Low rotational drag in high-temperature superconducting bearings

Bearings consisting of permanent magnets stably levitated over high-temperature superconductors exhibit low rotational drag and have the potential to enable high-efficiency flywheel energy storage. The coefficient of friction /spl mu/ for such storage systems is derived as a function of bearing parameters and is shown to be an appropriate figure of merit to describe bearing losses. Analysis shows that values of /spl mu/ <10/sup -6/ enable flywheel standby losses <0.1%/hr for high-speed flywheels. A vacuum-chamber experimental apparatus has been constructed to measure values of /spl mu/ for various experimental bearing designs. Experimental values for /spl mu/ at low velocity have been as low as 3/spl times/10/sup -7/ for an 89-mm-diameter ring permanent magnet stably levitated over an array of melt-textured Y-Ba-Cu-O. An important loss mechanism occurs from eddy currents induced in the rotating magnet due to the discrete nature of the superconductor array.<>