An analysis of the main factors on the wear of brushes for automotive small brush-type DC motor

Some critical components in motors and generators have sliding electrical contacts. Electrical brushes are commonly used in those contact points to conduct current between the stationary and moving parts of a motor. Brushes are exposed both to mechanical and electrical loading. In this paper, studies on the wear of brushes against copper commutator were briefly reviewed. The main influential factors of brush wear are associated with both mechanical and electrical wear. Brush wear is affected by various factors including temperature, material properties, sliding speed, contact force, and interfacial and environmental conditions. The mechanical wear of brushes is proportional to the brush spring pressure and sliding speed, while the electrical wear of brushes is associated with current and contact voltage drop. To characterize the wear, a brush wear test machine was designed, and influential factors were measured such as electrical contact resistance, temperature, wear mass loss, and so on. The wear tests were processed using a small brush type automotive DC motor.     

The effect of brush spring pressure on the wear behaviour of copper–graphite brushes with electrical current

Electrical brushes are used to conduct current between stationary part and moving part of a motor or a generator. To ensure proper current transfer and continuous contact, brushes must be loaded against the sliding contact surface with a sufficient force. High loads increase frictional losses and wear of the brushes and/or sliding surface. While relatively low contact pressure causes arcing and higher voltage drop.In this study, a novel pin-on-slip ring-type friction and wear test machine was designed and manufactured for the purpose of brush testing. Copper–graphite-based electrical brush containing 90 wt% copper and 10 wt% graphite was manufactured by powder metallurgy and the tribological behaviour and voltage drop were investigated at different brush spring pressures at 10–200 kPa with current.It was found that the specific wear curve showed three distinct wear rate regimes, such as low, mild, and severe. Severe wear was observed below 30 kPa and above 120 kPa brush spring pressures (BSP) (3 and 12 N loads, respectively). Arc erosion was the main wear mechanism below 30 kPa brush spring pressure while abrasion was dominant above 120 kPa BSP. Low and mild regimes were observed between 30–50 and 50–120 kPa BSP, respectively. SEM observations showed that a continuous surface layer was formed at the sliding surfaces of the wear samples in low and mild wear regimes. The wear debris was examined by SEM and X-ray diffractometer.     

Influence of high velocities and high current densities on the friction and wear behavior of copper-graphite brushes

The friction and wear behavior of Morganite CM1S powder metallurgy copper-graphite brushes at sliding velocities up to 160 m s^?1 and current densities up to 870 A cm^?2 is presented. The brushes had a cross-sectional area of 1.2 cm² and the loads employed ranged from 9 to 45 N. The wear rates as a function of velocity and the voltage drop per brush as a function of sliding velocity, brush pressure and current were determined. The wear rates under a current of 600 A are of the same order as those obtained under no current conditions. The minimum difference was obtained at a sliding velocity of 100 m s^?1 (4.1 × 10^?4 cm km^?1 with current compared with 3.4 × 10^?4 cm km^?1 without current). The wear rate exhibited by the positive brush was lower than that of the negative brush at any sliding velocity.At constant current and sliding velocity the contact voltage drop decreases with increasing brush load. The voltage drop exhibited by the positive brush is always lower than that of the negative brush. The contact voltage drop varies little with sliding speed when the current and the brush load are kept constant. At constant brush load and constant sliding speed the voltage drop increases monotonically when the current is increased. It has been determined that local rotor waviness, even of small amplitude, can produce sufficient brush bouncing to cause excessive sparking which results in pronounced damage to the brushes and rotor surface.     

Determination of accelerated condition for brush wear of small brush-type DC motor in using Design of Experiment (DOE) based on the Taguchi method

This research was conducted to understand friction and wear due to mechanical and electrical contact between a brush and a commutator, which play an important role in driving brush-type DC motors, and to predict the motor life through brush wear. To identify the influencing factors, the variables used in the experiments were the operating voltage, the current load, the rotational speed, and the environmental temperature. The design of experiment used for the analysis of brush wear was based on the Taguchi method. An independent condition was provided for each factor during the experiment. The results indicate the high contribution of current and temperature to the wear behavior. The operation voltage showed negligible influence on the wear behavior.     

