Why aircrafts are producing AC instead of DC ?
 

Hi guy I search all the forums but there is no clear explanation.

FYI there are/were aircraft around that “produce” DC .....and they then often have to have inverters installed to get the AC power needed for any components needing such a supply.

Leaving aside the school answer about AC and transmission losses (which may or may not be significant with airframes) I’m guessing AC has the advantage of being able to be transformed/rectified to other voltages and/or rectified to DC as and where required around the airframe...it might be harder/less convenient to do the process the other way.

Awaits incoming....

There are two types of AC power generators. Variable frequency and constant frequency. Constant frequency generators (or IDG's) use a variable speed transmission to maintain a constant speed at the generator as the engine drive speed varies.

Not related to the OP but high voltage DC is now being used for long range power transmission. Mr Tesla would be disappointed ;)

AC wiring is much thinner and lighter.

Wire needs the same dimensions to carry the same current - whether its AC or DC. The heating effect is I squared * R - doesn't make any difference to the direction of electrons.

DC motors and generators need brushes. Those wear out leading to increased maintenance and make sparks for a bonus fire hazard. Not something you want on an airplane.

You would rather have:
https://en.wikipedia.org/wiki/Induction_motor
https://en.wikipedia.org/wiki/Induction_generator

You could rectify the AC generator output but you would still need brushed motors and more importantly:
(https://en.wikipedia.org/wiki/Circui...rcuit_breakers)

Switching 115V DC (equivalent cabling requirements to 115V AC) is not trivial at all, and going lower voltage would mean much more weight for thicker copper wires carrying all the extra current. Also what i quoted above is not just for breakers but also for normal switches and relais.

Not sure about the weight of DC motors compared to AC 400Hz motors but i guess the 400Hz wins again and the brushes i already mentioned.


I'm pretty sure the extra problems with switching DC and of course not being able to put it through a transformer are both more or less instant killers without even looking at motors/generators.


Today you can transform DC with switch mode transformers so it's not so much an impossibility compared to 50 years ago but then comes reliability again.

and @andmiz AC wiring at the the same voltage and current is not thinner and lighter.
DC used on cars and trucks though is much lower voltage due to the switching problem and batteries being low voltage so that makes the wiring much heavier.

At low voltages, large DC currents must be carried by larger, heavier wires. The associated components are also heavier, so weight becomes a major disadvantage. Higher voltage AC is more practical with its light weight components. High voltage AC power is preferred over DC for large aircraft mainly due to weight and space savings and low current transmission. AC generators are lighter and smaller than DC generators of a similar output.

lower voltage means heavier cables that's correct. now that's not what you wrote, you wrote about AC vs DC not low vs high voltage.

of course it's kinda correct because DC used for these applications is usually lower voltage.

What seems to be missing out of this discussion is Power. Every electrical item the designers install use it.
As a review: Power (P) = Current (I) times Voltage (E) or P=IE.
That can then also be reworked to P/E = I, which is where the wire size comes into play.
For example,
If a landing light consumes 250 Watts, a typical value:
@ 14 V (AC or DC, it does not matter) that is 17 Amps (round values)
@ 28 V that is 9 Amps
@ 115 V that is 2 Amps
Now, let us look at the wire size to carry that. From the circuit breaker in the cockpit to the wing root on Boeing 707 (think back when this was first being done, and probably more like a DC 4) that wire run would be about 80 feet.
To carry 14 Volts @ 9 Amps and only have 1 volt drop (allowed for intermittent loads like landing lights) an 8 AWG wire is required, which weights in around 5 lbs and is 0.17 in diameter.
28 V at 9 Amps uses a 12 AWG wire and 2 volt drop which weights in around 2 lbs, and is .09 in diameter
115 V at 2 Amps is smaller than 24 AWG and an 8 volt drop. Back then about the smallest that was used was 20 AWG which weights in around 1/2 lbs at 0.04 in diameter.

As noted, designs have changed in time and we can now generate higher voltage DC systems. Back in the 707 day the only way was AC systems. In addition, the AC system was 3 phase, which helps efficiency in things like motors and power conversion systems. DC switching systems now change all of that.

