Lingo...s about Electricity...A320

[SRover40]

Always asking .... Hoping to have enough knowledge to answer back....


Reading FCOM abotu Electrical...

Generator 1 produces 90 KVA at 115 and 200 volts and 400 hertz..

Could anyone give me a interpretation..about this.... :confused:

Always sincere thanks.... :)

[Spanner Turner]

I will try to keep it as simple as possible because if you don't know what these mean, electrical theory can get real complex really quickly.

The 90KVa is kilovolt amperes. This is basically how much output the generator can supply. i.e a 90KVa gen will run more lights and equipment than a 60KVa generator.

Now the 115 and 200 volt part. I'll start with your average car alternator, which produces single phase AC (alternating current) power of 12-14 volts. This power is then 'changed' to 12 volt DC (direct current) power which in turn runs all your car elec devices.

An aircraft uses AC power which normally comes from a generator that has 3 seperate windings inside it. This gives 3 seperate outputs (or phases) from the one generator. If one was to measure the voltage between a single one of these phases and ground you would get 115volts. If you were to measure the voltage directly between 2 of the three phases you would get 200volts.

Now if you're still with me , the 400Hz is the amount of alternating current cycles the power experiences each second. (this is dependant upon the speed the generator is spinning) Normal household power is around 50Hz(in Australia anyway) Aircraft utilise 400Hz power predominately, I won't go into the reasons why, and are thus fitted with devices called 'contant speed drives' to maintain generator RPM at a speed to give 400Hz output.

I've tried to keep it simple - not sure how well I did. No doubt there will be further posts that will explain it clearer or make it as clear as mud!

:ok:

[west lakes]

90 KVA at 115 and 200 volts and 400 hertz


90 KVA = 90 thousand volt amps, this is the standard way of expressing power in alternating current systems.

115 & 200 volts, generator will have two outputs possibly connected internally.

If so this would give maximums of 782 amps at 115v or 450 amps at 200v or a combination of the two up to 90KVA.

400hz is the frequency of the AC supply. there is another thread discussing the reasons for this figure elsewhere (don't know how to post link) e.g. your domestic supply will be either 50 or 60 hertz

Power is normally expressed in watts (named after James Watt of steam engine fame) and is arrived by the same calculation volts x amps (this is valid for dc systems) but in alternating current terms a third factor arises i.e. Power Factor. this is the degree by which the current waveform leads or lags the voltage waveform e.g. in power distrubution systems a factor of .9 is commom. So to get a figure in watts the calculation is Volts X Amps X Power Factor.
i.e. at .9 power factor generator output would be 90 x .9 = 81watts
I am sure an a/c expert can confirm configuration of generator as suggested above, the alternative would be seperated windings for the two voltages.
Question of my own, how is an a/c generator driven from the engine(s) to maintain constant speed/output frequency as if mechanicay coupled generator speed and hence frequency will vary dependant on engine speed:confused:

[west lakes]

spanner turner
as you say, 3 phase:ugh:

and you answered my question as well:D

I was assuming a single phase unit, so currentb output I quoted is a total over the three phases.
as an aside for three phase the phase to phase voltage is 1.73 (square root of 3) times the phase to neutral voltage

[SRover40]

Para Siempre... Gracias...

[zerograv]

West Lakes,

To the question of your own. The generator is not in a direct mechanical
link with the engine. It is connect to the engine via, as Spanner Turner
mentions, a Constant Speed Drive Unit (CSDU) which serves the purpose
of maintaining the generator speed at, I guess, 400 rpm independently
of engine thrust (rpm). Don't ask me how it works as it has been a very
loooooooooong time since I last studied systems for the ATPL theory
(just me being a lazy bastart :O)

Cheers,
Zero

[hetfield]

RPM is 12.000

[zerograv]

Thanks!!!

(I guess I'm not very good at guessing)

Cheers

[Clarence Oveur]

Well, if you want 400 Hz the generator turns at 8000 RPM.

[hetfield]

n = f * 60/p

f = 400 Hz
p = 2 (2 coilpairs)

gives 12.000 RPM

[west lakes]

at 8000 rpm that makes for an interesting machine, essentially that means 3 coils per phase set at 40 degrees apart.

as a comparason UK power stations use 1 coil per phase at 3000 rpm (interstingly a 500Mva set has about 400t revolving at that speed)

most emergency sets (we have 2 100kva units here) are 2/ph = 1500 revs, the generators on these are about .8m x .7m x .7m.

so 12000 is also valid for a 2 coil/ph machine

all depends on how complicated you want to make the wiring in the machine!

[Spanner Turner]

12000 RPM is the answer.

From the maintenance manual.

"The airplane is equipped with four Integrated Drive Generators
(IDG). Each IDG is a combined generator and Constant Speed Drive
(CSD). The generator portion delivers 3 phase, 400 Hz, 115 volts ac
at 90 kva. The CSD portion maintains the generator at a constant
12,000 rpm during normal operating conditions. This ensures that the
generator delivers a steady 400 Hz."

[Loose rivets]

Can you say if the aircraft's electronics–that require an accurate 400htz–rely on the CSD, or do they have their own electronically derived source?

ie, 30 years ago we would have had a fairly constant drive speed, 6,000 on the 1-11 if memory serves, but that was considered 'wild' and would not have been pure enough for the on-board electronics.

