Missouri University of Science and Technology Scholars' Mine
UMR-MEC Conference on Energy
09 Oct 1975
Inexpensive Inertial Energy Storage Utilizing Homopolar Motor- Generators
W. F. Weldon
H. H. Woodson
H. G. Rylander
M. D. Driga
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Recommended Citation Weldon, W. F.; Woodson, H. H.; Rylander, H. G.; and Driga, M. D., "Inexpensive Inertial Energy Storage Utilizing Homopolar Motor-Generators" (1975). UMR-MEC Conference on Energy. 88. https://scholarsmine.mst.edu/umr-mec/88
This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in UMR-MEC Conference on Energy by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. INEXPENSIVE INERTIAL ENERGY STORAGE UTILIZING HOMOPOLAR MOTOR-GENERATORS
W.F. Weldon, H.H. Woodson, H.G. Rylander, M.D. Driga Energy Storage Group 167 Taylor Hall The University of Texas at Austin Austin, Texas 78712
Abstract
The pulsed power demands of the current generation of controlled thermonuclear fusion experiments have prompted a great interest in reliable, low cost, pulsed power systems. The Energy Storage Group at the University of Texas at Austin was created in response to this need and has worked for the past three years in developing inertial energy storage systems. 0.5 and 5 megajoule homopolar motor-generators have been designed, built and.tested at the University of Texas and design studies have been completed for several systems ranging in size up to 63 gigajoules. The performance of the two laboratory machines and the potential applications which have been investigated are discussed.
INTRODUCTION
Many processes in the scientific, industrial and the dead time and then deliver a high power, short military communities require relatively large duration pulse on demand. magnitude short duration pulses of electrical COMPARISON OF PULSED POWER SUPPLIES energy. Some such processes utilize pulsed power because it is not feasible or practical to produce A survey of pulsed power supplies in use in diff the extremely high power levels continously, others erent areas reveals a surprising diversity. because the pulsed power consuming process is only Functionally all such power supplies have two one of a sequence of operations resulting in "dead" features: the method for storing energy and the time between pulses, and still others because they mechanism for energy conversion and transfer. Due are inherently pulsed phenomena. Controlled ther to the magnitude of the power requirements for monuclear fusion experiments, lasers, resistance such systems, the cost per unit of stored energy welding, electrochemical processing, heat treating and the efficiency at which the energy is deliver and forming of metals, seismic wave generation, ed to the load are of prime importance. The dis down-hole formation fracturing, shock tunnel experi charge time (relative power level) required of the ments, and X-ray simulation experiments are all power supply is also an important factor both in examples of processes utilizing pulsed power. determining the proper system to use and the cost The periodic nature of most pulsed power require of the system. ments, that is regular pulses followed by "dead" Typical pulse power supplies include: times, makes energy storage systems attractive for (a) Capacitive Energy Storage these applications. Such systems can store energy (b) Inductive Energy Storage at a continuous, relatively low power rate during (c) Inertial Energy Storage (rotating elec-
316 trical machinery) 1948 - Progressive Welder Co., 7.0 volt, 50 KA 3 (d) Chemical Energy Storage (batteries) flash/spot welder. (e) Chemical Explosives 1949 - Electric Products Co., 400 KW, 18.7 Item (e) is included only as a matter of interest volt, 21 KA, homopolar power supply for since.it does not comply with the general notion of Carnegie Institute of Technology Syncro- 4 energy storage and is difficult to harness except Cyclotron magnet. in unusual applications such as liner implosions to 1952 - Allis Chalmers Mfg. Co., 100 KW, 4 volt, generate extremely high magnetic fields. Table 1 250 KA homopolar generator to power shows a detailed comparison of these various pulsed electromagnetic pumps for liquid metals.^ power supplies. 1956 - Atomic Energy Research Establishment, From Table 1 it becomes apparent that, especially 10 KW, 1.0 volt, 10 KA homopolar gener- . 6 in large sizes and for pulses of more than 1 msec, ator. duration, inertial energy storage utilizing homo- 1958 - General Elect. Co., 4 EA, 45 volt, 550 polar conversion, that is: systems in which the KA homopolar generators for arc dis flywheel is also the motor-generator armature, and charge wind tunnel at USAF Arnold Engr. very attractive in terms of size, cost, discharge Dev. Center.