Copper Slip Ring Research Articles

Influence of the contact parameters and several graphite materials on the tribological behaviour of graphite/copper two-disc electrical contacts

Abstract This paper reports on the performance and tribological mechanisms in a newly developed design for an electrical sliding contact in terms of sliding velocity, normal force and electrical current during 24-h experiments. Copper discs were run against hard-carbon graphite, electrographite and polymer-bonded graphite. The average wear rates of the copper discs against the three graphite materials were reduced to 10−8 and 10−7 mm3/Nm, and the graphite discs to 10−6 mm3/Nm, which is for 1 and 1–2 orders of magnitude less, than copper slip-rings and carbon brushes in conventional systems, respectively. The coefficient of friction was 0.2–0.4, depending on the conditions, which is comparable to the values of conventional electrical sliding contact. Tribofilms are generated in almost all contacts, and crucially determine the contact performance. The polymer-bonded-graphite/Cu formed the most compliant tribofilms that unify and decrease the local contact pressures and temperatures, and have the largest surface coverage that reduces the wear and contact resistance, which in turn reduces the contact instabilities and so the friction.

Effect of combined velocity and pressure on life time of carbon brushes

The value of contact resistance between copper sliprings and carbon brushes, in wound-rotor induction motors, has a profound effect on the performance of these machines. The increase of this resistance due to bad contact may be detrimental. In this paper, an important factor affecting the resistance is introduced. This is the PV factor (product of contact pressure and peripheral speed). It represents the frictional power density at the interface; and it is also an indication of the volumetric or linear wear rate. A relation between the operational life time of brushes and the PV factor is experimentally deduced.

Photoelastic visualization of contact phenomena between real tribological surfaces, with and without sliding

The design, construction and operation of a machine for visualizing real contact interfaces and testing sliding wear is described. The bodies that form the interface are both opaque—the slider is a carbon graphite sample (National Electrical Carbon Company brush grade 634) and the rotor surface is copper. The visualization technique uses contact indentation stresses and photoelasticity to generate optical fringes (shadows) directly beneath the contact interface. The resultant fringes indicate the size and location of the islands of real contact that exist within the apparent contact region. Resolution is sufficient to distinguish two neighbouring millimetre-sized contacts. For calibration and comparison purposes, photoelastic pictures of sharp graphite pencil points and hard plastic spheres of various diameter indenting the copper slip ring surface are presented. Photographs of photoelastic images of contact interfaces between an opaque carbon sample sliding against an opaque copper slip ring are also presented. Photographs are presented that show the formation of concentrated contact (and possible evolution to thermal mounding) on the carbon slider surface. Collective data from other photographs measures the relative occurrence of concentrated contact and suggests operating and surface conditions that can discourage concentrated contact and formation of thermal mounds.

Electric current effects in wear phenomena at a rotating Cu-Ag interface

Two silver wire brushes were held in contact with a rotating copper slip ring in an ultrahigh vacuum system. After initial sputter cleaning of the copper slip ring, the vacuum system was backfilled with humidified CO 2 to a pressure of 1 atm. A d.c. electric current was passed across the silver brushcopper slip ring-silver brush assembly and the resulting wear particles were analyzed using scanning electron microscopy and electron probe microanalysis. The slip ring surface was analyzed using Auger electron spectroscopy to determine the amount of silver transferred. Currents ranging from 0 to 50 A were used. In each case the numbers of pure copper, pure silver, copper-rich and silver-rich particles were recorded as a function of the current. The Auger signal from the slip ring and the concentrations of the elements in the wear debris were plotted versus the current to determine the role of current in the present Cu-Ag system.

Characterization of Ag Wire Brush/Cu Slip Ring Electrical Contacts Rotating in Wet CO<inf>2</inf>

Current-carrying silver wire brushes were run on the same track against a rotating copper slip ring in a conventional stainless steel ultra high vacuum system. The experiments were performed after sputter cleaning of the slip ring in a H 2 0-CO 2 ambient held at one atmosphere of pressure. The electrical contact resistance and the frictional coefficient were measured in situ. Subsequently, scanning electron microscopy, electron probe microanalysis, and elemental mapping studies were carried out ex situ on the slip ring, brush end, and wear particle surfaces. Both low (50 mA) and high (40 A) currents were used for comparison. Interestingly, it was found that not only was silver transferred to the copper slip ring, but copper was also transferred to the wire brush ends. Moreover, the wear particles were mainly of two types, one being rich in silver and the other rich in copper, the relative concentrations depending on the current flowing through the contacts. These results demonstrate clearly that mechanical mixing of brush and slip ring material and the incorporation of both materials in wear particles play important roles in rotating current-carrying electrical contact phenomena.

