Mechanism of a high-Tc superconducting flux pump: Using alternating magnetic field to trigger flux flow

High-T c Superconducting (HTS) magnets operating in persistent current mode suffer a current decay due to flux creep of superconductor and joint resistance. Flux pumps are able to inject direct current into superconducting circuit to compensate the current decay, without the thermal loss caused by current leads. In this work, we proposed a flux pumping mechanism for HTS coils, with an experimental verification and an analytical model. The basic principle we have used is that flux flow can be triggered when the superconductor carrying a direct current is subjected to a perpendicular AC magnetic field. Low frequency alternating current is induced in a loop of YBCO tape using an AC field. A portion of the tape which we refer to as the “bridge” shorts a superconducting coil. A high frequency AC field is applied perpendicular to the bridge tape when alternating current in the tape reaches one polarity. This triggers a net flux flow and results in a current increase in the coil. The proposed flux pump has clear physics and is easily controllable, C 2015 AIP Publishing LLC. which may make it promising in practical use.

APPLIED PHYSICS LETTERS 107, 142601 (2015)A Compact Superconducting Motor with Novel Stator Windings for Vehicle Applications

The aim of this paper is to investigate how compact a superconducting motor can achieve the same performance as a conventional permanent magnet (PM) moto r. A design of superconducting motor is proposed and the performance is calculated to meet the specifications of a conventional PM machine while keeping the size of the superconducting motor as compact as possible. The proposed superconducting motor uses YBCO racetrack coil on the stator and bulk superconductors on the rotor. The operating temperature is 22 K and liquid hydrogen is chosen as the agent. A numerical model is developed to calculate the performance of this motor and the resu lts show that it is able to deliver a power at 118 kW and a torque at 622 N m with a total volume as compact as 1.5 liter. The torque and power densities of this su perconducting motor are compared with those of the PM motor and the boundary performance of volume reduction that can be achieved by substituting conventional coppe r and permanent magnets by superconducting materials is discussed.

JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 23 (3):381-389 2010ac losses and field and current density distribution during a full cycle of a stack of superconducting tapes

Starting from an existing model by Clem et al., this paper has analyzed how the current density and magnetic field distribution of a stack of superconduc ting tapes with ac transport currents or applied fields will change in a full cycle. This paper assumes when the ac current or field starts to change in the other d irection, a new penetrated region will begin to penetrate from the superconductor surface. If we assume J(c) is constant in the critical region, this paper demonstr ates that the Claassen formula (7) can be used to calculate the exact ac losses. If J(c) depends on local B-z, we can use Eq. (9) to quickly predict the ac losses. This approach does not need to calculate a complete ac cycle. This saves considerably computation time while gives a result which is in close agreement with that ca lculated from a complete ac cycle. The calculation method can be applied for calculating a superconducting pancake coil if the coil radius is much larger than the t ape width. c 2010 American Institute of Physics.

JOURNAL OF APPLIED PHYSICS 107 (9): Art No. 093909 2010Comparison of AC losses, magnetic field/current distributions and critical currents of superconducting circular pancake coils and infinitely long stacks usi ng coated conductors

A model is presented for calculating the AC losses, magnetic field/current density distribution and critical currents of a circular superconducting panca ke coil. The assumption is that the magnetic flux lines will lie parallel to the wide faces of tapes in the unpenetrated area of the coil. Instead of using an infin itely long stack to approximate the circular coil, this paper gives an exact circular coil model using elliptic integrals. A new efficient numerical method is intro duced to yield more accurate and fast computation. The computation results are in good agreement with the assumptions. For a small value of the coil radius, there i s an asymmetry along the coil radius direction. As the coil radius increases, this asymmetry will gradually decrease, and the AC losses and penetration depth will i ncrease, but the critical current will decrease. We find that if the internal radius is equal to the winding thickness, the infinitely long stack approximation over estimates the loss by 10% and even if the internal radius is reduced to zero, the error is still only 60%. The infinitely long stack approximation is therefore adeq uate for most practical purposes. In addition, the comparison result shows that the infinitely long stack approximation saves computation time significantly.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 23 (8): Art No. 085011 2010Calorimetric method of ac loss measurement in a rotating magnetic field

A method is described for calorimetric ac-loss measurements of high-T-c superconductors (HTS) at 80 K. It is based on a technique used at 4.2 K for conve ntional superconducting wires that allows an easy loss measurement in parallel or perpendicular external field orientation. This paper focuses on ac loss measuremen t setup and calibration in a rotating magnetic field. This experimental setup is to demonstrate measuring loss using a temperature rise method under the influence o f a rotating magnetic field. The slight temperature increase of the sample in an ac-field is used as a measure of losses. The aim is to simulate the loss in rotatin g machines using HTS. This is a unique technique to measure total ac loss in HTS at power frequencies. The sample is mounted on to a cold finger extended from a liq uid nitrogen heat exchanger (HEX). The thermal insulation between the HEX and sample is provided by a material of low thermal conductivity, and low eddy current hea ting sample holder in vacuum vessel. A temperature sensor and noninductive heater have been incorporated in the sample holder allowing a rapid sample change. The ma in part of the data is obtained in the calorimetric measurement is used for calibration. The focus is on the accuracy and calibrations required to predict the actua l ac losses in HTS. This setup has the advantage of being able to measure the total ac loss under the influence of a continuous moving field as experienced by any r otating machines. (C) 2010 American Institute of Physics.

