Low Coercive Force Permanent Magnets Research Articles

Investigation of the over-load unintentional remagnetization effect in series hybrid magnet variable flux memory machine

Variable flux memory machines (VFMMs) have become a research hotspot due to their adjustable air-gap flux with the employment of low coercive force (LCF) permanent magnets (PMs). Series hybrid magnet VFMMs (SHM-VFMMs) utilizing series-magnetic-circuit high coercive force (HCF) and LCF PMs are extensively investigated due to their excellent unintentional demagnetization (UD) withstand capability. Nevertheless, the existing researches still focus on the UD of LCF PMs under heavy load operation, and the remagnetization issue remains unreported. Therefore, in this paper, the over-load unintentional remagnetization (UR) effect of the a dual-layer-SHM-VFMM (DLSHM-VFMM) is first revealed, and its mechanism is investigated and analyzed in depth based on finite element (FE) analyses. A DLSHM-VFMM prototype is manufactured and tested to verify the effectiveness of the theoretical analyses.

Development and comparison of double-stator memory machines with parallel hybrid magnets

PurposeThis paper aims to further develop stator permanent magnet (PM) type memory machines by providing generalized design guidelines for double-stator memory machines (DSMMs) with hybrid PMs. This paper discusses the design experience of DSMMs and presents a comparative study of radial magnetization (RM) and circumferential magnetization (CM) types.Design/methodology/approachIt begins with an introduction to RM and CM operating principles and magnetization mechanisms. Then, a comparative study is conducted for one of the RM-DSMM rotor pole pairs, inner and outer stator clamping angles and low coercive force PMs thickness. Finally, the two machines’ finite element simulation performance is compared. The validity of the proposed machine structure is demonstrated.FindingsIn this paper, the double-stator structure is extended to parallel hybrid PM memory machines, and two novel DSMMs with RM and CM configurations are proposed. Two types of DSMMs have PMs and magnetizing windings on the inner stator and armature windings on the outer stator. The main difference between the two is the arrangement of PMs on the inner stator.Originality/valueConventional stator PM memory machines have geometrical space conflicts between the PM and armature windings. The proposed double-stator structure can alleviate these conflicts and increase the torque density accordingly. In addition, this paper contributes to comparing the arrangement of hybrid PMs for DSMMs.

Analysis of flux barrier effect of LCF PM in series hybrid magnet variable flux memory machine

Variable flux memory machines (VFMMs) with series hybrid magnets using both low coercive force (LCF) and high coercive force (HCF) permanent magnets (PMs) have been recognized as a viable candidate for wide-speed-range industrial applications due to the advantages of high torque density and wide speed range. Nevertheless, the adverse effects of LCF PM on the HCF PM in series-type VFMM under different magnetization states (MSs) are still unreported. In this paper, the flux barrier effect (FBE) of the LCF PM existing in series hybrid magnet VFMM is first revealed, and its causes are discussed as well as analyzed in depth on the basis of the equivalent magnetic circuit method and finite-element (FE) analyses. A topology of VFMM with dual-layer PMs is further developed to alleviate the FBE induced from LCF PMs. It can be found that the FBE can be effectively suppressed by employing the dual-layer (DL) PM arrangement and additional leakage flux paths. A prototype of the proposed design is built, and the theoretical and FE results are experimentally verified.

A Novel Slot PM-Assisted Hybrid Magnet Memory Machine

This article presents a novel slot permanent-magnet-assisted hybrid-magnet memory machine (SPMA-HMMM) with hybrid permanent magnets (PMs) in both stator yoke and slot opening. The key is each pair of yoke PMs is composed of one pole of high coercive force (HCF) PM and one pole of low coercive force (LCF) PM with flexible flux regulation. LCF PMs can be freely magnetized and demagnetized through dc current pulses, which are supplied by dc excitation coils wound on LCF PMs. This design leads to high efficiency of machine due to negligible field excitation loss. In this article, the machine configuration and operation principle are illustrated in detail. An equivalent magnetic circuit is modeled to unveil flux regulation and parallel excitation effect. Then, the electromagnetic performances of SPMA-HMMM under different magnetization states (MSs) are investigated, which verify flux regulation ability of the proposed machine with changeable MSs of LCF yoke PMs.

