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DocLightning
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Gearboxes And Generators

Mon Nov 16, 2009 8:51 pm

So one thing I've been wondering about is the talk of gearboxes and generators in jet engines.

How does one extract the rotational energy of the shafts without interrupting the airflow?

Do both N1 and N2 have attached gears? What's the arrangement that allows for that?

And how is the generator handled? It needs to produce a constant supply of juice, but with the engine speed going up and down all the time how is this regulated?
-Doc Lightning-

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Woodreau
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RE: Gearboxes And Generators

Mon Nov 16, 2009 10:00 pm

In general, the gearbox is usually attached to the N2 compressor and all the accessories are attached to the gearbox (the starter, generator, oil pumps, fuel pump, hydraulic pump, generator for the FADEC - if it has a FADEC, etc.)

The generator itself is attached to a constant speed drive. The constant speed drive is attached to the gearbox. The generator requires a constant RPM, so that it can produce a steady 400Hz. It's the variation in the frequency that messes up electronics.

There are some generators out there that don't produce a constant frequency (they aren't attached to a constant speed drive, so they experience RPM changes as the engine accelerates and decelerates) - they're called "wild" generators, but I've only seen them in turboprops.

Again that's a generic explanation.
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DocLightning
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RE: Gearboxes And Generators

Mon Nov 16, 2009 10:17 pm



Quoting Woodreau (Reply 1):
In general, the gearbox is usually attached to the N2 compressor and all the accessories are attached to the gearbox (the starter, generator, oil pumps, fuel pump, hydraulic pump, generator for the FADEC - if it has a FADEC, etc.)

The generator itself is attached to a constant speed drive. The constant speed drive is attached to the gearbox. The generator requires a constant RPM, so that it can produce a steady 400Hz. It's the variation in the frequency that messes up electronics.

So these are the parts I'm specifically asking about.

How does one attach a gearbox to the N2 shaft, which lies in the center of a bunch of whirling blades? Where does the mechanical linkage sit so that it doesn't interrupt the airflow?

And how does a "constant speed drive" work? Do jet engines have some sort of continuously-variable transmission?
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"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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Woodreau
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RE: Gearboxes And Generators

Mon Nov 16, 2009 10:38 pm

It's probably easier to see than explain:

There is a power takeup unit which connects the two. It doesn't get in the way of the airflow if that's what you're wondering.

This is what I could find - googled accessory gearbox under images.

http://media.photobucket.com/image/a...ssory%20gearbox/evo65/accessor.jpg

The constant speed drive is a "box/device" which provides the constant speed that the generator requires, it's not part of the engine. How does it work - don't need to know how it works in order to fly the plane.  Smile It either works or it doesn't. If it doesn't work, you get to do some real life abnormals. disconnect the generator and start up the APU.

So - each thing is it's own "box/device" in between the arrows:

N2 compressor -> Power Takeup Unit -> Accessory Gearbox -> Constant Speed Drive -> Generator

again not talking about any specific jet engine
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NoWorries
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RE: Gearboxes And Generators

Mon Nov 16, 2009 10:38 pm

Quoting DocLightning (Reply 2):
And how does a "constant speed drive" work? Do jet engines have some sort of continuously-variable transmission?

There's folk a lot more knowledgeable than me that will probably jump in here, but essentially a gearbox not unlike a differential allows the shaft connected to the generator to rotate at constant speed. Look up CSD (constant speed drive) and IDG (integrated drive generator)

Newer systems power alternators that are rectified to DC and then inverted to whatever frequency is needed -- more efficient that using gear regulated velocity,

[Edited 2009-11-16 14:40:03]
 
rwessel
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RE: Gearboxes And Generators

Mon Nov 16, 2009 11:35 pm

The usual approach is to put a bevel gear onto the turbine shaft, and then run a shaft out the side of the engine. Usually it's the high pressure shaft, and often that shaft doubles as the drive shaft for the starter.

There are some good drawings here:

http://www.globalsecurity.org/milita...brary/policy/army/fm/1-506/Ch8.htm

The shaft does disrupt the airflow inside the engine, but so do the variety of structural members that hold the rotating parts in place. In at least some case, the shaft is run, wholly or partially, inside one of those structural members.

Usually the constant speed drive on a jet is not a CVT in the sense of varying-diameter pulleys, but rather a combination of a hydraulic pump and a hydraulic motor, and a bit of regulator to maintain the speed. In principal this is similar to the torque converter on an automatic transmission on a car.

For electricity generation, the more modern approach is to run the generator at variable speed, and then electronically convert that (varying output) to the required (fixed) output levels.
 