^ efficiency, and simplicity. In comparison, bat 1960 - Henschel Werke (Austria), 14 volt, 20 KA homopolar machine built for Techni- teries are inefficient, discharge slowly, and |# O generate explosive gases; inductive energy stores sche Hochschule. are inefficient unless superconducting windings are 1961 - N.A.S.A. Lewis Research Center, 4.5 MW, used, in which case they become more expensive and 37 volt, 300 KA homopolar supply for of questionable reliability. Capacitors are cap pulsed high field magnet.^ able of very rapid discharges, but their cost is 1962 - Australian National University, 500 Mj, high and because of their low energy density, 10 to 800 volt, 1.6 MA, pulsed homopolar 20 Mj per installation is a practical limit. power supply for air cored magnet.^ 1964 - University of Paris at Orsay, 1.5 Mj, HISTORICAL APPLICATIONS OF HOMOPOLAR MACHINES 1.1 MA, 7000 RPM, pulsed homopolar Although they may recently have been hailed as a power supply.^ new device to solve the problems associated with 1969 - International Research and Development providing pulsed power for fusion experiments, Co., 3250 hp. continuous duty homo- 12 homopolar generators have, in fact, been in use for polar motor for power plant pumping. some time. The following incomplete list serves to 1973 - University of Texas at Austin, 0.5 Mj, illustrate this point. i 17 volt, 14 KA, 6000 RPM pulsed homo- 13 1831 - Farady, M., first homopolar generator. polar motor-generator. 1906 - Westinghouse Elect. Co., 2000 KW, 260 1974 - University of Texas at Austin, 5 Mj , volt, 7.7 KA, 1200 RPM continuous duty 42 volt, 560 KA, 5680 RPM, pulsed 13 power supply for cement plant, used homopolar motor-generator. until conversion to purchased power in CURRENT PROGRAMS HOMOPOLAR MACHINES 1925.2 1934 - Westinghouse Elect. Co., 1125 KW, 7^ Although homopolar machines have been used as volt, 150 KA, 514 RPM resistance weld continuous duty devices in the past, all major ing supply for pipe manufacture for programs involve their use as pulsed power supp Youngstown Sheet and Tube Co. later lies with the exception of naval programs devel supplied currents up to 270 KA, still oping homopolar ship and submarine motors. 2 in use in 1956. The 500 Mj machine at the Australian National
317 Potential Energy Largest Existing Discharge System System Source Density Installation Times Efficiency Cost (Joule/cm3) (megajoules) mil 1iseconds) (%) ($/Joule
Inductive Energy 10 100 0.1 - 10 50 - 70 0.25 - 0.3 Storage (Frascatti)
Capacitive Energy 10 - 10 80 0.25 - 1 .0 Storage (Los Alamos) (Fast Bank) 25 (Lawrence Livermore (Slow Bank) Inertial Energy Storage (Motor-Flywheel-Alternator) 25 1400 103 + 40 0.015 - 0.04 (Garching) Inertial Energy Storage (Homopolar) 100 500 1 - 103 + 50 - 95 .001 - .01 (Canberra) Chemical Energy Storage (Batteries) 500 100 103 + .5 - 10 .03 - 3.0 (Frascatti) 1 1 CO * O Chemical Explosives 104 100 o 2 (Los Alamos)
Table 1 1 Comparison of Pulsed Power Supplies
♦See Text
318 University in Canberra is still in use and has erratic, operating torques experienced with the recently been used to power rail gun experiments rolling element bearings in the high magnetic field and intermediate inductive stores which in turn (See Fig. 1). 14 drive laser flash lamps. The 5 Mj machine at The 0.5 Mj machine self-motored to 6000 RPM in app Orsay, France has been superseded by a similar roximately 150 seconds with an armature supply 100 Mj unit which is still under development. Dr. current of 1000 amperes and produced a 7 second dis A. E. Robson at the Naval Research Laboratories charge pulse with a peak current in excess of 14,000 is developing a self excited 10 Mj homopolar amperes. This represents a current density of 723 generator in which the field coil acts as an amps/sq. cm. (4667 amps/sq. in.) under the brushes. intermediate inductive store, for use in liner Based on the success of the 0.5 Mj program, a 5 Mj implosion experiments.^ single rotor homopolar motor-generator was designed In 1972 the Energy Storage Group (ESG) was formed using the same basic concepts. Concurrent with at the University of Texas at Austin for the this effort a brush testing program was established purpose of developing low cost pulsed power to determine the brush materials most suitable for supplies primarily for use in controlled thermo use in pulsed homopolar motor generators. Figure 2 nuclear fusion experiments. Inertial energy shows a cutaway view of the 5 Mj homopolar motor- storage with homopolar conversion, in which the generator which was designed and built at the Univ flywheel acts as both the motor and generator ersity of Texas in a six month period during the armatures, was identified as the most promising spring and summer of 1974. candidate for this application. The accomplish The 5 Mj machine has a 61 cm. diameter, 28 cm. thick ments and current programs of the Energy Storage steel rotor supported by hydrostatic radial and Group are described in more detail in the follow axial bearings in a 1.6 Tesla magnetic field created ing section. by a room temperature, water cooled field coil. It, too, uses