| Summary: | With increasing concerns over environmental pollution and growing mobility demands of smart cities, the electrified automated ground vehicle (AGV) has been drawing extensive attention from both academia and industry as a more efficient propulsion system. As the heart of the propulsion system, the electric motor determines the performance of the whole system. There are mainly two kinds of motor systems being employed for low-speed AGV applications, namely, the high-speed motor with mechanical gearbox (i.e., geared-drive) systems and direct-drive (i.e., gearless) systems. Generally, high-speed motors are preferred to reduce machine size and overall weight at the same power rating. Therefore, geared-drive systems are predominant in industrial AGV applications. However, the mechanical gearbox would inevitably bring along issues of noise, vibration, and abrasion, resulting in low operation reliability and high maintenance costs. On the other hand, direct-drive systems outperform geared-drive systems by getting rid of mechanical gearboxes. Nevertheless, the existing direct-drive motors feature a large pole and slot number when operating under low-speed applications, which leads to bulky motor volume and deteriorated torque density. In this research work collaborated with Schaeffler (Singapore) Pte Ltd., the emerging field-modulated machines (FMMs) with high torque density are comprehensively investigated to facilitate their industrial direct-drive AGV applications.
The performance characteristics of various electric machines are reviewed in Chapter 2 to evaluate their potential for direct-drive applications. Attributed to the advances in high-energy magnet materials, the permanent-magnet synchronous machine (PMSM) is the most favorable direct-drive candidate which exhibits higher torque density and higher operation efficiency than the other conventional electric machines. By reviewing the development of PMSMs and examining the torque production mechanism, it is concluded that further torque density enhancement of PMSM is limited by steel and magnet material properties as well as thermal dissipation capability. This poses great challenges for the application of PMSM in modern AGVs. Known for high torque density, the FMM has been receiving wide interest from academia and is acknowledged as a promising candidate for direct-drive applications. Based on field-modulation theory, it is revealed that FMM can be perceived as the artful combination of a PMSM and virtual mechanical gear, exhibiting superior torque density and no mechanical contact. Nevertheless, the practical application of FMMs is rarely found. In Chapter 2, the asymmetrical issue, long end-windings, and low power factor are identified as the main prohibiting issues that hinder the industrial application of FMMs. This indicates the research directions of the following chapters.
The asymmetrical magnetic circuit issue of FMMs is investigated in Chapter 3. Based on magnetic field harmonic analysis, it is revealed the asymmetrical issue is an inherent problem of FMMs attributed to their special working principle, i.e., field-modulation mechanism. To alleviate the asymmetrical issue and reduce torque ripples, a complementary FMM with an axially shifted structure is investigated. Although the asymmetrical issue can be effectively mitigated, the complicated structure restricts the practical applications of the complementary FMM. To improve the situation, an FMM with the generic winding model is further developed, which allows for consideration of all potential winding layouts. The genetic algorithm (GA) is employed to optimize the winding in terms of minimizing the back EMF asymmetry. Finite element analysis (FEA) shows the proposed winding layout exhibits superior filtering capability for three-phase asymmetric magnetic field harmonics, without introducing extra manufacturing complexity. As compared with the conventional integral-slot distributed winding (ISDW) counterpart, the FMM with the proposed winding exhibits improved magnetic circuit symmetry, suppressed torque ripples, and reduced core losses. Finally, an experimental prototype is manufactured to validate the effectiveness of asymmetric magnetic field suppression for FMMs.
To reduce the end-winding volume, various feasible winding configurations are reviewed and compared in Chapter 4. The results indicate that two-slot pitch winding (TSPW) is the most suitable winding choice for FMMs, which can achieve a favorable trade-off between torque density and end-winding volume. The stator shifting technique (SST) is investigated to further improve the performance of FMMs equipped with TSPW. The effects of various shift angles on armature magnetomotive force (MMF) harmonic contents are explored. It shows that proper shift angles can be selected to preserve the working component while eliminating loss-producing harmonics. Besides, the implementation of various shift angles leads to uneven stator teeth structure, which affects field modulation. Therefore, the output torque capability, phase inductance, power factor, core losses, and fault-tolerance are affected by different shift angles. Chapter 4 intends to exploit the full potential of TSPW-FMMs by SST, considering the synthetic effects on harmonic reduction and field modulation. Through the exemplification of 24-slot, 19-rotor pole pair, and 5-armature pole pair TSPW-FMM, it indicates SST is a more general and flexible design method that can provide new design opportunities and improve various performance metrics by introducing shift angle as an additional design variable. In particular, SST shows great potential in improving the power factors of FMMs and providing target-tailored FMM designs for specific application scenarios. Finally, a prototype of the double-layer TSPW-FMM with the optimal shift angle is fabricated for concept validations.
