Electrical Breakdown Processes

Bob Hebner presented a paper “Factors Contributing to Streamer Morphology” at the 14th International Conference on Dielectric Liquids in Graz, Austria, in July 2002. Recent simulations of streamers have shown that good correlation with experiments can be obtained if the fast positive streamer stage of liquid breakdown can be modeled as stochastic growth of a branching fractal tree. The challenge of the model is that the key parameter, the voltage dependence of growth depends on electron production and recombination rates, which are not easily amenable to direct verification. This work discusses the affects of additives that are expected to modify these rates in predictable ways. The results are consistent with the model but insufficient to define the limits of the model’s applicability.

Presentation Honored

Sid Pratap received the award for the best oral presentation at the 11th Electromagnetic Launch Symposium in Saint-Louis France. The presentation summarized the work described in the paper, “An Assessment of Losses in the Field Coil of the Compulsator Under Dynamic Conditions” that Pratap wrote with M. Driga of the Electrical and Computer Engineering Department of the University of Texas at Austin. The presentation used animation to show how the temporally and spatially varying magnetic field affected the distribution of current in the conductors of a compensated pulsed alternator, i.e. a compulsator.

Electric Motor Efficiency

Angelo Gattozzi and Bob Hebner presented a paper titled, “Inconsistencies in Electric Motor Certification Requirements” at the Power and Energy Systems Conference in Marina Del Rey, California, in May 2002. The paper highlighted difference in the statistical model the Department of Energy requires manufacturers to use in certifying motor efficiencies and the model the Department uses to assure the certification was done properly. The differences mean that, in certain cases, although a motor manufacturer follows rigorously the specified procedure to determine the nameplate efficiency for a motor lot, the enforcement procedure could yield a rating that is higher or lower than the correct rating.

High Efficiency Launch

Jon Kitzmiller and Mircea Driga of the Electrical and Computer Engineering Department at the University of Texas at Austin presented a paper, “An Optimized Double Ramp Integrated Launch Package Design for Railguns” at the 11th Electromagnetic Launch Technology Symposium, in Saint-Louis, France, May 2002. This paper describes the design process to minimize the input energy requirements for a railgun launcher by optimizing the rail geometry and the packaging of the item to be launched. The process has been reduced to a computer-aided design approach and the results of the modeling agree well with experimental results.

Transient Losses in Generators

Sid Pratap and Mircea Driga, of the Electrical and Computer Engineering Department at the University of Texas at Austin presented a paper, “An Assessment of Losses in the Field Coil of the Compulsator under Dynamic Conditions” at the 11th Electromagnetic Launch Technology Symposium, in Saint-Louis, France, May 2002. The paper describes combining finite element and circuit analysis techniques to estimate transient losses. Transient currents in the armature winding produce magnetic fields that interact with the field coils producing additional losses. The calculation is complicated by the fact that there is relative motion between the conductors. The use of this technique has shown that despite higher loss ratios, faster charge rates improve efficiency.

Advances in Aerospace Flywheels

Staff from the Center for Electromechanics presented four talks at the Aerospace Flywheel Workshop in March 2002 in Cleveland. Bob Hebner presented the keynote address in which he highlighted the rapid technical progress in composite flywheels and the growing use in both terrestrial and aerospace applications. Joe Beno presented the design of a particular flywheel system intended for use on the International Space Station. Tony Pak discussed the design and construction of a composite arbor for a flywheel that should lead to a higher stored energy per unit mass, a critical parameter for system that must be launched into space. Finally Angelo Gattozzi discussed the feasibility of using magnetic nanoparticles to make lighter efficient flywheels.

Flywheels Go Underwater?

The Institute of Electrical and Electronics Engineers sponsored a workshop in San Antonio, TX, in June 2002, to explore future power and energy needs for autonomous underwater vehicles. Joe Beno, Richard Thompson, and Bob Hebner were asked to participate and present a paper to assure that anticipated future flywheel advances were appropriately considered. Their paper, “Flywheel Batteries for Vehicles” reports recent advances in the areas of cycle life and vibration management.

Series-Parallel Combinations of Power Semiconductors

John Pappas, Angelo Gattozzi, and Bob Hebner presented a paper titled, “Pulsed-Duty Characterization of Turn-Off for a Population of SCR’s in the Effect of Variation on Equalization Circuit Design,” at the 11th Electromagnetic Launch Technology Symposium, in Saint-Louis, France, May 2002. This paper reports on the investigation of the recovery characteristics of high power silicon rectifiers after they conduct a high power pulse. Within the device, these characteristics are determined by carrier lifetime, which is a sensitive function of impurities. Thus, these characteristics vary from device to device. This investigation measured the extent of the variation and developed an appropriate external circuit to minimize the effect of these variations.

Advances in Compensated Pulsed Alternators

Jon Kitzmiller, Sid Pratap, and Mircea Driga of the Electrical and Computer Engineering Department at the University of Texas at Austin presented a paper, “An Application Guide for Compulsators” at the 11th Electromagnetic Launch Technology Symposium, in Saint-Louis, France, May 2002. A compulsator is a pulsed alternator with a compensation winding to lower the source impedance and thus produce a faster current pulse. The paper summarizes the characteristics of the compulsators that have been constructed, including both iron-core and air-core types. It also summarizes the potential applications for the various types for military systems.

Cooling Compulsators

Hsing Liu and Jon Hahne presented a paper, “High-Speed Compulsator Stator Thermal Management” at the 11th Electromagnetic Launch Technology Symposium, in Saint-Louis, France, May 2002. The stator armature of a compensated pulsed alternator, a compulsator, provides a particular challenge in thermal management. The stator armature is made from litz wire, which is a bundle of smaller wires twisted to minimize inductance and losses. The electrical insulation and composite structural support provide low equivalent thermal conductivity. So, active cooling is required. This paper presents the results of thermal analysis, using a finite element code, to predict the performance of various cooling approaches.

Residual Air Pressure in High-Speed Generators

Mike Werst, Jon Hahne, Hsing Liu, and Chuck Penny presented a paper, “Design and Testing of a High Speed Spin Test for Evaluating Pulse Alternator Windage Loss Effects” at the 11th Electromagnetic Launch Technology Symposium, in Saint-Louis, France, May 2002. As rotor tip speeds are increased to values in excess of 1000 m/s, it becomes increasingly important to be able to predict the losses in the gap between the rotor and the stator as a function of residual air pressure. This work combines experimental measurements with approximate theoretical approaches to develop an empirical loss model.

Energy Future

Bob Hebner was invited to present a talk titled “Energy Futures” to the University’s continuing education program Learning Activities for Mature Adults (LAMP) in May 2002. The talk focused on the global energy future, programs at the University of Texas at Austin to improve the energy situation, and electricity deregulation in Texas.