Wear debris analysis in rotating Ag-Cu electrical contacts

A copper slip ring was rotated in contact with two current-carrying silver wire brushes in an ultrahigh vacuum system. The copper ring was initially sputter cleaned and the experiments were performed in humidified CO 2 at atmospheric pressure. The wear particles produced at the two contacts were collected for subsequent investigations by scanning electron microscopy and electron probe microanalysis. Currents ranging from 0 to 40 A were used and the slip ring was rotated at 150 rev min −1. In one zerocurrent experiment a speed of 15 rev min −1 was used for comparison. The wear particles consisted of pure silver, pure copper and mixed particles with the relative amounts depending on the current through the contact. Some of the characteristic features of the most frequently occurring wear particles are explained and the mechanisms for the generation of the particles are discussed.

Thermomechanical effects in high current density electrical slip rings

An experimental apparatus was constructed to study the contact of electrical brushes passing high currents. This apparatus consists of a 100 mm copper slip ring and a brush holder which can be loaded against it. The surface temperature of the brush, the extent of contact and the friction can be measured with this apparatus. Four brush materials were studied: one electrographitic brush and three silver-filled graphite brushes. The results of the experiments showed that above a particular threshold speed the brushes exhibited a sparking phenomenon which consisted of periodic motion of a cluster of small sparks on the brush surface. The peak temperature and the periodicity of the sparking were shown to be a function of the properties of the brush material. A theoretical model of the sparking phenomenon has been developed which shows good agreement with the sparking motion observed using the electrographitic brush.

High current brushes: VIII: Effect of electrical load

Performance data are presented from controlled experiments with Ag-graphite brushes sliding on copper slip rings in a continuous-operating mode over a wide range of electrical loads (0.8 – 8 MA m −2) in a humidified CO 2 environment. The observed contact resistance, friction and wear characteristics are explained on the basis in part of past and recent contact models and in part of thermophysical property data acquired for the specific Ag-graphite brush material (mass fraction of silver in graphite, 0.75). The phenomenon of electromigration of silver is invoked to explain observed consistent asymmetry in contact resistance between anode and cathode brushes. Direct evidence in support of the models and observed brush performance is offered from results of detailed contact surface morphology and structure analyses.

Frictional and electrical interactions in current collectors

Numerical results are given for electric current flow through a sliding contact that involves a graphite brush and a copper slip ring. The contact of length 2 l and unit depth moves with velocity c along the brush face. It is found that for a given fixed set of operating conditions (current I, sliding velocity V, contact force F N etc.) many values of c and l may exist; it is also found that c is linear in sliding velocity V.

In situ auger electron spectroscopy characterization of wet-CO 2-lubricated sliding copper electrical contact

The electrical contact resistance, elemental surface composition and friction of an OFHC copper slip ring rotating in contact with two high purity copper wire brushes on different tracks were investigated in situ for heavy and light normal contact forces under a wet CO 2 environment at atmospheric pressure. Scanning electron microscopy was also used to characterize the slip ring and brush surfaces. Previous work in ultrahigh vacuum showed that, as rotation proceeded, interfacial impurities were almost totally removed and the electrical contact resistance decreased until cold welding occurred. In the present work, the slip ring surface was sputter cleaned (more than 95% Cu) before contact rotation and was only slightly contaminated after rotating in wet CO 2. Both the contact resistance and the friction decreased quickly and reached steady state values almost simultaneously in the early stages of rotation. Also, cold welding phenomena did not occur. Scanning electron micrographs taken after each experiment showed that the surfaces of the slip ring tracks and the brush wire ends were much rougher when heavy normal contact forces were used than in the light normal force condition. All these results confirm that wet CO 2 is an effective lubricant for Cu-Cu electrical sliding contacts.

Microstructural characterization of rotating Cu-Cu electrical contacts in vacuum and wet CO 2 environments