Influence of Low-Coercive-Force Magnet Property on Electromagnetic Performance of Variable Flux Memory Machine

The magnetization properties of low-coercive-force (LCF) permanent magnets (PMs), i.e., remanent flux density, coercivity, and hysteresis curve slopes, are closely related to the magnetization characteristics of variable flux memory machine (VFMM). Therefore, this article investigates the inherent relationship between the properties of LCF PMs and electromagnetic characteristics of VFMM using a combined solution integrated with magnetic equivalent circuit (MEC) and graphical methods. First, the hysteresis curve of the LCF PM is analytically modeled by a simplified piecewise linear hysteresis model to characterize the mathematical relationships among the key parameters of the hysteresis model. Then, a simplified MEC method is used to obtain the permeability curves of VFMM, thereby further determining the intrinsic relationship between the dimensional parameters of the LCF PM and the electromagnetic characteristics of VFMM. Afterward, the flux regulation characteristics and on-load magnetization state (MS) stabilization capability of VFMM are qualitatively analyzed using a newly developed coupled approach. Furthermore, the electromagnetic characteristics of three VFMMs equipped with different version LCF PM materials are comprehensively compared using the finite-element (FE) method. Finally, a VFMM prototype using the selected AlNiCo magnet is manufactured and tested to verify the correctness of the theoretical and FE analyses.

Investigation of Axial Field Switched Flux Memory Machine by a Combined Analytical Method

This article proposes a novel axial field switched flux memory machine (AF-SFMM) which is analyzed by a combined analytical method. Compared with conventional axial field (AF) permanent magnet (PM) machines having magnets in the rotor side, the AF-SFMM benefits from good heat dissipation, high rotor robustness, and improved power density. Meanwhile, because of the utilization of low coercive force PM materials, the proposed machine can flexibly regulate air-gap flux. This analytical method is characterized by the combination of a quasi-3-D (quasi-3D) finite element method, a magnetomotive force–permeance-based method, and a virtual linear hysteresis model of low coercive force PM. In addition, compared with conventional time-consuming 3-D finite element analysis (FEA), the proposed method can improve the computational efficiency significantly with satisfactory accuracy. This makes the proposed combined analytical method suitable for preliminary sizing determination of AF-SFMM. Meanwhile, the effectiveness of the proposed analytical method is validated by FEA results.

On Unintentional Demagnetization Effect of Switched Flux Hybrid Magnet Memory Machine

This paper investigates the unintentional demagnetization (UD) characteristics of low-coercive-force (LCF) permanent magnets (PMs), in switched flux hybrid magnet memory machines (SF-HMMMs). Although the LCF PM field is magnetically in parallel to the magnetic fields produced by the NdFeB PM, as well as the armature reaction in the investigated machines, the UD phenomenon of LCF PMs still possibly occurs, particularly, under on-load operation due to the magnetic saturation effect. First, the UD effect is revealed by the frozen permeability method (FPM), and analytically explained via a magnetic circuit model. Various UD types are then identified with the finite-element (FE) method, coupled with a virtual linear hysteresis curve (VLHC) of LCF PM and FPM. In addition, the dimension and grade of the LCF PM are designed with the aid of VLHC, in order to prevent the UD effect. Finally, a fabricated SF-HMMM prototype is tested to verify the theoretical analyses.

Investigation of Torque Characteristics of Switched Flux Hybrid Magnet Memory Machine by a Coupled Solution

This article investigates the torque characteristics of a switched flux hybrid magnet memory machine (SF-HMMM) by employing a coupled solution combining a quasi-linear hysteresis model (QLHM) of low coercive force (LCF) magnet and frozen-permeability method (FPM). This provides an in-depth understanding of the torque production of SF-HMMM. The QLHM is utilized to characterize the repetitive remagnetizing/demagnetizing behaviors of LCF permanent magnets (PMs), while the FPM is employed to separate and quantify the torque contributions due to two sets of PMs and armature windings. The machine topology and analysis methodology are described, respectively. The torque segregation results accounting for magnetization state variation are analyzed by the proposed coupled solution. The influence of the LCF PM property on the torque segregation result is investigated to facilitate the establishment of general design guidelines. Finally, the prototype is tested to experimentally validate the theoretical analysis.