Friction and wear properties of two types of copper — Graphite brushes under severe sliding conditions

High speed dry friction experiments using two copper-graphite brushes against an AISI 4340 steel rotor were conducted at sliding velocities up to 230 m s⁻¹ and at current densities up to 526 A cm⁻². One brush was a commercial powder metallurgy (PM) specimen and the other was a graphite fiber-Cu/Sn matrix composite material. The composite brush was prepared by a proprietary process of liquid-metal infiltration and was run with the graphite fibers perpendicular to the rotor surface. The coefficient of friction was determined as a function of velocity, the wear rates were determined as a function of velocity and the voltage drop was determined as a function of velocity and current. The results show that the coefficient of friction exhibited by the PM brush is lower than that of the composite material at any velocity tested. The wear rates without current are much higher for the PM than for the composite brush, but they are of the same order when a current of 600 A is passed through. The voltage drop at the brush-rotor interface shows a similar variation with velocity for the two brushes, but the variation of the voltage drop as a function of current is different for the two specimens. The voltage drop increases almost linearly with increasing current for the PM brush. For the composite brush it exhibits a sharp increase up to about 50 A and then varies very little up to the maximum current of about 600 A. The damage done to the rotor surface in the case of heavy sparking is more pronounced with the PM brush than with the composite brush. It appears that the difference between the high temperature mechanical properties of the two types of brushes is responsible for their different behavior under severe sliding conditions.     

Effect of electrical current on the tribological behavior of the Cu-WS2-G composites in air and vacuum

As the traditional graphite-based composites cannot meet the requirement of rapid developing modern industry, novel sliding electrical contact materials with high self-lubricating performance in multiple environments are eagerly required. Herein a copper-based composite with WS2 and graphite as solid lubricant are fabricated by powder metallurgy hot-pressed method. The friction and wear behaviors of the composites with and without current are investigated under the condition with sliding velocity of 10 m/s and normal load of 2.5N/cm 2 in both air and vacuum. Morphologies of the worn surfaces are observed by optical microscope and compositions of the lubricating films are analyzed by XPS. Surface profile curves and roughness of the worn surfaces are obtained by 2205 surface profiler. The results of wear tests show that the friction coefficient and wear volume loss of the composites with current are greater than that without current in both air and vacuum due to the adverse effects of electrical current which damaged the lubricating film partially and roughed the worn surfaces. XPS results demonstrate that the lubricating film formed in air is composed of oxides of Cu, WS2 , elemental S and graphite, while the lubricating film formed in vacuum is composed of Cu, WS2 and graphite. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composites show low friction coefficient and wear volume loss in air condition. Owing to the fact that graphite loses its lubricity which makes WS2 become the only lubricant, severe adhesive and abrasive wear occur and result in a high value of wear rate in vacuum condition. The formation of the lubricating film on the contact interface between the brush and ring is one of the factors which can greatly affect the wear performance of the brushes. The low contact voltage drop of the composites in vacuum condition is attributed to the high content of Cu in the surface film. This study fabricated a kind of new sliding electrical contact self-lubricating composite with dual-lubricant which can work well in both air and vacuum environments and provides a comprehensive analysis on the lubrication mechanisms of the composite.     

碳纳米管-银-石墨复合材料的电磨损性能

采用粉末冶金工艺制备碳纳米管—银—石墨复合材料,初压压力200 Mpa,在H2保护气氛下烧结并保温1 h,复压压力400 Mpa制得复合材料,研究电流密度(0～25 A/cm2)对碳纳米管—银—石墨复合材料摩擦磨损性能的影响。结果表明：随着电流密度的增大,电磨损过程中的发热量增大,粘着磨损加剧,润滑膜遭到破坏,导致复合材料的摩擦因数、磨损量增大。比较正、负极电刷磨损量发现,接触表面理化反应和金属转移的存在,导致正刷磨损量大于负刷磨损量;正刷磨损量随电流密度增大而成倍增长,负刷磨损量与电流密度关系不明显,电流密度越大磨损极性的差异越明显。     