PGNCS
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RE: Gearboxes And Generators

Mon Nov 16, 2009 11:41 pm



Quoting Woodreau (Reply 1):
There are some generators out there that don't produce a constant frequency (they aren't attached to a constant speed drive, so they experience RPM changes as the engine accelerates and decelerates) - they're called "wild" generators, but I've only seen them in turboprops.

I have never heard that term, but have no turboprop time to speak of. I can tell you the MD-90 changed the rather bulletproof DC-9 CSD system that worked great to a Variable Speed Constant Frequency (VSCF) similar to what you describe. It is my least favorite feature of the MD-90, and one that was certainly not a step forward in my opinion.
 
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DocLightning
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RE: Gearboxes And Generators

Tue Nov 17, 2009 12:23 am



Quoting Rwessel (Reply 5):

http://www.globalsecurity.org/milita...8.htm

Ah, that answered a lot. So it seems that the gearbox sits within the diameter of the compressor disks so as to not impede airflow and a thin shaft (presumably in an aerodynamic fairing) passes out of the core to the gearbox.
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rwessel
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RE: Gearboxes And Generators

Tue Nov 17, 2009 1:13 am



Quoting DocLightning (Reply 7):
Ah, that answered a lot. So it seems that the gearbox sits within the diameter of the compressor disks so as to not impede airflow and a thin shaft (presumably in an aerodynamic fairing) passes out of the core to the gearbox.

Terminology quibble:

It's the bevel gears and shafts that are in the engine core, what's usually called the gearbox is outside the engine, on the other end of that shaft. The accessories then attach to the gearbox in various ways.
 
tdscanuck
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RE: Gearboxes And Generators

Tue Nov 17, 2009 1:29 am



Quoting DocLightning (Thread starter):

How does one extract the rotational energy of the shafts without interrupting the airflow?

You need to support the engine core, since that's where all the loads come in via the bearings. That means you need fixed structures between the outer case and the inner non-rotating structure...these take the forms of aerodynamic struts that go in between the rotating disks. The shaft is usually run inside one of these, so the shaft doesn't disrupt the airflow any more than it already was.

Quoting DocLightning (Thread starter):
And how is the generator handled? It needs to produce a constant supply of juice, but with the engine speed going up and down all the time how is this regulated?

787 just skips the whole problem and uses variable frequency generators.

Tom.
 
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DocLightning
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RE: Gearboxes And Generators

Tue Nov 17, 2009 2:01 am



Quoting Rwessel (Reply 8):

Terminology quibble:

Thanks for the correction.  Smile
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Fly2HMO
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RE: Gearboxes And Generators

Tue Nov 17, 2009 3:17 am

Here's a cutaway of the CF6 as seen from the bottom-rear:

http://www.flightglobal.com/airspace/photos/aeroenginesjetcutaways/images/5605/general-electric-ge-cf6-50-cutaway.jpg

On almost all (civilian a tleast) engines the gearbox is somewhere between the 4 and 8 o'clock position. It's hard to make out but you can see the gearbox's drive shaft just aft of the bypass duct guide vanes in wide vertical fairing at the 6 o'clock position. This engine is a GE CF6-50.

In this pic of a CFM 56, you'll note the gearbox and accessories are offset more to the side, due to the 737's short height and thus producing the distinctive bulge on the 737 cowlings.



[Edited 2009-11-16 19:19:20]
 
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jetmech
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RE: Gearboxes And Generators

Tue Nov 17, 2009 6:27 am



Quoting DocLightning (Thread starter):
How does one extract the rotational energy of the shafts without interrupting the airflow?

You can see the set-up in the following picture of an RB211. Basically, there is a bevel gear set (circled in blue) that takes rotational energy off the N3 spool. This is transmitted via a set of drive shafts (red lines) to the external gearbox (boxed in green). The gearbox then distributes this to the various accessories such as the IDG. The only part of the set-up intruding into the gas path is the housing for the vertically orientated drive shaft.

http://www.flightglobal.com/airspace/photos/aeroenginesjetcutaways/images/5645/rolls-royce-rb211-535-cutaway.jpg
http://i94.photobucket.com/albums/l118/Jet-Mech/Jet.jpg

http://www.flightglobal.com/airspace.../rolls-royce-rb211-535-cutaway.jpg

Quoting DocLightning (Reply 2):
Do jet engines have some sort of continuously-variable transmission?