Comparative Study on Variable Flux Memory Machines With Different Arrangements of Permanent Magnets

Compared with conventional permanent magnet synchronous motor, variable flux memory motor (VFMM) possesses the advantage of adjusting the magnetization state online. Because of its wide speed range and large high-efficiency region, it becomes a competitive candidate for the electric drive system. This paper presents a novel parallel hybrid magnets VFMM which is obtained by updating a conventional series hybrid permanent magnets VFMM. The proposed motor adopts a new permanent magnet arrangement and the combination of low coercive force permanent magnet and high coercive force permanent magnet is used. The proposed parallel hybrid permanent magnets VFMM and the series hybrid permanent magnets VFMM are compared. The equivalent magnetic circuit models of the motors are used to explain the principle of the motors. The finite element analysis method is used to study the fundamental properties, such as open circuit radial flux density, unintentional demagnetization with q-axis current, intentional demagnetization with d-axis current, torque capability, and efficiency characteristics. It was shown the efficiency of the parallel hybrid permanent magnets VFMM is higher in the high-speed region and the magnetization state adjustment range of the parallel hybrid permanent magnets VFMM is larger.

A Novel Hybrid-Magnetic-Circuit Variable Flux Memory Machine

This article proposes a novel hybrid-magnetic-circuit variable flux memory machine (HMC-VFMM) by combining series and parallel hybrid magnet structures. Thus, the synergies of wide flux regulation range in parallel type and excellent on-load demagnetization withstand capability in series type can be simultaneously obtained with the proposed HMC design. Meanwhile, two sets of the permanent magnets (PMs) with high coercive force and low coercive force (LCF), i.e., NdFeB and AlNiCo PMs, are employed to achieve high torque density and energy-efficient magnetization state adjustment. The topologies and tradeoffs of traditional parallel and series VFMMs are addressed first. In addition, the structure evolution, features and operating principle of the proposed HMC-VFMM are described, respectively. A simplified equivalent magnetic circuit is modeled to reveal the performance improvement of the machine. Then, the design improvements with q-axis barriers are presented to elevate the LCF PM working point for preventing the on-load demagnetizing effect, while maintaining the torque capability. The electromagnetic characteristics of the HMC design are investigated and compared with the parallel/series counterparts. Finally, the experiments have been carried out to validate the finite-element analyses.

Stepwise Magnetization Control Strategy for DC-Magnetized Memory Machine

A memory machine (MM) equipped with hybrid permanent magnets (PMs), i.e., NdFeB and low coercive force PMs, exhibits acceptable torque capability at low speeds and high efficiency at high speeds. Previous literatures have addressed that the constant power speed range (CPSR) of MMs can be further extended by online PM flux control and the requirement of the flux-weakening (FW) inverter current can be reduced as well. Nevertheless, how to coordinate the d -axis vector FW and PM magnetization control over a whole operating range in a reasonable manner remains unreported. Therefore, this paper proposes and implements a stepwise magnetization control strategy on a dc-magnetized MM based on the operating characteristics under various PM magnetization states and speed ranges. The configuration, principle, and mathematical model of the investigated machine are introduced first. Then, the proposed control strategy is established by dividing the operating envelop into several FW regions and an appropriate FW control scheme is utilized at each stage. It demonstrates that the CPSR can be effectively extended by simply applying the demagnetizing current pulses in several steps. This simplifies the control efforts considerably without resorting to continuous PM flux control and frequent actions of the switching devices. The effectiveness of the proposed control strategy is verified by experimental results.

Analysis of Axial Field Flux-Switching Memory Machine Based on 3-D Magnetic Equivalent Circuit Network Considering Magnetic Hysteresis

This paper presents a 3-D magnetic equivalent circuit (MEC) network that integrates the hysteresis model for the analysis of the axial field flux-switching (AFFS) memory (AFFSM) machine. Similar to finite element method, meshing is used to subdivide the model into smaller domains called elements, over which a set of element equations and a reluctance network equation are built. Newton–Raphson method is adopted to handle the nonlinear problems. The fixed-point technique is introduced to cope with the problem involving hysteresis. The proposed method is capable of predicting the operation of the low coercive force permanent magnet during flux regulation and the electromagnetic field distribution of the machine. Finally, the proposed 3-D MEC method is implemented for analysis of the AFFSM machine. MEC results are compared with 3-D finite element analysis and experimental results. The results show that the proposed method offers reasonable accuracy with moderate computational effort.

