IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014 3600405

Lateral Displacement Influence on the LevitationForce of YBCO Coated Conductor Linear Bearings

Felipe Sass, Daniel Henrique Nogueira Dias, Guilherme Gonçalves Sotelo, and Rubens de Andrade, Jr.

Abstract—Superconducting coated conductors have been usedfor several large-scale applications. One potential application forYBCO second-generation (2G) wire is the development of super-conducting magnetic bearings (SMB). The main advantages of 2Gwires are large scale production, high critical current density val-ues, lower manufacturing cost than first generation, and superiorperformance under high magnetic fields. This work proposes astudy of a linear SMB using 2G wires as a passive levitator forMagLev vehicles. The 2G block is conceived to replace the YBCObulk in the MagLev’s SMB prototype, which is being built atFederal University of Rio de Janeiro. In this context, a supercon-ducting block using stacked 2G wires was built to evaluate thebehavior of this SMB. The forces between the 2G block and a per-manent magnet were measured and compared with the results foran YBCO bulk with nearly the same dimensions. The experimentalrig used for this work allows vertical and lateral displacements anda load cell is used to evaluate the SMB levitation force decay dueto the lateral movement. The 2G block has presented lower forcelevels than ordinary YBCO bulks, but it presents potential to beapplied in a SMB application.

Index Terms—Coated conductors, second generation (2G) HTSwire, superconducting magnetic bearing (SMB).

I. INTRODUCTION

THE Superconducting Magnetic Bearing (SMB) has beenextensively studied in the literature. Linear or rotational

SMB can be designed and applied in the development ofMagLev vehicles [1]–[3] and rotational devices [4]–[6]. TheSMB has its operating principles based on the repulsive forcebetween superconductor bulks and the magnetic field of perma-nent magnets. The production of a set of YBCO bulks for SMBtakes about one week, in a complex and expensive process.For this reason it is difficult to upscale its production. Thedevelopment of devices for commercial purposes motivated ourteam to investigate new possibilities for SMB, allowing thereplacement of YBCO bulk. Recently, manufacturers are ableto produce high quality second generation (2G) superconductorwires hundreds of meters long [7], [8]. This fact has givennew perspectives for several engineering applications, such as:power system devices [9], power transmission and distributioncables [10], fault current limiters [11], electric machines [12],

Manuscript received July 15, 2013; accepted October 29, 2013. Date ofpublication November 7, 2013; date of current version November 21, 2013.This work was supported in part by the Brazilian agencies CAPES, CNPq,FAPERJ, BNDES, and FOPESQ/UFF.

F. Sass and R. de Andrade, Jr. are with Federal University of Rio deJaneiro, 21941-901 Rio de Janeiro-RJ, Brazil (e-mail: felipesass@ufrj.br;rubens.andrade@ufrj.br).

D. H. N. Dias and G. G. Sotelo are with the Fluminense Federal University,24220-008 Niteröi-RJ, Brazil (e-mail: dhndias@id.uff.br; gsotelo@id.uff.br).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TASC.2013.2289300

Fig. 1. Illustration of the maglev Cobra: (a) vehicle and (b) cryostat(unit: mm).

high field magnets coils [13], and others. In this context, thepresent work studies the construction of a new type of bearingusing several pieces of coated conductors stacked.

To verify the feasibility of 2G wires for the SMB appli-cations, the permanent magnet guideway (PMG) from theMagLev Cobra [1] project was used as source of externalmagnetic field. This paper presents an investigation of this SMBsystem considering the levitation force decay due to lateralmovements. During experiments, vertical and lateral displace-ments of the PMG in relation to the superconductor were made.The results show the potential of the 2G superconductor wiresfor SMB applications, especially considering the upscalingproduction of superconducting bearings, but its feasibility stilldepends on the cost reduction of the YBCO wires.

