1. Brush current density is too high. In actual operation, if one or two brushes are stuck due to mechanical reasons and spring pressure is insufficient, the current through other brushes will increase in order to maintain normal load. If there is a brush deflected by rubbing in the brush with increasing current, the contact area with the sliding ring will decrease and the current density on the contact surface will increase. According to Q=I*IRT, the calorific value is proportional to the square of the current. When the accumulated calorific value cannot be emitted, the temperature rises sharply and sparks will appear on the contact surface between the brush and the slip ring. In addition, the spark will make the working environment of the brush worse, which will lead to a vicious circle and make the spark of the brush worse. Circumferential fire will occur when several of these conditions are serious.
2. The brush pressure is not uniform or does not meet the requirements. When the brush leaves the factory, the manufacturer is related: the brush pressure is adjusted to the minimum pressure without spark. After scientific demonstration and practice summary, the minimum pressure is generally 200-300/c, that is, 0.02-0.03 MPa. It is required that the pressure between brushes should be uniform, and the difference between brushes should not exceed 10. In the long-term operation of the generator, if the friction of a brush is too short, or when the brush brushes the hysteresis card in the frame, the pressure will be uneven. In addition, due to the influence of machinery and the softening of the spring itself by heat, the pressure can not meet the minimum theoretical requirement that the brush does not spark. In addition, when we press and pull carbon brushes, sometimes we rely on feeling, without experience accumulation, and the operation error is very large.
3. The surface of the brush is dirty. This is the most neglectable point. Normal generator brushes are sealed in a closed cabinet. There are geared axial flow fans in the middle of the two poles brushes. The heat of graphite powder and brushes is pumped out in time by the rotation of the rotor to achieve the function of clean cooling. If the cabinet door is not closed properly or forgot to close during inspection, the dust outside is absorbed into the brush with hot air and attached to the brush column, which affects the heat dissipation of the brush on the one hand. On the other hand, when sparks occur, the insulation strength between brush columns is not enough because of the unclean surface of brush columns. In addition, there is a spiral groove on the sliding ring, which is to increase the heat dissipation area of the brush, improve the contact with the brush, make the brush more fully cooled, and more easily remove the carbon powder. If the cabinet door is not closed tightly, the fan suction is insufficient outside dust is easily attached to the spiral ditch, the consequences are conceivable. Exciter brushes are exposed because of their low voltage level. It is also recommended that they be cleaned regularly during overhaul. A few days ago, our factory had an excitation brush spark forced shutdown incident, the reasons are various, often small things lead to major disasters. We should pay enough attention to it.
4. Bearing vibration is large. The large axles of the first and second phases are equipped with exciters. Because of the long bearings and many additional equipments, the vibration is naturally difficult to control to the minimum. The vibration of the bearing is increased due to the complex factors such as long running of the unit, fatigue, aging and so on. With the vibration of the brush, the friction of the brush column will be uneven, the lead wire and spring will be loosened, and local sparking will slowly further affect the overall situation.
Other reasons, such as poor brush manufacturing technology, poor abrasion, wrong position of brush, uneven gap, excessive contact resistance, high load, short circuit and so on, may exist in practice, but in recent years, the production practice shows that the above four reasons are the most common.
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