The Application of High Temperature Superconductors in Brushless Ac Machines

Abstract

The practical application of high temperature superconductors in electrical machines has been impeded in part by the low critical current densities and sensitivity to magnetic field, which is typical of these materials. The situation is exacerbated by the difficulty of fabricating robust and flexible continuous conductor based on HTS; the manufacturing techniques required to overcome these difficulties result in a very expensive product. A final bar to their ready application is the strain-induced degradation, which limits the radius to which a conductor can be bent. The best commercial HTS tapes cannot be bent to radii less that about 100 mm. This inhibits their use in machine windings, except in very large machines, thus ruling out the construction of small experimental prototypes for developmental purposes. Current densities, which approach those typical of low temperature superconductors, i.e. in excess of 1010 Am-2, are achieved only in epitaxial thin films of high temperature superconductors. Bulk materials have critical current densities that are several orders of magnitude below this value. A remarkable exception is the seeded, melt-textured, single domain samples of the rare-earth barium copper oxide (REBCO) 123 phase. These materials contain about 20% of a fine dispersion of the 211 phase, which contributes both to flux pinning, and to mechanical integrity. The best examples of these materials can carry critical current densities of 108 Am-2, at 77K and in magnetic inductions of up to 1 Tesla. These materials can trap inductions of several Tesla at 77K, and may be regarded as rivals permanent magnets. Many interesting and novel applications have been suggested for these materials.