II. YBCO-COATED CONDUCTOR LINEAR BEARINGS

To start the mass production of Maglev vehicles, thousandsof YBCO melt-textured bulks would be necessary. For example,Fig. 1 shows an illustration of the Maglev Cobra vehicle that isbeing build at Federal University of Rio de Janeiro with its 24cryostats, each one with 24 YBCO bulks (total of 576 bulks).

There are some possibilities available to replace the bulkby coated conductors as passive levitators: coils with wiresterminals short circuited [14], one looping coil made by cutting

1051-8223 © 2013 IEEE

3600405 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014

Fig. 2. SMB used in this work: (a) 2G block (11× 38× 67 mm) builtwith three columns of 2G wire stacked; (b) YBCO bulk (14× 34× 67 mm)superconductor with three seeds; (c) PMG with dimensions in millimeter andflux lines.

longitudinally the 2G wire in its middle without cutting theterminal boarders [15] and the use of stacked 2G wires tomake a block [16]. In previous work [17], the idea of using2G wires to develop a linear SMB was originally introduced,evaluating both levitation and lateral forces. For that purpose,a superconducting block was built by stacking many pieces of2G superconductor wire. The levitation force between the PMGand the HTS samples (2G block and an YBCO bulk from ATZ[6]) were measured during vertical and lateral displacements.

III. SYSTEM DESCRIPTION

The HTS samples and the PMG, which are the basic com-ponents of the SMB tested in this work, are described in thissection.

The illustration of the HTS samples tested in this work arepresented in Fig. 2. A superconducting block was built bystacking more than 500 pieces of 2G (from SUPERPOWER,model SF12050) wire. This sample was named 2G block and itsbehavior in SMBs application was also analysed in [16], [17].The 2G block was built using the largest commercial 2G YBCOwire available (12 mm width) and it has 3 columns to obtainthe closest size of the commercial YBCO bulk manufacturedby ATZ [6], as presented in Fig. 2.

The PMG used in this work as the source of external fieldis the same of the MagLev Cobra project [18]. This PMG ispresented in Fig. 2(c), where two coordinate systems are de-fined: (CS1) and (CS2). The CS1 is placed above an iron piece,where the magnetic flux is mainly in z direction. The CS2 isplaced at the center of the permanent magnet guideway, wherethe magnetic flux is mainly in y direction. In the x direction theguideway is considered infinitely long, so that edge effects canbe neglected. In MagLev Cobra, YBCO bulks are placed abovedifferent places of the presented PMG. At different positionsthe HTS is exposed to different magnetic field configurations.For this reason, measurements in this work were taken atboth coordinate systems, representing typical positions of thesuperconducting blocks in the levitating cryostat. However, inthis work, the largest dimension of the HTS sample was placedin parallel with x axis.

An experimental rig was used to measure the levitationforce between the HTS sample and the PMG. In this rig, thesuperconductor is mounted on a 6 axis load cell (from ATI,model Omega 160), which is fixed at a structure capable ofperforming vertical movement driven by a software controlledstep motor. The PMG is placed below the HTS sample and is

Fig. 3. Vertical force measurements with YBCO bulk placed at CS1. First, avertical approach was performed from the initial ZFC position of z = 100 mmto the final position of z = 5 mm. Then, a lateral movement in a range of± 10 mm was performed.

capable of performing lateral movement. Vertical and lateralmovements occurs in steps of 1 mm, stopping 1 second duringeach movement step for data acquisition. During this time in-terval, about 10000 acquisitions were computed and an averagevalue was calculated, which reduces considerably the amountof noise acquired, leading to a more accurate measure. Thisprocedure is repeated until the pre-established final position isreached.

During the tests the samples were placed centered on bothCS1 and CS2. Two kinds of tests were performed at eachcoordinate system: In one of them, the superconductor wascooled at a vertical distance of a 100 mm away from thePMG (z = 100 mm), where the external magnetic flux canbe neglected. Because of that, these are called Zero FieldCooled (ZFC) tests; in the second kind of tests the sample wascooled at a vertical distance of 20 mm away from the PMG(z = 20 mm) and, therefore, the PMG’s magnetic flux cannotbe neglected anymore, so these are called Field Cooled (FC)tests. On both ZFC and FC tests, after the cooling, the HTSsample approached the PMG, which then performed a lateralmovement, in order to evaluate the levitation force behavior.

