series-segment thickness unchanged. Afterwards, the influence of middle-AlNiCo positon on electromagnetic torque and back EMF is investigated. It is found that moving the middle AlNiCo closer to the air gap can decrease the total harmonic distortion (THD) of back EMF at reverse magnetization state, and almost has no impact on electromagnetic torque and back EMF at forward magnetization state.The influence of PM ratio is analyzed, and it is indicated that VFM with larger PM ratio has wider speed-adjusting range, but the average torque is relatively low. With PM ratio kept unchanged, the average torque increases firstly, and then decreases with the increase of PM-pole width. Moreover, the difference between d- and q-axis inductances is found having the same variation law with the average torque. The reason is that changing PM-pole width will change the distance between poles, which will change the d- and q-axis magnetic circuits distribution of the machine. When the thickness of the PM pole is increased, the average torque is increased firstly, and then decreased, while the torque ripple is increased continuously.The efficiency maps of the proposed VFM at forward and reverse magnetization states are shown in Fig. 2. It is illustrated that when VFM is reversely magnetized, the maximum speed of the machine is increased, and the efficiency at high-speed region is increased. The maximum torque of the machine at high speed region is also increased.IV. ConclusionsA novel series-parallel VFM with segmented hybrid PMs is proposed, and the selection principles for PM parameters of the machine is proposed. The speed-adjusting ability of the machine is elaborated by analyzing the magnetization-regulation progress, and the speed-adjusting performance is revealed by efficiency maps at different magnetization states. **