Alternator

Alternator

An alternating current generator or alternating current generator is a special embodiment of an electrical generator which is used to generate single-phase alternating current . Since, in contrast to the direct current generator, there is no commutation with this type of generator , an alternating current is generated, the frequency of which is proportional to the rotor speed. For the power generation phase AC generators are in a big way today only of secondary importance. Nowadays they are only built for small outputs (e.g. emergency power supplies) up to around 2.2 kilowatts . The most widely used alternator is the bicycle dynamo , which works on the generator principle designed by Hippolyte Pixii .

history

Functional principle of an alternator
with permanent magnet

1820 André-Marie Ampère discovers that an electric current affects a magnetic field
1831 Michael Faraday discovered electromagnetic induction
1832 Hippolyte Pixii designs the first alternator with permanent magnet
1866 Werner Siemens builds the first practically usable generator

With the first alternator from the French Hippolyte Pixii, a horseshoe magnet rotated in front of two coils connected in series . The changing magnetic flux density induces an alternating voltage in the coils. Since at that time people were not interested in alternating current, but in direct current, a commutator was built in , which turned the alternating current into a direct current . This principle of direct current generation by means of collectors and carbon brushes is still used today in direct current generators ( dynamos ).

principle

By rotating a conductor loop in a magnetic field of magnetic flux density , the magnetic flux through the loop changes constantly. This induces a voltage in it . If we describe the position of the loop in terms of the angle , the magnetic flux passing through is: ${\ displaystyle B}$ ${\ displaystyle \ phi}$ ${\ displaystyle U_ {ind}}$ ${\ displaystyle \ gamma}$

{\ displaystyle \ phi = B \ cdot A \ cdot \ cos \ gamma}

If the loop rotates with the angular velocity ω, then

{\ displaystyle \ gamma = \ omega \ cdot t}

and the induced voltage

{\ displaystyle U_ {ind} = - {\ frac {d \ psi} {dt}} = U_ {s} \ cdot \ sin \ omega t}

Source:

The voltage induced by the rotation in the conductor can be tapped by brushes on the slip rings . If the conductor moves across the magnetic field, the Lorentz force acts on it when the current is drawn . This inhibits the movement of the loop and therefore mechanical work is necessary to move the loop.

Layout and function

Alternator Two-pole internal pole machine

Alternator Four-pole internal pole machine

The alternator consists of a stator and a rotor . The AC generators customary today are designed as single-phase synchronous machines in internal pole machine design, i.e. H. the rotor is magnetically excited and the excitation field induces a voltage in the stator. For energizing different systems are used. For example, the excitation field can be provided by permanent magnets . The advantage here is that no excitation power has to be supplied. Another method is excitation by means of a static excitation device , in which the excitation power is supplied via slip rings . A small accumulator is sometimes used as a buffer and to stabilize the excitation voltage . To start without an accumulator, a weak permanent magnetization of the rotor can also be used. After the start of the self-excitation process, the excitation current must be regulated in order to keep the output voltage constant.

The rotating field magnet induces a single-phase alternating voltage in the stator coils, the frequency of which depends on the speed of the driving machine and the number of pole pairs of the generator. In contrast to the DC generator , however, the exciting field rotates here and thus induces a current in the stationary stator winding arranged around the rotating field.

Since alternators generate a single-phase alternating voltage, they are only conditionally suitable for operation in the interconnected network . In Germany, for example, the generators for generating electricity for Deutsche Bahn belong to this category. In a three-phase network , the feed-in of this single-phase AC voltage can lead to asymmetry, which is why alternating current generators are otherwise only operated on the network in small hydropower plants.

Another type of AC voltage generator are asynchronous generators . A single-phase asynchronous machine, connected to the mains voltage, is able to work as a generator if it is rotated by the slip faster than the synchronous speed. If there is no mains connection available, capacitors can be used to compensate for the reactive power or to provide the excitation. Such self-excited asynchronous generators, however, require a residual magnetization of the squirrel cage or a starting current pulse for operation.

