Key Takeaways & Executive Findings
- •• • At 35 °C and 50% RH, the average friction coefficient is 0.1297, a 46.6% reduction from the maximum of 0.2427 at 25 °C and 55% RH. This translates to lower mechanical energy losses and reduced wear in hydroelectric generator brush systems, potentially extending maintenance intervals. • • The minimum average contact resistance of 1.52 Ω occurs at 25 °C/50% RH and 40 °C/45% RH, a 49.3% decrease from the maximum of 2.27 Ω at 25 °C/40% RH. Lower contact resistance reduces Joule heating and improves electrical efficiency, critical for high-current applications. • • Wear rate is minimized at 50% RH, with increased humidity reducing surface roughness and promoting water film formation that acts as a lubricant. This mitigates abrasive and adhesive wear, with adhesive wear lowest near 50% RH, enhancing brush longevity. • • Contact temperature follows an 'N'-shaped trend with increasing temperature at constant humidity, and at 30 °C, friction coefficient, wear rate, and contact resistance all reach relatively low values. This non-monotonic behavior underscores the need for precise environmental control to avoid performance degradation.
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Abstract
The operational reliability of hydroelectric generator units is critically constrained by the current-carrying tribological performance of carbon brush/collector ring systems, which are highly sensitive to ambient temperature and humidity. This study conducted controlled-atmosphere experiments on a 45 steel/carbon friction pair under temperatures of 20–40 °C and relative humidity (RH) of 40–60%. Key parameters including friction coefficient, wear rate, contact resistance, and contact temperature were measured, and surface damage mechanisms were analyzed. Results show that at 35 °C and 50% RH, the average friction coefficient reached a minimum of 0.1297, a 46.6% reduction compared to the maximum of 0.2427 at 25 °C and 55% RH. The lowest average contact resistance of 1.52 Ω was obtained at 25 °C/50% RH and 40 °C/45% RH, representing a 49.3% decrease from the maximum of 2.27 Ω at 25 °C/40% RH. Wear rate was minimized at 50% RH. Contact temperature exhibited an 'N'-shaped variation with increasing temperature at constant humidity. Elevated temperature promoted oxidation but reduced water vapor and induced desorption of water molecules, hindering water film formation. At 30 °C, friction coefficient, wear rate, and contact resistance all reached relatively low values. Increased humidity reduced surface roughness and smoothed the brush surface. Water vapor is a key factor influencing abrasive and adhesive wear, with adhesive wear minimized near 50% RH. High temperature or high humidity environments degrade current-carrying tribological performance. These findings provide optimal environmental parameters for enhancing the operational reliability of hydroelectric generator units.
1. Introduction
Current-carrying tribological systems in hydroelectric generator units face persistent challenges from variable ambient conditions, leading to excessive wear and electrical instability. Existing commercial carbon brush/collector ring assemblies often fail prematurely due to uncontrolled temperature and humidity, causing unplanned outages and increased maintenance costs. The lack of systematic understanding of how these environmental factors couple to affect friction, wear, and contact resistance has hindered the development of optimized operating protocols.
This study addresses the bottleneck by conducting controlled-atmosphere experiments on a 45 steel/carbon friction pair under a matrix of temperatures (20–40 °C) and relative humidity levels (40–60% RH). By measuring friction coefficient, wear rate, contact resistance, and contact temperature, and by analyzing surface damage mechanisms, the work establishes quantitative relationships and identifies optimal conditions. The findings provide a mechanistic framework for environment-specific design and operation of carbon brush/collector ring systems, moving beyond trial-and-error approaches.
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ZHAO Xinze, LI Yang, WU Hailin, LI Wanting, LI Chenshi, XU Xiang, ZHAO Meiyun, YANG Wei (2026). Effect of Ambient Temperature and Humidity on the Current-carrying Tribological Properties of Carbon Brushes/Collector Rings. Surface Technology (表面技术). https://doi.org/10.16490/j.cnki.issn.1001-3660.2026.11.004
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Frequently Asked Questions
What is the optimal ambient temperature and humidity combination to minimize friction coefficient in carbon brush/collector ring systems?
The minimum average friction coefficient of 0.1297 is achieved at 35 °C and 50% RH, which is 46.6% lower than the maximum of 0.2427 observed at 25 °C and 55% RH. This combination significantly reduces mechanical losses and wear, enhancing operational efficiency.
How does humidity affect contact resistance and wear rate, and what is the underlying mechanism?
Contact resistance reaches a minimum of 1.52 Ω at 25 °C/50% RH and 40 °C/45% RH, a 49.3% reduction from the maximum of 2.27 Ω at 25 °C/40% RH. Wear rate is also minimized at 50% RH. Increased humidity promotes water film formation on the contact surface, which acts as a lubricant, reducing abrasive and adhesive wear. However, excessive humidity can lead to increased oxidation and water desorption at high temperatures, degrading performance.
What are the failure mechanisms under high temperature or high humidity conditions?
High temperatures accelerate oxidation, forming conductive oxides that increase contact resistance, while also causing water desorption and reducing water film lubrication, leading to increased friction and wear. High humidity beyond 50% RH can cause excessive water accumulation, leading to hydrodynamic effects and potential electrical instability. Both extremes degrade current-carrying tribological performance.
How does contact temperature vary with ambient temperature, and what are the implications for system design?
Contact temperature exhibits an 'N'-shaped variation with increasing ambient temperature at constant humidity: initially increasing, then decreasing, then increasing again. This non-monotonic behavior suggests complex interactions between frictional heating, oxidation, and water film effects. Designers must avoid temperature ranges that exacerbate contact heating to prevent thermal runaway and material degradation.
What is the industrial significance of the optimal conditions identified, and how do they compare to current practices?
The optimal conditions (e.g., 35 °C/50% RH for friction, 25 °C/50% RH for contact resistance) offer a 46.6% reduction in friction coefficient and 49.3% reduction in contact resistance compared to worst-case scenarios. Implementing these conditions can extend brush life, reduce maintenance costs, and improve energy efficiency in hydroelectric generators, providing a clear pathway for operational optimization.
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