Commutator (electrical engineering)
In the electrical engineering is with the commutator (of lat. Commutare , - exchange) collector or commutator means for reversing the polarity ( commutation ) in electrical machines referred to.
history
As early as 1834, Moritz Hermann von Jacobi designed the first technically usable precursor of today's commutator for his motor, with which he propelled a ship on the Neva in Saint Petersburg on September 13, 1838 .
Mode of action
The direct current machine can be operated as an electric motor (driven by the power grid) or as a generator (driven e.g. by a turbine). The commutator enables the alternating current to be converted into direct current .
A direct current in the excitation windings creates a magnetic direct field that leads from pole to pole across the rotor. If the rotor is rotated, then voltages are induced in the conductors of the rotor winding, namely alternating voltages. In order for a direct voltage to be generated, the polarity of the connection between the individual winding elements and the machine connection terminals must always be reversed when the direction of the induced voltage changes. This is done with the help of the commutator.
Function and structure
The rotor (armature) of the machine consists of the laminated core firmly connected to the shaft, the armature winding and the commutator. The armature current I A is transmitted to the armature coils via carbon brushes that form a sliding contact with the commutator bars. The windings of the armature are connected via the commutator, which serves as a pole changer. Classically, commutators consist of a sliding contact between the lamellas of the collector and two or more brushes . The sliding contacts are arranged in such a way that they change the polarity of the armature windings during rotation in such a way that current in the corresponding direction always flows through those windings which move across the excitation field. The commutator works together with the brushes as a mechanical switch. The direct current I A is continuously distributed to the coils by this switch in such a way that the current direction is the same within one pole area and only changes from pole to pole. During the period in which the coil changes from one pole to the other, it is short-circuited by the carbon brush, the coil current changes direction during this time; this process is known as commutation or commutation.
The brushes are made of a material that offers good, low-wear contact (often self-lubricating graphite, sometimes mixed with copper powder; precious metal brushes are also used in small motors for cassette tape recorders). The lamellas are glued to an insulating cylinder or circular surface and have an air gap - the abrasion of the brushes can therefore not get stuck and does not affect the insulation.
business
Smaller DC machines up to approx. 1 kW only have the main poles enclosed by the excitation winding in the stator. With larger machines, brush fire occurs with this simple design, caused by switch-off induction, which tries to maintain the field ( Lenz's rule ). In order to counter these difficulties, reversible poles with the reversible pole winding are built into the stator between the main poles. In large machines from approx. 50 kW, a compensation winding is also accommodated in the pole pieces of the main poles.
Excessive spark formation on the commutator ( brush fire ) must be avoided, as the resulting heat would lead to wear. Therefore, as many lamellas and armature windings as possible and the narrowest possible brushes are used. The brushes wear out during operation and become shorter. They are therefore often accommodated in metal guides in which they are pressed onto the collector by means of a spring. For better contact between the brush and the power connection, it often has a pressed-in copper cable or it is soldered directly to a bronze leaf spring.
Carbon brushes are wearing parts and can therefore be replaced in most motors. The commutator bars also wear out, but much more slowly than the carbon brush. With a given load, thick lamellas can be used to increase the service life of the motor.
- Avoidance of sparks
In the case of motors running relatively slowly, when operating with 50 Hz alternating current, the current direction changes several times in the coils of the armature winding that are fed by the brushes . During the change of the currently current-carrying collector lamellas due to the rotation when passing through the brushes, there is usually no or only random coordination between the moment of lamellar change and the polarity change with the more favorable voltage minimum of the alternating current wave. The result is arcing, also known as brush fire, which causes considerable wear, especially with high engine outputs.
For this reason, at the beginning of the 20th century, the frequency of the traction current to low values between 15 and 16 ⅔ Hz with overhead line voltages of 10 to 15 kV for cost-effective long-distance transmission was found an operationally acceptable compromise for electrically operated mainline railways .
The brushless or electronic commutation works without wear. In the so-called brushless DC motor , for rotor position detection z. B. Hall sensors are used, which control the windings via power drivers ( transistors , thyristors , triacs ).
An electrolysis commutator that uses electrolytic current transmission can be used to avoid spark formation and brush wear. The principle uses the current flow between mutually approaching electrolysis electrodes , which means that conventional sliding contacts can be dispensed with. A useful side effect of this type of commutator is the simultaneous formation of hydrogen and oxygen during operation.
Web links
literature
- Gregor D. Häberle, Heinz O. Häberle: Transformers and electrical machines in power engineering systems. 2nd edition, Verlag Europa-Lehrmittel, Haan-Gruiten 1990, ISBN 3-8085-5002-3 .
- Gerd Fehmel, Horst Flachmann, Otto Mai: The master's examination in electrical machines. 12th edition, Vogel Buchverlag, Oldenburg / Würzburg 2000, ISBN 3-8023-1795-5 .
- Günter Springer: Expertise in electrical engineering. 18th edition, Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .
- Hermann Linse, Rolf Fischer: Electrical engineering for mechanical engineers, fundamentals and applications , 12th edition, Teubner Verlag Wiesbaden 2005
- Dubbel, paperback for mechanical engineering, 14th edition, Springer Verlag Berlin Heidelberg New York 1981
Individual evidence
- ^ Electric motor from Jacobi . Retrieved March 27, 2020 . on LEIFIphysics
- ↑ Floating electrolysis motor. In: dpma.de