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NUMERIČNO MODELIRANJE INDUKTIVNEGA SISTEMA ZA BREZŽIČNI PRENOS ELEKTRIČNE ENERGIJE
STRAUCH, LUDVIK (Author), Pavlin, Mojca (Mentor) More about this mentor... This link opens in a new window, Bregar, Vladimir Boštjan (Co-mentor)

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Abstract
Induktivni sistem za brezžicni prenos elektricne energije predstavlja tehnicno in tržno zelo zanimiv nacin prenosa elektricne energije. Tudi v podjetju Kolektor Sikom, PE Magma razvijamo induktivne sisteme za brezžicni prenos elektricne energije za polnjenje prenosnih naprav v avtomobilski industriji, medicini in ostalih tržno zanimivih podrocjih. Doktorska disertacija opisuje numericno modeliranje induktivnega sistema za brezžicni prenos elektricne energije, ki je sestavljen iz oddajne in sprejemne tuljave z dodanima feritnima jedroma. Induktivni prenos se loci na resonancni in neresonancni prenos. Podrobno je obravnavan induktivni neresonancni prenos, saj je podjetje Kolektor Sikom, PE Magma usmerjeno v izdelavo tuljav z dodanimi feritnimi jedri in ne v izdelavo elektricnih vezij, ki omogocajo tudi resonancni prenos. Poleg tega vecina tržno zanimivih aplikacij za induktivni prenos uporablja polnjenje prenosnih naprav na majhnih razdaljah nekaj milimetrov, kjer je neresonancni prenos dovolj dobro primerljiv z resonancnim. V literaturi so predstavljeni vecinoma induktivni sistemi, sestavljeni iz oddajne in sprejemne tuljave brez feritnih jeder, ki se lahko opisujejo z analiticnimi ali empiricnimi enacbami. Dodana feritna jedra bistveno izboljšajo brezžicni prenos in služijo tudi kot magnetna zašcita pred magnetnimi polji iz strani oddajne in sprejemne tuljave. Vendar pa v literaturi ni predstavljenih enacb za opis induktivnega sistema z dodanimi feritnimi jedri, saj zaradi kompleksne geometrije feritnih jeder in nelinearnih lastnosti feritnega materiala analiticen pristop ni vec mogoc. Zato je nujno potrebno uporabiti programska okolja za numericno modeliranje induktivnega sistema z dodanimi feritnimi jedri. Tudi obstojeci numericni modeli v literaturi ne opisujejo vpliva vseh parametrov, ki vplivajo na induktivni sistem. Namen tega doktorskega dela je z numericnimi modeli: opisati, ovrednotiti in analizirati vpliv položaja oddajne in sprejemne tuljave z dodanimi feritnimi jedri, navesti možnosti izbire feritnih materialov iz stališca relativne permeabilnosti in magnetne zašcite, preuciti vpliv bakrene folije na magnetno zašcito, poiskati optimalno obliko feritnih jeder, izracunati induktivnosti tuljav z dodanim feritnim jedrom, preuciti vpliv zlaganja feritnih plošc, dolociti izgube v pletenicah razlicnih tuljav, ovrednotiti izgube v feritnih jedrih in na koncu še analizirati segrevanje tuljav s feritnimi jedri. Numericni modeli so tudi primerjani z izmerjenimi vrednostmi. V uvodnih poglavjih doktorske disertacije je predstavljen induktivni sistem za brezžicni prenos elektricne energije, sestavljen iz oddajne in sprejemne tuljave. Induktivni sistem je predstavljen preko razlicnih enacb in kljucnih parametrov: faktor sklopitve, faktor kvalitete in izkoristek prenosa. Razložena je tudi primerjava med induktivnim resonancnim in neresonancnim prenosom za induktivni sistem za brezžicni prenos elektricne energije. Nadalje sta predstavljena parametra tuljav induktivnost in ohmska upornost. Induktivnost je predstavljena iz izracuna preko teoreticnih in empiricnih metod, kjer je tudi naveden pregled literature izracuna induktivnosti zracnih tuljav razlicnih avtorjev. Ohmska upornost pa je sestavljena iz ciste ohmske upornosti, upornosti zaradi kožnega in sosedstvenega pojava. V nadaljevanju doktorske disertacije so opisani uporabljeni materiali in metode. Izracuni numericnih modelov so bili izvedeni v programskih okoljih Comsol Multiphysics in Ansoft Maxwell. Lastnosti uporabljenih feritnih materialov v modelih so izbrani na podlagi realnih materialov, ki jih industrijsko uporabljamo za tovrstne aplikacije. Nazadnje so razložene tudi metode merjenja in navedeni uporabljeni merilni instrumenti s katerimi so bile verificirane simulacije v programskih okoljih. V poglavju 5. Rezultati in razprava so z numericnimi modeli analizirani vplivi razlicnih geometrij in parametrov: faktor sklopitve k, induktivnost L, faktor kvalitete Q in magnetne zašcite, ki imajo vpliv na delovanje in lastnosti induktivnega sistema za brezžicni prenos elektricne energije. Vpliv spreminjanja položaja na faktor sklopitve je bil narejen na primeru geometrije oddajne in sprejemne tuljave z dodanima feritnima jedroma v vodoravni smeri, navpicni smeri in kotu med tuljavama. Ugotovljeno je bilo, da se z vecanjem razdalj v navpicni, vodoravni smeri in povecevanju kota med tuljavama faktor sklopitve k zmanjšuje. Dodatno so bile