Generator brushes and exciter brushes have relatively small resistivity and relatively stable performance. However, they are affected by the size of the current flow, the pressure of the brush, the ambient temperature, humidity, cleanliness, the wear of the brush and the slip ring surface, and itself. Due to factors such as manufacturing process, in the long-term operation, if the hidden danger is not found in the first time and the judgment is made decisively, it is inevitable that the brush will spark, ring fire or even be forced to stop due to the brush.
There are many reasons for the occurrence of electric brush sparking, and related materials and regulations have made a detailed description of it. However, in actual production, the actual causes of electric brush sparking are probably the following:
1. The brush current density is too large. In actual operation, if one or two brushes are jammed due to mechanical reasons, insufficient spring pressure, etc., the current through other brushes will increase in order to maintain the normal load. For brushes that are deflected, the flow density of the contact surface increases due to the reduced contact area with the slip ring. According to Q=I*IRT, the heat generation is proportional to the square of the current. When the accumulated heat cannot be dissipated, the temperature rises sharply, and sparks will be generated on the contact surface of the brush and the slip ring. In addition, after the sparks are generated, the working environment of the brushes becomes worse, and the vicious circle makes the sparks of the brushes more serious. When this situation is serious, a ring fire will occur.
2. The brush pressure is uneven or does not meet the requirements. When the brushes leave the factory, the manufacturers have related: the brush pressure is adjusted to the lowest pressure that does not cause sparks. After scientific demonstration and practice summary, the lowest pressure is generally 200～300ɡ/c㎡, that is, 0.02～0.03MPa, and each brush is required The pressure between them should also be uniform, and the difference should not exceed 10. In the long-term operation of the generator, if the friction of a certain brush is too short, or the brush is stuck in the brush frame, the pressure will appear uneven. In addition, the pressure cannot reach the minimum theoretical requirement that the brush does not generate sparks due to factors such as the mechanical and the spring itself being heated and soft. In addition, when we press and pull the carbon brushes, we sometimes rely on feelings and have no experience to accumulate, and the operating errors are very large.
3. The surface of the brush is dirty. This point is most neglected. The generator brushes in normal operation are sealed in a closed cabinet. There is a gear-shaped axial flow fan sleeved on the shaft between the two-pole brushes. The graphite carbon powder and electric brushes are ground by the rotation of the rotor. The heat is removed in time to achieve the effect of cleaning and cooling. If the cabinet door is not closed tightly or the door is forgotten to close during the inspection, the dust from the outside will be sucked into the brush along with the hot air and attached to the brush column, which will affect the heat dissipation of the brush. On the other hand, when sparks occur in the brushes, the surface of the brush posts is not clean, and the insulation strength between the brush posts is not enough, so it is easy to ring fire. In addition, the slip ring has a spiral groove on the car, which is to increase the heat dissipation area of the brush, improve the contact with the brush, so that the brush can be more fully cooled, and the toner can be more easily removed. If the cabinet door is not closed tightly, the fan dust will easily adhere to the spiral groove if the fan’s suction is insufficient, and the consequences can be imagined. Considering that the voltage level of the exciter motor brush is relatively low, it is exposed to the outside, and it is also recommended to check and clean it regularly.
4. Large bearing vibration. The large shafts of the first and second phases are equipped with exciters. Due to the long bearings and more additional equipment, it is naturally difficult to control the vibration to the minimum. The long-term operation of the unit, fatigue, aging and other complex factors cause the bearing vibration to increase. With the vibration of the brush, the friction of the brush column will be uneven, the lead wire and the spring will be loose, and the occurrence of local sparks will gradually further affect the overall situation.
Post time: Jun-09-2021