Analysis of demagnetization field of line-start permanent magnet synchronous motor under abnormal operation conditions

To research the influence of the demagnetization field on the permanent magnet of line-start permanent magnet synchronous motor(LSPMSM) under the abnormal operation conditions,the time-stepping finite element model of LSPMSM,which took the irreversible demagnetization effect into account,was established to calculate and analyze the three abnormal operation conditions including out of step,three-phase short circuit and reclosing,and the irreversible demagnetization situations of each permanent magnet element under the three abnormal operation conditions were gained.Results show that the excess load causes the permanent magnet motor out of step and shock,and then the stator magnetic field which rotates relatively to the rotor causes the effective coercivity of the permanent magnet element reduced,and irreversible demagnetization occurred.The three-phase short circuit condition leads to the stall of the permanent magnet motor.During the stop process,the demagnetization field caused by the short-circuit currents results in the lower flux density of the permanent magnets,and the action time of the demagnetization field is longer when the load is smaller,which makes the permanent magnets more prone to be demagnetized.There is a transition state during the reclosing process,and the transition time and the transient inrushing current amplitude are affected by the switch voltage at the moment of reclosing.When a reclosing is performed with the larger switch voltage,the transient inrushing current is larger and the transition time is longer;the time the permanent magnet endure the demagnetization field is therefore longer,and then the irreversible demagnetization is severer.