R&D

Technical research on carbon brushes

Carbon brush installation

1. The carbon brush should be installed in the brush holder to ensure free movement up and down. The gap between the carbon brush and the inner wall of the brush holder is between 0.1-0.3 mm to avoid the swing of the carbon brush and the brush holder due to excessive clearance. The distance between the lower edge of the brush holder and the surface of the commutator should be kept at about 2 mm.

2. On the same motor, in principle, the same type of carbon brush should be used. However, for large and medium-sized motors with special difficulty in individual commutation, a double-sub-carbon brush can be used, which has good lubrication performance when sliding into the side, and the sliding edge is suppressed. A carbon brush with a strong sparking power, which improves the operation of the carbon brush.

3. The unit pressure stress applied to each carbon brush of the same motor should be uniform to avoid uneven current distribution, resulting in overheating and sparking of individual carbon brushes. The unit pressure of the carbon brush should be selected according to the “carbon brush technical performance table”. For motors with higher speed or motors operating under vibration conditions, the unit pressure should be properly increased to ensure normal operation. For example, the carbon brush unit pressure of the tractor motor is 0.4-0.6kgf/cm2. The sustained and rapid development of the national economy has widened the already severe shortage of power supply demand, and the installation and installation of self-provided power supply systems and self-contained engine systems for new units, various industries, such as “spring shoots” can be seen everywhere. The carbon brush operation and maintenance technology of the generator excitation system is a traditional technology, and there has been no substantial breakthrough in the process and technology level for many years. Therefore, all parties pay insufficient attention to the excitation system of the generator, and the input is limited, which leads to the carbon brush accident of the excitation system. In the past period of multi-phase safety notification, several large-scale generator sets were continuously reported, causing large economic losses to the enterprise due to the shutdown accident caused by the failure of the carbon brush or the rotor collector ring. When a fault occurs in the operation of the carbon brush that imports and exports current, how to distinguish the type and nature of the fault so as to eliminate the fault as soon as possible and ensure the stable operation of the equipment is something that everyone needs to seriously consider.

Carbon brush detection

1. From an intuitive point of view, the brush should be chamfered, the specifications are appropriate, the structural specifications, the cross-section and length of the wire meet the requirements, and there is no looseness, falling off, damage, falling edge, corner drop, and clamps.

2. From the point of view of use, there are mainly the following situations in which the brush has good performance:

(1) Long service life does not wear the commutator or collector ring.

(2) It has good commutation and current collecting performance, so that the spark suppression is within the allowable range and the energy loss is small.

(3) When the brush is running, it is not hot, the noise is small, and it is not damaged.

(4) During the operation, a uniform, moderate and stable oxide film can be formed on the surface of the commutator or the collector ring faster.

3. Detect the brush through the instrument. From a technical perspective, the brush should meet the standards issued by the Ministry of State. Through the instrument, the brush can be tested according to the performance required by the Brush Technical Performance Table. Such as brush resistivity, Rockwell hardness, bulk density, current density, contact voltage drop, friction coefficient, 50-hour wear, allowable peripheral speed, unit pressure, and even metal content.

4. On the same motor, you should choose to use the same model and the same manufacturer's brush. Because the performance of brushes produced by different manufacturers varies greatly, even the performance of brushes produced by the same manufacturer at different times is different. Therefore, for the same motor, the same manufacturer of the same model should be selected as much as possible, preferably the brush produced at the same time to prevent the imbalance of the current distribution of the parallel brush due to the difference in brush performance, affecting the normality of the motor. run.

5. For large and medium-sized motors with special difficulty in individual commutation, you can choose to use a split brush. The purpose is to improve the commutation performance by increasing the short-circuit current in the brush. For motors with difficult one-way rotation and commutation, a combination brush can be selected. That is, the sliding-in side selects a low-resistance graphite-based electrochemical graphite brush with good lubricity and strong film forming ability, and selects a high-resistivity carbon black-based electrochemical graphite brush with good commutation performance. This makes it easy to form an oxide film and greatly improves the commutation performance. Carbon fiber composite brushes can also be used on the sliding side to highlight the characteristics of strong arc extinguishing ability, so that the motor can achieve the best effect.

