Superconductor-based bearing undergoes testing

Nexans has supplied the world's first contactless bearing based on high-temperature superconductors, which is to undergo testing for use in an industrial application. The customer for this prototype is Siemens' Corporate Technology Division, which is evaluating it for use in electrical machinery. Siemens is carrying out intensive research on high-temperature superconductors (HTS), and recently started operation of a 4MVA generator designed for ships, containing a rotor with an HTS magnet. The high current densities of HTS result in compact designs for engine and generator applications. This is especially important in marine applications, where low weight and small dimensions allow efficient use of space and permit new types of ship design that cannot be realised using conventional engines. In this case, the high current densities help to keep a rotating shaft suspended in a magnetic field. With the help of a magnetic field, the HTS bearing holds a rotor equipped with permanent magnets in contactless suspension, and can bear a nominal load of 500 kg. The bearing was developed and built by Nexans SuperConductors (NSC), based in Hurth (Germany), in collaboration with Nexans' refrigeration technology experts in Hanover, and designed in cooperation with specialists at the IMAB Institute at the Brunswick Technical University. The superconductors were manufactured and assembled in Hurth. The copper stator that bears the superconductor and the rotor were also developed and produced at NSC. 'HTS bearings are inherently safe, something which distinguishes them from other contactless bearings', explained Dr Joachim Bock, the Managing Director of NSC. Permanent magnets alone cannot produce a stable state. Bearings based on electromagnets (active magnetic bearings) require a constant current supply to make the control adjustments to balance the load. The passive HTS bearing does not require this type of regulation as the yttrium barium copper oxide(YBCO) crystals react individually when they approach the rotor's permanent magnets - in this case arranged in a ring form. The superconductor becomes a permanent magnet under the influence of an external magnetic field. 'This HTS permanent magnet keeps the shaft in a stable position by levitation and also counterbalances radial and lateral forces', explained Jean-Maxime Saugrain, Nexans' superconductor activities Manager. The HTS cylinder consists of 270 individual YBCO bulk crystals produced by NSC, which are arranged on a copper body with a diameter of around 325mm and a height of 305mm. A refrigerating machine cools the copper cylinder containing the superconductors to around -210C. A 10 to 7mbar vacuum in the bearing prevents the icing that would occur caused by moisture freezing in the atmosphere. The stator's copper mass retains the low temperature, enabling the bearing to bear its load for several more hours even if the refrigerating machine breaks down. After pretesting at NSC, the HTS bearing is currently undergoing thorough tests at Siemens. Reliable operating data will be gathered and limit values for utilisation in rotating machinery will be produced. It has already proved its main qualities on the test bench: it can be cooled to -245C (the minimum requirement was -210C); can bear a load of up to 690kg (the minimum requirement was 500kg); and can support engine speeds of up to 3600 revolutions/min. The bearing gap also represents an outstanding achievement, as the rotor equipped with permanent magnets runs in the 'warm bore' in a gap just 1mm wide. There is also only 1mm distance between the insulating cryostat wall and the HTS material at a temperature of -245 C. Differential temperatures of up to 270C are insulated over this short distance.