Conventional Surface-mounted Permanent Magnet Research Articles

Design and Analysis of a Variable-Speed Constant-Amplitude Wind Generator for Stand-Alone DC Power Applications

This paper presents the design and analysis of a permanent magnet (PM) wind generator which consists of two sets of windings, and two rotors. The proposed PM wind generator is designed for variable-speed constant-amplitude voltage operation in DC power applications, in order to maximize the utilization of wind energy and make the electricity more accessible to stand-alone situations, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e.g.</i> , remote areas and offshore islands. The operating principle of the variable-speed constant-amplitude voltage operation of the proposed PM wind generator is demonstrated in detail. A comparative study is carried out among the proposed generator, a conventional surface-mounted PM synchronous generator, and an existing counterpart generator used for the same application. The results show that compared to the conventional PM synchronous generator and the existing counterpart, the proposed PM wind generator exhibits the advantages of high induced voltage, high torque/power density, high efficiency, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">etc.</i> Finally, the proposed PM wind generator is prototyped and manufactured. The validity of the variable-speed constant-amplitude operation of the proposed generator under both steady-state and dynamic conditions, is verified by experimental results.

Vibration Reduction Design of Consequent Pole PM Machine by Symmetrizing Local and Global Magnetic Field

The consequent pole permanent magnet (CPM) machine is well known for its high magnet utilization ratio and poor vibration performance. This article focuses on electromagnetic vibration and its reduction design in CPM machines, and the conventional surface-mounted permanent magnet (SPM) machine is used as a benchmark. First, the asymmetrical magnetic field and unbalanced radial force of the CPM machine are investigated with emphasis. The slot-pole combination and force modulation effect are the two reasons for the unbalanced radial force generation. Afterward, two new designs to reduce the vibration caused by the unbalanced radial force are proposed, and the reduction principles are described in detail. Finally, the prototypes of the 12-slot/10-pole SPM and proposed CPM machines are manufactured. The tests are conducted to validate the theoretical analysis and proposed vibration reduction design.

Comparative study of stator consequent‐pole permanent magnet machines

A comparative investigation is made of stator consequent-pole (SCP) machines with different permanent magnet (PM) arrangements. Compared with conventional surface-mounted PM (SPM) configurations, SCP configurations produce more orders of air-gap field harmonics. Due to asymmetrical PM magnetomotive force (MMF) distributions and the double-side slotting effect, SCP machines exhibit multiple air-gap field harmonics, especially relatively enhanced low-order field harmonics, for torque production. As a consequence, improved torque capability can be obtained over that of conventional SPM counterparts. Machine topologies and operating principles are described. The design parameters of four SCP machines with different magnet arrangements are globally optimised. The key electromagnetic performances, including open-circuit performance, torque quality, field-weakening capability, and efficiency, are comprehensively investigated and compared by the finite element (FE) method. Finally, some experimental measurements on an optimally selected SCP machine prototype are carried out to validate the theoretical and FE analyses.

Novel Hybrid-Pole Rotors for Consequent-Pole PM Machines Without Unipolar Leakage Flux

This paper presents two novel consequent-pole permanent magnet (PM) machines featuring N-iron-N-iron-N-S-iron-S-iron-S and iron-S-iron-S-iron-iron-N-iron-N-iron sequences. In order to provide assistant flux and suppress the subharmonics of the air-gap flux density, the tangential PMs are embedded into the proposed rotors of the consequent-pole PM (CPM) machines to form the novel hybrid-pole PM (HPM) machines. The principle of eliminating unipolar leakage flux and the performance improvement by hybrid pole are analyzed based on the simplified PM magnetic circuit. Then, the proposed rotors of CPM and HPM machines, as well as their conventional counterpart with an N-iron sequence, are optimized by finite-element analyses. Furthermore, the electromagnetic performances of the proposed machines, including the open-circuit air-gap flux density, average torque, torque ripple, loss, efficiency, unbalanced magnetic force, demagnetization withstand capability, and unipolar leakage flux, are compared with that of the conventional surface-mounted PM (SPM) and CPM machines. It is demonstrated that both the conventional CPM and proposed HPM machines have >20% higher PM material utilization ratio than the SPM machine. The proposed HPM machine can obtain almost equivalent torque and efficiency compared to the conventional CPM machine. Moreover, unipolar leakage flux does not exist in the end shaft, and thus the magnetization of the mechanical components can be effectively eliminated. Finally, a 9-slot/10-pole HPM machine is prototyped and measured to validate the analyses.

