Brush (electric)

The carbon brush ( or short brush, also carbon brush, motor carbon ) is a sliding contact in motors and generators, and makes electrical contact to the collector or to the slip rings of the rotating part of the machine ( rotor or rotor).

Structure and function

Carbon brushes are usually made of graphite. Depending on the specific application, they are enriched partly either with metal components (copper, silver, molybdenum ) or consist entirely of metal.

Sliding contacts in potentiometers, rotary switches and current collectors are made of just such materials, but are referred to in pantographs as an abrasive strip and potentiometers as a grinder.

For over one hundred years old, this current transmission technology is far from obsolete today. Carbon brushes are still to be found in very many electric motors in the world. Ranging from small motors in toys, electric kitchen appliances, electric windows, electric razors, washing machines, hair dryers, vacuum cleaners or power tools ( electric drills, angle grinders, trimmers, circular saws, etc.), the spectrum to large machines in electric locomotives, submarines, in power plant generators and also in wind turbines. Just as diverse are the geometrical and electrical dimensions of the carbon brushes. While the smallest variations in toys weigh only a few ounces and with the transfer of a few mA, carbon brushes are used with a weight of over 2 kg for the transfer of up to 1,000 A in electroplating.

The term carbon brush derives historically from the brushes from which you formerly used as sliding instead of graphite. Since the composition of carbon brushes must be matched to virtually any electric motor out their development effort is relatively high. Therefore, carbon brushes are developed only by a few specialized companies worldwide.

Production

• Mixture of components: natural graphite; Electro-graphite, copper as Leitungsverbesserer, rarely other metals (Fe, Mo, etc.) and binder ( pitch, resin or plastic powder)
• Pressing the powder into their own presses under defined pressure, in a separate form with upper and lower punches. For mass-produced final pressing technologies are preferred with crimped power cable. For smaller quantities, the carbon brushes of blocks are machined out.
• Annealing the pressed carbon brushes or blocks under oxygen exclusion and well-defined fire curve at up to 1,200 ° C. This " coked " the binder contained and connects the ingredients.
• For special materials a further thermal treatment is performed by inductive heating up to 3000 ° C. Here are the materials in artificial graphite, so-called electro-graphite, converted.
• For mixtures that require a high fire temperature (Cu -free), the wire ( cable) is subsequently scrapped, as well as the Cu wire from certain temperatures discoloration ( change in structure / attenuation ) and reactions with the sintering gases tends.
• Subsequent treatments such as impregnation with waxes, oils, resins and metals, which can be a further adjustment of the material properties to the specific requirements.
• Regrinding to measure: coals are in the installed state of close tolerance and usually need to be ( seen radially and tangentially in Motorachsrichtung ) subsequently ground to size, since the fire a certain distortion and shrinkage occurs.
• The power cable (electrical connection from superfine copper wire) is then secured in these brushes with a tamping process.

The world market for carbon brushes is by the companies Schunk carbon technology ( Germany ), Carbone Lorraine ( France) and Morgan Crucible ( UK), and TRIS Inc. (Japan, for automotive applications) dominates. But there are also many medium-sized and small manufacturers like PanTrac GmbH and Schmidt Hammer Elektrokohle GmbH ( Germany ).

Applications

Basically, two types are distinguished by the structure and electrical properties:

• Carbon brushes for dc, so for DC motors (eg starter motor, wiper motor, many types of servo motors, internal fan, petrol pump and radiator fan in cars, in alternators ) and for supplying the field winding of synchronous machines.

• Carbon brushes for AC motors ( eg in household appliances such as vacuum cleaners, blenders, etc.). Such brushes are mostly copper free.

At carbon brushes high demands are made: The brushes are trimmed by the engine manufacturer to a certain life back that needs to be proven in extensive preliminary experiments.

Coals for starters is, for example, withstand approximately 40,000 start cycles and tested in extensive additional tests: salt water spray test, heat and cold tests, dust and overcurrent etc. Each company has its own standardized test methods that simulate the extreme everyday use and the reliable operation of the to ensure the electric motor in practice.

In fuel pumps (approximately since 2000) and the commutator of graphite materials used in new developments. This is due to the favorable electrical properties and extended service life using self-lubrication of the running in the fuel components.

In addition to the life of the carbon brushes, such as required 4,000 wash cycles in washing machines, are also the noise behavior and especially the avoidance of brush fire in the foreground. Only specially adapted carbon brush materials, some with elaborate finishing, fulfill these requirements. In addition to the carbon brush material and the design of the carbon brush has a decisive influence on the behavior of the sliding contact.

When using not only the wear of the carbon brush has a role. Carbon brushes the counter material as commutator or slip ring conserve, since their replacement is significantly more expensive than the replacement of the wear part carbon brush. The replacement of worn carbon brushes of a motor does not put his age back to zero, as well as the commutator or slip rings and bearings wear.

However commutator motors generate electromagnetic disturbance, even in low brush fire. The brushes are therefore almost always provided with a Nahentstörung (parallel switched capacitor ). DC motors with carbon brushes have contributed a large part of the industrial revolution in the 20th century. Another application is the slip ring induction motors and three-phase generators with static excitation equipment ( exciter current passes through slip rings, such as automotive alternator), or Außenpolgeneratoren, in which the electricity is removed with carbon brushes on the rotor. Also in wind turbines partially generators are used with carbon brushes ( slip-ring induction generators, synchronous generators ). When used as the power supply of the excitation winding of generators in power plants because of the high currents, these are in larger machines ranging from a few kiloamperes switched many brushes per pole in parallel. The machine is so designed that it is possible to exchange individual carbon brushes on the fly.

An electric motor with a commutator, requires at least two brushes. In older model electric locomotives by Märklin with disc collector motor is running as a brush carbon brush graphite and the other of copper wire mesh, the contact surface cleaning.

Alternatives

In small DC motors partly of precious metal brushes are used.

In particular, at very high speeds it can affect the " brush fire ". For one reason engines have been developed with no carbon brushes. These have, instead of the commutator and the brushes for commutation of electronic components. The effort is very large, so this technology is currently mainly can be found in niches. In industrial and railway engines are the " three-phase induction motors " standard.

• Some generators in power plants use on the generator shaft mounted auxiliary generators ( exciter ) for generating and controlling the excitation power and do not require slip rings ( brushless excitation device ).

• The signals of the rotary head drum in helical scanning, such as used for video tapes or digital audio tape (DAT) to be transmitted with rotary transformers.

• Microwaves in a surveillance radars are transmitted via contactless waveguide flanges.