University Of Texas At Austin Center For Electromechanics Research Articles

Optimization of Gas Turbine Generator-Sets for Improved Power Density

Many future U.S. Navy ships will employ all-electric propulsion systems instead of mechanical drives. To help optimize performance of these systems, studies are under way at The University of Texas at Austin Center for Electromechanics (UT-CEM) to minimize the size of power generation components. These studies focus on increasing the power density of directly coupled gas turbines and generators (gen-sets). The approach adopted in this paper uses scaling laws of gas turbines and synchronous electrical generators to examine the possibility of increasing power density by operating at higher shaft speeds. Included is consideration of inlet and exhaust turbine ducts and issues involving power electronics. Study results indicate that if inlet and exhaust duct volumes are neglected, the power density of directly coupled gas turbine-generator sets can be significantly improved by scaling to higher operating speeds. However, the advantages of scaling to higher speeds are largely negated when duct volumes typically encountered on modern ships are included. This suggests locating power generation equipment near the ambient terminus of inlet and exhaust ducts, so that duct lengths are minimized and fully exploiting the power density advantages of scaling to higher shaft speeds becomes possible.

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.

Investigation of windage splits in an enclosed test fixture having a high-speed composite rotor in low air pressure environments

The University of Texas at Austin Center for Electromechanics (UT-CEM) has designed and conducted a series of composite rotor spin tests to measure the windage losses and temperature distributions of a test setup at high rotor speeds and low air pressures. The intent of the windage tests is to validate the windage loss predictions and investigate how the air-gap windage is distributed between the rotor and stator. The findings of the spin tests will then be used to perform windage-related thermal design and analysis of a high-speed electrical machine. The radial air-gap flows under the test conditions, a low rotor cavity air pressure of 1 torr and high rotor surface velocities of 333 and 614 m/s, were in a laminar flow regime. Transient rotor and stator finite-element thermal analyses, using the measured windage losses and predicted laminar-flow windage splits, have been carried out to analyze the rotor and stator temperature distributions. This paper shows the detailed thermal analysis and compares the predictions with the measurements. The predicted and measured transient rotor and stator temperatures are in good agreement.

An optimized double ramp integrated launch package design for railguns

The need to evolve and make more efficient design tools for use in complex railgun system trade studies has led to an enhanced design for a fully integrated hybrid metal contact launch package for railguns. During work on two successful railgun projects at The University of Texas at Austin Center for Electromechanics (UT-CEM), an algorithm was designed which has produced what appears to be a strong design candidate for an aluminum double ramp integrated launch package (ILP). Using only four user inputs, the code produces railgun bore configurations (simple or augmented) and detailed ILPs which minimize the energy requirements for the compulsator. The output design gives useful mass fractions very close to the pinnacle of what is possible without more extensive use of composites. This paper describes the design process used to optimize the railgun bore and ILP. Included is an advanced concept generated by the algorithm along with a detailed three-dimensional structural analysis of the ILP based upon a current profile that is relevant for high-performance applications.

Sensor development for compulsator driven railgun systems

As the technologies of rotating power supplies and thyristor switches advance, new methods of measuring various signals must be developed. The University of Texas at Austin Center for Electromechanics (UT-CEM) has developed new methods to measure compulsator position and speed and has made advancements in health monitoring of the thyristor switches. Technological advancements in machine design have enabled dramatic increases in machine speeds, which in turn increase electromagnetic interference. It is essential to a successful operation to have a method (reliable within this harsh environment) of measuring speed and sensing rotor position to generate gate signals for thyristor switch modules. As the modules switch larger amounts of current and voltage, it is correspondingly important to monitor the switching process, so that any damage to the system caused by a fault condition can be minimized. This paper describes sensors developed at UT-CEM to monitor speed, sense rotor position, and detect fault conditions.

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.