Production and performance of metal foil brushes

Electrical brushes which were made of silver, copper and aluminum foils of 12.5 and 25 μm thickness and were composed of 15–195 individual foils, were tested in purified argon on a polished copper rotor at a speed of 13 m s^?1. Brush pressures varied between 3.1 × 10³ and 2.8 × 10⁴ N m^?2 and current densities were up to about 700 A cm^?2 (about 4500 A in^?2).The observed dependence of the voltage drop across the brushes as a function of the current densities agreed closely with Holm 's contact theory as applied to foil brushes. The film resistivities were found to be near $\text{σ}{}_{\mathrm{F}}\text{= 10}{}^{\mathrm{?12}}\text{Ω}\text{m}{}^2$ for copper and silver and to be about $\text{3 × 10}{}^{\mathrm{?12}}\text{Ω}\text{m}{}^2$ for aluminum. The projected performance of foil brushes based on these results is very favorable and the future commercial use of foil brushes appears to be possible.The total loss, electrical and mechanical, through the brushes is independent of current density if the brush pressure is chosen to minimize the total loss. If so, the loss depends only on the brush speed, the hardness of the softer of the two materials involved (i.e. of foil and rotor or slip ring), the coefficient of friction and the film resistivity.Microscopic surface examinations of rotor and brushes show that the brush surface is smoothed through running the brush, whereas the rotor remains almost unaffected or is mildly roughened, as long as no arcing takes place. Arcing causes considerable surface roughening on both the brush and the rotor surface and debris is thus deposited on the rotor; this can score the brushes. Further experiments are required to determine the rate of brush wear.     

Chemical factors in carbon brush wear

Chemical factors involved in the wear of carbon brushes on metal slip rings are discussed. Under conditions of elevated temperature operation in an oxidizing atmosphere, a correlation is found between the oxidation characteristics and the wear rates of carbon brushes. It is demonstrated that temperatures in the oxidation range can readily be generated by current converging through high resistance constrictions in the contact zone. Copper and other metals present in the slip ring can act as catalysts for the oxidation, the metal oxide particles migrating into the interior of the brush and causing enhanced gasification rates and increased porosity. Treatment of the brush with oxidation inhibitors, such as phosphorus oxychloride, and surface alloying of the ring with non-catalytic materials, such as zinc, can reduce the wear rate at elevated temperatures under favorable conditions. Graphite surfaces are also attacked by reactive gaseous species such as atomic nitrogen and oxygen, ozone and nitrogen oxides which may form near the brush surface during arcing.     

Low wear metal sliding electrical contacts at high current density

Operation of a low wear (2 × 10^?5 mm³/(N-m)), low contact resistance copper sliding electrical contact was demonstrated. The wear rate of a lightly loaded copper–beryllium metal fiber sliding on a polished copper counterface was insensitive to (DC) current density values as great as 440 A/cm² (in a brush positive or anodic configuration). Low wear and relatively low friction (μ ～ 0.2 to 0.3) was achieved by operating the contact immersed in a liquid medium consisting of a hydrofluoroether with helium cover gas, inhibitingoxidationand providing cooling of the contact. Similar experiments performed in liquid mediums of ultrapure water and dilute (3%) hydrogen peroxide show an order of magnitude increase in wear rate and provide further insight on the role of electrochemically enhanced oxidation and the degraded contact resistance and tribological behavior of non-noble sliding electrical contacts in general. In contrast to high current density slidingin hydrofluoroether, an order of magnitude greater wear rate was observed for similar sliding conditionsin hydrogen peroxide or water without the aid of externally supplied electric potential. A conceptual model is proposed correlatingthe rate of brush wear to fatigue strength and electrochemically enhanced oxidation as a result of high current density transport through the contact. A mathematical expression was derived to calculate the approximate wear volume of a single fiber laterally contacting a slip-ring, based on direct measurement of the wear scar geometry.     

Effect of Operating Conditions on Tribological Response of Al–Al Sliding Electrical Interface

Aluminum is widely used in electrical contacts due to its electrical properties and inexpensiveness when compared to copper. In this study, we investigate the influence of operating conditions like contact load (pressure), sliding speed, current, and surface roughness on the electrical and tribological behavior of the interface. The tests are conducted on a linear, pin-on-flat tribo-simulator specially designed to investigate electrical contacts under high contact pressures and high current densities. Control parameters include sliding speed, load, current, and surface roughness. The response of the interface is evaluated in the light of coefficient of friction, contact resistance, contact voltage, mass loss of pins, and interfacial temperature rise. As compared to sliding speed, load, and roughness, current is found to have the greatest influence on the various measured parameters. Under certain test conditions, the interface operates in a “voltage saturation” regime, wherein increase in current do not result in any increase in contact voltage. Within the voltage saturation regime the coefficient of friction tends to be lower, a result that is attributed to the higher temperatures associated with the higher voltage (and resulting material softening). Higher interfacial temperatures also appear to be responsible for the higher wear rates observed at higher current levels as well as lower coefficients of friction for smoother surfaces in the presence of current.     