še primerjane med seboj vrednosti lastnih in medsebojnih induktivnosti tuljav z dodanima feritnima jedroma in razložen pojav negativnega faktorja sklopitve k. Iz stališca faktorja sklopitve k in magnetne zašcite je bila izvedena parametrizacija izbire razlicnih feritnih materialov preko vecanja relativne permeabilnosti na doloceni geometriji induktivnega sistema. Ugotovljeno je bilo, da ni smiselno višati relativne permeabilnosti preko dolocene vrednosti, saj s tem ne izboljšamo bistveno faktorja sklopitve k in magnetne zašcite. Nadalje je bila za izboljšanje magnetne zašcite na feritnih jedrih dodana tudi bakrena folija, ki še dodatno izboljša magnetno zašcito. Feritna jedra, ki se uporabljajo v induktivnem sistemu so razlicnih oblik kot so: feritni loncki, feritne plošce, E-jedra in U-jedra. V serijski proizvodnji je pri nacrtovanju induktivnih komponent pomemben kriterij poraba feritnega materiala. Zato je bila izvedena optimizacija induktivnega sistema sestavljenega iz tuljav z dodanimi feritnimi jedri iz feritnih palck. Ugotovljeno je bilo, da je za dosego visokega faktorja sklopitve k potrebno izbrati vecje število feritnih palck. S tem se doseže manjša poraba materiala in tudi manjša teža feritnega jedra. Poleg tega predstavlja vecje število feritnih palck dovolj dobro magnetno zašcito. Dodatno je bilo ugotovljeno, da je vpliv zamika kota na skupno razliko faktorja sklopitve k med feritnimi palckami na oddajni in sprejemni strani manjši v primeru vecjega števila feritnih palck. Iz stališca izracuna induktivnosti tuljav z dodanimi jedri, je bilo ugotovljeno, da so v literaturi predstavljene predvsem analiticne in empiricne enacbe za tuljave brez feritnih jeder. Zato je bil uveden nov faktor induktivnosti, ki omogoca hitrejši poenostavljeni izracun induktivnosti za tuljave z dodanimi feritnimi jedri v numericnih modelih. Zaradi omejitev pri izdelavi vecjih feritnih plošc v serijski proizvodnji je bila razložena tudi možnost izdelave preko tehnike zlaganja. Pri tem se sestavi vecjo feritno plošco iz vec majhnih feritnih plošc, kjer je potrebno izbrati kompromis med velikostjo zracne reže in številom majhnih feritnih plošc. Zlaganje feritnih plošc ne poslabša bistveno faktorja sklopitve k in magnetne zašcite. V induktivnem sistemu je zelo pomembno tudi dolociti izgube v tuljavah in feritnih jedrih. Tuljava, ki deluje v visokofrekvencnem obmocju nad 100 kHz je narejena iz pletenice, ki je sestavljena iz vec manjših vlaken. Iz stališca dolocitve izgub v pletenicah razlicnih tuljav je bilo narejenih vec analiz zracnih tuljav z razlicnim številom vlaken in številom ovojev. Izkaže se, da tuljava sestavljena iz pletenice predstavlja zelo kompleksno geometrijo za numericne modele, saj je le-ta sestavljena iz velikega števila med seboj prepletenih vlaken. Poleg tega geometrija pletenice ni enolicno dolocena in poznana, saj je zelo odvisna od izdelave proizvajalca. Ugotovljeno je bilo, da se je zaradi same kompleksnosti pletenice pri visokih frekvencah bolje odlociti za izdelavo konkretne tuljave in izvesti meritve upornosti ali pa uporabiti izracune upornosti in s tem izgub iz omenjene literature. Nadalje je bil pri doloceni geometriji izveden tudi izracun faktorja kvalitete Q obeh tuljav z dodanimi feritnimi jedri. Izgube v feritnih jedrih so bile dolocene preko numericnih modelov in kataloških podatkov feritnega proizvajalca. Iz primerjave velikosti prevladujocih izgub v induktivnem sistemu je bilo ugotovljeno, da se prevladujoce izgube nahajajo v tuljavah in ne v feritnih jedrih. Izracunane izgube v tuljavi in feritnih jedrih so bili nato vnesene še v termicni model. Ugotovljeno je bilo, da natancno poznavanje izgub v feritnih jedrih, ki najveckrat izhajajo iz kataloških podatkov materiala, ne prispeva pomembno k izboljšanju natancnosti modelov. Vse analize in izracuni razlicnih vplivov in parametrov na induktivni sistem so bili izvedeni s simulacijami v razlicnih numericnih modelih v programskih okoljih, ki so osnovani na metodi koncnih elementov. Rezultati specificnih numericnih modelov so bili primerjani z eksperimentalno dobljenimi vrednostmi. Razvoj numericnih modelov je omogocil natancen vpogled v delovanje sklopa oddajne in sprejemne tuljave z dodanima feritnima jedroma iz stališca elektromagnetnih in termicnih pojavov. Razviti numericni modeli bodo pripomogli k hitrejšemu in boljšemu nacrtovanju induktivnega sistema za brezžicni prenos elektricne energije.

Language:Slovenian
Keywords:induktivni sistem za brezžicni prenos ee, tuljave, feritna jedra, numericno modeliranje, programsko okolje Comsol Multiphysics, programsko okolje Ansoft Maxwell, programsko okolje Ansys, parametrizacija, optimizacija, faktor sklopitve, induktivnost, faktor kvalitete, magnetna zašcita, izgube v jedrih
Work type:Doctoral dissertation (mb31)
