Characterization & Analysis of the Magnetic Field of a Halbach Array and Its Utilization in Flywheel Energy Storage

Abstract

The development of High Temperature Superconductors (HTS) has resulted in the production of highly efficient magnetic bearings with frictional losses several hundred times lower than losses which occur in bearings where the two surfaces are in contact with each other. Because of their efficiency, these magnetic bearings are being used in the development of a flywheel energy storage system. In such a device, electrical energy is stored as rotational energy in a rotor which spins at speeds of several thousand revolutions per minute. An array of HTSs are cooled to a critical temperature using liquid nitrogen, at which point the material becomes superconducting and can levitate a magnet assembly, creating a nearly frictionless magnetic bearing. The assembly is attached to a rotor which, when rotated in a vacuum environment, virtually eliminates losses due to frictional forces. A Flywheel Energy Storage System utilizing the principles described above is currently being researched at Argonne National Laboratory. During the development of the device, the decision was made to move away from a conventional spin-up induction motor to a Halbach magnetic array, a dipole magnet which, in conjunction with a stator coil, would act as the motor and generator necessary for initially storing energy in the device and its later retrieval. A Halbach array is a dipole magnet constructed by using several permanent magnet segments whose permanent axes are aligned to yield a uniform field. Tests were conducted to measure the magnetic field and determine the magnitude and the deviation from the expected field for the Halbach array that would be used. A simple two-phase device was constructed utilizing basic electronic principles to test the Halbach array arrangement. In the stator assembly, Hall effect sensors are used to sense the magnetic field and produce a signal proportional to it. After amplification, the signal is fed back into the coil and generates a magnetic field perpendicular to the field of the Halbach, creating a torque on the array and the magnet-rotor assembly to which it is attached, thus accelerating the rotor.