IV. RESULTS

This section presents experimental results. The lateral dis-placements influence on the levitation force of superconductingmagnetic bearings was evaluated for ZFC and FC tests. Theperformance of the YBCO bulk was compared with the 2Gblock in each test. Due to the several layers presented in the2G block, it is possible to have screening currents only in thea-b plane (xy directions in Fig. 2). In the other hand, besidesthe current in the ab plane, the YBCO bulk can present currentin the c direction with lower critical current density. This factcan influence in the force result for the 2 coordinates systemsadopted: CS1 and CS2.

A. ZFC Tests in the YBCO Bulk

The first test performed with the YBCO bulk is presented inFig. 3. The sample was placed at CS1 and a maximum levitationforce of 289.0 N was achieved after a vertical approach (fromz = 100 mm to z = 5 mm). Afterwards, with the HTS fixed

SASS et al.: INFLUENCE ON LEVITATION FORCE OF CONDUCTOR LINEAR BEARINGS 3600405

Fig. 4. Vertical force measurements with YBCO bulk placed at CS2. First, avertical approach was performed from the initial ZFC position of z = 100 mmto the final position of z = 5 mm. Then, a lateral movement in a range of± 10 mm was performed.

vertically 5 mm away from the guideway, the sample wasmoved 10 mm in the positive direction of y1 axis. This lateraldisplacement leads to a decrease in the levitation force due toa smaller magnetic field profile in the z-direction in the newposition. Then, the sample was moved 20 mm in the negativedirection of y1 axis showing that the maximum levitation forceoccurs at a specific horizontal position, where the magnetic fieldin the z-direction is maximum. At last, the sample is moved10 mm in the positive direction of y1, returning to its originalposition. Since the lateral movement can pump some magneticflux inside the superconductor, due to a time varying magneticfield [19], the maximum levitation force presents a decay aftereach lateral movement cycle.

The YBCO bulk was also placed at CS2 for a ZFC test, asshown in Fig. 4. The levitation force after the vertical approachwas 131.5 N, a value quite small when compared with the289.0 N from Fig. 3. The superconductor levitation force (Fz)is proportional to the gradient of the z component of magneticfield and to the amplitude of the magnetic induction in the ydirection when a current density flows in the x direction (by theLorentz Force). Due to a much higher ∇Bz at CS1 than at CS2,the force increases when the sample is moved from CS2 to CS1.However, the lateral movement still leads to a levitation forcedecay after each cycle due to the flux pump inside the bulk.

B. ZFC Tests in the 2G Block

The levitation force measured in the ZFC test with the2G block placed at CS1 presented a similar behavior whencompared with the bulk superconductor, as shown in Fig. 5.The maximum levitation force after the vertical approach was130.1 N, which is about 45% of the value achieved with thebulk superconductor. At this point, it is worth mentioning thatthe total volume of YBCO in the 2G block is less than 2% ofthe bulk. This force reduction is also attributed to the design ofthe 2G block, since it has 3 totaly independent regions with athinner current loop, limited by the 12 mm wire width. In theother hand, the YBCO bulk presents 3 intragrains loops with34 mm width, and a larger intergrain loop [20], that can increasea lot the levitation force.

Fig. 5. Vertical force measurements with the 2G block placed at CS1. First, avertical approach was performed from the initial FC position of z = 20 mmto the final position of z = 5 mm. Then, a lateral movement in a range of± 10 mm was performed.

Fig. 6. Vertical force measurements with the 2G block placed at CS2. First, avertical approach was performed from the initial FC position of z = 20 mmto the final position of z = 5 mm. Then, a lateral movement in a range of± 10 mm was performed.

