Theoretical Analysis And Topology Investigation Of Multiple-working-MMF Flux Modulation Machines

A permanent magnet machine is an electromechanical device that uses permanent magnets as excitation sources to convert energy between electrical and mechanical forms.With the improved requirements for motor efficiency and the promotion of high-efficieny motors in all industries,high-efficiency,high-power-density permanent magnet machines are dominating the innovation and reformation of traditional industries such as industrial servos and household appliances.At the same time,they are also widely used in emerging fields such as transportation electrification,robots,and electric vehicles.With ambitious carbon targets published,China is embarking on a new round of revolution in the energy sector.Therefore,the role of novel permanent magnet machines in energy fields such as new energy power generation and pumped storage has also gained further attention.In recent years,research and applications of flux modulation machines—machines that adopts flux modulation theory—provide a new approach to further improving the torque quality and efficiency while reducing the volume/mass of the motor.With the deepening of research,the performance of conventional single-working-MMF flux modulation machines is gradually approaching its theoretical upper limit.Meanwhile,demand for higher power density and torque density of PM machines continues to rise in a variety of traditional and novel application fields.As a result,it is critical to investigate the theory and high torque density topology for flux modulation machines.Furthermore,there is a problem with the low calculation accuracy of the air gap flux density when using the MMFpermeance model,which restricts the understanding of the torque mechanism of flux modulation machines from the view of airgap maxwell stress tensor.Flux modulation machines that using multilayer permanent magnets,such as magnetic geared machines and dual PM vernier machines,have recently gained attention in order to further improve the torque density.Recently,flux reversal and flux switching machines with multiple working MMFs have emerged for optimizing the performance of stator PM flux modulation machines.The performance superiority of above-mentioned multiple-workingMMF flux modulation machines(MWM-FMMs)is gradually being recognized.However,current research on MWM-FMMs is dispersed and independent,with no unified theory to explain the common working principle of MWM-FMMs.Furthermore,practical design procedures as well as research on key common problems for this new type of machines,such as cogging torque and demagnetization,are still required.Taking the MWM-FMMs as the research object,the multiple MMF working mechanisms are deeply analysed,the optimization methods and novel high-torque-density topologies are thoroughly investigated in this thesis.The content is arranged as follows:The understanding and development of flux modulation theory in PM machines are reviewed.The topologies of flux modulation machines are summarized and reclassified according to the number of the modulation and excitation units.The concept of MWM-FMM is defined and the machine topologies it contains are clarified.Furthermore,the current research status of MWM-FMMs is summarized.The calculation error of airgap flux densities caused by pole-transition-over-slot effect in flux modulation machines is firstly analysed.Using the modified MMF-permeance model and Maxwell stress tensor method,the average torque in flux modulation machines(FMMs)are derived and calculated.The working criterion and several feasible conditions of multiple working MMFs in FMMs are systematically considered.The torque formulas of three MWM-FMMs are derived,the theoretical support of multipleworking-MMF principle in flux modulation machines is achieved,and the relationship between size parameters and torque in MWM-FMMs is revealed.Combining torque formulas and FEA methods,the torque performances between singleworking-MMF FMMs and MWM-FMMs are compared,which shows the multiple-workingMMF leads to higher torque density.The strategies to improve the torque density of MWMFMMs based on single-working-MMF and multiple-working-MMF are introduced.Four novel MWM-FMM structures with high torque density are proposed based on these strategies.Based on the theoretical analysis,three MWM-FMMs topologies are studied respectively.1.The magnetitude upper limit of field harmonics of PM array in stator-PM MWM-FMMs is given.The design method of a magnet array with flexible MMF parameters is proposed,and the effect of construction parameters is studied.Taking multi-working-MMF flux reversal machine(MWM-FRM)as the object,the condition and the parameter optimization that maximizing the back EMF and torque enhancement is studied.At last,an optimized MWMFRM prototype with a 40% increase in torque and back EMF has been built and tested.2.Using differential magnetic circuit model,the air gap field is dual-PM MWM-FMMs is calculated.The formulas of back-EMF and torque are derived,and verified with finite element analysis.Taking torque density as the target,the selection of stator/rotor pole pairs,pole ratios,and size parameters are studied.The cogging torque formula is first derived,and he cogging torque reduction methods are proposed from the perspective of air-gap field harmonics.The demagnetization mechanism is studied,and several demagnetization suppression methods are proposed and compared.3.The working principle,and the relationship between parameters and torque in magnetic gear are analysed.The electromagnetic performances of various MGM topologies are introduced and compared.The design optimization method of MGM is studied.The compound split ratio is defined,and the effect of pole pairs,slot-pole combination,and compound split ratio on the integrated output torque of MGM is studied.The design and optimization process of MGM is summarized under given thermal load.At last,the torque performances of proposed four novel MWM-FMM topologies are validated through the FEA methods and the experimental test of prototypes.