Dependence of Resistance and Number of Tapes on the Coupling AC Loss of Soldered REBCO Stacks

Abstract

HTS stack has high current carrying capacity which is widely used in motors or generators. Usually, the HTS tapes are stacked and soldered together at ends. Although the HTS tapes are insulated and have low loss, the soldering parts are conventional conductors with resistance. When the HTS stack is subject to a parallel ac magnetic field, the coupling currents are induced because of the soldered parts which would form current loops. And then, the coupling currents would cause coupling loss or been called resistance loss. The loss is not only related to the applied field but also depends on the number of tapes and resistance value. The minimum electromagnetic entropy production method is used to study the dependence of resistance and number of tapes on the magnetization loss systematically. A HTS stack with realistic resistance was prepared and measured to be verified the calculation. The results show that the superconductor loss increases at the beginning but then decreases later when more tapes are stacked. However, the resistance loss increases almost linearly. The resistance loss and the superconductor loss also depend on the resistance value. When the resistance is small, although there are coupling currents pass through the ending parts, the resistance value is so small that the resistance loss is very difficult to be observed. And the resistance loss would increase with the resistance linearly and the superconductor loss almost keeps constant. With the resistance increasing, there is a peak in the resistance loss and the superconductor loss drops dramatically. Later, the resistance loss decreases with the resistance and the superconductor loss keeps constant again, because the resistance is so large that no coupling current can pass through the soldering parts so that there is almost no resistance at all.