The Magnetic Field Analysis Of High-speed PM Brushless Motor Based On Soft Ferrite And A Novel Weaking Flux Configuration

With the development of high-speed bearings and motor control technique, the High-speed permanent-magnet motors, which have advantage of low rotor loss and high power density, have been applied more and more extensively. However, in the state of high speed, the frequency of alternating magnetic field is so high that the stator iron loss is large, which leads to the motor overheating. In order to reduce the stator iron loss, A permanent magnet brushless DC motor is designed, the material of which the stator is soft ferrite, which has the characteristics of high initial permeability, high resistivity and low cost. And then a depth study of this motor is carried out. The main contents are as following:Firstly, based on the characteristics of high permeability and high resistivity of soft ferrite, a slotless motor structure is constructed, which the stator core is made up of soft ferrite and the rotor of NdFeB magnetic ring. The sizes of stator and rotor are matched appropriately, which makes the soft ferrite and NdFeB run at the ideal operating point and the power density is increased while the motor iron loss is reduced greatly and efficiency improved significantly. Prototype experiments indicate that the design proposed in this paper is feasible.Secondly, the internal magnetic field of ferrite-based high-speed permanent magnet brushless DC motor is analyzed especially. On the one hand, by analyzing the characteristic of the distribution of slotless permanent magnet motor, the conclusion that the distribution of gap magnetic field is not homogeneous, i.e.the same conductor in a winding generates different electromotive force in different location. Besides, the motor air gap magnetic field is influenced greatly by direction of the magnetization of rotor magnets. Via calculation, the conclusion that the parallel magnetized rotor magnet can enhance the air gap magnetic field is proposed, and parallel magnetization is fit to the high-speed permanent magnet motor. On the other hand, based on the analytical results of the slotless motor air gap magnetic field, analytical emf formula of that kind of motor is deduced and verified by the result of finite element analysis, and then the formula can provide the basis for the design and calculation of motor. Additionally, due to the big air gap of slotless motor, the magnetic leakage of motor end is serious. By means of three-dimensional finite element analysis, the flux leakage of motor ends are analyzed, and the result indicates that magnetic flux leakage influences the air gap magnetic field seriously, which should be considered in the design of motor. In order to reduce the magnetic leakage of the motor ends, an improved structure of the rotor excitation is proposed and verified by three-dimension finite element analysis.Thirdly, with the combination of the soft ferrite's characteristics of low saturation magnetic flux density and isotropy, a novel weak magnetic structure of high-speed permanent magnet motor is proposed, the DC excitation winding of which generates an axial magnetic field and enhances the saturation of the stator yoke iron core and increases the magnetic reluctance of permanent magnet circuit, and the goal of weakening magnetic is achieved. The correctness and feasibility are verified by finite element analysis. In order to simplify the calculation of finite element analysis, a two-dimension finite element model of the weakening magnetic structure equivalent to three-dimension counterpart is proposed and compared with the result of three-dimension finite element analysis, and reach the conclusion that the two-dimension finite element model is accurate and reliable.Finally, based on sensorless brushless motor ASIC, a control system of high-speed permanent magnet brushless DC motor is designed and prototype testing is carried out, which verifies the feasibility of the control system.