We have measured the current-voltage (I-V} characteristics of several high-temperaturesuperconducting materials with widely difkrent morphologies {bulk Ag/Pb-Bi-Sr-Ca-Cu-0 tapes, thin films of Y-Ba-Cu-O, and melt-textured, bulk Y-Ba-Cu-0 samples). The I-V curves were taken at several magnetic fields ranging from 0 to 8 T. The measurements were carried out at three temperatures (4.2, 27, and 77 K) where the samples were immersed in liquid cryogens to ensure good thermal equilibrium. We compared our experimental results to the predictions of dissipation in superconductors made by the following physical models: modified Ambegaokar-Halperin, flux creep, vortex glass, collective flux creep, and a power law. The fits were extremely good for the first model and were not nearly as good for the others. Using the modified Ambegaokar-Halperin model, the critical current I"the normal-state resistance R",and y, which is proportional to the pinning potential U(H, T), were obtained for each material. Since the Ambegaokar-Halperin model is the only one which uniquely defines I"we conclude that its use puts this parameter on a solid physical basis.
Synchronous machines with superconducting field windings offer reduced losses, size and weight. Continuous advances in the HTS conductor development at American Superconductor Corporation enable the demonstration of machines with increasing power. The latest achievements include a > 200 h.p. (150 kW) motor operated on an inverter, built and tested at Reliance Electric Corporation, and coils for a 1 MW generator for airborne applications. The HTS coils provide MMFs of the order of 70 000 A turns per pole and operate between 20 and 27 K. At this temperature, gaseous helium or hydrogen is used as the coolant. The cooling is more efficient by a factor of 10 than for cooling systems required for LTS materials operating at liquid helium temperatures. The elevated operating temperature offers significant benefits for the overall efficiency as the coefficient of performance for removing the parasitic heat load (cryostat and current lead loss) is improved. On this path to commercially viable machine sizes, American Superconductor currently is developing and manufacturing HTS coils for a 1000 h.p. (750 kW) synchronous motor which will be delivered to Reliance Electric for demonstration in 1998.
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