Rare-earth PM Research Articles

Review of linear flux-switching machines for variable speed applications

Purpose This paper aims to present an overview of permanent magnet linear flux-switching machines (PMLFSM), field excited LFSM and hybrid excited LFSM (HELFSM) topologies as presented in literature for transportation systems such as high-speed trains and maglev systems. Design/methodology/approach The structural designs of different configurations are thoroughly investigated, and their respective advantages and disadvantages are examined. Based on the geometry and excitation sources, a detailed survey is carried out. Specific design and space issues, such as solid and modular structures, structure strength, excitation sources placement, utilization of PM materials, and flux leakage are investigated. Findings PMLFSM provide higher power density and efficiency than induction and DC machines because of the superior excitation capability of PMs. The cost of rare-earth PMs has risen sharply in the past few decades because of their frequent use, so the manufacturing cost of PMLFSM is increasing. Owing to the influence of high-energy PMs and magnetic flux concentration, the efficiency and power density are higher in such machines. PM is the only excitation source in PMLFSM and has constant remanence, limiting its applications in a wide speed operation range. Therefore, the field winding is added in the PMLFSM to flexibly regulate the magnetic field, making it a hybrid excited one. The HELFSM possess better flux linkage, high thrust force density and better flux controlling ability, leading to a wide speed range. However, the HELFSM have problems with the crowded mover, as PM, field excited and armature excitation are housed on a short mover. So, for better performance, the area of each excitation component has to compete with each other. Originality/value Transportation of goods and people by vehicles is becoming increasingly prevalent. As railways play a significant role in the transportation system and are an integral part of intercity transportation. So, this paper presents an overview of various linear machines that are presented in literature for rail transit systems to promote sustainable urban planning practices.

Improved calculation of core losses of high-speed motors with two different permanent magnets considering higher order current harmonics

In high-speed permanent magnet synchronous motor drives, time harmonics in the current occur owing to the carrier frequency of the inverter, which affects the core losses among the electromagnetic losses. According to Steinmetz’s equation, the core loss depends on the frequency and magnetic flux density, with each term influenced by harmonic currents. Therefore, to accurately predict core loss, it is necessary to consider harmonic currents. This study presents the effects of differences in the resistance, inductance, switching frequency, and harmonic current on the core loss for two models with different PMs at the same output power. Core losses are significantly affected by harmonic current content and magnetic field behavior, making the analysis of harmonic components and magnetic field behavior essential. To achieve this, the stator core was divided into segments to examine the magnetic field behavior owing to harmonic currents in each region, thus facilitating a comprehensive analysis of the core losses. When the results were compared with the experimental data, it was observed that the analysis of harmonic currents significantly improved the accuracy compared to the analysis based solely on sinusoidal currents; in particular, the error rate reduced from 37.8% to 4.1%. Furthermore, it was confirmed that ferrite PMs resulted in smaller core losses owing to harmonic currents than rare-earth PMs. This finding suggests that ferrite PMs are a suitable alternative to high-speed permanent-magnet synchronous motors.

A comparative study of electric motor for low-power density electric vehicles.

Due to the price uncertainty of rare-earth PM materials and limited availability around the globe, it is required to think about new electric motor candidate for automotive industries. According to the literature survey, it is observed that PMBLDC is the widely used in automotive industry for low power applications. There are some noticeable limitations reported with this motor such as high cost of PM, demagnetization, and complex control. Based on the comparative analysis of three motors, i.e., Synchronous Reluctance Motor (SynRM), Permanent Magnet Synchronous Motor (PMSM), and PM-assisted Synchronous Reluctance Motor (PMASynRM) through Finite Element Method (FEM) for the same design parameters, it is concluded the proposed alternative is a PMASynRM. As per the research gap findings, authors have designed the PMASynRM for low-power EV applications with novel rotor geometry. The simulation results of FE analysis validate the proposed motor design for the various performance parameters.

