In electrical engineering, brushes conduct current between stationary wires and moving parts, most commonly in a rotating shaft. Most importantly, in an electric motor, an alternator or electric generator, the coils of the rotor have to be connected. To accomplish this, two metal (copper or brass) 'slip rings' are affixed on the shaft and springs press braided copper wire 'brushes' onto the rings which conduct the current. Later, these copper wire brushes were replaced with carbon blocks — but these blocks are still called brushes. As the brushes are slowly abraded, they may have to be replaced, if this is possible. If the copper rings are split into parts with "interlaced" connections, the arrangement is called a commutator. Metal fiber brushes are currently being developed again. These brushes may have advantages over current carbon brushes, but have not yet seen wide implementation. Types of carbon brushes There are distinguished basically 3 types of carbon brushes: 1) brushes for automotive applications: DC current, voltage 12 - 48 Volts. 2) brushes for "household" applications: AC sinus current, voltage 110 / 220 Volts 3) brushes for industrial motors: both AC and DC current of various Voltages.
manufacturing process (simplified): 1) Mixture of components: main component: Graphite/Carbon powder; mostly Copper (CU) for better conductibility (rare for AC applications and not on automotive fuel pumps which run on carbon commutators); binders (mostly phenol- or other resins, pitch); other additives (Metal powders, MoS2, SiO2, etc). Much know-how and research is needed in order to define a brush grade mixture for each application/even for each motor. 2) Pressing of powder in a tool consisting of upper and lower punch and dye, on mechanical or hydraulical presses. In this step, depending on later processing, the Copper-wire (called shunt wire) can be inserted automatically through a hole in the upper punch and fixed into the pressed brush block by the powder pressed around. After this process, the brush is still very fragile and in professional jargon called 'green brush'. 3) Heat treatment of the 'green brushes' under artificial atmosphere (usually H2 + N2). Temperatures up to 1200° C. This process is called 'Sintering'. It causes the binder to carbonize and to form a cristallite structure between the Carbon, Copper and other Additives. The heat treatment is transformed by a temperature curve exactly defined for each material mixture. Besides the mixture composition, the used temperature curve is the second big 'secret' of each brush manufacturer. After the heat treatment, the brush structure is modified in a way which makes copying of the brush nearly impossible for competing companies. 4) Heat treatment usually causes the brushes to shrink and to bend. They must be grinded to an exact size defined by the motor constructor (usually on the running surface and on the back side of the wire). 5) Some companies use additional treatments in order to enlarge durability of brush (and therefore application), for example impregnation or the running surface by special oils and grease.
Manufacturing of carbon brushes requires very high knowledge of materials and experience in mixture compositions. Already very small changes in brush contents by few weight% of components can significantly change the properties of brushes on its applications. There are just a handful of brush developing companies in the world, which are mostly specialized on certain types of brushes. Companies: Schunk Group, D-Gießen (+ subsidiaries), TRIS Inc., JP-Matsusaka (+ subsidiaries), E.C.S. (former Morganite) I-Martinsicuro, LCL - Le Carbone Lorraine (+ subsidiaries) France. Carbon brushes are one of the least costly parts in an electro motor. On the other hand, they usually are the key part which delivers the durability ("life-time") and performance to the motor they are used in. There production requireds very high attention in quality control and production process control throughout all steps of the production process.
See also
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| Broad Motor Categories | Synchronous motor • AC motor • DC motor |  |
| Conventional Electric Motors | Induction • Brushed DC • Brushless DC • Stepper • Linear • Unipolar • Reluctance | |
| Novel Electric Motors | Ball bearing • Homopolar • Piezoelectric • Ultrasonic • Electrostatic | |
| Motor Controllers | Adjustable-speed drive • Amplidyne • Direct Torque Control • Direct on line starter • Electronic speed control • Metadyne • Motor controller • Variable-frequency drive • Ward Leonard control |
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| See also | Barlow's Wheel • Nanomotor • Traction motor • Lynch motor • Mendocino motor • Repulsion motor • Inchworm motor • Booster (electric power) • Brush (electric) | |
