Chau, KT

Li, WL

Lin, F

Liu, C

Qiu, C

Journal of Applied Physics

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TitleQuantitative comparison of dynamic flux distribution ofmagnetic couplers for roadway electric vehicle wirelesscharging systemAuthor(s) Qiu, C; Chau, KT; Liu, C; Li, WL; Lin, FCitation Journal of Applied Physics, 2014, v. 115 n. 17, article no. 17A334Issued Date 2014URL http://hdl.handle.net/10722/202865Rights Journal of Applied Physics. Copyright © American Institute ofPhysics.Quantitative comparison of dynamic flux distribution of magnetic couplers for roadwayelectric vehicle wireless charging systemChun Qiu, K. T. Chau, Chunhua Liu, Wenlong Li, and Fei Lin  Citation: Journal of Applied Physics 115, 17A334 (2014); doi: 10.1063/1.4866882 View online: http://dx.doi.org/10.1063/1.4866882 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/17?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Field weakening capability investigation of an axial flux permanent-magnet synchronous machine with radiallysliding permanent magnets used for electric vehicles J. Appl. Phys. 111, 07A719 (2012); 10.1063/1.3676230  Permanent magnet online magnetization performance analysis of a flux mnemonic double salient motor using animproved hysteresis model J. Appl. Phys. 111, 07D119 (2012); 10.1063/1.3676049  Design and analysis of new fault-tolerant permanent magnet motors for four-wheel-driving electric vehicles J. Appl. Phys. 111, 07E713 (2012); 10.1063/1.3672853  A flux-mnemonic permanent magnet brushless motor for electric vehicles J. Appl. Phys. 103, 07F103 (2008); 10.1063/1.2830553  Use of a thermophotovoltaic generator in a hybrid electric vehicle AIP Conf. Proc. 460, 488 (1999); 10.1063/1.57831    [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:147.8.31.43 On: Tue, 06 Jan 2015 07:14:57Quantitative comparison of dynamic flux distribution of magnetic couplersfor roadway electric vehicle wireless charging systemChun Qiu, K. T. Chau,a) Chunhua Liu, Wenlong Li, and Fei LinDepartment of Electrical and Electronic Engineering, The University of Hong Kong Pokfulam Road,Hong Kong, China(Presented 6 November 2013; received 23 September 2013; accepted 22 November 2013; publishedonline 27 February 2014)This paper gives a quantitative comparison of magnetic couplers for electric vehicle (EV) wirelesscharging applications. Circular pad with ferrite spokes and coreless rectangular coils are speciallyselected for analysis. The dynamic flux density between couplers under high misalignment isstudied by calculating the uncompensated power of the pick-up coupler. By using finite elementanalysis, the performance of each type of coupler is evaluated, and its adaptation to on-roadEV charging are compared according to the flux distribution and effective charging area. VC 2014AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4866882]I. INTRODUCTIONThe development of electric vehicles (EVs) has beenhindered by the costly battery and inconvenient batteryrecharging.1 A roadway charging system employing wire-less power transfer technology is anticipated to solve thisproblem. Two types of charging infrastructures are gener-ally used, namely, the lumped magnetic couplers andstraight line buried rail. The rail type is welcomed bythe move-and-charge application for its evenly distributedmagnetic field at each cross-section along the rail.However, its installation and arrangement are inflexible andcostly. The lumped couplers offer better flexibility andlower cost, and are mainly used for stationary chargingsince the mutual inductance of couplers varies significantlywith the vehicle position. Recently, some researches havebeen done on the misalignment adaption.2–4 With effectivecontrol strategies, the lumped magnetic couplers can sup-port the desired constant power for move-and-chargeapplication.In this paper, two most common lumped couplers areevaluated on the perspective of on-road charging application:the circular pad with ferrite spokes and flat coreless rectan-gular coils. The uncompensated power of the pick-up couplerPUN is chosen as a simple and intuitive way to evaluate theflux distribution and coupling performance. Then, two pairsof couplers with the same power supply, load rating, anddimension constraints are designed for comparison. Finiteelement analysis (FEA) is employed to directly evaluate theflux density distribution, and to calculate PUN at differenthorizontal offsets of the couplers. Finally, the effectivecharging ranges of two types of couplers are compared withemphasis on the power fluctuation and leakage flux density.The rectangular couplers show a much wider charging rangebut with a slightly higher leakage flux as compared with thecircular couplers.II. ROADWAYWIRELESS CHARGING SYSTEMFig. 