REVIEW OF SCIENTIFIC INSTRUMENTS 81 (7): Art No. 074702 201022nd International Symposium on Superconductivity (ISS 2009)

This paper investigates the electromagnetic properties of high-temperature superconductors with a particular focus on the AC loss in coils made from YBCO superconductors. The numerical analysis and finite element modelling of the YBCO superconductors used in Cambridge's superconducting permanent magnet synchronous m otor currently in development is described. The stack of tapes in the superconducting coils is modelled using the direct H formulation, a B-dependent critical curre nt density and a bulk approximation. Magnetic boundary conditions for this model are derived from a 2D finite element method (FEM) motor model. The combination of t hese models allows the total AC loss (combined transport and magnetisation losses) in the HTS coils used in an all-superconducting machine design to be estimated. T he raw AC loss figures are compared to the output power of the motor for two test cases, and it is found that the AC loss contributes significantly to the total los s and therefore efficiency. An experimental rig is also described, which has been built in order to test the electromagnetic properties and performance of the motor . It is explained how this rig will be used to investigate the magnetisation of the rotor and carry out AC loss measurements on the stator coils. (c) 2010 Elsevier B.V. All rights reserved.

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 470 (20):1752-1755 201022nd International Symposium on Superconductivity (ISS 2009)

The Electrical Power and Energy Conversion (EPEC) superconductivity group at Cambridge University has been working on the application of superconductivit y to large scale devices. This work is taking place over a range of areas which cover FCLs, motors and generators, SMES, accelerator magnets and MRI. The research i s underpinned by advanced modelling techniques using both pure Critical State models and E-J models to analyse the behaviour of the superconductors. As part of the device design we are concentrating on the analysis of AC losses in complicated geometries such as are found in motor windings and the magnetisation of bulk supercon ductors to enable their full potential to be realised. We are interested in the full range of high-temperature superconductors and have measured and predicted the p erformance of YBCO, MgB2 and BSCCO at a range of temperatures and in wire, tape and bulk forms. This paper concentrates on recent work which includes: modelling of coils using formulations based on H and A. A critical state model for the analysis of coils in SMES; crossed field effects in bulk superconductors; a magnetic model together with experimental results which explain and describe the method of flux pumping whereby a bulk superconductor can be magnetised to a high flux density usi ng a repeatedly applied field of low flux density and finally a new configuration for MRI magnets (C) 2010 Elsevier B.V. All rights reserved.

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 470 (20):1845-1852 201021st International Conference on Magnet Technology (MT-21)

This paper presents an SMES coil which has been designed and tested by University of Cambridge. The design gives the maximum stored energy in the coil wh ich has been wound by a certain length of second-generation high-temperature superconductors (2G HTS). A numerical model has been developed to analyse the current d ensity and magnetic field distribution and calculate the AC losses during the charge and discharge process of the coil. A cryostat has been designed and a test of t he I-V curve measurement of the coil has been accomplished. In addition, the power electronics control of the SMES coil has been simulated.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 20 (3):1379-1382 201021st International Conference on Magnet Technology (MT-21)

It is particularly attractive for High Temperature Superconductors (HTS), such as the melt-processed YBCO single-grain bulks, to trap strong magnetic fie lds at cryogenic temperatures. With the flux density orders of magnitude much greater than the rare earth magnets, the YBCO bulks can be formed as small and as comp act as them. As a result, YBCO bulks are used as magnets in magnetic bearings, MRI scanners and motors, etc. The trapped field by the YBCO magnet is decided by the magnetization, which normally includes three different ways. However, the traditional ways to magnetize the YBCO will always need the applied field to be as high as the expected field on the superconductor or much higher than it. In this paper, we will describe a technique which facilitates the creation of the high magnetic fi eld and utilize a normal permanent magnet in stead. By using this rare earth permanent magnet which is not as strong, this technique involves the superconducting fl ux pump concept, which means that a much larger field being trapped by the superconductor bulk is caused by a small field repeatedly applied to it. To achieve such pumping effect, an intermedia material is necessary and in the related experiments, Gadolinium is used as such an important thermally actuated material. Traveling m agnetic field through it will magnetize the YBCO in such a way that the flux density will be accumulated step by step on the surface on the bulk.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 20 (3):1823-1826 2010A model for calculating the AC losses of second-generation high temperature superconductor pancake coils