Quoting Rwessel (Reply 5):
but rather a combination of a hydraulic pump and a hydraulic motor

Pretty much. A CSD consists of a variable displacement hydraulic pump supplying fluid (usually engine oil) to a fixed displacement hydraulic pump. The variable displacement hydraulic pump is usually of the multi-piston type with a tilting swash-plate. The input from the gearbox rotates the shaft, pistons and the cylinder block containing the pistons. The swash-plate (yellow) does not rotate, but pivots on a fixed axis as shown below. The small slipper feet (dark green ) on the bottom of the pistons slide upon the surface of the swash-plate.



http://home.planet.nl/~brink494/axpp_v.htg/axpp_v.gif

http://www.emeraldinsight.com/fig/0180560404021.png

http://www.emeraldinsight.com/fig/0180560404021.png

You can see how the tilt of the swash-plate varies the strokes of the pistons, and hence the volumetric output from the pump. When the engine speed is high, the swash-plate tends to move to a neutral position to minimised the stroke of the pistons, and hence output of the pump.

When this fluid is fed to the constant displacement motor – which drives the generator - it will produce a gearing down effect, that is, the constant displacement hydraulic motor will spin at a reduced RPM compared to the variable displacement pump

When the engine speed is low, the swash-plate tends to move to a tilted position to maximise the stroke of the pistons, and hence output of the pump. When this fluid is fed to the constant displacement motor – which drives the generator - it will produce a gearing up effect, that is, the constant displacement hydraulic motor will spin at an increased RPM compared to the variable displacement pump.

The swash-plate constantly modulates between these two extremes to ensure the generator spins at a constant RPM whatever the engine RPM.

Regards, JetMech
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CelticMech
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RE: Gearboxes And Generators

Tue Nov 17, 2009 4:48 pm



Quoting JetMech (Reply 12):
Pretty much. A CSD consists of a variable displacement hydraulic pump supplying fluid (usually engine oil)........................

Ah yes...the days of learning about all this stuff from the basic licences now comes flooding back to me! I knew it was somewhere in my head!!!  Big grin
 
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DocLightning
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RE: Gearboxes And Generators

Wed Nov 18, 2009 6:52 am



Quoting JetMech (Reply 12):

The swash-plate constantly modulates between these two extremes to ensure the generator spins at a constant RPM whatever the engine RPM.

Wow. Fascinating! Hardly an obvious solution to a problem. Now, what regulates the swashplate? Is it an act of physics, or is there a motor in the shaft that adjusts the angle to keep the output constant?
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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rwessel
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RE: Gearboxes And Generators

Wed Nov 18, 2009 8:24 am



Quoting DocLightning (Reply 14):
Now, what regulates the swashplate? Is it an act of physics, or is there a motor in the shaft that adjusts the angle to keep the output constant?

The swashplate is invariably moved hydraulically (after all, there's plenty of hydraulic pressure around). The regulation for that is more variable. In simple designs that are more concerned with constant output pressure, it can be simple feedback from the pressure on the output side pushing against a spring - if the output pressure drops, the swashplate angle is increased by the spring - if the output pressure rises, the hydraulics will push the swashplate towards flat. For better output flow control, you can attach a small constant displacement pump to the motor being driven, and use the pressure from that to move the swashplate (if the motor slows down, the pressure will be reduced). Or you can go highly electronic.
 
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jetmech
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RE: Gearboxes And Generators

Wed Nov 18, 2009 9:18 am

Quoting CelticMech (Reply 13):

I think one of the most interesting parts is how some of the pressurised fluid is bled off to lubricate the slipper foot. I think there is also a small amount of leakage between the piston and cylinder for lubrication purposes also. I presume both of these fluid streams make up the case drain fluid.

Quoting DocLightning (Reply 14):



Quoting Rwessel (Reply 15):

As mentioned by Rwessel, the multi-piston axial pump with tilting swash plate is also the same configuration of device used for aircraft hydraulic pumps, in this case, the swash plate tilts to modulate pressure.

Quoting Rwessel (Reply 15):
if the output pressure drops, the swashplate angle is increased by the spring - if the output pressure rises, the hydraulics will push the swashplate towards flat.

http://i94.photobucket.com/albums/l118/Jet-Mech/CSDpic.jpg

http://home.planet.nl/~brink494/axpp_v.htg/ppmpreg.jpg

With respect to the CSD, GE quotes that the N2 spool of the GE90 spins at 6610 RPM during ground idle, and 10.850 RPM at max power. It thus seems that the maximum speed range that needs to be accommodated by a CSD is a somewhat modest 2:1.

http://www.geae.com/education/engines101/

Regards, JetMech

[Edited 2009-11-18 01:38:31]
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MD11Engineer
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RE: Gearboxes And Generators

Wed Nov 18, 2009 12:07 pm



Quoting CelticMech (Reply 13):
Quoting JetMech (Reply 12):
Pretty much. A CSD consists of a variable displacement hydraulic pump supplying fluid (usually engine oil)........................