Effect of PV factor on the wear of carbon brushes for micromotors

In this paper the effect of the PV factor (product of contact pressure and peripheral speed) on the wear of carbon brushes was investigated in detail. The wear tests were done on an alternate current (ac) motor. The results show that the wear rate of brushes is proportional to PV factor when PV factor is less than 0.5 MW/m², but it increases exponentially when PV factor is greater than 0.5 MW/m². SEM (scanning electron microscope) and EDS (energy dispersive X-ray analysis) were adopted to analyze the worn surface of brushes for different PV factors and a thermocouple was used to measure the temperature of the surface of brushes. These characterizations infer that the mechanical wear of brushes is proportional to PV factor and the electrical wear is mainly caused by damaging the lubricating film, oxidizing the resin binder, weakening the structure of brushes and a low “relative humidity” due to a high temperature on the surface of brushes.     

Thermal analysis on voltage responses of high-T c superconducting thin-films exposed to a pulse laser beam

One-dimensional radiation/conduction heat transfer model is employed to investigate thermal responses of HTSC thin-film detectors exposed to a pulse laser beam. The theoretical model includes local radiation absorption in the HTSC film based on the electromagnetic theory, thermal contact resistances at the interface between film and substrate, and nonuniform initial condition for the film/substrate temperature incurred by the Joule heating due to the bias current and inherent electrical resistance even before the radiation exposure. Using the steady-state conduction equation, the experimental resistance-temperature curve is corrected based on the real film temperatures instead of the substrate temperatures. The error involved in the estimation of the voltage jump based on the model without initial Joule heating could be significant near the transition temperature. Still there is a big discrepancy between the theoretical and experimental results, though the nonuniform initial condition model reduces the gap.     

Brush wear detection by continuous wavelet transform

This paper is concerned with the singularity detection of a motor current signal. These singularities are related to the brushes condition, which are one of the sensible parts of the motor to wear out. As the mechanical contact between brushes and commutation segments increases, the rising parts of the current commutation waves become wider and this is a signal of brush wear. We successfully implement an algorithm to localise the singular points of the commutation waves based on the wavelet maxima lines. The width of the rising part of the commutation waves is then calculated from these maxima lines.     

空间导电滑环技术研究与发展综述

空间导电滑环是通过弹性电刷在导电环道内的滑动电接触来实现航天器连续转动部分和相对固定部分间传输电功率和电信号的关键部件,目前工程上对空间服役环境下导电滑环磨损量、摩擦力矩波动、接触电阻及电噪声等方面性能表现不稳定尚缺乏有效解决方法。空间导电滑环技术研究涉及材料、机械、物理、化学、空间环境等多学科概念及理论,通过梳理导电滑环近些年来的研究热点和难点,发现其性能实现与接触表面粗糙度、材料硬度、电刷压力、耐磨性、抗疲劳性、耐腐蚀性、真空下的自润滑、微重力环境等相关。目前对空间导电滑环电接触表面的微观特征及其演化过程、滑动电接触的磨损机制及磨屑运动路径和电传输性能及其控制方面的研究还存在不足,现有的知识储备和技术储备不足以支撑在轨更长寿命、更高可靠、更稳定运行的空间导电滑环研制,亟需提高对导电滑环在空间极端工况及多场耦合条件下相关特性的科学认识,以期为保障航天器在轨安全运行提供更多的理论指导和技术支持。     

Effect of arc discharge on the wear rate and wear mode transition of a copper-impregnated metallized carbon contact strip sliding against a copper disk

Wear of the contact strip on the pantograph of electric railway vehicles is governed mainly by arc discharge occurring simultaneously with break of contact between the strip and trolley wire. As a step to clarify the wear mechanism of metallized carbon contact strips under the occurrence of contact break arc discharge, a detailed sliding wear test of 30 min duration was carried out for the combination of a copper-impregnated carbon strip and a copper disk at a sliding speed of 100 km/h. The worn surfaces of the strip and disk were observed every 5 min. The voltage drop and electric current were measured throughout the test, and the occurrence and energy of the arc discharge were evaluated for each revolution of the disk. The wear process is considered in terms of the wear mode transition, and the effect of arc discharge on the mode transition and wear rate of the strip is discussed.     

