J. H. Gully

Compact homopolar generator developed at CEM-UT

For electromagnetic launchers (EMLs) to become practical devices, they must evolve from laboratory test beds to field-portable systems. Such systems require the development of compact, lightweight, high-energy, high-current power supplies. Investigation of the candidate systems -- flux compressors, capacitors, inductors, batteries, and rotating machines -- showed the homopolar generator (HPG) to be a device with immediate potential for development. HPGs were selected because of their demonstrated ability to produce the high-energy, high-current electrical pulse required of an EML power supply from a relatively compact light-weight machine. By taking state-of-the-art HPG technology and integrating it with a machine designed specifically for high energy density, a field-portable HPG-powered EML system can be realized.

HPG operating experience at CEM-UT

The Center for Electromechanics at the University of Texas at Austin (CEM-UT) has developed iron-core homopolar generators (HPGs) as inexpensive, high energy density pulsed power supplies since 1973. Three generations of these machines have been built and tested - each new generation demonstrates higher energy and power densities. Until 1985, all three HPGs were operated in the CEM-UT laboratory for a variety of pulsed power experiments. Because of our recent move into a new facility, all the machines were disassembled, inspected and rebuilt. The original machine, a disk type, 10-MJ HPG was last rebuilt in May 1981. The second machine, the compact HPG (CHPG) has had its performance limits investigated with a cryogenic coaxial inductor as a load and the machine has been used as a power supply for opening switch experiments. The final machine, the HPG system tester, is now used as a full scale, high speed bearing and brush test facility. A summary of our operating experience with these machines is presented. Funding for the CHPG and HPG system tester was provided by the Defense Advanced Research Projects Agency and the U.S. Army Research and Development Center. Funding for the 10-MJ HPG has come from numerous industrial contracts.

The high voltage homopolar generator

A limitation of iron-core homopolar generators (HPG) is that the magnetic field strength and thus terminal voltage of the generator is dependent on the saturation limit of the material in the magnetic flux path. The Center for Electromechanics at The University of Texas at Austin (CEM-UT), in cooperation with GA Technologies, Inc. in San Diego, California, has designed and fabricated a 500 V, 500,000 A, 3.25 MJ, air-core pulsed homopolar generator. GA Technologies designed and constructed the 5 T, superconducting, solenoidal field coil. The stator subassembly, consisting of the rotor, bearings, stator, and output current conductors was designed and fabricated at CEM-UT. This experimental machine will be the first pulsed HPG with a superconducting field coil. Aspects of the machine design as well as the machine test program are discussed. Brushgear and bearing performance in high magnetic fields are also covered.

Fabrication of a compact storage inductor for railguns

A liquid nitrogen-cooled, coaxial, energy storage inductor has been designed and built to be used in conjunction with a compact homopolar generator to form a high-energy-density power supply for use with electro-magnetic accelerators. The low-resistance, lightweight aluminum inductor stores 3.1 MJ at a peak current of 1.0 MA. Minimizing weight rather than size was emphasized in the design, resulting in a 1.23-m (48.5-in.) diameter by 0.91 m (36 in.) long inductor weighing 14.7 kN (3,300 lb). A coaxial design was chosen to eliminate high external magnetic fields without the necessity for shielding. External magnetic fields are undesirable because of effects on nearby components and the possibility of detection. Also, attention has been given to minimizing the partial flux linkages or internal inductance of the coil, thereby maximizing the overall transfer efficiency into a railgun. Details of the design, fabrication, and predicted performance will be presented.

Effect of temperature on wear rate of homopolar pulse consolidated electrical brushes

Abstract Binderless copper-graphite composite electrical brushes are being developed using a high-energy, high-rate pulse sintering technique by the Center for Electromechanics at The University of Texas at Austin (CEM-UT). Experiments were done to investigate temperatures effect on the homopolar pulse consolidated (HPC) brush wear rate for an apparent brush current density of 180 A cm −2 , a brush downforce of 44.5 N, and rotor surface sliding speeds of 10 m s −1 and 40 m s −1 . At a sliding speed of 10 m s −1 , it was found that brush wear rate dropped steeply as the brush bulk temperature increased from 80 °C to 103 °C. Other than this unusual wear finding in this particular sliding speed and temperature range, test results indicated that brush wear rate generally increased with increasing brush bulk temperature. At a sliding speed of 40 m s −1 , it was found that brush wear rate suddenly increased by several times as the brush bulk temperature approached 149 °C. In the case of 10 m s −1 sliding speed, no stepwise rise in brush wear rate was observed even as the brush bulk temperature reached 156 °C.

Improved energy density homopolar generator

The preliminary design of a self excited, air-core (SEAC) homopolar generator (HPG) which stores about 250 MJ inertially and is4 capable of delivering 3.2 MA current pulses is presented. In aiming for maximum energy density in an HPG and inductor power supply for electromagnetic (EM) accelerators, the improved energy density (IED) machine uses its self-excited field coils as energy storage inductors and a lightweight graphite reinforced flywheel for inertial energy storage. Weighing approximately 5,000 kg, the design represents a twenty-fold increase in mass energy density over the state of the art and addresses the problem of trapping flux in the rotor during discharge by separating the voltage generating and energy storage functions. Voltage is generated across a squirrel-cage rotor armature by an opposed pair of five-turn cryogenically cooled field Coils/inductors. Inertial energy is stored in a graphite-reinforced epoxy flywheel which will operate at a maximum tip speed of 1,100 m/s. Current collection is accomplished at the smaller radius of the squirrel-cage armature which implies brush slip speeds of no more than 300 m/s at the design speed. The machine is expected to develop about 500 V at half speed while charging the coils to 130 MJ at 3.2 MA. Peak output voltage during discharge of coils will be roughly 10 kV.