What exactly is a generator field?...also 744 IRU on battery wheel well warning horn
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What exactly is a generator field?...also 744 IRU on battery wheel well warning horn
Hi,
Coming from a background of airplanes with DC power I see many references to "generator fields" in boeing manuals and cannot visualize the mechanism. The -400 has guarded generator field reset switches on the overhead maintenance panel and items such as the engine fire switches and generator control switches are said to "open the generator field" as well as the "generator control breaker".
I assume opening the field is analogous to depressurizing a hydraulic pump, i.e. it stops generation of electrical current whereas the open GCB isolates the generator from the electrical system.
Does anyone have helpful explanations, links, or diagrams to aid in understanding this? Thanks
Somewhat unrelated question:
I remember being told at some point that a horn sounds in one of the 744's gear wells if the IRU's are operating on DC power. Can anyone elaborate on which wheel well the horn sounds in (nose gear I presume) and how long it goes off (5 minutes I think, due to being associated with the center IRU?).
I appreciate it.
Coming from a background of airplanes with DC power I see many references to "generator fields" in boeing manuals and cannot visualize the mechanism. The -400 has guarded generator field reset switches on the overhead maintenance panel and items such as the engine fire switches and generator control switches are said to "open the generator field" as well as the "generator control breaker".
I assume opening the field is analogous to depressurizing a hydraulic pump, i.e. it stops generation of electrical current whereas the open GCB isolates the generator from the electrical system.
Does anyone have helpful explanations, links, or diagrams to aid in understanding this? Thanks
Somewhat unrelated question:
I remember being told at some point that a horn sounds in one of the 744's gear wells if the IRU's are operating on DC power. Can anyone elaborate on which wheel well the horn sounds in (nose gear I presume) and how long it goes off (5 minutes I think, due to being associated with the center IRU?).
I appreciate it.
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Gen field
Hi,
In a generator, a spool of wiring is rotated inside a magnetic field. Inside the wiring elctron movement is induced, in other words, electricity.
The magnetic field can be the one from a permanent magnet, the Gen is then a Permanent Magnet Generator (PMG), or the magnet can be an electro-magnet, usually a weak-iron piece with electric wire coils wrapped around it. The current that is passed through the coils of the electro-magnet is the so-called "field" that can be switched off to stop the rotating spool from generating electricity.
Hope this explanation helps you.
Cannot help you on the horn question.
In a generator, a spool of wiring is rotated inside a magnetic field. Inside the wiring elctron movement is induced, in other words, electricity.
The magnetic field can be the one from a permanent magnet, the Gen is then a Permanent Magnet Generator (PMG), or the magnet can be an electro-magnet, usually a weak-iron piece with electric wire coils wrapped around it. The current that is passed through the coils of the electro-magnet is the so-called "field" that can be switched off to stop the rotating spool from generating electricity.
Hope this explanation helps you.
Cannot help you on the horn question.
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Adding on to the generator field;
Without the field breaker closed you'll only be able to pull the 10-18 residual volts off the generator. The residual voltage is essentially "fed back" into the permanent magnet in the generator to create the electro magnet that then pumps out your 120VAC power.
Most FE orals I've been involved with go into detail about 10 times greater on the generator, surprised they don't teach this stuff anymore.
Without the field breaker closed you'll only be able to pull the 10-18 residual volts off the generator. The residual voltage is essentially "fed back" into the permanent magnet in the generator to create the electro magnet that then pumps out your 120VAC power.
Most FE orals I've been involved with go into detail about 10 times greater on the generator, surprised they don't teach this stuff anymore.
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Edit: I submitted this before the above reply was written; it answers my follow up question....apparently the IDG has a permanent magnet which is used to power the electromagnet. Frankly I'm also surprised at how little generator operation is emphasized during training, granted I'm not an engineer and in an automated airplane such as the -400 there's really not much application for the knowledge. Thanks.
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Thanks for the explanations so far, very helpful.
So for a generator to have a "field" which can be turned off, it must use an electromagnet instead of a permanent magnet? Obviously, if a permanent magnet is used, the field is always there.
Does anyone know where the power for the electromagnets comes from on a 744/GE CF6 IDG? Does it come from the same power source as that of the EECs (although our manuals don't discuss it I've been told there are separate generators in each engine which power the EEC's independently of the regular aircraft electrical system)? Does it come from the DC electrical system (I recall the "IDG disconnect switches" being located on the "Main Battery Bus")?
