Electrical Power Generation Questions

csirac

Newbie
Jul 9, 2004
5
0
I'm studying "Micro-" Electronic Engineering, and as such, I'm fairly ignorant of power electronics in general, save for design concepts found in switch-mode/linear power supplies you might find in puny domestic (mains) powered stuff. And even then anything >100W is considered "high power" for us!

My question regards commercial jetliners, and electrical power generation/distribution. I've been reading through the fascinating stuff over at b737.org.uk. From the (surely) over-simplified schematic here I can see that the B737 has five main power sources for its various AC 115V 400Hz (and other) buses.

1) Ground power
2) APU
3) No. 1 engine
4) No. 2 engine
5) Batteries: "static inverter". Granted, this one probably can't even power hydraulics/pumps/etc...

Question: How on earth are all these alternating-current sources kept in-phase?

... or perhaps I should first ask this question: is there, in fact, any time when there is more than one source at a time connected to a single bus? I'm assuming that this must occur at least sometimes, at least momentarily - such as when switching between APU/ground/engines.

If the answer to the above is "no, one source at a time", then how do all the different systems cope having their power cut in and out to a source that could suddenly be completely out of phase? I see from the schematic that the APU can power "No. 1" and "No. 2" gen. busses during take-off. At some point, the APU has to hand things over to the engine-powered generators in-flight, right? How "smooth" is this transition of power sources?

As for DC power, are the batteries running in parallel the whole time helping "filter" the spikes? Or do sensitive systems have to assume "dirty", discontinuous power here as well and do their own filtering/conditioning?

Obviously I have no idea about aircraft systems, just curious!
 
Hmm... I found this site via a google search on an unrelated topic that had threads with actual technical content, however it seems most forum traffic here is political...

Sorry for the "noise", from the forum description it seemed like a good place to post, at the time anyway B)
 
No this is a good place to ask Questions! You are correct, it is political in nature, USaviation has decided to move us to this area. It may have been more appropriate to start another area of Union vs. Union.

You may want to visit here; http://forum.amtonline.com/ or go to the boeing site to ask the Question.
 
csirac said:
I'm studying "Micro-" Electronic Engineering, and as such, I'm fairly ignorant of power electronics in general, save for design concepts found in switch-mode/linear power supplies you might find in puny domestic (mains) powered stuff. And even then anything >100W is considered "high power" for us!

My question regards commercial jetliners, and electrical power generation/distribution. I've been reading through the fascinating stuff over at b737.org.uk. From the (surely) over-simplified schematic here I can see that the B737 has five main power sources for its various AC 115V 400Hz (and other) buses.

1) Ground power
2) APU
3) No. 1 engine
4) No. 2 engine
5) Batteries: "static inverter". Granted, this one probably can't even power hydraulics/pumps/etc...

Question: How on earth are all these alternating-current sources kept in-phase?

... or perhaps I should first ask this question: is there, in fact, any time when there is more than one source at a time connected to a single bus? I'm assuming that this must occur at least sometimes, at least momentarily - such as when switching between APU/ground/engines.

If the answer to the above is "no, one source at a time", then how do all the different systems cope having their power cut in and out to a source that could suddenly be completely out of phase? I see from the schematic that the APU can power "No. 1" and "No. 2" gen. busses during take-off. At some point, the APU has to hand things over to the engine-powered generators in-flight, right? How "smooth" is this transition of power sources?

As for DC power, are the batteries running in parallel the whole time helping "filter" the spikes? Or do sensitive systems have to assume "dirty", discontinuous power here as well and do their own filtering/conditioning?

Obviously I have no idea about aircraft systems, just curious!
I can't tell you much about the 737 but on the MD80 which is probably similar you have one generator powering one bus. Generally speaking the Capts instruments run off one bus and the FO's on another. If a gen fails there is a "cross tie relay" that allows one generator to power both buses. Ground Power and APU power feeds both buses when the engines are not turning. I don't think there's a situation where you would have two sources powering one bus. When a switch is made like lets say from APU to Eng Gen it is not really that smooth as everthing dips for a sec and every cockpit relay clicks off and back on again. But the phases are all handled by a "Black Box". All we do is throw a toggle swith.
As for batteries they are used for starting the APU on the MD80 and for the Emergency power bus. I doubt they are in Parallel.
Don't have a diagram in front of me so I'm only about 90% sure of this info. Maybe someone else can add to this or correct me?
 
AAmech, thanks for your insight!

I guess the period of the supply signal (1 / 400Hz = 2.5ms) is going to be much smaller than the "glitch time", so really when changing from one source to the other in a break-before-make switching scheme, the phase of the new source is not relevant. I guess I'm wondering how everything can cope with discontinuous power.

