J M Ceballos

Current distribution and ac loss for a superconducting rectangular strip with in-phase alternating current and applied field

The case of ac transport at in-phase alternating applied magnetic fields for a superconducting rectangular strip with finite thickness is investigated. The applied magnetic field is considered to be perpendicular to the current flow. We present numerical calculations assuming the critical-state model of the current distribution and ac loss for various values of aspect ratio, transport current and applied field amplitude. A rich phenomenology is obtained due to the highly nonlinear nature of the critical state. We perform a detailed comparison with the analytical limits and we discuss their applicability for the actual geometry of superconducting conductors. A dissipation factor is defined, which allows a more detailed analysis of the ac behaviour than the ac loss. Finally, we measure the ac loss and compare it with the calculations, showing a significant qualitative and quantitative agreement without any fitting parameters.

Predicting AC loss in practical superconductors

Recent progress in the development of methods used to predict AC loss in superconducting conductors is summarized. It is underlined that the loss is just one of the electromagnetic characteristics controlled by the time evolution of magnetic field and current distribution inside the conductor. Powerful methods for the simulation of magnetic flux penetration, like Brandts method and the method of minimal magnetic energy variation, allow us to model the interaction of the conductor with an external magnetic field or a transport current, or with both of them. The case of a coincident action of AC field and AC transport current is of prime importance for practical applications. Numerical simulation methods allow us to expand the prediction range from simplified shapes like a (infinitely high) slab or (infinitely thin) strip to more realistic forms like strips with finite rectangular or elliptic cross-section. Another substantial feature of these methods is that the real composite structure containing an array of superconducting filaments can be taken into account. Also, the case of a ferromagnetic matrix can be considered, with the simulations showing a dramatic impact on the local field. In all these circumstances, it is possible to indicate how the AC loss can be reduced by a proper architecture of the composite. On the other hand, the multifilamentary arrangement brings about a presence of coupling currents and coupling loss. Simulation of this phenomenon requires 3D formulation with corresponding growth of the problem complexity and computation time.

Study of ac loss in Bi-2223/Ag tape under the simultaneous action of ac transport current and ac magnetic field shifted in phase

Investigation of ac loss under the simultaneous action of the transport ac current and the external ac magnetic field is of prime importance for the reliable prediction of dissipation in electric power devices such as motors/generators, transformers and transmission cables. An experimental rig allowing us to perform ac loss measurements in such conditions, on short (10 cm) tape samples of high-temperature superconductor Bi-2223/Ag, was designed and tested. Both the electromagnetic and thermal methods were incorporated, allowing us to combine the better sensitivity of the former and the higher reliability of the latter. Our main aim was to see how the ac loss depends on the phase shift between the transport current and the external magnetic field. Such a shift could have different values in various applications. While in a transformer winding, the maximum phase shift at full load will probably not exceed a few degrees, in a three phase transmission cable in tri-axial configuration it is around 120°. Therefore, we explored the whole range of phase shifts from 0 to 360°. Surprisingly, the maxima of dissipation did not coincide with zero shift as expected from qualitative considerations.

AC losses in a toroidal superconducting transformer

In order to study the viability of coreless AC coupled coils, a superconductor transformer based on BSCCO-2223 PIT tapes was constructed. To achieve the minimum flux leakage, a toroidal geometry was selected. Both secondary and primary coils were wound around a glass fiber reinforced epoxy torus, obtaining a solid system. The field inside the transformer, the coupling factor, and the losses in the system were computed and measured, providing suitable parameters for new improvements in these systems.

Disk-shaped superconducting rotor for an axial flux induction motor

Abstract Most work on bulk-based superconducting electrical motors has been done with superconducting materials in the rotor only, due to the difficulty in machining the material into the conventional coil shape. As part of the design of a superconducting induction motor with superconductors in both rotor and stator, we made a disk-shaped rotor from the same YBCO bulks that we use to fabricate ceramic coils in constructing a modular stator for a biphasic axial flux motor, and studied this rotor’s working behaviour. To this end we constructed a system to simulate the working environment of a YBCO disk within the motor, and measured the magnetic field in the disk and the speed–torque characteristic.

