Why is alternating current used

Alternating current

Lexicon> Letter W> Alternating Current

Acronym: AC = alternating current

Definition: electric current that periodically changes direction

Counter-term: direct current

English: alternating current

Category: electrical energy

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 04/26/2010; last change: 08/15/2020

URL: https://www.energie-lexikon.info/wechselstrom.html

Alternating current is an electric current that periodically changes direction. Corresponding is one AC voltage an oscillating electrical voltage. Typically the current strength and voltage oscillate sinusoidally with a fixed one frequency. For example, the network frequency of 50 Hertz (50 Hz) used in the European network means that the voltage oscillates 50 times per second. In contrast, 16.7 Hz is used for the railroad.

Peak values ​​and effective values

Since the level of the electrical voltage oscillates, the question arises as to how such a voltage should be characterized. One possibility would be to specify the Peak value. However, it is more common to specify the Rms value. This is the value that a DC voltage, connected to an ohmic resistor, would have to have in order to produce the same electrical output. In the case of sinusoidal voltages, the rms value results as the peak value divided by the square root of 2 (approx. 1.414). He corresponds Not the mean amount of the oscillating voltage, but the square root of the mean square of the voltage.

As an example, consider the voltage at a household socket (see Figure 1). Its rms value is 230 V, and the peak value is around 1.414 230 V = 325 V.

For currents, rms values ​​are given completely analogously. Note that the current intensity does not oscillate sinusoidally for all consumers. However, it is also true that the rms value is the square root of the mean square of the current intensity.

Three-phase current

The combination of several phase-shifted alternating currents results in three-phase current. Households and businesses are normally supplied with three-phase alternating current, and various single-phase alternating current consumers are e.g. B. connected to one of the three phases (and of course the neutral conductor) via household sockets. Typically, currents of up to 16 A can be drawn, which results in a maximum output of approx. 3.7 kW. Devices with higher power (e.g. powerful electric motors) are usually connected via a three-phase socket that offers all three phases and can therefore deliver three times more power with the same current strengths. Of course, a direct connection without a socket is also possible, e.g. B. for an electric stove.

Complex number calculations

Complex numbers are often used to advantage for calculations in connection with alternating voltages and currents. Here a single complex number represents a sinusoidal voltage curve. It therefore contains the information about the amplitude and phase (position in time) of the oscillating voltage. The same can be done for current intensities as long as these also have a sinusoidal course. The connection between voltage and current strength is established with the help of complex impedances, which also deal with the phenomenon of reactive currents. Complex numbers are particularly useful in connection with three-phase current, where you have to deal with several voltages and currents with different phase values.

Alternating and three-phase current in power engineering

Today almost the entire electrical energy supply is based on low-frequency alternating current and three-phase current. The great advantage of alternating current (and three-phase current) over direct current is that the voltage and current level can be adjusted with the help of transformers with low losses. In particular, in a power plant, the moderate voltage of the generator can be brought to a high level of hundreds of kilovolts with a transformer in order to enable low-loss transmission with high-voltage lines. For the consumers, the high voltage is then converted back into medium or low voltage using additional transformers.

This reclamping was previously only practically possible with alternating or three-phase current, whereas today there are very powerful direct voltage converters that also work with low losses.

Alternating and three-phase current can be generated with the help of Rectifiers can easily be converted into direct current. Reversing can be made from direct current with the help of Inverters Alternating current or three-phase current can be produced, although the technology here is more complex than that of rectifiers.

A major disadvantage of alternating and three-phase current is that all generators in a large supply network not only have to work with the same frequency, but also have to work precisely synchronously. This causes a high level of technical effort that would not be necessary in the case of direct voltage networks. Also require so-called Reactive currents further measures and cause additional line losses, especially with underground and submarine cables. There are other advantages of direct current, which are explained in the article on high voltage direct current transmission.

AC voltages (especially with frequencies that are common in energy transmission) are more dangerous for people than DC voltages in the event of electric shocks. This is because pulsating currents can more easily lead to ventricular fibrillation. These major hazards were used as an argument in favor of using direct current rather than alternating current in the early days of electrical energy supply.

Questions and comments from readers


In the USA, AC power supply was preferred to DC voltage, and certainly not without reason.

Interconnecting multiple generators is relatively straightforward. It can be done automatically or manually. The voltage must have the same level, the phase position must be the same and the frequency must be synchronous. The phase position is checked by means of a zero voltmeter and the frequency by means of frequency meters. The connection takes place when the zero voltmeter shows no voltage.

The advantage of AC voltage is that a much smaller conductor cross-section is required to transport the current.

Answer from the author:

In the early days of electrification, there were indeed very good reasons for alternating current - especially the fact that there were transformers for it that could be used to step the voltage up and down.

The synchronization of alternating voltage generators is of course technically possible, but definitely more complex than with direct current.

The assertion with the smaller conductor cross-sections is not true - today, the most efficient way to use high power is high-voltage direct current transmission.

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See also: direct current, three-phase current, effective value of voltage and current, electrical energy, rectifier, inverter, reactive current, power factor
as well as other items in the electrical energy category