pjflysfast
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AC Verses DC

Tue Jul 04, 2006 2:59 pm

What are the advantages and disadvantages for designing an aircraft with AC verses DC power?
 
bri2k1
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RE: AC Verses DC

Tue Jul 04, 2006 4:39 pm

The main reason AC is used in our homes is because it's less likely to be lethal. As the current literally reverses direction 50 or 60 times a second (depending on where you live), if you do take a shock, it will likely not stop your heart because it's not in a constant direction through your body. (This may be more accurately described as stopping your heart but restarting it quickly for as long as you keep those fingers stuffed into the outlet.) Since passengers on airliners are not likely to ever come in contact with wiring, this is probably not it.

Computer equipment and radio equipment typically require DC. These components require precise voltage levels to operate IC chips, modulate and demodulate waveforms, and amplify signals, and AC's reversing precludes all of those. Your home PC has an AC to DC inverter either built in (desktop) or attached (laptop). To supply one of these for every such component on an airliner would add far too much weight.

AC is somewhat easier to generate. All electric generators (and motors) work by moving a magnetic field relative to a wire. The wire is coiled to produce more "bang for the buck" of each motion. Since this process has to be repeated continously, it's done by rotating the components relative to each other; most often, the magnet rotates inside the stationary coil. Magnets have polarity, though; so, through the first half of the rotation, current is generated in one direction, and through the other half, it's generated in the opposite direction. By controlling the speed of the rotation, the phase of the current is regulated (60 times a second in North America, 50 most other places). DC generation requires some additional complexity to convert that AC into DC. It can be accomplished inside the generator by reversing the way the rotating brushes contact the housing as it rotates or via an external rectifier. Both of these introduce complexity and, hence, decrease reliability (statistically speaking only; DC generation has come a long way and may be comparably reliable.)

One problem with generating power from an engine that does something else too is the varying speed with which that engine rotates. Many turbine engines include a Constant Speed Drive (CSD) shaft which rotates at the same speed regardless of turbine RPM; this is obviously a complex mechanism adding weight and maintenance requirements. It's a necessity though since turbine engines have an enormous delta between idle and maximum RPM, but producing adequate power throughout this range is a requirement. Also, as the RPM of the generator changes, the phase of the AC being generated changes.

Some equipment, such as heater coils (for A/C, de-ice or anti-ice, coffee makers) and even some lighting systems don't require a constant phase. They may be designed to work well over a wide range from, say, 50Hz to 500Hz. Current technology used for computers could never cope without a supplemental power supply, and it's doubtful for radio equipment as well. To that end, an aircraft fitted with variable-phase power generation could simply have one large DC inverter connected to a DC bus and the DC or constant-phase-dependant equipment could be fed from it. This would surely seem to be less expensive than completely redesigning all the computer, radio, and other equipment on board that requires constant phase or DC.

CAVEAT: Not being in the aircraft design industry, but engineering electrical systems for other applications, most of this is general electrical theory with lots of speculation thrown in. Comments, requests for clarification or examples, and corrections are welcome.
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pilotaydin
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RE: AC Verses DC

Tue Jul 04, 2006 4:42 pm

awesome explanation thanks
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PurdueAv2003
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RE: AC Verses DC

Tue Jul 04, 2006 4:53 pm

The same advantages and disadvantages as there are for AC vs DC in every application. AC is easy to generate by rotational means (i.e. a spinning shaft from an engine gearbox), and can easily be stepped up or stepped down in voltage. However, almost all digital circuits require the use of DC. DC can also be backed up by the use of a battery. Typically, small aircraft have been designed with DC generators or alternators so all systems can be run off the battery in an engine failure and to save the weight of additional rectifiers or inverters. With so many different system requirements on larger aircraft, most airliners use AC with rectifiers or inverters as needed to adjust the electrical output to specific devices. A ram air turbine, or RAT, is deployed and used to generate AC in the case of a multiple engine failure.
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n8076u
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RE: AC Verses DC

Wed Jul 05, 2006 2:41 am

Quoting Bri2k1 (Reply 1):
The main reason AC is used in our homes

AC can be transmitted over long distances with minimal power losses, when stepped up to very high voltages with the use of transformers. Not so easy to do with DC. Thomas Edison and his DC lost out to Westinghouse and their AC way back when for this very reason.

