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Coated Conductors for the Magnetic Bearing Application
Available online at www.sciencedirect.com
Physics Procedia 36 (2012) 1008 – 1013
Superconductivity Centennial Conference
Coated conductors for the magnetic bearing application Felipe Sassa , Daniel Henrique Nogueira Diasb , Guilherme Gonçalves Sotelob , Rubens de Andrade Júniora a COPPE/UFRJ
- Rio de Janeiro Federal University, Av. Athos da Silveira Ramos, 149, CT/UFRJ, I-148, Rio de Janeiro-RJ, 21941-209, Brazil b UFF - Fluminense Federal University, Rua Passo da Pátria, 156, Niterói-RJ, 24210-240, Brasil
Abstract The second generation (2G) of superconductor wires have been considered for several applications lately. This work presents a preliminary study of superconducting magnetic bearings (SMB) using 2G wires as passive levitators. A superconducting block using stacked 2G wires was built to evaluate the magnetic bearing behavior, thought levitation force measurements, when a permanent magnet cylinder approaches or moves away from the block. The superconducting block was compared with an YBCO bulk with nearly the same dimensions and the results show a promising potential for this application.
© Published Elsevier Selection peer-review under responsibility of the Editors. c 2012 2011 Published byby Elsevier Ltd.B.V. Selection and/or and/or peer-review under responsibility of Horst Rogalla andGuest Peter Kes. Keywords: High-temperature superconductors, Coated conductors, Magnetic bearings;
1. Introduction The YBCO superconductor stands out among the high temperature superconductors (HTS) for being able to withstand magnetic fields of over 5 T when cooled with liquid nitrogen at 77 K [1], what makes this superconductor very attractive for the magnetic bearings application. The YBCO bulk superconductors are already being used in some real scale prototypes of MagLev vehicles [2]. This work proposes an attempt to replace the YBCO bulks for 2G wires in the superconducting magnetic bearings application. The superconductor properties of the 2G wire are considerable better than in the YBCO bulks, but the superconductor material in the coated conductor represents only a small fraction of the wire volume [3]. The 2G wires have been considered for the most general applications recently, such as electrical energy transmission cables [4], fault current limiters [5], MagLev vehicles [6], electrical machines [7], high field magnets [8], bearings for flywheel [9], superconducting magnetic energy storage [10], etc. The main advantage of the 2G wire over the the 1G is the higher background field that it can work [11]. The main purpose of this study is to verify the feasibility of a superconducting magnetic bearing using permanent magnets and 2G wires acting passively. In this way, more than 500 pieces of 2G wire were stacked in order to form a block physically similar to a bulk superconductor. The levitation force measurements show that this block is able to work as a passive magnetic bearing. The levitation force measurements behavior was compared with the behavior of an YBCO bulk with nearly the same dimensions, showing a promising potential of 2G wires for this application.
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. doi:10.1016/j.phpro.2012.06.097
Felipe Sass et al. / Physics Procedia 36 (2012) 1008 – 1013
2. Samples preparation and experimental procedures This section presents the procedures adopted for the construction of the superconducting block and measurements. The superconducting block was built with 3 columns of stacked 2G wire. The coated conductor was cut into 531 pieces of 66 mm each and placed in compartments slightly larger than its size, as can be observed in Fig 1.a. A preliminar test with a small block had its superconducting properties degraded due to handling and storage conditions. To avoid deterioration of the 2G wire properties, the block was potted with an especial epoxy resin, as shown in Fig 1.b. The resin protects the wire against mechanical stress and from humidity. Additional information about the block is provided in Table 1. Fig 1.c shows the YBCO bulk (67 mm × 36 mm × 13 mm) that was used for comparison.
Fig. 1. Superconducting block built with stacked 2G wire: (a) picture during the construction; (b) picture after fixing the block with an especial epoxy resin and a bulk superconductor with similar dimensions (c).
