Study of bulk current leads for an axial type of HTS propulsion motor

The high-temperature superconductor (HTS) bulk current lead (BCL) is a significant device for a large number of current applications into the propulsion motors with HTS windings. The coexistence of low thermal conductivity and superconductivity enables us to shrink the dimension of the power transmission system. For the effective usage of the BCL, one has to optimize the dimension and the temperature gradient across the BCL length. We have made the BCL with a melt growth bulk crystal of GdBa₂Cu₂O_7−z by ourselves, serving for the introduction of the excitation current to the coreless Bi2223 superconductor winding field-pole magnets applied to the axial-gap type motor. We successfully installed the BCL and obtained a thermal gradient of 40 K across the BCL with 75 mm length under the current flow of 200 A, necessary to operate the superconductor windings at 30 K below Tc. The use of the BCL enabled us to suppress the thermal transfer into the coil vital part down to 6 W from the external power source. The present result shows that BCLs effectively operate while being connected to superconductor coils in propulsion motors.