Slip Ring Surface Research Articles

Carbon Nanotubes-Titanium Diboride/Copper (CNTs-TiB2/Cu) Laser Cladding Layer with Enhanced Tribo-Electrical Property on Wind Turbine Pitch Slip Ring

Wind turbine pitch slip ring requires excellent tribo-electrical properties, including low friction and wear, high current transmission efficiency, low power loss, and contact resistance. To improve the tribo-electrical property, a carbon nanotubes-titanium diboride/copper (CNTs-TiB2/Cu) coating was prepared on slip ring surface by laser cladding. Microstructure, dilution rate, microhardness, tribo-electrical property, and power transmission property of cladding layer were analyzed. Optimal metallurgical bonding was achieved at a laser power of 900 W, resulting in a dilution rate of 14.5%. Elemental Ti diffused from the substrate into the coating, forming a hard titanium carbon (TiC) phase that significantly improved both friction reduction and wear resistance. Additionally, the formation of Cu3Ti increased the electrical conductivity of the cladding layer. The contact resistance was reduced by 30% to below 0.06 Ω, while the current transmission efficiency improved by 25%, exceeding 90%.

Increasing operational life of brush-contact device in the turbine generator due to using lubricating molybdenum disulphide brushes

A way of reducing irregular current distribution in multi-brush systems of sliding current transfer with its wear reduction due to installing lubricating molybdenum disulphide brushes on slip rings to ensure a greasing nano-sized cover on the slip ring surface is proposed. The authors give the results of industrial tests estimated on the performance effectiveness of lubricating brushes on slip rings of the TBB-320-2UZ-type turbine generator. The results showed that the lubricating brushes reduce a) the wear of 6110 OM+M and EG2AF+M brushes by 1.2 and 2.1 times respectively, b) current distribution irregularity in parallel operating brushes due to stabilizing the contact arc, and c) the temperature of the electrical brush-contact device due to the friction reduction in brushes.

Research on the fault diagnosis method for slip ring device in doubly‐fed induction generators based on vibration

The brush and slip ring system is an important part of excitation system in doubly-fed induction generators, but the excitation instability caused by the surface damage of slip ring often occurs, which brings a great security trouble to the stable operation in the generator. To solve this problem, the method that the vibration signal is used to carry out diagnosis of slip ring surface in different fault states was proposed. First, the principle based on vibration, the transmission route and failure mechanism of the slip ring were analysed. Then, the experiment platform of the brush and slip ring was set up; the real-time vibration signal was collected before and after fault; the wavelet energy spectrum was applied to the vibration signal processing. The abnormal vibration band was found by the contrast and proportion analysis of the wavelet energy spectrum distribution of vibration signals before and after fault, which can be the basis for fault diagnosis of the slip ring device.

High-speed electrical sliding wear behaviors of Cu-WS2-graphite-WS2 nanotubes composite

Abstract Cu-WS2-graphite-WS2 nanotubes composite was fabricated by the powder metallurgy hot-pressed method. The effects of electrical current (5–15 A/cm2) and sliding velocity (5–15 m/s) on the electrical wear behaviors of the composite were investigated using a block-on-slip ring wear tester rubbing against Cu-5 wt% Ag alloy ring under 2.5 N/cm2 of applied load. The lubricating effect of WS2 nanotubes and composition of tribo-film were analyzed. The results demonstrated that the contact resistance decreases but the wear rate increases as electrical current increases, because the adverse effects of electrical current soften the materials at “a-spots” and damage the tribo-film. Due to the adsorption of gaseous molecule film on the tangential direction of slip ring surface, with the rise of sliding velocity, the contact resistance increases while the wear rate reaches the minimum at a sliding velocity of 10 m/s. The reasonable addition of WS2 nanotubes into the Cu-WS2-graphite composite to replace WS2 powder can result in a reduction of both contact resistance and wear rate. X-ray photoelectron spectroscopy (XPS) analyses revealed that copper oxides, graphite, WS2 and WS2 nanotubes in the tribo-film play the main lubrication action at the tribo-interface.

