HTS Motor Performance Evaluation by Different Pulsed Field Magnetization Strategies

2019

Materials

In an attempt to unveil the impact of the material law selection on the numerical modelling and analysis of the electromagnetic properties of superconducting coils, in this paper we compare the four most common approaches to the E-J power laws that serve as a modelling tool for the conductivity properties of the second generation of high-temperature superconducting (2G-HTS) tapes. The material laws considered are: (i) the celebrated E-J critical-state like-model, with constant critical current density and no dependence with the magnetic field; (ii) the classical Kim’s model which introduces an isotropic dependence with the environment magnetic field; (iii) a semi-empirical Kim-like model with an orthonormal field dependence, J c ( B ) , widely used for the modelling of HTS thin films; and (iv) the experimentally measured E–J material law for SuperPower Inc. 2G-HTS tapes, which account for the magneto-angular anisotropy of the in-field critical current density J c ( B ; θ ) , with a derived function similar to Kim’s model but taking into account some microstructural parameters, such as the electron mass anisotropy ratio ( γ ) of the superconducting layer. Particular attention has been given to those physical quantities which within a macroscopic approach can be measured by well-established experimental setups, such as the measurement of the critical current density for each of the turns of the superconducting coil, the resulting distribution of magnetic field, and the curve of hysteretic losses for different amplitudes of an applied alternating transport current at self-field conditions. We demonstrate that although all these superconducting material laws are equally valid from a purely qualitative perspective, the critical state-like model is incapable of predicting the local variation of the critical current density across each of the turns of the superconducting coil, or its non-homogeneous distribution along the width of the superconducting tape. However, depending on the physical quantity of interest and the error tolerance allowed between the numerical predictions and the experimental measurements, in this paper decision criteria are established for different regimes of the applied current, where the suitability of one or another model could be ensured, regardless of whether the actual magneto angular anisotropy properties of the superconducting tape are known.

Section: Introductionmentioning

confidence: 99%

How to Choose the Superconducting Material Law for the Modelling of 2G-HTS Coils

Robert

2019

Materials

“…al. 10,[55][56][57][58][59][60][61] , in this paper we extend the integral formulation of the critical state theory introduced in Ref. 45, by including a multipole fields expansion which enables the straightforward inclusion of the magnetostatic coupling between a SC and a rounded SFM sheath (Sec.…”

Section: Introductionmentioning

confidence: 99%

Why energy lossless ferromagnetic materials can add energy losses onto type-II superconductors?

2021

Preprint

Understanding the physical coupling between the macroscopic electromagnetic properties of type II superconductors (SC) and soft ferromagnetic materials (SFM), is root for progressing onto the application of SC-SFM metastructures in scenarios such as magnetic cloaking, magnetic shielding, and loss free current transmission systems. However, in the latter case understanding the origin of the rise in the hysteresis losses of the superconductor by effect of the coupling with the SFM has historically resulted in a notable challenge, it because this rise in the AC losses is simply counterintuitive due to the fact that the SFM itself does not add magnetization losses to the system and furthermore, there is no evidence of electrical current sharing between these two materials. Thus, aimed to resolve this long-standing problem, in this paper, we present a semi-analytical model for monocore SC-SFM heterostructures of cylindrical cross-section and self-field conditions, showing the first known map of AC-losses for SC-SFM magnetically shielded wires, with magnetic relative permeabilities for the SFM ranging from mur=5 (NiZn ferrites) to mur =350000 (pure Iron). The distribution of current density and magnetic field inside the SC-SFM metastructure is shown in great detail, revealing a remarkable agreement with the intriguing magneto optical imaging observations that were originally questioning the validness of the critical state theory. In this sense, we have extended the critical state theory within its variational formalism, incorporating a multipole functional approach which allows the direct finding of the coupling terms between a SC current and a SFM sheath, proving that all reported phenomena for the self-filed hysteretic behavior of SC-SFM heterostructures can be understood within the classical critical state model without the need to recur to the ansatz of overcritical currents.

“…Within these kind of studies, one of the most determining factors which needs to be considered when fostering a superconducting application is the actual estimation of its AC losses, as it utterly defines the optimal design, performance, and cost assessment of a superconducting machine [12]. However, it is well-known that the type-II superconducting materials used for the fabrication of 2G-HTS tapes are extremely prone to be affected by external and induced changes in the electromagnetic excitations, what results in very complex local distributions of current density and flux front profiles, both being the physical source of the hysteresis losses [13,14]. In fact, in the case of a superconducting coil, each one of its turns which carries a certain amount of current, then affects the electromagnetic performance of the other turns by magnetic induction where not only profiles of transport current must appear, but also local profiles with magnetization currents.…”

Section: Introductionmentioning

confidence: 99%

Flux front dynamics and energy losses of magnetically anisotropic 2G-HTS pancake coils under prospective winding deformations

Robert

2019

Eng. Res. Express

In this paper, a comprehensive analysis on the local electrodynamics at micron level for the second generation of magnetically anisotropic high temperature superconducting (2G-HTS) pancake coils is presented. Special attention has been paid to the influence of prospective winding misalignment factors onto macroscopical quantities such as the hysteresis losses and critical current density inside each turns of a generic 2G-HTS coil, and their relation with the magneto-angular dependence of the infield critical current density J c (B, θ) of the 2G-HTS tape. It has been shown that for low amplitudes of the applied transport current,  I I 0.2 a c 0 , the flux-front profile for perfectly aligned pancake coils develops a well-shaped flux-free core with a complete absence of magnetization currents in ∼30% the middle turns, that are enclosed by transport current profiles in similar fashion to what would be observed in superconducting bulks, but with the outer turns showing a clear dominance of magnetization currents. However, the misaligned pancake coils show a notorious deflection of the flux-front semi-elliptical vertices which induces a repositioning of the co-vertices, shaping then a 'worm-like' flux-front profile that is caused by the breakdown in the ordinal symmetry of the mutual inductances. This phenomenon leads to an increment in the Lorentz force between the transport and magnetization currents, what causes an additional source of hysteretic losses which cannot be accounted by classical changes in the size of the flux-front profile. Thus, we have obtained that for relatively large deformations of the pancake coil, up to a 19% increment in the AC losses can be achieved for moderate to low intensities of I a , whilst for currents greater than 0.7I c0 , a striking although low reduction on the AC losses can be achieved, whose main physical signature has been connected to the actual disappearance of the flux-free core inside the superconducting coil.