R.J. Hayes

Combination Radial-Axial Magnetic Bearing

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

Active magnetic bearings for energy storage systems for combat vehicles

Advanced energy storage systems for electric guns and other pulsed weapons on combat vehicles present significant challenges for rotor bearing design, Active magnetic bearings (AMBs) present one emerging bearing option with major advantages in terms of lifetime and rotational speed, and also favorably integrate into high-speed flywheel systems. The Department of Defense Combat Hybrid Power Systems (CHPS) program serves as a case study for magnetic bearing applications on combat vehicles. The University of Texas at Austin Center for Electromechanics (UT-CEM) has designed active magnetic bearing actuators for use in a 5 MW flywheel alternator with a 318 kg (700 lb), 20000 rpm rotor. To minimize CHPS flywheel size and mass, a topology was chosen in which the rotating portion of the flywheel is located outside the stationary components. Accordingly, magnetic bearing actuators are required which share this configuration. Because of inherent low loss and nearly linear force characteristics, UT-CEM has designed and analyzed permanent magnet bias bearing actuators for this application. To verify actuator performance, a nonrotating bearing test fixture was designed and built which permits measurement of static and dynamic force. An AMB control system was designed to provide robust, efficient magnetic levitation of the CHPS rotor over a wide range of operating speeds and disturbance inputs, while minimizing the occurrence of backup bearing touchdowns. This paper discusses bearing system requirements, actuator and controller design, and predicted performance; it also compares theoretical vs. measured actuator characteristics.

Design and Testing of a Flywheel Battery for a Transit Bus

The University of Texas at Austin Center for Electromechanics (UT-CEM) has designed and tested a flywheel energy storage system conventionally referred to as a flywheel battery (FWB) for power averaging on a hybrid electric transit bus. The system incorporates a high speed (40,000 rpm) 150 kW permanent magnet motor generator with magnetic bearings to levitate a 2 kWh composite flywheel. This paper summarizes: design goals, required operating parameters, system design, analysis completed prior to fabrication, and initial performance testing completed in the laboratory. The paper includes information on the motor/generator, power electronics, magnetic bearing sensors and controls, and FWB subsystems (including containment). Finally, recommendations for continued testing are made along with recommendations for improvements to the existing design.

Current status of the Hobby-Eberly Telescope wide-field upgrade

The Hobby-Eberly Telescope (HET) is an innovative large telescope of 9.2 meter aperture, located in West Texas at the McDonald Observatory (MDO). The HET operates with a fixed segmented primary and has a tracker which moves the four-mirror corrector and prime focus instrument package to track the sidereal and non-sidereal motions of objects. A major upgrade of the HET is in progress that will increase the pupil size to 10 meters and the field of view to 22′ by replacing the corrector, tracker and prime focus instrument package. In addition to supporting the existing suite of instruments, this wide field upgrade will feed a revolutionary new integral field spectrograph called VIRUS, in support of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEXχ). This paper discusses the current status of this upgrade.

High voltage stator for a flywheel energy storage system

The University of Texas at Austin Center for Electromechanics (UT-CEM) has designed and conducted component test/development for a flywheel energy storage system for pulsed loads and mobility load leveling in a tactical vehicle. Performance goals of this machine dictate that the stator windings will see continuous voltages of 6.7 kV and spikes to 10 kV. To manage the thermal loads produced by the intermittent 5 MW output pulses, and continuous 350 kW duty, the generator is cooled by oil at a maximum temperature of 90/spl deg/C. UT-CEM designed and developed a novel insulation system and dual coolant passage arrays for the stator. Design performance was verified in laboratory prototype testing. This paper summarizes design goals, analysis, and mockup testing of the motor-generator stator.

Experimental results from CEM-UT's single shot 9 MJ railgun

A 10-m-long, 90-mm bore railgun has been designed and fabricated. During the test program, a number of solid armatures and projectile packages have been tested in a 50-m-deep vertical test range. The experiments are powered by six homopolar generator (HPG) charged inductive stores, sequentially staged to provide the desired acceleration profile. Prior to testing, computer simulations are run to determine the preferred current profile and predict system performance. During projectile flight, high-speed films, X-rays, muzzle volts, and velocity/acceleration profiles are recorded along with power supply operating parameters. Postshot diagnostics include bore wear analysis and armature and target recovery. Comparisons of predicted and recorded shot performance are also made. On selected tests, an energy balance is performed to determine efficiencies of the various components. A summary of all 90-mm gun shots is presented along the critical data collected from selected tests. >

Lightweight containment for high-energy rotating machines

Developed a lightweight containment system for high-speed composite rotors. The containment device, consisting of a rotatable, composite structure, has been demonstrated to contain the high-energy release from a rotor burst event and is applicable to composite rotors for pulsed power applications. The most important aspect of this design is that the free-floating containment structure dissipates the major loads (radial, torque, and axial) encountered during the burst event, greatly reducing the loads that pass through the stator structure to its attachments. The design results in significant system-level weight savings for the entire rotating machine when compared to a system with an all-metallic containment. Of equal interest to the containment design, the experimental design and instrumentation was very challenging and resulted in significant lessons learned. This paper describes the containment system design, rotor burst test setup, instrumentation for measuring loads induced by the burst event, and a detailed explanation of the successful containment test results and conclusions.

