The Study Of Permanent Magnet Demagnetization In Permanent Magnet Synchronous Motor

Rare earth permanent magnet electrical machines are widely used because of the high efficiency, big torque-quality ratio, large power density and good control performance; moreover, they also have outstanding advantages of simple structure and reliable operation. However, motor heating and bad torque property are caused by filed fluctuation and demagnetization of the permanent magnet and the mutual interaction of current, temperature rise and power angle, motors could be scrapped under serious circumstances, which significantly restrict further application of permanent magnet machines. Therefore, the demagnetization analysis of permanent magnet machine becomes a widespread concern of scientific study area and research subjects.This thesis is primarily concerned the method of demagnetization analysis, risk estimation and partial phenomenon of demagnetization, under asynchronous starting operation and steady-state operation these two working condition, the main contents are as follows:(1) From the demagnetization principle of permanent magnet material, the properties and characteristics are thoroughly described. Combining with the mutual interaction of current, temperature rise, losses and demagnetization, the methods and main reasons of demagnetization of permanent magnet in the machine are discussed.(2) Based on the fundamental harmonic distribution of MMF in the air gap, the increasing magnetization effect and demagnetization effect on permanent magnet field produced by the synthesis MMF of stator and rotor. The model of self-starting synchronous motors are modeled by the methods of circus and filed. By comparing two simulating calculation results, complementary and shielded effects on the stator winding MMF caused by the MMF in squirrel-cage conductor of the rotor are analyzed, at the same time, the changes of the angle between the stator and rotor synthesis field and the permanent magnet field and its cosine value under different loading circumstances are discussed, too.(3) Under normal temperature, by exerting different loadings and giving parameterized different rotor starting positions, the demagnetization principles of the self-starting synchronous permanent magnet machine in the starting operation are generalized, analyzed and summarized.(4) Under rated operation temperature, by the electromagnetic torque and power factor angle characteristic and giving different sudden overloads, the demagnetization conditions in steady state operation and out-of-step operation are analyzed.(5) Based on the two most serious working conditions in starting and steady state operations, combing with the changes of demagnetization inflection point of the NdFeB permanent magnet material caused by temperature rises in the motor, the magnet model, which under the situation that the partial demagnetization area has been produced, is created. According to this model, by comparing the conditions before and after demagnetization, the changes of electromagnetic parameters and properties of the machine are analyzed; tendency of the demagnetization deterioration is under further investigation.