In order to evaluate the feasibility of the block built with coated conductors in the magnetic bearings application an experimental rig was used. It is able to measure the levitation force between the block and a permanent magnet cylinder with 75 mm diameter and 20 mm height, as presented in Fig 2. An automated system controls the relative vertical position between the permanent magnet and the block and also acquires the force over the magnet, which was measured by a load cell. The permanent magnet movement occurs along the vertical direction (z direction) in steps of 1 mm, stopping 1 second during each movement step until a pre-determined final vertical position is reached. An aluminum cylindrical piece with a cavity to carry the permanent magnet was used to fix it to the load cell.
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Felipe Sass et al. / Physics Procedia 36 (2012) 1008 – 1013 Table 1. Superconducting Block Information
2G wire model Minimum critical current Length of each 2G wire piece Number of columns of 2G wire Number of 2G wire pieces Dimensions
SF12050 from SUPERPOWER 276 A 66 mm 3 531 (176 + 179 + 176) 67 mm × 38 mm × 11 mm
Fig. 2. Experimental rig used during the levitation force measurements: (a) illustration; (b) picture.
The measurements were carried out using either the zero field cooled (ZFC) or the field cooled (FC) processes. The ZFC test consists in cooling the superconductor without the presence of any external field, while the in the FC test the superconductor is cooled in the presence of an external field. 3. Results and Discussion This section presents the results of the levitation force measurements. The first one was a ZFC test at the distance of 100 mm between the superconducting block and the magnet. After the block has been cooled, the magnet was moved closer to the block until the gap of 5 mm and then returned to its original position. The measured ZFC force for this superconductors can be seen in Fig 3 and one important result is the hysteresis effect. When the magnet was moving from the initial to the final gap, the measured force followed a different path then when the magnet was returning to its original position. This means that magnetic flux was trapped in the superconducting block, as it was expected [12]. The block built with 2G wire shows a more pronounced hysteresis effect than the bulk superconductor. The hysteresis effect can also be observed when the block is cooled in the presence of an external field. The FC test shows different results depending of the external field applied while cooling. In practice, the closer the magnet is from the superconductor during the cooling process, smaller will be the levitation force, but there will be more lateral stability. In this work, the FC tests were done by cooling the superconductor at three different distances from the magnet. Firstly, the magnet was positioned at a distance of 30 mm from the block and the levitation force measured can be seen in Fig 4. It is important to note that some magnetic flux was trapped [12] while the magnet was moving along Path 1 and that is the reason why the levitation force returned from a different path to the distance of 100 mm (Path 2). Finally, while approaching the magnet to a gap of 5 mm (Path 3) the levitation force measured was very similar than during Path 1 between
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Felipe Sass et al. / Physics Procedia 36 (2012) 1008 – 1013
2G wire Block
(a)
YBCO Bulk
(b)
Fig. 3. Zero field cooled test: (a) block built with 2G wire and (b) YBCO bulk.
the gaps of 30 mm and 5 mm. It means that, along Path 3, the block arrived to the distance of 30 mm with a field configuration similar than when it was cooled. 2G wire Block
(a)
YBCO Bulk
(b)
Fig. 4. Field cooled test at a gap of 30 mm: (a) block built with 2G wire and (b) YBCO bulk.
The second FC test was for a initial gap of 20 mm and the results are presented in Fig 5. The analysis is very similar to the one that was done in Fig 4, but in this case the block returned along Path 3 showing a higher maximum levitation force than in Path 1. The third and last FC test done in this work was for a gap of 10 mm. The hysteresis effect was very strong in this test, as can be seen in Fig 6. It is interesting to note that between the distances of 10 mm and 5 mm, Path 1 and Path 2 are basically the same because at this band the flux pinning was strong enough to avoid a significative change in the trapped flux. Since the superconducting block built with coated conductors showed more hysteresis effects than the bulk superconductor it was done a force decayment test. This experiment consists in a ZFC test where the magnetic approaches the superconductor from the distance of 100 mm to 5 mm at a constant speed. The levitation force was measured during the movement and after the magnet has reached its final position, as presented in Fig 7. It can be observed that both blocks present the same relaxation behavior on the measured levitation force, but the 2G wire block has reached only about 55% of the force measured with the YBCO bulk.
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Felipe Sass et al. / Physics Procedia 36 (2012) 1008 – 1013
2G wire Block
YBCO Bulk
(a)
(b)
Fig. 5. Field cooled test at a gap of 20 mm: (a) block built with 2G wire and (b) YBCO bulk.