A model is presented for calculating the AC losses of a stack of second-generation high temperature superconductor tapes. This model takes as a starting point the model of Clem and co-workers for a stack in which each tape carries the same current. It is based on the assumption that the magnetic flux lines lie paral lel to the tapes within the part of the stack where the flux has not penetrated. In this paper we allow for the depth of penetration of field to vary across the sta ck, and use the Kim model to allow for the variation of J(c) with B. The model is applied to the cases of a transport current and an applied field. For a transport current the calculated result differs from the Norris expression for a single tape carrying a uniform current and it does not seem possible to define a suitable ave rage J(c) which could be used. Our method also gives a more accurate value for the critical current of the stack than other methods. For an applied field the stack behaves as a solid superconductor with the J(c) averaged locally over several tapes, but still allowed to vary throughout the stack on a larger scale. For up to abo ut ten tapes the losses rise rapidly with the number of tapes, but in thicker stacks the tapes shield each other and the losses become that of a slab with a field p arallel to the faces.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 22 (7): Art No. 075028 2009Numerical Analysis of the Demagnetization Effect in a Superconducting Machine With Bulk HTS Material on the Rotor

When a bulk superconducting material is used as the magnets of an electric motor, the magnetized superconducting pucks are subjected to a varying magneti c field which has a detrimental effect on the trapped field. This may lead to a long term decay of the magnetization of the bulk superconductors. In this paper, we analyze numerically the demagnetization effect on bulk superconductors subjected to a magnetic field applied orthogonally to the initial direction of the trapped fl ux. The condition of a superconducting sample being exposed to a transverse field with varying magnitude is simulated and is very close to the operating environment in a synchronous machine. The decay of magnetization is calculated. The mechanism is explained in terms of the distortion of current distribution on the cross sect ion of the sample. The long term effect of the application of a transverse field is predicted by the model.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 19 (3):2897-2900 2009Numerical and Experimental Analysis of I-c and AC Loss for Bent 2G HTS Wires Used in an Electric Machine

Application of the second generation High Temperature Superconducting (HTS) YBCO tapes has been increasingly popular since the low-cost superconducting m aterials were discovered. This paper mainly presents the properties of two types of 2G YBCO tapes from American Superconductor Corporation (AMSC) and SuperPower, In c, respectively. All superconducting materials are cooled by liquid nitrogen (77 K). At first, we will introduce the basic design strategy for synchronous motor and characteristics of superconducting materials. Then, the basic principle and algorithm for calculation and measurement of Ic and AC transport loss will be explained . Secondly, the novel measurement system using a high-precision digital lock-in amplifier is presented. Finally, we present and analyze simulation and experimental results for two types of YBCO tapes in bending condition.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 19 (3):3356-3360 2009Magnetization Process of an HTS Motor and the Torque Ripple Suppression

This paper describes in detail the magnetization process of a synchronous motor, constructed with High Temperature Superconducting (HTS) elements on the rotor. The Pulse Field Magnetization (PFM) method is considered to magnetize the HTS rotor as a four-pole trapped flux magnet. A 2D model is built for the magnetiza tion process. During the machine operation, intolerable torque ripples occur, which are mainly caused by the irregular field distribution from the magnetized HTS ro tor. In order to suppress the large torque ripples, an iterative learning control method is used in addition to the traditional field-oriented control method.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 19 (3):1644-1647 2009The Next Generation of Superconducting Permanent Magnets: The Flux Pumping Method

Magnets made from bulk YBCO are as small and as compact as the rare earth magnets but potentially have magnetic flux densities orders of magnitude greate r than those of the rare earths. In this paper a simple technique is proposed for magnetizing the superconductors. This technique involves repeatedly applying a sma ll magnetic field which gets trapped in the superconductor and thus builds up and up. Thus a very small magnetic field such as one available from a rare earth magne t can be used to create a very large magnetic field. This technique which is applied using no moving parts is implemented by generating a traveling magnetic wave wh ich moves across the superconductor. As it travels across the superconductor it trails flux lines behind it which get caught inside the superconductor. With each su ccessive wave more flux lines get caught and the field builds up and up. The wave could be generated in many different ways but the preferred way is simply to heat a material whose permeability changes with temperature at its edge. As the heat travels across the material so the permeability changes and a magnetic wave is gener ated. It is in effect the first novel heat pump in a very long time and one which will enable the enormous potential available from these unique and highly versatil e superconducting magnets to be fully realized. Within this paper we present results showing the superconductor being progressively magnetized by sequentially appli ed "heat" pulses. We also demonstrate that the sign of the magnetization is reversed if "cold" pulses are applied instead of heat pulses. These experimental res ults are supported by modeling.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 19 (3):2169-2173 2009Thermally actuated magnetization flux pump in single-grain YBCO bulk