Copper–beryllium metal fiber brushes in high current density sliding electrical contacts

The use of copper–beryllium (UNS C17200) was demonstrated as a low wear, highly compliant material for the construction of high current density metal fiber brushes. Testing was conducted in a humidified carbon dioxide environment. Brush linear wear rates of 4 × 10 −11 m/m were achieved at sliding speeds and current densities as high as 7.5 m/s and 215 A/cm 2 (2 MA/m 2). Contact resistance values were on the order of 10 mΩ and friction coefficients varied from 0.30 at 2.5 m/s to 0.15 at 7.5 m/s. Characterization of the brush and slip-ring surfaces using SEM as well as the slip-ring subsurface using TEM is presented as evidence in favor of fatigue dominated wear of the copper–beryllium fiber tips. Debris flakes were observed forming on the fiber tips with characteristic widths in the range 10–100 μm and thickness on the order of 1 μm, in stark contrast to 1–10 μm wide flakes of thickness on the order of 1 μm observed forming on the slip-ring surface. Relatively clean contact resistance data at the end of the test is indicative of the robustness of the brush design and the ability of the fiber tips to remain in contact with the evolving slip-ring surface. The importance of material selection, particularly in regard to fatigue and yield strength, is highlighted in this work as key factors in improving brush design and understanding the wear phenomena observed in high current density sliding electrical contacts.

Asymmetric wear behavior of self-mated copper fiber brush and slip-ring sliding electrical contacts in a humid carbon dioxide environment

This study examines wear phenomenon occurring in high-current density self-mated metal copper sliding contacts in humid carbon dioxide environments. A custom tribometer built to test macroscopic (lightly loaded) brush and slip-ring sliding electrical contacts at relatively high speed and current density is described in the text. Data is presented as further evidence that at some elevated current density (between 120 and 180A/cm2) there is a transition in the wear mechanism as the brush wear rates, friction coefficients, and contact resistances bifurcate. In situ white light interferometry provides new insight on the evolution of slip-ring surfaces as a function of current flow direction through the contact. Ex situ SEM of the brush fiber bundle surfaces shows a strong dissimilitude in the wear of contacting fibers as a result of the current flow direction. Steady-state friction coefficients in the range of 0.20–0.25, linear wear rates of 3–5×10−11m/m, and contact resistances of 0.6mΩ are reported for copper fiber brushes in water saturated carbon dioxide environments at 2.5m/s sliding speed and up to 120A/cm2. A bifurcation occurs at 180A/cm2 where the electron receiving brush (positive bias) exhibits an increase in friction coefficient to a range of 0.3–0.4, and the electron donating brush (negative bias) a drop to 0.18. The positive brush also exhibits an increase in wear rate to 2.3×10−10m/m and a sharp decrease in contact resistance to 0.2mΩ. The negative brush shows an increase in wear rate but only to half the value of the positive brush, and the negative brush contact resistance increased sharply to 0.6–0.8mΩ. An uncertainty analysis of wear rate is presented that demonstrates the need for long (100s of km) sliding distances to achieve a reasonable measurement uncertainty for such low wear contacts.

Characteristics of surface film on slip-ring in the low atmospheric pressure

The surface him of slip-ring is important for good sliding contact of brush and slip-ring. This film formation is influenced by the brush material, the atmospheric temperature, humidity and atmospheric pressure. The condition of sliding contact and surface film can be estimated by the measurement of contact voltage drops. The contact resistance is classified into constriction resistance and film resistance. From the results of this paper, in the low atmospheric pressure, the oxidized film scarcely grew on the slip-ring surface, so the contact resistance is thought to be generated mainly by the current constriction. Further, it can be found that the resistance in the low atmospheric pressure was only constriction resistance, and that the resistance in the air was composed of the constriction resistance and him resistance. The wear length of the test brushes at reduced atmospheric pressure, depending upon the brush grade, was many times larger than the wear in the air.

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.

Thermoelastic effects on slipring surface leading to brush vibrations

A novel interpretation of the brush vibration phenomenon is presented for current collection systems consisting of carbon brushes and steel sliprings. For the investigation of this phenomenon, a mechanical model with a rigid brush was used, and the thermal deformation of the slipring surface due to the losses generated by the vibrating brush was taken into consideration. The calculations show that at certain parameter configurations the brush vibration reinforces the thermal deformation and the closed-loop system becomes unstable, leading to pronounced brush vibration referred to as regenerative brush chatter. Measurements of the slipring surface temperature confirm this theory.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fundamental Analysis of Cu Brush-Ag Slip Ring Sliding Electrical Contacts

