Ronghai Qu

High-Power-Factor Vernier Permanent-Magnet Machines

Vernier permanent-magnet (VPM) machines are well known for high torque density but low power factor. This paper deals with the low power factor of VPM machines. The goal is not obtained by reducing the electrical loading or adjusting current advance angle but by proposing a novel vernier topology, i.e., a dual-stator spoke-array (DSSA) VPM topology. In this paper, the characteristics of the DSSA VPM topology, such as active part, auxiliary mechanical structure, and rotor anisotropy, are analyzed in detail. Performances are evaluated based on finite-element analysis, including power factor, torque density, and cogging torque. The results show that the DSSA VPM topology exhibits high power factor, viz., ~0.9, and significantly high torque capability. The verification of the mechanical structure scheme is also done in this paper. Finally, theoretical analyses are validated by the experimental results by a 44-rotor pole 24-slot DSSA VPM prototype.

Analysis and modeling of air-gap and zigzag leakage fluxes in a surface-mounted permanent-magnet Machine

In this paper the magnetic characteristics of surface-mounted permanent-magnet machines are analyzed and modeled. The air-gap and zigzag leakage fluxes are analytically expressed in terms of the magnetic material properties and the motor dimensions. Both factors are essential quantities for the accurate prediction of the flux distribution within the machine and of the machine torque. Therefore, they are desired for the purpose of machine design and optimization. In order to evaluate the validity of the proposed models, the finite-element method (FEM) analysis is used. The results show that the errors between the FEM results and analytical predictions are less than 7% for the nonsaturated tooth flux and less than 17% for the saturated case. Finally, the models are applied to a novel permanent-magnet machine design.

Review of Superconducting Generator Topologies for Direct-Drive Wind Turbines

Wind energy, as a clean and renewable energy, is now being widely developed to reduce carbon dioxide production and mitigate the energy crisis. The urgent needs for wind energy motivate larger generators with lower cost, lower weight, and higher reliability. A popular solution is the direct-drive generator concept, such as a permanent magnet generator and superconducting (SC) generator. When referring to weight, volume, and cost, SC generators are superior to permanent magnet generators for wind turbines with rated power of 8 MW or more according to a report from the American National Renewable Energy Laboratory. In order to find out the suitable topology for megawatt-class direct-drive wind turbine generators, various designs of SC machines in literatures are carefully reviewed; advantages and disadvantages are discussed and a few ways to benefit from their advantages are pointed out. Electromagnetic, mechanical, and thermal structures, including excitation system, SC support system, cryogenic cooling system etc., are reviewed for wind SC machines. Design challenges and possible solutions are also summarized.

Relationship between magnetic gears and vernier machines

It is demonstrated that a vernier machine and a magnetic gear work under a same principle, and the vernier machine teeth perform the flux modulation function, same as the ferromagnetic pieces do in a magnetic gear. A vernier machine is equal to a system consisting of a magnetic gear and a normal machine. Moreover, this paper presents how to convert a magnetic gear machine system to a vernier machine, and discusses structure differences among vernier machine, magnetic gear, and many other magnetic gear based topologies. It is illustrated that all these topologies share a same operation principle. This conclusion is verified by FEA simulation results. Finally, advantages and disadvantages of each topology are discussed and some application suggestions are given as well.

Cogging torque minimization technique for multiple-rotor, axial-flux, surface-mounted-PM motors: alternating magnet pole-arcs in facing rotors

A variety of techniques exist for reducing the cogging torque of conventional radial flux PM machines. Even though some of these techniques can be applied to axial flux machines, manufacturing cost is especially high due to the unique construction of the axial flux machine stator. Consequently, new low cost techniques are desirable for use with axial flux PM machines. This paper introduces a new cogging torque minimization technique for axial flux multiple rotor surface magnet PM motors. First, basic principles of the new technique are explored in this paper. A 3-kW, 8-pole axial flux surface-magnet disc type machine with double-rotor-single-stator is then designed and optimized in order to apply the proposed new method. Optimization of the adjacent magnet pole-arc which results in minimum cogging torque as well as assessment of the effect on the maximum available torque using 3D finite element analysis (FEA) is investigated. The minimized cogging torque is compared with several existing actual machine data and some important conclusions are drawn.

