Carbon brush
A brush is a device which conducts current between stationary wires and moving parts, most commonly in a rotating shaft. Typical applications include electric motors, alternators and electric generators.
For certain types of electric motors or generators to function, the coils of the rotor must be connected to complete an electrical circuit. To accomplish this, a copper or brass commutator or 'slip rings' are affixed to the shaft, and springs press braided copper wire 'brushes' onto the rings which conduct the current. Such brushes provided poor commutation as they moved from one commutator segment to the next. The cure was the introduction of 'high resistance brushes' made from graphite (sometimes with added copper). Although the resistance was of the order of tens of milliohms, they were high resistance enough to provide a gradual shift of current from one commutator segment to the next. The term brush remains in use. Since the brushes wear out, they can be replaced in products intended to allow maintenance.
Metal fiber brushes are currently being developed again. These brushes may have advantages over current carbon brushes, but have not yet seen wide implementation.
Exact composition of the brush depends on the application. Graphite/carbon powder is commonly used. Copper is used for better conductance (rare for AC applications and not on automotive fuel pumps which run on carbon commutators).In order to maximize electrical conductivity and green strength, highly dendritic (electrolytic) copper powder is used . Binders, mostly phenol- or other resins or pitch, are mixed in so the powder holds its shape when compacted. Other additives include metal powders, and solid lubricants like MoS2, WS2. Much know-how and research is needed in order to define a brush grade mixture for each application or motor.
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Wikipedia