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.