Large aircraft electrical systems usually use 400 Hz alternating current (AC), while common household electrical outlets deliver AC at 50 or 60 Hz. Why the difference?

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    $\begingroup$ The first question is NOT a duplicate question, although the answer to this IS addressed in one of the ANSWERS there. The second question IS a duplicate question, although the answer there is terrible, and neither one shows up when looking for "400 Hz" as a search. If I were searching for the answer to 400 Hz vs 60 Hz, "Why do airplanes use AC" wouldn't be an obvious place to look for the answer. $\endgroup$
    – Ralph J
    Commented Mar 18, 2017 at 18:00
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    $\begingroup$ @RalphJ Still, now that you know, since you self-answered, the appropriate next action would be to first improve that second question with an edit to make it more searchable, then to post your answer there. Furthermore, if you can find a way to make that 2nd link not be a dupe of the 1st (which it is currently marked as), you could find a way to reopen it, thus allowing you to add your answer there. Or if you can find a way to add your answer to the 1st link (currently open), that works too. $\endgroup$
    – Jason C
    Commented Mar 18, 2017 at 18:52
  • $\begingroup$ @JasonC, the first question is perfectly fine as is, but it is fundamentally a different question than this one. For the second question to be marked as a duplicate of the first was absolutely a mistake, IMHO -- "why do acft use AC power not DC power" and "why do aircraft use 400Hz not 60Hz AC" are NOT the same question. You're right, I could edit the second question (which currently has a single, incorrect, answer) to try to get it reopened, and then post this answer to it, but THIS question is more findable in search & lacks the incorrect answer. JMHO. $\endgroup$
    – Ralph J
    Commented Mar 18, 2017 at 21:07
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    $\begingroup$ @RalphJ Hm, well, it seems responsible to do something, no matter what that something is. We could perhaps petition a mod to re-close that second question as a dupe of this one, or merge them. There is no doubt that your Q & A are the better of the two. $\endgroup$
    – Jason C
    Commented Mar 18, 2017 at 21:11
  • $\begingroup$ Actually, one of the answers to the first question is the answer to this one (as I pointed out in my original comment, which has since been deleted). That's also why the second question (which doesn't really have a valid answer to the question which was asked) has the first question as its' duplicate. See this answer which addresses your question: aviation.stackexchange.com/a/3108/69 (Keep in mind that the reason we keep questions like this around when there is a duplicate is sometimes they are hard to find and this acts as a "sign post" for others.) $\endgroup$
    – Lnafziger
    Commented Mar 19, 2017 at 2:10

2 Answers 2


Induction motors turn at a speed proportional to frequency, so a high frequency power supply allows more power to be obtained for the same motor volume and mass. Transformers and motors for 400 Hz are much smaller and lighter than at 50 or 60 Hz, which is an advantage in aircraft (and ships). Transformers can be made smaller because the magnetic core can be much smaller for the same power level. Thus, a United States military standard MIL-STD-704 exists for aircraft use of 400 Hz power.

So why not use 400 Hz everywhere? Such high frequencies cannot be economically transmitted long distances, since the increased frequency greatly increases series impedance due to the inductance of transmission lines, making power transmission difficult. Consequently, 400 Hz power systems are usually confined to a building or vehicle.


Further discussion of aircraft electrical systems

Further, further discussion of aircraft electrical systems

Added: cool discussion of the use of 400hz with Cray supercomputers on another S.E. forum

  • $\begingroup$ Marine vessels (military, not civilian) usually have 400 Hz as well. Same reason. $\endgroup$
    – Mast
    Commented Mar 18, 2017 at 20:46
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    $\begingroup$ A bit OT from 50+ years ago in Norway... When melting aluminum with electrolysis it's beneficial with AC-power with a low frequency - which of course would be supplied by a local power-plant. However the small towns often built around such smelters would get their power from the same plant... And with AC-frequency of maybe just 10-30 Hz, that meant flickering lights and extra large transformers (because of the iron required) in appliances. Higher frequency so you could use less iron, makes much more sense! $\endgroup$ Commented Mar 18, 2017 at 21:37

Inductors and transformers at 50 Hz are much larger and heavier than the 400 Hz case.

Aviation likes small weights.


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