Another result from Fig. 5 that should be commented is theintersections of the levitation force during the lateral movementleads small hysteresis loops. Since the HTS sample is fixed inthe load cell, the liquid nitrogen evaporation could affect theresults. However, this effect can be neglected for the order ofmagnitude of the measurements. The force intersection couldalso be attributed to the flux creep effect, that is proportionallygreater in the 2G block than in the YBCO bulk [16]. Whenthe 2G block is moved from the cooling position to z = 5 mm,some flux is pumped inside the material. After that, the sampleis moved along y1 and this flux naturally relax. When thedirection of the lateral movement is changed (from +y1 to−y1), the 2G block sees a larger external field than that onewas trapped previously, resulting in a slightly higher levitationforce.

When the 2G block was placed at CS2 the sample wasexposed to a magnetic flux from the guideway that is mostly iny2 direction, which is parallel to the 2G wire current conductionplane. In this way, flux lines are able to pass between thesuperconductor material at each 2G wire. Since the 2G blockis built with several layers with no persistent superconductingcurrent, its levitation force at CS2 is expected to be muchsmaller than the one presented by the bulk. This result ispresented in Fig. 6.

3600405 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 24, NO. 3, JUNE 2014

Fig. 7. Vertical force measurements with YBCO bulk placed at CS1. First, avertical approach was performed from the initial FC position of z = 20 mmto the final position of z = 5 mm. Then, a lateral movement in a range of± 10 mm was performed.

Another difference between the YBCO bulk and the 2Gblock measurements at CS2 observed in Fig. 4 and 6 is thenew minimum force point after the first half circle. The YBCObulk has the new minimum force in y2 � 1 mm while the 2Gblock presented Fz = 0 for y2 2̃ mm. It can be explained bythe different sizes of the current loops. The width bulk region is34 mm and a displacement of 10 mm cannot change so muchthe current distribution inside the sample. In the other hand, the2G block has 3 regions having 12 mm width each one. Duringa displacement of y2 = 10 mm, for a area with higher magneticfield in the z direction, that covers almost one wholly currentloop, changing the new minimum point to y2 > 0.

C. FC Tests in the YBCO Bulk

In the FC test the sample was cooled in the presence of anexternal magnetic field, which means that some magnetic fluxwas trapped inside the HTS sample. As a result, Fig. 7 showsthat the maximum levitation force achieved by the YBCO bulkplaced at CS1 after the vertical approach (191.2 N) was about66% of the levitation force achieved in the ZFC test (Fig. 3).However, the levitation force behavior is the same on both tests(ZFC and FC).

With the YBCO bulk placed at CS2, the behavior of thelevitation force measured in the FC test was also similar to theresults for the ZFC test, as shown in Fig. 8.

D. FC Tests in the 2G Block

The last set of measurements performed were FC tests for the2G block placed at CS1 and CS2. First, for the sample placed atCS1, Fig. 9 shows a maximum levitation force of 106.5 N afterthe vertical approach, which is about 55% of the value achievedwith the bulk superconductor. The hysteresis loops formed bythe levitation force intersections points are wider than the onespresented in Fig. 5, reinforcing the fact that this phenomenonis not solely related to liquid nitrogen evaporation, but it couldattributed to the flux creep too. In the FC process more flux istrapped inside the 2G block, increasing the flux creep effect.

For the FC test with the 2G block placed at CS2, Fig. 10shows that the levitation force behavior and order of magnitudeare similar of the results presented in Fig. 6.

Fig. 8. Vertical force measurements with YBCO bulk placed at CS2. First, avertical approach was performed from the initial FC position of z = 20 mmto the final position of z = 5 mm. Then, a lateral movement in a range of± 10 mm was performed.

Fig. 9. Vertical force measurements with the 2G block placed at CS1. First, avertical approach was performed from the initial FC position of z = 20 mmto the final position of z = 5 mm. Then, a lateral movement in a range of± 10 mm was performed.

