Made of carbon, but not a brush.
Those not fully versed in engineering or car mechanics may be puzzled by the term ‘carbon brush’. Is it a brush to clean up soot and coal dust? Is it a brush made of some type of carbon? The brush under consideration doesn’t have hairs or bristles and doesn’t look like a brush for sweeping up detritus.
A carbon brush in electrical engineering terms is a ‘device that conducts current between stationary wires and moving parts’ (Wikipedia). Most commonly, carbon brushes are found in motors, generators and alternators.
Originally, engineers had problems making the drive shaft of an engine turn through 360 degrees. The solution was a copper or brass commutator ring that was added to the shaft. As the shaft rotated the commutator came into contact with a series of braided copper wires that allowed for current to be provided to the shaft as it turned.
The design worked but was inefficient. The solution was not to use braided copper wires but instead small blocks of graphite. Although these do not look like brushes the original name stuck.
The advantage of graphite over copper is that graphite has a higher electrical resistance that allows for the gradual transference of power as the drive shaft makes a rotation. Using carbon for the brushes was a major breakthrough in engineering that allowed for the automobile industry to really take off as a commercial enterprise.
Nowadays there are three types of brush: for alternating current, for direct current and for industrial motors that use both AC and DC. It is interesting to note that Edison’s direct current would have been useless for moving vehicles without carbon brushes. This is not the case with Tesla’s much maligned at the time AC system.
The preparation of brushes requires the measuring out of exact proportions of graphite, copper and other components. The brush is made and then subjected to extreme heat. This is called sintering. Different companies use slightly different ranges of heat in their sintering process. These are heavily guarded secrets as the smallest difference in the finished carbon brushes can have a big impact on performance.
Hard carbon brushes are used with machines that require speeds below 20 meters per second. These are usually small machines with flush mica that requires brushes with strong polishing. These brushes can only take a low thermal and electrical load.
Carbon graphite brushes can take a higher thermal and electrical load. They have better commutating properties and high polishing abilities. Typically these types of brushes are found on small thyristor fed DC machines and F.H.P motors.
Resin bonded brushes use graphite and resin in varying quantities. These have high commutating ability but tend to waste electricity. They are used for machines with a high commutating but low brush current requirements such as three-phase commutator motors.
When graphite is heated to 2500 degrees centigrade it becomes crystalline and forms what is known as electrographite. This makes the resulting brush ideal for applications with high thermal and electrical loads such as large engines.
For specialist functions graphite is mixed with metal – commonly copper, and occasionally silver, lead or tin. They are made for machines with high brush loads but not high commutating requirements. Metal graphite brushes can only manage speeds of up to 30 meters a second.
The next time you get in a car or elevator or you turn on a generator consider how the simple carbon brush has made such a machine possible, how it has helped define the technological environment that we live in.