Alfredo Álvarez

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.

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.

A test rig for thrust force measurements of an all HTS linear synchronous motor

This paper presents the design of a test rig for an all HTS linear synchronous motor. Although this motor showed to have several unattractive characteristics, its design raised a number of problems which must be considered in future HTS machines design. HTS electromagnetic properties led to the development of new paradigms in electrical machines and power systems, as e. g. in some cases iron removal and consequent assembly of lighter devices. This is due to superconductors ability to carry high currents with minimum losses and consequent generation in the surrounding air of flux densities much higher than the allowed by ferromagnetic saturation. However, severe restrictions in HTS power devices design that goes further beyond cryogenic considerations must be accounted in. This is usually the case when BSCCO tapes are used as conductors. Its bending limitations and the presence of flux components perpendicular to tape surface, due to the absence of iron, have to be considered for it may turn some possible applications not so attractive or even practically unfeasible. An all HTS linear synchronous motor built by BSCCO tapes as armature conductors and two trapped-flux YBCO bulks in the mover was constructed and thrust force measurements are starting to be performed. Although the device presents severe restrictions due to the exposed and other reasons, it allowed systematising its design. A pulsed-field magnetiser to generate opposite fluxes for both YBCO bulks is also detailed. Thrust force numerical predictions were already derived and presented.

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.

Sand Pile Modeling of Multiseeded HTS Bulk Superconductors: Current Densities Identification by Genetic Algorithms

The sand pile model, in conjunction with Bean model, is often applied to describe single grain bulk superconductors. However, in several applications such as electric motors, multiseeded bulks are needed, due to the need to increase sample dimensions. In this paper, an extension of the sand pile model is presented in order to manage this type of materials. Multiseeded HTS bulk superconductors, produced, e.g., by the top-seeded melt growth process, are characterized by intra- and intergrain currents, and these are reflected in the model. However, identifying these currents from flux density measurements is not straightforward, when considering more than one grain. In fact, the number of currents increases with the number of grains, and these have to be identified from the measured field surface. A method to identify these currents based on genetic algorithms is validated with artificial data and then used in real measurements.

Numerical Design Methodology for an All Superconducting Linear Synchronous Motor

One potential advantage of the application of superconducting materials in electrical machines is the possibility to build lighter and compact devices by removing iron. These machines find applications, e.g., in systems where cryogenics is already available, or in naturally cryogenic environments. The design of motors with high temperature superconductors (HTS) presents issues unconsidered in classical machines, besides considerations on cryogenics, such as HTS brittleness or mechanical restrictions. Moreover, HTS’ electromagnetic properties also degrade due to flux density components, which arise if there is no iron to guide magnetic flux. Several aspects must thus be considered in the design stage, as applications may turn less attractive or even unfeasible. In this paper these issues are detailed, and a numerical methodology for the design of an all superconducting (without iron or conventional conductors) linear synchronous motor is presented.

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.