Wear, 59 (1980) 259 - 276 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

259

RECENT ADVANCES INELECTRICALCURRENTCOLLECTION*

I. R. McNAB

Tribology and Magnetofluidynamics, Electrotechnology Department, Westinghouse R &D Center, Pittsburgh, Pa. 15235 (U.S.A.)

(Received November 13,1979)

Summary

During the last few years several advanced rotating and linear electrical machine concepts have been developed which depend for their success on the efficient transfer of electrical current across sliding interfaces. The steady state operation of brushes at very high current densities (up to 7.75 MA mT2) is the goal for advanced land or sea propulsion units. Even higher cur- rent densities (above 18 MA mm2) and high sliding speeds (300 m s-l) are re- quired for the subsecond operation of inertial storage pulsed power sources. Speeds and current densities up to an order of magnitude higher, but for millisecond pulses, may be necessary for the linear electromagnetic accelera- tion of projectiles. Fundamental investigations and laboratory demonstra- tions of several forms of advanced current collectors for these applications are described in this paper. Monolithic brushes, metal fiber brushes and carbon fiber brushes are mentioned.

l.Background

The requirement to transfer current from moving metal surfaces to stationary conductors is essential for many electrical machines. It is there- fore appropriate that the history of current collection goes back to the earliest days of electrical research. In 1832 Michael Faraday, while working on the fundamental laws of electromagnetic induction, used copper rods wetted with mercury to collect current from a metal disc which was being rotated in a magnetic field. One might mention in passing that this configura- tion was basically that of the homopolar or acyclic generator in which there has been a resurgence of interest in recent years as will be described later.

*Presented at the International Workshop on Thermoelastic Instability, Annapolis, Maryland, June 1979.

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During the subsequent growth of the electrical industry in the mid- nineteenth century current collectors were usually made from bundles of copper wire or foil. Indeed this gave rise to the expression which is still in common use today for a current collector, i.e. a brush, because that is what they actually resembled. However, high temperatures, high friction and high wear characterized the use of such brushes and as the design of electrical machinery progressed it became apparent that machine performance was being limited by this type of collector.

The introduction of graphite as a brush material was first suggested by a Professor Forbes in England in 1883 for use on homopolar machines but it was not particularly successful. However, some six years later a carbon block was tried again by van Depeole, one of Elihu Thomson’s colleagues, this time for traction motors on rail cars. In this case, which involved commutation, the high resistance of carbon proved to be very successful. Subsequently car- bon and graphite brushes became widely used and laid the foundation for the so-called “carbon” brush industry which exists today.

Initially a block of solid carbon was used as the brush material, but in subsequent years natural or electrographite were used to enhance the lubri- cating properties of the brush. Later, metal was also added to improve the current-carrying capability and a wide variety of different grades are now manufactured for many slip-ring and commutator applications.

Some considerations in conventional brush practice include whether to use monoblock or .multi-piece brush construction, the requirements for flex- ible connections to transfer current to the stator conductors, the necessity for spring loading to maintain the required contact pressure and the impor- tance of constraining the brush mechanically in a way which will not permit instabilities or resonant vibrations to be excited. All these are essential for good current collection. Slip rings for power machines are generally manu- factured from copper or copper. alloys or steel although special surfaces and surface treatments are also used. Slip rings may be smooth or (for high speed operation) helically grooved to reduce selectivity; commutators may have flush or undercut insulation usually of mica or a mica-based material.

Most machines operate in air and under these conditions a film is formed on the slip ring or commutator. In the case of a copper ring this film is composed of cuprous oxide (CuzO) together with some metal and carbon or graphite pick-up from brush wear debris. The condition of this film is often critical for good brush operation. A considerable body of empirical evidence relates to film formation and guidance is available from brush manufacturers on the rectification of the causes of poor filming behavior.

Applications for conventional brushes are almost too numerous to men- tion, ranging from very large industrial machines, traction and automobile applications to fractional horsepower applications of the type often found in households, even down to electric toothbrushes.

