Surface Permanent Magnet Machines Research Articles

Optimal Design of a Surface Permanent Magnet Machine for Electric Power Steering Systems in Electric Vehicle Applications Using a Gaussian Process-Based Approach

The efficient design optimization of electric machines for electric power steering (EPS) applications poses challenges in meeting demanding performance criteria, including high power density, efficiency, and low vibration. Traditional optimization approaches often fail to find a global solution or suffer from excessive computation time. In response to the limitations of traditional approaches, this paper introduces a novel methodology by incorporating a Gaussian process-based adaptive sampling technique into a surrogate-assisted optimization process using a metaheuristic algorithm. Validation on a 72-slot/8-pole interior permanent magnet (IPM) machine demonstrates the superiority of the proposed approach, showcasing improved exploitation–exploration balance, faster convergence, and enhanced repeatability compared to conventional optimization methods. The proposed design process is then applied to two surface PM (SPM) machine configurations with 9-slot/6-pole and 12-slot/10-pole combinations for EPS applications. The results indicate that the 12-slot/10-pole SPM design surpasses the alternative design in torque density, efficiency, cogging torque, torque ripple, and manufacturability.

Investigation of Six-Phase Surface Permanent Magnet Machine With Typical Slot/Pole Combinations for Integrated Onboard Chargers Through Methodical Design Optimization

This article presents an analytical magnetic equivalent circuit (MEC) modeling approach for a six-phase surface-mounted permanent magnet (SPM) machine equipped with fractional slot concentrated winding (FSCW) for integrated onboard chargers. For the sake of comparison, the selected asymmetrical six-phase slot/pole combinations with the same design specifications and constraints are first designed based on the parametric MEC model and then optimized using a multiobjective genetic algorithm (MOGA). The commercial BMW i3 design specifications are adopted in this article. The main focus of this study is to achieve optimal design of the SPM machine considering both the propulsion and charging performances. Thus, a comparative study of the optimization cost functions, including the peak-to-peak torque ripple and core losses under both motoring and charging modes and electromagnetic forces (EMFs) under charging, is conducted. In addition, the demagnetization capability in the charging mode and the overall cost of the employed machines are optimized. Since the average propulsion torque is crucial in electric vehicle (EV) applications, it is maintained through the design optimization process. Furthermore, finite element (FE) simulations have been carried out to verify the results obtained from the analytical MEC model. Eventually, the effectiveness of the proposed design optimization process is corroborated by experimental tests on a 2-kW prototype system.

Efficiency modeling and comparison of surface and interior permanent magnet machines for electric vehicle

This paper presents the analysis and comparisons between the Surface Permanent Magnet (SPM) and Interior Permanent Magnet (IPM) Machines about their loss and efficiency characteristics. To calculate the airgap flux density and torque, an analytic model is created. The loss functions are derived using the key design parameters which have a great influence on the loss characteristic. As a result, the efficiency can be calculated. The relationship between the design parameters and efficiency characteristic is investigated adopting the proposed method. The efficiency of different driving cycles is also calculated and compared. Finally, some principles are obtained to help the initial design to fit the actual loading requirement. The analytic results are compared with those by Finite Elements Analysis (FEA), and a prototype is made and tested to validate the proposed method and analysis.

On Torque Improvement by Current Harmonic Injection in Isotropic and Anisotropic Multiphase Machines

Multiphase machines give the possibility to increase the torque injecting and controlling current harmonics of different orders. This work investigates the current harmonic injection techniques for both isotropic and anisotropic machines. The proposed analysis shows that for anisotropic machines, both amplitudes and angles of the optimal current injection should not be predicted based on the back electromotive force only. An analytical model is used to assess the differences in terms of no load and load voltage spectra. Considering these differences, a current injection technique based on the harmonic content of the voltage waveforms is proposed. In addition, the analysis to determine the optimum amplitudes and angles to maximize the torque capability is carried out by means of finite-element analysis (FEA) and compared with the techniques based on the analytical model. The FEA is carried out for both surface permanent magnet and V-shaped interior permanent magnet (IPM) machines with dual-three-phase winding layouts and injecting the fifth harmonic of current. Finally, the proposed concept is validated via an experimental test on a dual-three-phase V-shaped IPM machine. The results show that current harmonic injection based on the voltage vectors is applicable for both isotropic and anisotropic machines with good accuracy.