Comparison of End Effect in Series and Parallel Hybrid Permanent-Magnet Variable-Flux Memory Machines

Hybrid permanent-magnet (PM) variable-flux memory (VFM) machines employ both high coercive force PM, i.e., the constant PM (CPM), and low coercive force PM, i.e., the variable PM (VPM). The high energy-product CPM guarantees the benefits of high torque density of the conventional PM machines, while the PM magnetization state of the VPM is flexibly regulated to match various operation requirements. In fact, the two kinds of PMs can be magnetically connected in either parallel or series, with which the machine characteristics are quite different. In this paper, based on the two-dimensional and three-dimensional finite-element analyses, a pair of VFM machines with parallel and series connections, respectively, are investigated with specific reference to identify their difference in end effects. The comparison results show that the series hybrid PM VFM machine suffers more serious end leakage flux. The prototype machines are manufactured and tested to validate the analyses and predictions.

Magnetization State Regulation Characteristic Study of Series Hybrid Permanent Magnet Axial Field Flux-Switching Memory Machine

Due to the short axis, high torque density, and wide permanent magnet (PM) flux linkage regulation range, the series hybrid PM axial field flux-switching memory machine (SHPM-AFFSMM) has been paid much attention. In order to extend its speed range, the way to generate an effective current pulse to regulate the magnetization state of the low coercive force PM is the key issue. The magnetization state regulation (MSR) method characterized with bidirectional and quantitative current pulse is presented to investigate the MSR characteristics of the SHPM-AFFSMM. Based on the proposed MSR method, the MSR characteristics of the researched machine, including the inductance characteristics, cogging torque, and permanent magnet flux density, are investigated by simulation and experiment.

Comparative Study of Hybrid PM Memory Machines Having Single- and Dual-Stator Configurations

In this paper, the memory flux principle is extended to switched flux structures, forming two newly emerged switched flux memory machines (SFMMs) with single-stator (SS) and dual-stator (DS) configurations. Two types of permanent magnets (PMs), i.e., NdFeB and low coercive force PMs, are located in the stationary part. Thus, the developed machines can achieve easy online PM magnetization control, excellent air-gap flux control, and acceptable torque capability. In order to address the issue about the limited stator space encompassing dual PMs and magnetizing coils in the SS-SFMM, a DS design is further developed, where all excitations are placed on a separate inner stator to improve the torque density. A comparative study between the SFMMs with SS and DS structures is established. The investigated machine topologies and operating principle are described first based on a “U”-shaped hybrid PM arrangement, and the PM sizing of the DS machine is optimized with a simplified magnetic circuit model. In addition, the electromagnetic characteristics of the SFMMs with SS and DS structures are investigated and compared by a finite-element (FE) method. The FE results are validated by the experiments on two fabricated prototypes.

Variable Flux Permanent Magnet Synchronous Machine (VF-PMSM) Design Methodologies to Meet Electric Vehicle Traction Requirements with Reduced Losses

Variable flux permanent magnet synchronous machines (VF-PMSMs) in which the magnetization state of low coercive force permanent magnets can be actively controlled to reduce losses in applications that require wide-speed operation have been proposed recently. While prior focus has been on achieving magnetization state manipulation without oversizing the inverter and obtaining higher torque capability, this paper extends the design objectives to include the power requirements of an electric vehicle traction motor over its entire speed range. Finite-element methods are used to study the effect of combinations of low coercive-force and high coercive-force permanent magnets arranged in either series or parallel on the performance of VF-PMSMs. While both configurations help improve the torque density, only the series configuration can help improve the high speed power capability. Experimental results showing the variable magnetization state property, torque-speed capability, and loss reduction capability of a series magnet configuration VF-PMSM test machine are presented.

Design Synthesis of Switched Flux Hybrid-Permanent Magnet Memory Machines

This paper develops a design methodology for switched flux (SF) hybrid permanent magnet memory machines (HPMMMs). The memorable flux is achieved due to the variable magnetization level of low coercive force (LCF) permanent magnets (PMs) Thus, the associated excitation loss is negligible, resulting in high efficiency operation over a wide speed range. A general hybrid PM magnetic circuit is modeled, which is characterized by spoke-array NdFeB PMs and LCF PMs sandwiched between an outer stator ring and an inner stator pole. Based on the magnetic circuit, the design conflicts within the stationary side are unveiled. Thereafter, the machine configuration is introduced, followed by a description of the design procedure. First, the optimal stator pole number is determined based on one-phase machine models. In addition, a simplified permeance function is utilized to analytically optimize the rotor number, accounting for different ratios of the slot opening to NdFeB PM thickness. The electromagnetic performance of the machines with alternate stator/rotor pole combinations are compared. The design guidelines for LCF PMs are presented. Finally, the theoretical analysis is verified experimentally on the prototype machine.