Optimum working temperature of power devices based on Bi-2223 superconductors

Transformers, motors and magnets based on Bi-2223 tapes have an optimum working temperature where the um of investment and exploitation costs has a minimum. The investment cost is lower at temperatures below 77 K due to the higher critical current of the superconductor. The exploitation cost is mainly determined by the ac loss, which is dominated by the magnetization loss. The magnetization loss of a Bi-2223 tape at 48 Hz is measured for various temperatures between 65 and 77 K. The temperature dependence of the magnetization loss is fully explained by the change in critical current and agrees with the critical-state model. At high magnetic-field amplitudes the magnetization loss normalized with the critical current is independent of the temperature. Nevertheless, at lower temperatures the exploitation cost is higher due to the higher cooling penalty factor. A relation for the optimum temperature is developed, based on realistic expressions for the cooling penalty factor and the critical current as a function of the temperature. The influence of the other relevant parameters is summarized into a single dimensionless number. It is concluded that the optimum temperature of a power device based on Bi-2223 tapes is in the range 65-75 K. (author)