Novel Permanent-magnet Machines Based On The Flux-Modulating Theory:Topology,Modeling,and Design

There are a lot of low-speed high-torque applications in the modern industry,where mechanical gearboxes are widely used to match the speed and torque between the electrical machines and the loads.For mechanical gear pairs,torque is transmitted via engagement of teeth.In result,several problems are inevitable such as friction losses,abrasion wear,lubrication requirement,mechanical vibration and acoustic noise.Mechanical gearboxes are also vulnerable to irregular loads and overload condition,which may cause system failure,reducing the overall efficiency and reliability.Consequently,the direct drive technique which removes mechanical gearboxes has become increasingly popular.According to the machine design experiences,in a given range of power and speed,the output torque of an electrical machine is proportional to its volume.This means a conventional high-torque direct-drive machine is very bulky with a high material consumption and a high cost.The idea of coaxial magnetic gears has provided an alternative solution for this problem.By introducing the idea of flux-modulating theory into the machine design,we can get several new topologies which have very high torque density.This area has gained growing attention in recent years.Under the background mentioned above,this dissertation studies PM machines which are based on the flux-modulating theory.According to the state of the art,torque capability,structural complexity and cost issues have been the three main problems confronted in this area.This study aims to give contributions to solving these problems.The methods used include topology innovation and parametric optimization.The means are finite element method(FEM),analytical modeling and prototype test.The study starts from the two-dimensional(2D)topology innovation and then extended to three-dimensional(3D)innovation.The main content of this dissertation are summarized below:1.A generalized model for PM machines based on the flux-modulating theory has been established.The relationship between four types of specific machines and this generalized model is explained.The advantages and disadvantages of different machines are compared.Their applications are introduced.2.A novel consequent-pole 'pseudo' direct drive machine topology is proposed.The torque generation principle and the limiting factors of output torque are derived analytically.The concept of'consequent-pole' is introduced into 'pseudo' direct drive machine.The advantages are twofold:first,the PM assumption and the manufacturing cost are reduced;second,the torque matching of the magnetic part and the PM machine part of the 'pseudo' direct drive machine is improved,and consequently,the overall output torque is increased.In addition,the consequent-pole structure is also beneficial in terms of mechanical robustness.3.On the basis of the study above,a novel vernier 'pseudo' direct drive machine topology is proposed,aiming to solve the low power factor problem of the traditional vernier PM machine.The new topology features both characteristics of the 'pseudo' direct drive machine and the vernier PM machine.Compared with conventional vernier PM machines,it has a higher power factor.Compared with the conventional 'pseudo' direct drive machine,it has a simpler structure.An exact subdomain analytical modelis established for the proposed topology.A two-stepped optimization method is adopted combining the benefits of the analytical method and the FEM,giving considerations to both efficiency and accuracy.4.The topology innovation is then extended from 2D to 3D.A novel axial-transverse flux-focusing magnetic gear topology is proposed.The torque capability of different existing topologies are compared.Influences of flux orientation and pole type on torque performances are analyzed.An axial flux-focusing magnetic gear features a very high volume torque density and is chosen as the benchmark of the study.Problems of this topology are analyzed followed by an improve solution.By utilizing both axial and transverse flux orientations,the flux leakage and saturation effects in the original topology are reduced.Consequently,the problem of sharply reduced torque density with axial length in the original topology is improved.5.The same idea is then introduced from the magnetic gear to the flux-modulated machine.A novel axial-radial flux-modulated machine topology is proposed.Disadvantages of conventional radial flux-modulated machines are analyzed and an improved solution is given.In the new topology,the cup-shaped rotor is no longer necessary,which reduces manufacture difficulty and improves mechanical robustness.The axial space caused by the end windings is used to accommodate the PM stators,and the flux focusing effects are utilized.Consequently,the torque density with a small length-diameter ratio is significantly improved.An illustrative prototype is designed and tested.6.Advantages and disadvantages of different topologies are compared and their perferred applications are introduced.Guidelines are provided for the industry to refer to.