With rapid development of China’s national economy in recent years, the self-contained power supply system springs up in various industries. However, as a conventional technology, the maintenance of generator carbon brushes for many years has no substantive breakthrough. Due to limited investment in generator excitation system, carbon brush accidents occur in succession.
Several cases of carbon brush failure leading to the generator failure have been reported and many manufacturing enterprises have suffered high economic losses. When the carbon brushes fail to work, how to diagnose the cause of failure and bring the brushes to normal operation is a question that we shall try to solve.
Hazards of Carbon Brush Failure
It was reported that carbon brushes installed on the rotor slip rings of two large-scale generator sets were damaged successively. After the brush accidents, the technical personnel tried to replace the damaged brushes but failed. Then the slip rings caught on fire and the entire generator sets were forced to split. Similarly, certain power plant of Northwest China Power Grid suffered emergency shutdown due to the carbon brush damage of QFS—125 generator. Another accident happened in certain power plant of Southern China in which the inner ring carbon brushes of generator suffered from undue wear, the collector ring brush holder was found seriously polluted and the temperature of the contact area of collector ring inner surface and carbon brushes was up to 230 ℃ ~ 360 ℃ (the normal temperature shall be 60 ℃ ~ 70 ℃). In the end, the entire generator set was forced into splitting. Frequent occurrence of carbon brush accidents has caused huge economic losses to the power plant and electrical system.
Carbon brushes, also known as electric brushes, if used improperly will cause an accident resulting in reduced load, or even shutdown of the generator set.
Causes of Carbon Brush Failure
Electromagnetic factors: With excitation current adjustment, carbon brush spark may have significant changes. When the generator excitation system changes the direction of the electric current, poor contact of carbon brushes and commutator segments, or uneven oxide film of commutator or slip ring often causes imbalanced distribution of carbon brush current; sudden load changes, or sudden short circuit leads to the abnormal distribution of commutation segments voltage; generator overload or voltage imbalance; uneven spacing of carbon brushes; quality problems of brushes, etc..
Mechanical factors: Deflected commutator center contributes to the imbalance of armature; high vibration of generator set; commutator segments convex; roughness of commutator surface resulting in poor contact of carbon brushes and commutator; stained commutator surface; imbalanced pressure of brush spring; carbon brushes getting loose or getting stuck in the brush holder; speed reduction of generator set leading to shrinking of brush sparks.
Chemical factors: When the generator set runs in corrosive gas or in a space lack of oxygen, the oxide film on the contact area of commutator and carbon brushes will be destroyed and the already formed commutation no longer exists. As the oxide film re-forms on the contact area, the spark of commutator will be intensified; commutator or slip ring getting corroded by acid gas or grease; carbon brushes or commutator being polluted, etc..
A. Runtime checks: Under normal circumstances, technical personnel are required to check the generator carbon brushes twice (one check in the morning, another in the afternoon). Use the infrared thermometer to measure the temperature of collector ring and carbon brushes. The interval of temperature checks in summer should not be long.
B. Repair and Replacement: Inspect newly-purchased carbon brushes to determine acceptance. Test the self-resistance values of carbon brushes to determine if the resistance values conform to the national standards. Ensure that the carbon brushes installed on the same generator set are consistent. Before replacing carbon brushes abrade the brush surface carefully and allow 0.2-0.4mm clearance when installing the brushes on the brush holder, permitting the brushes to move flexibly in the brush carrier. Note that the lower edge of the brush holder should have a distance of 2-3mm from the working surface of the commutator. If the distance is too small, the commutator surface can be easily damaged. If the distance is too large, the beating of brushes may produce sparks. Replace the carbon brushes regularly, but the replaced brushes shall not exceed 10% of the total number. And the newly-installed carbon brushes shall be measured by infrared thermometer to prevent the brushes from overheating. For the concave and convex of slip ring or commutator segments, tighten or abrade during the checks. Strengthen maintenance quality and operational control to avoid the engine oil leak which contributes to the increase of contact resistance between the collector ring and the carbon brushes. Adjust the brushes and brush holder, and guarantee that the slip-in edge and slip-out edge must be parallel to the commutator.
C. Routine maintenance: Keep the carbon brushes and commutator slip rings tidy; in case of windy weather, cleaning shall be done immediately; adjust the pressure of brush springs regularly, assuring that the brush spring pressure is in conformity with the manufacturing standards; prevent some carbon brushes from overheating or producing sparks. Ensure that the carbon brushes installed on the same generator set are consistent. When the technical personnel is conducting maintenance work, assure that hair is covered with a cap and sleeve cuffs fastened, avoid the clothes and cleaning material being caught by the machine. During the generator set checks, the technical personnel shall stand on the insulation mat and are prohibited to touch poles or grounding parts. Cleaning and adjusting carbon brushes and slip rings when the motor is running requires shall be conducted by experienced technical personnel.