# Frequency, Electrical

*The Great Soviet Encyclopedia*(1979). It might be outdated or ideologically biased.

## Frequency, Electrical

the time parameter of a periodically (cyclically) varying electric current, expressed by the ratio of the number of complete cycles of current variation to unit of time; the quantity is the inverse of the period of current variation. It is measured in hertz (Hz). For a sinusoidal alternating current the concept of angular frequency is used, which is related to the electrical frequency by the formula ω = 2π*f* (where ω is the angular frequency and *f* is the electrical frequency).

In many countries of the world, including the USSR, the frequency of the current generated by electric power stations and commercially distributed to industry is 50 Hz; in the USA it is 60 Hz. Railroads in many countries use current with a frequency of 16⅔ Hz (for electric traction) as well as frequencies of 25 and 75 Hz (for automatic block signaling, for example, on track circuits). In aviation, the current used has a frequency of 400 Hz (for independent power-supply systems). In certain industrial and agricultural equipment the operating frequency is raised to 200–400 Hz.