How does a rotating movement generate electricity?
The heart of all power plants: a rotating magnet in a coil
As soon as a magnet is moved in a copper wire coil, the pointer of the connected measuring device deflects - electrical voltage is generated.
Electricity can be generated in a number of ways. For example, through chemical processes in a battery: There is then a voltage gradient between the plus and minus poles of the battery. If the two poles are connected to a lamp with wires, a current flows through the filament of the lamp, which heats up and glows. If you also attach a device with which the current flow can be switched on and off, you basically have a flashlight.
The battery was invented around 1800. Until then, electricity could only be generated using so-called induction machines, which separated electrical charges through friction between certain materials. They did supply high voltages with which sparks could be generated and harmless "blows", but no practically usable currents.
With the battery, the researchers had for the first time a power source that enabled a more detailed study of electricity: For example, the relationship between current, voltage and resistance, as formulated by Ohm's law. Or the discovery of the magnetic field that every conductor through which current flows. In addition, batteries enabled the first practical applications of electricity, for example to operate telegraphs, telephones, bells or incandescent lamps.
However, today's power supply would by no means be possible with batteries. Rather, it is based on the principle of "induction", which was discovered in the first half of the 19th century and developed in the second half into the most productive and common form of electricity generation to date. It is based on the conversion of mechanical energy into electrical energy by moving a magnet inside a copper wire coil. The movement of the magnet "induces" a voltage in the coil, which can be tapped at its ends.
Direct and alternating current through induction
For example, if you were to push a hundred magnets quickly one after the other through a coil made of copper wire, the ends of which are connected to a voltmeter, you would get a pointer deflection between the plus and minus poles of the coil a hundred times - proof that a voltage is generated in the coil .
We have tacitly assumed that the magnets always have the same polarity when they move through the coil, e.g. always immerse into the coil with the north pole. Under this condition, a direct current is generated in the coil, in which the plus and minus poles of the power source do not change. However, this current does not have a constant voltage, like that of a battery, but its voltage precisely follows the movement of the magnet through the coil, increasing from zero to a maximum value and then falling back to zero.
If the magnets were to be passed through the coil with the polarity reversed, a pulsating direct current would also come about - but with the difference that the plus and minus poles are interchanged.
If you were to move the magnets alternately through the coil with different polarities, you would get two pulsating direct currents, which replace each other with reversed polarity - a so-called alternating current.
The simplest solution: The magnet rotates
Now it would be extremely laborious and expensive to push hundreds, thousands and millions of magnets through such a coil in quick succession in order to generate electricity. In practice, one simply lets a magnet rotate inside the coil: every 180 degree rotation of the magnet then has the same effect as if a new magnet were pushed through the coil. However, the polarity of the magnet changes every time, since the north and south poles alternately move past the coil. So there is an alternating current. The ups and downs of this current from plus to minus and back to plus during one full revolution of the electromagnet is called "phase".
Such a machine for generating electricity can be found on every bicycle: the "dynamo" contains a permanent magnet inside, which is rotatably mounted in a coil. A friction wheel that protrudes from the housing sits on the extended axis of the magnet. As soon as the friction wheel is pressed against the tire and thereby driven, the magnet rotates within the coil and generates a "single-phase" alternating current for the supply of the headlights and taillights.
Synchronous and asynchronous generators
The large generators in the power plants are constructed in a similar way to a bicycle dynamo. With them, however, the rotatable magnet does not consist of an iron permanent magnet, but of an electromagnet. If the electromagnet is supplied with direct current, a "synchronous generator" is created in which the generated alternating current follows the rotation of the rotor exactly. On the other hand, if you send alternating current through the electromagnet, you have an "asynchronous generator" in which the alternating current generated follows the rotary motion of the rotor with a greater or lesser delay.
The generators in the power stations of the public power supply are all synchronous generators. The rotating electromagnet receives the direct current for building up the magnetic field from an additional direct current generator, which sits on the same axis and is referred to as the "exciter machine". In our illustration, the power is supplied from the exciter to the rotating electromagnet via slip rings on the axis. There is also another solution that is based on the asynchronous generator and uses an AC exciter with subsequent rectification of the current. This offers structural advantages, because in an alternating current exciter the current can be taken directly from the rotor, which sits on the same axis as the electromagnet to be supplied with it. This eliminates the sliding contacts. The necessary conversion of the alternating current into direct current is done by diodes that rotate along the axis.
The three-phase construction
The power plant generators of the public power supply are built in such a way that they not only generate a single alternating current - like the bicycle dynamo - but three alternating currents at the same time: their electromagnet moves past three coils with every full rotation (360 degrees), the are each attached offset by 120 degrees. As a result, the "phases" of the three alternating currents that each rotation of the electromagnet generates in the coils are offset by 120 degrees from one another. This "three-phase" alternating current is also referred to as three-phase current.
In order to generate direct current with generators, additional effort is required: For example a mechanical commutator, which reverses the change of the current from plus and minus via radial sliding contacts on the axis of the rotor by reversing the change of the current from plus and minus halfway through each phase Plus and minus interchanged, thus converting the alternating current into a pulsating direct current.
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