Flux-Focusing Permanent Magnet Machines With Modular Consequent-Pole Rotor

In this paper, the modular consequent-pole (MCP) rotor is adopted in the fractional-slot interior permanent magnet (IPM) machine to utilize the strong flux-focusing effect and reduce the leakage flux. Hence, the torque density and utilization ratio of permanent magnet (PM) material can be improved. However, the unacceptable unbalanced magnetic force (UMF) may arise due to asymmetric airgap flux density produced by consequent-pole topology. Thus, a pole-shaping method based on three-sectional arcs is proposed to suppress it. Due to the relatively complex rotor structure, the multiobjective optimization combined finite-element method with genetic algorithm is performed in this paper. Furthermore, the electromagnetic performance of the IPM machine with MCP rotor (namely IPM-MCP machine), including the open-circuit airgap flux density, phase back EMF, torque, PM utilization ratio, efficiency, flux-weakening capability and UMF, are compared with the conventional surface-mounted PM (SPM), IPM, and consequent-pole IPM (IPM-CP) machines. It is demonstrated that the IPM-MCP machine obtains the largest output torque in these machines. Meanwhile, the IPM-MCP and IPM-CP machines respectively have >30% and >20% higher PM utilization ratio than the conventional SPM and IPM machines while they obtain similar torque ripple and efficiency. Moreover, the IPM-MCP machine can obtain much lower UMF than the IPM-CP machine due to the effective suppression of asymmetric airgap flux density. Finally, a 12-slot/10-pole IPM-MCP machine is fabricated to verify the analyses.

Optimal Design and Experimental Test of a SPM Motor With Cost-Effective Magnet Utilization to Suppress Torque Pulsations

This paper presents an optimal design for a surface-mounted permanent magnet (SPM) motor to suppress torque pulsations and save the magnet cost, by using multi-grade permanent magnets. Based on a conventional SPM motor with single-grade bonded NdFeB magnets, the proposed SPM motor is designed with two-grade bonded NdFeB and one-grade ferrite magnets, and then optimized by combining the Kriging method with a genetic algorithm, for further minimizing torque pulsations and saving the magnet cost by maintaining the high average torque. As a result, the cogging torque, torque ripple, and magnet cost of the optimized SPM motor are highly reduced compared to those of the conventional SPM motor. Furthermore, the optimized SPM motor is verified to have superior endurance against the magnet's irreversible demagnetization within three times of the rated current excitation. All motor characteristics, including airgap flux density, cogging torque, electromagnetic torque, and demagnetization ratio are first predicted with the aid of a finite-element method. Finally, the optimal design and analysis results are validated by the experimental results.

Analytical Determination of Optimal PM-Arc Ratio of Consequent-Pole Permanent Magnet Machines

This paper analytically derives the optimal permanent magnet (PM) arc ratio of the consequent-pole PM (CPM) machines to improve the utilization of rare earth PM material and maximize the output torque. The relationship between the output torque and the PM-arc ratio is established. Moreover, the optimal PM-arc ratio is analytically derived and confirmed by finite element (FE) analysis. It is found that the optimal PM-arc ratio is determined by the ratio of the airgap length to the PM thickness, and its influence on the optimal PM-arc ratio is investigated as well. The electromagnetic performances of the CPM machine with the optimal PM-arc ratio, including the open-circuit airgap flux density, the back EMFs, the average torque, and the torque ripple, are compared with those of the conventional surface-mounted PM (SPM) machine. It is demonstrated that the CPM machine with optimal PM-arc ratio features 33% reduction of PM volume and its torque ripple is almost unchanged compared with the conventional SPM machine. Although the output torque is decreased by 8%, the torque per PM volume is improved by 38%. Finally, a 12-slot/10-pole CPM machine is prototyped and tested to validate the analyses.