Experimental study on arc ablation occurring in a contact strip rubbing against a contact wire with electrical current

A series of tests on arc discharge were carried out to better understand the wear mechanism of a contact strip rubbing against a contact wire with electrical current. The arc discharge process was recorded with a high-speed camera. The arc voltage drop and electric current were measured throughout the test. The accumulated arc discharge energy was evaluated. Experimental results show that the wear rate of the contact strip is approximately directly proportional to the accumulated arc discharge energy in logarithmic coordinates. Increasing the normal force can suppress arc discharge and decrease wear of the contact strip.     

基于DSP的新型电参数测量仪设计

针对电机型式试验中的电参数测试,设计了一种基于DSP的新型电参数测量仪。详细论述了整个系统的硬件构架与软件算法,该系统通过采用高精度A/D转换器、DSP芯片以及上位机来完成电压、电流、功率、功率因数及频率的测量与数据的输出显示。对于中小型电机,测量仪既可测三相交流电参数,又可同时测量单相交流与直流电参数;当被测对象为直流有刷电机时,还能测量其转速;测试量程根据输入电流、电压的大小自动进行切换。测试结果表明该系统精度高,运行稳定可靠,能够满足实际需求。     

Sliding wear behavior of copper alloy contact wire against copper-based strip for high-speed electrified railways

This paper reports the study of the sliding wear behavior of the Cu–Ag–Cr wire. Cu–Ag–Cr alloy is a promising contact wire material for high-speed electrified railways, which has an excellent combination of mechanical strength and electrical conductivity. Wear tests were conducted under laboratory with a special sliding wear apparatus, which simulated the tribological conditions of sliding current collectors on overhead wires in the railway system. The Cu–Ag–Cr alloy wire was slid against a copper-based powder metallurgy strip under unlubricated conditions. The same strip as those in the train systems were used. Worn surfaces of the Cu–Ag–Cr alloy wire were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectrum (EDS). Within the studied range of electrical current, normal pressure and sliding speed, the wear rate increased with the increasing electrical current and the sliding distance. Compared with a Cu–Ag contact wire under the same conditions, the Cu–Ag–Cr alloy wire had much better wear resistance. Adhesive wear, abrasive wear and arc erosion were the dominant mechanisms during the electrical sliding processes.     

A procedure for the wear prediction of collector strip and contact wire in pantograph–catenary system

The effects of friction and electrical phenomena like arcing and sparking govern the wear rate in the sliding contact between the contact wire and the collector strip, these two effects are mutually interconnected in a complex manner. A means of investigating the wear of the collector strip and the contact wire is to carry out laboratory tests that allow to perform comparative tests between different material combinations and to establish the dependence on the main parameters such as sliding speed, contact force and current intensity. A problem to be considered in the application of the laboratory test results is their extrapolation to the real operating conditions, in order to assess the effective benefit among different solutions. In this paper, a procedure that combines a wear model for the contact between collector strip and contact wire with the simulation of the dynamic interaction between pantograph and catenary is proposed. The adopted wear model is based on the wear map concept, including the effect of electrical current flow, and it is tuned by means of the results obtained on laboratory test rig.The dependence of the electrical contact resistance on the contact force between each contact strip of the pantograph and the contact wire of the overhead line is considered and the corresponding electrical current on each of the two collectors of the pantograph is evaluated. Instantaneous values of contact forces and electrical current are then fed into the wear model and the amount of the wear of the collector strips and of the contact wire along the overhead line is calculated, generating an irregular profile of the contact wire.The proposed procedure is applied to two cases: in the first one the wear of the contact wire using copper collector strips and graphite collector strips for dc line are compared. In the second one, the consequence of the variation of the mechanical tension of the contact wire on the wear levels is predicted.