B Pérez

Superconducting armature for induction motor of axial flux based on YBCO bulks

Abstract Several small-scale SC rotating machines have been developed based on HTS bulk materials. Up to now, the role of the SC has been restricted to the rotor where no coil has been required. Despite the difficulty of building classical coils with ceramic materials, we have developed suitable geometries which can be built by machining melt textured YBCO pellets. In this work, we describe a multiphase armature designed to be made starting HTS ceramic pellets. The field created is designed to interact with a disc-shaped rotor producing both the torque and the forces to maintain it in working conditions. Calculations of the field in the rotor cavity generated by the armature in the working conditions are also reported.

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.

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.

Loss and Transition Studies of Shunted Free-Stabilized YBCO Tape for SFCL Applications

Superconducting fault current limiters (SFCL) use the transition of superconducting materials as the mechanism to introduce high impedance in the line to be protected when the current in the line surpasses the fixed maximum value. When the normal working current passes through the line, the device exhibits almost zero impedance. The transition of the superconductor may be due to the current crossing the material or the magnetic field surrounding the material. In order to evaluate the possibilities of externally shunted non-stabilized YBCO tape for the design of resistive SFCL with transition by current and magnetic field, a study of this tape was made. First, the AC loss was determined as a function of the rms value of the transport current. This permits us to know what cooling power is required in the normal state and the value of the critical current that makes the material transit. Second, we studied the tape stabilized by external resistances, to get information about the behavior of the tape carrying current greater than the critical current. Finally, the variation of the critical current and AC loss in the presence of magnetic fields was studied. Some comments about the relation between all these parameters and the possibility of designing SFCL using this tape are included as conclusions.

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.

Superconducting tape characterization under flexion

Abstract Electrotechnical applications of high temperature superconducting materials are limited by the difficulty of constructing classical windings with ceramic materials. While Bi-2223 tape may be a solution, it cannot be bent to radii less than a certain value since its superconducting capacity disappears. We describe an automated measurement system of the characteristics of this tape under flexion. It consists of a device that coils the tape over cylinders with different radii. At the same time, the parameters of its superconducting behaviour (e.g. resistance) are taken and processed. This system was developed at the “Benito Mahedero Laboratory of Superconducting Electrical Applications” in the University of Extremadura.

Device for Measuring the Thermal Cycling Degradation in 2G Tapes for Electrical Power Applications

The standard electrical engineering applications of high-temperature superconductors (HTSs) as coils (SMESs), motors, power cables, fault current limiters (SFCLs), etc., usually involve operating conditions under which the critical current of HTS tapes might be degraded. The dependence of such magnitude on mechanical properties, frequency variations, or thermal changes is a key concern for the accurate design of those devices. Two of the parameters affecting the critical current when the superconductor operates in broad temperature ranges are the number and the speed of transitions from the superconducting to the normal state. In order to determine how these parameters affect the critical current, we have designed an automated dipping setup in liquid nitrogen to reproduce heating and cooling cycles on superconducting tapes. In this paper, this device is described, and several tests on a short sample of YBCO coated conductor are reported and analyzed.