Technical question: 747 Integrated Drive Generator (IDG) synching. How is it achieved?

Ok, long subject title and short question. Its been a while since I last started a thread here. This one is for the real engineers on this site (no offence to enthusiasts intended).

I was recently on a 747-400 conversion course and one thing came up that I did not quite get and was unable to clarify due to time constraints, and errr... to many christmas beers with the engineers at the Rowers club at Mascot in Sydney.

How the hell do they synchronise the 4 independant Integrated Drive Generators (IDG's), (one per engine) so that they are all in exact phase with each other at 115V 400Hz? If I remember correctly it is via a mechanical means using a swash plate and bobweights and is reffered to as generator "jogging". Unfortunately neither my hand written notes nor the Boeing and airline manuals gives a description of how it's done, and I feel a bit of a goose for not knowing. If someone out there cares to give me an engineers description I will be most happy.

Regards, a budding engineer who is prepared to admit ignorance.

LEE.

[edit on 28-12-2007 by thebozeian]

WOW, 100+ views and nobody can answer me yet! I would have thought someone like kilcoo316 or Zaphod could have nailed that one, or at least know someone who can. Well I'll keep an eye on this thread and bump it till someone can come up with an answer.

In the meantime I would like to wish everyone here a happy new year and thank them for a productive and informative discussion over the last 12 or so months I have been here. It's been fun guys, looking forward to another challenging year!!

LEE.

[edit on 29-12-2007 by thebozeian]

Okay here you go. As I assume you are aware, each generator has a GCU "Generator control unit". In present generation 747's the paralleling of the generators is performed automatically by the respective gcu before it is allowed to go "on-line". This is accomplished by comparing the off line generator's phase rotation with that of the "line". A signal is then sent to CSD, "constant speed drive" that is basically the transmission component of the IDG which is made up of the CSD and the generator. A component inside the CSD is called a governer. This is basically a spinning set of flyweights that sit atop a sliding shaft. In normal mode this stabilizes output rpm to produce 400 +/- 20 cycle operation. The shaft has slots that act to port oil pressure to either an increase or decrease rpm port. In paralleling mode, the shaft can be shifted by electrical solenoid input. As oil pressure is redirected, it affects the positioning of the wobbler. This component is the mechanical equivalent of a tilted swashplate that depending on tilt angle either increases or decreases output rpm. Constant comparision and adjustments are being made throughout operation and all generators are only "happy" when everyone is operating at the same frequency and each ABC phase rotation cycle of all generators are matched. There is some ability built in to "electronically "smooze" slight variations by adding or subtracting capacitance or inductance values but that is a whole nuther lesson. Back in the stone age the flight engineer did the initial setup manually by watching a neon bulb on his panel and turning a rheostat until is slowed to a steady on state, he then flipped the contactor switch and placed the generator on line. Hope this helps.

Thankyou mizzu, you know your stuff. That explains it rather well and your explanation of the relationship between the GCU and CSD has jogged my memory. So I was right with the swashplate reference, but wasn't aware of how the governor actually worked. You have prompted me to dig through my 747-400 parts locator manual and see if there a diagram to help me visualize it in operation.

One thing though you talked about the offline generators phase rotation being compared with the line. I thought all four IDG's came on line simultaneously when they take over from the APU's IDG's, or is that what you mean?

LEE.

Glad I could help. To answer your question, whoever is first on line is in essence the "line". Most often this is the APU. The only exception is an external power source which will trip aircraft systems offline once energized. Back to your question, during normal engine start following a pushback, the apu generator (IDG) is online and powering the aircraft. Following the first engine start, that engine's IDG will assume the aircraft load from the APU once it is placed online. Subsequent engine starts then use that engine's IDG output as the system reference in order to sychronize phase relationship & voltage. On older aircraft there was always the problem of one generator (IDG) "hogging" the load. This was usually due to it having a slightly higher voltage output than its brethren. Series 300 and later aircraft solve these problems automatically by utilizing Arinc 429/629 databus sharing and interaction of all the related aircraft supervisory systemology. Take note that the above and preceeding is just a basic system overview and does not take into consideration the different aircraft AC Bus configurations that happen during emergency or less than "all engines operating" conditions. It is also not meant to provide an accurate illustration. Careful study of the aircraft flight manual and systems suppliments pertinent to the particular aircraft you are flying will offer you a world of knowledge that will prove invaluable to you as a pilot/engineer. Sad to say, but a lot of pilots and flight engineers are limited to being "switch flippers" and are completely helpless in a situation that is not covered in the emergency procedures manual. IMOA good basic knowledge of ALL aircraft systems including hydraulics, pneumatics and avionics is a must.