Brush/slip ring current collector performance and anomalies during compulsator commissioning

During recent tests associated with a model scale compulsator development program at The University of Texas at Austin Center for Electromechanics (UT-CEM), the performance of severe duty trailing arm brush current collectors was evaluated. The original material of choice for the brush slip ring design was 70075-T6 aluminum to minimize machine mass and simplify slip ring assembly on the rotor shaft. The slip rings have a nominal diameter of 5.5 in. The combination of CM1S copper, trailing arm brushes, and aluminum slip rings performed well up to about 80% full speed, although the design did not appear to be very damage tolerant. As testing continued at higher speeds, one of the slip rings began to exhibit small shallow arc pits in the brush track on the slip ring outer diameter. This was observed on only one slip ring and had no apparent affect on machine performance. Left unchecked, this shallow arc damage can cause a high speed brush to "bounce" excessively during a run and cause more severe damage, as well as increase the circuit impedance from the arcing. This paper presents a general description of the observed brush performance during normal and fault conditions. It also includes a description of mechanical analysis and field strength measurements done around the brushes in an effort to explain the "one brush" bouncing. Brush and slip ring design upgrades (including a reinforced brush actuator for increased brush down-force and a newly implemented two-layer aluminum and steel slip ring design) are discussed.

Power converters for railguns

The University of Texas at Austin Center for Electromechanics (UT-CEM) has been developing power converters for use with DC railguns. Special design rules have been established and demonstrated in the construction of laboratory hardware. Power converter requirements for electric guns are dependent on the characteristics of the power supply, load, circuit topology, and switching devices used. This paper discusses some of the design requirements particular to these converters and shows why simple figures of merit (FOMs) developed in the early design of a converter are not valid. An alternate FOM that can be used to compare converters and judge development progress is offered. Use of this FOM requires that an adequate amount of design work be completed on a converter before it is applied. The FOM is applied to previously demonstrated converters. Possibilities for near term development of devices and converters are examined and the potential results of that development are compared to past work by using the new FOM.

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.

Cannon-caliber electromagnetic launcher

The University of Texas at Austin, Center for Electromechanics (UT-CEM) designed and fabricated a rapid-fire railgun launcher. Three, five-round salvos of 185-g launch packages are to be accelerated to 1,850 m/s at a rate of 5 Hz. The 2.25-m-long series-augmented launcher has a 30-mm round-bore equivalent rectangular geometry and is water-glycol cooled. Rapid-fire operation is achieved by driving the launcher with multiple 835-kA peak pulses provided by the Cannon-Caliber Electromagnetic Gun (CCEMG) compulsator. A single-shot launcher prototype was tested for the CCEMG system at UT-CEM and at the US Army Research Laboratory (ARL). The launcher has demonstrated an efficiency over 50%. A high CCFEMG system efficiency is in part due to the use of a solid armature and is enhanced by having a structurally stiff railgun. Historically, a railgun's stiffness was proportional to its weight. Laboratory-based railguns that have respectable mechanical properties have required massive structures that are nowhere near meeting the requirements of future vehicle integration and weaponization. The CCEMG railgun design incorporates a directional preloading mechanism, ceramic sidewalls, and a composite overwrap, which together give it a structural stiffness dominated by a high-modulus ceramic. The overall mass of the launcher is 273 kg. These characteristics make the cannon-caliber launcher one of the most fieldable railguns built to date.

Laboratory testing of the pulse power system for the cannon caliber electromagnetic gun system (CCEMG)

The team of (prime contractor) United Defense LP (UDLP) and The University of Texas at Austin Center for Electromechanics (UT-CEM) has completed a significant portion of the testing phase of a trailer mounted compulsator driven 35 mm (round bore equivalent) rapid fire railgun system. The objective of the program is to develop a compact, lightweight pulse power test bed capable of launching 3, 5 round salvos of 185-g integrated launch packages to 1.85 km/s at a firing rate of 5 Hz. Per contractual requirements, the pulse power system is also size compatible with the Amphibious Assault Vehicle (AAV). The pulse power system is developed around a second generation air-core, 4-pole rotating armature, self-excited, compulsator design. The 40 MJ at 12,000 rpm composite rotor stores all 15 shots inertially and is capable of 2.5 GW performance into the 2.21 m long series augmented railgun. This paper describes the CCEMG pulse power supply configuration and highlights important features of the commissioning test plan. The paper then presents test results from mechanical runs, stand alone compulsator (CPA) rectifier tests, short circuit tests, and single shot live fire tests. Finally, CPA performance is compared with predictions for the single shot tests presented.