So I suppose the big loads on the AC buses would be trim motors, hydraulic pumps, heaters, etc. right? Which I guess could handle a few hundred ms interruption. So the "sensitive" stuff must condition power locally using batteries? Are there many batteries littered around the place, or just in the avionics bay/cockpit? For instance, what powers the bus-transfer relays during the transfer? :)

I'm guessing the CVR and FDR, various computers and navigation electronics would be battery-backed, conditioned to have continuous power... but I suspect from your reply that aviation stuff in general is just designed to accept intermittent power and like it! So as a whole, it would be bad to design aircraft systems that depended on continuous power, eg. it'd be unfortunate for faulty wiring to cause a runaway trim condition.

So in that case, if you lost all generators on your jet, apart from instruments/radio, what's the most critical thing you'd want to keep power to? Aren't some of the fuel/oil pumps electronic?

How you generate a constant RPM from varying shaft speeds is another unsolved mystery for me! The fact turbines are doing 6-digit RPMs helps a lot in that the actual generator only wants a few thousand RPMs (?) I guess, but then again RPM of turbines is much more stable than piston engines...

Are the generators even direct-driven using a gearbox? Obviously you need a clutch system (or torque converter, or...) to get the engine initially "motored" and spooled-up at start before you engage the (magical constant-speed) transmission to the generator; on the other hand perhaps an unloaded gen is no burden at all to the air-start process?

Hmmm well one thing's for sure, I sure do respect the folks who have to trouble-shoot intermittent wiring faults on just about any decent sized aircraft! Does Boeing, MD, etc. install "spare" wires in those looms at the factory to save tearing up all the floor panels? :p
 
csirac Posted on Jul 9 2004, 10:09 AM


I guess the period of the supply signal (1 / 400Hz = 2.5ms) is going to be much smaller than the "glitch time", so really when changing from one source to the other in a break-before-make switching scheme, the phase of the new source is not relevant. I guess I'm wondering how everything can cope with discontinuous power.

So I suppose the big loads on the AC buses would be trim motors, hydraulic pumps, heaters, etc. right? Which I guess could handle a few hundred ms interruption. So the "sensitive" stuff must condition power locally using batteries? Are there many batteries littered around the place, or just in the avionics bay/cockpit? For instance, what powers the bus-transfer relays during the transfer?

I'm guessing the CVR and FDR, various computers and navigation electronics would be battery-backed, conditioned to have continuous power... but I suspect from your reply that aviation stuff in general is just designed to accept intermittent power and like it! So as a whole, it would be bad to design aircraft systems that depended on continuous power, eg. it'd be unfortunate for faulty wiring to cause a runaway trim condition.

So in that case, if you lost all generators on your jet, apart from instruments/radio, what's the most critical thing you'd want to keep power to? Aren't some of the fuel/oil pumps electronic?



How you generate a constant RPM from varying shaft speeds is another unsolved mystery for me! The fact turbines are doing 6-digit RPMs helps a lot in that the actual generator only wants a few thousand RPMs (?) I guess, but then again RPM of turbines is much more stable than piston engines...

Are the generators even direct-driven using a gearbox? Obviously you need a clutch system (or torque converter, or...) to get the engine initially "motored" and spooled-up at start before you engage the (magical constant-speed) transmission to the generator; on the other hand perhaps an unloaded gen is no burden at all to the air-start process?

Hmmm well one thing's for sure, I sure do respect the folks who have to trouble-shoot intermittent wiring faults on just about any decent sized aircraft! Does Boeing, MD, etc. install "spare" wires in those looms at the factory to save tearing up all the floor panels?
 
Regarding;

Question: How on earth are all these alternating-current sources kept in-phase?

ANswer: Depends on the engineering design of the system. Some designs do have provisions to perfectly match the phases (blends/mixes the electicals sources. This is normaly only done with the combining of the engine driven generators. On the ground you power (via switching) the entire airplane.

Other designs have segregated electrical distribution per the MD80 example above.

737 uses segregated system with switching available to keep all components powered in event of generater failure.

Example of mixing of electrical sources are generaly older designs, 727, DC-8.
 
csirac Posted on Jul 9 2004, 10:09 AM


How you generate a constant RPM from varying shaft speeds is another unsolved mystery for me! The fact turbines are doing 6-digit RPMs helps a lot in that the actual generator only wants a few thousand RPMs (?) I guess, but then again RPM of turbines is much more stable than piston engines...

MD-80 uses a CSD (Constant Speed Drive) which is between the engine gearbox and generator. CSD is a type of transmission. What ever the speed of the engine is, from idle to takeoff power the generator is turning the same speed.