Performance Entitlement by Using Novel High Strength Electrical Steels and Copper Alloys for High-Speed Laminated Rotor Induction Machines

The laminated rotor Induction Machine (IM), with its simple construction and manufacturing, robustness, ease of control and comparatively lower cost remains by far the most utilized electromechanical energy converter. At very high speeds, traditionally its use is considered to be limited to the previously established operational limits of 2.5 × 105 rpm√kW, beyond which the surface Permanent Magnet (PM) Machine and the solid rotor Induction Machine become the machines available for consideration. The aforesaid limits are derived from the use of classic materials. This paper reviews the recent developments in electrical steels and copper alloys and translates these into the resulting performance entitlement and operational limits through a case study involving a marine application, for which an existing rare-earth PM machine is in use. It is concluded that with novel materials, laminated rotor induction machines can be operated up to 6 × 105 rpm√kW, thus opening the use of the rare-earth free Induction Machine for a wider application range previously limited to PM machines.

Development of an external rotor surface permanent magnet machine with high continuous torque density

In-wheel motor fundamentally simplifies the mechanical transmission structure and is an ideal choice for the motor of electric vehicle. In the motor design process, not only the motor need excellent electromagnetic performance, but also the motor need good heat dissipation design. In this paper, the external rotor SPM machines are optimized for electromagnetic and thermal performance under the space limitation. By improving the heat dissipation structure of the end winding, the overall temperature rise of the external rotor SPM motor is reduced. The experimental results show that at the winding temperature of 140 degrees centigrade the continuous torque is increased by 25.2% with the improved cooling structure. The purpose of improving the continuous torque capability of the motor is achieved.

Significance of Anisotropic Thermal Expansion in High Speed Electric Machines Employing NdFeB Permanent Magnets

Many high speed applications employ a surface permanent magnet (PM) machine topology with a retaining sleeve due to its robustness and ability to achieve high overall peripheral speeds as well as efficiencies. One often overlooked feature in the mechanical design of such machines, which has not achieved sufficient attention to date is the anisotropic thermal expansion of rare earth magnets, the degree of which varies for different magnet technologies. This paper investigates the effects of the aforementioned on the mechanical design of a high speed PM spindle machine with NdFeB magnets. The maximum allowable interference is found to be limited by the working temperature of the magnets while the minimum required interference is increased due to their anisotropic thermal expansion. Based on this, appropriate conditions are formulated to integrate a Neodymium Iron Boron (NdFeB) PM in high speed rotors. These modifications considering the shaft together with the magnet anisotropic thermal expansion are included in a proposed rotor design and validated using simulations in ANSYS mechanical environment.

Torque Production Limit of Surface Permanent Magnet Synchronous Machines and Their Electromagnetic Scalability

In view of the increasing demand in torque density, this article propounds the idea of looking for an upper bound of the torque production and average shear stress for surface permanent magnet (SPM) synchronous machines. The derivation is based on the assumption of an infinite permeability of the iron core and employs the transfer relation between the normal magnetic flux density and the tangential magnetic strength. The result is written as functions of the machine's major geometries and excitation conditions. The ratio of the actual value and this upper bound may be used as a metric of measuring the usage of materials' electromagnetic capability, or reversely as an indicator of the marginal gain of the iron core of higher relative permeability. The result is further investigated to discuss the electromagnetic scalability and the sizing law of SPM machines. Specifically, the reason for increasing volumetric torque density as the machine size goes up is revealed. The optimal remanence flux density of permanent magnets is also predicted at 1.91 T, providing that the lamination saturates at 2 T.

