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Why new generators are lighter than old generators

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Why new generators are lighter than old generators

Old 8th Feb 2019, 03:38
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Why new generators are lighter than old generators

Yet they are putting out more power...

“The primary in-flight electrical source on the 777 is a 120 kVA constant speed two-pole brushless alternator. This represents a 33% increase in capacity over the next largest electrical source on previous Boeing commercial jet transports, the 747-400, 757 and 767. The alternator in these machines is integrated with an axial gear differential/hydraulic constant speed drive. The speed on the 120 kVA machine maintains an average 24,000 rpm plus or minus about 1%. This is currently the lightest weight technology in kVA per pound for supplying a 400 Hz constant frequency system, actually weighing just slightly less than the 40 kVA drive/alternator on the earlier Model 727.

Minimizing weight has been achieved partly by increasing the alternator speed, partly by improved packaging and partly by switching from air cooling to oil cooling. The progression on Boeing airplanes has been from air-cooled 6000 rpm machines on the 707, 727, 737-100/200/300/400's and 8000 rpm machines on the 747-100/200, to oil-cooled 12,000 rpm machines on the 767, 757 and 740-400, and ultimately to 24,000 rpm machines on the 777. The 24,000 rpm constant speed Integrated Drive Generator, or IDG as it is referred to, was a mature technology also in service on the McDonnell Douglas MD-11 and Airbus A330.”

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Old 8th Feb 2019, 16:07
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Not from an electrical background, but advances in materials could play a part, just like in engines.

The strength of the magnets in the generators has probably increased over the years.
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Old 8th Feb 2019, 17:03
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Originally Posted by renard View Post
Not from an electrical background, but advances in materials could play a part, just like in engines.

To a degree. But the main advances really are in rotor speed. Voltage produced in a winding is proportional to the number of turns of wire times the speed of magnetic flux change. Increase the speed of change (by upping the RPM) and you need fewer turns of wire. Fewer turns of wire need smaller slots in the generator stators and therefore smaller stators. The stators are the big (heavy) magnetic pieces. So weight savings here is significant.

On the 787, the main generators are connected directly to the engine gearbox and produce a frequency dependent on engine speed. Solid state converters are used to take this variable frequency power and produce 400Hz. The biggest weight savings here is the elimination of the hydro-mechanical constant speed drive either as a separate unit on older models or an IDG (Integrated Drive Generator) on the 767 and on.
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Old 9th Feb 2019, 12:01
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Thanks,

I would be curious to know when Boeing went to brushless generators.
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Old 9th Feb 2019, 14:28
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A very long time ago. You can't use brushes without sparks and the less dense the atmosphere the bigger the spark. Aircraft Gennys have been brushless since the 50/60s.
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Old 9th Feb 2019, 17:07
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brushless generators.
Alternators, to be precise. When silicon diodes became available to rectify AC and produce DC, brushed machines disappeared. For some time, alternators were used for larger AC loads and motor-generator sets (an AC motor driving a brush commutated DC generator) supplied smaller DC loads.

Slip ring connection of field coils also disappeared by producing alternators with a small excitation generator on the same rotor shaft. Basically, an inside-out alternator, the field is on the outside (fixed) producing and controlling a small amount of AC power on the stator, which is attached to the shaft and rotates. A small rectifier takes this and produces DC, which feeds the main generator field on the rotor. So you now have a system with only magnetic field connections between the spinning and non-spinning parts for low maintenance.
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Old 10th Feb 2019, 00:09
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EEngr... Can you comment on Sundstrands apparently very proprietary technology which actually squeezes 250 KVAR out of each of the six 787’s (light weight?) VF motor/generators on-board? Seems they’ve really “one upped” the entire electrical industry with these fantastic machines, which, by the way, we’re designed and first built back in 2008/2009.
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Old 10th Feb 2019, 00:26
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The biggest weight savings here is the elimination of the hydro-mechanical constant speed drive either as a separate unit on older models or an IDG (Integrated Drive Generator) on the 767 and on.
Boeing used the same IDG on the 757, 767, and 747-400 (regardless of engine type). But the reliability of the IDG varied greatly between the installations - for example the MTBF on the 757/PW2000 was less than half that of the 757/RB211-535.
This was initially quite puzzling until someone thought to look at the typical and peak IDG oil temperatures between the various installations - turns out there was an almost linear inverse relationship between the oil temp and the IDG MTBF - the higher the oil temp, the worse the IDG reliability For reasons that I don't recall, the PW2000 IDG had by far the highest operating oil temperatures.
It was bad enough that back around year 2000 we looked at what it would take to get the IDG oil temp down around that for the Rolls installation. Unfortunately all the identified fixes would be so expensive to retrofit that it was cheaper to simply keep replacing the IDG.