In recent years there has been a renewal of interest in sophisticated ver- sions of Faraday’s original homopolar generator. The main features of such a machine are typified in the brush test machine shown in Fig. 1. The essential

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Segmented Excitation Magnet

MachineTerminal Leads

Stator Conductor Drum

-Water Outlet

Solid Brush Current Collection Arm 1

- Cover Gas Inlet

Brush Area Service Covers

Fig. 1. Homopolar machine used for brush testing.

features are a current-carrying segmented excitation magnet coil which pro- duces a magnetic field and a rotor conductor drum which moves through this magnetic field and thus generates a voltage. An appropriately located brush current-collection area allows the generated voltage to be tapped and the resulting current to be fed into an external load.

The initial impetus for the development of these homopolar machines has come from the application to naval propulsion systems which use gas turbine prime movers. Most naval ships, while requiring the capability for full-speed operation, actually spend most of their time at half-speed or less which corresponds to less than one-eighth of the maximum power. Since part-load operation of gas turbines is rather inefficient, a direct drive through gears significantly penalizes the fuel utilization.

An electric drive system of the type illustrated in Fig. 2, in which switching would permit from one to four turbogenerators each operating at optimum efficiency to feed both motors, appears to offer appreciable operating flexibility and economic advantages.

In addition, however, several other possible applications are foreseen for homopolar machines, both for continuous and pulsed power, as discussed in Section 6. Among these, the production of very high power pulses (mega- joules to gigajoules) for periods from milliseconds to seconds for devices such as electromagnetic launchers is of special interest.

The common feature of all these advanced homopolar generators is that they can only be successful if a brush technology can be developed which is well in advance of conventional practice. To support these developments a research and development program has been funded at the Westinghouse R & D Center by the Materials Science Office of the Advanced Research Projects Agency of the U.S. Department of Defense and monitored by the Office of Naval Research. This program has had the following objectives:

(1) the achievement of an improved understanding of the fundamental aspects of current transfer across sliding interfaces;

262

SW-8

Fig. 2. Four-turbogenerator dual motor electric ship propulsion system showing the switching arrangements (Sl - S16).

(2) the development of improved monolithic brush materials for high current density applications;

(3) the development of innovative techniques for improved current transfer;

(4) application-related studies in the areas of (a) vehicle propulsion, (b) inertial storage pulsed power generators and (c) electromagnetic projec- tile launchers.

This program has involveduniversity and industry collaboration in the several interdisciplinary fields which play an important role in these devel- opments. Some of the highlights of these studies are described in Sections 3 - 6; however, prior to that the present understanding of current transfer is summarized.

2. Physics of current transfer

As with any two real solid surfaces in contact, the presence of micro- scopic surface roughness on both the brush and the slip ring or commutator causes the real area of mechanical contact to be very much smaller than the apparent contact area. If the bodies were completely rigid only three points of contact, each of infinitesimal size, would be present. In practice elastic or plastic deformation occurs and the real area of contact may be only l/1000 or l/10 000th of the apparent area. As well as can be estimated some 10 - 50 points of mechanical contact occur at a brush/slip-ring inter- face under normal conditions, but particular characteristics of brush ma- terials such as differences in surface texture may affect this.

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A significant film may be formed on the sliding surface. Oxides are present on copper together with mechanically embedded graphite particles and brush and slip-ring wear debris. Since such films are basically insulating, the local electric field which is present between the brush and slip ring is sufficient to cause electromigration and electrical breakdown within small portions of the mechanical contact areas. Such areas of electrical contact are called “alpha spots”.

In addition to the solid film adsorbed films of gases and vapors are also present. Even though these may be only monolayers thick they can have a marked effect on friction and brush wear. Their absence can lead to cata- strophic brush wear, called dusting, because strong adhesive bonds can devel- op on the unsaturated edge sites of the graphite crystallites.

The necessity for all the current to pass through the alpha spots intro- duces a resistance to the current flow which is called the constriction resis- tance. In addition to this conduction has also to occur through the adsorbed films which are present on the alpha spots. The total resistance therefore includes both of these contributions.