Design of Low-Cost Synchronous Machine to Prevent Demagnetization

The request for high efficiency motor paves the way for the replacement of induction motors with permanent magnet synchronous motors. Although the efficiency is increased, for medium and high power, the current ripple causes significant additional losses in the magnet and lamination; and, high temperature can lead to demagnetization. In this paper, a new rotor topology is proposed and compared to a traditional surface permanent magnet rotor to reduce the magnet losses and protect them from demagnetization. A reference surface permanent magnet machine is compared with the proposed one in terms of performance and magnet losses. Both analytical and experimental analysis are carried out and discussed.

Estimation of PM Machine Efficiency Maps From Limited Data

This article investigates the accuracy of the estimation of efficiency maps for permanent magnet (PM) machines using the stator resistance, d - and q -axis flux-linkages versus the corresponding axis current and the iron loss versus the speed characteristic. The ultimate goal is to apply this approach to the experimental measurements, but this article performs initial investigation using only the finite-element (FE) data. Detailed FE data for 50-kW surface PM (SPM) and interior PM (IPM) machines are used to determine the “actual” or exact efficiency map and, hence, the accuracy of using approximations. This article examines the effect on the torque–speed capability curve when ignoring cross-saturation effects. It also examines the modeling of the variation of iron losses as a function of load in the constant torque and power regions. A novel approach based on scaling the no-load (NL) losses as a function of load is proposed and shown to give promising results. FE results from two other machines are also provided, which show good correspondence.

Investigation Into Multi-Layer Fractional-Slot Concentrated Windings With Unconventional Slot-Pole Combinations

Fractional-slot concentrated windings (FSCWs) are an attractive option for the design of synchronous permanent-magnet machines. It is commonly assumed in the existing literature that a symmetrical three-phase FSCW is feasible only on a condition that the number of slots Z is an integer multiple of three times the maximum common divisor between Z and the number of pole pairs p . Slot-pole combinations satisfying this rule can be defined conventionally, the others unconventionally. In contrast to the common belief, this paper shows that, using a multi-layer arrangement, it is possible to synthetize a symmetrical FSCW having unconventional slot-pole combinations. A general design methodology for this purpose is presented and validated by finite element analysis. The pros and contras of FSCWs with unconventional slot-pole combinations are examined. Finally, the application of an unconventional FSCW to a shipboard surface permanent-magnet machine prototype is presented to illustrate the possible practical convenience of this kind of winding and tests on the prototype are reported for experimental validation.

Design for Reliability: The Case of Fractional-Slot Surface Permanent-Magnet Machines

Surface permanent-magnet machines are widely used in different applications, from industrial automation to home appliance and electrical traction. Among any possible machine topology, the fractional-slot surface permanent-magnet one has gained increasing importance, because of its high torque density, low cogging torque, extended flux weakening capability and high efficiency. In addition, fractional-slot machines are attractive for tooth concentrated windings, which allow some optimized manufacturing solutions such as modular stator tooth and high slot filling factor, which result in copper volume reduction; cost reduction, and lower stator parasitic resistances. The slot–pole combination is one of the most important design parameter and, as shown in this paper, it affects performances and the robustness of the machine with respect to the manufacturing imperfections. In the literature, slot–pole combinations are optimized at design phase by finite-element analysis relying on a healthy machine model. The original contribution of this paper is a design for reliability method that models manufacturing defects and includes them at design phase in the optimization process of slot–pole combinations. A method is presented that allows defining the optimal design parameters for maximum performances and robustness towards unavoidable imperfections caused by tolerances of the manufacturing process.