1 shows a typical schematic of the EV wirelesscharging system. The power is drawn from a single-phase orthree-phase AC supply. The active front end rectifier is usedto achieve unity power factor. The primary coil L1 is fed bya current source converter operating at tens of kilohertz toseveral hundred kilohertz. The secondary coil L2 whichlocates close to the primary coil should be able to capturesufficient amount of magnetic flux. For EV application, thegap between these two coils should be 150mm to 200mm totolerate actual road conditions. Two capacitors C1 and C2 areconnected in series with L1 and L2, respectively, to form aresonant compensation network, hence maximizing the loadpower and lowering the volt-ampere (VA) rating of thepower supply.5 The series-series topology is selected becausethe optimal operating frequency is independent of the mag-netic coupling and load condition, which is especially con-venient for the roadway application.For magnetic couplers, the performance is related toboth self-inductance and mutual inductance on both sides. Itcan be simply evaluated using the magnetic coupling coeffi-cient k as given byk ¼ MffiffiffiffiffiffiffiffiffiL1L2p ; (1)where M is the mutual inductance which is subject to the rel-ative position of the couplers. The power transferred to theload PL is defined asPL ¼ M2L2xIP2Q ¼ VOCISCQSC ¼ PUNQ; (2)where x is the operating frequency, IP is the primary current,VOC and ISC are the open-circuit voltage and short-circuitcurrent of the secondary coupler, and Q is the secondaryquality factor which can be tuned by controlling the equiva-lent load impedance. While L1 and L2 are relatively constantat different horizontal offsets, especially with a large air gap,a)Electronic mail: ktchau@eee.hku.hk.0021-8979/2014/115(17)/17A334/3/$30.00 VC 2014 AIP Publishing LLC115, 17A334-1JOURNAL OF APPLIED PHYSICS 115, 17A334 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:147.8.31.43 On: Tue, 06 Jan 2015 07:14:57k has a square relationship with PL. Thus, the mostly relatedand invariant parameter to k is PUN which is the product ofVOC and ISC, and it is easy to be measured. Therefore, PUN ischosen as the index to evaluate the performance of thecouplers.In this paper, a roadway EV wireless charging system hav-ing the power of 2 kW and air gap of 200mm is used for exem-plification, which is suitable for most passenger EVs. Twotypes of magnetic couplers are designed based on the samepower and performance demands, as shown in Figs. 2 and 3.As the optimal air gap is close to 1/4 of the diameter,3 thediameter of the circular couplers is set to 700mm. Ferritespokes are used to guide the magnetic flux, thus enhancing thecoupling performance. However, a large number of narrowspokes may reduce the mechanical strength. The selection ofprimary and secondary turn ratio should consider the loadpower, load voltage, supply current, and operating frequency.Since the purpose of this paper is to evaluate the performanceof couplers, the turn ratio is simply selected as 1:1.The design of rectangular couplers is simple. Each cou-pler has a square coil with the outside length of 700mm,which is essentially the same size as the circular coupler.Detailed specifications of both couplers are shown in Table. I.III. COMPARISON AND RESULTSTo evaluate the dynamic flux distribution of the two typesof couplers, a three-dimensional FEA tool JMAG is adopted.The simulation is carried out at different horizontal offsetsbetween the primary and secondary couplers. Namely, PUN isevaluated at each position by separately calculating VOC andISC of the secondary coupler.Fig. 4 shows the characteristics of PUN versus differenthorizontal offsets with an air gap of 200mm. It can be observedthat the rectangular coupler shows better performance in bothpower rating and misalignment tolerance. It is noteworthy thatthe circular coupler begins to offer insignificant PUN when thehorizontal offset is larger than 300mm, indicating that the loadwill not receive any power regardless of the regulation. Sincethe radius of the coupler is larger than 300mm, there will be anabsence of power on the load even all primary couplers are putFIG. 