Recent progress in material processing has proved that high temperature superconductors (HTS) have a great potential to trap large magnetic fields at cry ogenic temperatures. For example, HTS are widely used in MRI scanners and in magnetic bearings. However, using traditional ways to magnetize, the YBCO will always n eed the applied field to be as high as the expected field on the superconductor or much higher than it, leading to a much higher cost than that of using permanent m agnets. In this paper, we find a method of YBCO magnetization in liquid nitrogen that only requires the applied field to be at the level of a permanent magnet. More over, rather than applying a pulsed high current field on the YBCO, we use a thermally actuated material ( gadolinium) as an intermedia and create a travelling magn etic field through it by changing the partial temperature so that the partial permeability is changed to build up the magnetization of the YBCO gradually after mult iple pumps. The gadolinium bulk is located between the YBCO and the permanent magnet and is heated and cooled repeatedly from the outer surface to generate a travel ling thermal wave inwards. In the subsequent experiment, an obvious accumulation of the flux density is detected on the surface of the YBCO bulk.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 22 (10): Art No. 105011 20098th International Conference on Power Electronics and Drive Systems (PEDS 2009)

Wind power generation as one of the most popular renewable energy applications is absorbing more and more attention all over the world. However, output p ower fluctuations of wind farm due to random variations of wind speed can cause network frequency and voltage flicker in power systems. The power quality consequent ly declines, particularly in an isolated power system such as the power system in a remote community or a small island. This paper proposes an application of superc onducting magnetic energy storage (SMES) to minimize output fluctuations of an isolated power system with wind farm. The isolated power system is fed by a diesel ge nerator and a wind generator consisting of a wind turbine and squirrel cage induction machine. The control strategy is detailed and the proposed system is evaluated by simulation in Matlab/Simulink.

2009 INTERNATIONAL CONFERENCE ON POWER ELECTRONICS AND DRIVE SYSTEMS, VOLS 1 AND 2807-810 20098th European Conference on Applied Superconductivity

The potential of bulk melt-processed YBCO single domains to trap significant magnetic fields (Tomita and Murakami 2003 Nature 421 517-20; Fuchs et al 200 0 Appl. Phys. Lett. 76 2107-9) at cryogenic temperatures makes them particularly attractive for a variety of engineering applications including superconducting magn ets, magnetic bearings and motors ( Coombs et al 1999 IEEE Trans. Appl. Supercond. 9 968-71; Coombs et al 2005 IEEE Trans. Appl. Supercond. 15 2312-5). It has alrea dy been shown that large fields can be obtained in single domain samples at 77 K. A range of possible applications exist in the design of high power density electri c motors (Jiang et al 2006 Supercond. Sci. Technol. 19 1164-8). Before such devices can be created a major problem needs to be overcome. Even though all of these de vices use a superconductor in the role of a permanent magnet and even though the superconductor can trap potentially huge magnetic fields ( greater than 10 T) the p roblem is how to induce the magnetic fields. There are four possible known methods: ( 1) cooling in field; ( 2) zero field cooling, followed by slowly applied field ; ( 3) pulse magnetization; ( 4) flux pumping.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 21 (3): Art No. 034001 200820th International Conference on Magnet Technology

This paper presents a new synchronous motor whose stator uses 2 High Temperature Superconducting (HTS) pancake-coils wound from 2G HTS tapes and 4 conven tional copper coils, while the rotor contains 75 bulk HTS elements. All superconducting materials are cooled by liquid nitrogen (77 K). The HTS and copper armature includes 3 phase circuit and is mounted in the slots of a nonmagnetic and insulating material. Each phase winding, fabricated in the form of a pancake-coil, is dist ributed in slots of 60-degree intervals. YBCO melt-textured pucks were incorporated into the rotor and the reluctance was tested. In addition, a unique cooling syst em using a copper conductor is designed, which is similar to that of a superconducting flywheel. Finally, we present and analyse results on measuring DC critical cu rrent and self-field AC loss in HTS tape supplied by American Superconductor. The measurement systems are described. The results on the AC loss measurement of YBCO samples are presented and analyzed within the framework of the critical state model.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 18 (2):1236-1239 200820th International Conference on Magnet Technology

In this paper, the crossed field demagnetization effect of the bulk type-II superconducting materials in a variety of external magnetic field condition h as been investigated both numerically and experimentally. A melt-textured YBCO bulk sample was magnetized along the axial direction and after that a transverse fiel d perpendicular to the central axis applied. The collapse of the original trapped field was measured using a Quantum Design Physical Property Measurement System. A numerical solver based on critical state model is proposed to simulate the measured results.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 18 (2):1561-1564 2008The design, magnetization and control of a superconducting permanent magnet synchronous motor

