Computation And Analysis Of 3-D Open-boundary Magnetic Fields Of Ironless Permanent Magnet Motor

Developing and manufacturing ironless permanent magnet (PM) motor is an important subject in current electric machine region. The problems of heaviness, high iron loss, vibration and noise existing in conventional PM motor can be resolved by adopting ironless motor structure. Developing a new type ironless PM motor requires accurately calculating the performance parameters. An important prerequisite to this purpose is the acquirement of correct magnetic field distribution. There is no ferromagnetic material in the outside of ironless PM motor's airgap. Because of this characteristic, airgap magnetic field of ironless PM motor belongs to 3-dimentions (3-D) open boundary magnetic field. Engineering problem of open boundary magnetic field is a hot research issue in the computation electromagnetics field recently.The contents of this thesis are of key technologies in national high-technology research and development (863) program of China (New type rare-earth PM motor design and integration technology). The calculation method for practical engineering situations should not only achieve high precision, but also need memory allocation and CPU time. After comparing with many techniques available for dealing with the open boundary problems, the asymptotic boundary conditions (ABCs) combined with the finite element method (FEM) are employed for the calculation of 3-D open boundary magnetic fields in ironless PM motor.Four parts of this thesis are included as follows:Firstly, 3-D open region magnetic field computation methods are investigated. An ABCs based on scalar magnetic potential is proposed and the correspondent mathematical models in spherical boundaries are established. In order to reduce the number of nodes as much as possible for 3-D ironless PM motor structure, the first-order and the second-order ABCs operators for box-shaped and cylinder-shaped truncating boundary conditions are derived according to the structure of ironless PM motor. The operators are simple and easy to be implemented in the FEM. The mathematical models for the combination of ABCs and FEM are investigated and established. The concrete implement methods are put forward and correspondent discrete equations are derived. By calculating a 3-D open boundary magnetic field of a cuboid permanent magnet, the ABCs method and the calculation program are proved to be correct. The ABCs method and the truncation method are compared in precision and artificial outer boundary distance: With the same artificial boundary conditions, ABCs have higher precision. Second-order ABCs are superior to first-order ABCs. Comparing with truncation method, ABCs method needs less memory allocation and CPU time. So ABCs settle the contradiction between computation time and precision more successfully.Secondly, the 3-D open region magnetic fields of inner rotor in ironless PM motor with Halbach cylinder are calculated quantitatively by employing the ABCs method. The regularities of magnetic field spatial distribution of ironless PM motor with Halbach array are generalized.Thirdly, magnetic fields of different topology Halbach array motor are investigated comparatively by a large number of calculations. Influencing factors of airgap flux density in Halbach array PM motor without a permeable rotor backing are discussed. The influences of Halbach magnet axial length on the leakage flux density are analyzed. The relationship between the key design parameters of motor, such as leakage coefficient and the calculated length of the armature, and the structure parameters of motor are obtained.Finally, the magnetic fields of three ironless PM motor prototypes, in which the experiment data have been obtained, are calculated and analysed. The calculated magnetic fields coincide with the experiment data. So the effectiveness and practicability of the ABCs method solving 3D open region problems are verified.