A Consequent Pole Single Rotor Single Stator Vernier Design to Effectively Improve Torque Density of an Industrial PM Drive

The Vernier machine can utilize magnetic-gear effect, which conspicuously improves the torque production over other type of synchronous machines. However, a dual-rotor/dual-stator structure enabled high torque density Vernier design is often unfavorable for industrial applications, due to manufacturing complexity and thermal management challenges. This article successfully addresses this problem, by building magnetic-gear effect into a consequent pole enabled alternating flux barrier structure with single-rotor single-stator (SRSS). In this way, the proposed machine can achieve a superior torque density as a Vernier machine does, while maintaining an equivalent manufacturing complexity as a conventional SRSS synchronous machine. This article begins with elaborating generic analytical equations, which can transform into a Vernier machine or an SRSS synchronous machine. More specifically, the magnetic-gear effect is manifested in the equations, via stator teeth originating flux modulation concept and resulting multiharmonic field coupling effects. An SRSS Vernier topology with consequent pole and V-shape magnet is then proposed. A considerable 52.6% improvement on the torque production with comparable efficiency is validated by experiments, where the comparison is made over that of a benchmark rare earth PM machine. Power factor would fall behind with increasing current load. Overall, this new design achieves an outstanding torque density, showing solid potential and feasibility for industry adoption.

A Novel Slot-PM-Assisted Consequent-Pole-PM Machine With Hybrid Magnets of Ferrite PMs and Rare Earth PMs

In this article, slot-PM-assisted consequent-pole-PM machine (SPMA-CPMM) is proposed. The key is that the consequent-pole ferrite PMs are employed in the stator yoke to reduce the PM material cost largely, while a small amount of tangential magnetized rare-earth PMs is placed in the slot opening to improve the torque production. This proposed hybrid magnet design can greatly enhance the PM utilization ratio and reduce the PM cost while maintaining a relatively large torque density. In this article, the configurations of ferrite-PM machine (FPMM), consequent-pole ferrite-PM machine (CFPMM), and SPMA-CPMM are illustrated, while equivalent magnetic circuits (EMCs) of them are compared. Then, the electromagnetic performances of three machines are investigated and compared, which verifies the validity of the proposed machine.

Analysis of Linear Hybrid Excited Flux Switching Machines with Low-Cost Ferrite Magnets

Linear hybrid excited flux switching machines (LHEFSM) combine the features of permanent magnet flux switching machines (PMFSM) and field excited flux switching machines (FEFSM). Because of the widespread usage of rare-earth PM materials, their costs are steadily rising. This study proposes an LHEFSM, a dual stator LHEFSM (DSLHEFSM), and a dual mover LHEFSM (DMLHEFSM) to solve this issue. The employment of ferrite magnets rather than rare-earth PM in these suggested designs is significant. Compared to traditional designs, the proposed designs feature greater thrust force, power density, reduced normal force, and a 25% decrease in PM volume. A yokeless primary structure was used in a DSLHEFSM to minimize the volume of the mover, increasing the thrust force density. In DMLHELFSM, on the other hand, a yokeless secondary structure was used to lower the secondary volume and the machine’s total cost. Single variable optimization was used to optimize all of the proposed designs. By completing a 3D study, the electromagnetic performances acquired from the 2D analysis were confirmed. Compared to conventional designs, the average thrust force in LHEFSM, DSLHEFSM, and DMLHEFSM was enhanced by 15%, 16.8%, and 15.6%, respectively. Overall, the presented machines had a high thrust force density, a high-power density, a high no-load electromotive force, and a low normal force, allowing them to be used in long-stroke applications.

Experimental Validations of Hybrid Excited Linear Flux Switching Machine

Linear Flux Switching Machines (LFSMs) possess the capability to generate adhesive thrust force, thus problems associated with conventional rotatory electric machines and mechanical conversion assemblies can be eliminated. Additionally, the unique features of high force/power density, efficiency, and a robust secondary structure make LFSMs a suitable candidate for linear motion applications. However, deficiency of controllable air-gap flux, risk of PM demagnetization, and increasing cost of rare earth PM materials in case of PMLFSMs, and inherent low thrust force capability of Field Excited LFSMs compels researchers to investigate new hybrid topologies. In this paper, a novel Double-Sided Hybrid Excited LFSM (DSHELFSM) with all three excitation sources, i.e., PMs, DC, and AC windings confined to short moving primary and segmented secondary providing short flux paths is designed, investigated, and optimized. Secondly, unequal primary tooth width optimization and additional end-teeth at all four corners of the primary equip proposed design with balanced magnetic circuit and reduced end-effect and thrust force ripples. Thirdly, the measured experimental results of the manufactured proposed machine prototype are compared with corresponding simulated model results and shows good agreements, thus validating the theoretical study.