Now, long distance transmission of power may not be much of an issue with an aircraft, but too many things already require AC on an aircraft without redesigning each of those sysems. Synchros inside instruments require the three-phase 400hz the aircraft uses. Any CRT screens require AC. Most avionics boxes require it as well. The ground power at most airports is already set up for AC. So it is easier to start out with 120v 400hz 3-phase power, which is already a fairly efficient way to go, than DC.

Chris
Don't blame me, I don't work here...
 
SlamClick
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RE: AC Verses DC

Wed Jul 05, 2006 5:15 am

Quoting Bri2k1 (Reply 1):
AC is used in our homes is because it's less likely to be lethal. As the current literally reverses direction 50 or 60 times a second

This is a remarkable statement given the original debate over AC vs. DC between Thomas Edison and Nicola Tesla. Edison's main argument was the lethality of AC. He cooked hot dogs on the wires.

The electric chair was AC.

The standard for jet airliners during my career has been engine driven and identical APU AC generators producing 115 volt, 400 cycle, three-phase AC power. Pretty far upstream in this supply system ('generator' or 'main' bus typically) there would be Transformer-Rectifiers producing 28 volt DC. This system kept one or two 24V NiCd batteries charged but except for starting the APU from a dark airplane, the 'battery' bus is rarely powered by the actual battery.

Various instruments used the full 115V 400Hz or stepped it down, or just used one or two phases of it. Others used the 28V DC. On some aircraft there was also 56V DC. It was a hodgepodge but the common characteristic was that with the exception of the main AC supply cables, which were as thick as your thumb, all the longer wires tended to be AC and the DC components tended to be wired closer to the source.

For that reason I'll have to side with this statement:

Quoting N8076U (Reply 4):
AC can be transmitted over long distances with minimal power losses, when stepped up to very high voltages with the use of transformers. Not so easy to do with DC.

It is my understanding that to transmit DC over longer distances you must run the voltage off the top of the scale - thousands of volts.
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rmd11
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RE: AC Verses DC

Wed Jul 05, 2006 11:25 am

Quoting N8076U (Reply 4):
120v 400hz 3-phase power

You wont find any 120vac 3 phase ground power 3 phase comes in 208vac and up (at least in the us)
none
 
seanp11
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RE: AC Verses DC

Wed Jul 05, 2006 11:32 am

Quoting SlamClick (Reply 5):
It is my understanding that to transmit DC over longer distances you must run the voltage off the top of the scale - thousands of volts.

Both modes of electricity require you to pump up the voltage and lower the amperage to transmit over long distances. The difference is that transformers only work with AC, to change the voltage of a DC supply you'd first have to run it through an inverter. If you run DC through a transformer, you'll most likely fry it.

And its easier to go from AC to DC, all you need in simplicity is a diode, although a real rectifier is more complicated.
 
n8076u
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RE: AC Verses DC

Wed Jul 05, 2006 12:05 pm

Quoting RMD11 (Reply 6):
You wont find any 120vac 3 phase ground power 3 phase comes in 208vac and up (at least in the us)

208 volts is what you get if you measure the RMS voltage between any two of the three phases, and 120 volts is what you get measuring from any one of those phases to "ground".

Even if you consider what I say to be a matter of symantics, I must point out that the AC voltage meters/readouts on all the commercial aircraft I've ever worked on all show "normal" to be in the 120v range.

Chris
Don't blame me, I don't work here...
 
bri2k1
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RE: AC Verses DC

Wed Jul 05, 2006 12:43 pm

Quoting SlamClick (Reply 5):
Edison's main argument was the lethality of AC. He cooked hot dogs on the wires.

The electric chair was AC.

Well, sure, but times have changed (although the chair is still AC powered). Of course, I wasn't around at the time to be sure  Smile but...There was no NEMA or NEC. Edison claimed AC was more lethal, and it was one of his employees (Brown) who invented the first electric chair. Edison discovered DC first, and developed the first distribution system for it, so it's easy to see why he might have championed its uses, although the voltage and current requirements of household appliances, lights, etc. of the time were nowhere near as large as today's. But the chair was a different story. It used around 2000V initially, far greater than what you could encounter in any residence. The disruption of central nervous system functions probably happens very quickly, but even if that much electricity was quickly removed, the subject would probably live. Further current passing through the body heats it substantially, causing irreperable damage to internal organs, resulting in certain death.

