Alternating current is the name for an electric current whose direction of flow changes in regular repetition. Positive and negative instantaneous values complement each other in such a way that the current is zero on time average. The English designation is “alternating current” or the abbreviation AC. In contrast, DC (“direct current”) denotes direct current.
Worldwide, alternating current with a sinusoidal waveform is most frequently used for power supply. In Europe, the mains frequency is 50 Hertz, which means that the direction of the current changes 50 times per second in each direction (i.e. 100 times). However, there are also networks with other frequencies. In Germany, Austria and Switzerland, for example, the railroads run at a reduced frequency of 16.7 hertz. This has historical reasons.
Transformers are one of the main reasons for the success of alternating current.
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Alternating current is generated by means of generators based on the dynamoelectric principle. As with a bicycle dynamo, a coil is moved in a magnetic field at rest.
Compared to direct current, alternating current has the advantage that it can be easily brought to a different voltage level and, above all, without great losses. This is ensured by the transformers, which were developed in parallel by various researchers in the years around 1900. They are one of the main reasons for the success of AC grids.
In contrast, direct current has advantages for long transmission distances from about 750 km or even for submarine cables. The transmission losses are significantly lower here. However, the direct current must be converted back into alternating current by converter stations before it can be fed into the power grid. However, such plants are expensive. High-voltage grids with direct current are therefore only worthwhile in special applications.
