Carbon Brushes Give DC Motors A New Lease Of Life

Jon O’Brien, Application Engineer at Morgan Advanced Materials, discusses the evolution of carbon brushes and the maintenance techniques that can extend the life span, improved motor performance and reliability of these essential components, as well as providing guidance on the material choices available.

What are carbon brushes?

Although they may differ in size, shape and technical composition, carbon brushes used in DC motors and generators all fulfil the same basic function – to transfer electrical current from a moving device to a stationary point and vice versa, within a circuit.

The main function of the brush is to conduct current without interruption, meaning the point at which the brush transfers current from its surface to the moving collector is critical. During operation, a film – or patina – is automatically formed on the surface of the collector, and this is critical in conducting current and reducing friction.

Carbon brushes were first patented in 1885 by Scottish inventor Professor George Forbes. They proved a more effective conductor than the earlier brushes which were made of strands of copper and frequently left deep scratches and grooves in the collector, resulting in the need for regular resurfacing.

Additionally, the hard metal brushes would themselves often wear away, releasing debris which could short circuit the collector by becoming wedged in moving parts. Gauze brushes, an early alternative to copper, provided improved surface contact and reduced segment wear, but were much more expensive than their copper counterparts.

The use of carbon brushes minimises sparking and arc damage at the point of contact, while the low friction characteristics and lubrication properties of carbon minimise damage to the collector (commutator or slip ring) surface, reducing the amount of wear debris that can collect within the machine.

Carbon brushes are also more versatile than copper alone, as they can be adjusted for individual applications by altering the ratio of carbon to metal powder – brushes with a high metal content perform best with low voltage and high current, while a higher carbon content results in a brush ideal for high voltage and low current density.

Despite the benefits of carbon brushes over their copper predecessors, 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.

The life of the brush can also be affected by factors including the 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 both unnecessary downtime and premature replacement.

General maintenance points

Routine condition-based maintenance begins with a visual inspection of the carbon brush and associated components. Brushes, holders, collectors and connections should all be regularly checked for general condition, cleanliness and contamination from excessive dust or grease.

When inspecting collectors, the patina (an oxidised film which forms on the surface) should be closely monitored as this will reduce frictional wear and so improve the life of the brush, as well as minimising the wear on the collector. Brushes fitted to a freshly-turned collector are liable to wear faster than brushes running on an established patina as the patina takes a while to fully form, and this should also be taken into account during inspection.

The alignment of brush holders should be measured to ensure there has been no slippage or shifting which could hasten wear and damage, 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 is indicative of good performance.

Influencing factors

Damage to carbon brushes can occur due to mechanical, electrical or environmental factors, and inspection schedules can be designed around common faults in these areas. Mechanical problems which must be identified quickly include poorly mounted or aligned holders; worn springs; eccentricities in wear patterns on the collector; and incorrect pressure.

Contamination, temperature and humidity are among the environmental factors which may cause premature damage or quicken wear, and the causes of any ambient changes should be identified and rectified to prevent further damage. Poor commutation, leading to sparking, as well as a lack of a fully formed patina, are electrical factors which can lead to wear of both the brush and commutator.

Vital checks

If a brush is found to be wearing prematurely, maintenance engineers should work through a list of vital checks which will uncover the causes of shortened brush life and inform the corrective strategy. The following should be ascertained:

• 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
• 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, whether they are mechanical, environmental or electrical.

Mechanical causes and solutions

As previously discussed, mechanical issues can cause problems such as ill-fitting brushes in the holder. If the fit is too tight, it 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 the brushes are fitted correctly, check that brushes 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.

Brush holder alignment

Carbon brushes are fitted into holders which are designed to keep them in the correct position and allow the brush to run on the surface of the collector to transfer the maximum current and so deliver optimum performance. There are two main types of holders; one in which the brush is rigidly attached to a swivel arm, and the other in which 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 as per Figure 2.

To ensure the brush is at the optimum distance from the collector – and so prevent brush movement issues or the brush holder arcing current to the collector – the holder should be positioned approximately 2.5mm from the collector. If distance falls below 2mm or exceeds 3 mm, the window holder needs to be reset as shown in Figure 3.

Springs and spring pressure

The final area that can cause mechanical and also electrical issues, is that of spring pressure. To prevent inequalities the spring pressures should be the same across all the brushes in the motor or generator. Such inequalities – where some brushes are optimally configured while others are at too low or high a spring pressure – are likely to result in rapid mechanical or electrical wear, and may also cause 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 remainconstant 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 location of spring with damping to reduce any potential vibration issues.

Common electrical issues and solutions

Poor commutation, leading to sparking, as well as a lack of a fully formed patina, are electrical factors which can lead to wear of both the brush and commutator. This is frequently evidenced by ghost marked surfaces on the brush (Figure 5), which could indicate issues such as an incorrect neutral point or interpole 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 to the brushes, holders or collector. Overload current will be shown by burnt flexes or a pitted brush surface of the brush (Figure 6). If the brush is overloaded, it is advisable to check the brush material and ensure use of 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.

Environmental issues and proposed solutions

Interruption to the formation of the patina can be caused not only by mechanical and electrical issues but environmental issues such as; ambient temperature, humidity, contamination, vibration and maintenance standards – all of which can have an impact on the film formation. For the film or patina to form correctly, optimum conditions are required.

Moisture is essential, with humidity levels of less than 4.6 mg/L (1.5 grains/ft³) potentially disrupting 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 the film formation and ultimately on the brush performance.

Grades of brush material

The brush/ holder/ collector system plays a key role in optimising motor uptime. Ensuring continuous, even current flow is paramount and correct brush grade selection is vital to that process.

Various grades are available depending on the specific application requirements. Entity in a steel mill motor, for example, a combination of issues may be present such as contamination, high temperatures, varying current loadings with periods of light load running and vibration caused from other equipment in the area.

For this application, an electrographitic grade, able to cover a spectrum of current densities, coupled with a CF spring and rubber Tufnol top design to counter vibration, is a recommended solution.

As well as electrographites, the most commonly used brush grades are metal graphites, natural graphites, resin-bonded products, metal impregnated products, and hard carbon/ carbon graphite products.

Electrographite, used for both commutation and collection, consists of carbon which has been partially transformed into graphite by heating the material to temperatures between 2200°C and 2800°C. Electrographite is suitable for a range of applications including DC drives and AC rings, with current densities normally in the range 4 to 16A/cm² and surface speeds up to 50m/sec.

Metal graphite products are mainly used for collection, except in low voltage applications. Specifiers typically choose between copper, which is best at current densities 12 to 30A/cm² and surface speeds up to 30m/sec, and silver which is suitable for the same speed range but for low voltage applications with good signal integrity, and varied current densities from just above zero to 30A/cm².

Natural graphite products are also suitable for collection applications, particularly Turbine Alternator (TA) exciters with steel sliprings, current densities up to 12A/cm² and surface speeds up to 80m/sec.

For commutation applications, resin bonded products are ideal for applications where there is high contact drop (poor commutation) or in low current/ temperature applications with current densities up to 8A/cm² and surface speeds up to 30m/sec.

At higher current densities up to 30A/cm² and surface speeds up to 40m/sec, metal impregnated products are the best choice. The final option is hard carbon/ carbon graphite, which is suitable for current densities up to 10A/cm² and surface speeds up to 30m/sec in older applications or where commutation is difficult for any reason.