Fig. 10. Vertical force measurements with the 2G block placed at CS2. First,a vertical approach was performed from the initial FC position of z = 20 mmto the final position of z = 5 mm. Then, a lateral movement in a range of± 10 mm was performed.

V. CONCLUSION

Lateral displacements influence on the levitation force ofYBCO coated conductor bearings was evaluated by measure-ments with two kinds of HTS samples. In these tests, the 2Gwire presented similar behavior of the YBCO bulk. Exceptionswere basically due to the high anisotropic properties of the 2Gwire, where there is only one current conduction plane. In this

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way, 2G wire can be used to replace YBCO bulk in SBMs, but itis important to optimize the design of the bearing according tothe magnetic field profile, avoiding a magnetic field parallel tothe 2G block, in a sense that it will have a very weak levitationalforce.

In general, the 2G block has achieved levitation forces levelsof about half of the values measured with the YBCO bulk.However, the volume of YBCO in the 2G block is less than2% of the bulk. The bulk superconductor showed a smallerpercentual levitation force decay due to lateral displacements.

As a final conclusion, the 2G block is a possible substituteto the YBCO bulk, and may be used as a linear SMB in theMagLev Cobra project. The feasibility of this application isengaged with the relation between the maximum current thatthe 2G wire can carry per unit price and, fortunately, there aregood expectations about improvements in the performance ofcoated conductors and also in its price decreasing.

ACKNOWLEDGMENT

The authors would like to thank the LASUP staff for techni-cal support.

REFERENCES

[1] G. Sotelo, R. de Andrade, D. Dias, A. Ferreira, F. Costa, O. Machado,R. de Oliveira, M. Santos, and R. Stephan, “Tests with one module of theBrazilian Maglev-Cobra vehicle,” IEEE Trans. Appl. Supercond., vol. 23,no. 3, p. 3601204, Jun. 2013.

[2] L. Kühn, O. De Haas, D. Berger, L. Schultz, H. Olsen, and S. Röhlig,“Supratrans II—Test drive facility for a superconductor-based Maglevtrain,” eb—Elekt. Bahnen, vol. 110, no. 8/9, pp. 461–469, Jul. 2012.

[3] Z. Deng, J. Wang, J. Zheng, Y. Zhang, and S. Wang, “Feasibility ofintroducing ferromagnetic materials to onboard bulk high-Tc supercon-ductors to enhance the performance of present Maglev systems,” Phys. C,Supercond., vol. 485, pp. 20–23, Feb. 2013.

[4] G. G. Sotelo, R. De Andrade, Jr., and A. C. Ferreira, “Test and simulationof superconducting magnetic bearings,” IEEE Trans. Appl. Supercond.,vol. 19, no. 3, pp. 2083–2086, Jun. 2009.

[5] A. Cansiz, E. A. Oral, and O. Gundogdu, “Driving stability of supercon-ducting magnetic bearing system,” in Proc. AIP Conf., 2012, vol. 1434,no. 1, pp. 1389–1395.

[6] F. N. Werfel, U. Floegel-Delor, R. Rothfeld, T. Riedel, B. Goebel,D. Wippich, and P. Schirrmeister, “Superconductor bearings, flywheelsand transportation,” Supercond. Sci. Technol., vol. 25, no. 1, p. 014007,Jan. 2012.

[7] M. Rupich, X. Li, S. Sathyamurthy, C. Thieme, K. DeMoranville,J. Gannon, and S. Fleshler, “Second generation wire development atAMSC,” IEEE Trans. Appl. Supercond., vol. 23, no. 3, p. 6601205,Jun. 2013.

[8] H. Song, P. Brownsey, Y. Zhang, J. Waterman, T. Fukushima, andD. Hazelton, “2G HTS coil technology development at SuperPower,”IEEE Trans. Appl. Supercond., vol. 23, no. 3, p. 4600806, Jun. 2013.