solid, metallic brushes, each being 2.5 THE ESG PROGRAM cm. square, with 108 brushes on the rotor and 36 on In the winter of 1972-73 a 0.5 Mj homopolar motor- the shafts (shaft brushes were later increased to generator was built using a 38 cm. diameter, 18 64). The individual brush interface pressure can cm. thick ferromagnetic steel rotor shrunk fit on a be externally controlled. The rotor bearings nonmagnetic stainless steel shaft. A water cooled, were made extremely stiff to allow detailed study room temperature, field coil produced a uniform of rotor dynamics during discharge. axial magnetic field of 16 kilogauss across the face The 5 Mj machine motors to 5680 RPM in approxi of the rotor. Unlike many homopolar devices, this mately 15 minutes with an armature supply current machine used solid (sintered copper-graphite) of 1200 amperes or in approximately 3 minutes brushes to collect the current from the rotor sur with a supply of 3000 amperes. It has been dis face. This can be accomplished at much higher charged from half speed (2800 RPM) into a short current densities than in conventional D.C. machines circuit (total circuit resistance of 34 microhms) since there is no commutation loss in a homopolar producing a 0.7 second pulse with a maximum cur machine. Also, the added friction due to solid rent of over 560,000 amperes. This represents a brushes is not of serious consequence in pulsed peak current of over 2300 amps/sq. cm. (15,000 operation where the brushes are in contact with the amps/sq. in.) under the shaft brushes. This rotor for only a short period of time. The machine generator has now undergone a year of extensive initially used rolling element bearings for both testing and will soon be operating in a repetit axial and radial location of the rotor, but the ive discharging mode using a room temperature radial bearings were later changed to hydrostatic load inductor to demonstrate the machine's dura- units and the axial bearings were placed on an ex bil ity. tended shaft to eliminate problems of very high,
319 The testing of the 5 Mj machine required the be necessary for some mobile applications. design and development of a 500,000 ampere making The work was supported by the U.S. Energy Research switch which was also accomplished by the Energy and Development Administration, the Electric Power Storage Group of the University of Texas at Research Institute, and the Texas Atomic Energy Austin. Additionally the group has conducted or Research Foundation. The authors and personnel of participated in design studies for several pulsed the Energy Storage Group wish to express their homopolar systems ranging in size up to 63 giga- gratitude to those agencies for making this work joules. The ESG is presently involved in the possible. design of three new homopolar systems: a 200 Mj,
400 volt homopolar power supply for a new Texas REFERENCES Tokamak, a 0.5 Mj, 200 volt machine intended to (1) Knoepfel, H., Pulsed High Magnetic Fields, explore the fundamental limits to discharge times North-Holland Pub. Co., Amsterdam, 1970, p. 3. of homopolar machines, and the reversible 22 KV, (2) Meyer, E. H., "The Unipolar Generator" Westing 63 Gj compression power supply for Los Alamos house Engineer, March, 1956, pp. 59-61. Scientific Laboratories' Reference Theta Pinch (3) Crawford, T. J., "Kinetic Energy Storage for Reactor. Resistance Welding", The Welding Engineer, These last two machines are expected to open an April, 1948, pp. 36-39. entirely new area of performance for pulsed homo- (4) "Homopolar Generator to Excite Synchro-Cyclo polar generators; that of discharge times in the tron at Carnegie Tech", Power Generation, Nov. 1 to 100 millisecond range. In the spring of 1949, p. 70. 1974 a theoretical program was established within (5) Young, E. L., "Development of Electromagnetic the ESG to determine the fundamental limitations Pumps Suitable for Pumping Liquid Metals", to discharge times of homopolar machines. For a USAEC file no. NP-3962, July 1, 1952. properly designed machine this limit has been (6) Watt, D. A., "10 KW Homopolar Generator with identified as the time required for the electro Mercury Brushes", IEE Journal, London, p. 233, magnetic wave to diffuse into the rotor and 1958. stator conductors. For practical sized machines (7) "Direct Current Without Commutation", Steel, this limit is around 1 millisecond. This problem V149, N24, p. 96-98, Dec. 1961 . has been treated in detail analytically^ and (8) Klaudy, P., "The Graz Homopolar Machine", detailed engineering designs are now underway for Proc. of Int'l Conf. on High Magnetic Fields, machines to operate in this regiem. M.I.T. , Nov. 1-4, 1961. In conclusion, our studies and experiments to date (9) Fakan, J. C., "The Homopolar Generator as an indicate that inertial energy storage using homo- Electromagnet Power Supply", Ibid. polar conversion is a practical, low cost alter (10) Hibbard, L. V., "The