The Demagnetization Mechanism And Anti-demagnetization Method For Line-start Permanent Magnet Synchronous Motors

In the development of line-start permanent magnet synchronous motors (LSPMSM), the key problems need to be solved at the present stage are to study the demagnetization mechanism by establishing a demagnetization field calculation model with a variety of nonlinear factors and complex structures being considered, and then to proposed anti-demagnetization measures by studying the relationship between different factors and the demagnetization field. In this paper, the demagnetization mechanism and the anti-demagnetization measures of a line-start permanent magnet synchronous motor are systematically studied by a combined method of theoretical analysis, numerical calculation and experimental study. The main works are in the following.1. The demagnetization field calculation models which took account of the permanent magnet field, the squirrel-cage asynchronous motor effect and the variable-frequency permanent magnet generator effect comprehensively and separately are proposed, and the influences of the starting conditions on the demagnetization field are systematically studied. The relationships between the load and the rotor speed when the largest demagnetization during the start process appeares are revealed. It is found that the largest demagnetization field during the start with heavy load appears when the speed is close to the synchronous speed. When the LSPMSM starts with light load, the larger demagnetization field appears only once near the half of the synchronous speed.2. Through the time-stepping finite element analysis of the irreversible demagnetization effect of each permanent magnet element, the effects of the abnormal operation conditions, such as the open phase fault, the out of step, the reclosing and short circuit faults, on the permanent magnet demagnetization are obtained. The relationships between the control parameters and the demagnetization extent of permanent magnets are revealed, and the control parameters include the load, the stator current amplitude, and the reclosing time.3. In view of the problem that the local demagnetization cannot be considered in the calculation of the maximum demagnetization working point by the magnetic circuit method, a quasi-steady state model is proposed. By an example calculation based on the proposed model, the local position of the minimum working point in permanent magnet under the most serious demagnetization condition is confirmed, and it can be used for demagnetization risk checking.4. A new squirrel-cage rotor design using composite material for line-start permanent magnet synchronous motors is proposed, and the optimization algorithms, such as the orthogonal test method and the genetic algorithm, are adapted to optimize the size of the bar structure and the material characteristics. The anti-demagnetization ability of the motor is significantly improved by the optimization on the structure. At the same time, the impacts of the rotor end-ring impedance, the permanent magnet parameters and the ambient temperature on the anti-demagnetization ability of the motor are analyzed.5. The experimental measurement methods on the static demagnetization field and the dynamic demagnetization field of the line-start permanent magnet synchronous motor are proposed, and the test systems are designed. A special structure of permanent magnet motor is designed for the experimental study, and a lot of experiments are conducted. The Results of the static measurement and dynamic measurement on the surface magnetic field of the permanent magnet agree with the calculation results based on finite element and time-stepping finite element method, which verify the accuracy of the calculation model and the effectiveness of anti-demagnetization measures.