The chemical, electrical and wear properties of the rotating interface between OFHC copper slip rings and two high purity copper wire brushes were investigated in situ in ultrahigh vacuum (UHV) and in 1 atm wet CO 2. The chemical composition of the slip ring surface was determined by Auger electron spectroscopy. The contact resistance was measured by a potentiometric four-point probe technique while the wear properties of the interface and the morphology of the debris were studied by frictional force, scanning electron microscopy (SEM), transmission electron diffraction (TED) and X-ray diffraction (XRD) measurements. Rotation in UHV of a conventionally cleaned (CC) slip ring produced a much cleaner surface. The contact resistance of both brush interfaces decreased and the frictional force increased with increasing number of revolutions. After many revolutions the brush and slip ring welded. The decrease in contact resistance with the number of slip ring revolutions more or less paralleled the decrease in total impurities. Rotation in wet CO 2 of a CC slip ring and brushes also produced much cleaner surfaces. In contrast, initially argon ion sputter-cleaned surfaces became slightly contaminated (mainly carbon and sulfur) when rotated. The contact resistance at both interfaces and the coefficient of friction decreased with increasing number of slip ring revolutions, finally reaching steady state values. After each experiment, SEM examination of vacuum rotated surfaces showed deep ridges and broken pieces of material on the slip ring surface and badly deformed brush wire ends. Wet CO 2 rotated surfaces were relatively smooth and shallow ridges were seen. SEM examination of wear particles collected during rotation indicated that they may have come from both the slip ring and brush wire materials and were rolled in the regions between the brush and slip ring. XRD and TED from individual particles showed a randomly oriented polycrystalline microstructure. The particles collected from the wet CO 2 experiments were much smaller in size than those collected in vacuum experiments. In wet CO 2, the contact resistance was interpreted as being predominantly due to an electron tunneling mechanism through the CO 2-H 2O molecular layer at the interface. As expected, the thickness of the layer appeared to vary with the contact pressure. Friction would then arise largely when the molecular layer was occasionally broken, allowing intimate contact and temporary welding of the brush and slip ring surfaces. Subsequent fracture of these welds during continued rotation would initiate the formation of wear particles.

Characterization of Thick Films Formed on Slip Rings During High Current Density Operation

The thick film that is formed on the surface of a copper slip ring sliding against monolithic silver-graphite brushes in a humidified carbon dioxide atmosphere has been investigated experimentally. The object was to study the nature and method of formation of the thick film since the films play a major role in the electrical, frictional, and wear performances of sliding electrical contacts. The morphology and composition of the thick films were studied by a combination of metallographic and electron optical techniques. The principal mechanism by which the films are formed was found to be weld formation between the silver in the brush and the copper in the slip ring followed by the rupture of a weaker interface in the brush material. The "film" deposited on the copper slip ring was, therefore, a composite of individual lumps of material, rather than a smooth layer-by-layer atomic deposit. The thickness of the film was highly variable from location to location; values as thin as 0.5 µm and as thick as 150 µm were observed. The collected wear debris were much smaller than the original particles used to form the brush composite, and the graphite content of the wear fragments was higher than that of the brush. It is believed that the thick film initially had the same composition as the brush material, but an enrichment in silver occurred during operation. This is attributed to the preferential removal of exposed graphite from both the thick film itself and from the brush surface during continued wear.

High Current Brush Material Development, Part II: Metal- Solid Lubricant Coated Wires

The preparation and testing of an electrical contact material, metal-solid lubricant coated copper wires for fabrication of multielement high current brushes, are described. The composite coating consisted of a metal, copper or silver, and an overall 1-5 weight percent (wt %) of a fine solid lubricant powder (graphite, WS 2, or WSe2) evenly dispersed and well oriented in the metal matrix. The coatings were obtained by an electrolytic codeposition technique. By sliding a bundle of the coated wires against a rotating silver or copper slip ring at constant speed of 16.75 m/s, the sliding characteristics-- contact voltage drop and friction of the testing system-were measured as a function of current density up to 12.4 MA/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in several different gas environments. Several brush life tests were made at 12.4 MA/m 2. Results indicate that the coated wire possesses high currentcarrying capability under a wide range of environmental conditions. The tests also showed that vapor lubrication is applicable to all solid lubricants with lameilar structure for reduction of wear rate. A tribochemical background of the sliding system is discussed.

In Situ AES Characterization of Rotating Electrical Contacts

The electrical contact resistance and surface chemical composition of a rotating copper slip ring in contact with two wire brushes were investigated in situ under ultrahigh vacuum (UHV) conditions as a function of the number of revolutions of the slip ring. The initial surface of the copper slip ring was examined by Auger electron spectroscopy (AES) techniques and found to be almost completely covered by surface impurities largely consisting of carbon. As the slip ring was rotated in contact with the brushes, the impurity concentration declined sharply to less than ten at% after several hundred revolutions. A corresponding decrease in electrical contact resistance and a sharp increase in friction and wear were also observed. Regardless of polarity, the brush with the higher contact pressure had the lower contact resistance. The in situ experiments were terminated when the motor turning the slip ring could no longer overcome the adhesive forces between the brushes and slip ring. Subsequent scanning electron microscopy observations of the brush and slip ring surfaces gave supplementary information on the mechanisms of friction, wear and surface cleaning during rotation. For a given experiment the change in contact resistance during rotation divided by the change in impurity concentration on the surface of the slip ring is approximately the same for the positive and negative interfaces. This parallelism of contact resistance and surface impurity concentration suggests that the former is caused to a significant extent by the latter. Since the surface impurities are mostly carbon atoms, it is concluded that carbon impurities are largely responsible for the observed contact resistance.

