Cause Analysis of Carbon Brush Failure of Diesel Generator

Generally, some small diesel generator sets also use alternator with carbon brushes. Alternator with carbon brushes should be maintained and replaced regularly. Today this article is mainly about the analysis of carbon brush failure of diesel generator. 

Factors leading to carbon brush failure:

Electromagnetic factors:

1. When the reactive power or excitation current is adjusted, the spark of carbon brush changes obviously. When the exciter is commutated, the carbon brush is in poor contact with the commutator, and the contact resistance is too large;

2. Uneven thickness of oxide film of commutator or slip ring causes unbalanced distribution of carbon brush current;

3. Or sudden load change and sudden short circuit lead to abnormal voltage distribution between commutators;

4. Unit overload and imbalance;

5. The selection of carbon brushes is unreasonable, and the spacing of carbon brushes is different;

6. Carbon brush quality problems, etc.

Mechanical factors:

1. The center of the commutator is not correct and the rotor is unbalanced;

2. Large vibration of the unit;

3. Insulation between commutators protrudes or the commutator protrudes;

4. The contact surface of carbon brush is not polished smoothly, or the surface of commutator is rough, resulting in poor contact;

5. The commutator surface is not clean;

6. The air gap under each commutation pole is different;

7. The spring pressure on the carbon brush is uneven or the size is inappropriate;

8. 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.

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

Maintenance of carbon brush

A. Operation inspection. Strengthen regular and irregular equipment patrol inspection. Under normal circumstances, the staff must check the generator carbon brush twice a day (once in the morning and once in the afternoon), and measure the temperature of the collector ring and carbon brush with an infrared thermometer. During peak load in summer and when reactive power and voltage fluctuate greatly, the temperature measurement interval shall be shortened, and the replaced new carbon brush shall be subject to key inspection. Conditional users should regularly measure the temperature of collector ring and carbon brush with infrared thermometer. Record the operation conditions of patrol inspection equipment.

B. Repair and replace. Check and accept the newly purchased carbon brush. Measure the inherent resistance value 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. There should be a gap of 0.2 - 0.4mm in the brush holder, and the brush can move up and down freely 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 and be easy to be damaged. If the distance is too large, the electric brush is easy to jump and produce sparks. Strive to make the contact surface of the carbon brush greater than 80% of the cross section of the carbon brush. Replace frequently, but the carbon brushes should not be replaced too many times. The number of carbon brushes replaced at one time shall not exceed 10% of the total number of single poles. The carbon brush whose top is 3mm lower than the top of the brush holder shall be replaced as soon as possible. Each time the carbon brush is replaced, the carbon brush of the same model must be used, but pay attention to saving and making full use of the carbon brush. The carbon brush after replacement must be measured by DC caliper meter, and the temperature test shall be carried out by infrared thermometer to prevent individual carbon brushes from overheating due to overcurrent. For obvious equipment problems such as protrusion and depression of slip ring or commutator commutator commutator, the opportunity of unit maintenance shall be used for fastening and turning and grinding. Strengthen the maintenance quality and operation control to avoid the leakage of turbine oil onto the collector ring during the operation of the unit due to poor maintenance quality or improper operation adjustment, and increase the contact resistance between the carbon brush and the collector ring. The brush holder and brush holder shall be carefully adjusted during major and minor maintenance of the unit. When putting back and installing the brush holder, the angle and geometric position shall be in the original state, and the sliding in edge and sliding out edge of the carbon brush must be parallel to the commutator.

C. Routine maintenance. Clean frequently and keep the smooth surface of carbon brush and commutator slip ring clean. In case of windy weather, it must be cleaned in time. Adjust the spring pressure frequently. The pressure of the carbon brush spring shall comply with the regulations of generator manufacturer to make the carbon brush bear uniform pressure. Prevent individual carbon brushes from overheating or sparks, and brush braids from burning. Problems in the operation of carbon brushes 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 hair braid 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. The technician must be experienced in adjusting and cleaning the slip ring of the motor.