106 research outputs found
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Modeling of Trapped Fields by Stacked (RE)BCO Tape Using Angular Transversal Field Dependence
Stacks of superconducting (RE)BCO tape are gaining popularity as a potential alternative for superconducting bulks for trapped field applications. This is partly due to versatility and uniformity of the starting material, allowing for more deterministic prediction of field profile and magnitude. However, most FEM models of trapped field magnets do not incorporate parameters such as critical current and n-value dependence on the angle of applied magnetic field, leading to only qualitative modeling results. More quantitative results can be obtained from incorporating more data for superconductivity and thermal properties of the material. Such models can be used as a starting point for most geometries and both trapped field and current transport modeling problems. An FEM model of a stack of tapes was constructed using the H formulation, incorporating goniometric critical current and n-value measurements. The modeling results were compared to field cooling experiments for stacks of different heights. The experiment and modeling show good agreement.This work was supported in part by the Engineering and Physical Sciences
Research Council, U.K., and in part by SKF S2M, France
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Computation of Superconducting Stacks Magnetization in an Electrical Machine
Superconducting technology offers the prospect of sharply increase the power density of rotating electrical machines, especially in the low speed, high torque range, with impact in applications such as wind energy and aircraft propulsion. Among the enabling technologies, stacks consisting of piling up layers of high temperature superconductor may provide a source of magnetic flux density for torque production, without the complexity of superconducting wound rotor poles. For this to happen, careful designs, optimizing electromagnetic, mechanical and thermal aspects at the same time, must be developed. In that sense, this work applies a recently developed combined electromagnetic formulation to compute the magnetization level of high temperature superconductor stacks installed in the airgap of an electrical motor after field cooling magnetization. The results are congruent with the applied field, show a strong interaction between teeth and stacks and provide a way of initializing the state of the machine prior to operation.Horizon 2020 research innovation programme under grant agreement No 7231119 (ASuMED consortium) and EPSRC grant EP/P000738/
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Magnetic levitation using a stack of high temperature superconducting tape annuli
Stacks of large width superconducting tape can carry persistent currents over similar length scales to bulk superconductors, therefore giving them potential for trapped field magnets and magnetic levitation. 46 mm wide high temperature superconducting tape has previously been cut into square annuli to create a 3.5 T persistent mode magnet. The same tape pieces were used here to form a composite bulk hollow cylinder with an inner bore of 26 mm. Magnetic levitation was achieved by field cooling with a pair of rare-earth magnets. This paper reports the axial levitation force properties of the stack of annuli, showing that the same axial forces expected for a uniform bulk cylinder of infinite can be generated at 20 K. Levitation forces up to 550 N were measured between the rare-earth magnets and stack. Finite element modelling in COMSOL Multiphysics using the H-formulation was also performed including a full critical state model for induced currents, with temperature and field dependent properties as well as the influence of the ferromagnetic substrate which enhances the force. Spark erosion was used for the first time to machine the stack of tapes proving that large stacks can be easily machined to high geometric tolerance. The stack geometry tested is a possible candidate for a rotary superconducting bearing.The authors would like to acknowledge the financial support of SKF S2M, the magnetic bearing division of SKF, the Isaac Newton Trust, Cambridge and EPSRC
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Toward Uniform Trapped Field Magnets Using a Stack of Roebel Cable Offcuts
Stacks of high temperature superconducting tape can be magnetized to produce a variety of different trapped field profiles in addition to the most common conical or pyramidal profiles. Stacks of tape using discarded Roebel cable offcuts were created to investigate various stacking arrangements with the aim of creating a stack that can be magnetized to produce a uniform trapped field for potential applications such as NMR. A new angled stacking arrangement proved to produce the flattest, most uniform field of all the overlapping stacking arrangements and has the potential to be scaled up. FEM modeling in COMSOL was also performed to complement the measurements and explain the limitations and advantages of the stacking arrangements tested.This work was supported in part by the Engineering and Physical Sciences Research Council, U.K., and in part by SKF S2M, France.This is the author accepted manuscript. The final version is available from IEEE via http://dx.doi.org/10.1109/TASC.2016.251899
Cross-field demagnetization of stacks of tapes: 3D modeling and measurements
