Leilei Wu

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.

Design of Three-Phase Flux-Reversal Machines With Fractional-Slot Windings

This paper proposes several slot-pole combinations of flux-reversal machines (FRMs) with fractional-slot windings (FSWs) to achieve a larger torque density and a lower pulsating torque. First, the general rules for the determinations of the winding pole pair and slot per pole per phase (SPP) are introduced, and then the feasible slot–pole combinations of three-phase FRMs with FSWs are given. Second, the analytical expressions for the back electromotive force (EMF) and torque of fractional-slot FRMs are derived and expressed in terms of machine dimensions. Based on the analytical equations, the effects of rotor pole number, split ratio, slot opening ratio, permanent magnet (PM) thickness, and rotor pole arc on back EMF are investigated and analyzed, which gives a prediction for maximal achievable back EMF and power density of the FRM with FSWs. Third, the proposed 12-slot/14-pole FRM is compared to a conventional 12-slot/16-pole FRM in terms of cogging torque, flux linkage, back EMF, average torque, torque ripple, and efficiency. It is found that the cogging torque and torque ripple of the proposed FRM are 55% and 48% lower than that of the conventional one, respectively. Moreover, the rated torque of the former is also 4% higher than the latter. Finally, the analysis results are verified by the experiments on a 12-slot/14-pole FRM prototype with FSWs.

Analysis of torque capability and quality in vernier permanent magnet machines

This paper presents theoretical analysis and comprehensive simulations about average torque and pulsation torque of vernier permanent magnet (VPM) machine. A general torque equation is proposed which is used to analyze torque features and the parameters effect on torque performance of VPM machines in this paper. Based on the general torque equation and finite element algorithm, it is found that smooth torque is the inherent performance of VPM machines. Furthermore, It is demonstrated that the torque density of the VPM machine is larger than that of a regular commercial PM machine by almost 60%. All these advantages demonstrate that the VPM machines can obtain much better steady and dynamic drive performance. In order to validate theoretical analysis, a prototype is built, and more experiments will be available.

Design of three-phase flux reversal machines with fractional-slot windings

The conventional three-phase flux reversal machines (FRMs) are with integer slot windings (ISWs) and the most significant drawback is large cogging torque. This paper proposes several slot-pole combinations of FRMs with fractional slot windings (FSWs), taking advantage of the FSW with low cogging torque. First, the general rules for the determination of the FSW pole pair are introduced, and the feasible slot-pole combinations of three-phase FRMs with FSWs are predicted and analyzed. Second, the winding connections of the proposed combinations are obtained with the employment of star of slots method. Third, based on finite element analyses (FEA), the effects of rotor pole number, split ratio, slot opening ratio, PM thickness and rotor pole arc on cogging torque and rated torque are investigated and analyzed, which gives a prediction for optimum achievable torque waveforms of FRM with FSWs. Finally, a proposed 12-slot/14-pole FRM with FSWs is compared with a conventional 12-slot/16-pole FRM with ISWs in terms of cogging torque, flux linkage, back-EMF, average torque, torque ripple and efficiency. It is found that the cogging torque and torque ripple of the proposed FRM are 55% and 48% lower than that of the conventional one, respectively.

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.

Analysis of FSCW SPM servo motor with static, dynamic and mixed eccentricity in aspects of radial force and vibration

This paper deeply discusses the effect of eccentricity, including static, dynamic and mixed eccentricity on the performance of FSCW SPM servo motor analytically and using finite element analysis (FEA) to validation. The effect of different kinds of eccentricity on magnetic field including magnet field and armature reaction field is described with three factor called modulation factor. The frequency and spatial mode order of radial force for static, dynamic and mixed eccentricity are analyzed and summarized with the analytical method and validated by FEA. It is found that compared with health machine, the frequency and mode order of radial force distort for eccentric machine: the static eccentricity (SE) extends the mode order but the frequency invariant while the dynamic eccentricity (DE) and mixed eccentricity (ME) change both the frequency and mode order of radial force. With the electromagnetic radial force obtained, we predict the vibration of the stator housing, and acoustic noise around the machine. The results show that due to the smallest spatial mode decrease and frequency spectra extension, the eccentric machines will have a large vibration both on load and without load for easier to resonance at some natural frequency compared with health machine. As a consequence, the noise level of eccentric machine will also enlarge. The analysis results is verified by experiment and can be explained reasonably. The work of this paper is helpful to design and analyze the motor which is used in the servo drive system and other similar occasions.

Reduction of sub-harmonic effect on the fractional slot concentrated winding interior PM machines by using spoke-type magnets

In this paper, a spoke-type IPM motor (buried with circumferentially magnetized magnets) with fractional-slot concentrated windings is designed to block the two-pole sub-harmonic and, thus, reduce the iron core losses and torque ripple. This is demonstrated by the comparison of a conventional V-type IPM configuration and a designed spoke-type machine. Both of them have 12-slot/10-pole configurations with the same stator structure. The simulation results indicate that under the same design specifications, the spoke-type motor has less magnet consumption and exhibits much smaller iron core losses and torque ripple than that of its V-type counterpart, especially when operated at high speed. Finally, the mechanical stress of the designed spoke-type machine rotor is analyzed by finite element analysis.

Analysis of eddy current losses in surface-mounted permanent magnet vernier machines

This paper mainly focuses on the analysis of the eddy current loss in permanent magnet (PM) of the surface-mounted PM vernier machine (SPMVM). The eddy current losses under open-circuit, armature reaction and load conditions are detailed investigated by the analytical method and finite-element analysis (FEA). The influence of the pole ratio, which is defined as the ratio of the rotor PM pole-pairs to winding pole-pairs, and stator winding pole-pairs on eddy current loss are discussed. The results reveal that the SPMVM has large eddy current loss, especially on the load condition. The eddy current losses reduction method by PM segmentation circumferentially is investigated in this paper.

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.