To maintain the film portion of this loss at a low value we have found it to be greatly beneficial to operate in non-oxidizing atmospheres as de- scribed later. We expect this to be most advantageous for all advanced brush systems.

The concentration of current at the alpha spots causes high local tem- peratures to occur. These are often called supertemperatures. In symmetric contacts the maximum temperature occurs at the interface, but in an asym- metric contact the maximum temperature may occur in the brush itself because of the lower electrical and thermal conductivity there.

Heat is also generated at the contact surface as a result of friction during sliding. The total heat generation is proportional to the friction coefficient, the normal force and the relative speed of the two surfaces. At low speed the relative heat inputs to the brush and slip ring from this source vary with their thermal conductivities. However, at high speed the incoming slip ring is cooler than the brush and an increasing fraction of the heat goes into a thin layer on the surface of the ring. Phenomena such as thermoelastic in- stabilities may modify this distribution by favoring particular alpha spots.

The value of the friction coefficient is determined by the characteristics of the film on the areas of mechanical contact. This is affected not only by the characteristics of the solid film but by the adsorption of gases and vapors on the basal and edge sites of the graphite crystallites. In particular, adsorbed hydrocarbon vapors have been shown to be particularly effective in provid- ing lubrication even at very low concentrations. Experimental work on brush contact resistance with a variety of adsorbed long-chain hydrocarbon vapor films has been undertaken by Lee and Johnson [l] .

3. Recent fundamental investigations

The fundamental investigations undertaken in the DARPA advanced current-collection program have centered on the analysis and evaluation of

264

Fig. 3. Brush wear particle adhering to slip-ring surface.

the nature of the brush/slip-ring interface. It is at this interface, or in the close vicinity, where most of the “action” takes place in the sliding process - friction, wear and current transfer. It is also this interface that has tra- ditionally been most difficult to study using macroscopic techniques. In this program modem diagnostic techniques have been used to provide a new look at the interface region and at the processes which are taking place. Thus at Westinghouse and in subcontracted work at Syracuse University scanning electron microscopy (SEM) and Auger spectroscopy techniques have been employed to study thick and thin films formed on slip-ring sur- faces and brushes. Initial experiments were undertaken on specimens which had been operated in standard brush-testing machines. However, more recent- ly in situ measurements have been made inside the Auger and SEM chambers using specially developed equipment. With this equipment slip rings can be rotated at low speeds within the high vacuum of these instruments, and the build-up of surface films or modification of the slip-ring surfaces can be examined as they occur.

Several important observations have resulted from this work. Research at Syracuse [2] has shown that at the elevated temperatures that are likely to occur on a transient basis at the contact spots the surface composition of the copper slip ring may differ significantly from that which would be ex- pected. Thus significant amounts of carbon may be present on the slip-ring surface in certain cases. Increasing the temperature causes the amount of carbon and other elements to change. In addition, changing the base material from OFHC copper to silver-bearing copper or 99.9999% pure copper may cause significant surface changes. These results are discussed further in ref. 2. Additional study is required to establish the significance of the observed ma- terial segregation to the electrical and mechanical properties of the contact interface.

265

i r CU

x0.4

I I 1 I

0 0.1 0.2 0.3 0.4 0.5 0.9 E lkeVi

Fig. 4. Auger spectrum of adsorbed film on copper showing unusual features (two arrows).

Work at Westinghouse [ 31 has concerned both thick- and thin-film development. The thick films (typically of the order of micrometers) which result from the bulk transfer of brush material to the reacted slip ring sur- face appear to be mainly responsible for the brush wear. Direct evidence for strongly adherent macroscopic brush particles on the slip ring surface has been obtained (Fig. 3). The thin films (less than about 3 nm) which form continuously on all solid surfaces exposed to gaseous atmospheres have been analyzed in detail using Auger techniques.