...but these are just semantics  biggrin  After all, the typical electric shock we've all received from touching a doorknob or the like is on the order of 100,000 volts per inch gap and can be thought of as direct current, though the current is very, very small. Regardless of DC or AC, it's the current that ultimately kills. You can stick a flashlight battery just about anywhere on your body (DO NOT TRY THIS -- I AM NOT RESPONSIBLE IF YOU DO) and its DC Is not going to kill you. But one Ampere can kill you, AC or DC.

There have been some good responses, filling in some holes I left in my description -- especially the bit about the battery. Chemical batteries, fuel cells, in fact all of today's known methods of storing electrical energy without physical motion, produce only DC. I think DC could be efficiently transmitted over long distances with different materials, but their low availability and resulting high cost probably precluded this, and may still today.

The bottom line for aircraft, in my mind, seems to still be the fact that the vast majority of generation, transmission, and consumption technology is adapted to the existing methods. This means that AC is generated rotationally, used by some components, and others use a converted DC feed from this AC source. Barring major unforeseen technological advancements, I think we'll continue to see this for a long time. Even the Space Shuttle, arguably the most complex machine ever built, uses relatively old Hydrogen fuel cell technology for its power needs.
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kaddyuk
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RE: AC Verses DC

Wed Jul 05, 2006 7:15 pm

Quoting Bri2k1 (Reply 9):
But one Ampere can kill you, AC or DC.

50mA thats 0.050 of an ampere will frazzle your body.
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lorm
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RE: AC Verses DC

Wed Jul 05, 2006 8:45 pm

Quoting Bri2k1 (Reply 9):
Regardless of DC or AC, it's the current that ultimately kills. You can stick a flashlight battery just about anywhere on your body (DO NOT TRY THIS -- I AM NOT RESPONSIBLE IF YOU DO) and its DC Is not going to kill you. But one Ampere can kill you, AC or DC.

 checkmark 

Another good example is to think of "less than lethal" stun guns/tasers/prods that produce hundreds of kVs. But the amp current is very low usually around 1-5 mAs.
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Jetlagged
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RE: AC Verses DC

Wed Jul 05, 2006 9:35 pm

Quoting SlamClick (Reply 5):
The electric chair was AC.

Probably explains why the chair was sometimes so gruesomely inefficient. DC would have been more instantly lethal.

3 phase AC is good for applications which require power. It's easy to get DC from AC, so usually aircraft generate 3 phase AC, then use transformer/rectifiers to create DC supplies from that. For standby purposes single phase AC can be produced from DC using an inverter, but it's not ideal for primary purposes and inverter AC is usually a much less clean signal.
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411A
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RE: AC Verses DC

Wed Jul 05, 2006 10:16 pm

Most have really beat around the bush here with regard to AC vs DC in aircraft.

Small general aviation piston (and some turbine) aircraft use DC simply because generally only one voltage is required, and most starts are done from the ships battery.
Easy and simple...less costly.

Large piston aircraft (DC-6, Constellation etc) were also primarily DC powered aircraft, and where AC was required (some instruments) an inverter was used.
Not a static inverter either, a DC motor driven small AC generator.
Engine starts from the ships battery were also often used, especially at more remote locations.

Large turbine aircraft use primarily AC for one very important reason, the weight penalty is significantly reduced, as very large currents are required for many applications.
Any DC that is required is provided by transformer/rectifier units.

The lethal discussion, simply does not enter into the picture...unless of course one was to put his/her hands where they didn't belong.
 
bri2k1
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RE: AC Verses DC

Wed Jul 05, 2006 10:49 pm

Quoting Bri2k1 (Reply 1):
Since passengers on airliners are not likely to ever come in contact with wiring, this is probably not it.



Quoting 411A (Reply 13):
The lethal discussion, simply does not enter into the picture...unless of course one was to put his/her hands where they didn't belong.

 yes 

But we know the importance of safety in aviation, and no electrical discussion is complete without a mention of safety. I think it's relevant.