Until the 1960s or so, most cars had "generators," which produced DC. They had heavy permanent magnets, and brushes and commutators that wore out. Around that decade, cars switched to "alternators," which generate AC using field coils, with no permanent magnets or brushes. They're smaller, lighter, more reliable, and generally have higher output. In a car, the AC output is rectified to DC by diodes contained in the alternator itself. In many aircraft, the power is distributed as AC for the reasons others have pointed out.

Just noticed no one said this directly: AC can simply and efficiently be converted to a higher or lower voltage through a simple transformer. So in an aircraft, power can be distributed at a relatively high voltage, permitting lighter wiring, and then be converted to a lower voltage (and then to DC) where needed. This is the same reason utilities transmit power over long distances at tens of thousands of volts, covert that to thousands of volts at substations, and then to 120 or 240V at transformers on street corners.

As others have pointed out, its now much easier to convert DC between voltages, but it's still not as simple or efficient as with AC.

And in AC you have the power factor to consider. AC/DC not an apple to apple comparison.

https://en.wikipedia.org/wiki/Power_factor

AC= brushless generation( less rf noise, no sparks, longer life, lighter weight)
115v by convention
400hz, well latest trend is (back to )variable frequency but high because it saves weight on rectification.

high frequency means smaller motors and smaller generators as far as i know.

You've got 3 options for electrical power distribution:

DC
Constant Frequency AC
Frequency Wild AC

Lots of relevant individual points made above already regarding the different options. There are advantages and disadvantages to each. There are essentially 3 questions to ask when looking at power system design. They all relate to your consumers (i.e. the equipment that needs the power).

1. Do they need a lot of power or just a little bit?
2. Are they resistive in nature (i.e. do they tend to draw current in phase with the voltage) or are they reactive (i.e. draw current out of phase).
3. How much stored power is needed in the event of the loss of power generation.

If you don't need a lot of power then the simplicity of DC wins, and your emergency supply is a battery.

If you need a lot of power then the various advantages of AC win.

If you have a reasonable amount of reactive load to deal with, then your system is best optimised by balancing the load to improve the power factor (as mentioned earlier) but this requires a constant frequency system. Change the freq and the balancing goes out of whack. Hence we have the overhead of things like CSDUs / IDGS. We're hauling extra weight, but it is worth it in this case.

However if the majority of your load is resistive then frequency wild can be a good solution. e.g. turbo props. Might need a lot of power for ice protection due to medium level ops.

Where there is a small need for constant frequency AC the most practical way to provide it might be via DC driven inverters

Aircraft electrical system design mixes and matches these three types of power system according to the requirements of the power consumers.

Some of the power requirements are probably legacy, too. I remember the Dash 7 and 8 having a mainly DC system with variable AC used for de-icing and some pumps only, but they still needed transformers and TRUs for some sytems.

Another relevant factor is that Constant Speed Drive generators are more complex and probably heavier than variable AC and DC generators.

Power factor is a red herring in this discussion.

Weight and Reliability are where it's at.
High voltage means lighter cables
High frequency means less metal in the motors and TRUs

These would suggest otherwise.

https://www.casa.gov.au/file/152026/...token=XwhMsWDd

http://www.martekpower.com/pdfs/en/72.pdf

Analysis of a C-130 electrical system. PF gets quite a mention.

http://www.collectionscanada.gc.ca/o...01/MQ44836.pdf

We use to say :
AC is the muscle (high power needed, like pumps )
DC is the brain (like avionics- computers)

Can you provide an example?

https://en.wikipedia.org/wiki/High-v...direct_current

whatever the reason google didn't work for you there.

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.

if the cable would have to care about the conductivity of saltwater then you would have a problem :)

as described in the link i posted above the capacitance of the under sea cable is the problem.

for over land it's only done for significantly longer distances.

For the same reason wy Cars shifted from DYNAMO's to Alternators,

Alternators are far more efficient than Dynamos.How ever it does depend upon the electrical design of the airplane.


For the B737 the engines drive Electrical AC Generators which in-tuurn power the electrical system and DC is generated via Rectifiers...

How ever on the ATR the Engines have DC generators which use the inverters to create AC (Constant frequency) and the Props drive Variable frequency AC alternators.

Also another reason for AC is Higher the frequency smaller is the design shape/size/weight of the electrical system/component.

The issue is that AT THE SAME VOLTAGE you get less losses from DC than AC and thus in principle it is better for long distance transmission.

However, transmission at Higher Voltage is very strongly beneficial.