BTW radken, IIRC the 787 alternators are 150 KVA each, not 250...
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Old 10th Feb 2019, 01:23
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Originally Posted by tdracer View Post
Boeing used the same IDG on the 757, 767, and 747-400 (regardless of engine type). But the reliability of the IDG varied greatly between the installations - for example the MTBF on the 757/PW2000 was less than half that of the 757/RB211-535.
This was initially quite puzzling until someone thought to look at the typical and peak IDG oil temperatures between the various installations - turns out there was an almost linear inverse relationship between the oil temp and the IDG MTBF - the higher the oil temp, the worse the IDG reliability For reasons that I don't recall, the PW2000 IDG had by far the highest operating oil temperatures.
It was bad enough that back around year 2000 we looked at what it would take to get the IDG oil temp down around that for the Rolls installation. Unfortunately all the identified fixes would be so expensive to retrofit that it was cheaper to simply keep replacing the IDG.

BTW radken, IIRC the 787 alternators are 150 KVA each, not 250...
Interesting. I had a couple of lost generator situations on the 727. That was when the last item on the checklist(actually it was a post checklist thing we did) was to place an empty coffee cup on the flap handle....for later reminder to not select any flaps until a pack had been turned off. All so that the remaining genrators wouldn't be lost when the high electrical draw pack cooling fans started operating when flaps were extended(hope my memory is correct).

Anyways, I remember one particular generator failure was a on a flight where it had been noted earlier in cruise that the CSD oil temp was higher than the others. Can't remember if it was the In or Rise temp. 40 KVA generators if I remember correctly.
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Old 10th Feb 2019, 04:23
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well not military ,all the ones I worked on had brushes,albeit special hard carbon ones to minimise the sparking effect.(1959 to 1968)
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Old 10th Feb 2019, 12:35
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Originally Posted by FE Hoppy View Post
. . . the less dense the atmosphere the bigger the spark.
That makes vacuum circuit breakers seem kind of pointless. But maybe the effect reverses itself below a certain density?
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Old 10th Feb 2019, 16:30
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Originally Posted by Chu Chu View Post
That makes vacuum circuit breakers seem kind of pointless. But maybe the effect reverses itself below a certain density?
Yes. See Paschen's Law
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Old 11th Feb 2019, 14:25
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Originally Posted by tdracer View Post
BTW radken, IIRC the 787 alternators are 150 KVA each, not 250...
Nope, 250 kVA for the four engine starter/generators and 225kVA for the two APU starter/generators. 10kVA for the RAT generator.
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Old 11th Feb 2019, 23:47
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Originally Posted by tttoon View Post
Nope, 250 kVA for the four engine starter/generators and 225kVA for the two APU starter/generators. 10kVA for the RAT generator.
Yea, I stand corrected. My memory was faulty... (I didn't work the 787, I was busy with the 747-8).
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Old 16th Feb 2019, 07:42
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Oil cooling

Removing heat from the interior of an electrical machine is difficult with air, and the dimensions of the stator and rotor are governed as much by the need to keep eddy current density in check to keep heat production sufficiently low.

This is why Teslas canít keep doing their party piece acceleration all the time; the motor gets too hot and it takes time for the heat to bleed away.

I can see oil cooling being a lot better at heat removal, allowing the stator and rotor to be smaller and thence lighter. I wonder if they flood the genny with oil; quite impressive for something spinning that fast!
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