Primary electric power on larger airplanes is provided as AC. What aspects of aircraft make AC a better choice than DC?
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.
Then, depending on the instrument, the current is either used directly or, by the use of converters, in DC form as not only it is easier to produce, it is also easier to convert. So electronic devices that uses 12, 5 or 3V may contain their own voltage converter (though this is less and less true with modern efficient switch-mode converters). If DC conversion was as much efficient as AC conversion, we wouldn't see high-voltage lines. Last but not least, it's easier to switch, because the current is null twice per cycle. DC must be switched at full current, which is expensive and weight costly. (thanks mins)
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).
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.
In a balanced wye configuration the 4 wires come from the transformer. Each 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.
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.
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.
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.
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 uncommon). But maybe the standard has settled before this complex electronics became available, or maybe because higher frequencies may be source of interference.
This goes back to the 1930-s and has been kept because it was a good solution and still works.
In the 1930-s the big electronic revolution started and accelerated in the second world war. This is of course before the transistor, so vacuum tubes where used in radios, radars and so on. The generators on the engines were DC-generators and feeding batteries (remember, no diodes available for AC generators). But the vacuum tube elecronics boxen needed several different voltages inside: something like 6.3V for heating and several other voltages, say 12V and 400V. In order to produce these transformers were the best solution. But transformers need to be fed AC.
To create AC an inverter were used. In those days, a DC motor connected to an AC generator. The RPM of the DC generator was, somewhat, regulated to get in the ballpark correct AC frequency.
As the weight of a transformer is, somewhat again, inversely proportional to the frequency you would want a high frequency in order to keep weight down. As losses increase, again somewhat, proportional to frequency, you would want to have a low frequency. Somewhat in the middle of frequencies the engineers, who incidentally knew exactly what the were doing, landed on 400Hz as a reaonable compromise.
And now we keep on using 400Hz, simply because it works. All of the components and lots of the engineering decisions has changed, but 400Hz AC still is a good in the middle compromise.