Canberra Homopolar Gen native for meeting the pulsed power needs of fusion erator", Atomic Energy in Australia, July, experiments. Initial studies indicate that the same 1962. technology can be applied to meeting the pulsed (11) Rioux, C., "Experiment of Pulsating Unipolar power requirements of industry and the military. Generator Without Ferromagnetic Materials", Units as large as 63 Gj, discharging in 30 msec, Sc. D. thesis, University of Paris, June, 1964. with 95% efficiency, do appear practical and for (12) Appleton, A. D., "Motors, Generators, and Flux special applications, units discharging in less than Pumps", Cryogenics, Vol. 9, No. 3, pp. 147-157, 1 msec, can be built. Of course such units require June, 1969. very high magnetic fields which can be most econom (13) Weldon, W. F., et. al, "The Design, Fabrication, ically produced with superconducting coils. New and Testing of a Five Megajoule Homopolar Motor brush materials and techniques must be developed to Generator", Int’l Conf. on Energy Storage, allow operation at very high speeds and current Compression, and Switching, Torino, Italy, densities for long periods of time. To date very Nov. 5-7, 1974. little work has been done on increasing the energy (14) Inall, E. K., "An Improved Method of Triggering to weight ratio of homopolar devices, but improve Flash Lamps Powered from an Energy Storage In ments of an order of magnitude or more do not appear ductor", Australian National University, Can unreasonable. This improvement, of course, would berra, Australia, 1975.
320 (15) Robson, A. E., et. al., "A Multi-Megajoule consulting assignments with more than 20 major Inertial-Inductive Energy Storage System", corporations and has 35 technical publications. Int'l. Conf. on Energy Storage, Compression, He is a registered professional engineer in the and Switching, Torino, Italy, Nov. 5-7, 1974. state of Texas. (16) Driga, M. D., et. al, "Fundamental Limitations William Forrest Weldon received his M.S. in 1970 and Topological Considerations for Fast Dis from the University of Texas at Austin. Mr. Weldon charge Homopolar Machines", IEEE Trans, on worked 5 years in industry involved in design and Plasma Science, December, 1975. development of mechanical and electromechanical de (17) Ortloff, D. J., "The Design of a Brush Test vices before joining the University of Texas at Machine", Masters' Thesis in Mechanicl Engin Austin as a research engineer in 1973. He has eering, The University of Texas at Austin, served as Chief Engineer of the Energy Storage January, 1975. Group during the testing and development of the 0.5 Mj. homopolar machine and the design, fabrication, BIOGRAPHIES assembly and testing of the 5 Mj. homopolar machine. He is a member of Pi Tau Sigma honorary fraternity Herbert H. Woodson received his Sc.D. degree in and is a registered professional engineer in the Electrical Engineering at MIT in 1956. Dr. Woodson state of Texas. taught at MIT from 1954 to 1971 where he was also Philip Sporn Professor of Energy Processing, 1967- Mircea D. Driga received his Dr. Eng. in Electrical 71, and Director, Electrical Power Systems Engin Engineering from Polytechnical Institute of Bucha eering Laboratory, 1968-71. Since 1971 he has rest, Romania in 1972. Dr. Driga has been a been associated with the University of Texas at lecturer in Electrical Engineering at the University Austin as Chairman of the Department of Electrical of Texas at Austin since 1973. For three years he Engineering, Alcoa Foundation Professor, 1971-75, has been a Fulbright scholar in engineering at Director, Center for Energy Studies, and Associate Ploiesti, Romania as an Associate Professor. Dr. Director, Fusion Research Center. He presently Driga has 6 years of experience in designing serves on the board of directors of 3 firms and electrical machines and holds 8 patents in the U.S. has acted as consultant to over 20 other major and abroad. firms. He is a member of the National Academy of Engineering, and Eta Kappa Nu, Tau Beta Pi, Sigma Xi, and Phi Kappa Phi honorary fraternities and is listed in Who's Who in America and American Men of Science.
Henry Grady Rylander received his Ph.D. in 1965 from Georgia Institute of Technology. Dr. Rylander has been a professor in Mechanical Engineering at the University of Texas at Austin since 1947. He has taught over twenty different courses in the mechanical systems group as well as supervising 24 master's theses and 6 doctoral dissertations. He has been honored by membership in Tau Beta Pi, Pi Tau Sigma, Sigma Xi, and Phi Kappa Phi. He received the Teacher's Excellence Fund Grant in i960, the Mobil Oil Foundation Grant in 1968, 1969, 1970, and 1973, and the Engineering Foundation Faculty Award in 1970 and 1973. He is listed in
American Men of Science and Who's Who in Engineer ing. Dr. Rylander has been involved in research and
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