Organization:FE - Faculty of Electrical Engineering
Year:2016
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Secondary language

Language:English
Title:NUMERICAL MODELLING OF INDUCTIVE SYSTEM FOR WIRELESS POWER TRANSFER
Abstract:
An inductive system for wireless power transfer is technically and commercially very interesting way of transmitting electrical energy. Also in company Kolektor Sikom, PE Magma we develop systems for inductive wireless power transfer to charge portable devices in the automotive industry, medicine and other market interesting areas. Doctoral dissertation describes the numerical modeling of the inductive system for wireless power transfer, which consists of the transmitter and the receiver coil with ferrite cores. Inductive wireless power transfer can be resonant or non-resonant. Since the company Kolektor Sikom, PE Magma has been focused on production of coils with ferrite cores we analyze in detail non-resonant transfer while for applications in power electronics resonant transfer is also relevant. Also, most interesting market applications for charging portable devices use small distances in the range of a few millimeters, where non-resonant transfer is comparable with the resonant transfer. In the literature, inductive systems mainly consist of a transmitter and receiver coils without ferrite cores. Such simple systems, can be analyzed by analytical or empirical equations. In order to improve inductive wireless power transfer, ferrite cores are also used that in parallel also serve as a magnetic shield against magnetic fields from the transmitter and receiver coils. However, due to the complex geometry of the ferrite cores and nonlinear properties of the ferrite material the analytical approach is no longer possible. It is therefore essential to use finite element software for numerical modeling of the inductive system with ferrite cores. Studies, which describe the inductive system with numerical modeling also use simplified models which do not describe the influence of all parameters. The aim of this work was to build numerical models: which describe, evaluate and analyze the impact of position between the transmitter and receiver coils with ferrite cores, to specify ferrite materials from the relative permeability and magnetic shield, to consider the impact of copper foil on the magnetic shield, to find the optimal shape of the ferrite cores, to calculate inductance of coils with ferrite cores, to examine the effect of assembled ferrite plates, to determine the losses in different coils made from litz wires, to evaluate the losses in ferrite cores, and finally to analyze the impact of the heating on transmitter and receiver coils with ferrite cores. Numerical models are also compared with the measurements. In the introductory chapters an inductive system for wireless power transfer is presented, which consists of transmitter and receiver coils. Inductive system is presented through various equations and parameters: the coupling coefficient, quality factor and efficiency. A comparison between inductive resonant and non-resonant wireless power transfer is also presented. Furthermore, the parameters inductance and ohmic resistance are defined. Inductance is presented from theoretical and empirical methods, where also an overview of the literature for inductance calculations of different authors is presented. The ohmic resistance consists of pure ohmic resistance, resistance due to skin and proximity effect. Further, the materials and methods are explained. The calculations of numerical models were made in the programming environments Comsol Multiphysics and Ansoft Maxwell. Characteristics of ferrite materials in the models were selected on the basis of realistic materials that can be used or are used for such industrial applications. Finally, the measuring methods and measuring instruments which were used for verification of simulations in programming environments are also discussed. In the 5. Results and discussion chapter results of numerical models which analyzed the effects of different geometries and parameters are presented: the coupling coefficient k, the inductance L, the quality factor Q and magnetic shielding. All these parameters have an impact on the performance and characteristics of an inductive system for wireless power transfer. The impact of varying position on coupling coefficient is presented in example of geometry with transmitter and receiver coil with additional ferrite cores in horizontal direction, vertical direction and angle of rotation between coils. It has been found that by increasing the distances in the vertical and horizontal direction and by increasing the angle between coils the coupling coefficient k decreases. Additionally, values of self and