Precautions

1. If the lead wire of the carbon brush is covered with an insulating tube, it should be installed in the insulated carbon brush holder; the lead wire is bare copper wire and should be installed in the ground carbon brush holder.

2. When the carbon brush is placed on the carbon brush holder, pay attention to the direction of the curved surface. If the reverse contact surface is too small, the power generation is weak or does not generate electricity.

3. The carbon brush should be able to rise and fall freely in the carbon brush holder. If there is a card issue, the excess should be removed.

4. The carbon brush spring should be pressed in the middle of the carbon brush to prevent eccentric wear due to uneven force.

5. The contact area between the carbon brush and the commutator should not be less than 3/4 of the total contact surface. The carbon brush should not be contaminated.

replace

Precautions

(1) It is strictly forbidden for two people to work at the same time;

(2) The clothes and cuffs must be fastened during work to avoid being twisted by the rotating part;

(3) Standing on the insulating mat during work;

(4) It is strictly forbidden to touch two poles or one pole to the ground at the same time;

(5) The same type of carbon brush should be used for the same unit; if the source is difficult, different types of carbon brushes can be used (the same type still guarantees the same model), but the prior approval of the leader must be obtained;

(6) Replace the brush immediately below the brush holder or as flat as the brush holder.

Replacement situation

When the carbon brush appears in the following situations, it should be replaced: 1. The wear is serious (less than 1/3 of the original length); 2. The damage is serious; 3. The contact surface of the carbon brush and the slip ring is less than 70%.

Troubleshooting

1. In order to ensure the normal operation of the motor, it is very important to choose the correct brush type. Because of the different raw materials and processes used in the manufacture of the brushes, the technical performance is also different. Therefore, when selecting a brush, the performance of the brush and the requirements of the motor for the brush should be considered comprehensively.

The performance of the brush should be good: the brush has a long service life and does not wear the commutator or the collector ring; the brush has good commutation and current collecting properties, so that the spark is suppressed within the allowable range, and the energy The loss is small; when the brush is running, it is not hot, the noise is small, the assembly is reliable, and it is not damaged.

2. The brush should be installed in the brush holder to ensure that it can move up and down freely. The gap between the brush and the inner wall of the brush holder is between 0.1 and 0.3 mm to avoid swinging due to excessive gap between the brush and the brush holder. The distance between the lower edge of the brush holder and the surface of the commutator should be kept at about 2 mm. If the distance is too small, the brush grip is easy to touch the commutator, and the distance is too large, and the brush is easy to vibrate and cause damage.

3. On the same motor, in principle, the same type of brush should be used. However, for large and medium-sized motors with special difficulty in individual commutation, the double-sub-brush can be used. The sliding-in side has good lubrication performance, and the sliding edge is suppressed. A brush with a strong sparking power, which improves the operation of the brush.

4, the brush wear to a certain extent to replace the new brush, the brush is best to replace all at once, if the old and new mixed, there may be uneven current distribution. For large units, the replacement of brushes will inevitably affect production, and you can choose not to stop. We usually recommend that customers replace 20% of the brushes (20% of each brush of each motor) each time. The interval is 1-2 weeks. After the operation is completed, the remaining brushes are gradually replaced to ensure the normal continuous operation of the unit.

5. In order to make the brush and the commutator in good contact, the new brush should be subjected to arcing, and the arcing is generally performed on the motor. Place a piece of fine glass sandpaper between the brush and the commutator. Under normal spring pressure, grind the brush along the direction of motor rotation. The sandpaper should be as close as possible to the commutator until the arc of the brush is anastomosed, then removed. Sandpaper, use compressed air to blow the dust, and then wipe it with a soft cloth. Grinding brushes should not be used with emery paper to prevent emery particles from being embedded in the commutator slots. When the motor is running, the brushes and commutator surfaces are scratched. After arcing, the motor first runs 20-30% under load for several hours to make the brush and commutator run in and create a uniform oxide film. Then gradually increase the current to the rated load.

6. Apply uniform pressure to each brush of the same motor to avoid uneven current distribution, resulting in overheating and sparking of individual brushes. The unit pressure of the brush should be selected according to the “brush technical performance table”.