Ah yes...the days of learning about all this stuff from the basic licences now comes flooding back to me! I knew it was somewhere in my head!!! Big grin

When you BAEC part 6 exams, did you also have to describe the internal workings of a CSD in detail?  Silly

Jan
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LMP737
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RE: Gearboxes And Generators

Wed Nov 18, 2009 5:01 pm



Quoting PGNCS (Reply 6):
I can tell you the MD-90 changed the rather bulletproof DC-9 CSD system that worked great to a Variable Speed Constant Frequency (VSCF) similar to what you describe. It is my least favorite feature of the MD-90, and one that was certainly not a step forward in my opinion.

The problem with the VSCF system on the MD-90 is that it was way too sensitive. The system would latch onto faults way to easily. When AA had the ex-Reno MD-90's it was common practice during the day to keep the APU running and not hook up ground power. Gate power had a tendency to fault the system, something you don't have to worry about with an MD-80.

The concept of a VSCF system is a good on. Unfortunately Douglas made a big mess. If they had done it right they would have sold a lot more MD-90's. Or if they had stuck an IDG on the aircraft instead. And put a new wing on it.

I hear Delta is getting some second hand MD-90's for their fleet. I'm not sure whether to pity or envy the A&P's over at Delta. Pity them because the 90 can be a handful, envy because it's almost guaranteed overtime.  Wink
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MD11Engineer
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RE: Gearboxes And Generators

Wed Nov 18, 2009 5:14 pm



Quoting LMP737 (Reply 18):
The concept of a VSCF system is a good on. Unfortunately Douglas made a big mess. If they had done it right they would have sold a lot more MD-90's. Or if they had stuck an IDG on the aircraft instead. And put a new wing on it.

Back in the 1990s LH tried VSCF systems with their 737 classics. It was a big fail. The systems constantly went U/S. in the end they went back to conventional CSD / generator combinations.

Jan
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NoWorries
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RE: Gearboxes And Generators

Wed Nov 18, 2009 6:53 pm

Do any new-design commercial jets use these types of systems. The 787 doesn't, I don't thin

Quoting Tdscanuck (Reply 9):
787 just skips the whole problem and uses variable frequency generators.

Will any new designs still use these types of constant speed systems? I didn't think the A380 used them either.
 
ex52tech
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RE: Gearboxes And Generators

Thu Nov 19, 2009 12:43 am



Quoting Rwessel (Reply 8):
It's the bevel gears and shafts that are in the engine core, what's usually called the gearbox is outside the engine, on the other end of that shaft.



Quoting DocLightning (Reply 7):
Ah, that answered a lot. So it seems that the gearbox sits within the diameter of the compressor disks so as to not impede airflow and a thin shaft (presumably in an aerodynamic fairing) passes out of the core to the gearbox.

GE calls that gearbox inside the compressor case (frame) an inlet gearbox, it is the power takeoff for the main gearbox shaft. They were touchy to build, quite a few measurements and lash checks to accomplish. On a CF6 that shaft passageway through the case strut is also used to scavenge the oil from the N1 compressor rear bearing, and the N2 compressor front bearing, or the "B" sump.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
tdscanuck
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RE: Gearboxes And Generators

Thu Nov 19, 2009 3:26 am

In one of those lovely twists of fate, I found a lovely cross-section of a Trent 1000 that I was able to liberate from it's home this morning. It is an excellent overview of the major engine components and layout, and has a great view of the point inside where the power is extracted from the spools.

Here's the full view:


And here's a close-up of the bevel gears:


The right gear is the N3 spool, the left gear is the N2 spool. As you can see, there are actually separate output shafts from each spool that go into the gearbox just under the core, which sends it out to the main gearbox on the bottom of the fan case.

I have the full resolution gory detail scan (~200 MB as a TIFF) if anyone wants it. Send me a msg and we'll figure something out.

Tom.
 
ex52tech
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RE: Gearboxes And Generators

Thu Nov 19, 2009 4:12 am



Quoting Tdscanuck (Reply 22):

The right gear is the N3 spool, the left gear is the N2 spool. As you can see, there are actually separate output shafts from each spool that go into the gearbox just under the core, which sends it out to the main gearbox on the bottom of the fan case.

They must be operating different portions of the main gearbox my guess would be the more loaded components would be powered by the N3, but it is just a guess. Does anyone know which spool is powering what components, and why they are operating from two different inputs?  scratchchin 
"Saddest thing I ever witnessed....an airplane being scrapped"
 
LMP737
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RE: Gearboxes And Generators

Thu Nov 19, 2009 4:56 pm



Quoting MD11Engineer (Reply 19):
Back in the 1990s LH tried VSCF systems with their 737 classics. It was a big fail. The systems constantly went U/S. in the end they went back to conventional CSD / generator combinations.