Your answers also assume current technology, existing requirements, etc. Breakthroughs in engineering (such as glass-panels, Xenon landing lights, and bleedless engines) come when designers look past limitations of today's technology.
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pjflysfast
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RE: AC Verses DC

Sat Jul 08, 2006 12:15 pm

A lot of good information here! I have another question "or still dont quite get it yet" in a challenger 604 I saw on the over head the buttons for AC and DC power. When do you use either one of them or can you use them both at the same time.
 
flymatt2bermud
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RE: AC Verses DC

Sat Jul 08, 2006 9:57 pm

AC & DC is used simultaneously powering specific buses. There are separate inputs for AC & DC when the aircraft is on the ground and requires either from a ground power source. The switches for ground AC & DC power are independant (these are probably the switches you are referring to).
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
 
delta-flyer
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RE: AC Verses DC

Sun Jul 09, 2006 8:31 am

Quoting 411A (Reply 13):
Most have really beat around the bush here with regard to AC vs DC in aircraft.

411 is correct ...
On larger aircraft, there are many electric motor driven equipment, for example, the hydraulic pumps ....
Take a 747, 757, 747, 767 .... they use 2 - 4 motor driven hydraulic pumps with input of 17 kW. Using a 28 Vdc motor, it would draw 600 amperes. Imagine the inrush current - 4 - 5 times that! Imagine the size wires you would need - they would be half-inch rods!
With 115V/3phase AC, the current is only 50 A per phase, so you need 3 no. 8 or 10 wires. The generator and motor are much smaller, too.

Cheers,
Pete
"In God we trust, everyone else bring data"
 
n8076u
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RE: AC Verses DC

Sun Jul 09, 2006 9:45 am

Just to clarify something, the 747 has four turbine-engine-driven hydraulic pumps (each engine has one mounted to its gearbox), one for each of the four hydraulic systems. There are also 4 air-driven hydraulic pumps (in the pylon above each engine), one for each of the four hydraulic systems. There are no electric hydraulic pumps for use in flight on the 747.

However, there is a small electric motor driven pump with very limited capacity in the #4 pylon for powering the primary brake source which is the #4 hyd. system. There also may be an optional but identical electrically powered pump that is located at the #1 pylon, for powering system #1, which is the alternate brake source and also powers the body gear steering.

Chris
Don't blame me, I don't work here...
 
113312
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RE: AC Verses DC

Sun Jul 09, 2006 10:07 am

SlamClick is 100% correct in pointing out the original debate about AC vs. DC between Edison and Tesla. Edison designed the first municipal electric generation and transmission system around DC. One of the big arguments about the danger of AC was that if you touched a live wire carrying AC, your muscles wouldn't allow you to let go of the wire whereas a person could easily let go of wire carrying DC current.

The main reason for using AC in our homes and in aircraft is the ease with which AC allows for the changing of voltage using transformers. Transformers allow stepping up or stepping down of voltage to almost any value desired. Use of high voltage allows for a large amount of electrical energy to be transmitted on thinner wires thus saving weight and space. Where low voltage is needed, it can be converted locally with a transformer.

Rectifiers also allow AC to easily be converted into DC.

Although automobiles use DC for most of their systems, the weight and inefficiency of DC generators were long ago abandoned in favor of alternators. An Alternator is an AC generator with a built in rectifier to produce an output of DC.
 
sprout5199
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RE: AC Verses DC

Sun Jul 09, 2006 10:24 am

Quoting Seanp11 (Reply 7):
And its easier to go from AC to DC, all you need in simplicity is a diode, although a real rectifier is more complicated

Not to nic-pic---well ok I am, a "real rectifier", which you mean a full-wave rectifier, is just 4 diodes, just a little bit more complicated.

Quoting Bri2k1 (Reply 1):
The main reason AC is used in our homes is because it's less likely to be lethal. As the current literally reverses direction 50 or 60 times a second (depending on where you live), if you do take a shock, it will likely not stop your heart because it's not in a constant direction through your body

As was said before, this is false. It may not be in a constant direction but changes so fast your body still "sees" the current. But as we use to say in the navy, High voltage A/C throws you, high voltage D/C grabs you and wont let go. this is from the muscles reacting to the current. but any way you are dead, just D/C will cook you for longer.