Optimal rotor poles and structure for design of consequent pole permanent magnet flux switching machine

Permanent magnet flux switching machines (PMFSM) have attracted significant research interest and are considered as competent candidates when higher torque density is primary requirement. However, conventional PMFSMs uses excessive rare earth PM volumes which ultimately increases machine the machine weight and PM cost. Moreover, the PMs extended at the stator yoke results in stator leakage flux which degrades the performance. To suppress the leakage flux and diminish the PM volume, the consequent pole PMFSM (CPPMFSM) with flux bridges and barriers encompassing partitioned circumferential and radial magnetized PMs is proposed, thereby ensuring an alternate magnetic path for the working harmonics which improves the modulation effect and flux distribution. Moreover, the influence of the rotor pole number on seven different rotor structures namely, curved rotor, trapezoidal rotor, wide rotor tooth tip, wide rotor base width, rectangular segmented and eccentric rotors are investigated based on the electromagnetic performance and stress distribution. Finite element analysis (FEA) reveals that the 12S-13P CPPMFSM with a wider rotor base offers comparatively better electromagnetic performance. Compare to the conventional PMFSM, the proposed CPPMFSM reduces the PM volume which minimizes the overall machine cost and weight, suppresses the torque ripples by 16.49%, diminishes total harmonic distortion (THD) by 35.24% and decreases cogging torque by 32.88%. Furthermore, the torque and power density are enhanced by 7.028% and 7.025% respectively.

Analytical Airgap Field Model and Experimental Validation of Double Sided Hybrid Excited Linear Flux Switching Machine

Linear Flux Switching Machines (LFSMs) are suitable candidates for long stroke applications as they confines all excitation sources to primary thus leaving completely passive, robust, and low cost secondary. Permanent Magnet LFSMs (PMLFSMs) enables high thrust force density and efficiency. However, deficiency of controllable air-gap flux, risk of PM demagnetization, and increasing cost of rare earth PM materials diverted researchers towards Field Excited LFSMs (FELFSMs). FELFSMs wiped out aforementioned PMLFSM’s shortcomings at the cost of low thrust force density. In this paper, merits of PMLFSM and FELFSM are combined by proposing a novel Hybrid Excited LFSM (HELFSM). Proposed machine is excited by PMs, Field Excitation Coils (FECs), and Armature Windings (AWs). However, complex magnetic circuit of poly-excited HELFSM compels designers to adopt FE Analysis (FEA) for design, analysis, and optimization. To decrease dependency on computationally complex and time consuming FEA, an analytical model combining lumped parameter magnetic equivalent circuit, Fourier analysis, Laplace equation, and Maxwell Stress Tensor method is proposed to predict open-circuit flux linkage, B-EMF, normal and tangential components of no-load and on-load magnetic flux density, detent, and thrust force performance. Finally, predictions of the developed analytical model are validated with corresponding FEA and experimental results.

Potentials of locally manufactured wound-field flux switching wind generator in South Africa

The China-based monopoly of high-energy permanent magnet materials used in modern wind generators impact the economic viability and local content value of most wind turbines installed in South Africa, especially large installations. It is possible to design with less expensive excitation technologies using locally-sourced wound-field electromagnets, which might promote local content. This study involves the optimum design performance comparison of the wound-field flux switching machine (WF-FSM) technology based on two variants – Design I and II (D-I and D-II) – the difference being in the arrangement of their DC wound-field coils. The machines are evaluated using finite element analyses (FEA) with optimum performance emphasised on design parameters such as torque density, efficiency and power factor. The selected design targets are meant to improve the performance to cost fidelity of the proposed wind generator variants. In 2D FEA, D-II can produce up to 18.8% higher torque density (kNm/m3) and 17.1% lesser loss per active volume (kW/m3) than D-I. In 3D FEA, the torque density of D-II remains higher at 10.6%, but its loss per active volume increases by 15% compared to D-I. The discrepancy observed in 2D and 3D FEA is due to an underestimation of the end-winding effects in D-II. The power factor of D-II is higher than D-I, both in 2D and 3D FEA, which may translate to lower kVA ratings and inverter costs. A higher total active mass ensues for the studied WF-FSMs than a conventional direct-drive PMSG, but avoiding rare earth PMs translate to significantly lower costs.