Sorry for taking so long to get back to you mizzu but again thanks for the reply. It was more or less what I thought must be happening as like most things engineering it is logical.

Yes I agree that more pilots or in my case maintenance engineers, should look deeper into the systems that they work with. Alas some airlines are better it seems than others when it comes to this.

An Important note:

This week on a flight from London on 07/01/08 a Qantas 747-400 suffered an inflight shutdown of ALL its Integrated Drive Generators (IDG's) whilst 15 minutes out from Bangkok. The standby emergency system which consists of a main aircraft battery driven inverter kicked in and supplied critical systems power whilst the aircraft was on final approach. The cause of the incident is being investigated but preliminary findings are that it was caused by a water leak from an overhead galley. It appears that water leaking from a galley pipe (it was not stated if it was waste water or fresh) collected in the leakage pan that sits below the galley. Unfortunately there appears to have been a crack in the pan which allowed water to then drop onto the equipment centre housing the Generator Control Units (GCU's). The end result was a fried set of GCU's and the emergency kicking in. However the static inverter can only supply enough power at 110V to power a single phase so only some systems will work.

Of course it goes without saying that the flight landed safely. However if the incident had happened earlier or over water the battery is only capable of supplying 30-60 minutes of power so it could have been a potentially much more serious affair.

And this incident brings to mind a conversation I had with a senior engineering instructor a few weeks back when I posed the question, "Shouldn't there be emergency provision for an inflight restart of the APU and seperation (when required) of it's IDG's and GCU from the main system?" Of course until this week I believe there had only ever been one or two total generator failures in the entire life of the 747. Maybe Boeing will now look to providing this option as it seems simple enough to do?

LEE.

reply to post by thebozeian


Awesome to see a more technical discussion that makes me think a bit on here lol. Thanks for your answers mizzu they help someone who eats sleeps and breaths as much about aviation as i can and this subject doesn't come up much. wonder why lol

Anyways just wanted to say that flight global has a news article on the 747 loss of power.

It says now following intense media interest: “As a precaution, Qantas has inspected its entire 747-400 fleet and all of these aircraft have been cleared to fly.” It also says Boeing will be notifying all 747-400 operators of the incident.

Qantas says the aircraft is now back in normal operation following “stringent inspections and testing in Bangkok before being cleared to fly”.

www.flightglobal.com...

Originally posted by thebozeian
WOW, 100+ views and nobody can answer me yet! I would have thought someone like kilcoo316 or Zaphod could have nailed that one, or at least know someone who can. Well I'll keep an eye on this thread and bump it till someone can come up with an answer.

In the meantime I would like to wish everyone here a happy new year and thank them for a productive and informative discussion over the last 12 or so months I have been here. It's been fun guys, looking forward to another challenging year!!

LEE.

[edit on 29-12-2007 by thebozeian]

I just found this thread while browsing through the forums. Sorry, but I was on vacation from ATS at the time, so wasn't really paying attention.

Mizzu, watch it with the stone age comments. I remember watching the flight engineers sitting there watching the lights and turning the nobs.

Mizzu, watch it with the stone age comments. I remember watching the flight engineers sitting there watching the lights and turning the nobs.

Ahhh those were the days Zaph, when men where men (or women )and aircraft where REAL aircraft.

Two things to update here,
First there have been another bunch of Qantas 747's found with cracked leak pans (aint offshoring some maintenance or using contractors a great idea? ) so the potential for this kind of mass GCU failure is perhaps greater than would first be thought. Makes one wonder what similar problems are lurking with less reputable operators.