2G wire Block
YBCO Bulk
(a)
(b)
Fig. 6. Field cooled test at a gap of 10 mm: (a) block built with 2G wire and (b) YBCO bulk.
Fig. 7. Force decayment along time.
Felipe Sass et al. / Physics Procedia 36 (2012) 1008 – 1013
4. Conclusions The constructed superconducting block achieved satisfactory levitation force results in each experiment. It also presented a similar behavior to that of the YBCO bulks during the experiments. Considering that the volume of superconductor in the built block is about 1.5 % of its total volume, this work shows that coated conductors are very suitable for the magnetic bearings application. However, the 2G wire manufacturers are optimizing their products for other applications, since the YBCO bulks are still a good solution for the superconducting magnetic bearings applications and because, from an economical point of view, the 2G wire is still very expensive. On the other hand, there are good expectations for the 2G wires price decreasing and also good expectations about increasing their critical current density. 5. Acknowledgments The authors would like to thank CAPES, CNPq and FAPERJ for financial assistance. References [1] David Larbalestier, Alex Gurevich, D. Matthew Feldmann, Anatoly Polyanskii. High-Tc superconducting materials for electric power applications. Nature 2001; p.368-377. [2] Frank N. Werfel, Uta Floegel - Delor, Rolf Rothfeld, Thomas Riedel, Bernd Goebel, Dieter Wippich, P. Schirrmeister. Recent Up-Scaling in HTS Magnetic Device Technology. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2011; vol. 21, No. 3, p.1473-1476. [3] Xiaoping Li, Martin W. Rupich, Cornelis L. H. Thieme, M. Teplitsky, Srivatsan Sathyamurthy, E. Thompson, D. Buczek, J. Schreiber, K. DeMoranville, J. Lynch, J. Inch, D. Tucker, R. Savoy, S. Fleshler. The Development of Second Generation HTS Wire at American Superconductor. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2009; vol. 19, No. 3, p.3231-3235. [4] V. Selvamanickam et. al. High Performance 2G Wires: From R&D to Pilot-Scale Manufacturing. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2009; vol. 19, No. 3, p.3225-3230. [5] C. A. Baldan, J. S. Lamas, C. Y. Shigue and Ernesto Ruppert Filho. Fault Current Limiter Using YBCO Coated Conductor - The Limiting Factor and Its Recovery Time. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2009; vol. 19, No. 3, p.1810-1813. [6] M. Ogata, Y. Miyazaki, H. Hasegawa, T. Sasakawa and K. Nagashima. Basic Study of HTS Magnet Using 2G Wires for Maglev Train. Physica C 2010; vol. 470, p.1782-1786. [7] R. Pei et.al. Numerical and Experimental Analysis of IC and AC Loss for Bent 2G HTS Wires Used in an Electric Machine. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2009; vol. 19, No. 3, p.3356-3360. [8] D. W. Hazelton and V. Selvamanickam. SuperPowerŠs YBCO Coated High-Temperature Superconducting (HTS) Wire and Magnet Applications. Proceedings of the IEEE 2009; vol. 97, No. 11, p.1831-1836. [9] K. Nagashima, H. Seino, N. Sakai and M. Murakami. Superconducting Magnetic Bearing for a Flywheel Energy Storage System Using Superconducting Coils and Bulk Superconductors. Physica C 2009; vol. 469, p.1244-1249. [10] W. Yuan et. al. Design and Test of a Superconducting Magnetic Energy Storage (SMES) Coil. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2010; vol. 20, No. 3, p.1379-1382. [11] D. Hazelton, V. Selvamanickam, J. Duval, D. Larbalestier, W. Markiewicz, H. Weijers, R. Holtz. Recent Developments in 2G HTS Coil Technology. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2009; vol. 19, No. 3, p.2218-2222. [12] D. H. N. Dias, E. S. Motta, G. G. Sotelo, R. de Andrade Jr. Experimental validation of field cooling simulations for linear superconducting magnetic bearings. Superconductor Science and Technology 2010; vol. 23, No. 3, p. 07501(6pp).
