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How to Recognize Brush of Diesel Generator Set.

Oct. 30, 2018

How to Recognize Brush of Diesel Generator Set

The materials of generator brushes are mostly made of graphite. In order to increase electrical conductivity, graphite has good conductivity, soft texture and wear resistance. The motor brush only exists in DC generator, which is used to realize mechanical commutation with commutator, that is, to ensure that the input to DC motor or DC generator output is DC current.

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With the development of power electronic technology, the brushless DC machine which uses the electronic device commutation also begins to appear widely, and the development is mature. Such as asynchronous motor and synchronous generator, are AC motor, no brush structure. The effect of the generator brush is to connect the current between two objects with relative motion. The most common are commutators for motors, trams with linear motion and brushes for electric locomotives.


The shape of the brush is different used on the motor is mostly rectangular, placed on the brush rack. Brush and commutator or sliding ring between a certain pressure, large brush structure is more complex, small brush is relatively simple.


The current source of the brush is also different. Large brushes have wires, and small ones are often drawn from elastic conductors. Cranes and subways, urban iron collector boots, can also be attributed to the brush, but the material and structure and graphite brush is very different.


The main cause of sparking in generator brush


The resistance coefficient of generator brush and exciter brush is small and the performance is stable. However, due to its current size, brush pressure, ambient temperature, humidity, cleanliness, wear on the surface of the brush and slip ring, and its own manufacturing process, etc., it is in operation for a long time. If the hidden trouble is not found in the first time and the decision is made decisively, the brush sparks and the ring fire will occur. Even because of brush reasons, forced to shut down events are inevitable. The following is the main reason why the generator really causes brush sparks in actual production.


1. Brush current density is too high.


In actual operation, if one or two brushes are stuck for mechanical reasons, the spring pressure is insufficient, and so on, in order to maintain the normal load, the current through other brushes will increase. Due to the decrease of contact area, the flux density of contact surface increases. According to QT, the heat is proportional to the square of the current. When the accumulated heat cannot be emitted, the temperature rises sharply, and the contact surface between the brush and the slip ring produces sparks. In addition, after the generation of spark, the working environment of electric brush is worse, thus the vicious circle, so that the brush sparks are more serious, several of them will have a ring fire when the situation is serious.


2. Brush pressure is uneven or does not meet the requirements.


When the brush comes out of the factory, the factory has the correlation. The brush pressure is adjusted to the minimum pressure without sparks. Through scientific demonstration and practice summary, the minimum pressure is generally 200 ~ 300 / c, i.e. 0.02 ~ 0.03 MPa, which requires that the pressure between each brush should be uniform, and the difference is not more than 10. If the brush friction is too short in the long run, or if the brush is stuck in the brush frame, the pressure will be uneven. In addition, the pressure caused by the mechanical and spring itself being heated and softened can not reach the minimum theoretical requirement of the brush without sparks. There are also we in the compression, drawing carbon brush, sometimes by feeling, no experience accumulation, the operation error is very large.


3. The surface of the brush is dirty.


This is the most important point that many users would ignore. In normal operation, the generator brush seal is in the closed cabinet, and the two poles brush has a gear shaped axial flow fan in the middle of the shaft. The graphite powder is removed by rotating the rotor, and the heat of the electric brush is removed in time to achieve the function of cleaning and cooling. If the door is not closed strictly, or forget to close the door during inspection, dust outside with hot air into the brush, adhere to the brush column, on the one hand affect the brush heat dissipation.


On the other hand, when the brush sparks, the brush column surface is not clean, the insulation between the brushes is enough easy to ring fire. In addition, there are spiral grooves on the slide ring, which is designed to increase the heat dissipation area of the brush, improve contact with the brush, make the brush cooling more fully, and make it easier for the carbon powder to be removed. If the cabinet door is not strict, fan pumping is not enough outside dust is easily attached to the spiral ditch, the consequences can be imagined. Exciter brush considering its low voltage level exposed in the outside, here is also recommended to repair regular cleaning.


4. Bearing vibration


Because the bearing is long and the additional equipment is more, it is difficult to control the vibration to the minimum. Complex factors such as long time operation fatigue aging and so on cause bearing vibration to increase. Brush with vibration will make brush-column friction uneven lead and spring loosening and local sparking will gradually affect the whole situation.


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