Internal Pump Method for Convective Cooling of the Rotor of a Superconducting Motor

Abstract

Electrification has been proposed as a route to decarbonizing air travel. Conventional electric motors are too heavy to achieve the power densities required for aviation so superconducting motors for aircraft are being developed. Superconductors however, work at cryogenic temperatures, which indispensably require a reliable and efficient cooling mechanism. The Robinson Research Institute is developing a 3 MW superconducting motor using an HTS (High Temperature Superconductor) rotor operating at 50 K. The motor is intended to run at a shaft speed of 4500–6000 rpm to directly drive a ducted fan. A key challenge is the removal of heat from a cryogenic spinning rotor to a stationary refrigerator so it can be rejected at ambient temperatures. Previous motors have utilized externally pumped or thermo-syphoned cryogens to cool the rotor, which require complex rotary seals and a heavy external cold box for the refrigerator. The aircraft application demands a compact and low weight solution. In a synchronous superconducting motor, a popular configuration is to have the rotor with direct current (DC) field coils and the stator with alternating current (AC) coils. This configuration significantly reduces cooling load on the rotor as DC superconducting coils have few losses. Most of the heat to be lifted to ambient from the rotor comes from conduction down the shaft(s), especially if the DC coils are energised with a flux pump. However, the rotor is rotating, so the heat needs to be transferred across an interface to the non-rotating (stationary) world. This work proposes using a novel concept using a stationary cryocooler's cold heat exchanger as part of a pump to circulate gaseous helium inside the rotor and to use the rotor's spinning action to perform the pumping. In this way the rotating interface for heat transfer becomes the pump. A rotating gas seal is still necessary, but this can be at near ambient temperature and pressure. Computational fluid dynamics (CFD) analysis has indicated that the system will work with acceptable losses and requires less than 20 W of cooling to keep a motor's rotor at 50 K. This paper presents the CFD modelling, results of the proof-of-concept experiments that validated the CFD model, and will present further improvements to the concept, demonstrating a feasible cooling method and its application to a superconducting rotor.