This page describes the tips, experience and knowledge for carbon brush maintenance and troubleshooting, as well as tips for slip ring maintenance, and how they work together.

Newyard Carbon are not only specialized manufacturing carbon brush raw graphite materials such as eletrographite, natural graphite, metal graphite and resin-bonded graphite but also the finished products for wind turbine, generator, industrial and traction motors and power tools as well.

tips for Carbon Brush Mainantence and slip Ring maintenance

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Carbon Brush Selection, maintenance and troubleshooting

Good preventive maintenance measures can ensure the normal operation of the motor and carbon brushes. The preventive and maintenance measures depend on the model and type of the motor, especially the working environment of the motor, the period of intermittent operation, the load condition and other related parameters are very important.

Importance of design and selection of slip rings and carbon brushes

(1) The carbon brush material should be selected to match the conductive ring material, motor working condition, and operating environment. The main carbon brush should be made of copper-graphite material and grounded carbon brush made of silver-graphite material, and the structure is double-joined.
(2) The slip ring conductive ring is made of martensitic stainless steel, which is quenched and tempered to increase wear resistance. Coastal areas should take anti-salt spray measures. Metal materials meet mechanical properties of the the working conditions of the motor, and the insulation material is grade F. The overall structure must be reliable and safe.

Precautions for assembly of slip ring and carbon brush

 (1) Use a dial indicator to detect the radial runout of the assembled slip ring. Generally, it is not allowed to exceed 0.05mm; 
(2) Detect the on-off of the carbon brush alarm switch line, which is normally closed under normal conditions to avoid the occurrence of no alarm after the carbon brush is worn out; 
(3) The constant pressure spring should be assembled in place, the spring pressure should be checked, and the spring beyond the standard pressure range should be replaced in time.
(4) Check whether the contact surface of the carbon brush is qualified, whether the brush braid is broken, whether the joint is good, whether the brush braid is firmly connected to the brush holder, and whether the ground wire is connected,
whether the contact parts are clean to prevent poor contact between the carbon brush and the sllip ring brush holder.
(5) Check the distance between the brush holder and the ring surface of the slip ring, the distance should be 2.5mm-3mm; the installation angle of the brush holder is concentric with the slip ring, and whether the gap between the carbon brush and the brush holder is qualified.

Things to pay attention to during the shutdown inspection

Cut off the power supply of the motor, and detect the working conditions from when the motor is decelerated to when it stops completely.
Check when slowing down:
(1) Observe the vibration and noise of the motor during vulgar operation. Such conditions may affect the damage of the bearings or carbon brushes in the motor;
(2) Check whether the coupling has an impact sound when the motor is running;
(3) Measure the surface temperature of the commutator or slip ring immediately after the motor stops;

Important steps to clean the motor

Regularly clean the toner accumulated in the slip ring chamber, sliding surface, brush holder, and brush grip gap to avoid serious carbon deposits. Keep the slip ring surface clean and free of oil or attachments.
(1) Thoroughly blow clean the stator and rotor from both ends of the motor with dry compressed air.
(2) To clean the motor effectively, foreign matter and dust should be blown out of the motor thoroughly.
(3) Clean the air filter with detergent and blow dry.
(4) Wipe the insulating part with a dry cloth.
(5) Use glass fiber or nylon brush to remove the deposits between the commutator segments.
(6) Regularly replace the filter in the toner collector to keep the cooling air path of the slip ring chamber unobstructed.

Electrical testing and mechanical inspection

(1) Measure the concentricity of the motor commutator or slip ring. The ideal value should be 0.01mm. The concentricity of the repaired commutator or slip ring should not exceed 0.03mm.
(2) Measure the insulation resistance of the winding four times; heating/cooling, before cleaning/after cleaning.
(3) Measure the pressure of the carbon brush compression spring.
(4) Measure the wear of the bearing.
(5) Measure the length of the carbon brush on each motor to check whether the wear is normal.