Braking a Variable Flux-Intensifying IPMSM in Minimal Time

This paper proposes a method for braking the variable flux-intensifying interior permanent-magnet synchronous motor (FI IPMSM) under a minimal time condition while operating in the high-speed region. The rare-earth PMs retain its magnetization as the flux weakening current is decreased. This inherent feature of the PMs makes the braking torque maximization possible for IPMSMs, thus minimal-time braking can be achieved. In contrast, the low-coercive magnets utilized in variable-flux IPMSMs are partially demagnetized in the high-speed region. This means that the variable-flux IPMSM takes a longer time to stop if compared to the IPMSM. Supplying a magnetizing current that ranges from 1 to 3 p.u. in the high-speed region while the inverter is running out of voltage is quite challenging. In this paper, an analytical solution to the amount of magnetizing current required to maximize the braking torque based on the available voltage to ensure minimal-time braking is presented. The simulated and experimental results obtained using a 5-hp variable FI IPMSM show the validity of the proposed braking scheme. A front-end active rectifier is utilized to recuperate the braking energy and maintain the dc bus voltage constant.

Performance evaluation of a permanent magnet-assisted synchronous reluctance machine with hybrid magnets of ferrite magnets and rare-earth PMs

In this paper, a novel permanent magnet-assisted synchronous reluctance machine (PMASynRM) with rare-earth PMs and ferrite magnets is proposed. The performance of PMASynRM is discussed with respected to the different magnet ratio of rare-earth PMs and ferrite magnets. Some characteristics including the flux density, output torque, cogging torque, output power, power factor, torque ripple, loss, efficiency, and demagnetization are calculated by 2-D finite element analysis (FEA). The analysis results show that the excellent performance can be obtained by using hybrid magnet of rare-earth PMs and ferrite magnets with the suitable magnet ratio, and provide some desirable cost-performance trade-off.

Design and Analysis of a Spoke-Type Hybrid Permanent Magnet Motor for Electric Vehicles

This paper proposes a new spoke-type hybrid permanent magnet brushless (H-PMBL) motor for potential applications in electric vehicles, where the high-energy rare-earth PM and the cost-effective ferrite-PM are combined. In order to realize low-cost design and improve the operating points of the rare-earth PM and the ferrite-PM, the proposed motor is designed based on the calculation of PM cost and analysis on the operating points. Moreover, to fairly estimate electromagnetic performances of the H-PMBL motor, a conventional spoke-type non-rare-earth ferrite-PMBL motor with the same overall dimensions is also analyzed and compared using the finite element method. The simulation results indicate that the H-PMBL motor cannot only retain low PM cost, but also exhibit better flux-weakening ability and enhanced demagnetization withstand capability.

Analysis and design of a high power density permanent magnet-assisted synchronous reluctance machine with low-cost ferrite magnets for EVs/HEVs

Purpose The purpose of this paper is to propose a permanent magnet-assisted synchronous reluctance machine (PMASynRM) using ferrite magnets with the same power density as rare-earth PM synchronous motors used in Toyota Prius 2010. Design/methodology/approach A novel rotor structure with rectangular PMs is discussed with respect to the demagnetization of ferrite magnets and mechanical strength. Some electromagnetic characteristics including torque, output power, loss and efficiency are calculated by 2D finite element analysis. Findings The results of the analysis show that a high power density and high efficiency for PMASynRM can be achieved using ferrite magnets. Originality/value This paper proposes a novel rotor structure of PMASynRM with low-cost ferrite magnets that achieves high power density as permanent machines with rare-earth PMs.

Design and Analysis of Low-Cost Tubular Fault-Tolerant Interior Permanent-Magnet Motor

Tubular motors are suitable for active vehicle suspension due to their zero net radial force and volumetrically efficient. This paper proposes a new low-cost tubular fault-tolerant (TFT) interior permanent-magnet (IPM) motor for the high reliability operation of the active vehicle suspension system. The key is that the proposed motor incorporates the concept of fault-tolerant teeth to provide the desired decoupling among phases and adopts the ferrite magnet material to reduce the usage of rare-earth PMs. Its electromagnetic performances are investigated, such as flux linkage, back electromotive force, mutual- and self-inductances, and detent and thrust forces. A finite-element method is employed to verify the theoretical analysis, showing that the proposed TFT PM motor not only has the merits of low-cost, strong mechanical integrity, but also offers good fault-tolerant ability and high thrust force capability.