Chapter 5 is dedicated to improving the power factors of FMMs. Even though there are extensive efforts and attempts to resolve the low power factor issue of FMMs, the existing solutions are mainly focused on increasing magnetic loading by sophisticated magnetic flux path design or using massive magnet materials. The increased structural complexity and production cost limit their practical values. By analyzing the effects of various inductance components on machine performance, it is found the mutual inductances between different coils of single phase and that between three-phase windings can be eliminated to enhance the power factor without impairing output torque. As a result, a novel hybrid concentrated winding (CW) with low-coupling properties is proposed to improve the power factor of FMMs. FEA simulation and experimental results validate the power factor of FMM can be significantly improved by the proposed low-coupling hybrid-CW, through the meticulous selection of short coil pitch and low-coupling design. Firstly, through the meticulous design of the short coil pitch, the inductance is reduced more significantly than the magnet flux linkage, resulting in a substantially improved power factor. Secondly, the proposed low-coupling design leads to further improvement in power factor without impairing the magnet flux linkage. The low-coupling design has twofold essences, namely the coupling between different coils of single phase and the mutual coupling among three phases are reduced to a large extent. As a result, the inductance is further reduced and the power factor is improved, with retained magnet flux linkage and torque production capability. The electromagnetic performances of the investigated hybrid-CW FMM are compared with two conventional FMMs with open-slot and split-tooth structures, showing the power factor can be improved to 0.96 from 0.83 and 0.76, respectively. Finally, a prototype is manufactured to verify the proposed concepts.
In Chapter 6, a demonstration of FMM is designed based on the solutions proposed in Chapter 3 to Chapter 5. The practical value and market potential of the developed FMM are objectively evaluated, so as to indicate the future research directions for FMMs. Firstly, the performance of industrial geared-drive and direct-drive AGV motors is reviewed, which serves as the benchmark for evaluating the developed FMM. In addition, the existing academic-researched FMMs are examined as well. It is found that complicated structure, excessive PM usage, and bulky end-winding volume are the three prohibiting problems that restrict practical applications of the academic-researched FMMs. The performance of the proposed FMM is compared with the industrial AGV motors and academic-researched FMMs. Comparison results reveal that the proposed FMM exhibit almost doubled active torque density than industrial AGV motors. However, due to the special field-modulation mechanism, the end-windings of the proposed FMM are still longer than that of industrial products, i.e., two-slot pitch vs. one-slot pitch. When the aspect ratio of outer diameter to axial length is designed to be lower than two, the proposed FMM can provide higher systematic torque density than the industrial AGV motors. This implies the proposed FMM is not suitable for pancake-shape design and in-wheel installation, which are popular choices for conventional low-speed high-torque machines. It is worth noting that the superior torque density and the elimination of mechanical gearboxes still make the proposed FMM a promising candidate for industrial AGV applications.
Finally, the research work presented in this thesis is summarized in Chapter 7. It can be concluded that the three prohibiting issues of FMMs, i.e., asymmetrical issue, long end-windings, and low power factor, are effectively resolved by the solutions investigated in Chapter 3 to Chapter 5. In addition, the comparison in Chapter 6 shows the proposed FMM has great potential for industrial AGV applications. Based on the preliminary research work and comparison results, several potential future research directions are discussed, including employing advanced winding technologies in FMMs, developing novel design methods for FMMs based on low-coupling winding concept, enhancing field-modulation effect for FMMs, and developing less/non-rare-earth magnets FMMs.
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