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Primary electric power on larger airplanes is provided as AC. What aspects of aircraft make AC a better choice than DC?

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if you search for "dc ac spike" or "dc ac Voltage spike" you will see that this property alone, from the AC power, can be enough to justify the adoption of it against the DC. –  user2485710 Apr 8 at 0:18
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@user2485710 I have no idea what you're talking about. Care to share a link? You can put voltage spikes on any power line, it doesn't matter if it's DC, AC, or 12-phase –  Nick T Apr 8 at 0:50
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@user2485710 I have over 9000 rep on that site and am an electrical engineer. I don't know what you're talking about. –  Nick T Apr 8 at 1:02
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Something interesting (to me at least) is that in the Dassault Falcon aircraft, they eliminated the AC bus in newer aircraft and generate 100% DC. Where AC is needed for specific instruments, they replaced the instruments with versions that had built-in inverters. So I wouldn't assume that all airplanes are using AC. Perhaps (just a guess) larger airplanes are going to AC because of the longer wire runs? –  Lnafziger Apr 8 at 4:35
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I haven't seen anyone here mention the interesting artifact of "DC dies and AC lies" -- that is, DC powered instruments tend to drop to zero when power fails, AC instruments (like the torque gauges on the King Air) freeze wherever they were last, which is NOT a very friendly failure mode. Incidentally the King Airs have all of: 28V DC, 28VC AC, and 110V AC. –  RobP Apr 10 at 5:12

6 Answers 6

up vote 11 down vote accepted

AC is easier to produce with the engines, that act as generators. The engines have a rotating shaft that it is easily equipped with magnetic dipoles all around.

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Then, depending on the instrument, the current is either used directly or, by the use of converters, in DC form.

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Not entirely correct, I think. The AC needs typically be 400 Hz constant frequency, regardless of how fast the engine is spinning. So the engine and the generator are connected with a Constant Speed Drive unit, which in essence an automatic transmission that keeps the generator at constant speed. –  Hilmar Apr 7 at 18:48
    
@Hilmar I left that out on purpose, it is not required to understand "why AC instead of DC". –  Federico Apr 7 at 19:03
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@Hilmar That's not always true-some aircraft have variable "wild" frequency power 350-800Hz depending on engine speed. –  Spehro Pefhany Apr 7 at 19:07
    
@Federico: Except it's kind of essential. There are no DC generators, only AC generators with converter. But when there is a converter, the frequency does not matter. When there is not, the frequency and the phase must match, which is quite a bit of complication. –  Jan Hudec Apr 7 at 21:56
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That's an inefficient diagram you have there, 2 south poles, 4 north poles on the rotor! –  Nick T Apr 8 at 0:49

What the other answers have failed to note is that on a plane, its not just AC power, but 3 phase AC power.

Depending on how the plane is wired up, you will either get the benefit of reduced cable weight or higher reliability (or some blend of the two).

Delta

If the plane is wired up with a Delta transformer, then 3 wires are used to carry the electricity. However, what is crazy is that the Delta transformer (or alternator) will continue to run (albeit at a lower capacity) with one of the windings destroyed.

Wye

In a balanced wye configuration the 4 wires come from the transformer. Each carry of the first three wires carry current from the transformer, with the 4th as the common "return". However since each of the wires are "balanced", the actual current on the common is approximately zero (and not the 3 * current of the output). That means your 3 "live" wires only needs to be thick enough to carry the current one way. (typically with DC you would need to have wire thickness to carry the current there then back again). So in effect we can achieve the same power transmission with half the weight of a DC setup.

Since on planes, both redundancy (safety) and weight (economy) are both massive factors in cost. It is actually economical to run power on AC, and convert to DC where necessary.

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The size, weight, and cost of switchgear for DC-DC conversion at any given power is much higher than AC-AC and AC-DC conversion. So whether a piece of equipment requires AC or DC, it can be converted more easily from an AC source than from a DC source - but more importantly, it can be done with less weight, volume, and cost.

It's the same reason 400Hz is used rather than 50Hz or 60Hz - weight. A generator or transformer that can handle a given load is physically smaller and lighter at higher frequencies due to issues with core saturation at lower frequencies. A smaller, lighter core can be used for higher frequencies.

While switchgear has evolved over the decades, and weight and cost aren't as big an issue as they were in the past, the generator itself still has to have a stator and windings, and these are still physically smaller and lighter for a given power output than the equivalent DC, or low frequency AC generator.