This paper describes in detail the method of magnetization of a superconducting permanent magnet synchronous motor. The rotor of the motor consists of 60 superconducting pucks, which are magnetized by two additional copper windings. The pulse field magnetization (PFM) method is considered and the resulted distributi on of the magnetizing flux linkage from the rotor is not a perfect sine wave in the air gap, which leads to a large torque ripple and harmonics of the stator curren ts. In order to suppress the torque ripple, an iterative learning control (ILC) algorithm is used in addition to the former field-oriented control method. The resul ts show the ILC algorithm can largely reduce the torque ripple.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 21 (6): Art No. 065011 2008Numerical estimation of AC loss in MgB2 wires in self-field condition

In order to utilize MgB2 wires in AC electrical devices, it is very important to be able to understand the characteristics of MgB2 materials in the AC el ectromagnetic conditions and give an accurate estimate of the AC loss. A numerical method is proposed in this paper to estimate the AC loss in MgB2 wires. This meth od is based on solving a set of partial differential equations in which the magnetic field is used as the unknowns to get the current and electric field distributio ns in the cross sections of the wires, and hence the AC loss can be calculated. A commercial FEM solver is used to give an easy and fast solution for many complex g eometries. This method is used to model a monocore MgB2 wire and a multifilamentary MgB2 wire. The results demonstrate that the multifilamentary MgB2 wire has a low er AC loss than monocore one when carrying the same amount of current.

JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 21 (3):205-211 20083rd IEEE International Conference of Nano/Micro Engineered and Molecular Systems

In this piece of research we have designed and fabricated micro-actuators that are magnetically actuated for biological and biomedical applications. Thes e devices are fabricated by electro-deposition of magnetic materials onto silicon substrates. It has been demonstrated that they are capable of delivering high forc es over large actuation distances, and that their use will not jeopardize living cells compared to conventional thermal actuators by featuring much lower power cons umption and working temperature. Furthermore they can be suitable for applications in conductive fluidic environments. Finite-element and analytical models are used to simulate the device performance and magnetic actuation tests are implemented.

2008 3RD IEEE INTERNATIONAL CONFERENCE ON NANO/MICRO ENGINEERED AND MOLECULAR SYSTEMS, VOLS 1-3673-676 20088th European Conference on Applied Superconductivity

In order to utilize HTS conductors in AC electrical devices, it is very important to be able to understand the characteristics of HTS materials in the AC electromagnetic conditions and give an accurate estimate of the AC loss. A numerical method is proposed in this paper to estimate the AC loss in superconducting co nductors including MgB2 wires and YBCO coated conductors. This method is based on solving a set of partial differential equations in which the magnetic field is use d as the state variable to get the current and electric field distributions in the cross sections of the conductors and hence the AC loss can be calculated. This me thod is used to model a single-element and a multi-element MgB2 B wires. The results demonstrate that the multi-element MgB2 wire has a lower AC loss than a single- element one when carrying the same current. The model is also used to simulate YBCO coated conductors by simplifying the superconducting thin tape into a one-dimens ional region where the thickness of the coated conductor can be ignored. The results show a good agreement with the measurement.

8TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'07) 97 Art No. 012117 20088th European Conference on Applied Superconductivity

This paper gives a detailed description of the design of a high temperature superconducting (HTS) motor. The stator of the motor consists of six air core d HTS racetrack windings, together with an iron shield. The rotor is made of 80 superconducting YBCO pucks, which can be magnetized and equates to a four- pole perm anent magnet. The whole HTS motor is cooled by liquid nitrogen to 77K, and acts as a permanent magnet synchronous motor with the power rate of 15.7 kW.

8TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'07) 97 Art No. 012123 2008Behavior of bulk high-temperature superconductors of finite thickness subjected to crossed magnetic fields: Experiment and model

Crossed-magnetic-field effects on bulk high-temperature superconductors have been studied both experimentally and numerically. The sample geometry invest igated involves finite-size effects along both (crossed-)magnetic-field directions. The experiments were carried out on bulk melt-processed Y-Ba-Cu-O single domains that had been premagnetized with the applied field parallel to their shortest direction (i.e., the c axis) and then subjected to several cycles of the application of a transverse magnetic field parallel to the sample ab plane. The magnetic properties were measured using orthogonal pickup coils, a Hall probe placed against the sample surface, and magneto-optical imaging. We show that all principal features of the experimental data can be reproduced qualitatively using a two-dimensional f inite-element numerical model based on an E-J power law and in which the current density flows perpendicularly to the plane within which the two components of magne tic field are varied. The results of this study suggest that the suppression of the magnetic moment under the action of a transverse field can be predicted successf ully by ignoring the existence of flux-free configurations or flux-cutting effects. These investigations show that the observed decay in magnetization results from the intricate modification of current distribution within the sample cross section. The current amplitude is altered significantly only if a field-dependent critica l current density J(c)(B) is assumed. Our model is shown to be quite appropriate to describe the cross-flow effects in bulk superconductors. It is also shown that t his model does not predict any saturation of the magnetic induction, even after a large number (similar to 100) of transverse field cycles. These features are shown to be consistent with the experimental data.