An adsorbed thin film of water vapor on the surface of a copper slip ring appears to be essential for the successful low-friction operation of both multi-element and monolithic brushes. This film appears to have little effect on the conduction mechanism, either because it is periodically disrupted and re-formed as asperities on the brush and slip ring contact each other or be- cause it is so thin that effective tunneling can occur without significant elec- trical losses. Unusual features of the Auger spectrum of oxygen on copper surfaces have been found (Fig. 4) and are thought to provide a clue to the unique properties of the water film. It is thought that Cu,O* Ha0 complexes (Fig. 5) may be formed at the metal surfaces exposed to humid atmospheres. Such a film should have excellent frictional properties (as observed) since the water molecules are weakly bound and can shear easily. The compressibil- ity will be characteristic of a typical liquid layer while separating the contact surfaces by only 2 - 3 nm, so that conduction can take place with low losses (also as observed). Further exploration of this hypothesis using surface techniques which are sensitive to the presence of hydrogen are required. A more detailed description of the techniques and observations is included in ref. 3.

266

1 ATMOSPHERE ]

Fig. 5. Possible CU~O-H,O complex on the slip-ring surface.

Analytical studies have also been used to establish a physical model of the brush contact. Thermoelastic deformations at the contact interface were studied at Northwestern University. A possible unfavorable effect of cool- ing the brush holder on the uniformity of the contact surface is described in ref. 4. A study of the effect of rotor eccentricity on the wear pattern of the brush and the resulting nature of the mechanical and electrical con- tact has been conducted at Westinghouse. These results are being used to optimize new multicomponent brush designs.

4. Monolithic brush materials

During the earlier portion of the DARPA-funded program greatest emphasis was laid on research, development and testing of monolithic brush materials. Generally, these materials are mixtures of graphite, carbon and a metal, the latter constituent varying from zero to almost 100 wt.%. The production is generally by a powder metallurgy process in which small particles of graphite, carbon and metal (usually copper or silver) are pressed and sintered to form a solid block. An alternate production route which offers improved conduction is through the vacuum impregnation of a porous graphite block with molten metal. However, typical graphite matrix pore volumes limit the metal content of these materials to about 55 wt.%.

A comprehensive evaluation was made of the effect of the metal con- tent of conventional smtered metal-graphite materials on the important parameters of brush performance. The results of this experimental program have been presented elsewhere [ 51. Figure 6 shows the wear performance of silver-graphite materials as a function of the mass fraction of silver present in the matrix.

These initial experiments were undertaken at a current density of 0.78 MA mB2 (500 A inm2) which at that time was a significant increase over con- ventional operating practice (50 - 100 A inF2). However, optimization of materials and operating conditions has provided even more significant ad-

267

10&----J 0 0.2 0.4 0. 6 0. 8 1.0

Mass Fraction d Silver

Fig. 6. Wear of silver-graphite brushes running on a copper slip ring in a humidifi_d CO2 atmosphere: apparent brush surface area, 1 cm2; ring peripheral speed, 12.7 m s .

vances in achievable current densities. In this optimization a multidimen- sional matrix of environmental gases, vapor additives, brush material com- positions, ring materials and operating temperatures has been investigated. A significant achievement has been the operation at a current density of 1.6 MA mV2 (1000 A inm2) of a multibrush (92) machine environment tester of the type shown in Fig. 1 (see ref. 6). Recently, current densities as high as 3.1 MA mW2 have been achieved under simulated machine con- ditions in the same multibrush tester. Most recently, steady state experi- ments have been undertaken on small bench testers in carefully controlled environments at current densities up to 7.75 MA mm2 (5000 A inm2). The encouraging result of these experiments is the finding that both the friction coefficient and the contact resistance apparently decrease with increasing current density beyond about 3.9 MA rnq2 (2500 A inm2) although the con- duction process becomes less stable. Further work is required to establish in more detail the processes involved in these very high current regimes. It is possible that local melting may occur at the transient contact spot asper- ities and that this is responsible for the observed behavior.