High Torque Density and Low Torque Ripple Shaped-Magnet Machines Using Sinusoidal Plus Third Harmonic Shaped Magnets

This paper proposes a novel shaped-magnet surface permanent magnet machine design concept featuring magnets shaped according to a sinusoid plus a third harmonic along the axial direction, and full-pole-pitched distributed stator windings with a unity winding factor to provide high torque density with essentially zero torque ripple for high-performance motor applications. The magnet thickness is uniform along the radial direction so as not to impair the magnet's ability to handle the demagnetization force. The proposed design was compared against a full-pole-pitched magnet design with maximum magnetic loading and a conventional bread-loaf magnet design. The results demonstrate that the proposed shaped-magnet design uses the magnet materials efficiently to achieve torque density comparable to that of the full-pole-pitched magnet design, and essentially zero torque ripple without compromising its resistivity against a demagnetization force, which conventional bread-loaf magnets cannot offer.

A New Perspective on the PM Vernier Machine Mechanism

Permanent magnet vernier (PMV) machines have attracted more and more attention for their merits of high torque density and simple structure. Also, the principle of electromechanical energy conversion is the most common way to investigate the PMV machine by calculating back electromotive force and electromagnetic torque. In this paper, a new perspective on the mechanism of PMV machines based on the Maxwell stress tensor method is presented to deepen the insight into the reason why the force on the rotor of a PMV machine is larger than that of an surface permanent magnet (SPM) machine. Based on the finite element analysis (FEA) method, three machines with exactly the same rotor are analyzed and compared, namely 24-slot/20-pole SPM, 12-slot/20-pole surface PMV, and 6-slot/20-pole split-tooth PMV machines. The radial and tangential flux densities and force distributions along the airgap are illustrated. The influence of pole ratio on the performance of PMV machines is also investigated. It is shown that the improvement of tangential flux density in the PMV machine plays a primary role in the higher torque density, which shows a promising way to improve the torque density of machines.

Permanent-Magnet Machine Flux and Torque Response Under the Influence of Turn Fault

This paper analyzes different stator windings configurations for permanent-magnet synchronous machines during the presence of a stator turn fault. Due to the inverter pulsewidth modulation, the fault commonly occurs in variable-frequency permanent-magnet machine drives. In this paper, two types of windings are compared to identify key machine topologies that can achieve the reduced influence of the turn fault. When the machine is operated in real time, the windings with separated neutral points reduce the torque ripple caused by fault-reflected flux harmonics. Analytical models based on equivalent circuits are developed to predict the machine flux and torque waveform during the presence of the turn fault. All developed analytical models are verified by the finite-element analysis and experimental results on a 750-W surface permanent-magnet machine prototype.

Implementation Issues of Flux Linkage Estimation on Permanent Magnet Machine Position Sensorless Drive at Low Speed

This paper improves the surface permanent magnet (PM) machine position sensorless drive at low speed. Considering the surface PM machine (SPM) drive, EMF voltage or flux linkage should be estimated for the sensorless drive. Different from EMF voltage, the flux linkage based on the voltage integration is theoretically independent to speed which is suited for the low speed position estimation. In this paper, several improvements on the flux-based sensorless drive are proposed to enhance the low speed dynamic performance. First, a modified voltage integration is develop to remove the flux estimation drift caused by voltage or current offset. This integration contains a high-pass filter (HPF) for the DC drift elimination. In addtion, the filter delay is compensated to maintain the flux phase. Second, inverter deadtime harmonics are decoupled with the knowledge of actual machine phase voltages. It is shown that the position estimation error is decreased for the better low speed performance. According to experimental results, SPM machine sensorless drive is enhanced at 4%~6% speed region from many aspects. They include position signal SNR, position error and drive dynamic response. More importantly, the overall current regulation bandwidth can increase to 1kHz at low speed. It is compatible to standard encoder-based field oriented control (FOC) drives.