[Clarence Oveur]

12000 RPM is the answer.

For an IDG that is. For a Gen+CSD setup it is 8000 RPM.

From the maintenance manual.

"The main source of electrical power is constituted by two 400 Hz constant frequency 90 KVA AC generators.
Each generator is linked mechanically to a constant speed drive (CSD) driven by an engine through an accessory gearbox. The AC frequency is maintained within a range of 396 to 404 Hz at a constant speed of 8000 RPM"

[Bolty McBolt]

Generator speeds V frequency is decided by number of poles the Generator has.

The old Gens on a 747 classic and apu gens on a 744 are spun at 8000 RPM the newer type aircraft use 12,000 RPM. The newer type have 6 poles are more compact 12.000 rpm the older have 4 poles 8000 RPM.

This reply is rushed but I hope you get the idea.

[hetfield]

It' either 12.000 or 24.000 RPM, depending on number of pole (coil) pairs

n = f * 60/p

f = 400 Hz
p = 2 (2 coilpairs)

gives 12.000 RPM

1 coil pair
gives 24.000 RPM

[Spanner Turner]

Bolty is correct,
I was hoping not to introduce more and more facts and figures - it does become hard though.




FREQ = (Num of poles X RPM) divided by 120(i.e 3 phases seperated by 120 degrees)






400Hz = (4pole X 12,000RPM) divide by 120




400Hz = (6pole X 8,000RPM) divide by 120

[west lakes]

As my earlier post all are valid. even zerograv theoretically is correct with 400 rpm with 60 poles!:) not having a go here

I guess that as materials have improved rpm has been increased and the coils reduced. makes for a simpler more compact (lighter) machine.

as an aside 90kva would be demed sufficiant to supply 35 - 40 average uk houses

[IFixPlanes]

Looking in my fundamental training documents i found:
Most aircraft generators need an input speed of 12,000 RPM because they are equipped with two pole pairs.
But aircraft generators with input speeds of 24,000, 8,000 or 6,000 RPM are also used. This means that the number of pole pairs can vary from one to four.



... please try to learn the difference between fundamentals and aircraft related data.
Is it so difficult to give us the related Aircraft when you quote the AMM?

[deeceethree]

zerograv,

The CSDUs on the previous old jet I flew operated much like a car automatic gearbox - it was a fluid drive clutch thingy ....... yeah I can't be arsed to read the notes either, ha ha!

[Clarence Oveur]

... please try to learn the difference between fundamentals and aircraft related data.

Well excuse me Sir! Quoting the RPM of every generator ever used is hardly "fundamentals" either.

west lakes,

The innards of a CSD looks something like this:

http://i20.photobucket.com/albums/b211/Clarence_Oveur/CSD.jpg

[west lakes]

Roger oveur - yeah good film:D

regarding the rpm figures is it fair to say that design depends on materials as I posted, space available etc. I would also (carefully - I know more about electricity than mechanicals & electricity is scary:confused:) suggest that the higher the speed differential being handled by the csd the higher the mechanical effort to operate it on top of the load imposed on the engine by the generators. End of the day engines prime function is to power a/c in flight and keep it there so reducing losses for other uses is preferable.

Loose rivits
the frequency range quoted by Clarence (may I call you that) is =/-1%. this matches most public supply systems.

Tolerances
as in an engine the gap between the rotor and the stator needs to be as small as possible to improve efficiency am sure that as engines developed their rpm increased and the clearance between parts decreased. which is possibly why generator rpms are increasing. also the fewer the coil pairs the lower weight and the smaller the unit - you don' have to fit as much wire in.

As always if the two bits collide - bad.

Another aside
In the late 1950's a power station was commisioning a new 90mVA set. steam added to turbine and run up to speed, connected to grid and throttled up to generate - BANG!
Part of the turbine took flight vertically, throught the roof, over an adjacent building and landed on a shed in an adjacent field.
Cause a 1/2 pint tin mug left in a steam pipe!:ugh:

[ARINC]

I can't elaborate to much further on the topic..!! however..

Genny and CSDU are usually a matched pair and changed together...

[balsa model]

In case someone is wandering why 400 Hz and not 50 or 60 Hz as household equipment: probably because by the time they were designing it, the magnetic materials could handle such frequency without excessive losses. The motivation for going faster is that you end up with a smaller & lighter unit to do the same job: convert certain amount of mechanical power to electrical power.
Does anyone know what sort of generator frequencies are used on modern fighter jets where weight is at a premium? I imagine that such designs start with a clean sheet, without any need for legacy compatibilities. Or am I dellusional?
BM

[hetfield]

Don't know about fighters, but about A 380 which has great concerns about weight....

It has a free (variable) AC power generation which requires no CSD/IDG.

Frequency is 380 Hz - 800 Hz.

[balsa model]

Interesting..
Slower than I expected but it makes sense that they relaxed frequency tolerance. Nowadays, designing small and light rectifiers that can take a wide frequency range is easy. Input transformers can be altogether dispensed with.

And now that I think of it again, maybe fighter jets don't bother with AC bus at all.

[barit1]

There's a previous thread here. (http://www.pprune.org/forums/showthread.php?t=275868)