A new monolithic material which was developed as part of the DARPA current-collection research program consists of pressed and sintered metal- coated graphite particles of the type shown in Fig. 7. The metal content is controlled by the thickness of the particle coating. Other options available include the addition of elemental graphite and/or metal powders to the basic

268

(a)

Fig. 7. Silver-coated graphite particles: (a) magnification 33 X ; (b) magnification 132 x

Cu-Graphite

0 Commercial Product

1 This Work

Metal Content. ~4%

Fig. 8. Electrical resistivity of copper-graphite brush materials.

metal-coated graphite powder. Efficient use of metal, i.e. solid metal phase throughout, permits lower bulk electrical resistivities to be achieved through- out the metal content range up to nearly 100 wt.% as shown in Fig. 8. A description of the fabrication technique and resistivity measurements for the new materials using silver- and copper-plated powders is given in ref. 7. Brushes made from these materials performed best when natural graphite powder was used, although brushes made from synthetic and oleophilic graphites were also tested. The material demonstrated improved electrical performance relative to conventional metal-graphite brushes with the same

269

metal content; further development is underway to increase its mechanical strength.

In another materials study oleophilic graphite and silver powders were successfully pressed and sintered into solid blocks from which test brushes were produced. Brushes were also produced from composite materials of silver and selected dichalcogenide-type solid lubricants. The dichalcogenides were molybdenum disulfide and niobium diselenide. These lubricants were used either alone with silver or as a mixture with natural graphite and silver.

At the Massachusetts Institute of Technology an experimental study evaluated the performance of a silver-graphite pin material on discs of various noble and rare metals. Friction, wear and electrical resistance were measured and the results are in refs. 8 and 9. Low wear was found to correlate with metallurgical incompatibility of the disc metal with the graphite rather than with the silver of the pin material. Another unexpected result compared with earlier studies [lo] was that high wear rate was associated with lower values of friction. Low wear was found for rhodium, palladium and platinum. This work and the studies at Westinghouse described earlier provide a com- prehensive background on monolithic brush materials.

5. Multi-element brushes

Early in the study of the fundamental features of current transfer across interfaces it was apparent that, excluding body and circuit resistance, the contact resistance was composed of the constriction and film resistances. A major result deriving from this is that when the constriction resistance component is dominant the contact resistance can be reduced by subdivid- ing the contact into a larger number of independently loaded units.

The first brush of this type to be investigated which was based on earlier experience [ 111 was the metal-plated carbon fiber brush in which lo5 - lo6 individual fibers per square centimeter are used to contact the slip ring surface. The technology for producing individually plated fibers in relatively large quantities (millions of fiber meters) and in assembling the fibers into brush units was developed. With these techniques fiber brushes with area packing factors of 0.25 - 0.75 or more can be fabricated. The reduced constriction resistance of the fiber brush and its improved ability to follow slip-ring surface irregularities permit the use of lighter contact loads than with monolithic brushes (typically 7 kPa (1 lbf inw2)) and thus reduces frictional as well as electrical contact power loss.

Brushes of metal-plated carbon fibers of diameter 10 pm have been tested in air and in helium over a wide range of humidity levels [ 121. Stable brush operation was achieved at current densities up to 3.9 MA mm2 with short-term tests limited by inadequate cooling to 4.65 MA rns2 (3000 A ins2). Total power losses (frictional plus electrical) as low as 0.2 W A-’ per brush were measured (see Fig. 9) with brush life typically 1000 - 3000 h cm-‘. Further information is given in ref. 12.

I I I I

50 100 !%I laxl xna Humidity (PPM)

Fig. 9. Metal-plated carbon fiber brush loss us. humidity in air environment: 1.55 MA mP2, 15 m 6-l and silver ring.

;; t o.2 k,=l,~kl=1/2

t

i

0

1 ill II+ u? Id ,"5 ,"6 ,d __ __

Fig. 10. Effect of brush subdivision on contact resistance (constant total force).

