Carbon brush
The carbon brush (or brush for short, also contact carbon , motor carbon ) is a sliding contact in motors and generators and establishes the electrical contact to the collector or to the slip rings of the rotating part of the machine ( rotor or rotor).
Layout and function
Carbon brushes are mostly made of graphite . Depending on the application, they are partially enriched with metallic components ( copper , silver , molybdenum ) or consist entirely of metal.
Sliding contacts in potentiometers , rotary switches and on current collectors are made of the same materials, but are referred to as sliding strips in current collectors and as wipers in potentiometers.
This power transmission technology is already over a hundred years old and is still widely used today. Carbon brushes can still be found in many electric motors. Starting with small motors in toys, electric kitchen appliances, electric window regulators, razors, washing machines , hair dryers, vacuum cleaners or power tools (electric drills, angle grinders, hedge trimmers, circular saws, etc.), the spectrum ranges to large machines in electric locomotives , submarines, and power plant generators and also in wind turbines . The geometric and electrical dimensions of the carbon brushes are correspondingly diverse. While the smallest variants in toys only weigh a few grams and carry out the transmission of a few milliamps, carbon brushes weighing over two kilograms are used in electroplating for the transmission of up to 1000 amperes .
The term carbon brush is historically derived from the brush brushes that were used in the past as a sliding contact instead of graphite. Since the composition of carbon brushes has to be matched to practically every electric motor, their development effort is relatively high. Therefore, carbon brushes are only manufactured by a few specialized companies worldwide.
Manufacturing
- Mixture of components: natural graphite, electrographite, copper as a conduction improver, more rarely other metals (Fe, Mo etc.), as well as binders (pitch, resins or plastic powder)
- Compression of the powder under a defined pressure, in its own form with an upper and lower punch. For mass-produced items, pre-press technologies with a pressed-in power cable are preferred. For smaller quantities, the carbon brushes are machined out of blocks.
- Annealing of the pressed carbon brushes or blocks with exclusion of oxygen and a precisely defined fire curve at up to 1200 ° C. The binder contained "cokes" and connects the ingredients.
- For special materials, inductive heating is used for further temperature treatment at up to 3000 ° C. The materials are converted into artificial graphite, so-called electrographite.
- In the case of mixtures that require a high fire temperature (Cu-free), the strand (cable) is subsequently tamped as the copper strand also tends to discolour (structural change / weakening) and to react with the sintering gases above certain temperatures.
- Subsequent post-treatments such as impregnation with waxes, oils, resins and metals allow the material properties to be further adapted to the specific requirements.
- Regrinding to size: Coals are subject to a tight tolerance when installed and usually have to be subsequently ground to size (viewed radially and tangentially in the direction of the motor axis), as a certain amount of distortion and shrinkage occurs during fire.
- The power cable (electrical connection made from fine-wire copper braid) is then attached to these carbon brushes using a tamping process.
Applications
Basically, carbon brushes are divided into two types in terms of structure and their electrical properties:
- Carbon brushes for direct current, i.e. for direct current motors (e.g. starters , wiper motors, many types of servomotors, interior fans , gasoline pumps and cooling fans in vehicles, in alternators ) and for supplying the excitation winding of synchronous machines .
- Carbon brushes for alternating current, i.e. for universal motors (e.g. in household appliances such as vacuum cleaners, mixers, etc.). Such brushes are mostly free of copper.
High demands are placed on carbon brushes: The brushes are trimmed by the motor manufacturer for a certain service life, which must be proven in extensive preliminary tests.
Coals for automotive starters must z. B. can withstand approx. 40,000 start cycles and are tested in extensive additional tests: salt water spray test, heat and cold tests, dust and overcurrent, etc. Each company has its own standardized test procedures that simulate extreme everyday use and are intended to ensure the reliable function of the electric motor in practice .
