| Due to the permanent magnet machine is widely applied in wind power,rail transit,aircraft,electrical vehicle,robot and other areas,the reliability and stability of the permanent magneti machine is more important than before.And it is significant for engineering to reduce the probability of failure and improve the reliability of machine by motor design.At present,the researches about the permanent magnet machine is mostly focus on cogging torque,unbalanced magnetic pull,radial fore wave,vibration and noise.However,a net asymmetry flux and shaft voltage will be induced when there are rotor eccentricity and inter-turn short circuit or other faults.When the shaft voltage is too high to exceed the threshold,the oil film will be broken and the shaft voltage will also be induced,and irrecoverable electric corrosion will be conducted on the bearing inner surface.The electric corrosion will reduced the stability of the bearing and aggravate the vibration of the machine,and even threaten the stability and reliability of the whole motor system.But the present investigations of the shaft voltage are mostly on induction machines or synchronous machines,some analysis and conclusion can not fully apply to permanent magnet machines.Through the analysis of the electromagnetic field in surface.-mounted permanent magnet machines,the shaft voltage will be induced in fractional slot permanent magnet machines with some certain pole and slot number combinations,and we define that shaft voltage as"intrinsic shaft voltage" in fractional slot permanent magnet machines.This paper firstly investigate the relationship of pole and slot number combinations and intrinsic shaft voltage in fractional slot permanent magnet machines.Then the mitigation method for intrinsic shaft voltage by pole-arc coefficient selection and step-skew method.While,the influence of rotor eccentricity on the shaft voltage in integer slot and fractional slot perrmanent magnet machines are investigated,and tihe effect of segmented stator on large permanent magnet machines and the mitigation method are also researched.The detailed research contents are listed as following:(1)The principle of the shaft voltage induced by the slot effect in permanent magnet machines with some certain pole and slot number combinations are explained ar first.And the effect of slot on shaft voltage in permanent magnet mahcines with different pole and slot number combinations are investigated by analytica method,then the concept and analytical expression of the intrinsic shaft voltage is proposed.The harmonics of shaft voltage in permanent magnet machines with different pole and slot number combinations are summarized,the results show that there are no shaft voltage induced in the machines with the combinations whose slot number of the unit motor is even.The shaft voltage in 8 permanent magnet machines with different pole and slot number combinations are calculated by the finite element method(FEM)to verify the accuracy and effectiveness of the analytical method.To avoid that intrinsic shaft voltage in FSPMMs,an 110kW fractional slot permanent machine with 40-pole 48-slot is designed for the factory,the shaft voltage of the prototype motor is tested to verify the accuracy of the theoretical analysis.(2)The mitigation method for intrinsic shaft votage with pole-arc coefficient selection is proposed by the aid of the analytical method and the FEM.By investigating the relationship of the pole-arc coefficient and the shaft voltage,the expression for calculating the optimal pole-arc coefficient for suppress the fundamental component of the intrinsic shaft voltage is deduced.Then,the FEM is used to verified the effectiveness of the mitigation method,the optimal value by analytical method is corrected constantly by the iterative optimization by the FEM,and output an optimal value which can nearly suppress the intrinsic shaft voltage to zero.The effect of the parameters of the stator slot on the optimal pole-arc coefficient is studied to explain the necessary of the iterative optimization by the FEM.(3)The influence of the conventional step-skewing method on the cogging torque and intrinsic shaft voltage is investigated by the FEM at first,and the results show that the conventional step-skewing method can suppress the cogging torque effectively,but have little effect on the intrinsic shaft voltage.The expressions of the shaft voltage in each rotor step and the whole shaft voltage are all deduced.By the vector superposition diagram,the reason for the liminations of the conventional method is inllustrated.The improved step-skewing method for mitigating the intrinsic shaft voltage is proposed by the analytical method,and the influence of the skew angle and step number by the improved method on the intrinsic shaft voltage is also illustrated by the vector superposition diagram.And the result shows that,all the harmonics of the shaft voltage can be mitigated effectively expect the component whose order is same as the step number.Then,the influence of the improve step-skewing method on the intrinsic shaft voltage in 8-pole 12-slot fractional slot permanent magnet machine is also calculate by the FEM,and the results show that,the intrinsic shaft voltage can be mitigated effectively by well skew angle and step number selection.(4)The influence of the static and dynamic eccentricity on the magnetic flied in permanent magnet machines are investigated by the analytical method,the analytical expression of the shaft voltage by the static and dynamic eccentricity in permanent magnet machines are deduced respectively,the harmonics of the shaft voltges by no eccentricity,static and dynamic eccentricity are all summarized in integer slot and fractional slot permanent magnet machines.Then,the magnetic fiield and the shaft voltage in different machines with eccentricities are also investigated by the FEM,the results verify the accuracy of the analytical deductions and the feasibility of fault diagnosis by the shaft voltage signal.(5)By simplifying the permeance of the joints between segments,the influence of the segmented stator on the shaft votlage in large permanent magnet machines are investigated,the expression of the shaft voltage considering segmented stator and offset segments.The relationship of the segmented stators number D,the offset segments number and the pole pair number p is studied,the conclusion shows that when the numerator of the lowest term of D/p or DQ/pis an evennumber,no shaft voltage will be induced.By the FEM,the shaft voltge in 24-pole 144-slot permanent magnet machine with segmented stator is calculated,the effect of different position of the joints on the shaft voltage is also studied,the results verify the effectiveness and accuracy of the analytical deducitons.And the effect of the different width of joints on the shaft voltage is also investigated.The shaft voltage in the fractional slot permanent magnet machines with segmented stators are also investitage by the FEM,with is further verify the analytical results. |
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