The carbon brush is the electric brush. The operation and maintenance technology of the carbon brush of the generator excitation system is a traditional technology, and there has been no substantive breakthrough in the process and technical level for many years. In addition, all parties paid insufficient attention to the generator excitation system and limited investment, which led to the carbon brush accident of the excitation system. When the carbon brush introducing and exporting current fails in operation, how to distinguish the fault type and nature, so as to eliminate the fault as soon as possible and ensure the stable operation of the equipment.

1、 Factors leading to carbon brush failure

1. Electromagnetic factor: when the reactive power or excitation current is adjusted, the spark of carbon brush changes obviously. When the exciter is commutated, the carbon brush and the commutator are in poor contact, and the contact resistance is too large; The oxide film of commutator or slip ring is uneven, which causes the unbalanced distribution of carbon brush current; Or sudden load change and sudden short circuit lead to abnormal voltage distribution between commutators; Unit overload and imbalance; Unreasonable selection of carbon brushes and unequal spacing of carbon brushes; Carbon brush quality problems and so on.

2. Mechanical factors: the center of the commutator is not correct and the rotor is unbalanced; Large unit vibration; Insulation between commutators protrudes or the commutator protrudes; The contact surface of carbon brush is not polished smoothly, or the surface of commutator is rough, resulting in poor contact; The commutator surface is not clean; The air gap under each commutation pole is different; The spring pressure on the carbon brush is uneven or the size is inappropriate; The carbon brush is too loose in the brush holder and jumps, or too tight, and the carbon brush is stuck in the brush holder. The spark will be reduced when the running speed of the unit is reduced or the vibration is improved.

3. Chemical factors: when the unit operates in corrosive gas or the unit operation space lacks oxygen, a naturally formed copper oxide film on the surface of the commutator in contact with the carbon brush is damaged, the formed commutation of approximate linear resistance no longer exists, and the commutator spark intensifies in the process of re forming oxide film on the contact surface. The commutator (or slip ring) is corroded by acid gas or grease. Carbon brush and commutator are polluted.

2、 Maintenance of carbon brush

1. Operation inspection: strengthen regular and irregular equipment inspection. Under normal circumstances, the staff must inspect the generator carbon brush twice a day (once in the morning and once in the afternoon), measure the temperature of collector ring and carbon brush with infrared thermometer, and shorten the temperature measurement interval during peak load in summer and when reactive power and voltage fluctuate greatly, Replace the new carbon brush and make key inspection. Units with conditions shall regularly measure the temperature of collector ring and carbon brush with infrared thermal imager, and make working condition records.

2. Maintenance and replacement: check and accept the newly purchased carbon brush. Measure the inherent resistance of the carbon brush and the contact resistance of the carbon brush lead. The resistance value shall comply with the manufacturer and national standards. Strictly grasp the process of replacing carbon brushes. The carbon brushes used in the same unit must be consistent and cannot be mixed. Before replacing the carbon brush, carefully grind the carbon brush to make its surface smooth. The electric brush shall have a clearance of 0.2 – 0.4mm in the brush holder and move freely up and down in the brush holder. The distance between the lower edge of the brush holder and the working surface of the commutator is 2-3mm. If the distance is too small, it will collide with the commutator surface, which is easy to be damaged. If the distance is too large, the electric brush will jump and produce sparks. Strive to achieve that the contact surface of the carbon brush is greater than 80% of the cross section of the carbon brush. The carbon brushes shall be replaced frequently, but not too many times. The number of carbon brushes to be replaced at one time shall not exceed 10% of the total number of single poles. The carbon brushes with the top of the carbon brush 3mm lower than the top of the brush holder shall be replaced as soon as possible. The carbon brushes of the same model must be used each time, but attention shall be paid to saving and making full use of the carbon brushes.

3. Daily maintenance: clean and blow soot frequently, keep the smooth surface of carbon brush and commutator slip ring clean, and clean it in time in case of windy weather or after. The spring pressure shall be adjusted frequently, and the problems in the operation of carbon brush must be eliminated in time to avoid vicious cycle and endanger the normal operation of the unit. The carbon brushes used in the same unit must be consistent and cannot be mixed. The maintenance personnel shall be particularly careful during inspection and maintenance. The braids shall be placed in the hat and the cuffs shall be fastened to prevent the clothes and wiping materials from being hung by the machine. When working, stand on the insulating pad and do not contact the two poles or one pole and the grounding part at the same time, nor do two people work at the same time.