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Physics Procedia 36 (2012) 1008 – 1013
Superconductivity Centennial Conference
Coated conductors for the magnetic bearing application Felipe Sassa , Daniel Henrique Nogueira Diasb , Guilherme Gonçalves Sotelob , Rubens de Andrade Júniora a COPPE/UFRJ
- Rio de Janeiro Federal University, Av. Athos da Silveira Ramos, 149, CT/UFRJ, I-148, Rio de Janeiro-RJ, 21941-209, Brazil b UFF - Fluminense Federal University, Rua Passo da Pátria, 156, Niterói-RJ, 24210-240, Brasil
Abstract The second generation (2G) of superconductor wires have been considered for several applications lately. This work presents a preliminary study of superconducting magnetic bearings (SMB) using 2G wires as passive levitators. A superconducting block using stacked 2G wires was built to evaluate the magnetic bearing behavior, thought levitation force measurements, when a permanent magnet cylinder approaches or moves away from the block. The superconducting block was compared with an YBCO bulk with nearly the same dimensions and the results show a promising potential for this application.
© Published Elsevier Selection peer-review under responsibility of the Editors. c 2012 2011 Published byby Elsevier Ltd.B.V. Selection and/or and/or peer-review under responsibility of Horst Rogalla andGuest Peter Kes. Keywords: High-temperature superconductors, Coated conductors, Magnetic bearings;
1. Introduction The YBCO superconductor stands out among the high temperature superconductors (HTS) for being able to withstand magnetic fields of over 5 T when cooled with liquid nitrogen at 77 K [1], what makes this superconductor very attractive for the magnetic bearings application. The YBCO bulk superconductors are already being used in some real scale prototypes of MagLev vehicles [2]. This work proposes an attempt to replace the YBCO bulks for 2G wires in the superconducting magnetic bearings application. The superconductor properties of the 2G wire are considerable better than in the YBCO bulks, but the superconductor material in the coated conductor represents only a small fraction of the wire volume [3]. The 2G wires have been considered for the most general applications recently, such as electrical energy transmission cables [4], fault current limiters [5], MagLev vehicles [6], electrical machines [7], high field magnets [8], bearings for flywheel [9], superconducting magnetic energy storage [10], etc. The main advantage of the 2G wire over the the 1G is the higher background field that it can work [11]. The main purpose of this study is to verify the feasibility of a superconducting magnetic bearing using permanent magnets and 2G wires acting passively. In this way, more than 500 pieces of 2G wire were stacked in order to form a block physically similar to a bulk superconductor. The levitation force measurements show that this block is able to work as a passive magnetic bearing. The levitation force measurements behavior was compared with the behavior of an YBCO bulk with nearly the same dimensions, showing a promising potential of 2G wires for this application.
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. doi:10.1016/j.phpro.2012.06.097
Felipe Sass et al. / Physics Procedia 36 (2012) 1008 – 1013
2. Samples preparation and experimental procedures This section presents the procedures adopted for the construction of the superconducting block and measurements. The superconducting block was built with 3 columns of stacked 2G wire. The coated conductor was cut into 531 pieces of 66 mm each and placed in compartments slightly larger than its size, as can be observed in Fig 1.a. A preliminar test with a small block had its superconducting properties degraded due to handling and storage conditions. To avoid deterioration of the 2G wire properties, the block was potted with an especial epoxy resin, as shown in Fig 1.b. The resin protects the wire against mechanical stress and from humidity. Additional information about the block is provided in Table 1. Fig 1.c shows the YBCO bulk (67 mm × 36 mm × 13 mm) that was used for comparison.
Fig. 1. Superconducting block built with stacked 2G wire: (a) picture during the construction; (b) picture after fixing the block with an especial epoxy resin and a bulk superconductor with similar dimensions (c).
In order to evaluate the feasibility of the block built with coated conductors in the magnetic bearings application an experimental rig was used. It is able to measure the levitation force between the block and a permanent magnet cylinder with 75 mm diameter and 20 mm height, as presented in Fig 2. An automated system controls the relative vertical position between the permanent magnet and the block and also acquires the force over the magnet, which was measured by a load cell. The permanent magnet movement occurs along the vertical direction (z direction) in steps of 1 mm, stopping 1 second during each movement step until a pre-determined final vertical position is reached. An aluminum cylindrical piece with a cavity to carry the permanent magnet was used to fix it to the load cell.