Check the surface roughness of the commutator or slip ring

The surface roughness Ra of the commutator or slip ring should be avoided to be less than 0.2μm. If it is less than 0.2μm, a medium-grain grindstone or 120 mesh (not higher than 150 mesh) should be used.
Sandpaper treatment. If the surface roughness exceeds the Ra value of 2μm, the carbon brush will wear out too quickly.
(1) The roughness Ra of the slip ring motor is required to be between 0.75 and 1.25 μm.
(2) The roughness Ra value of the commutator surface of ordinary industrial motors is required to be between 0.9 and 1.8 μm.

(3) The surface roughness Ra value of the commutator of the motor below 1KW requires 0.5~1.8μm.

If there are pits and bumps on the surface of the slip ring, use oilstone to polish off the large bumps first, and then polish it with No. 0 polishing sandpaper to until the slip ring is restored smooth. If the damage is serious, it is recommended to remove the sliding ring to process the torus. When processing, the inner hole of the slip ring to be coaxial with the outer circle  the degree should be controlled within 0.03mm, and the toroidal surface roughness Ra is 0.8μm-1.2μm.

Check before starting

A: commutator/slip ring
(1) Check the chamfer of the commutator segment and whether the commutator segment has dents, ablation marks, oil leakage marks, etc. (If the oxide film is too thick, it is best to deal with it with sandpaper) 
(2) Check whether the groove edge of the slip ring is damaged or ablated. If the edge of the slip ring is very sharp, it will cause rapid wear or production of the carbon brush, create issues with sparks and ablation.
(3) Check the commutator or slip ring and confirm that it is not contaminated by oil.
Note: The number of commutating blades between any two adjacent rows of brush holders must be equal.
B: Brush holder and carbon brush
(1) The distance between the brush holder and the commutator should be uniformly 2~3mm.
(2) Check and confirm that there is no copper silt inside and outside the brush holder.
(3) Check whether the contact edge of the carbon brush is damaged, missing corners, and whether there is ablation or vibration on the contact surface of the carbon brush.
(4) Check whether the inner surface of the brush box is smooth and clean.
(5) Check whether the carbon brush wire is oxidized, ablated or damaged.

Steps to be done after reinstalling the carbon brush into the brush holder

 (1) After replacing the carbon brush, the contact surface must be pre-grinded with No. 100 sandpaper, and the motor is idling for about 30 minutes to continue to run in the contact surface until it reaches more than 80% be connected to the grid. When replacing the grounding carbon brushes, the grounding carbon brushes of the double-piece structure should follow the direction of motor rotation, and the sliding end of the carbon brushes is a carbon-containing one and the sliding end are the side containing silver, following the principle of lubricating first and then conducting electricity.
(2) The carbon brush can slide up and down normally in the brush box.
(3) The compression spring should be pressed at the correct position in the middle of the top of the carbon brush.
(4) The wire cannot be squeezed by the compression spring.
(5) The carbon brush grades on the same motor must be the same, and it is absolutely not allowed to mix and use carbon brushes of different manufacturers and grades.
Measure whether the pressure on each carbon brush is equal:
(1) The pressure requirement of the slip ring motor on the compression spring is generally 17-20 kPa.
(2) Ordinary industrial motors generally require 17-20 kPa for compression spring pressure (bad working conditions, large vibration and impregnation materials need to be increased to 25 kPa).
(3) Traction motors generally require 25 to 45 kPa for compression spring pressure.

Special treatment when the motor is stopped for a long time

Use paper or a clean soft cloth to protect the motor commutator or slip ring from damage or oil, dust, and air pollution.
If the motor needs to be placed in a humid, salty, acid-base or chemically polluted place for a long time, all the carbon brushes should be removed; if not, it is best to wrap insulation material between carbon brush and commutator/slip ring.
For areas with high humidity and low temperature in plateaus and coastal environments, the heater in the slip ring chamber should be turned on before the motor is shut down for a long time to avoid condensation on the surface of the slip ring, which may cause ignition of the slip ring and carbon brush.

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