Investigation of the End Leakage Flux in Fractional Slot Concentrated-Winding Surface Permanent Magnet Machines

When the end leakage flux or end leakage inductance cannot be ignored in a permanent magnet (PM) machine (e.g., a PM machine with a short axial length), the analysis of the machine is a time-consuming three-dimensional (3-D) issue. Existing research has made the end leakage inductance no longer to hinder that the performance calculation of fractional slot concentrated-winding surface permanent magnet (FSCWSPM) machines is simplified into a time-saving 2-D issue. However, no such mature research exists for end leakage flux. This paper proposes a novel end leakage flux function that can be used to accurately quantify the end leakage flux of FSCWSPM machines. Then, based on sensitivity research and regression analysis, a parametric model is established to quickly construct the end leakage flux function. Thus, even for FSCWSPM machines with nonnegligible end leakage flux, the results comparable to those from the 3-D finite element method (FEM) can be easily obtained using the proposed end leakage flux function and parametric model to revise the results from 2-D FEM. Moreover, the application of the above function and model is briefly described. Finally, FEM and experimental results are used to verify the calculation precision, adaptability, and timeliness of the proposed methodology.

Guest Editorial Emerging Electric Machines and Drives for Smart Energy Conversion

Energy conversion is one of the key steps for human utilizing the energy. In particular, as the key component for electromechanical energy conversion, electric machines play a significant role and greatly promote the development of modern civilization. Actually, the conventional types of electric machines, namely DC machines, induction machines, and synchronous machines, as well as surface permanent-magnet (SPM) machines and inserted-PM (IPM) machines, have been widely applied for various aspects in human lives. In recent years, emerging electric machines, including the double-salient type, hybrid-excitation type, PM memory type, magnetic-gearing type, PM vernier type, advanced magnetless type, multiport type, and some newly developing types, are proposed for new application occasions such as electric vehicles (EVs), electric ships, electric aircraft, electric robotics, wind power generation, unmanned propulsion systems, and so on. In addition, the corresponding advanced control strategies have presented for electric drives, which include the direct torque control (DTC), field oriented control (FOC), finite control set model predictive control (FCS-MPC), fault-tolerant control, and sensorless control. Thus, emerging electric machines and drives take active roles for applications with more electric or full electric. This special section brings new findings of electric machines and drives on their topologies, structures, analysis approaches, advanced control strategies, and new applications.

Modular Spoke-Type Permanent-Magnet Machine for In-Wheel Traction Applications

This paper proposes a novel modular spoke-type permanent-magnet machine for in-wheel traction applications. First, the topology and the operating principle are briefly introduced. Then, an analytical model of the proposed machine is deduced based on conformal mapping. A three-phase 48-slot 52-pole modular spoke-type permanent-magnet (PM) machine is designed based on the deduced mathematical model where the dimensions and power source are constrained the same as those of a commercial three-phase surface PM machine in electric motorcycles. Electromagnetic performance comparisons among the proposed machine, the commercial machine, and a conventional spoke-type PM machine are conducted based on finite-element analysis (FEA) with respect to magnetic field, back electromotive force (EMF), flux-weakening capability, torque capability, machine efficiency, etc. The results indicate that the proposed machine has an improved torque, a higher efficiency, and an extremely better flux-weakening capability than the conventional in-wheel machine. Finally, a prototype machine is manufactured and tested to verify both the analytically and FEA predicted results.

On-Load Analysis of Saturated Surface Permanent Magnet Machines Using Conformal Mapping and Magnetic Equivalent Circuits

This paper presents an iterative method based on conformal mapping and magnetic equivalent circuits for on-load analysis of saturated surface permanent magnet machines with integer slot and fractional slot windings. Magnetic saturation is taken into account by defining additional point wires in the stator slots and interpolar region between the magnets. The added point wires create magnetomotive force equal to the magnetic voltage drops across iron core flux tubes that are calculated using magnetic equivalent circuit. An improved stator tooth model has been introduced that enables more accurate field calculations for tooth geometries with highly saturated tooth tips. The model also takes into account variation of permanent magnet magnetization due to external magnetic field. The proposed method has been implemented on a saturated 12 slot, 10 pole surface permanent magnet machine with nonoverlapping concentrated winding and shows a very good agreement with finite element calculations in terms of flux density, back electromotive force, and torque waveforms as well as $d$ - and $q$ -axis inductances, cross-saturation inductance, and permanent magnet flux linkage in both axes.