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Felipe Sass et al. / Physics Procedia 36 (2012) 1008 – 1013 Table 1. Superconducting Block Information
2G wire model Minimum critical current Length of each 2G wire piece Number of columns of 2G wire Number of 2G wire pieces Dimensions
SF12050 from SUPERPOWER 276 A 66 mm 3 531 (176 + 179 + 176) 67 mm × 38 mm × 11 mm
Fig. 2. Experimental rig used during the levitation force measurements: (a) illustration; (b) picture.
The measurements were carried out using either the zero field cooled (ZFC) or the field cooled (FC) processes. The ZFC test consists in cooling the superconductor without the presence of any external field, while the in the FC test the superconductor is cooled in the presence of an external field. 3. Results and Discussion This section presents the results of the levitation force measurements. The first one was a ZFC test at the distance of 100 mm between the superconducting block and the magnet. After the block has been cooled, the magnet was moved closer to the block until the gap of 5 mm and then returned to its original position. The measured ZFC force for this superconductors can be seen in Fig 3 and one important result is the hysteresis effect. When the magnet was moving from the initial to the final gap, the measured force followed a different path then when the magnet was returning to its original position. This means that magnetic flux was trapped in the superconducting block, as it was expected [12]. The block built with 2G wire shows a more pronounced hysteresis effect than the bulk superconductor. The hysteresis effect can also be observed when the block is cooled in the presence of an external field. The FC test shows different results depending of the external field applied while cooling. In practice, the closer the magnet is from the superconductor during the cooling process, smaller will be the levitation force, but there will be more lateral stability. In this work, the FC tests were done by cooling the superconductor at three different distances from the magnet. Firstly, the magnet was positioned at a distance of 30 mm from the block and the levitation force measured can be seen in Fig 4. It is important to note that some magnetic flux was trapped [12] while the magnet was moving along Path 1 and that is the reason why the levitation force returned from a different path to the distance of 100 mm (Path 2). Finally, while approaching the magnet to a gap of 5 mm (Path 3) the levitation force measured was very similar than during Path 1 between
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Felipe Sass et al. / Physics Procedia 36 (2012) 1008 – 1013
2G wire Block
(a)
YBCO Bulk
(b)
Fig. 3. Zero field cooled test: (a) block built with 2G wire and (b) YBCO bulk.
the gaps of 30 mm and 5 mm. It means that, along Path 3, the block arrived to the distance of 30 mm with a field configuration similar than when it was cooled. 2G wire Block
(a)
YBCO Bulk
(b)
Fig. 4. Field cooled test at a gap of 30 mm: (a) block built with 2G wire and (b) YBCO bulk.
The second FC test was for a initial gap of 20 mm and the results are presented in Fig 5. The analysis is very similar to the one that was done in Fig 4, but in this case the block returned along Path 3 showing a higher maximum levitation force than in Path 1. The third and last FC test done in this work was for a gap of 10 mm. The hysteresis effect was very strong in this test, as can be seen in Fig 6. It is interesting to note that between the distances of 10 mm and 5 mm, Path 1 and Path 2 are basically the same because at this band the flux pinning was strong enough to avoid a significative change in the trapped flux. Since the superconducting block built with coated conductors showed more hysteresis effects than the bulk superconductor it was done a force decayment test. This experiment consists in a ZFC test where the magnetic approaches the superconductor from the distance of 100 mm to 5 mm at a constant speed. The levitation force was measured during the movement and after the magnet has reached its final position, as presented in Fig 7. It can be observed that both blocks present the same relaxation behavior on the measured levitation force, but the 2G wire block has reached only about 55% of the force measured with the YBCO bulk.
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Felipe Sass et al. / Physics Procedia 36 (2012) 1008 – 1013
2G wire Block
YBCO Bulk
(a)
(b)
Fig. 5. Field cooled test at a gap of 20 mm: (a) block built with 2G wire and (b) YBCO bulk.