A Fast Algorithm for Initial Design of HTS Coils for SMES Applications

Superconducting magnetic energy storage (SMES) is characterized by low-energy density but high-power density, making this an unfeasible approach for bulk energy storage. Nevertheless, there are applications where high amounts of power must be available for a short period of time, like power quality applications. The core element of SMES is the superconducting coil. Different approaches are found in the literature considering the modeling of this component, either for design or simulation purposes. These usually consist of analytical or numerical approaches. The former allows fast results, but only considers geometric effects. The latter provides accurate results, considering, besides electromagnetic, also mechanical and thermal effects. In this paper, a review of these models is performed, and analytical models are used in an algorithm that allows optimizing equivalent inductance for a specified length of tape. Two small prototypes are fabricated, and experimental measurements carried out, in order to validate the models that are in the base of the proposed algorithm.

Influence of the shape in the losses of solenoidal air-core transformers

The losses in an HTS tape depend strongly on the perpendicular magnetic field. In order to avoid this magnetic field component in an air core transformer, a toroidal geometry was proposed and studied in previous work. Due to the difficulties that one finds in constructing toroidal coils, the straight solenoidal geometry is now under study. In this case, the magnetic field close to the ends of the coil is not parallel to the axis and a perpendicular component appears. In the present work, the losses due to this component are studied as a function of the coil geometry-i.e., the ratio between length and diameter-and a practical formulation is found.

Disk-shaped Superconducting rotor under a rotating magnetic field: speed dependence

As part of the design of small axial-flux induction motors using disk-shaped rotors made from YBCO bulks, in previous work we studied the behavior of such disks under a rotating 2-pole magnetic field and gave an explanation of the results. That work yields information on such internal parameters as pinning forces and pull-out torque, and good estimates of the damping and Magnus coefficients. As in conventional motors, one expects to find dependence of these parameters on the magnetic field speed-the synchronous speed. In the present work, this dependence is studied and the previous model is improved.

Losses in 2G Tapes Wound Close Together: Comparison With Similar 1G Tape Configurations

In multilayer and magnetically coupled coils made from tape, the loss in each segment of tape in a coil depends on the parallel segments in the adjacent layers. In the case of a single multilayer coil, the current in all the layers is the same, but in magnetically coupled coils, the current in adjacent layer from different coils can be different both in amplitude and phase-usually 180deg out of phase one with respect to the other. In previous work, we have studied the influence of the proximity between tapes by considering the total loss in a segment as the sum of three components: the transport current in it, the global magnetic field due to the complete coil (or coils), and the local magnetic field due to the current in the tape wound just over or under the segment in question. To measure the last component, an experimental method has been proposed and carried out with Bi-2223 tape, showing that the loss in the tape can be increased or reduced by the proximity of another tape, depending on the current, if any, that the latter carries. By means of the loss variation, we have shown how the variation of transport currents (and, therefore, of the associated magnetic fields) influences the practical critical current of the tape under test. Advances in YBCO tape (2G tape) fabrication have led to increases in the field tolerance of the tape, and the dependences of loss and practical critical current on the proximity of an adjacent tape needed to be revised. In the present work, we study the behavior of the loss in 2G tapes under the influence of other tapes carrying zero or different currents. A comparison between Bi-2223 and YBCO tapes is shown.

Influence of the current through one turn of a multilayer coil on the nearest turn in a consecutive layer

Many references on AC losses can be found for straight superconducting tapes with or without an external magnetic field. There are fewer references on AC losses for bent tapes such as we find it in a spire or solenoid. But even fewer are the references on the study of AC losses in multilayer coils or magnetically coupled coils wound close together. In these cases, the loss in each piece of tape depends on three factors: the transport current in it, the global magnetic field due to the complete coil, and the local magnetic field due to the current in the tape wound just over or under the piece in question -the main difference between multilayer coils and magnetically coupled coils is that the current in the former is the same in all the layers and the currents in magnetically coupled coils are different in amplitude and phase. In order to determine the losses due to the third factor above, the local magnetic fields, we propose in this paper an experiment that consists of the measurement of losses in two straight insulated superconducting tapes located one over the other as close together as possible. In this way, the effect of the global magnetic field of the coil disappears because the coil does not exist. Furthermore, one of the tapes is made to be twice as long as the other so that we can measure the part of the transport losses in the part of the tape independent on the influence of the other. This permits us to distinguish the component of the losses due to the interaction between the pair of tapes. BSCCO tape was used and the pieces were fed with two different power supplies each one giving a current adjustable in amplitude. Measurements of the voltages between taps and in contact-less loops were taken both between the tapes and, in the longer tape, away from the influence of the shorter one. The losses were calculated from the wave forms of the contact and contact-less voltages and the currents. The influence of the proximity of the tapes was determined.