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A new lease of life for carbon brushes

10 October 2015

Jon O’Brien takes a closer look at carbon brushes - the common electrical and mechanical issues that affect their use in dc motors and generators, and how a simple but rigorous maintenance regime will ensure long and trouble-free life.

Carbon brushes in dc motors and generators all fulfil the same basic task – to transfer electrical current from a moving device to a stationary point and vice versa, within a circuit. Their main function is to conduct current without interruption, making the point at which the brush transfers current from its surface to the moving collector critical. During operation, a film – or patina - automatically forms on the collector surface; this is critical in conducting current and reducing friction. 

Friction between the brush and collector is inevitable and eventually causes wear to the surface of both. As carbon is the softer of the two, it will wear faster, and is therefore designed to be easily replaced. Brush life can also be affected by factors including holder design and alignment; temperature and humidity variations; contamination by debris or lubricants; vibration; or changes in current, speed or pressure. Effective inspection and maintenance are therefore critical in avoiding unnecessary downtime and premature replacement. 

General maintenance
Brushes, holders, collectors and connections should all be regularly checked for general condition, cleanliness and dust or grease contamination. The patina should be closely monitored as this will reduce frictional wear and so improve brush life, while minimising collector wear. Brushes fitted to a freshly-turned collector may wear faster than brushes running on an established patina as the patina takes a while to fully form. Brush holder alignment should be measured to ensure there has been no slippage or shifting, while brushes themselves should be thoroughly checked from terminal to contact surface – with particular attention paid to contact faces, where a smooth, well-polished surface indicates good performance.

If a brush is found to be wearing prematurely, maintenance engineers should work through a list of vital checks; these include: current density, collector run-out, surface speed, collector temperature, humidity, brush pressure, amount of sparking, duty cycle, level of vibration during operation, appearance of the patina/ collector film, brush set-up in terms of whether it is correctly aligned, spacing of brush holder, and the presence of any contaminants nearby. Discovering if any of these factors is beyond acceptable limits, combined with the brush life history taken from maintenance records, should lead engineers to diagnose and rectify problems.

Mechanical problems and solutions
Mechanical issues can cause problems such as ill-fitting brushes in the holder. Too tight a fit will cause sticking which in turn will lift off the collector, breaking the current path. Too loose a fit allows movement of the brush in the holder which in turn will cause intermittent breaking of the current flow. When fitting new brushes, it is advised to bed the brushes into the shape of the collector as illustrated in Figure 1.

To ensure brushes are fitted correctly, check they are free to move easily in their boxes after bedding in and cleaning. Brushes and holders manufactured to either IEC 136 or DIN 4300 tolerances should always be selected.

Carbon brushes are fitted into holders designed to keep them in the correct position and allow them to run on the collector surface to transfer maximum current and so deliver optimum performance. There are two main holder types; one where the brush is rigidly attached to a swivel arm, and the other where the brush is free to slide in a supporting box. Slide type holders are generally classified according to the angle at which the brush meets the collector (see Figure 2). To ensure the brush is at the optimum distance from the collector the holder should be positioned approximately 2.5mm from the collector. If distance falls below 2mm or exceeds 3mm, the window holder should be reset as shown in Figure 3.

Another area that can cause both mechanical and electrical issues is spring pressure.  Spring pressures should be the same across all brushes in the motor or generator. Any inequalities may cause rapid mechanical or electrical wear, or selective action across the whole system. Due to the radial travel of the brush through the holder as it wears, it is advantageous to use constant force (CF) springs. Figure 4 shows the force in these springs is constant over a wide range of spring extensions. As the spring travels down the brush holder, the force remains constant until it has reached 0.8 times the diameter of the coil. To optimise performance, a rubber tufnol top with a locating radius should be fitted to allow best possible spring location with damping to reduce any potential vibration issues.

Electrical problems and solutions
Poor commutation, leading to sparking, as well as a lack of a fully formed patina, are electrical factors which can cause brush and commutator wear. This is frequently evidenced by ghost marked surfaces on the brush, which could indicate issues such as an incorrect neutral point or inter-pole problems. The neutral point should be checked with a neutral meter before any further issues are investigated.

Overload, low load or varying load can also cause damage. Overload current will be shown by burnt flexes or a pitted brush surface. If the brush is overloaded, it is advisable to check the brush material. Always use a brush with the optimum current density for the application. Low load – often evidenced on variable load motors seen in steel mills by streaking on the commutator - can potentially be overcome by selecting a brush grade suitable for a broad spectrum of loads including light load running.

Interruption to patina formation can be caused not only by mechanical and electrical issues but by environmental issues such as ambient temperature, humidity, contamination, vibration and maintenance standards. For the film or patina to form correctly, optimum conditions are required. Moisture has a big impact, and low humidity levels can potentially disrupt this process. Cooling air is often supplied from outside and is generally forced directly onto the collector, but problems can arise if the air is too hot or too cold. Furthermore, at low load, the collector may never achieve normal operating temperature – which will also impact on film formation and ultimately on brush performance.

Jon O’Brien is an application engineer at Morgan Advanced Materials

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