Nitin Kulkarni
Technical Services Committee Member
Helwig Carbon Products, Inc.
The worn carbon brush face indicates the operating conditions. Therefore, it can be utilized by brush experts as a highly effective diagnostic tool for troubleshooting and determination of root causes. If these warning signs shown at the brush face can be identified and proactively addressed in a timely manner, then major unexpected expensive catastrophic failures like flashover or repair of the contact surface can be avoided.
Far too often when a motor or generator comes out of service or is sent in for repair, the used brushes are considered worthless as a replaceable item. As warning signs at brush faces are missed with root cause left unaddressed, there can be a repeat of failures and high-cost maintenance.
What is Happening at Brush Face?
Theoretically, contact area of the brush, Thickness X Width, is considered for current density (Amps/cm2 or Amps/in2). Although practically, at microscopic levels (Figure 1) the two contact surfaces are not making full contact. At any moment then the current is flowing through few contact points based on quality of the contact between the two surfaces (Figure 2). Other parameters that contribute significantly to the number of contact points at the brush face are current level, surface finish, total indicated runout (TIR), spring force and vibration level, etc.
The commutator or ring contact surface can also be used as a good diagnostic tool. The main challenge for the experts is the surface film or patina formation (Figure 3), often referred to as brush track, will vary for different grades, current density, humidity and other atmospheric conditions.
In many cases, it can be premature to use contact surface as the only reference for troubleshooting an application issue. Knowing the type of carbon grade while looking at the brush face is a valuable piece of information for better performance and to extend the life of the equipment.
Let us look at a few of the most common brush face warnings where there is an opportunity to make corrections.
Threading
These brush faces (Figure 4) indicate threading or hairlining. The most common root causes of this condition are lower, or under load or weak spring pressure. Vibrations and contamination can aggravate the situation. Dusting and contact surface damage are more severe symptoms. Please do note that some carbon grades, based on how they are made or their ingredients like high metal percentage, show more threading than others. Therefore, this condition can be minimized proactively with optimal spring and grade before the contact surface is damaged.
Chipping
These brush faces (Figure 5) indicate issues typically related to excess brush movement. Surface roughness, high points on contact surface, out of round condition or high frequency chatter due to light load are common causes. Inadequate holder support, weak springs and excess vibration could also cause this. Many carbon grades are brittle, hard or very weak by design and therefore experience chipping or fracture the edges. This can be minimized by addressing the contact surface, minimizing vibrations, optimum carbon grade and spring force combination.
Edge Burning
Trail edge arcing (Figure 6) in DC motors is certainly common. It is natural for more arcing on the exiting edge of the brush, as this breaks the conducting circuit when the commutator bar leaves the brush face. Brushes out of neutral is the most common root cause. Along with this, weak interpoles, excess bar coverage of the brush, or weak springs can contribute or worsen the situation. A better commutating grade and brush design can help minimize these problems, but machine adjustment is often needed to eliminate it.
Contamination
The below brush face pictures (Figure 7) show the influence of dust, oil, grease or any other foreign particles in the environment. Note metal brushes on slow-speed applications only show localized burning that appears as black spots due to high resistance. Metal brushes help clean the contact surface. If the brushes are non-metal and there are extreme cases of severe long-term contamination, the result can be high-intensity arcing with damage to the contact surface including hairlining or grooving as shown.
Double Facing and Lamination Lines
Double facing (Figure 8a) on the face of the brush is typically caused by reversing action in a dual direction motor, while lamination of the sliding face (Figure 8b) is caused by a winding fault resulting in a voltage surge during commutation.
Copper Picking
Copper picking (Figure 9) on the brush face is caused by poor or high resistance contact, abrasive grade and weak spring force, or high intensity arcing resulting in the pick up of softened copper from the contact surface. Other causes may include vibration from external or internal sources.
Sometimes in extreme conditions, the copper pick up from the contact surface is deposited in the brush face. This copper then becomes work hardened and causes streaking or threading of the contact surface in addition to the brush face.
Conclusions
It would be premature to blame the brushes or the carbon grade for fast wear problems. The appearance of the worn brush is telling you a story about operating conditions. Identify the warning signs by looking at the used brush face. Instead of throwing away used brushes, take clear pictures of some or save a few that best represent all other brushes. Involve an experienced and logical carbon brush expert, preferably one who knows how different carbons are made at ingredient level. This will help you to resolve issues with operating conditions outside the scope of carbon brushes and extend the life of your motor or generator.
Please note during troubleshooting that it is not personal emotion or opinion but objective information that is more valuable. Also, carbon brush technology is not an unknown nor pseudo-science. It is a logical advanced science that includes getting hints through curiosity, while focusing on a solution!
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