Despite these promising results recent work has offered even more attractive routes for the development of multi-element brushes. This effort has centered around a general analysis of the total contact resistance for multi-element brushes which operate in the plastic or elastic asperity de- formation mode. This analysis has shown that the benefits to be gained by increasing the number of individual brush contacts accrue relatively rapidly so that it is likely to be unnecessary in most circumstances to have more than lo2 - 10s individual contacts (see Fig. 10). Based on this evaluation, brushes made of multiple metal fibers have been designed, built and tested under a range of operating conditions (Fig. 11). The results obtained have been outstanding, including operation at high current densities (7.75 MA mm2) with low losses (less than 0.25 W A-l) and low dimensionless wear (less than 10-l’). Recent results with small brush units have shown that cur- rent densities up to 100 MA mm2 (65 kA inP2) can be stably transferred through individual elements of fiber bundles at sliding speeds of 15 m s-l. These current density levels are remarkably high and are more typical of

271

Fig. 11. Metal fiber brush.

values found in solid superconductors than in sliding electrical contacts. A description of the metal-fiber brush configurations, the initial test results and the analytical model applicable to multi-element metallic contacts are included in ref. 13.

Subdivided monolithic brushes have also demonstrated the capability of carrying very high current density, although at higher mechanical loads and wear rates. An experimental study is presented in ref. 14 in which a silver-graphite brush was tested with segments ranging from 9 to 45 mm2 in cross-sectional area. The study demonstrates the more complex nature of the contact interface model that is required when a metal-graphite film is formed on the slip ring.

A major feature of the experiments that have been undertaken at Westinghouse is the finding that in controlled environments low dimension- less wear (less than 10-l’) and low friction (less than 0.5) can be achieved by metal fibers sliding on metal surfaces. It is this result which taken in conjunction with the very high current-carrying capability offers the pros- pect for an extremely effective high current brush.

6. Applications

In Section 1 the possible use of high current density brushes for electric ship propulsion machinery was mentioned. Two additional areas of interest are discussed below.

6.1. Pulsed power sources During the last one to two years it has become apparent that a signif-

icant number of applications may exist for pulsed power sources. These applications include electroforming, resistance welding, electromagnetic launch- ers (see Section 6.2) and high power supplies for laser fusion experiments.

’ l 75 m/s, 35 ms Pulse

0 150 m/s, 40 ms Pulse

o 225 m/s, 60 ms Pulse

n 277 m/s, 55 ms Pulse

Current i?r Brush (A) 600

Fig. 12. High speed pulsed brush test results.

One of the most effective techniques available for storing large quan- tities of energy is inertial storage in high speed rotors. Thus a cubic meter of steel fashioned into a cylindrical rotor moving with a peripheral velocity of 300 m s-l has astored energy of 175 MJ. Release of this energy in 0.25 s will therefore provide an instantaneous power level of 700 MW. In many cases it is difficult to see how such a power level could be achieved in any other way. The key to extracting this inertially stored energy and transform- ing it into useful electric energy-lies in the development of an effective, efficient and high current density brush technology.

Up to 1978 data on pulsed brush operation was relatively sparse [ 15, 161, but advances in this area have recently been made during the develop- ment and testing of pulsed brush systems for a homopolar energy transfer system for a fusion reactor experiment [ 171. In 1978 a subdivided brush concept was successfully tested at current densities up to 17 MA rnw2 (11 kA inm2) at speeds from zero up to 277 m s-l (see Fig. 12). In line with the application, pulse durations of 0.030 - 0.065 s were employed with pulsed currents up to 14 kA (see ref. 18 for further details).

Major objectives of future development are to increase the operating current density, to increase the total current and to investigate the influence of collector thermal shock on the design of high speed rotors. Operational constraints for pulsed homopolar machines, especially size and weight, necessitate very high sliding speeds. Under these conditions with simulta- neous current transfer new regimes of physical phenomena (such as thermo- elastic instabilities) are expected to dominate the contact resistance, fric- tion and transfer processes.

213

6.2. Electromagnetic launchers During recent years interest in the use of pulsed d.c. electromagnetic

techniques to launch macroparticles to high velocities has grown considerably. This has largely occurred as a result of the work of Barber [ 191 and Marshall [ 201 and their colleagues at the Australian National University in Canberra, and work at Westinghouse on advanced current collection, homopolar gen- erators and energy storage coils. Earlier attempts to accelerate solid objects to high velocities date back to the 1920s and several attempts were made during World War II. However, relatively little success was achieved although it was demonstrated that highly ionized plasmas could be accelerated to velocities in excess of 100 km s- ’ for space propulsion applications [21] . In this area plasma masses were in the microgram range, whereas Barber and Marshall demonstrated that objects of up to a few grams could be accelerated to velocities near to 6000 m s-l.