I remember reading about the VSCF system being used on some 737 classics at one point. I did not know LH had it on theirs.

The 777 uses a VSCF system for backup power. Each engine has a Back Up Generator mounted on the lower r/h fan case, at least on the Trent. The two BUG's supply power to a backup power converter which converts the variable frequency to 115/400. From what I've seen it's a pretty reliable system. There was a brief period of time when the BUG's had a higher than normal fail rate. That problem seems to be rectified. Which is good because I had to change one in freezing drizzle about four years ago.

Quoting NoWorries (Reply 20):
Will any new designs still use these types of constant speed systems? I didn't think the A380 used them either.

I believe both the A380 and 787 use the VSCF system.
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tdscanuck
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RE: Gearboxes And Generators

Fri Nov 20, 2009 2:02 am



Quoting LMP737 (Reply 24):
I believe both the A380 and 787 use the VSCF system.

A380 and 787 are not VSCF, they're VSVF. Variable speed-variable frequency.

Tom.
 
NoWorries
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RE: Gearboxes And Generators

Fri Nov 20, 2009 12:55 pm



Quoting Tdscanuck (Reply 25):
A380 and 787 are not VSCF, they're VSVF. Variable speed-variable frequency.

As I undestand it then, this means that the AC generators are geared directly to the engine and their speed varies with engine speed. It's strictly solid-state electronics that tranform this varying output to whatever frequencies (or DC) and appropriate voltages are needed.
 
bri2k1
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RE: Gearboxes And Generators

Fri Nov 20, 2009 2:37 pm



Quoting NoWorries (Reply 26):

There are some electrical components that don't care about the frequency, and IIRC there are both variable frequency and constant frequency buses. Isolating those systems which require constant frequency probably reduces the cost and complexity of the solid state systems. I can imagine things like anti-ice heaters don't care about frequency, and potentially lighting systems. Probably many others too.
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tdscanuck
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RE: Gearboxes And Generators

Sat Nov 21, 2009 4:17 am



Quoting NoWorries (Reply 26):
As I undestand it then, this means that the AC generators are geared directly to the engine and their speed varies with engine speed.

Yes.

Quoting NoWorries (Reply 26):
It's strictly solid-state electronics that tranform this varying output to whatever frequencies (or DC) and appropriate voltages are needed.

For systems that need it, yes. As Bri2k1 noted, many systems don't care so you don't have to bother feeding them constant frequency power.

Tom.
 
NoWorries
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RE: Gearboxes And Generators

Sat Nov 21, 2009 1:37 pm

It looks like there could be quite a few trade-offs when considering CSD vs VSVF systems -- systems cost, installation, space, weight, maintenance cost, reliability, durability, efficiency, what else? To me it seems like VSVF would have the edge in most of these categories -- not sure about efficiency, possibly reliability (though VSVF seems overall so much simpler).
 
tdscanuck
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RE: Gearboxes And Generators

Sat Nov 21, 2009 5:14 pm



Quoting NoWorries (Reply 29):
It looks like there could be quite a few trade-offs when considering CSD vs VSVF systems -- systems cost, installation, space, weight, maintenance cost, reliability, durability, efficiency, what else?

The big enabler for VF systems was good, reliable, high power-density solid-state electronics. The CSD is a mechanical nightmare and took a long time to get right. However, on the flip side, they sunk so much effort into getting it right that it stuck around for a long time once the reliability was there.

Now that it's easier and more reliable to do your power conditioning with solid-state, the pendulum swung to complexity on a power-handling side, dead simplicity on the mechanical side.

Tom.
 
Tristarsteve
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RE: Gearboxes And Generators

Sat Nov 21, 2009 8:57 pm



Quoting LMP737 (Reply 24):
I remember reading about the VSCF system being used on some 737 classics at one point. I did not know LH had it on theirs.

The British Airways B734 fleet was also delivered with VSCF units. But due to unreliability they eventually gave up and converted them to conventional IDGs.
I think the problem was locating the electronics on the engine.

Quoting LMP737 (Reply 24):
The 777 uses a VSCF system for backup power. Each engine has a Back Up Generator mounted on the lower r/h fan case, at least on the Trent. The two BUG's supply power to a backup power converter which converts the variable frequency to 115/400

The B777 moved the electronics to the Electronic Bay. But it was still a hefty unit with a lot of cooling required and didn't produce a lot of power.

The A380 and B787 seem to have solved this with more modern electronics.

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