Areas of application

Small hydropower plants
Small wind turbines
Emergency power generators (up to 2.2 kW)
AC alternator on bicycles and some motorcycles

Statutory provisions and other regulations

EN 60 034 Part 1 General provisions for rotating electrical machines
DIN IEC 34 Part 7 Types of rotating electrical machines
EN 60034-5 Degrees of protection of rotating electrical machines
EN 60034-6 Types of cooling, rotating electrical machines

Individual evidence

↑ Hans-Günter Boy, Horst Flachmann: The master's examination - electrical machines and control technology. 4th edition. Vogel, Würzburg 1983, ISBN 3-8023-0725-9 , p. 151.
↑ ^a ^b Wilhelm Lehmann, Ramon Geisweid: Electrical engineering and electrical drives . 7th, completely revised edition. Springer Verlag, Berlin / Heidelberg 1973, pp. 350–354.
↑ ^a ^b RWE AG: From Amber Magnets to Alternating Current Generators: Development History of Electric Generators . In: RWE Magazin. No. 01, 2008.
↑ Hans-Joachim Braun: The 101 most important inventions in world history . 2nd updated edition. Verlag CH Beck, Munich 2007, ISBN 978-3-406-56466-6 , pp. 66-67.
^ Günter Springer: Electrical engineering. 18th edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 , pp. 93-98, 296.
^ Wilhelm Raith: Textbook of Experimental Physics . Volume 2: Electromagnetism. 9th edition. Walter de Gruyter, Berlin 2006, ISBN 3-11-018898-8 .
↑ Gisbert Kapp: Dynamo machines for direct and alternating current . Third, increased and improved edition. Verlag R. Oldenbourg, Munich 1899, pp. 286-298.
↑ Ekbert Hering, Rolf Martin, Martin Stohrer: Pocket book of mathematics and physics . 5th, updated and expanded edition. Springer Verlag, Berlin / Heidelberg 2009, ISBN 978-3-540-78684-9 , p. 369.
↑ ^a ^b ^c Wolfgang Courtin: Electrical energy technology . Introduction for all courses, Verlag Friedrich & Sohn, Wiesbaden 1999, ISBN 3-528-03856-X , pp. 57–58.
↑ Engelbert Arnold (Ed.): The windings of the alternating current machines . Third volume: the windings of the alternating current machines. Springer Verlag, Berlin / Heidelberg 1904, pp. 1–11.

Web links

Commons : Alternators - collection of images, videos and audio files

Functional model of an alternator (accessed August 5, 2016)
Animation model of an alternator (accessed August 5, 2016)
Experimental setup of an alternator (accessed on August 5, 2016)

[Quelle_1-1] Hans-Günter Boy, Horst Flachmann: The master's examination - electrical machines and control technology. 4th edition. Vogel, Würzburg 1983, ISBN 3-8023-0725-9 , p. 151.

[Quelle_10-2] Wilhelm Lehmann, Ramon Geisweid: Electrical engineering and electrical drives . 7th, completely revised edition. Springer Verlag, Berlin / Heidelberg 1973, pp. 350–354.

[Quelle_2-3] RWE AG: From Amber Magnets to Alternating Current Generators: Development History of Electric Generators . In: RWE Magazin. No. 01, 2008.

[Quelle_6-4] Hans-Joachim Braun: The 101 most important inventions in world history . 2nd updated edition. Verlag CH Beck, Munich 2007, ISBN 978-3-406-56466-6 , pp. 66-67.

[Quelle_3-5] Günter Springer: Electrical engineering. 18th edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 , pp. 93-98, 296.

[Quelle_9-6] Wilhelm Raith: Textbook of Experimental Physics . Volume 2: Electromagnetism. 9th edition. Walter de Gruyter, Berlin 2006, ISBN 3-11-018898-8 .

[Quelle_4-7] Gisbert Kapp: Dynamo machines for direct and alternating current . Third, increased and improved edition. Verlag R. Oldenbourg, Munich 1899, pp. 286-298.

[Quelle_7-8] Ekbert Hering, Rolf Martin, Martin Stohrer: Pocket book of mathematics and physics . 5th, updated and expanded edition. Springer Verlag, Berlin / Heidelberg 2009, ISBN 978-3-540-78684-9 , p. 369.

[Quelle_5-9] Wolfgang Courtin: Electrical energy technology . Introduction for all courses, Verlag Friedrich & Sohn, Wiesbaden 1999, ISBN 3-528-03856-X , pp. 57–58.

[Quelle_8-10] Engelbert Arnold (Ed.): The windings of the alternating current machines . Third volume: the windings of the alternating current machines. Springer Verlag, Berlin / Heidelberg 1904, pp. 1–11.