Homopolar Generator Research Articles

Microstructure/Processing Relationships in High-Energy High-Rate Consolidated Powder Composites of Nb-Stabilized Ti3Al + TiAl

ABSTRACTA new approach to powder processing is employed in forming titanium aluminide composites. The processing consists of internal heating of a customized powder blend by a fast electrical discharge of a homopolar generator. The high-energy high-rate “1MJ in 1s” pulse permits rapid heating of an electrically conducting powder mixture in a cold wall die. This short time at temperature approach offers the opportunity to control phase transformations and the degree of microstructural coarsening not readily possible with standard powder processing approaches. This paper describes the consolidation results of titanium aluminide-based powder composite materials. The focus of this study was the definition of microstructure/processing relationships for each of the composite constituents, first as monoliths and then in composite forms. Non-equilibrium phases present in rapidly solidified TiAl powders are transformed to metastable intermediates en route to the equilibrium gamma phase. The initial single phase beta in Nb-stabilized Ti3Al is transformed to alpha two with an intermediate beta two phase. In composite blends of TiAl powders mixed with Nb-stabilized Ti3Al powders a 10 μm thick interfacial layer is formed on the dispersed TiAl. Limited control of post-pulse heat extraction prevents full retention of the rapidly solidified powder microstructures.

High-Energy High-Rate Processing of High-Temperature Metal-Matrix Composites

AbstractAdvances in kinetic energy storage devices have opened up a new approach to powder processing of High Temperature Composites. The processing consists of internal heating of a customized powder blend by a fast electrical discharge of a homopolar generator. The high-energy high-rate “lMJ in 1s” pulse permits rapid heating of a conducting powder in a cold wall die. This short time at temperature approach offers the opportunity to control phase transformations and the degree of microstructural coarsening not readily possible using standard powder processing approaches. This paper will describe the consolidation results of two high temperature composite materials, (W-Ni-Fe)/B4C and (Ti3Al+Nb)/SiC. The focus of this study was the identification of the reaction products formed at the matrix/reinforcement interface as a function of input energy and applied stress. Input energies beyond a threshold value for each system were required to produce detectable reaction products. In the (W-Ni-Fe)/B4C system, the reaction products formed at 4000 kJ/kg input energy under 420 MPa applied stress were a series of complex carbides and borides including W2C, FeWB, Fe3C, Fe6W6C and Ni4B3. The intermetallic Fe7W6was also observed. In the (Ti3Al+Nb)/SiC system, the reaction products observed at 3400 kJ/kg and 210 MPa were TiC and TiSi2.

Applied superconductivity work at the Institute of Electrical Engineering, Academia Sinica

The superconducting magnet development work at the Institute of Electrical Engineering, Academia Sinica, is described. Progress with the main projects: high energy physics, fusion, MHD power generation, superconducting homopolar generator, magnets for scientific instruments, high field magnets and magnetic separation, is summarized. The near-future programmes are outlined.

Two laboratory experiments involving the homopolar generator

In these experiments numerical integration is used to verify Faraday’s law of induction for the case of an open-circuit homopolar generator. The computed emfs are compared with experimentally measured values.

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.

Design of a homopolar generator for 400 m/s slip ring velocity brush testing

Continued development of homopolar generator (HPG) technology by the Center for Electromechanics at the University of Texas at Austin (CEM-UT) will be aided by the installation of a 400 m/s slip ring velocity, controlled atmosphere, high current HPG for brush testing purposes. This HPG will be rated at 4.6 MJ, 36 V, 1 MA and have the capability of testing actively cooled brushes in varying environments. Inductive and resistive loads will be used for output current pulse shaping with an extensive battery of instrumentation and control equipment available for data collection and evaluation. Operating characteristics, brush testing envelope dimensions, and HPG design parameters are presented. Additional information on component designs and status of fabrication of the HPG is included.

Self excitation of iron core homopolar generators

In the interest of reducing homopolar generator (HPG) auxiliary requirements, a self-excited field coil for pulsed duty, iron-core HPG has been developed and tested at the Center for Electromechanics at The University of Texas at Austin (CEM-UT). In order to minimize rotor energy expended during excitation, a low-resistance, low-inductance coil was desired to allow field current to rise as rapidly as possible. A seven-turn field coil, having a nominal resistance of 500 µΩ was fabricated for the CEM-UT compact HPG system tester and subsequently tested. At 6,000 rpm, a field current rise time of 1 s was achieved and resulted in an average peak field density of 1.94 T. Only 300 kJ, about 13% of the 2.27 MJ stored in the rotor was required to fully excite the generator.

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.

