AC loss measurements in high-T-c superconductors

Low a.c. loss is a key issue for electrical engineering applications of high-T-c superconductors at industrial frequencies. Different material options are characterized by three different a.c. loss measurement techniques which approach as much as possible the application conditions of the conductors. Melt-textured-growth YBa2Cu3O7-x bulk samples, suitable for magnetic bearings, are characterized in an external a.c. field by magnetization measurements. Melt-casted Bi2Sr2Ca2Cu3O8-x samples and powder-in-tube Bi2Sr2Ca2Cu3O10-x tapes are suitable for current leads or power cables. In these applications the conductors are exposed to a.c. transport currents. The associated losses in self field are measured by a very sensitive electrical measurement technique. Finally, a calorimetric method is necessary when larger conductors (for instance Bi2Sr2Ca2Cu3O8-x tubes) are tested under transport currents I-t, generating a transverse magnetic field H proportional to I-t, as encountered in magnet windings for SMES, transformers or generators. The results show that the a.c. losses are sufficiently low for self field applications at industrial frequencies and a comparison of the different high T-c superconductors is given. The results show further that the a.c. losses are essentially hysteretic and can be modeled using the Bean model.