AC Losses in Superconducting Magnetic Bearings

Since the discovery of high-temperature superconductors, superconducting magnetic bearings have become an icon of applied superconductivity. Their ability to provide passive stable levitation/suspension has attracted much attention for industrial applications such as maglev trains, flywheel energy storage systems, and high speed electrical machines.

Paihau-Robinson Research Institute has been working on the design and construction of a 22 kW, 30000 rpm homopolar superconducting motor for a hybrid aircraft propulsion system. Superconducting magnetic bearings seem to be a potential solution to overcome the speed limits imposed by friction and heat in mechanical bearings. Considering the lack of commercial availability of superconducting bearings, the Institute has decided to investigate the challenges involved in developing suitable bearings for ongoing and future projects. The focus of this thesis is to explore the development of suitable low-loss bearings from bulk high-temperature superconductors, and to develop suitable test methodologies that allow the properties and behaviour of high-temperature superconducting bearings to be explored. 

In order to accurately predict the behaviour of superconducting magnetic bearings, a precise knowledge about the non-linear voltage-current (E-J) relationshipis required for input to simulations. Unlike coated conductors, this cannot be achieved by transport measurements in superconducting bulks. In this thesis, a convenient non-destructive method based on levitation force decay in superconducting bearings is introduced and evaluated to map the non-linear electrical properties in superconducting bulks. Furthermore, fibre Bragg grating sensors are employed in an actual superconducting bearing environment to monitor temperature at the bearing surface. Accurate cryogenic temperature monitoring with large electromagnetic fields present is of great importance in practical applications, and is problematic with conventional electronic sensors.

A barrier to commercial application is suspected to be the effects of inhomogeneous magnet, or superconducting, properties on the efficiency of bearings. The key drivers of this, AC loss mechanisms in superconducting bearings are investigated and a new analytical expression is proposed for hysteresis loss in superconducting bearings which takes magnetic field inhomogeneity, magnetic field periodicity, and loss surface distribution into account. This new expression is tested by performing spin-down experiments, and the results are found to be in good agreement.

Lastly, a high-speed superconducting motor/generator demonstration is built, which sets a record for the fastest superconducting bearing made in New Zealand.

This will enable applications such as flywheel energy storage systems, direct-drive generators and high speed microfans to be explored whilst also providing the opportunity to demonstrate key technology elements such as flux-pumped hightemperature superconducting field windings and low-loss high-speed bearings. AC loss mechanisms at kHz-rated rotational frequencies are also studied with this prototype. The results show that at high rotational frequencies the superconducting bulks are pushed into a resistive flux-flow regime, where AC losses are createdby an eddy-current-like mechanism.

This thesis explores two phenomena of practical importance in superconducting bearings, and proves the principle of the application of superconducting bearings for ongoing and future projects in New Zealand.