Electrical contacts involving current-carrying copper-wire brushes sliding on a sputter-cleaned silver slip ring and rotating in an ambient of 1 atm of wet CO 2 were characterized by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), X-ray energy spectroscopy (XES), reflection high-energy electron diffraction (RHEED), and in situ measurements of contact resistance. AES analysis of the slip rings showed 1) carbon formation at the contact zone and 2) increasing amounts of Cu transfer from the brushes to the slip ring with increasing contact current. SEM and XES analyses of the brushes, slip ring, and wear particles showed that the copper content of the wear particles and the copper concentration of the slip ring surface increased with increasing contact current. RHEED studies of the wear particles showed that higher currents resulted in larger grain sizes. Initially, high values of the contact resistance decreased to fractions of a milliohm, and then after approximately 2000 cycles, gradually became more erratic as the contact deteriorated. This deterioration was caused by the accumulation of Ag debris on the Cu brush ends.

Characterization of sliding Al-Cu Electrical contacts

Two sets of Al-Cu electrical contact systems have been investigated. Current-carrying Al (Cu) wire brushes were in contact with a rotating Cu (Al) slip ring in a high-vacuum (2 X 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> torr) system which was backfilled to 1 atm of humidified CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . Measurements were made of 1) the electrical contact resistances of Cu brush-Al slip ring and Al brushCu slip ring systems, 2) the surface topographies of the contact faces of the brush, the slip ring wear track, and the wear particles by scanning electron microscopy (SEM), and 3) the transfer of material across the interface and to the wear particles by electron probe microanalysis (EPM). Currents up to 30-A de were used. After an initial "wear-in" the contact resistance dropped to the order of milliohms. The contact resistance of the Al brush-Cu slip ring was consistently higher than for the Cu brush-Al slip ring system. After several thousand revolutions the contact resistance began rising erratically as the slip ring surface roughened and the wear particles became larger. In both systems Al was transferred across the interface and to the wear particles. However, only in the Al brush-Cu slip ring case did Cu transfer across the interface.

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&amp;lt;inf&amp;gt;2&amp;lt;/inf&amp;gt;

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.

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 copper slip ring-wire brush electrical contacts

The chemical composition and the electrical and mechanical properties of the interface between a Cu slip ring that rotates in contact with two Cu wire brushes were investigated by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), contact resistance, and frictional force measurements. The experiments were carried out in an ultra-high vacuum system and in an environment of 1 × 10 −4 torr of wet C0 2 . The contact resistance at both the positive and negative interfaces decreased with increasing number of slip ring revolutions while the frictional force increased. Under the wet 10 −4 torr C0 2 environment the increase in frictional force was smaller than that in high vacuum, which suggests that wet CO 2 has a lubrication effect even at these relatively low pressures. In situ AES measurements showed that the composition of the slip ring surface, which was initially covered by about 50 atm% of carbon, changed drastically during rotation. After many revolutions it approached that of a clean Cu surface (total impurities < 10 atm%). The decrease in contact resistance with the number of slip ring revolutions more or less paralleled the decrease in total impurities in the high vacuum experiments. This parallelism suggests that the contact resistance is caused predominantly by the surface impurities. No systematic relationship between contact resistance and brush current density was observed. SEM observations showed that the surface material was smeared out in the brush track areas and that the initial surface impurities were buried during rotation.

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 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.

Electrodeposition of Rhodium and Ruthenium for Slip-ring Surfaces

SummaryThe mechanical wear of rhodium and ruthenium electrodeposits operating as slip-ring surfaces with palladium-silver alloy brushes is shown to be a function of electrolyte composition, plating conditions, and thickness of coating. Within selected thickness ranges the deposits function very satisfactorily and are resistant to wear. Types of roughness which should be avoided are discussed, and methods of measuring and controlling these are recommended. It is concluded that for slip-rings a thickness of 40 x 10−6 in should be sufficient to withstand the conditions involved in the majority of applications, and that for such duty ruthenium provides a more suitable surface than rhodium.

Microwaves used to observe commutator and slip ring surfaces during operation

A method of using short-wave radio (microwaves) and waveguide components for observing and recording the surface irregularities of commutators and slip rings during the operation of the associated machines is presented. If reasonable care is exercised, measurements of surface irregularities in the order of 1/2 mil or less may be obtained.