Effect of Number of Phases on Losses in Conducting Sleeves of Surface PM Machine Rotors Equipped With Fractional-Slot Concentrated Windings

High-speed machines with a solid rotor or a high- strength retaining sleeve could offer design and performance advantages. For a specific application, if the use of a high-strength nonmagnetic metallic retaining sleeve is more advantageous than a nonmetallic (e.g., carbon fiber) one, one needs to evaluate the eddy-current losses due to armature reaction space and time harmonics and/or tooth ripple, as they can be significant. This problem is aggravated furthermore in fractional-slot concentrated-winding machines due to their inherent sub- and super nonsynchronous MMF harmonic components. In this paper, the impact of the number of phases is quantified to help design a lower eddy loss rotor sleeve for a high-speed surface-mounted permanent-magnet rotor machine with fractional-slot concentrated armature winding, FSPCW-SPM. The goal of this paper is to provide a general method for laying out preferred FSPCW configurations. Also, a general method for screening and choosing the optimal slot/pole combinations is presented.

Performance comparison of dual-rotor radial-flux and axial-flux permanent-magnet BLDC machines

A novel machine family-dual-rotor, Radial-Flux, Permanent-Magnet (RFPM) machines-has demonstrated that it can substantially improve machine torque density and efficiency. The objective of this paper is to provide a performance comparison between two major alternatives of this technology: Surface-mounted dual-rotor RFPM machines and Axial-Flux Permanent-Magnet (AFPM) machines. The comparison is accomplished at four power levels ranging from 3 to 50 HP at a constant speed of 1800 RPM. The comparison includes material weights and costs, copper and iron losses torque and power per unit active volume, torque per unit active material weight, magnet material effect, losses per unit airgap area, and machine efficiency. Pole number effects on both machine types are investigated as well. The results reveal an Indication of the machine best suited with respect to performance criterion for a particular design requirement.

Comparison of Halbach and Dual-Side Vernier Permanent Magnet Machines

A closed-form analytical model is developed to build the special rules for vernier permanent magnet (VPM) machines comparison. Based on the finite element analysis (FEA), comparisons of some key parameters including power factor are made among three novel VPM topologies, i.e., Halbach, dual-stator (DS), and DS spoke-array (DSSA) structures. The results from both the analytical model and FEA model demonstrate that the DSSA VPM machine exhibits highest power factor, viz., ~0.91, highest torque density and fewest magnet usage.

Synthesis of Flux Switching Permanent Magnet Machines

This paper investigates the nature of a flux switching permanent magnet (FSPM) machine based on the flux harmonic theory. First, analytical expressions for air-gap flux density distribution, back-electromagnetic force (EMF), and torque are developed to analyze the electromagnetic features of FSPM machines. It is found that the conventional winding theory, such as star of slots, can be directly employed to analyze FSPM machines without any modification. Combinations of stator and rotor teeth, winding connection, and conditions for symmetrical-phase back-EMF waveform are analyzed and obtained using star of slots. The armature winding pole pairs of FSPM machines are redefined. Based on this new concept and the proposed analytical expressions, slots per phase per pole and winding factor in FSPM machines are derived, a new type winding configuration with overlapping windings for FSPM machines is predicted and verified to be with ~90% larger back EMF than those of conventional FSPM machines with nonoverlapping windings. All these analysis results are validated by finite element analyzes.

Influence of Pole Ratio and Winding Pole Numbers on Performance and Optimal Design Parameters of Surface Permanent-Magnet Vernier Machines

In recent years, surface permanent-magnet vernier machines (SPMVM) have been attracting more and more attention for several advantages, including a high torque density and a simple mechanical structure. However, the influence of the pole ratio, which is defined as the ratio of rotor pole pair numbers to winding pole pair numbers, and the winding pole pair numbers on the performance of SPMVMs has not been investigated in literature. This paper mainly focuses on the effects of the pole ratio and the winding pole pair numbers on the torque capacity, power factor, torque ripple, and cogging torque of an SPMVM. The variations of optimal design parameters for maximum torque, such as the split ratio (the ratio of the stator inner diameter to the outer diameter), the slot opening width, and the permanent-magnet thickness, with the pole ratio and the winding pole pair numbers are also investigated by finite-element analysis. The analysis results reveal that the pole ratio and the winding pole pair numbers significantly influence the performance and optimal design parameters of the SPMVM. Finally, an SPMVM prototype is built, and experiments are conducted to validate the aforementioned results.