Design and Analysis of the New High-Reliability Motors With Hybrid Permanent Magnet Material

This paper proposes a new type of motor with hybrid permanent magnet material (HPMM); termed HPMM motor. The key of the proposed HPMM motor is the excitations, which combined rare-earth PM (REPM) with ferrite PM (FPM). To investigate the HPMM motor extensively, the HPMM motors with parallel magnetic circuit (PMC) and serial magnetic circuit (SMC) are designed. Because of the property difference between the REPM and the FPM, both PMC-HPMM motor and SMC-HPMM motor can offer low total harmonic distortion at high winding factor. Besides, the PMC-HPMM motor needs low consumption of REPM and offers low cogging torque, while the SMC-HPMM motor needs the extraordinary consumption of FPM, but offers little lower cogging torque. In addition, the reliabilities of both PMC-HPMM motor and SMC-HPMM motor are improved because of the single-layer concentrated winding and fault-tolerant teeth. Finally, the demagnetization of two motors has been studied, which indicates that the partial demagnetization will occur in the FPM. All these theoretical analyses are evaluated by the time-stepping finite element method.

평행방향/할박 자화 영구자석을 갖는 마그네틱 기어의 설계변수에 따른 토크특성 해석 및 비교

Magnetic gear can transmit torque without any mechanical contact among rotational parts in rotating mechanical systems. Especially, magnetic gear using rare-earth PMs can be used in variety of industry application because of their great power efficiency. Thus, recent trend shows that magnetic gear can be replaced with common mechanical gear. This paper deals with comparison and torque analysis for magnetic gears with parallel and Halbach magnetization according to various design parameters. Using a two dimensional (2D) finite element (FE) analysis, suitable gear ratio is selected. We performed analysis for magnetically connected inner and outer torque with respect to various design parameters including thickness of inner and outer PM, steel pole angle, segments of Halbach array and magnetization pattern of inner and outer PMs. Finally, we can obtain improved design model having parallel and Halbach magnetization with larger torque, compared with an initial design model.

Automotive Electric Motors, Generators, and Actuator Drive Systems With Reduced or No Permanent Magnets and Innovative Design Concepts

Due to environmental and energy concerns, electric vehicles (EVs) and hybrid EVs (HEVs) are gaining increased attention. At the heart of the drive system of these vehicles is the electric machine and drive. For these applications, high power density, high torque density, wide speed range, and efficiency are of primary importance. To meet these demands, interior-permanent-magnet (IPM) machines have been widely used. However, IPM machines use rare-earth PMs (usually neodymium-iron-boron magnets) in their structure. The price of these types of rare-earth PMs has increased by orders of magnitude over several years. Due to the high cost and limited supply of this material, the use of IPM machines in HEV applications is now being challenged. In fact, many researchers are working on other machines that can compete with IPM machines in terms of size, efficiency, power, and torque density. With consideration of the above points, there is a timeliness to this important topic; thus, we are pleased to present this Special Section on Automotive Electric Motors, Generators, and Actuator Drive Systems With Reduced or No Permanent Magnets and Innovative Design Concepts of the IEEE Transactions on Industrial Electronics that consists of 12 papers reporting on the most recent research on automotive electric drive machines. Most of these papers focus on machines with less or no rare-earth PM material in their structure, although control is also important, and there are some papers related to this topic.

Comparison of Test Result and Design Stage Prediction of Switched Reluctance Motor Competitive With 60-kW Rare-Earth PM Motor

A switched reluctance motor has been designed with identical outer dimensions, maximum torque, operating area, and maximum efficiency as rare-earth permanent-magnet motors used in the Toyota Prius. In this paper, a test machine has been constructed, and test results are presented over the entire speed range. The targets are torque of 207 N·m, a shaft output of 60 kW, and the maximum efficiency of 96%, as well as a speed range of 2768-13 900 r/min with an output of 60 kW and an outer diameter and an axial length of 264 and 112 mm, respectively. It is found that a shaft output of 100 kW is possible at high rotational speed under the voltage and current ratings. The possible operation area in a torque-speed plane is found to be enhanced. It is also found that the design stage prediction is close to the test results, except in two operation regions.

Cost-Effectiveness Comparison of Coaxial Magnetic Gears With Different Magnet Materials

This paper presents a comparative study between the nonrare-earth PM and rare-earth PM-based coaxial magnetic gears. First, using finite-element analysis, the electromagnetic performances of four coaxial magnetic gears, which are installed with nonrare- or rare-earth PMs, are analyzed and quantitatively compared based on the same structure. Then, the natural magnetic properties of nonrare- and rare-earth PMs are evaluated and discussed. Finally, the cost effectiveness of coaxial magnetic gears adopting different types of PMs is assessed. The results show that the nonrare-earth PMs, especially the aluminum-nickel-cobalt, are preferred for application to coaxial magnetic gears with emphasis on the cost effectiveness.