In civil and military aviation, a frequency of 400 Hz is used in aircraft electrical systems. The main reason is that transformers, motors and other electrical components can be designed significantly smaller and lighter at 400 Hz than at the 50 or 60 Hz frequencies commonly used in public power grids. This enables compact and weight-optimised electrical equipment, which is particularly important in aviation.
Why are electrical components smaller and lighter at 400 Hz?
The electromotive force (EMF) generated in a coil depends, among other factors, on the frequency and magnetic flux. At a higher frequency, the same voltage can be achieved with a lower magnetic flux. This allows the iron core of a transformer to be designed smaller. The number of primary and secondary windings, and therefore the amount of copper required, can also be reduced. As a result, transformers and other electrical components can be designed more compactly and with less weight.
Why aren't public power grids operated at 400 Hz?
A high frequency such as 400 Hz is less suitable for transmitting electrical energy over longer distances. For public power grids, low-loss transmission over long distances is therefore a key consideration. This is why grid frequencies of 50 Hz and 60 Hz have become established internationally. The 50 Hz and 60 Hz frequencies provide a suitable compromise between the requirements of electrical machines and transformers and the efficient transmission of electrical energy. In aircraft, however, transmission distances are comparatively short, meaning that the advantages of a higher frequency for compact and lightweight electrical equipment are more significant.
What are the advantages of 400 Hz in aircraft?
Another advantage of 400 Hz is the compact and weight-optimized design of electrical equipment. Transformers, motors and other electrical components can be designed smaller and lighter at higher frequencies. In aircraft, every kilogram of weight matters, as lower weight can help reduce energy consumption and increase range.
The use of 400 Hz is particularly suitable for applications with comparatively short transmission distances. Unlike public power grids, electrical power in an aircraft only needs to be distributed over relatively short distances. This means that the advantages of a higher frequency can be utilized without the disadvantages associated with transmitting electrical energy over long distances being equally significant.
The combination of compact design, low weight and efficient electrical power supply therefore makes 400 Hz particularly suitable for aviation.
Some railway networks, including Deutsche Bahn, still operate at 16.7 Hz. This comparatively low frequency is the result of the technological capabilities of the first electrical machines at the beginning of the 20th century, which made it possible to build powerful machines with low losses at low frequencies. Switching to higher frequencies would involve significant technical and economic modifications to the existing infrastructure.
In aircraft, different electrical voltage levels are used. Find out what role 28 V DC, 270 V DC and 115/200 V AC play in connection with 400 Hz aircraft electrical systems and how these voltage levels differ in our overview of the different voltage levels used in aviation.
What role does 400 Hz play in aircraft ground power?
400 Hz technology is not only used in aircraft electrical systems but also plays an important role in aircraft ground power. While an aircraft is on the ground at an airport, it can be supplied with electrical power by a Ground Power Unit (GPU). This allows various electrical systems on the aircraft to be operated while on the ground without continuously using the aircraft's onboard power sources.
For ground power supply, electrical power can, for example, be supplied from a low-voltage grid at 400 V / 230 V and 50 Hz and converted to 115/200 V at 400 Hz using suitable frequency converters. Ground Power Units are available in different power ratings. The requirements for aircraft ground electrical supplies are defined, among other standards, by international standards such as DFS 400 and ISO 6858.
Different electrical voltage levels and systems are used for aircraft ground power. These include, among others, 115/200 V AC at 400 Hz and 28 V DC. The appropriate system depends on the aircraft type and the intended application. Find out what role 28 V DC, 270 V DC and 115/200 V AC at 400 Hz play and how these voltage levels differ in our overview of the different voltage levels used in aviation.
SAB Bröckskes offers an extensive range of 400 Hz, 28 V DC and 270 V DC cables for aircraft ground power applications, including fixed, flexible and reeling applications.
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Further information
General Information on 400Hz Cables
You can also find the most important technical information about 400 Hz and its applications summarized in our PDF.