PHYSICAL REVIEW B 75 (17): Art No. 174515 2007Applied Superconductivity Conference 2006

The apparatus described here is designed to measure the total ac loss in 2G tape subjected to a rotating magnetic field of up to 1 Tesla applied perpendi cular to the tape axis and at speeds of up to 3600 rpm, while carrying a transport current. Both thermal and electrical measurement techniques are. incorporated in the apparatus. The data collected from the apparatus will be used to assist the design of superconducting machines in which the superconductor is used for the stato r as well as the rotor and where the minimization of AC losses is crucial to the success of the machine. The measurement of ac losses will give results that are dif ficult to predict from mathematical modeling.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 17 (2):3199-3202 2007A numerical method to estimate AC loss in superconducting coated conductors by finite element modelling

A finite element method code based on the critical state model is proposed to solve the AC loss problem in YBCO coated conductors. This numerical method is based on a set of partial differential equations (PDEs) in which the magnetic field is used as the state variable. The AC loss problems have been investigated bo th in self-field condition and external field condition. Two numerical approaches have been introduced: the first model is configured on the cross-section plane of the YBCO tape to simulate an infinitely long superconducting tape. The second model represents the plane of the critical current flowing and is able to simulate the YBCO tape with finite length where the end effect is accounted. An AC loss measurement has been done to verify the numerical results and shows a good agreement wit h the numerical solution.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 20 (4):331-337 2007Applied Superconductivity Conference 2006

A numerical method is proposed to analyse the electromagnetic behavior of systems including high-temperature superconductors (HTSC) in time-varying exter nal fields. The E-J constitutive law together with an H-formulation is used to calculate the current distribution then the magnetisation hysteresis loop of the supe rconductor can be obtained. This numerical method is adapted to commercial finite element software to solve the partial differential equations time dependently and is able to give accurate results to many problems in complex geometries.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 17 (2):3761-3764 2007Control of a superconducting synchronous motor

This paper presents a control algorithm for starting up a high temperature superconducting synchronous motor. The mathematical model of the motor has bee n established in m-file in Matlab and the parameters have been identified by means of the finite-element analysis method. Different starting methods for the motor h ave been compared and discussed, and eventually a hybrid control algorithm is proposed.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 20 (4):392-396 2007Design and control of a superconducting permanent magnet synchronous motor

This paper gives a detailed description of the design of a superconducting permanent magnet synchronous motor. The parameters of the motor have been iden tified, and the torque equation has been stated. A direct torque control algorithm is introduced and applied to a traditional permanent magnet synchronous motor and the superconducting permanent magnet synchronous motor described in this paper. The motor performance shows that the direct torque control algorithm provides excel lent control to the superconducting motor, and guarantees that the magnitude of the operational armature currents is smaller than the value of the critical current of the superconducting tape used for stator winding.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 20 (7):585-591 2007Applied Superconductivity Conference 2006

We have investigated magnesium diboride wires as an element in a resistive superconducting fault current limiter using a pulsed system, which could deliv er a fixed number of AC cycles to avoid burnout due to thermal instabilities. Experiments have been carried out in both liquid helium and at 27 K on magnesium dibor ide mono-core wires in stainless steel tubes, in a CuNi sheath with an Fe barrier and also on multifilamentary wires with Nb barriers. Experiments showed good curre nt limiting in the first cycle and no damage after six cycles. Also wires with a number of voltage contacts spaced at about 1 cm intervals to assess the effect of i nhomogeneity were tested. Relatively small differences in local J(c) led to large differences in the local temperature rise although in no case did this lead to fai lure of the conductor. The results were compared with those on Bi-2223/Ag tapes and YBCO coated conductors measured in liquid nitrogen.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 17 (2):1764-1767 2007Applied Superconductivity Conference 2006

Transport ac losses of parallel arrays of superconducting tapes with an elliptical cross-section carrying mutually anti-parallel currents were modeled nu merically by finite element method. Two different configurations-tapes positioned edge to edge (x-array) and tapes positioned face to face (y-array) were considered . It was found that an x-array increases the losses while a y-array decreases them-relative to infinitely distant tapes. The highest ac loss decrease is observed wh en the tapes are close to each other in y-array. The AC loss of a y-array made of various numbers of tapes was compared with the loss of an array made of round wire s. It was found that y-arrays of tapes have lower loss than arrays of round wires. Bringing y-arrays of tapes close to each other in the x direction causes a signif icant increase in ac loss. The present analysis may be useful if considering fault current limiters made of straight conductors in a meandering configuration. The r esults are qualitatively valid also for a fault current limiter made of non-inductive (bifilar) windings in the form of several pancake coils, each coil wound non-i nductively (i.e. with anti-parallel currents).

