The Study Of Permanent Magnet Demagnetization In Line-start Permanent Magnet Synchronous Motor

With the improvement of permanent magnet material and the development of design and manufacturing technology of the permanent magnet motor, the line-start permanent magnet synchronous motor (LSPMSM) is used in more and more applications. Compared with electrically-excited synchronous motor and induction motor, LSPMSM has higher efficiency, power factor and energy density, etc. Special operation conditions may occur during the normal usage of LSPMSM, such as: overload, out of step, super-synchronous, sudden reversal, etc., which will result in large currents in stator windings and rotor squirrel-cage and severe demagnetization field is generated. The permanent magnets in LSPMSM, which is reasonably designed, may be irreversibly demagnetized. In addition, the moment that the severest demagnetization field occurs during the normal starting process of LSPMSM is worth studying.Based on the two-dimensional finite element simulation, demagnetization characteristics of permanent magnets in LSPMSM under the above four operating conditions-starting process, out of step, super-synchronous and sudden reversal are analyzed. The main contents include:(1)The demagnetization mechanism of permanent magnet is detailed introduced, and the main reasons for the demagnetization of permanent magnet are analyzed. At the same time, research achievements of the demagnetization of permanent magnet achieved by researchers are summarized.(2)The average operating point of permanent magnets can’t reflect the local demagnetization of permanent magnets. In order to accurately reflect the demagnetization status of permanent magnets, the calculation method of the minimum operating point of the permanent magnets is introduced. And the calculation method of the magnet motive force (MMF) of stator winding and the rotor squirrel-cage is also introduced. (3)The influence of stator and rotor MMFs on the operating point of permanent magnets in LSPMSM during starting process is analyzed. The demagnetization characteristics of permanent magnets when LSPMSM starts under different conditions and the rule that magnets’demagnetization point occurs are studied. The calculation method of the severest demagnetization point of permanent magnets during starting process is also introduced.(4)The operation of LSPMSM when large load is applied, which will cause out of step or super-synchronous condition is analyzed. The influence of mutation moment, mutation load and initial load condition on demagnetization of permanent magnets and demagnetization characteristics of permanent magnets in LSPMSM under out of step and super-synchronous conditions are studied, and the calculation method of the severest demagnetization point is introduced.(5)The reversal process of LSPMSM caused by swapping the voltages of two phases of stator winding is studied. The influence of swap moment and initial load condition on demagnetization of permanent magnets and the demagnetization characteristics of permanent magnets during reversal process are analyzed.