2. Layout of 700mm circular couplers with ferrite spokes.FIG. 3. Layout of 700mm rectangular couplers.FIG. 1. Schematic of typical wirelesspower transfer system.TABLE I. Design specifications.Circular couplers Rectangular couplersNo. of primary coil turns 26 25No. of secondary coil turns 26 25Coil thickness 4 mm 4 mmAir gap 200mm 200mmPrimary current 23 A 23 AOperating frequency 20 kHz 20 kHzFerrite width 28 mm …Ferrite length 279 mm …Ferrite thickness 16 mm …Aluminum plate width 700 mm 700 mmAluminum plate thickness 5 mm 5 mmCoil central diameter 400 mm …Coil length … 700 mm17A334-2 Qiu et al. J. Appl. Phys. 115, 17A334 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:147.8.31.43 On: Tue, 06 Jan 2015 07:14:57close to each other. Moreover, unlike the rectangular couplers,there are inevitable holes between the circular pads. This willfurther increase the zero power zones. Consequently, if adopt-ing the circular couplers, the multilayer arrangement will berequired to achieve the desired power transfer. For the rectan-gular coupler, the power does not die out until the offsetreaches 500mm. The secondary quality factor is normally from4 to 6, which is governed by the electrical stress of circuit com-ponents. Thus, the load power can reach the rated value of2 kW even with a horizontal offset of 350mm (Q¼ 4). So ifthe adjacent couplers are close to each other, the power will beconstant along the road.As the unwanted leakage flux may bring up safetyissues, the International Commission on Non-IonizingRadiation Protection (ICNIRP) suggests that the acceptablehuman exposure to electromagnetic field should be less than6.25 lT at frequency 0.8–150 kHz.6 When the couplers areat the horizontal offset of 200 mm and the load power is2 kW, the leakage flux densities at different locations arecalculated by using FEA. As shown in Fig. 5, within the hor-izontal and vertical displacements of 1500 mm, the leakageflux densities of both circular and rectangular couplers aremostly greater than the threshold of 6.25 lT. This is antici-pated as there is no shielding adopted in this analysis, and itcan readily be improved by adding shielding rings4 or reac-tive loops.7 Meanwhile, it can be observed that although therectangular coupler can offer good performance on misalign-ment tolerance, its leakage flux is generally larger than thatof the circular coupler, especially at the location with verticaldisplacement, which is a concern for EV users.IV. CONCLUSIONThis paper evaluates and compares two different typesof magnetic couplers for EV move-and-charge application.Two sets of couplers are designed for a 2 kW system with anair gap of 200mm. The uncompensated secondary power isselected as the index for performance evaluation. The char-acteristics at different offsets and the magnetic field distribu-tions are simulated by using finite element analysis. Theresults show that the coreless rectangular coupler can offerbetter performance than the commonly adopted circular cou-pler with ferrite spokes for EV charging application, thoughit exhibits higher leakage flux in the absence of shielding.ACKNOWLEDGMENTSThis work was supported by a Grant (Project No. HKUSPF 201109176034) from the Committee on Research andConference Grants, The University of Hong Kong, HongKong.1C. C. Chan and K. T. Chau, Modern Electric Vehicle Technology (OxfordUniversity Press, 2001).2T. Misawa, T. Sato, T. Takura, F. Sato, and H. Matsuki, J. Appl. Phys. 111,07E720 (2012).3M. Budhia, G. A. Covic, and J. T. Boys, IEEE Trans. Power Electron. 26,3096 (2011).4J. Sallan, J. L. Villa, A. Llombart, and J. F. Sanz, IEEE Trans. Ind.Electron. 56, 2140 (2009).5G. A. Covic and J. T. Boys, Proc. IEEE. 101, 1276 (2013).6ICNIRP Guidelines, Health Phys. 74, 494 (1998).7J. Kim, J. Kim, S. Kong, H. Kim, I. S. Suh, N. P. Suh, D. H. Cho, J. Kim,and S. Ahn, Proc. IEEE. 101, 1332 (2013).FIG. 5. Leakage flux density analysis at 200mm horizontal offset.FIG. 4. Characteristics of uncompensated secondary power versus horizon-tal offset with 200mm air gap.17A334-3 Qiu et al. J. Appl. Phys. 115, 17A334 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:147.8.31.43 On: Tue, 06 Jan 2015 07:14:57

Quantitative comparison of dynamic flux distribution of magnetic couplers for roadway electric vehicle wireless charging system

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