Design of the fiber optic support system and fiber bundle accelerated life test for VIRUS

The quantity and length of optical fibers required for the Hobby-Eberly Telescope* Dark Energy eXperiment (HETDEX) create unique fiber handling challenges. For HETDEX‡, at least 33,600 fibers will transmit light from the focal surface of the telescope to an array of spectrographs making up the Visible Integral-Field Replicable Unit Spectrograph (VIRUS). Up to 96 Integral Field Unit (IFU) bundles, each containing 448 fibers, hang suspended from the telescopes moving tracker located more than 15 meters above the VIRUS instruments. A specialized mechanical system is being developed to support fiber optic assemblies onboard the telescope. The discrete behavior of 448 fibers within a conduit is also of primary concern. A life cycle test must be conducted to study fiber behavior and measure Focal Ratio Degradation (FRD) as a function of time. This paper focuses on the technical requirements and design of the HETDEX fiber optic support system, the electro-mechanical test apparatus for accelerated life testing of optical fiber assemblies. Results generated from the test will be of great interest to designers of robotic fiber handling systems for major telescopes. There is concern that friction, localized contact, entanglement, and excessive tension will be present within each IFU conduit and contribute to FRD. The test apparatus design utilizes six linear actuators to replicate the movement of the telescope over 65,000 accelerated cycles, simulating five years of actual operation.

HETDEX tracker control system design and implementation

To enable the Hobby-Eberly Telescope Dark Energy Experiment, The University of Texas at Austin Center for Electromechanics and McDonald Observatory developed a precision tracker and control system – an 18,000 kg robot to position a 3,100 kg payload within 10 microns of a desired dynamic track. Performance requirements to meet science needs and safety requirements that emerged from detailed Failure Modes and Effects Analysis resulted in a system of 13 precision controlled actuators and 100 additional analog and digital devices (primarily sensors and safety limit switches). Due to this complexity, demanding accuracy requirements, and stringent safety requirements, two independent control systems were developed. First, a versatile and easily configurable centralized control system that links with modeling and simulation tools during the hardware and software design process was deemed essential for normal operation including motion control. A second, parallel, control system, the Hardware Fault Controller (HFC) provides independent monitoring and fault control through a dedicated microcontroller to force a safe, controlled shutdown of the entire system in the event a fault is detected. Motion controls were developed in a Matlab-Simulink simulation environment, and coupled with dSPACE controller hardware. The dSPACE real-time operating system collects sensor information; motor commands are transmitted over a PROFIBUS network to servo amplifiers and drive motor status is received over the same network. To interface the dSPACE controller directly to absolute Heidenhain sensors with EnDat 2.2 protocol, a custom communication board was developed. This paper covers details of operational control software, the HFC, algorithms, tuning, debugging, testing, and lessons learned.

Response of an urban bus flywheel battery to a rapid loss-of-vacuum event

The University of Texas at Austin Center for Electromechanics (UT-CEM) has developed a 2 kW-hr flywheel battery for energy management on a hybrid electric urban bus. The battery recovers braking energy and stores excess energy generated by the prime mover (e.g., internal combustion engine). The flywheel rotor, fabricated from high-strength composites, spins at 40,000 rpm at full charge (∼900 m/s tip speed), and is housed in a vacuum enclosure to minimize windage drag. Also integrated into the enclosure is a composite containment system that has been prooftested to provide additional safety. Ensuring flywheel safety is a major issue that must be addressed in using flywheels for transportation applications. A large leak caused by a service failure of the vacuum system could damage the flywheel before the energy dump system has time to act. A rapid loss-of-vacuum test on a rotor similar to that planned for the urban bus flywheel was conducted. Instrumentation, during the flywheel spin test recorded increasing flywheel surface temperature (>316°C or 600°F) following an intentional and abrupt loss of vacuum. No severe damage was noted on the surface of the flywheel, which was later retested to a higher speed to assess structural integrity. This paper provides an analysis of the data from that test and discusses the experimental results as they pertain to safety of the bus flywheel.