How Did 400Hz Become a Standard?

Commercial aircraft rely heavily upon electrical systems to power flight instruments, entertainment services, and other operationally necessary components. This electrical system is supported by batteries, engine-driven alternators, and auxiliary power units, as well as external sources while not in flight. Like everything else in aviation, the electrical elements in modern aircraft are chosen carefully to optimize safety and performance, while also achieving industry standardization. One aspect of aircraft electrical systems that is markedly different from other industries is the use of 400 Hz power. In this blog, we will discuss the rationale behind the 400 Hz standard, while also describing alternator components in detail.

Generator-only electrical systems have largely been phased out on most commercial aircraft due to the fact that they are unable to produce a sustainable energy output until the engine is well into use. This dependence on higher RPMs means that critical systems might experience a transient decrease in power during periods of engine abatement. Generators are also heavier and have lower amperage ratings when compared to alternators. Further, most generators contain a carbon-brush component which is particularly prone to trapping dirt. Over time, this buildup can cause electrical noise that can impede the function of nearby sensitive avionics.

Alternators have the capacity to produce a fully-operational power output at very low RPMs, helping ensure stable energy throughout the entire flight. Due to their reliable nature, most modern aircraft have replaced all of their vacuum-driven systems with electrical instruments, showing confidence in this tested power-delivery component. Another important distinction between alternators and power sources is their usage of alternating current (AC ) power.

Direct current (DC) charges flow in a single direction along an electrical circuit. When plotted on a current vs time graph, the resultant line is flat and constant at a given "y" value. Although many avionics are powered using low-voltage DC, it is typically distributed by AC coupled with a transformer. Alternating current periodically changes direction and magnitude over time, resulting in a sinusoidal pattern when plotted on a current vs time graph. Compared to DC, AC is much more efficient to convert into a given voltage; therefore, it is made the standard for power generation in aircraft. When discussing alternating currents, it is important to take note of several characteristics, including the cycle, period, and hertz. These values refer to various components of the current vs time graph and are all dependent on each other. Hertz (Hz), in particular, is a standard measurement that varies among AC units depending on the application.

In general, the frequency of Hz is defined as having one cycle per second. As such, cycles refer to the complete wave of alternating current and include both positive and negative cycles. With this in mind, typical wall outlets in home or business settings often provide a 50Hz to 60Hz frequency range, whereas most aircraft use a 400Hz frequency. This drastic difference primarily comes down to the relative weight and size of the transformers and generators used. As the speed of voltage change decreases, the size of the associated components increases, along with its weight. After several aircraft manufacturers agreed upon this standard, it became an international custom, allowing for reliable service at all sites.

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