Design and construction of a two-stage opening switch

Homopolar generators (HPGs) are modern day energy stores (megajoules) capable of large currents (megamps). The generators are typically low voltage and large capacitance, disallowing loads requiring fast rise times to be driven directly by the HPG. An opening switch must be provided to first, in the closed state, transfer energy from the high current source to an inductive energy store and then open to commutate the current to the load. To charge an inductive energy store with large currents for long times (hundreds of milliseconds) indicates a massive switch with many low resistance contacts. At the same time the switch must commutate current to the load in a short time interval (tens of microseconds) indicating a light, fast-acting device. A two-stage opening switch addresses these two conflicting requirements. The first stage is a massive mechanical switch of coaxial geometry with many low resistance galvanic contacts, which commutates through a low inductance to an explosive element second stage. The second-stage switch is then interrupted explosively to provide the microsecond commutation. The design, manufacture, and testing of the two-stage opening switch were performed at the Center for Electromechanics at The University of Texas at Austin (CEM-UT) Taylor Hall facility in Austin, TX, using a compact homopolar generator (CHPG) and a five-turn coaxial, cryogenic inductor as the pulsed power supply. Initial testing included mechanical actuations and electrical measurements to verify fundamental operation and proper assembly. Testing continued with the CHPG charging the inductor through the switch statically (no commutation) to characterize the current carrying capability of the primary galvanic contacts. Next, tests to check the ability of the first stage to commutate to the second stage by sliding the primary contacts onto the armature insulator were conducted. The fourth phase of testing was a series of two-stage (double) commutations to a fixed load to characterize performance and timing of triggers, voltage hold-off capability, and fixed load commutation times. The fifth and last phase of testing involved the two-stage switch commutating current to a variable impedance (railgun) load.

Final design of the control and auxiliary systems for the Balcones 60 MJ homopolar pulse power supply

The Center for Electromechanics at The University of Texas at Austin (CEM-UT) has completed the final design of a 60 megajoule (MJ) laboratory pulsed-power facility. Six identical 10-MJ homopolar generators are utilized in various series and/or parallel configurations to provide maximum output current and voltage ratings ranging from 1.5 MA at 660 V DC to 9.0 MA at 110 V DC. An overview of the guidelines and constraints governing the final design of this versatile system are presented. Topics include the homopolar generator terminal voltage matching via rotor speed and magnetic field control, control and hardware features, power supply configurations and output terminal ratings, failsafe fault protection, computer automated measurement and control (CAMAC) data recording system, radio frequency and electromagnetic interference (RFI/EMI) shielding considerations, signal processing, as well as the mechanical and electrical subsystems required for generator operation and protection.

Small caliber mobile EML

The Center for Electromechanics at the University of Texas at Austin (CEM-UT) is conducting a feasibility and conceptual design study of a vehicle mounted small bore railgun. The study is funded by the U.S. Army Armament Research and Development Center (Joint Services Small Arms Program), under subcontract to Battelle Columbus Labs. Details of the power supply feasibility study, including evaluations of batteries, capacitors, homopolar generators (HPG), and compensated pulsed alternators (compulsators or CPAs) are discussed. System parameter trade offs to be considered are total system weight, rate of fire, and complexity of required support auxiliaries. The system to be designed was initially a tactical railgun device of approximately 30 caliber to be capable of accelerating a 2.5-g projectile to near 1,500 m/s using a 1-m rail and solid armature. At this time, due to considerations which will be presented in this paper, this has evolved into a railgun system to accelerate a 20-g projectile to 2.5 km/s in a three shot burst or accelerate an 80-g projectile to 2.5 km/s in a single shot. The power supply selected for this system is an air-core compulsator weighing approximately 480 kg (1,060 Ib) which stores 7.3 MJ of inertial energy.

Superconducting field coil for high-voltage homopolar generator

A homopolar generator with 500 volts output is being designed and built as a pulsed power source. A key component used in achieving this high output voltage is the 5 tesla field coil. A superconducting coil is required for a compact configuration that operates above the saturation of iron. The coil case is 158 cm long and has an outside diameter of 137 cm with a 79 cm warm bore for installation of the generator rotor assembly. This large, intrinsically stable coil was designed to achieve a uniform magnetic field in the area of the rotor brushes. The coil was wound with 87.5 km of copper-stabilized niobium/titanium superconductor in an epoxy matrix. Fabrication of the coil and its containment cryostat is complete, and it is being used in the high-voltage homopolar generator test program.

Testing of a homopolar generator, energy storage inductor, opening-switch railgun system

The first homopolar-fired railgun system was operated in Canberra in 1972 [1,2]. Since that time two additional systems have been tested, the EMACK homopolar generator (HPG) built by Westinghouse and operated by Army Research and Development Center (ARDC) [3] and the joint effort by The Center for Electromechanics of The University of Texas (CEM-UT) and General Dynamics in 1982 [4]. This paper describes the homopolar railgun system operated by CEM-UT in 1985. The aspect that makes this system different from the previous experiments is the design of a fast opening switch (10 to 20 µs) that will allow operation of the HPG into hypervelocity launchers.