The success of these experiments has led to several developments in this field including an evaluation of potential applications for such technology. This could include applications as diverse as the low speed (10 - 100 m s-l) launching of aircraft or the attainment of velocities so high (above 100 km s-l) that impact fusion may be possible. Intermediate in this range, but beyond the capabilities of rotating machinery, are sliding speeds of 1000 - 3000 m s-l with the simultaneous transfer of very high currents in the range 10 kA - 1 MA or more.

Recently a small laboratory system of this type (ELF-l) has been as- sembled. This comprises a capacitive power supply (ultimately replaceable by a homopolar inertial storage system), an inductive storage coil, switching mechanisms, a launcher barrel and projectile, and a down-range diagnostic and catcher section. A view of the 2 m long barrel during assembly is shown in Fig. 13. With this facility it is hoped that ultra-high pulsed current transfer experiments can be made on a variety of rail materials at sliding speeds in excess of 1000 m s-l. At these high sliding speeds even small rail surface irregularities can result in high inertial forces at the contact. In addition, exceptionally high electromagnetic forces also make control of contact force more difficult. With the short current pulse duration of typical pro- jectile launchers current flow in the rail is limited to the skin depth. This concentrates the current at the trailing edge and may prevent the effective utilization of the full contact area. The non-destructive recovery of the projectile will be attempted so that the sliding surfaces can be examined and flash X-ray techniques will be employed to examine in-bore stability and projectile integrity.

7. Conclusions

Major scientific and technical advances have been made in the present DARPA-funded advanced current-collection program. Highlights of the program include the following:

Fig. 13. Linear electromagnetic launcher under assembly.

(1) the steady state operation of optimized monolithic brush materials at 7.75 MA rnw2 (5000 A inF2);

(2) the.steady state operation of multi-element brushes with low losses (less than 0.25 W A-l) and low dimensionless wear (greater than 10V1’) at 7.75 MA me2 (5000 A inw2);

(3) the operation of multi-element brush units at very high current densities, i.e. 100 MA mm2 (65 000 A inF2);

(4) the development of new monolithic brush materials having enhanced electrical conduction;

(5) the development and use of in situ techniques for Auger spectroscopy and SEM analysis of slip-ring film composition;

(6) the demonstration of environmental control impact on brush be- havior;

(7) the development of new concepts important to the understanding and ability to model sliding electrical contacts;

(8) the identification of technological areas which will benefit from these advances.

275

Acknowledgments

The work reported here would not have been possible without (1) the financial support provided by the Materials Science Office of the Advanced Research Projects Agency under Contract N00014-79-C-0110 monitored by the Office of Naval Research and (2) the outstanding scientific and technical commitment of the members of the joint university-industry team who were involved in this work and whose contributions are, in part, cited in this paper.

References

1

2

3

4

5

c:

7

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10

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P. K. Lee and J. L. Johnson, High current brushes II - effects of gases and hydrocar- bon vapors, IEEE Trans. Components, Hybrids Manuf. Technol., 1 (1978) 40 - 45. R. W. Vook, B. Singh, E. A. Knabbe, J. H. Ho and D. K. Bhavsar, Elemental surface composition of slip ring copper as a function of temperature, Electrical Contacts 1979, Proc. 25th Holm Conf. on Electrical Contacts, Chicago, Sept. 10 - 12, 1979, Illinois Institute of Technology, Chicago, 1979, pp. 17 - 21. J. Schreurs, J. L. Johnson and I. R. McNab, High current brushes, Part VI: Evalua- tion of slip ring surface films, Electrical Contacts 1979, Proc 25th Holm Conf. on Electrical Contacts, Chicago, Sept. 10 - 12, 1979, Illinois Institute of Technology, Chicago, 1979, pp. 145 - 151. C. P. Chen and R. A. Burton, Tbermoelastic effects in brushes with high current and high sliding speeds, Electrical Contacts 1978, Proc. 24th Holm Conf. on Electrical Contacts, Sept. 11 - 15,1978, Illinois Institute of Technology, Chicago, pp. 571 - 575. I. R. McNab and J. L. Johnson, High current brushes, Part III: Performance evalua- tion for sintered silver graphite grades, Electrical Contacts 1978, Proc. 24th Holm Conf. on Electrical Contacts, Sept. 11 - 15, 1978, Illinois Institute of Technology, Chicago, pp. 493 - 499. J. L. Johnson and 0. S. Taylor, High current brushes, Part IV: Machine environment tests, Electrical Contacts 1979, Proc. 25th Holm Conf. on Electrical Contacts, Chicago, Sept. 10 - 12, 1979, Illinois Institute of Technology, Chicago, 1979, pp. 129 - 135. P. K. Lee, High current brush material development; I. Yintered metal-coated graphite, Electrical Contacts 1979, Proc. 25th Holm Conf. on Electrical Contacts, Chicago, Sept. 10 - 12, 1979, Illinois Institute of Technology, Chicago, 1979, pp. 153 - 158. P. C. Chan, Materials for advanced electric current collecting technique, M. S. Thesis, Mechanical Engineering Department, Massachusetts Institute of Technology, May 1979. E. Rabinowicz and P. Chan, Wear of silver-graphite brushes against various ring materials at high current densities, Electrical Contacts 1979, Proc. 25th Holm Conf. on Electrical Contacts, Chicago, Sept. 10 - 12, 1979, Illinois Institute of Technology, Chicago, 1979, pp. 123 - 127. N. Ohmae and E. Rabinowicz, The wear of the noble metals, ASLE Reprint 78-LC- 2C-1, 1978. I. R, McNab and G. A. Wilkin, Carbon fiber brushes for superconducting machines, IEE J. Electron. Power, (Jan. 1972) 8. I. R. McNab and W. R. Gass, High current density carbon fiber brush experiments in humified air and helium, Electrical Contacts 1979, Proc. 25th Holm Cod on Electrical Contacts, Chicago, Sept. 10 - 12, 1979, Illinois Institute of Technology, Chicago, 1979, pp. 159 - 163.

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13 P. Reichner, Metallic brushes for extreme high current applications, Electrical Contacts 1979, Proc. 25th Holm Conf. on Electrical Contacts, Chicago, Sept. 10 12, 1979, Illinois Institute of Technology, Chicago, 1979, pp. 191 - 197.

14 R. M. Slepian, High Current Brushes. V - Subdivided monolithic brushes at very high current levels, Electrical Contacts 1979, Proc. 25th Holm Conf. on Electrical Contacts, Chicago, Sept. 10 - 12, 1979, Illinois Institute of Technology, Chicago, 19’79, pp. 137 - 144.

15 I. R. McNab, Pulsed high power brush research, IEEE Trans. Components, Hybrids Manuf. Technol., 1 (1978) 30 - 35.

16 I. R. McNab, Pulsed high power brush research II. Interpretation of experimental data, Proc. Seminar on Energy Storage, Compression and Switching, Canberra, November 1977, Australian National University Press, 1978, pp. 201 _ 215.

17 C. J. Mole and E. Mullan, Design of a 10 MJ fast discharging homopolar machine, Proc. Seminar on Energy Storage, Compression and Switching, Canberra, November 1977, Australian National University Press, 1978, pp. 157 - 168.

18 R. A. Marshall and R. M. Slepian, Pulsed high-power brush research, Part III: Experi- ments at 15.5 MA/m2 and 277 m/s, IEEE Trans. Components, Hybrids Manuf. Technol., 2 (1) (1979) 100 - 107.

19 J. P. Barber, The acceleration of macroparticles and a hypervelocity electromagnetic accelerator, Ph.D. Thesis, Australian National University, March 1972.

20 S. C. Rashleigh and R. A. Marshall, Electromagnetic acceleration of macroparticles to high velocities, J. Appl. Phys., 49 (1978) 2540.

21 W. Bostick, Phys. Rev., 104 (1956) 242.