Dan in Jupiter
 
delta-flyer
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RE: AC Verses DC

Mon Jul 10, 2006 12:14 am

Quoting N8076U (Reply 18):
the 747 has four turbine-engine-driven hydraulic pumps

That's correct, but I believe some variants substitute electric pumps for some (or all) the air driven pumps. I can check which airlines use these variants, because we sell them the electric pumps - unless I am hugely mistaken.

As for the safety, that is not a factor, as both require extensive safety measures.

As for the transmission advantages of AC, that is not a factor on aircraft. The driving factor is the ability to run high power motors.

Now, to continue where I left off last night due to another activity .....

Having said that AC power has advantages for the high power motor drive requirements, the trend now is toward high voltage DC - mainly in the military, but also emerging in civilan platforms, most notably on the 787.

To generate high power 400 Hz AC requires a constant speed drive (CSD), which consists of a large, expensive hydro-mechanical transmission to convert the varying engine speed to a constant speed. This is necessary since the output frequency of an alternator is a function of input speed.

High voltage DC motors and generators are brushless, as brushes would wear excessively due to arcing, not to mention the challenging RFI (radio frequency interference) suppression requirements. But this concept eliminates the CSD, removing a lot of mechanical equipment that is a reliabilty and maintainability headache, but adds a lot of electronics and software. This is highly integrated in the design, although the controller is usually packaged in a separate box.

How does this work? You start with a generator that resembles a 3 phase alternator. The output goes into a generator controller, and is transformed to the correct voltage and rectified to DC (commonly 270 V). This is then transmitted throughout the aircraft. At a motor, the DC is taken by a motor controller, electronically "chopped up" into 3 phase AC, then fed to a 3 phase motor. The motor has a relatively conventional stator, but usually a permanent magnet rotor.

The advantages are weight savings and higher efficiency through control optimization, as the controllers are software controlled and can be tailored to any operating condition.


Pete
"In God we trust, everyone else bring data"
 
flymatt2bermud
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RE: AC Verses DC

Mon Jul 10, 2006 4:35 am

Pete,

Your

Quoting Delta-flyer (Reply 21):

is a very well written contribution on this subject!
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
 
delta-flyer
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RE: AC Verses DC

Mon Jul 10, 2006 6:21 am

Thanks, FlyMatt2Bermud, I appreciate your kind words.

Electric motorpumps have been a large part of my career as an engineer over the past 35 years. (This fall will be my 35th college reunion.) I've been in sales the last few years, but still keep my nose close to the design world.

This is a bit of topic, but on the subject of the 747 air driven versus electric driven hydraulic pumps .....


Quoting N8076U (Reply 18):
There are also 4 air-driven hydraulic pumps

I found some reliability data on our high power AC motorpumps used on 56 British and 44 JAL 747-400's. These aircraft have 2 of our ACMP's on each ship. I am speculating that they therefore have 2 instead of 4 air driven pumps.

These ACMP's deliver 6 gpm at 2850 psi and 12 gpm at 1200 psi in constant power mode. The maximum current draw is 50 A per phase, for a total power draw of 17.25 kVA.

The pumps that are mounted on the engine gearboxes are much larger, over 30 gpm each at 2850 psi, but their maximum output depends on engine speed.

(By the way, in case you're wondering, I work for Vickers, now part of Eaton Aerospace.)

Cheers,
Pete
"In God we trust, everyone else bring data"
 
n8076u
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RE: AC Verses DC

Mon Jul 10, 2006 6:48 am

Pete,

Thanks for posting some of the info regarding the 747 pumps you deal with, as it is very interesting.

So are these ACMPs used in lieu of some of the ADPs, or in addition to them? Or are these to replace the low-output electrically powered hydraulic pumps I mentioned systems #1 and #4 have, as an upgrade/replacement?


Chris
Don't blame me, I don't work here...
 
delta-flyer
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RE: AC Verses DC

Mon Jul 10, 2006 7:40 am

Chris, I'm not familiar with the 747 architecture. Perhaps what you call "low-output" are the ones I consider "high power" - they're low compared to the engine pumps, but high compared to the AC power available.

Well, I just thought of something - lets look what AIR5005 says .....

Hmmm - interesting -

AIR5005 says - for the 747-400 - there are 2 ADP's - system 1 and 4.
I also see the ADP's are the same as the EDP's - so there are 6 identical pumps per aircraft, ne on each engine gearbox, and 2 driven by air turbines. The EDP's run at 3750 rpm max., while the ADP's run at 3200 rpm.

