Investigation Of Three-Phase Short Circuit And Partial Irreversible Demagnetization In Surface-Mounted PM Synchronous Machines

With the development of rare earth permanent magnets（PMs）,PM synchronous machines（PMSMs）show higher torque density,efficiency and control accuracy than induction machines.Surface-mounted PMSMs（SPMSMs）have simple structure and are esay to control,which are widely used in wind power generation,industrial servo drive,and household appliances,etc.However,the application of PMs also brings risks and challenges to SPMSMs.Irreversible demagnetization is one of them,which heavily influences machine reliabilities.In addition,three-phase short circuit,which is also called symmetrical short circuit（SSC）fault,is more likely to happen and usually has larger currents than other short circuit types.Therefore,it is one of the most important reasons for demagnetization.Hence,this thesis investigates SSC current and the resultant partial irreversible demagnetization of SPMSMs.In order to investigate the influence of start rotor position on the d-and q-axis short circuit current,an SSC current analytical model considering PM linkage harmonics is developed.It is found the variation of the d-and q-axis SSC current with start rotor position repeats 6 times within one electrical cycle.The start rotor position resulting in the largest peak SSC current heavily depends on the harmonic amplitudes and phases.In addition,the influences of angular speed,flux linkage harmonic amplitude and initial current are investigated.Furthermore,the analytical model is verified by the finite element simulation and experiments.On this basis,the influences of design parameters and load characteristics on SSC current and demagnetization are further investigated.Firstly,the influence of design parameters on SSC current and irreversible demagnetization in SPMSMs is investigated.For the internal rotor SPMSMs,the d-axis peak SSC current decreases when air-gap length increases whereas the demagnetization aggravates.When the pole-arc to pole-pitch ratio,slot-opening width,rotor yoke thickness increases or tooth-tip height decreases,the d-axis peak SSC current increases and the demagnetization becomes more serious.When the split ratio,tooth width or stator yoke thickness increases,the d-axis peak SSC current first increases and then decreases,which makes demagnetization first becomes severer and then eased.In addition,radial magnetization has stronger demagnetization withstand capability than parallel magnetization.Furthermore,at the same rotor speed,the SSC current of star winding connection is generally larger than that of delta connection,and thus the demagnetization is severer.In addition,the conclusions in different winding configurations and rotor structures are compared.The analysis is also verified by experiments.Then,the influence of load characteristics on SSC current and irreversible demagnetization is investigated.For the gravitational lift torque（GLT）load,there is a threshold value for the load torque.When the load torque is less than the threshold,the rotor speed becomes stable during SSC.Otherwise,the SPMSM accelerates in the reverse direction.In addition,the possibility that the d-axis peak SSC curent under GLT load exceeds that under constant speed increases when the total inertia decreases.For dry friction torque load,viscous friction torque load,torque load square to speed,and constant power load,the rotor speed drops to 0 and the current decreases to 0 during SSC.In addition,under these four load types,smaller load torque or larger load inertia leads to the larger d-axis peak current.However,the d-axis peak current under these four loads is always smaller than that under constant speed.Furthermore,the experiments are conducted to verify the above analysis.Afterwards,the post-demagnetization unbalanced magnetic force（UMF）after SSC in SPMSMs is investigted.Neglecting possible eccentricity faults,for rotationally symmetrical winding configurations,the demagnetization distribution pattern is also rotationally symmetrical.Therefore,there is still no UMF after demagnetization.For the rotationally asymmetric winding configuration,the demagnetization distribution is also asymmetric and thus the additional 1st order component occurs in the post-demagnetization UMF.When eccentricity fault is considered,the irreversible demagnetization tends to gather around the PMs facing larger air-gap.Therefore,under static eccentricity fault,the UMF original harmonic components are reduced after demagnetization whereas the additional 1st component is mainly added.Under rotating eccentricity fault,for rotationally symmetrical winding configurations,the amplitude of the original UMF 1st component increases.However,for the rotationally asymmetric ones,the original 1st component decreases.Furthermore,the experimental verification is carried out.Finally,since dual three-phase（DTP）winding configurations are more and more widely used in SPMSMs,the SSC and irreversible demagnetization in dual three-phase SPMSMs（DTP-SPMSPMs）are investigated.An SSC current analytical model is developed for the DTP-SPMSMs where the mutual inductances in a single winding group are unbalanced.It is found that variation of the d-and q-axis SSC current with start rotor position repeats twice in an electrical cycle.For the DTP-SPMSMs with smaller M_ac,M_bc,and M_ab in a single winding group,the SSC start rotor positions leading to the largest d-axis peak SSC currents areπ/6+kπ,π/2+kπ,5π/6+kπ（k=0,1,2…）,respectively.The start rotor positions leading to the largest q-axis peak SSC currents are 7π/24+kπ,5π/8+kπ,and 23π/24+kπ,respectively.In addition,the post-demagnetization UMF of DTP-SPMSMs is also investigated.It is found that there is still no UMF after demagnetization when a single winding group is rotationally symmetrical.However,when a single winding group is rotationally asymmetric,even if the eccentricity fault is not considered,the post-demagnetization UMF occurs.When considering the eccentricity fault,the variation rules of post-demagnetization UMF are similar to that in single three-phase SPMSMs,which are rotationally symmetrical.