2G wire Block
YBCO Bulk
(a)
(b)
Fig. 6. Field cooled test at a gap of 10 mm: (a) block built with 2G wire and (b) YBCO bulk.
Fig. 7. Force decayment along time.
Felipe Sass et al. / Physics Procedia 36 (2012) 1008 – 1013
4. Conclusions The constructed superconducting block achieved satisfactory levitation force results in each experiment. It also presented a similar behavior to that of the YBCO bulks during the experiments. Considering that the volume of superconductor in the built block is about 1.5 % of its total volume, this work shows that coated conductors are very suitable for the magnetic bearings application. However, the 2G wire manufacturers are optimizing their products for other applications, since the YBCO bulks are still a good solution for the superconducting magnetic bearings applications and because, from an economical point of view, the 2G wire is still very expensive. On the other hand, there are good expectations for the 2G wires price decreasing and also good expectations about increasing their critical current density. 5. Acknowledgments The authors would like to thank CAPES, CNPq and FAPERJ for financial assistance. References [1] David Larbalestier, Alex Gurevich, D. Matthew Feldmann, Anatoly Polyanskii. High-Tc superconducting materials for electric power applications. Nature 2001; p.368-377. [2] Frank N. Werfel, Uta Floegel - Delor, Rolf Rothfeld, Thomas Riedel, Bernd Goebel, Dieter Wippich, P. Schirrmeister. Recent Up-Scaling in HTS Magnetic Device Technology. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2011; vol. 21, No. 3, p.1473-1476. [3] Xiaoping Li, Martin W. Rupich, Cornelis L. H. Thieme, M. Teplitsky, Srivatsan Sathyamurthy, E. Thompson, D. Buczek, J. Schreiber, K. DeMoranville, J. Lynch, J. Inch, D. Tucker, R. Savoy, S. Fleshler. The Development of Second Generation HTS Wire at American Superconductor. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2009; vol. 19, No. 3, p.3231-3235. [4] V. Selvamanickam et. al. High Performance 2G Wires: From R&D to Pilot-Scale Manufacturing. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2009; vol. 19, No. 3, p.3225-3230. [5] C. A. Baldan, J. S. Lamas, C. Y. Shigue and Ernesto Ruppert Filho. Fault Current Limiter Using YBCO Coated Conductor - The Limiting Factor and Its Recovery Time. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2009; vol. 19, No. 3, p.1810-1813. [6] M. Ogata, Y. Miyazaki, H. Hasegawa, T. Sasakawa and K. Nagashima. Basic Study of HTS Magnet Using 2G Wires for Maglev Train. Physica C 2010; vol. 470, p.1782-1786. [7] R. Pei et.al. Numerical and Experimental Analysis of IC and AC Loss for Bent 2G HTS Wires Used in an Electric Machine. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2009; vol. 19, No. 3, p.3356-3360. [8] D. W. Hazelton and V. Selvamanickam. SuperPowerŠs YBCO Coated High-Temperature Superconducting (HTS) Wire and Magnet Applications. Proceedings of the IEEE 2009; vol. 97, No. 11, p.1831-1836. [9] K. Nagashima, H. Seino, N. Sakai and M. Murakami. Superconducting Magnetic Bearing for a Flywheel Energy Storage System Using Superconducting Coils and Bulk Superconductors. Physica C 2009; vol. 469, p.1244-1249. [10] W. Yuan et. al. Design and Test of a Superconducting Magnetic Energy Storage (SMES) Coil. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2010; vol. 20, No. 3, p.1379-1382. [11] D. Hazelton, V. Selvamanickam, J. Duval, D. Larbalestier, W. Markiewicz, H. Weijers, R. Holtz. Recent Developments in 2G HTS Coil Technology. IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 2009; vol. 19, No. 3, p.2218-2222. [12] D. H. N. Dias, E. S. Motta, G. G. Sotelo, R. de Andrade Jr. Experimental validation of field cooling simulations for linear superconducting magnetic bearings. Superconductor Science and Technology 2010; vol. 23, No. 3, p. 07501(6pp).
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