400-Hz Power Systems for Air, Sea and Space

A 400-Hz AC-based approach enables jet airplanes’ power systems to be smaller and lighter than possible with a more conventional alternative 50- or 60-Hz scheme.
Published By

It may be a surprise to at least some of you to learn that mass transportation systems such as jet airplanes, ships, submarines, and spacecraft don’t use the 50 Hz (220-240V) or 60 Hz (100-127V) AC standards that are nearly universal for “mains” power nowadays (and that worldwide-compatible AC-to-DC converters handle with aplomb, in conjunction with a set of AC plug adapters). Instead, these transportation options rely on 400 Hz AC power transmission; US Military Standard MIL-STD-704, for example, specifies an 115V 400 Hz AC approach. Why?

Utility History

The early days of AC utility power were actually marked by a diversity of voltage, frequency, current and other specification combinations. This is because they all attempted to optimize and compromise across often-contradictory requirements of power:

1. Generation at the source
2. Transmission from source to destination, and
3. Usage at the destination

Early systems, for example, somewhat arbitrarily picked a frequency that was optimum for whatever steam engine, water turbine or other electrical generator was in use. Another important factor to consider is the “skin” effect, which manifests as the tendency for electrons to increasingly traverse a wire using only its outer region—versus its entire cross-section area—as the transmission frequency increases. The resultant increase in series impedance with increase in frequency—by virtue of the inefficient non-use of the wire’s core—leads to increased transmission losses over long distances. For utility companies motivated to provide as much power to their customers as possible at as low a cost as possible, high-efficiency transmission is critical.

Motors at the destination also historically tended to prefer lower transmission frequencies. This is because the inductance of the motor’s magnetic field opposed rapid changes of current. Materials used in early motor designs of the late 19th and early 20th centuries in particular did not work well above 60 Hz. Ironically, these historical materials issues have largely been ameliorated and it’s now the case that 50 or 60 Hz (versus 400 Hz or some other higher voltage) puts an upper-limit cap on an induction motor’s maximum RPM.

AC lighting at the destination conversely has historically preferred higher transmission frequencies. This is because an incandescent lamp’s filament cools on each alternating current half-cycle; the slower the frequency (i.e. the longer the cycle), the more noticeable the resulting lamp flicker. The mish-mash of incompatible power standards—sometimes between contending utilities within the same city, far from intra- and inter-country—took many decades to sort out, with consolidation fueled by factors such as multi-utility mergers, mass-production commoditization of electricity-powered appliances, and consumers’ understandable desires to not need to re-purchase those appliances when they moved. By the post-WWII era, the bulk of the world had largely settled on today’s two dominant standards.

Closed-System Divergence

Why, then, have the aforementioned transportation systems deviated from this convergence? For one thing, as this section’s title tips off, they’re closed systems. They don’t need to interoperate, for example, with the nearby power grids assembled by other utilities. About the only (inefficient) nod they need to provide to worldwide “mains” standards is, for example, in providing 50 and 60 Hz-compatible power outlets for passengers to use when powering equipment they’ve brought on board, and generated by AC-to-DC rectifiers followed by DC-to-AC inverters. And on that note, 400 Hz has advantages over 50 Hz and 60 Hz from a DC-generation standpoint; the shorter cycle time allows for use of a smaller “droop” capacitor as part of the rectifier circuit.

Keep in mind, too, that the power-carrying wiring harnesses in such transportation systems are hundreds of feet long, not hundreds of miles long; the attenuating effects of series inductance at higher AC frequencies are less of a concern in such cases. Of great concern, conversely, is weight. 

Ships, submarines, spacecraft and other vehicles have the same concerns and motivations as do aircraft; they want to minimize weight in order to maximize the range they can travel for a given amount of onboard-stored fuel.

As such, the transformers and other circuitry necessary to convert inconsistent engine-driven alternator outputs into consistent AC power—and to vary the voltage in order to power various airplane or other vehicle subsystems—are inherently smaller and lighter in a 400-Hz approach than with a 50- or 60-Hz alternative. Since the EMF (electromotive force) generated in a coil is proportional to both to flux and frequency, higher frequencies require less flux, therefore less iron is needed in the transformer core. And the more transformers that are in use, the greater the cumulative weight savings effect will be.

Specifically, when Faraday's Law of Induction is applied to describe a transformer:

VP = -NP (dΦ/dt)
VS = -NS (dΦ/dt)

It reveals (among other things) that a transformer’s EMF (electromotive force) varies with the derivative of magnetic flux of the core between the primary and secondary terminals with respect to time, where VP and VS are respectively the primary and secondary voltages, NP and NS are the number of “turns” of the primary and secondary windings, Φ is magnetic flux, and t is time. Therefore, said another way, the EMF of a transformer at a given flux density increases with frequency...or said a third way, for a given EMF needed to implement a particular primary-to-secondary voltage and current transformation, both:

• The amount of flux density i.e. core material, and
• The number of primary and secondary winding turns i.e. the total amount of wire coiled around that core can be decreased in size (and weight) as the operating frequency increases.

Related news articles

Latest News

Sorry, your filter selection returned no results.

We've updated our privacy policy. Please take a moment to review these changes. By clicking I Agree to Arrow Electronics Terms Of Use  and have read and understand the Privacy Policy and Cookie Policy.

Our website places cookies on your device to improve your experience and to improve our site. Read more about the cookies we use and how to disable them here. Cookies and tracking technologies may be used for marketing purposes.
By clicking “Accept”, you are consenting to placement of cookies on your device and to our use of tracking technologies. Click “Read More” below for more information and instructions on how to disable cookies and tracking technologies. While acceptance of cookies and tracking technologies is voluntary, disabling them may result in the website not working properly, and certain advertisements may be less relevant to you.
We respect your privacy. Read our privacy policy here