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Since DC generator is just an AC generator with converter, it can be made from high-frequency one and thus have the same low weight. It is even advantageous, because less stabilization is needed. –  Jan Hudec Apr 7 at 22:01
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@JanHudec "DC generator is just an AC generator with converter" I suppose you can make it this way, rather than using a real DC generator. But why? An AC generator without an AC-DC converter is still lighter and cheaper than an AC generator with a converter. Further, what are you going to do with that DC? You're going to have more expensive and heavier DC-DC and DC-AC converters throughout the plane. AC-DC and AC-AC converters are still cheaper and lighter. –  Adam Davis Apr 7 at 22:09
    
@AdamDavis: You'll need some conversion electronics by the generators in both cases -- the rotation of the several engines that may provide power is not synchronized, so just feeding the raw output from each AC generator into a common bus wouldn't end well anyway. (And in contrast to terrestial power plants, producing electricity is not the main task of a plane's engines so you cannot just adjust the engine speeds to lock frequency between the generators). –  Henning Makholm Apr 7 at 22:22
    
There is no "real DC generator". Electromagnetic induction only works with changing magnetic fields and thus produces changing current. So a DC generator is either wound generator with commutator instead of two separate rings or an induction generator with diode converter. There is no reason why it couldn't run at the same rpm as the AC generator and consequently no reason to be heavier. –  Jan Hudec Apr 7 at 22:28
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@HenningMakholm That's true, particularly on modern and larger multi-engine planes. The reason we do it today is largely historical. Lots of planes now have DC and AC buses, multiple generators, buses, and converters that will power one or more buses from the others depending on circumstances. They could change the systems completely over to DC, but it would require a significant change in the industry to justify the cost. We are now building planes based on the decisions made years ago due to the weight difference of direct AC and DC generators –  Adam Davis Apr 7 at 22:29

Does it have to be aircraft-specific reasons? I would imagine the reasons are not necessarily very aircraft related:

  • Different subsystems need different working voltages internally, and supplying AC makes it easy for each component to transform down to whatever it needs.
  • Where mechanical power is needed, asynchronous AC motors are a lot simpler (especially in terms of maintenance requirements) than DC motors.
  • Engineering inertia. All kinds of components already come expecting AC; even if DC is in isolation superior for some uses (as it might well be for electronics, with today's solid-state voltage converters) supplying DC to aircraft systems would mean that you need to have a DC supply network in addition to the AC supply for those components that are not yet available and certified in DC versions. That extra wiring alone might easily offset the advantages of having some of power supplies be lighter.
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I just didn't want an answer with AC vs DC. All of your points there are great answers. –  fooot Apr 7 at 17:09

I assume that aeroplanes use both AC and DC for powering the various subsystems:

If energy has to be stored in accumulators - well, they only store DC current. But if generators are involved you won't have a chance of totally avoiding AC since a rotating movement is essentially sinusoidal:

  • If you put a rotating magnet into a coil it will produce an alternating magnetic field in the coil => the coil will produce AC
  • and if you put a rotating coil into a magnetic field the field will alternate between going through the coil in one direction and in the other again producing AC current.

One can now put the AC through a rectifier or use a wiper (a switch that can reverse the current and is mechanically triggered twice at every revolution of the generator). But if you rectify a sinus you still get a voltage that goes up and down, only it no more passes the zero level. This means you will sometimes draw too much energy from the generator (when the voltage is high) and at every revolution the voltage will shortly drop to zero meaning you won't be able to draw any energy at all.

If you don't constantly draw energy from a generator, the generator will vibrate. You wouldn't like that in an aeroplane but there is a remedy:

Using three-phase AC: You can generate three sine waves with the same generator using coils that are a third of a revolution apart. When the voltage from one of these coils drops, the voltage at the two other coils rises - and the power produced (which is linear to the voltages squared) will add up to a constant flow.

The next thing one has to know that if you want a constant stream of power from a motor the same reasons make you use three AC waves here, too.

Normally this AC is generated from a DC voltage, though, because you might want to adjust the frequency the motor is running at or the amount of power it provides hence you want to adjust the frequency of the voltage the motor gets.

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A transformer that converts the same power on a higher frequency is much smaller and lighter.

A modern design of the compact isolating power supply (say 220 V 50 Hz to 12 V DC) would often include a frequency converter, then a high frequency transformer, then a rectifier (a big 50 Hz transformer is now less uncommon). But maybe the standard has settled before this complex electronics became available, or maybe because higher frequencies may be source of interference.

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