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.

Thermal design and development of actively cooled brushes for compact homopolar generators

The thermal and hydraulic design of actively cooled current transfer brushes for compact homopolar generators (HPG's) is described. The development of high-energy-density HPG'S at the Center for Electromechanics at the University of Texas at Austin (CEM/UT) requires brushgear capable of handling high current densities and large thermal loads. Platelet technology has been applied in the design of actively cooled brushes with coolant injection directly from the brush contact face into the brush-rotor interface. Coolant channels a few thousandths of an inch in diameter have been designed in brushes constructed of photoetched platelets of copper (0.020 to 0.005 in. thick) and then bonded to form the cooled brush. Platelet construction of brushes also permits the incorporation of internal instrumentation for temperature measurement. The brushes have been designed to provide data on the effects of rotor speed, current level, coolant flow rate, and coolant injection pattern. These brushes will establish the data baseline required for future cooled-brush designs.

Simulation and predicted performance of the Balcones GEDI experiment

The guided electromagnetic defense interceptor (GEDI) railgun launcher system is powered by six Balcones homopolar generators (BHPG), each of which stores 10 MJ in kinetic energy. Each BHPG is connected to a coaxial inductor which stores about half of the BHPG's energy when charged to 1.2 MA. Six two-stage opening switches open in sequence or simultaneously after current peak, directing the current to the railgun armature. As much as 7.2 MA can be delivered to the railgun. A computer simulation code is developed to perform design parameter studies, sensitivity studies, and to predict GEDI system behaviors. The code is general enough for analyzing any railgun driven by multi-HPG inductors coupled with two-stage opening switches. A Gear type, multistep algorithm is used to overcome stiffness of the differential system caused by the way the switches are modeled. GEDI system performances are predicted with the code based on most updated component parameters.

System design of the ultrahigh velocity GEDI experiment

System and component design for the ultrahigh velocity, guided electromagnetic defensive interceptor (GEDI) experiment is underway and initial fabrication has begun. Figure 1 is an artist's sketch of the experimental facility. The objective of the GEDI experiment is to accelerate a 1-g mass to 50 km/s. Prime power for the experiment will be supplied by the 60 MJ, 9-HA Balcones homopolar generator (BHPG) power supply located at the Center for Electromechanics at The University of Texas at Austin (CEM-UT). This power supply consists of six 10-MJ HPGs which can be connected in various series/parallel configurations or sequentially discharged for pulse shaping. Power conditioning consists of six room-temperature copper coaxial inductors, six two-stage opening switches, and six crowbar switches with a 1 m diameter × 40 m long vacuum chamber, and high speed diagnostics completing the system. The system which will be used to develop and test ultrahigh velocity electromagnetic accelerators (EMAs) was designed to be a versatile, easily maintained laboratory system. This power supply represents a substantial capability and can be used to drive a variety of experiments such as a large bore tactical-type railgun which is being designed to launch a 2-kg projectile at 3 km/s. Design parameters, component specifications, and program status are presented.

The installation and preliminary test programme of REMGUN III-a 6.7 MJ HPG driven railgun

REMGUN III is a railgun system built around a 6.7 MJ homopolar generator and is part of the RARDE research programme into electromagnetic launch systems. This paper reviews the installation of this system within a range complex, describes the associated range and gun instrumentation and gives the results of the initial sub-system and system trials.

A 10-MJ compact homopolar generator

The design and initial testing of a light-weight (5 kJ/kg) iron-cored homopolar generator is described. The machine employs an external power supply to motor up to operating speed (12,500 rpm) at which point 10-MJ of energy is stored in the all-steel rotor. Copper-graphite brushes in the stator, actuated by pneumatic actuators, make contact with the rotor surface and permit the inertial energy to be transferred to a load circuit at current levels up to 1.5 MA and voltages up to 60 V. A companion paper describes a 10-MJ cryogenic inductor which, with this machine, provides a 10-MJ power source suitable for electromagnetic launcher component testing.

Six megajoule rail gun test facility

CHECMATE, a high-energy rail gun user test facility, is currently operational at Maxwell Laboratories, Inc. in San Diego, California. Sponsored by the Strategic Defense Initiative Office (SDIO) and the Defense Nuclear Agency (DNA), it utilizes Maxwell's newly-developed high energy density capacitor technology. It thus represents the first rail gun facility where the high reliability, fast turnaround, and versatility of capacitor banks are combined with the multimegajoule energy previously available only with explosive flux compressors or large homopolar generators.