Combination Radial-Axial Magnetic Bearing

Abstract

The design and application of a patented active actuator design that combines the radial and axial actuator into one combination radial-axial actuator is presented. The theory of operation of the actuator is defined and magnetic finite element analysis modeling shown verifying operation. Its mechanical construction is then presented to show how this actuator configuration is successfully integrated into a mechanical system. Applications up to 60,000 rpm are then presented, with a detailed presentation of a 42,000 rpm energy storage flywheel currently under test at the University of Texas Center for Electromechanics. INTRODUCTION Conventional active magnetic bearing systems, whether electromagnet bias or permanent magnet bias, typically utilize three actuators for a 5-axis system (shown in Figure 1). This would be in the form of two radial actuators, each supporting and controlling two radial axes, and a thrust actuator supporting and controlling a single axial axis. Each actuator axis functions independently to provide forces in its defined axis for stable support and control of the levitated rotor. FIGURE 1. Three Bearing, Five Axis System The elimination of one actuator can simplify the system, reduce overall system size, possibly improve rotor dynamics and simplify control, and inherently reduce cost. A five axis, two bearing system is shown in Figure 2. These benefits have been realized to some extent with conical active radial bearings utilized to provide axial centering. However, this type of system presents manufacturing, control and integration challenges, in addition to issues with the long distance between the two radial bearings providing the axial control. What is needed for most applications is a full five active axis system to meet the load capacity and control requirements in all five axes adequately. FIGURE 2. Two Bearing, Five Axis System The combination bearing presented in Figure 3 reduces the total actuators required for a five axis active system from three to two, with one active actuator supporting and controlling three axes. This combination actuator configuration offers high radial and axial load capacities typically required for all active magnetic bearing systems. The simple construction and the elimination of a separate thrust actuator minimizes the space necessary to integrate the design and also minimizes rotor diameter, making it well suited for high-speed applications. FIGURE 3. Three Dimensional View of Combination Bearing MAGNETIC DESIGN This novel bearing design is constructed in a homopolar configuration such that the bias field is one polarity on all the radial poles, and the opposite polarity on the axial poles (i.e. the bias field enters the rotor through the radial air gaps and exits the rotor through the axial air gaps). This eliminates field polarity changes in the radial air gap to minimize rotor losses [1,2]. The design utilizes a permanent magnet or electromagnet to provide both radial and axial bias fields. The permanent magnet provides the linear negative stiffness benefit as present in the radial homopolar magnetic bearing [3]. Control coils for each radial axis and the axial axis act independently to modulate forces in each of the independent axes. The control field boosts the bias field in the direction of added force, and bucks the bias in the opposite pole. This difference in opposite pole fields provides the net force in the direction desired. The combination radial/thrust bearing utilizes a single radially polarized permanent-magnet ring to energize the radial and axial magnetic air gaps. The packaging of the control coils and ferromagnetic pole pieces results in virtually all of the volume being utilized functionally, leaving very little unused space within the confines of the bearing module. This highly efficient use of volume results in maximum spatial, magnetic, and electrical efficiencies. Figure 4 identifies the primary components of the bearing. Pole