Abstract: Stacks of superconducting (SC) tapes can trap much higher magnetic fields than conventional magnets. This makes them very promising for motors and generators. However, ripple magnetic fields in these machines present a cross-field component that demagnetizes the stacks. At present, there is no quantitative agreement between measurements and modeling of cross-field demagnetization, mainly due to the need for a 3D model that takes the end effects and real micron-thick SC layer into account. This article presents 3D modeling and measurements of cross-field demagnetization in stacks of up to 5 tapes and initial magnetization modeling of stacks of up to 15 tapes. 3D modeling of the cross-field demagnetization explicitly shows that the critical current density, Jc, in the direction perpendicular to the tape surface does not play a role in cross-field demagnetization. When taking the measured anisotropic magnetic field dependence of Jc into account, 3D calculations agree with measurements with less than a 4% deviation, while the error of 2D modeling is much higher. Then, our 3D numerical methods can realistically predict cross-field demagnetization. Due to the force-free configuration of part of the current density, J, in the stack, better agreement with experiments will probably require measuring the Jc anisotropy for the whole solid angle range, including J parallel to the magnetic field
Cross-field demagnetization of stacks of tapes: 3D modelling and measurements
Stacks of superconducting (SC) tapes can trap much higher magnetic fields than conventional magnets. This makes them very promising for motors and generators. However, ripple magnetic fields in these machines present a cross-field component that demagnetizes the stacks. At present, there is no quantitative agreement between measurements and modeling of cross-field demagnetization, mainly due to the need for a 3D model that takes the end effects and real micron-thick SC layer into account. This article presents 3D modeling and measurements of cross-field demagnetization in stacks of up to 5 tapes and initial magnetization modeling of stacks of up to 15 tapes. 3D modeling of the cross-field demagnetization explicitly shows that the critical current density, J, in the direction perpendicular to the tape surface does not play a role in cross-field demagnetization. When taking the measured anisotropic magnetic field dependence of J into account, 3D calculations agree with measurements with less than a 4% deviation, while the error of 2D modeling is much higher. Then, our 3D numerical methods can realistically predict cross-field demagnetization. Due to the force-free configuration of part of the current density, J, in the stack, better agreement with experiments will probably require measuring the J anisotropy for the whole solid angle range, including J parallel to the magnetic field
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Design considerations for electric motors using stacks of high temperature superconducting tape as permanent magnets
High temperature superconducting (HTS) tape can be cut and stacked to
form composite bulks capable of generating fields as high as 17.7 T, the highest of any trapped field magnet. This makes them the most powerful permanent magnets accounting for the need to maintain a cryogenic temperature. This cryogenic penalty is increasingly being justified due to the significantly higher power densities (>10kW/Kg) fully superconducting motors can enable by using magnetized stacks of tape on the rotor
and HTS coils on the stator. Design considerations for a motor using magnetized stacks for aerospace applications will be presented including FEM modelling in COMSOL and experimental prototype results for candidate designs. The rotor AC loss due to heating
by ripple fields will be discussed based on these results and its interdependence with stator AC loss in a fully superconducting motor which has not always been appreciated in previous partially superconducting motor designs.EPSRC grant EP/P000738/
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Magnetization and losses for an improved architecture of trapped-flux superconducting rotor
A hybrid electric configuration for aircraft propulsion yields several advantages, such as reducing the fuel consumption and take-off distance, improving control, and decreasing emissions. For such a scenario to occur, advances designed to increase the power-to-weight ratio of actual electric motors must be developed. Superconducting technologies offer the prospect of achieving such performances but at a cost of increased design and construction complexities. In that sense, stacks of high temperature superconductors have proven to trap high-current vortexes that provide a source of magnetic flux density for torque production without the need of current leads or other equipment in the rotor. However, these macroscopic currents must be induced prior to operation and remain undisturbed by variations in the magnetic flux density of the airgap, such as the ones caused by heating and demagnetization. This work presents the results of numerical computations on a new rotor architecture designed to facilitate the magnetization of stacks from a superconducting stator and prevent their demagnetization during torque production. The machine performance is assessed, and the expected survivability of the trapped-flux in stacks is compared to laboratory measurements.This research is financially supported by the European Union’s Horizon 2020 research innovation programme under grant agreement No 7231119 (ASuMED consortium) and EPSRC grant EP/P000738/1
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