There are 2 ACMP's - in sys. 2 and 3, as I described, but seemingly a hair larger (15%), while there is one in sys. 4 exactly as I described. I think that's an error, since every one complies with the same Boeing spec number.

Now, the 100/200/300's have 4 ADP's and 1 ACMP in #4, as you described. The ACMP is the same as the ones on the 400. So, looks like what you consider a "small" motorpump I consider "large"

So, this leaves one question still unanswered - how come our reliability data says there are 2 per aircraft, not 3? I assume there may variations that the AIR5005 does not include.

I hope I didn't confuse everyone!

Pete
"In God we trust, everyone else bring data"
 
n8076u
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RE: AC Verses DC

Mon Jul 10, 2006 9:41 am

Pete,

Honestly, I will admit, although I am very familiar with UA 747-400s, I cannot say what BA or JAL 747-400s have, as I never had the pleasure of working on them, so please pardon my ignorance. I do know all of UA's fleet of 747-400s have 4 EDPs, 4 ADPs, and one electric pump for system 4, which is basically for pressurizing the brakes, hence my calling it small, and it isn't even powerable in flight, and it is run from a bus that is only hot on the ground. And now that you mentioned it, I do recall that the pump itself at the EDP and ADP positions is identical.

I was under the impression that all 747-400s were the same in this way, but apparently I was wrong.  Wink I do know that at least some of the Air China and Singapore 747-400s I have worked on/around had 4 ADPs as well. Perhaps the 2 ADP/2 ACMP setup is an option BA and JAL chose? I also wonder if this is something all newer 747s come with and was retrofitted on the older aircraft?

Since systems #2 and #3 basically power flight controls, excluding the trailing edge flaps, it makes sense if Boeing went electric/smaller on those systems for the demand/backup pumps as a very high rate of flow just isn't required on those two systems, which is also why the reservoirs on those two are smaller than on #1 and #4. But, I would have to think these ACMPs for #2 and #3 are bigger than the #4 electric pump, or at least spin it far faster in order to do the job.

And thanks again for all the additional info!

Chris
Don't blame me, I don't work here...
 
mandargb
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RE: AC Verses DC

Mon Jul 10, 2006 3:10 pm

AC and DC :
As any engineering practise, it is used where it is convenient to use.
Convinience includes lots and lots of factors like
1] cost of equipment
2] cost of production (generation)
3] ease of use
4] maintainance
5] Interferance to neighbouring equipment / circuitry
6] reliablity and backup (high availablity factors) etc.

Most of the people have put really good information above.
So basically when your produce Electricity by means of a rotating magnetic field it is always AC. (Even in DC generators)
So the most convenient way to produce is used.
The size of AC equipment is usually inversaly proportional to some power or frequency.
So on most modern aircraft 400 Hz is used.
This 400 Hz 3 Phase will be typically generated by a constant rotating magnetic field. (You can accurately control the output voltage by controlling the excitation in this magnet if you want). Most modern automibiles also use the same method. And we call it alternator. (Dynamo were in the much older cars if you remember)
So this AC is used typically till the point of use and then converted to required DC voltage of DC by electronic bridge rectifiers.

So in short always GENERATE AC (since it is economical to do so)
And USE AC or DC based on the need.
 
flymatt2bermud
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RE: AC Verses DC

Sun Jul 16, 2006 5:24 am

Quoting Delta-flyer (Reply 21):
To generate high power 400 Hz AC requires a constant speed drive (CSD), which consists of a large, expensive hydro-mechanical transmission to convert the varying engine speed to a constant speed.

On an aircraft I used to fly frequently the left CSD developed a humming noise? I could alter the frequency of the sound by adding or removing the AC load. I wrote it up, the unit was under warranty. The manufacture of the CSD said, "if it's working, leave it on until it fails?"

Sorry, we do too many extended overwater trips to remote locations (by remote I mean it would take 3 days to get parts and a tech to us). I had the CSD removed and returned with the understanding, no problem-it's our dime, find a problem-it's your dime. The unit was returned, its been a while so I don't recall what they replaced but they even sent a note thanking us because, had the unit failed (as it was about to do) the cost to repair would have been much much more expensive.
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci

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