mutual inductances of coils with ferrite cores are compared between each other and also the effect of negative coupling coefficient k is explained. From the view of the coupling coefficient k and magnetic shielding the parameterization of various ferrite materials with increasing the relative permeability was also made for the specific geometry of the inductive system. It has been found that using materials with higher relative permeability does not significantly improve the value of coupling coefficient k and magnetic shielding. Futhermore, the copper foil added on ferrite core significantly improves magnetic shielding. Furthermore, ferrite cores, which are used in inductive system are of different shapes: ferrite pots, ferrite plates, the E-core and the U-core. In a serial production of inductive components consumption of ferrite material is of great importance. Therefore, optimization of the inductive system made of coils with ferrite cores which were made from ferrite bars was analyzed. We obtained that to reach high value of coupling coefficient k it is important to select a higher number of ferrite bars. This results in less material used and also less weight of a ferrite core. In addition, higher number of ferrite bars provide sufficient magnetic shielding. Further, it has been found that the effect of the angle between ferrite bars on the transmitter and receiver side on the overall coupling coefficient is smaller in the case of higher number of ferrite bars. The inductance of coils with ferrite cores as presented in the literature, is primarily based on analytical and empirical equations for coils without ferrite cores. Therefore, a new inductance factor was defined which allows faster and simplified calculation of inductance of coils with ferrite cores in numerical models. Due to the limitations in manufacturing larger ferrite plates in serial production also the possibility of making through stacking was analyzed. The larger ferrite plate is therefore made from small ferrite plates where it is necessary to choose some compromise between the size of an air gap and the number of ferrite plates. Stacking of ferrite plates does not deteriorate significantly the coupling coefficient k and magnetic shielding. Moreover, it is very important to determine the losses in the coils and in ferrite cores. The coil, which is used in high frequency range above 100 kHz, is made of litz wire which consists of number of smaller strands. For the determination of losses in coils, which are made of litz wire, more air coils with the different number of turns and strands were analyzed. It was concluded that coil made of litz wire represents very complex structure in numerical models, since it is made of the large number of twisted strands. In addition, the geometry of litz wire is not uniquely determined and known as it is very dependent on the manufacturing process. It has been found that due to the complex structure of litz wire in high frequencies, it is better to make a real coil and to make measurements of resistivity or to use resistivity formulas and thus losses from the literature. Further, there was performed at certain geometry also the calculation of the quality factor Q of the two coils with the addition of ferrite cores. The losses in ferrite cores were determined through numerical models and catalogue data of ferrite manufacturer. In the analysis of losses in the inductive system, it has been found that the dominant losses are in the coils and not in the ferrite cores. Both calculated losses in coils and ferrite cores were then inserted also in a thermal model. It was found that the accurate knowledge of the losses in ferrite cores, which are usually derived from catalogue data of material, does not significantly contribute to improve the accuracy of the models. All analyzes and calculations of different impacts and parameters on inductive system were made through simulations in different numerical models in the programming environments which are based on finite element method. Results of specific numerical models were compared to experimentally obtained values. The development of numerical models provides a detailed insight into the functioning of the inductive system made of transmitter and receiver coils with ferrite cores from view of electromagnetic and thermal phenomena. Developed numerical models will contribute to a faster and better design of an inductive system for wireless power transfer.

Keywords:inductive system, wireless power transfer, coils, ferrite cores, numerical modeling, Comsol Multiphysics, Ansoft Maxwell, Ansys, parameterization, coupling factor, inductance, quality factor, magnetic shield, losses in litz wire coils, losses in ferrite cores

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