Are the generators even direct-driven using a gearbox? Obviously you need a clutch system (or torque converter, or...) to get the engine initially "motored" and spooled-up at start before you engage the (magical constant-speed) transmission to the generator; on the other hand perhaps an unloaded gen is no burden at all to the air-start process?


The generator for the APU is direct-driven, that because the APU run at a constant RPM. For the main engines the CSD is always engaging the generator. So the generator is turning at start up. All engines are started with the generator unloaded. When a engine is up and running then the generator can be put on-line (on) only if the GCU (Generator Control Unit) says its ok. GCU looks at the frequency, volt, and amps. If the volts or frequency is low or high it will take the generator off line.

So in that case, if you lost all generators on your jet, apart from instruments/radio, what's the most critical thing you'd want to keep power to? Aren't some of the fuel/oil pumps electronic?

On the MD-80 you have 7 boost pumps 6 for the three main tanks (2 each tank)and one for APU when starting. The 6 pumps for main fuel tanks are AC while the one pump for the APU is DC. (The DC pump is just used to start the APU.) If you lost AC power in flight, fuel pumps are not high on the critical list. On the MD-80 the main engines have there own engine driven fuel pumps, so does the APU. So fuel can be drawn from each tank in case of an emergency.
 
fix_airplanes said:
Regarding;

Question: How on earth are all these alternating-current sources kept in-phase?

ANswer: Depends on the engineering design of the system. Some designs do have provisions to perfectly match the phases (blends/mixes the electicals sources. This is normaly only done with the combining of the engine driven generators. On the ground you power (via switching) the entire airplane.

Other designs have segregated electrical distribution per the MD80 example above.

737 uses segregated system with switching available to keep all components powered in event of generater failure.

Example of mixing of electrical sources are generaly older designs, 727, DC-8.
I remember the DC-8 and the 727s. With the 727 there was a knob to adjust the phase, the light would blink slower as you got closer to having it in phase, once it stoped you would throw the switch and the Generator would be running parallel. You would do that for each generator after the first so you could have three Generators running in parallel.

Todays systems are much easier and more reliable.
 
Bob Owens said:
With the 727 there was a knob to adjust the phase, the light would blink slower as you got closer to having it in phase, once it stoped you would throw the switch and the Generator would be running parallel.
Holy cow, manual phase alignment! So on the 727, once running in parallel, I take it there was at least some automatic governing to keep the generators in sync? If not, you'd just start blowing circuit breakers I suppose! I take it combing generators on this aircraft was only done in emergencies?

On the ground, what troubleshooting tools do you have available? If you have a short in a bundle, how do you find where it is? Can you use a "Time Domain Reflectometry" tool to send a pulse down the wire and use the reflection to get the distance to the short? Or is it all well-educated guesses from experience, now that the planes you mentioned have been in service for some time?

So, what do you think of "kapton" insulated wires, do you think the various media reports on their dangers are overrated or do you think one of these old planes is just going to catch fire one day :D

I'm asking too many questions, but just one more B) Airlines must be quite nervous (greedy?) about having any aircraft out of service for longer than it "has to". So I take it there's pretty high pressure to race against the clock, and judging by these forums, with little appreciation? What's the typical/longest time you've seen aircraft stay in the hangar? Are you expected to "quote" with accuracy how long unscheduled jobs are supposed to take, or do things often take longer than expected?

Thanks for all your feedback and patience! :)
 
csirac, we do use a time domain reflector to find flaws in coaxial cables. As far as finding a short in a bundle, we normally will disconnect the cannon plugs in the suspected shorted area and check for a short using a multimeter. If we do find a short, we have to search "comb" that bundle for a chafed area or we just pull the wire(s) out and replace it. Kapton wires are okay in non vibration areas, but they can chafe badly when installed on a engine or apu. Engine harnesses are normally repaired or replaced at overhaul so the chance of a short causing an inflight shutdown is very small.
 
PRINCESS KIDAGAKASH said:
csirac, we do use a time domain reflector to find flaws in coaxial cables. As far as finding a short in a bundle, we normally will disconnect the cannon plugs in the suspected shorted area and check for a short using a multimeter. If we do find a short, we have to search "comb" that bundle for a chafed area or we just pull the wire(s) out and replace it. Kapton wires are okay in non vibration areas, but they can chafe badly when installed on a engine or apu. Engine harnesses are normally repaired or replaced at overhaul so the chance of a short causing an inflight shutdown is very small.
Hey CIO, TWUer, and Nightwatch.

It seems that the lady that you claim is a stock clerk is quite knowledgable about maintenance!
 
Wow.........It would appear that some people are interested in answering technical questions about their skilled professions, rather than chewing each other out about union BS..........Hurrah! Could this be a turning point in this forum? Probably not! If you want an answer to your question, try......... www.airmech.co.uk