Field-Weakening Capability of Interior Permanent-Magnet Machines With Salient Pole Shoe Rotors

Brushless permanent-magnet (BLPM) machines, with inherent advantages of high-power density and high efficiency, have been widely employed to achieve traction characteristics for traction applications. Generally, traction characteristics require high torque at low speed and wide field-weakening region keeping constant power. However, both conventional interior permanent-magnet (IPM) and surface-mounted permanent-magnet (SPM) machines suffer from high-speed issues in the field-weakening region. A different BLPM machine topology, the pole shoe topology, is proposed in this paper. Although the pole shoe machine is common in industrial variable speed drives employing constant torque regimes, it has not been previously considered for machines designed for a wide field-weakening region. For analysis and comparison, a conventional IPM machine, which is employed as the Nissan Leaf vehicle traction machine, is studied as a reference benchmark machine. Experimental results from this machine are used to validate the analysis presented in this paper. The design results show that the proposed pole shoe machine achieves better field-weakening performance, compared with the conventional IPM and SPM machine topologies.

Optimal Design of a Novel Asymmetrical Rotor Structure to Obtain Torque and Efficiency Improvement in Surface Inset PM Motors

This paper proposes an optimal design of a novel asymmetrical rotor structure for surface inset permanent magnet (PM) motors to obtain torque and efficiency improvement. Different from the conventional approach, the proposed design of asymmetrical rotor structures is employed to improve the torque production by creating rotor asymmetry to allow the reluctance torque and the magnetic torque to reach a maximum at the same current phase angle. To evaluate the contribution, the frozen permeability method is utilized to segregate the torque into its reluctance and magnetic torque components. For demonstrating the design concept and obtaining a criterion for improving torque by making full use of torque components, an optimal design by iterative computation is first to be performed utilizing the finite-element method. Based on the obtained criterion, the optimal design by algorithms, such as the Kriging method and genetic algorithm, is applied to further improve the torque and efficiency. As a result, the performance of the proposed surface inset PM motor by two-step optimization is dramatically improved compared with that of the conventional surface inset PM motor. Furthermore, a comparison between the optimized surface inset PM motor and a conventional surface-mounted PM (SPM) motor is also performed under the same operating condition, which demonstrates that the optimized surface inset PM motor can significantly save the magnet amount compared with the conventional SPM motor for producing the same output torque.

Analytical Calculation of Magnetic Field and Cogging Torque in Surface-Mounted Permanent Magnet Machines Accounting for Any Eccentric Rotor Shape

This paper presents an improved analytical method for calculating the magnetic field and cogging torque in the surface-mounted permanent-magnet (PM) machines accounting for any eccentric rotor shape. The magnetic field in the surface-mounted PM machines is predicted according to the surface-current method of the PM, the subdomain model, and the superposition principle of vector potential. The cogging torque is calculated based on Maxwell stress theory and the air-gap magnetic field. The investigation shows that harmonic contents of radial flux density can be reduced a lot by changing eccentric distance of eccentric magnet poles compared with conventional surface-mounted PM machines with concentric magnet poles. Moreover, the analysis has made it clear that the cogging torque of motor with eccentric magnet poles is much smaller than the cogging torque of motor with concentric magnet poles. The finite-element results confirm that the developed analytical method has high accuracy for predicting the magnetic field and cogging torque.

Modeling of Novel Permanent Magnet Pole Shape SPM Motor for Reducing Torque Pulsation

This paper deals with the modeling of a novel permanent magnet (PM) pole shape for a surface-mounted permanent magnet (SPM) motor so as to generate sinusoidal back EMF and linear electromagnetic torque. Conventional SPM motors have a square-shaped PM pole structure; therefore, square back EMF is generated along with the torque pulsations. This novel SPM motor has sinusoidal PM pole shape in the axial direction; therefore, sinusoidal back EMF is generated due to the sinusoidal magnetic flux distribution. Sinusoidal back EMF eliminates most of the cogging torque; thus, a linear electromagnetic torque is generated. Square and sinusoidal current excitation is provided as to analyze the electromagnetic torque characteristics of SPM motor models. As compared to square current excitation, sinusoidal current excitation shows better linearity as well as average electromagnetic torque for the novel SPM motor model. 3-D finite element analysis (FEA) is utilized to analyze and compare the back EMF, cogging torque, and electromagnetic torque of conventional and proposed novel PM pole shaped SPM motor models for both square and sinusoidal current excitation.