Why is 115 VAC at 400Hz use for airplane circuits?

Why is 115 VAC at 400Hz use for airplane circuits?

Everything in the cockpit of airplanes use 115 VAC at 400Hz, any reason why?

Why at 400Hz?

Any reason why they didn't use 120VAC at 60hz?

why is it at 115VAC and not 120VAC?

Why is it at 400hz? why not higher? why did they pick it at 400hz?

The higher the frequency, the less iron you need in the transformer for any given power level. So a 400Hz transformer will be smaller and weigh less than a 50 or 60Hz transformer.

Yes I know that

But I don't get why they choose to use 400hz

Is it just because of the size and weight? and cost less?

There must be a reason why they use 400hz instead of 100hz or 8hz , they use 400hz for a reason it seems

Like I said on the other website, ..... never mind.

Let's get technical: https://www.airliners.net/aviation-forums/tech_ops/read.main/429/

So the AC generator is lighter, Less Ripple, Less Filtering, Small Transformer

Almost any choice in engineering is a compromise between pros and cons. Some factors get better with higher frequency, some get worse. You need to look at all of the factors that affect what you are doing and pick the best balance of good and bad.

As for transformers, eddy current losses will increase with higher frequency. You can use thinner laminations, but there comes a limit to how thin is practical. Transformers for modern high frequency switchmode supplies are only possible by using ferrite cores because it is has a very fine grain structure. So the eddy current losses at 100s of kHz are low enough not to be prohibitive. And why, as better ferrite materials are being developed, switching frequencies are going up too.

Another factor that would have driven the choice is where the AC comes from. The frequency of the AC from an alternator is directly controlled by the rotation speed and the number of poles it is wound with. A higher frequency would require a faster rotation speed and/or more poles. So, the practical limits of mechanical design will limit the frequency.

Distribution losses also go up with higher frequency due to inductance and capacitance.

There were probably other factors that would have been considered.

Billy Mayo said:

Why is 115 VAC at 400Hz use for airplane circuits?
Everything in the cockpit of airplanes use 115 VAC at 400Hz, any reason why?...

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Nothing in either of my airplanes is 115V at any frequency. Just good old 14V with a little alternator ripple for free...

Note that they mentioned 3 phase, primarily for motors.

Billy M
did you get a satisfactory answer?

MikeMl said:

Nothing in either of my airplanes is 115V at any frequency. Just good old 14V with a little alternator ripple for free...

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My airplanes use a 7.4V Li-po battery.

And, an awful lot of airplanes use 24 to 28 VDC.

As to the 400 Hz question, remember that when that frequency was selected, converters were mechanical. 400X60 = 24,000 rpm. I am not suggesting that the generators ran at 24,000 rpm. I only want to illustrate the mechanical equivalent of 400 Hz. You can configure the generator in different ways (e.g., increase the number of poles to decrease the rpm needed), but getting much higher frequencies will present problems. Wikipedia gives an example of LF RF generated mechanically, but that is not particularly relevant to power systems in aircraft.

Some things just happen, they work, and are then adopted as a standard. For example, ask yourself why most screws are inserted turning to the right? (There actually is a reason usually given for that, but I am not sure it influenced the decision at the time RH screws became dominant.)

Edit: Maybe these are better examples of the "it worked" principle. Why do most American aircraft engines turn counterclockwise (when viewed from the front looking toward the tail) and British engines turn clockwise? Why do the Brits drive on the left and the rest of us (mostly) drive on the right?

John

jpanhalt said:

Why do most American aircraft engines turn counterclockwise (when viewed from the front looking toward the tail) and British engines turn clockwise?

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Really?? I wonder why.

an awful lot of airplanes use 24 to 28 VDC.

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So these planes use a mechanical generator on the air plane that is 24 to 28VDC?

Billy Mayo said:

So these planes use a mechanical generator on the air plane that is 24 to 28VDC?

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Yes. I am not aware of any airplane that up-converts 12V to 24 to 28V for system voltage. On an exception basis, a 12 to 14V airplane may up-convert to the higher voltage for a particular piece of avionics.

John

audioguru said:

Really?? I wonder why.

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Notice the direction the propellors on these two aircraft are designed to turn. The left is British; the right is American:


One must be be aware of that difference as it causes different flight characteristics, particularly during takeoff and descent.

The reason is probably like I mentioned, "it worked." The most common exception is in twin-engine aircraft in which one of the engines may be counter-rotated relative to the other. The American P-38 is one of the more famous/notorious examples of that.

John

also it isn't 400Hz (not anymore ) its 360Hz --> 800Hz and is directly related to engine speed.

Billy Mayo said:

So these planes use a mechanical generator on the air plane that is 24 to 28VDC?

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No, most use an alternator, similar to the one in your car. About 50% of Piston powered aircraft are 14V and the others are 28V. Jets and turbine powered aircraft are likely to use 400Hz AC systems.

most use an alternator, similar to the one in your car. About 50% of Piston powered aircraft are 14V and the others are 28V. Jets and turbine powered aircraft are likely to use 400Hz AC systems.

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So what are they using than inside an airplane or aircraft? a AC generator or an AC alternator? a car uses a 12VDC battery with an alternator , do planes have a battery connected to the alternator?

Billy Mayo said:

So what are they using than inside an airplane or aircraft? a AC generator or an AC alternator? a car uses a 12VDC battery with an alternator , do planes have a battery connected to the alternator?

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Yes. What other arrangement would your consider?

Perhaps I was too general in my earlier comment. I considered your use of the term "generator" as including alternators. I have not seen an airplane with something other than an alternator in a very long time.

John