Tianjie Zou

Analysis and design of a dual-rotor axial-flux vernier permanent magnet machine

This paper proposes a dual-rotor, toroidal-winding, axial-flux vernier permanent magnet (VPM) machine. By the combination of toroidal windings with the rotor-stator-rotor topology, the end winding length of the machine is significantly reduced when compared with the regular VPM machine. Based on the airgap permeance function, the back-EMF and torque expressions are derived, through which the nature of this machine is revealed. The influence of pole ratio (ratio of rotor pole pair number to stator pole pair number) and main geometric parameters such as slot opening, magnet thickness etc., on torque performance is then analytically investigated. Both the quasi-3-dimensional (quasi-3D) finite element analysis (FEA) and 3D FEA are applied to verify the theoretical analysis. With the current density of 4.2 A/mm2, the torque density of the proposed machine can reach 32.6 kNm/m3. A prototype has been designed and is in manufacturing process. Experimental validation will be presented in the future.

Advanced high torque density non-overlapping winding PM vernier machines

In recent years, permanent magnet (PM) vernier machines have gained more and more attentions due to their high torque density and simple mechanical structure. However, vernier PM machines with lap windings always suffer from long end winding length, and regular non-overlapping winding may result in torque reduction for PM vernier (PMV) machines. In this paper, an advanced PM vernier (APMV) machine topology with multi working permeance harmonics is proposed. This topology is equipped with non-overlapping winding, i.e. short end winding length, and could achieve ∼20% higher torque density than that of regular non-overlapping winding PMV machine, with the same magnet usage. Furthermore, both the theoretical analysis and finite element algorithm (FEA) are used to study the performances of this topology, such as back EMF, output torque as well as structure parameter effect. Finally, analysis results are verified by experimental test on a 21Nm prototype, which is designed to have similar volume and weight with a 14Nm regular commercial PM machine.

A consequent pole, dual rotor, axial flux vernier permanent magnet machine

A consequent pole, dual rotor, axial flux vernier permanent magnet (VPM) machine is developed to reduce magnet usage and increase torque density. Its end winding length is much shorter than that of regular VPM machines due to its toroidal winding configuration. The configurations and features of the proposed machine are discussed. Benefited from its vernier and consequent pole structure, this new machine exhibits much higher back-EMF and torque density than that of a regular dual rotor axial flux machine, while the magnet usage is halved. The influence of main design parameters, such as slot opening, ratio of inner to outer stator diameter, magnet thickness etc., on torque performance is analyzed based on the quasi-3-dimensional (quasi-3D) finite element analysis (FEA). The analyzing results are validated by real 3D FEA.

Performance Comparison of Surface and Spoke Type Flux-Modulation Machines with Different Pole Ratios

In this paper, performance comparison between surface and spoke-type flux modulation (FM) permanent magnet (PM) machines is presented. Generally, spoke-array magnet arrangement is capable of increasing the torque density of PM machines due to its flux-focusing effect. Nevertheless, a flux-barrier effect is found when this magnet topology is applied in FM machines, which may offset the advantage in torque capability when the pole ratio is high. By flux distribution comparison of these two-machine topologies, the flux-barrier effect is visually explained. Through numerical FEA, this effect is further investigated in spoke-type vernier PM machines with a series of pole ratios. Finally, compared with surface-type FM machines, considerable reduction in modulated magnetic field as well as output torque capability is verified in high pole ratio, spoke-type FM machines.

Analysis of a Dual-Rotor, Toroidal-Winding, Axial-Flux Vernier Permanent Magnet Machine

In this paper, a vernier permanent magnet (VPM) machine with dual-rotor toroidal-winding axial-flux topology is analyzed. Through the combination of toroidal windings with the rotor-stator-rotor topology, the end winding length of the proposed machine is significantly reduced compared with that of the regular VPM machine. Based on the airgap permeance function, the back-EMF and torque expressions are derived and the nature of the machine is revealed. The influence of pole ratio (ratio of rotor pole pair number to stator pole pair number) and main geometric parameters, such as slot opening, magnet thickness, etc., on electromagnetic performance is then analytically investigated in detail. Both the quasi-three-dimensional (quasi-3D) finite element analysis (FEA) and 3-D FEA are applied to verify the derived equations. Finally, a prototype of the proposed machine has been manufactured and experimentally evaluated. The test results agree well with both theoretical and FEA analyses. With the current density of 5.8 A/mm 2 and slot fill factor of 0.35, the torque density of the proposed machine prototype can reach 31.9 kNm/m 3.