As AC is way easier to step up and down (voltage) then AC wins for general power transmission. However for very long distances the balance shifts back to DC.

I would be interested to know which large aircraft currently in service have DC generators?

Aircraft switched to AC in the 40/50s due to the higher power demands. Try finding a 75kw dc genny?

Actually not entirely true at least theoretically: surrounding wire carrying AC with a conductive medium will increase losses due to induced currents. On the other hand if the cable is balanced then the induced currents will mostly cancel.

Whether this is a real concern or not at the distances and frequencies in question I do not know.

Be careful of Google searches on this since you will find all sorts of (mis)information from the 'golden ears' audiophiles who can hear the difference in $500 vs $5000 speaker cables, although usually not in double blind tests.

One other advantage of long distance power transmission by DC is that it eliminates the need to keep the interconnected grids in phase.

As others have noted mechanically switching DC at much over 48 Volts is much more challenging due to the constant arc.

While published by the FAA and focused more on GA aircraft, the following document (PDF) provides more than enough detail (100 pages) on aircraft electrical theory and use:

Chapter 9 - Aircraft Electrical System - PDF.

I provided my take over decade ago on the comparison, although I focused on actual voltages used and practical considerations rather than going into theoretical arguments of the use of DC vs AC:

https://www.pprune.org/tech-log/2216...ml#post2517630

I didn't consider the absence of reactive power and skin effect in the DC scheme. It's obvious now that I think about it.

I found it interesting that a refitted DC transmission circuit can carry approximately 40% more voltage than the AC transmission circuit it replaces. Since DC is a constant voltage, assuming the current remains the same it can be transmitted at the former peak AC voltage rather than the RMS voltage capacity of the conductor while in AC service. That had not occurred to me.

I didn't think of checking Wikipedia when I asked my dumb question. :uhoh:

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. DC systems carrying the same current would require six wires. Since weight equals cost on an aircraft, the choice is obvious.


However, dc has the advantage that multiple generators do not need to be synchronised in frequency and phase.

Hate to disappoint you but there are 4 feeder cables coming out of my VFGs.

Need to get up to date guys ! Go and look at the A380 system !

Or the 787.

Frequency wild 235v AC generation converted to square wave +/-270v DC, variable controlled 'frequency'.

Clever innit.

Would you mind explaining the square wave part?
is it DC or is it square wave?

I have to admit my last physics class was in the last millennium (and not even the last decade of that). But square wave DC sounds like an oxymoron.

I had a hard time finding detailed information on the 787 electrical system on Google. What I did find, though, seems to indicate that the +-270 V means that there are three conductors: one at ground potential, one at +270 V compared to that, and one at -270V. So, sort of like with the 120/240 VAC system common in the U.S., a specific piece of equipment can be connected to run at either 270 or 540 volts.

Voltages supplied by the 787 electrical system are as follows:

28VDC - Traditional, through the batteries and TRUs, which are powered by the 235VAC system.

110VAC - Supplied through solid state PSUs, powered by the 235VAC system or ground power when connected. The 28VDC and 110V/400hz PSUs provide for the majority of electrical loads on the aircraft.

+/- 270VDC - Supplied through TRUs, 3 wire system as Chu Chu posts. Mainly supplies large electric motor loads such as hydraulic pumps, cabin air compressors, and center tank fuel pumps.

235VAC - Frequency wild as Turin states, 360-800hz, according to engine gearbox speed. Supplies starter/generators, wing anti-ice, flight control actuators, cabin air recirculating fans among others. These buses can also be supplied from the GPU 110VAC current through dedicated PSUs.

The flight control system electronics are powered by 3 independent, dedicated generators, two on the left engine, one on the right. As a backup, 28VDC from the TRUs or aircraft main batteries can also provide power, as can, temporarily, dedicated batteries.

vapilot,

How do you like it? Why did they use DC to drive pumps and the compressors? It would seem that AC would be better.

GF

I am with you, GF, it does seem odd. Not typed in nor have I flown in one (although our group was afforded a tour early on) but two of our mechs are fully fluent across Boeing's fleet.

My guess would be the frequency wild 235V main AC bus power would not be appropriate for motor operation and high power electronic devices work most efficiently with DC currents. In addition, most of the high power 270VDC loads are variable speed motors and the use of high voltage reduces the conductor size requirements, saving weight.