[9] A. Malozemoff, “Second-generation high-temperature superconductorwires for the electric power grid,” AMSC—Annu. Rev. Mater. Res., vol. 42,pp. 373–397, Aug. 2012.

[10] H. Piekarz, S. Hays, J. Blowers, and V. Shiltsev, “A measurement ofHTS cable power loss in a sweeping magnetic field,” IEEE Trans. Appl.Supercond., vol. 22, no. 3, p. 5800105, Jun. 2012.

[11] J. C. Llambes, D. Hazelton, J. Duval, M. Albertini, S. Repnoy,V. Selvamanickam, G. Majkic, I. Kesign, J. Langston, M. Steurer,F. Bogdan, J. Hauer, D. Crook, S. Ranner, T. Williams, and M. Coleman,“Performance of 2G hts tapes in sub-cooled LN2 for superconducting faultcurrent limiting applications,” IEEE Trans. Appl. Supercond., vol. 21,no. 3, pp. 1206–1208, Jun. 2011.

[12] J.-K. Lee, S. H. Park, Y. Kim, S. Lee, W.-S. Kim, K. Choi, andS.-Y. Hahn, “Electrical properties analysis and test result of windingsfor a fully superconducting 10 HP homopolar motor,” IEEE Trans. Appl.Supercond., vol. 22, no. 3, p. 5201405, Jun. 2012.

[13] S. D. Chen, Y. T. Yu, J. C. Jan, F. Y. Lin, C. S. Hwang, I. G. Chen,C. H. Du, T. M. Uen, and C. Y. Chang, “A superconducting magnetizationsystem with hybrid superconducting wire for the study and applicationof superconductivity,” IEEE Trans. Appl. Supercond., vol. 22, no. 3,p. 4301104, Jun. 2012.

[14] F. Sass, G. G. Sotelo, A. Polasek, and R. de Andrade, Jr., “Applicationof 2G-tape for passive and controlled superconducting levitation,” IEEETrans. Appl. Supercond., vol. 21, no. 3, pp. 1511–1514, Jun. 2011.

[15] G. A. Levin, P. N. Barnes, J. Murphy, L. Brunke, J. D. Long, J. Horwath,and Z. Turgut, “Persistent current in coils made out of second generationhigh temperature superconductor wire,” Appl. Phys. Lett., vol. 93, no. 6,pp. 062504-1–062504-3, Aug. 2008.

[16] F. Sass, D. H. N. Dias, G. G. Sotelo, and R. de Andrade, Jr., “Coatedconductors for the magnetic bearing application,” Phys. Proc., vol. 36,pp. 1008–1013, 2012.

[17] F. Sass, D. H. Dias, G. Sotelo, and R. de Andrade, Jr., “Superconduct-ing levitation using coated conductors,” IEEE Trans. Appl. Supercond.,vol. 23, no. 3, p. 3600905, Jun. 2013.

[18] E. S. Motta, D. H. N. Dias, G. G. Sotelo, H. O. C. Ramos, J. H. Norman,and R. M. Stephan, “Optimization of a linear superconducting levitationsystem,” IEEE Trans. Appl. Supercond., vol. 21, no. 5, pp. 3548–3554,Oct. 2011.

[19] T. A. Coombs, O. Hadeler, M. Zhang, and K. Matsuda, “Flux pumping,fluctuations and climbing fields,” Supercond. Sci. Technol., vol. 25, no. 10,pp. 104007-1–104007-7, Oct. 2012.

[20] Z. Deng, M. Izumi, M. Miki, B. Felder, K. Tsuzuki, S. Hara, T. Uetake,U. Floegel-Delor, and F. N. Werfel, “Trapped flux and levitation propertiesof multiseeded YBCO bulks for HTS magnetic device applications—Part I: Grain and current features,” IEEE Trans. Appl. Supercond., vol. 22,no. 2, p. 6800110, Apr. 2012.