Thomas M. Mulcahy

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.<<ETX>>

Levitation force and magnetic stiffness in bulk high-temperature superconductors

Levitation forces between a small permanent magnet and a disk of bulk high‐temperature superconductor at 77 K were measured as a function of vertical separation for disks of composition Y‐Ba‐Cu‐O, Ag/Y‐Ba‐Cu‐O, (Pb,Bi)‐Sr‐Ca‐Cu‐O, and Tl‐Ba‐Ca‐Cu‐O. The forces were highly hysteretic; however, for all samples, on the initial descent of the magnet toward the disk, the force was unique, independent of magnet speed, and varied approximately as the negative exponential of the separation distance. Magnetic stiffness, associated with minor hysteresis loops, was found to be approximately proportional to the levitation force, and nearly independent of magnet configuration and superconductor composition.

Amplitude dependence of magnetic stiffness in bulk high‐temperature superconductors

Magnetic stiffness has been measured in several vibratory systems composed of a permanent magnet elastically suspended above a stationary high‐temperature superconductor at 77 K. For Y‐Ba‐Cu‐O, Ag/Y‐Ba‐Cu‐O, (Pb‐Bi)‐Sr‐Ca‐Cu‐O, and Tl‐Ba‐Ca‐Cu‐O sintered disks, both the vertical and horizontal magnetic stiffnesses increase strongly as the magnet’s oscillation decays, down to amplitudes of 1 μm. Vertical stiffness is about twice as strong as horizontal stiffness, and both are much stronger than the gradient of the levitation force for large monotonic changes in magnet height over the superconductor. These results are not deducible from contemporary magnetization measurements, and elastic pinning of flux lines are believed to contribute significantly to the stiffness.

Test results of 2-kWh flywheel using passive PM and HTS bearings

Toward demonstrating the potential of flywheel energy storage systems that use high-temperature superconductors (HTSs) and permanent magnets (PMs) as passive rotor bearings, a flywheel system was developed and tested with a 165-kg cylindrical carbon- and glass-fiber rotor to rim speeds of 400 m/s (19,000 rpm) and stored energies of >2.25 kWh. The main bearings internal stack of PM rings was passively stabilized by HTS bearings at each end of the rotor. The stator portion of the HTS bearing consisted of an array of melt-textured YBCO pellets bathed in liquid nitrogen inside a nonconducting cryochamber. The motor/generator (M/G) was based on an internal-dipole Halbach array and could produce 1.5 Nm of torque. Each bearing and the M/G included multipiece banded PM rings secured to the rotor inside diameter with flexible urethane rings. In a vacuum enclosure at 10/sup -4/ Pa pressure, rotational drag on the rotor was hysteretic and at low speeds the coefficient of friction was well below 10/sup -6/.

Low friction in mixed-mu superconducting bearings

Individual magnetic steel rotors were levitated by combining the attractive force between permanent magnets and the steel with the repulsive force between high‐temperature superconductors and the steel. The free spindown of several rotors was observed, and the effective coefficient of friction for the bearing was calculated as a function of geometry. Low‐speed coefficients of <10−8 were observed, and the velocity dependence of MnZn ferrite rotors suggest that coefficients of <10−6 are attainable at bearing rim velocities of 100 m/s.

Flywheel energy storage advances using HTS bearings

High-temperature-superconducting (HTS) bearings have the potential to reduce rotor idling losses and make flywheel energy storage economical. Demonstration of large, high-speed flywheels is key to market penetration, Toward this goal, we have developed and tested a flywheel system with 5- to 15-kg disk-shaped rotors. Rim speeds exceeded 400 m/s, and stored energies were >80 Wh. Test implementation required technological advances in nearly all aspects of the flywheel system, Features and limitations of the design and tests are discussed, especially those related to achieving greater energy storage levels.

Velocity dependence of rotational loss in Evershed-type superconducting bearings

Results of free spin down in vacuum are reported for an Evershed-type superconducting bearing in which a permanent magnet (PM) ring is levitated over an array of high-temperature superconductors (HTSs) and under a similar PM ring in magnetic attraction. The velocity dependence of the rotational loss strongly suggests that the observed velocity-dependent losses are primarily due to eddy currents induced in the PM by inhomogeneity of the field produced by the magnetized HTS array. The results show that the Evershed-type bearing is capable of reducing these eddy-current losses to an extremely low level, so that at a maximum magnet rim velocity of 28 m/s, the fractional kinetic-energy loss per hour was 2.4×10−4. Significant levitation heights are also possible, and at a 23 mm height, we measured a low-speed coefficient of friction of 3×10−8.

Magnetic levitation and stiffness in melt‐textured Y‐Ba‐Cu‐O

Magnetic levitation and stiffness have been measured in several systems composed of a permanent magnet elastically suspended above a stationary melt‐textured sample of Y‐Ba‐Cu‐O. The levitation force and vertical stiffness have been calculated on the basis of magnetization measurements of the same system, and the calculated results showed excellent agreement with the experimental measurements. Based on the force and magnetization measurements, it is predicted that the same Y‐Ba‐Cu‐O material configured in a geometry suitable for magnetic bearings could produce a levitation pressure of 100–400 kPa at 20 K.

Effect of size and geometry on levitation force measurements between permanent magnets and high‐temperature superconductors

A series of experiments measuring the levitation force between a permanent magnet (PM) and a high‐temperature superconductor (HTS) and between pairs of PMs, coupled with finite‐element calculations of the forces and fields, has identified factors that influence the levitation force. The self‐demagnetizing factor within the HTS and, to some extent, within the PM has a profound effect on magnetic pressure. For large HTSs with strong flux‐pinning, the demagnetizing effect of the diamagnetic image of the PM is substantial. For short distances between the HTS and PM, compression of magnetic flux produces a dependence on PM diameter.

Stable levitation of steel rotors using permanent magnets and high-temperature superconductors

Individual freely spinning magnetic steel rotors were levitated by combining the attractive force between permanent magnets and the rotor with the repulsive force between high‐temperature superconductors and the steel. The levitation force and stiffness of several configurations are presented, and the application of this levitation method to high‐speed bearings is discussed.