Design and Performance of a “Squirrel-Cage” Dynamo-Type HTS Flux Pump

Abstract

Dynamo-type high-temperature superconductor (HTS) flux pumps are fully superconducting dc current sources for HTS magnet coils, which eliminate the need for external current leads. Previous work has focussed on dynamos employing a single-coated conductor stator wire, and the maximum output current is found to be limited by the effective internal resistance of the stator wire. It is expected that adding additional stator wires in parallel will increase the maximum output current. However, there have been few experimental reports of this effect, nor of the influence of the rotor magnet configuration in a multiply parallel stator arrangement. In this paper, we report on the design and experimental characterization of an HTS dynamo employing eight parallel HTS stator wires, which are arranged in a cylindrical geometry and excited by an axially concentric rotor carrying one or more permanent Nd-Fe-B magnets. We investigate the effect of changing the number of rotor magnets, and find that the maximum output current is achieved when the number of rotor magnets is less than half the number of stator wires. We have demonstrated output currents of up to 700 A from this device. This is the highest reported output to date from a flux pump operating at liquid nitrogen temperatures. Furthermore, our results indicate that the squirrel-cage architecture used here is capable of achieving a maximum output current of ~ 1.3 kA. At present, the heat load associated with a kA+ conduction cooled current leads prevents such large currents being injected into present-day cryo-cooled HTS magnets. Our results now indicate that this issue could be circumvented through employing an HTS dynamo, opening up new opportunities to consider novel designs of very high-current cryo-cooled HTS magnets.