High Current Tests of Metal Fiber Brushes

Simultaneous measurements Were made of the friction force and electrical contact drop for small metal fiber brushes at high current densities (12.4 MA/m2). Comparative testing was performed with both polarities of copper, silver-plated copper, and silver fiber brushes on copper and On silver-plated copper slip rings in humidified carbon dioxide. Friction increased from 0.3 to 1.4, while the contact drop decreased as more silver was added. At a contact force of 0.23 N and a sliding speed of 12 m/s, minimum total power loss was found for the bare and the silver-plated copper fibers on the copper sllp ring. However, the optimum material selection will depend on current density, sliding speed, and contact force Used for a specific application. Maximum test duration has been extended to almost 500 h, with indications of slight improvement of both contact drop and wear rate as the operating time is increased. Dimensionless wear rate was only 1.9 x 10-11. With added cooling capability, successful Shortterm operation has been achieved with current density in the individual fibers as high as 100 MA/m2 (65 000 A/in2).

High-Current Brushes Part VI: Evaluation of Slip Ring Surface Films

The thin film that is formed on the surface of an oxygen-free high-conductivity (OFHC) Copper slip ring sliding against monolithic silver-graphite brushes in a humidified carbon dioxide atmosphere has been investigatéd experimentally. The object was to study the nature and method of formation of the film since the films play a major role in the electrical, frictional, and wear performance of sliding electrical contacts. The techniques used included optical and electron microscopy and Auger electron spectroscopy. With the latter, two techniques were employed, both of which involved placing complete (83-mm diameter) slip rings within an Auger high vacuum chamber. In the first case externally operated slip rings were transferred into the Auger chamber for film analysis. In the second case a complete rotatable slip ring assembly, including brushes, was mounted within the Auger chamber. Low-speed rotational operation of this ring then permitted brush/slip ring sliding to take place so that film build-up could be analyzed in situ. The nature of the film formed on a copper slip ring was found to be dependent on the surface preparation and the ambient atmosphere, but it was found that the thin films of adsorbed gases did not form an oxide of appreciable thickness. Water vapor was identified and is known to play a critical role in the successful operation of brushes containing graphite. The adsorbed thin films were also found to contain significant quantities of sulfur and chlorine. In the presence of water vapor the adsorbed film on copper surfaces appears to have a special character. A hypothesis is suggested to explain the nature of this adsorbed film and the way in which it is bonded to the slip ring surface.

High-Current Brushes, Part III: Performance Evaluation for Sintered Silver-Graphite Grades

The results of varying the material composition, specifically the metal mass fraction, on the performance of silver-graphite brushes at high-current densities (0.8 MA/m2) in a carbon dioxide atmosphere, have been evaluated. Material properties which were measured included the brush density, electrical conductivity, and hardness. These properties were used in the evaluation of the brush performance when sliding on copper slip rings at a speed of 12.7 mis. Measured parameters included the contact resistance, friction coefficient, and wear rate. The results of this analysis indicate that the resistive behavior was dominated by constriction effects, with the number of contact spots apparently ranging from 2-15. The film contribution to the total resistance was small, the resistivity being less than 6 X 10-13 \Omega ·m2. Friction measurements indicated a specific shear strength of 10-24 MN/m2which, if caused by an adsorbed water film, would correspond to a film thickness in the range from about 0.2-0.4 nm. Wear rates well into the microwear regime have been observed, even at the high-current densities used. However, a satisfactory explanation for the strong dependence of wear on metal content (from 0.7-60 mm3/Mm) as the metal mass fraction varied from 0.4-0.9 has not yet been evolved.

Peculiarities of formation and properties of metal-polymer sliding electrical contact

The properties of collector films formed on copper slip rings during rubbing by various types of brush have been investigated. Structural peculiarities are discussed and a model is suggested for the structure of collector films formed during the operation of metal-polymer brushes.

Relationship Between Contact Resistance and Wear in Sliding Contacts

The wear characteristics Of metal graphite brushes on copper slip-rings are described in this paper. The influence of different parameters on wear and contact resistance is explained by means of the mechanism of current conduction. Wear as well as contact resistance is determined by the mechanism of current conduction. Therefore, it is possible to measure the wear by determining the contact resistance. Thus life tests can be shortened considerably.

Electrical-Power Brushes for Dry Inert-Gas Atmospheres

Conventional carbon brushes developed for earth's surface applications were found to be inadequate in terms of wearing life when applied in electrical contact with copper slip rings operating in dry inert-gas atmospheres. Under similar conditions, however, the life of such brushes was vastly increased when special treatments were incorporated in their body structure. One of these treatments (40 W/o MoS 2 ) imparted life ability and electrical voltage stability comparable to those of present-day industrial grade brushes operating in normal air atmospheres.