| With the continuous expansion of long-distance DC power transmission and gridconnected energy storage systems in China,High Frequency Transformer,which realizes DC voltage conversion and energy exchange through high-power power electronic devices and their control technology,has shown great advantages and gained extensive attention.However,as the operating frequency gradually increases to kilohertz,the core loss at the high frequency complex magnetic field will greatly increase.The small volume of High Frequency Transformer,is not conducive to heat dissipation,resulting in its own temperature rising.This puts forward a higher demand on the performance of the core materials used in High Frequency Transformer.Nanocrystalline soft magnetic alloy(FINEMET,NANOPERM,HITPERM)has excellent comprehensive soft magnetic properties such as high saturation magnetic induction intensity,high effective permeability,low dissipation and low coercivity,and low cost,simple preparation process and high heat resistance,so it is likely to become the most widely used ferromagnetic material.However,due to its complex internal microstructure and unclear action mechanism of high-frequency magnetization dominant mode,there is still a certain distance for it to meet the application requirements of high power and low loss of High Frequency Transformer.In order to realize the popularization of nanocrystalline soft magnetic alloy in the Electronic and Electrical Engineering,especially used in the High Frequency Transformer Core.In this thesis,based on the basic theory of micromagnetism,FINEMET nanocrystalline soft magnetic alloy is taken as the research object to explore the internal magnetic moment deflection of the nanocrystalline soft magnetic alloy during saturation magnetization under high frequency excitation and the effect mechanism of grain microstructure on the high frequency magnetic loss of the alloy.The research contents of this thesis are as follows:(1)The process of constructing the micromagnetic model of mesospheric spherical nanocrystals under high frequency excitation is introduced.In order to investigate the motion of the internal magnetic moment of the nanocrystalline alloy during magnetization,the micromagnetic simulation software OOMMF was used to model and analyze the nanocrystalline alloy.The difference between cube nanocrystalline cells and spherical nanocrystalline cells was compared when the model was established.It is found that compared with cube cells,spherical cells have the following advantages:On the one hand,if the nanocrystalline cell is assumed to have a cube structure,that is,the spherical nanocrystalline phase is intricately cut into the cuboid amorphous phase,then the maximum volume fraction of the nanocrystalline phase in the whole cell is only 52.35%,which is not in accordance with the actual situation(60-70%).The spherical cell can adjust the size of the radius R1 and R2 to change the volume fraction of nanocrystalline phase,making it conform to the actual situation(6070%).On the other hand,the unique edges and corners of cube cells will produce highly uneven stray magnetic fields,and the demagnetization effect generated by the stray fields will lead to the weakening of exchange and coupling between adjacent cells.Finally,the micromagnetic model of mesoscopic spherical nanocrystalline cells is constructed by using micromagnetic simulation software.After the construction of the model,in order to verify the correctness of the micromagnetic model,the main DC magnetic properties(static characteristic parameters)of the nanocrystalline soft magnetic alloy were measured by the direct magnetic testing device.At the same time,the magnetization curve of the model during the magnetization process is drawn and compared with the curve measured by experiment.It is found that the static magnetic characteristic parameters and magnetization curve of the micromagnetic model of nanocrystalline alloy are not much different from the data of experimental samples,which verifies the correctness of the micromagnetic model of nanocry stalline alloy.(2)The saturation magnetization of nanocrystalline alloys under high frequency excitation is studied.In order to investigate the rotation of magnetic moment inside the material under high frequency sinusoidal excitation,the relationship between the saturation magnetization process and the external alternating magnetic field is clarified.Based on the three-dimensional model of nanocrystalline alloy,a sinusoidal alternating magnetic field with frequency f of 1kHz10kHz and amplitude H of 0.1T-0.8T was applied to the model.The magnetic moment movement in magnetization process is investigated from mesoscopic and macroscopic levels by defining the magnetic moment deflection velocity ω and magnetization rate v respectively.The results show that the ω is positively correlated with the H and f of alternating magnetic field,and the increase of f has a particularly significant effect on the increase of ω compared with the increase of H.Then,the function relation between the ω and the H and f are obtained.The f coefficient is much larger than the H coefficient,which is about 2.5 times of the H coefficient.Finally,the magnetization curve is measured by an AC measuring device,and the functional relationship between the v and the f and H of the alternating magnetic field is obtained.The coefficient of f is much larger than the coefficient of H,which is about 2.75 times of the H coefficient.The multiple is approximately the same as the deflection angular velocityω,and the relative error is only 9.1%.realizing the mutual verification between micro and macro.(3)The influence of grain size and volume fraction variation on the high frequency magnetic loss of nanocrystalline alloys is investigated.In order to clarify the relationship between high-frequency magnetic loss and internal microstructure of nanocrystalline alloy,a sinusoidal alternating magnetic field with frequency of 1 kHz to 10 kHz and amplitude of 0.7T was applied to the three-dimensional model of nanocrystalline alloy at mesoscopic scale.Taking volume fraction V and grain size d as research parameters,the influence of microstructure changes on high frequency magnetic loss(P)was investigated from the mesoscopic level.The results show that t P increases with the increase of V and d.Among them,d has a more significant effect on P.When the external magnetic field frequency f=1kHz and V=60%remain unchanged,and d increases from 6nm to 15nm,the growth rate of P is 35.46%.Correspondingly,when the external magnetic field frequency f=1kHz and d=6nm remain unchanged,and the V increases from 60%to 80%,the P increases by 25.76%.When the external magnetic field frequency f=20kHz and V=60%remain unchanged,and d increases from 6nm to 15nm,the growth rate of P is 52.32%.Correspondingly,when the external magnetic field frequency f=20kHz and d=6nm remain unchanged,and V increases from 60%to 80%,the P increases by 27.42%.This is because the P are mainly composed of eddy current losses.The eddy current loss is positively correlated with d and V.Therefore,when d and V increase,the high frequency loss of the material will increase.In this thesis,the magnetization process of FINEMET nanocrystalline soft magnetic alloy was simulated by OOMMF.The deflection of internal magnetic moment during magnetization of nanocrystalline alloys under different working conditions and the influence of internal volume fraction and grain size on high frequency magnetic loss were investigated.It can lay the theoretical foundation and provide the key technology for the application of nanocrystalline alloy in high frequency transformer core.However,due to the limitation of experimental platform and research time,the magnetic moment deflection and high frequency magnetic loss in the magnetization process of nanocrystalline alloy under non-sinusoidal wave and higher frequency excitation containing a large number of harmonics can be considered in the future. |
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