Why aircrafts are producing AC instead of DC ?

[megan]

The reason ac is used is that the power transmission is 3-phase. That means that , for a balanced system, i.e. each phase equally loaded, the current in the neutral wire is zero. so you have tree wires, each carrying one third of the power, and you don't need a return wire

You need the forth (return) wire for the case where the three phases not balanced, which is most of the time. Extremely rare to have all phases balanced.

DC systems carrying the same current would require six wires

That's not how it works, AC vis a vis DC. In a DC system the return is via the airframe.

[Gauges and Dials]

Wow!

The level of misinformation in this thread is staggering.

Each one of us could make a significant contribution to the professionalism and quality of this site by pledging to refrain from answering questions when we aren't thoroughly sure we know the answer.

[msbbarratt]

Originally Posted by Dairyground

DC is used for long-distance (tens of miles or more) under(salt)water power links such as those between England and France. I have not investigated the rationale for choosing DC, but assume that an AC link would suffer significant losses because of the conductivity of the surrounding medium.

As others have hinted at, it's the capacitance between the undersea cable and the earth that makes AC a problem and DC worthwhile. The capacitance causes transmission losses for AC.

However, to make a DC cable efficient enough to be worthwhile requires very high voltages. Getting the balance right between the working voltage and the reliability of the cable's insulator is the tricky part. In cables such as that, any hint of a breakdown in the insulator is immediately, and explosively, disasterous. Especially as what leaks in is salt water.

It's basically the same argument in favour of using higher pressure hydraulics on an aircraft. Higher pressures are desirable (smaller pipes, more efficiency) but more difficult (preventing leaks).

Back in the 1960s when the first cable was laid, the AC-DC conversion was done with mercury-arc rectifiers; these were terrifying things that would scorch your skin off (they're prodigious UV emitters), and could explode in a cloud of boiling mercury droplets if anything went wrong. Suffice to say it's all solid state electronics these days.

AFAIK the cable has almost never conducted power from the UK to France (the original intention back in the coal rich 1960's UK). It's always flowed the other way (lots of nuclear power).

Superconducting Grid

Back in the 1960s the Central Electricity Generating Board researched the use of superconductors for the UK national grid. They concluded that it would be worthwhile, but would require switching the grid over from AC to DC; superconductors don't eliminate reactive AC loses. That conversion would simply have been overwhelmingly expensive, which is why they didn't do it.

Grid lines strung between pylons tens of meters above the ground are far less lossy at AC than a cable buried in a trench, but it's still a significant source of transmission loss even then. And in case you've ever wondered why grid cables are in bundles of 4, that's to approximate a tube and take some advantage of the skin effect that occurs in AC transmission. 50Hz is a reasonable compomise between AC and DC loses. A lot of power generators actually work at 3000Hz, and the power station has to do a conversion to 50Hz.

Explosively Formed High Temperature Superconductor Grids

These days the attraction of high temperature super conductors is high; liquid nitrogen is all that is required for cooling. Trouble is they're all ceramic; they don't bend, so cannot be rolled out into a trench. There is a technique that has been tried (successfully AFAIK!) to explosively form the ceramis in-situ; a copper tube is filled with the powdered ingredients, and wrapped in a layer of explosives. This is then rolled out into the trench. The explosives are set off, boom! Et voila, one long rod of high temperature superconductor is formed. I think this technique was used somewhere in the US to connect a (hydro?) generating plant up to the grid without putting a string of pylons through some area of outstanding natural beauty.

I don't think we'll be using that in aircraft any time soon...

DC DC Conversion

Mention has been made of efficient DC-DC conversion being a thing these days, and indeed it is (90% is possible). However it involves converting the DC to AC (say, 10kHz), putting it through a small transformer (hence the 10kHz), and recitfying it back to DC. This is what a switched mode power supply does.

[nicolai]

Originally Posted by msbbarratt

. A lot of power generators actually work at 3000Hz, and the power station has to do a conversion to 50Hz

You may be thinking of a typical power generator rotating at 3000 revolutions per minute, thereby generating 50Hz (cycles per second) electricity. 3600rpm in the USA, parts of Japan, and so on for 60Hz power.

There's typically no frequency conversion in large-scale power generation. Instead, the spinning alternator shaft is brought to the correct frequency and phase for grid synchronisation, and maintained at that frequency and phase while supplying a load to the grid. Large electricity generating turbines and alternators produce electricity at a voltage of around 15kV, which is then stepped up by transformers to the grid voltage (275kV or 400kV in the UK).

If the load is too great, the shafts in all generators in the UK spin more slowly, causing a drop in frequency. Therefore total grid load can be signalled to the consumer by a small drop in frequency, and load shedding on the UK National Grid is performed partly by very large electricity consumers (such as heavy industry plants) having frequency-sensitive switchgear which disconnects their equipment from the grid as the frequency decreases, thereby decreasing grid load.

We're a bit off the topic of aircraft now

On-topic: I am somewhat surprised that the 787 would use the frequency-wild 235V AC to run things that are not in the engines or wings. The starter and wing anti ice makes some sense, but the cabin recirculation fans being run from 235V AC frequency-wild when there is either 110V AC 400Hz or the +/- 270V DC available seems a bit odd to me. It would be a very fine level of optimisation to find that extra wiring and complexity of the recirc fans being frequency-wild AC would be lighter and cheaper than providing additional stabilised 110V AC or +/- 270V DC to run those fans.

[Chu Chu]

A lot of industrial motors are now variable speed to save electricity. They're generally either induction (AC) motors run on an inverter-based variable frequency drive, or brushless DC motors. In either case, the AC supply current is first rectified to DC, then chopped up as needed to run the motor at the required speed. The rectifier wouldn't care about the frequency of the AC supply (within reason, anyway).

That's a long way of saying that if the recirc fans are variable speed, it's probably just as easy to run them off the frequency-wild 235 VAC.

[custardpsc]

Here is a very clear answer why AC is used - because it can be run at 400Hz. The weight savings in copper inside equipment, inductors and other windings is the upside of that vs DC. I remain unconvinced by the arguments about motor/alternator speed being faster being any sort of reason. It certainly is faster if the same number of poles are used for 400hz vs say 50 hz but the net amount of work done is determined by the power supplied in Watts not the speed of revolution, so that is a red herring. There is also some saving in weight to be had by running three phase cabling - and I am guessing that some stuff is run that way - the heavier loads such as any electric heating for deice probably are connected between phases and so 3 cables only are required for 3 heaters.

What I don't know is why the aircraft industry standardised on 115V, almost certainly a USA-led thing. If they had standardised on 200V or 230V - they could have saved half the weight of copper again.

[megan]

The level of misinformation in this thread is staggering

Rather than posters because they may be incorrect with their facts, you might educate us all with your wisdom and insight. If we "pledged to refrain from answering questions when we aren't thoroughly sure we know the answer" there would be little to no discussion.

Back to regular programming.