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 17 (2):1803-1806 2007Applied Superconductivity Conference 2006

Second generation YBCO tapes have extremely high (similar to 10000) geometric aspect ratios (width to thickness), which require huge computer resource s and thus renders the simulation of fault current limiters (FCLs) based on them very difficult. In this paper we offer a solution to the aforementioned problem by assuming a linear scaling of the thermal and electrical properties with the geometrical size. The results obtained in proprietary software are in very good agreemen t with those from commercial FEM software. We also compare the limiting properties of YBCO and BSCCO, and find that in order to enable BSCCO to carry the same rated current as YBCO, one needs more than 40 times larger volume of BSCCO. In addition, YBCO limits the current quicker and much more efficiently than BSCCO. Finally, w e study the influence of defects on the current limiting properties of HTS tapes and find that defects lead to stronger heating and more non-uniform current and tem perature distributions, especially in BSCCO.

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 17 (2):1780-1783 2007Conference on Flux Pinning and Other Related Phenomena in Type-II Superconductors

Bulk melt-processed Y-Ba-Cu-O (YBCO) has significant potential for a variety of high-field permanent-magnet-like applications, such as the rotor of a bru shless motor. When used in rotating devices of this kind, however, the YBCO can be subjected to both transient and alternating magnetic fields that are not parallel to the direction of magnetization and which have a detrimental effect on the trapped field. These effects may lead to long-term decay of the magnetization of the b ulk sample. In the present work, we analyze both experimentally and numerically the remagnetization process of a melt-processed YBCO single domain that has been par tially demagnetized by a magnetic field applied orthogonal to the initial direction of trapped flux. Magnetic torque measurements are used as a tool to probe change s in the remanent magnetization during various sequences of applied field. The application of a small magnetic field between the transverse cycles parallel to the d irection of original magnetization results in partial remagnetization of the sample. Rotating the applied field, however, is found to be much more efficient at rema gnetizing the bulk material than applying a magnetizing field pulse of the same amplitude. The principal features of the experimental data can be reproduced qualita tively using a two-dimensional finite-element numerical model based on an E-J power law. Finally, the remagnetization process is shown to result from the complex mo dification of current distribution within the cross-section of the bulk sample.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 20 (9):S174-S183 2007Conference on Flux Pinning and Other Related Phenomena in Type-II Superconductors

Transport AC loss measurements have been made on YBCO-coated conductors prepared on two different substrate templates-RABiTS (rolling-assisted biaxially textured substrate) and IBAD (ion-beam-assisted deposition). RABiTS samples show higher losses compared with the theoretical values obtained from the critical state model, with constant critical current density, at currents lower than the critical current. An origin of this extra AC loss was demonstrated experimentally by comp arison of the AC loss of two samples with different I-V curves. Despite a difference in I-V curves and in the critical currents, their measured losses, as well as t he normalized losses, were practically the same. However, the functional dependence of the losses was affected by the ferromagnetic substrate. An influence of the p resence of a ferromagnetic substrate on transport AC losses in YBCO film was calculated numerically by the finite element method. The presence of a ferromagnetic su bstrate increases transport AC losses in YBCO films depending on its relative magnetic permeability. The two loss contributions-transport AC loss in YBCO films and ferromagnetic loss in the substrate-cannot be considered as mutually independent. (Some figures in this article are in colour only in the electronic version).

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 20 (9):S299-S304 20077th European Conference on Applied Superconductivity

Advanced silicon processing techniques developed for the Very Large Scale Integration (VLSI) industry have been exploited in recent years to enable the p roduction of micro-fabricated moving mechanical systems known as Micro Electro Mechanical Systems (MEMS). These devices offer advantages in terms of cost, scalabili ty and robustness over their preceding equivalents. Cambridge University have worked for many years on the investigation of high temperature superconductors (HTS) i n flywheel energy storage applications. This experience is now being used to research into superconducting Micro-Bearings for MEMS, whereby circular permanent magne t arrays are levitated and spun above a superconductor to produce bearings suitable for motors and other micron scale devices. The novelty in the device lies in the fact that the rotor is levitated into position by Meissner flux exclusion, whilst stability is provided by flux pinned within the body of the superconductor. Th is work includes: the investigation of the properties of various magnetic materials, their fabrication processes and their suitability for MEMS; finite element anal ysis to analyse the interaction between the magnetic materials and YBCO to determine the stiffness and height of levitation. Finally a micro-motor with the above pr inciples is currently being fabricated within the group.

7TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'05) 431011-1014 2006Numerical solution of critical state in superconductivity by finite element software

A numerical method is proposed to analyse the electromagnetic behaviour of systems including high-temperature superconductors (HTSCs) in time-varying ext ernal fields and superconducting cables carrying AC transport current. The E-J constitutive law together with an H-formulation is used to calculate the current dist ribution and electromagnetic fields in HTSCs, and the magnetization of HTSCs; then the forces in the interaction between the electromagnet and the superconductor an d the AC loss of the superconducting cable can be obtained. This numerical method is based on solving the partial differential equations time dependently and is ada pted to the commercial finite element software Comsol Multiphysics 3.2. The advantage of this method is to make the modelling of the superconductivity simple, flexi ble and extendable.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 19 (12):1246-1252 2006Design and AC loss analysis of a superconducting synchronous motor

This paper gives a conceptual design of a superconducting synchronous motor consisting of both high-temperature superconducting rotating field winding an d armature winding. The AC losses of the armature winding of the motor have been investigated experimentally and numerically, by considering the self-field of the s uperconducting coils and the rotating magnetic field exposed on the armature winding. The recent developments of YBCO-coated conductors present the possibility of a chieving a wholly superconducting machine of significantly smaller size and weight than a conventional machine. Both the rotating field winding and the armature win ding are composed of YBCO high-temperature superconducting (HTS) coils. A low AC loss armature winding design has been developed for this superconducting synchronou s motor. The performance of the machine was investigated by modelling with the finite-element method. The machine's torque is calculated from first principles by co nsidering the angle between the field and the armature main flux lines.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 19 (11):1164-1168 2006Dynamics of superconductor bearings in a cryogenic failure

The dynamics of superconductor bearings in a cryogenic failure scenario have been analyzed. As the superconductor warms up, the rotor goes through multip le resonance frequencies, begins to slow down and finally touches down when the superconductor goes through its transition temperature. The bearing can be modelled as a system of springs with axial, radial and cross stiffiiess. These springs go through various resonant modes as the temperature of the superconductor begins to r ise. We have presented possible explanations for such behaviour. (c) 2006 Elsevier B.V. All rights reserved.

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 442 (1):9-12 20067th European Conference on Applied Superconductivity

The dynamics of superconductor bearings in a cryogenic failure scenario have been analyzed. As the superconductor warms up, the rotor goes through multip le resonance frequencies, begins to slow down and finally touches down when the superconductor goes through its transition temperature. The bearing can be modelled as a system of springs with axial, radial and cross stiffness. These springs go through various resonant modes as the temperature of the superconductor begins to ri se. We have presented possible explanations for such behaviour.

7TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'05) 431003-1006 2006High aspect ratio problem in simulation of a fault current limiter based on superconducting tapes

We are offering a solution for the high-aspect-ratio problem relevant to the numerical simulation of AC loss in superconductors and metals with high aspe ct ( width-to-thickness) ratio. This is particularly relevant to simulation of fault current limiters (FCLs) based on second generation YBCO tapes on RABiTS. By ass uming a linear scaling of the electric and thermal properties with the size of the structure, we can replace the real sample with an effective sample of a reduced a spect ratio by introducing size multipliers into the equations that govern the physics of the system. The simulation is performed using both a proprietary equivalen t circuit software and a commercial FEM software. The correctness of the procedure is verified by simulating temperature and current distributions for samples with all three dimensions varying within 10(-3) - 10(3) of the original size. Qualitatively the distributions for the original and scaled samples are indistinguishable, whereas quantitative differences in the worst case do not exceed 10%.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 19 (6):629-634 2006High aspect ratio problem in simulation of a fault current limiter based on superconducting tapes

We are offering a solution for the high-aspect-ratio problem relevant to the numerical simulation of AC loss in superconductors and metals with high aspe ct ( width-to-thickness) ratio. This is particularly relevant to simulation of fault current limiters (FCLs) based on second generation YBCO tapes on RABiTS. By ass uming a linear scaling of the electric and thermal properties with the size of the structure, we can replace the real sample with an effective sample of a reduced a spect ratio by introducing size multipliers into the equations that govern the physics of the system. The simulation is performed using both a proprietary equivalen t circuit software and a commercial FEM software. The correctness of the procedure is verified by simulating temperature and current distributions for samples with all three dimensions varying within 10(-3) - 10(3) of the original size. Qualitatively the distributions for the original and scaled samples are indistinguishable, whereas quantitative differences in the worst case do not exceed 10%.

SUPERCONDUCTOR SCIENCE & TECHNOLOGY 19 (6):629-634 2006