Measurements of crossed-field demagnetisation rate of trapped field magnets at high frequencies and below 77 K

Design requirements of next generation electric aircraft place stringent requirements on the power density required from electric motors. Future prototype planned in the scope of European project “Advanced Superconducting Motor Experimental Demonstrator” ASuMED considers a permanent magnet synchronous motor, where the conventional ferromagnets are replaced with superconducting trapped field magnets, which promise higher flux densities and thus higher output power without adding weight. Previous work has indicated that stacks of tape show lower cross-field demagnetisation rate to bulk (RE)BCO whilst retaining similar performance for their size, however the crossed-field demagnetisation rate has not been studied in the temperature, magnetic field and frequency range that is relevant for the operational prototype motor. This work investigates crossed-field demagnetisation in 2G HTS stacks at temperatures below 77 K and frequency range above 10 Hz. This information is crucial in developing designs and determining operational time before re-magnetisation could be required

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

Analysis of an on-line superconducting cryofan motor for indirect cooling by LH2

This work relates to the study of an electrically powered cryofan for circulating close-loop cooling helium gas for superconducting applications with the following features: - Absence of any seal that can leak the pumped fluid or provide a path for heat transfer and require maintenance and/or is prone to failures. - The use of high temperature superconducting (HTS) stacks on the fan-rotor that, below critical temperature, can be magnetized contributing to the driving torque. The absence of electrically connected equipment as well as the lack of any seal, makes this arrangement especially suitable for reliable cryogenic helium gas circulation. Because HTS stacks cannot provide magnetic flux above Tc, during the initial stages of operation, in the presented study we analyse torque that will be provided by the passive iron components of the machine (reluctance torque, due to the saliency of the rotor) and by auxiliary permanent magnets or alternatively magnetizing coils

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/

Superconducting Magnetic Heterostructured Components for Electric Motor Applications

Trapped flux magnets made by stacking high temperature superconducting tape portray an easy assembly with already available materials, high mechanical resistance, provided by the substrate and improved thermal stability, which enhances the trapped flux compared to bulks. This allowed reaching the world record of 17.7 T. The presented analysis expands previous work with wide superconducting tapes showing further pos-sible improvements in this kind of devices in order to be used as source of magnetic flux in electrical motors, substituting permanent magnets. The aims are to increase the trapped magnetic flux during magnetization and decrease the leakage flux at the edges of the magnet during the operation of the machine. This is expected to be achieved by either introducing new materials in-between the individual tapes, such as ferromagnetic layers, or modifying the composition of the substrate. The results of simulations using the H-formulation as well as experimental measurements are presented.EPSRC grant EP/P000738/

Testing of Surface Mounted Superconducting Stacks as Trapped-Flux Magnets in a Synchronous Machine

Stacks of high temperature superconducting tapes may offer a technically affordable solution for the application of superconducting materials as trapped-flux magnets in the rotor of synchronous electrical machines. Nevertheless, several concerns must be first addressed, among others, the optimal procedure to induce the current vortexes previously to operation (magnetization), the survivability of these vortexes in the electromagnetic environment present in an electrical machine and the accuracy of recently developed numerical models. With the aim of exploring such practicalities, this paper presents a modified synchronous machine to test under liquid nitrogen conditions thin stacks of superconducting tapes. The machine is run under realistic conditions: currents in the stacks are induced from the stator, then the shaft is rotated and finally the machine is connected to a load, working as a generator. The results confirm previous numerical and experimental studies and establish a procedure for assessing the behavior of stacks in their actual operational environment.Also EPSRC EP/P000738/

Experimental system for testing a superconducting motor at temperatures close to 15 K

Integrating superconducting elements in an electric motor can greatly increase its power density. By doing so, lighter and more powerful machines can be produced for applications such as aviation, wind turbines and marine propulsion. Superconducting tapes can be stacked and magnetised to produce powerful trapped-flux magnets. The experimental setup was designed to allow measurements in a low temperature environment, providing tightness for the rotating part. We report on the design and construction of the setup and results from its first operation. The temperatures achieved during the experimental measurements reached 15 K, close to the actual operational temperatures of a liquid hydrogen cooled motor. The application of hydrogen onboard an aircraft is highly advantageous due to the possibility of its application as both coolant and fuel. Examination of the operation of the trapped-flux magnets at such temperatures is crucial for the understanding of the behavior of fully superconducting motors for future applications

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 $\textit{J c}$ 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

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

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/