In the case of fuel pumps, the commutator made of graphite materials has also been used in new developments (around since 2000) . The reasons for this are the favorable electrical properties and increased service life due to the self-lubrication of the components running in the fuel.
In addition to the service life of the carbon brushes, such as B. the required 4000 washing cycles in washing machines, the noise behavior and especially the avoidance of brush fire are in the foreground. Only specially adapted carbon brush materials, some with complex post-treatment, meet these requirements. In addition to the material, the design of the carbon brush also has a decisive influence on the behavior of the sliding contact.
During use, it is not just the wear and tear of the carbon brush that plays a role. Carbon brushes must protect the counter-rotating material such as the commutator or slip ring, as replacing them is significantly more expensive than replacing the wear part, the carbon brush. Replacing worn carbon brushes of a motor does not reset its age to zero, because the commutator or slip rings and bearings also wear out.
Commutator motors cause electromagnetic interference even when there is little brush fire . The brushes are therefore almost always provided with local interference suppression ( capacitor connected in parallel ).
DC machines with carbon brushes made a major contribution to the industrial revolution in the 20th century . Until the 1970s, DC alternators were present in every car. Another application is slip-ring asynchronous motors and three-phase generators with static excitation equipment ( excitation current runs through slip rings, e.g. automotive alternators ) or external pole generators , in which the electrical energy is drawn off with carbon brushes on the rotor. Generators with carbon brushes are also sometimes used in wind turbines (slip-ring asynchronous generators, synchronous generators). In the case of high currents, such as for supplying the excitation winding of generators in power plants in the range of a few kiloamps , several carbon brushes are connected in parallel per pole. The machine is designed in such a way that individual carbon brushes can be exchanged during operation.
An electric motor with a commutator requires at least two brushes. In older model electric locomotives with disc-collector motors, one brush is often designed as a carbon brush made of graphite, the other made of copper wire mesh for cleaning contact surfaces.
Alternatives
Brushes made of precious metals are sometimes used in small DC motors, for example in the form of a thin spring plate that rests against the commutator with a pressed dome or a more massive metal contact. There are versions of at least two-armed springs.
Brush fire can have a negative effect, particularly at very high speeds. This is one of the reasons why motors without carbon brushes were developed. Instead of the collector and the carbon brushes, these have electronic components for commutation. The effort is very high, so that this technology is currently v. a. can be found in niches. Three-phase asynchronous machines are standard for industrial and rail motors .
- Some generators in power plants use auxiliary generators ( excitation machines ) attached to the generator shaft to generate and control the excitation power and do not require slip rings ( brushless excitation equipment ).
- The signals from the rotating head drum in helical track recording , as used in video tapes or digital audio tape (DAT), are transmitted using rotary transformers .
- Microwave in Rundsicht- radars are contactless waveguide - flanges transmitted.
- In the case of brushless motors with semiconductor AC inverters, a magnetic rotor rotates between stator coils that are electronically controlled with AC current of the appropriate frequency. Such motors have been used in direct drive turntables since around 1985 and increasingly in washing machines and cordless hand tools as well as drives for balance boards and electric bicycles since around 2018.
literature
- Hans Fischer: Materials in electrical engineering , 2nd edition. Carl Hanser Verlag, Munich, Vienna 1982, ISBN 3-446-13553-7
- A. Senner: Electrical engineering , 4th edition. Verlag Europa-Lehrmittel, 1965
- Werner Schröter, Karl-Heinz Lautenschläger, Hildegard Bibrack: Taschenbuch der Chemie , 9th edition. Verlag Harry Deutsch, Frankfurt am Main 1981, ISBN 3-87144-308-5
- Günter Springer: Electrical engineering , 18th edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9
- Gregor D. Häberle, Heinz O. Häberle, Armin Schonard: Electrical drives and energy distribution , 5th edition. Verlag Europa-Lehrmittel, Haan-Gruiten 2006, ISBN 978-3-8085-5005-2