And second this kind of problem should be almost impossible with the A-380 due to some very well thought out systems redundancy design work.

LEE.

[edit on 24-2-2008 by thebozeian]

Ah yeah, those were the days. I remember doing an engine start with a crew one day, I don't remember the type of plane, I THINK it was a C-137. My job was to sit there and watch one specific gauge. It would start to rotate, then snap to a certain number. When it did, I would tell the pilot "In". After a minute when certain perameters on that engine were met, it would snap back, and I would tell the pilot "Out". Had to do that for all four engines.

I'm kinda not surprised that Qantas is having these problems. Once you start to see one plane having something like that happen, I can almost guarantee you're going to see more of them having a similar problem.

reply to post by thebozeian


As I'm sure most aviation nuts around here know most of the more modern airliners use what are called RATs (Ram Air Turbines) for auxiliary power if a similar incident were to happen.

Do the airliner RATS have a short life when operating or can they operate for a couple hours at a time?

Seems like an F4 driver told me that when their RAT was deployed it had a 20 minute life.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Interesting stuff on the generator synchronization bit.

The "stone-age" aircraft where the flight engineer was watching a light was waiting to see synchronization before he paralleled a generator to the system.

Very much the same is done in the electric bulk power industry.
Although we had the advantage of a synchronization meter that would spin fast or slow in either direction indicating out of synchronization between two generating units as well as lights.


Best was to get the meter needle turning very slow (two rpm if I remember right) and then close the circuit breaker when the needle was just short of being straight up and then the system would pull the unit into synchronization with the system.
HZ -or speed - adjustments were made at the generating plant.

Make a bad shot (synchronization attempt) and not only could you knock the oncoming unit off line, you may knock another one off line a hundred miles away.


Real interesting back in the good ol daze to parallel California and Arizona and watch the amp, volt and even the frequency meters swing slowly back and forth until the two systems settled down.

All that taking place at 60hz of course.
Looks like a whole other ball game at 400hz.

I'm curious as to why the very high frequency on aircraft?

By using 400hz, they get smaller generators and power supplies. Weight is key on aircraft, and having lighter and smaller power supplies is a good thing.

Yep, 400Hz means much better transformer action in the LRU's meaning less current requirements and thus smaller wire, smaller transformers and smaller motors. As far as RAT's are concerned, the units used on military fighter aircraft were propeller driven brushless AC generators. Their output was primarily used for keeping the flight instruments energized. They would have probably been able to run forever but usually once it came down to deploying the rat, the "driver" was looking for somewhere long hard and flat to put that sucker down on. On Civilian passenger aircraft, the RAT is a propeller driven Hydraulic pump. It is used to provide hydraulic power to the primary flight controls, charge the brake accumulators and thru selection of power transfer units it can be used to lower flaps, etc. It is a neat unit that maintains a system pressure against demands in volume by varying propeller pitch to increase or decrease rpm. Same system design as the old Ham-Std 43d50 & 45H60 Hydromatic propeller systems.[Ring any bells Zaph?) Also once it is deployed it stays deployed until the mechs restow it. As far as 747 GCU failures, Coffee spillage in the galleys used to be a major factor when I was crawling around in the belly.

God mizzu, talk about dredging up memories. I haven't seen or heard of one of those in ages. You're making me feel old, and I ain't THAT old yet! Just experienced. *laugh*

Thanks again for further input mizzu (and of course you too Zaph).

Its funny, when you were explaining how the RAT works to varying loads and said...

It is a neat unit that maintains a system pressure against demands in volume by varying propeller pitch to increase or decrease rpm

I thought hmmm, sounds analogous to a hydraulic prop pitch control... Which you then promptly confirmed with

Same system design as the old Ham-Std 43d50 & 45H60 Hydromatic propeller systems

Funnily enough I was looking at one of those old Ham-Std types the other day. Such an elegant piece of equipment I was thinking, I doubted they could come up with anything better today. Seems you have confirmed my suspicion.

LEE.