Failsafe bearing `floats` on superconductors

Siemens has started to test the world`s first non-contact bearing based on high-temperature superconductor (HTS) materials. The prototype bearing, developed by a team lead by German-based Nexans SuperConductors, can support a load of 500kg spinning at up to 3,600 rpm. Unlike other non-contact bearings, the HTS bearing is said to be inherently safe.

The bearing takes the form of a 325mm-diameter cylindrical copper stator (shown above) on which 270 HTS crystals are arranged, and a rotor fitted with permanent magnet rings. The copper cylinder is cooled to around -210°C and kept in a vacuum to prevent ice crystals forming on it. Even if the refrigeration equipment breaks down, the copper cylinder will stay cold enough to support the load for several hours.

The gap between the rotor and stator is just 1mm wide. A similar distance separates the HTS cylinder from the cooling cryostat wall. The photo below shows the cryogenic system that houses the bearing.

Unlike active magnetic bearings which require a continuous power supply to power the control systems that balance the loads to keep the rotor suspended, the passive HTS bearing does not. Instead, the HTS crystals (made from yttrium barium copper oxide) react individually when they approach the rotor`s permanent magnets. In effect, the crystals become permanent magnets themselves under the influence of the external magnetic field.

"The HTS `permanent magnet` keeps the shaft in a stable position by levitation, and also counterbalances radial and lateral forces," explains Jean-Maxime Saugrain, Nexans` superconductor activities manager.

In tests conducted by the Nexans team, the HTS bearing has carried loads of up to 690kg. Siemens is now performing more tests on the bearing to see how well it would work in rotating machines such as motors and generators.

Siemens, which built a 400kW superconducting motor in 2001, has recently started testing the world`s first HTS generator - a 4MW machine which spins at 3,600 rpm (shown above). Such generators could be half the size and weight of conventional machines. They would also be much more efficient because the HTS winding excitation exhibits almost no losses, and the copper stator air-gap winding has no iron teeth.

Another advantage of HTS generators is that they are more electrically stable with better reactive power capacities than conventional generators. They also provide better voltage stability in the event of transient load variations, making them ideal for isolated power systems such as those found on board ships. Future "all-electric" ships could therefore benefit from space- and weight-saving HTS propulsion systems which would cost much less to run than today`s systems.