Demagnetization Study Of Line-start Permanent Magnet Synchronous Motor With Composite Rotor

Line-start permanent magnet synchronous motor with composite rotor combines the structural characteristics of the solid rotor permanent magnet synchronous motor and cage rotor permanent magnet synchronous motor,so it has both good starting performance,high efficiency and high power factor.But as a kind of permanent magnet motor,permanent magnet material demagnetization problem also restricts its application.Therefore,it is necessary to study the demagnetization of permanent magnets under various operating conditions.The finite element model of a 355kW 10kV 6 pole line-start permanent magnet synchronous motor with composite rotor considering saturation,eddy currents and other factors were established to analysis the demagnetization law and the reasons of the motor’s starting process,three-phase short circuit and sudden increase in load torque.And the influence of rotor structure parameters on demagnetization during start-up is also analyzed.The basic structure and working principle of the line-start permanent magnet synchronous motor with composite rotor are briefly introduced and the magnetomotive forces of the motor are calculated.At the same time,a method of judging the operating point of permanent magnet is introduced,and related concepts are defined,such as minimum operating point,average operating point,the smallest of minimum operating points,the smallest of average operating point.The analysis of demagnetization during motor’s starting process shows that the minimum operating point of the permanent magnet is related to the synthetic magnetomotive force position.When the angle between the magnetic field of the permanent magnets and the magnetic field synthesized by stator and rotor is about 110°or 240°,the corners near air gap of permanent magnets faces great risk of demagnetization.The smallest of minimum operating points of the permanent magnets occurs at low speed,rather than ordinary line-start permanent magnet synchronous motor that appears in the near synchronous speed.Synthetic direct-axis of magnetomotive force is able to reflect the strength of the permanent magnet’demagnetization field.The load is heavier,the smallest of average operating point is lower,which corresponding speed is closer to synchronous speed,and the probability of most severe demagnetization is greater.The analysis of rotor structure found that the presence of the copper wedge and the copper bar have a protective effect on the permanent magnets.When there is no copper wedge and copper bar,the demagnetization point of permanent magnet decreases,and the moment of partial demagnetization more forward.And the smaller size of the copper wedge,the higher demagnetization point is,help to reduce the permanent magnet demagnetization,and it is conducive to reducing the permanent magnet demagnetization.The copper bar can also improve the motor starting performance,and lessen the possibility of demagnetization.The demagnetization under three-phase short circuit is analyzed.It is found that the minimum operating point of the permanent magnet is related to the direct-axis of the armature magnetomotive force and the average operating point is affected by the direct-axis component of the synthetic magnetomotive force.At the same time,the influences of short circuit moment,initial load torque and moment of inertia on the operating point of permanent magnet are analyzed,and the law of demagnetization of permanent magnet in three-phase short circuit is studied.It is found that whether it is partial demagnetization or overall demagnetization,demagnetization of three-phase short circuit is far from the start of the process of serious.The demagnetization when the load torque suddenly increases is analyzed.It is found that when the sudden load result in out-of-step,operating point changes greatly and it is easy to demagnetize.The influences of sudden load size,time of the load mutation,initial load torque and moment of inertia on the operating point of permanent magnet are analyzed,and the law of demagnetization of permanent magnet in out-of-step is studied.It is found that when the sudden load result in out-of-step,whether it is partial demagnetization or overall demagnetization,demagnetization of out-of-step is far from the start of the process of serious.