Advanced High Torque Density PM Vernier Machine With Multiple Working Harmonics

In recent years, permanent magnet (PM) vernier machines have gained more and more attention due to their high torque density and simple mechanical structure. However, PM vernier (PMV) machines with lap windings always suffer from long end winding length, and regular nonoverlapping winding may result in torque reduction for PMV machines. In this paper, an advanced PMV machine topology with multiple working harmonics is proposed. With specially designed stator auxiliary teeth, this topology could achieve ∼20% higher torque density than that of a regular nonoverlapping winding PMV machine, with the same magnet usage. Through finite element algorithm and theoretical analysis, the production of additional flux density harmonics and their contributions to back-electromotive force (EMF) are verified. Moreover, the electromagnetic performances of this novel machine topology, such as back-EMF and output torque, are quantitatively investigated with the geometric parameters’ effect considered. Finally, analysis results are verified by experimental test on a 21 Nm prototype, which is designed to have similar volume and weight with a 14 Nm regular commercial PM machine.

Synthesis of fractional-slot vernier permanent magnet machines

Vernier permanent magnet (VPM) machines have gained growing interest in recent years due to several advantages, including high torque density and simple mechanical structure. This paper presents a thorough investigation on the nature of fractional slot vernier permanent magnet (FS-VPM) machines with regular stator topology. Through analytical derivation, the configuration and performance of two winding types, i.e., fractional slot concentrated winding (FSCW) and FS winding with two-slot coil pitch are evaluated for VPM machines. The FS winding VPM machine with two-slot coil pitch is newly proposed, and turns out a promising candidate in direct-drive application with competitive torque density and improved power factor. All the electromagnetic performances are calculated and compared through finite element analysis (FEA).

Comparison of surface PM vernier motors with interior PM motors for traction application

Surface permanent magnet vernier (SPMV) motors are gaining more and more attentions over recent years due to their high torque density. Generally, most researches focuses on their applications in low speed, high torque area, and the study on their high speed operation is rarely seen. In this paper, a SPMV motor with 12 slot, 20 rotor pole is evaluated for traction application. The optimal designed SPMV motor is compared with the Prius-2004 interior permanent magnet (IPM) motor in terms of back-EMF, cogging torque, torque, field weakening ability, losses, efficiency and fault tolerance. The analyzing results show that the SPMV machine produces 12% higher torque under the same DC copper loss. However, the core loss and magnet loss of the SPMV motor are 2.5 and 6.25 times that of the IPM motor. Besides, the SPMV motor has inherently optimal flux-weakening capability despite its surface-mounted PM structure. Moreover, the anti- demagnetization capability of the SPMV motor with pole shoes to protect the magnets, is similar to that of the Prius PM motor.

A Brushless Dual-Mechanical-Port Dual-Electrical-Port Machine With Spoke Array Magnets in Flux Modulator

Brushless dual-mechanical-port dual-electrical-port (BLDD) permanent magnet (PM) machines have been gaining more and more attentions in recent years, with the merits of two decoupled rotors and contactless torque transmission. However, existing BLDD machines tend to suffer from low torque density due to low working flux density. In this paper, a BLDD machine with spoke array PMs in flux modular is proposed, which improves the torque density significantly. The structure and operation principle of the proposed machine is introduced. Detailed performance comparison between three different BLDD machine topologies, i.e., surface-mounted PM BLDD machine, flux-bidirectional modulation BLDD machine, and the proposed BLDD machine, is presented through finite-element analysis. The analyzing results show that although the modulated magnetic field coupled with the modulation winding is slightly reduced, the torque transmission capability of the regular winding in the proposed BLDD machine is significantly enhanced when compared with that of its two counterparts.

Modeling and sensorless vector control for vernier PM machines

Vernier permanent magnet (VPM) machines have been popular in recent years, which can own high torque density and simple mechanical structure. This paper develops the mathematical model of VPM machine in d-q axis reference frame with consideration of saturation and cross-coupling saturation. Then, the optimal inductance matrix of a VPM machine model is predicted by incorporating finite element analysis (FEA) with linear regression. Further, a back-EMF based sensorless control method is implemented in this VPM machine model with design of controllers and observer. Simulation results show that the VPM machine can be well controlled for steady state and dynamic state operation. Finally, the proposed sensorless control algorithm is validated through experimental test of a 1-kW VPM prototype machine.