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Thanks for the explanations so far, very helpful.
So for a generator to have a "field" which can be turned off, it must use an electromagnet instead of a permanent magnet? Obviously, if a permanent magnet is used, the field is always there.
Does anyone know where the power for the electromagnets comes from on a 744/GE CF6 IDG? Does it come from the same power source as that of the EECs (although our manuals don't discuss it I've been told there are separate generators in each engine which power the EEC's independently of the regular aircraft electrical system)? Does it come from the DC electrical system (I recall the "IDG disconnect switches" being located on the "Main Battery Bus")?
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Don't be to quick to discount the "need" for knowledge. On an aircraft as large and complex as the 747 (never flown it, but vaguely familiar with the systems), which has an electrical system capable of parallel operation knowing what the generators are doing and how they're doing it can and has proved invaluable. The same goes for hydraulic, pneumatic, fuel, and other systems.
My personal opinion is anybody who wants to call themselves "captain" need not only have the knowledge, judgement and experience to fill the seat, but should be able to rebuild the airplane systems from scratch. Remember, they removed the engineer from the 744 (and many other aircraft) because computers were able to do his job, but what replaced the knowledge?
My personal opinion is anybody who wants to call themselves "captain" need not only have the knowledge, judgement and experience to fill the seat, but should be able to rebuild the airplane systems from scratch. Remember, they removed the engineer from the 744 (and many other aircraft) because computers were able to do his job, but what replaced the knowledge?
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One of my favorite "gotchas" is on the old CSD units (as fitted to the old GTCP 85-98 APUs);
If the frequency is moving past 390 hz without any voltage indication and the starter still engaged you would be wise to immediately terminate the start, unless you want to wreck your APU starter.
But I digress, if you're interested, though, here's a diagram of the CSD, the Generator and the Generator Protection. I'm sure there are differences between this and the 747, but the general idea is the same.
If the frequency is moving past 390 hz without any voltage indication and the starter still engaged you would be wise to immediately terminate the start, unless you want to wreck your APU starter.
But I digress, if you're interested, though, here's a diagram of the CSD, the Generator and the Generator Protection. I'm sure there are differences between this and the 747, but the general idea is the same.
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I remember being told at some point that a horn sounds in one of the 744's gear wells if the IRU's are operating on DC power.
Many crews hear the horn, and turn off the IRUs. This is usually not necessary.
If the horn is sounding because the GPU has dropped off line, and you are starting the APU to restore ground power, then let the horn sound.
Depends on the aircraft but usually at OAT below 35degC, the IRU will happily run without cooling.
So many times I see crew turning off thr IRUs, then having to wait 10 mins to wind them up again, when there is no need for it.
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Random Person, the scene is confusingly set by the infuriating habit of some manufacturers to refer to Alternators as "Generators".
A Turbocharger is a Supercharger but referring to one as such in a specific technical discussion is less than helpful and is often totally misleading.
Alternators (ie AC Generators) have field coils that energise the magnetic field - ie an electromagnet instead of the permanent magnet found in a DC Generator. Switch that off, no more magnet, no more wigglies. Wigglies stop, try re-energising the field coil, hence the switchery.
But then they wear their pants as an outer garment over there too, so if not confusion what can you expect?
A Turbocharger is a Supercharger but referring to one as such in a specific technical discussion is less than helpful and is often totally misleading.
Alternators (ie AC Generators) have field coils that energise the magnetic field - ie an electromagnet instead of the permanent magnet found in a DC Generator. Switch that off, no more magnet, no more wigglies. Wigglies stop, try re-energising the field coil, hence the switchery.
But then they wear their pants as an outer garment over there too, so if not confusion what can you expect?
the generator field requires excitation current from an external source, from either a battery or other synchronous machines online, in order to produce lines of magnetic force...
That schematic posted by aviatorhi is close, but not quite the same as the -400 IDGs. But it'll do for a quick explanation.
Starting at the top of the ac generator block, there are the main stator windings. These produce the 115V 400 Hz power. In order to do so, a rotating magnetic field must sweep across them. So on the rotor (just below on the diagram), you see the "generator rotating field" a DC current through this (basically an electromagnet) produces that field. But here's an interesting problem: how do you get DC into the rotating rotor? Back in the old days (and in automotive alternators), they used brushes to feed the dc in from the regulator. But brushes are a maintenance problem, so here they use a neat trick. Just below the aforementioned field, there is a 6 diode rectifier and a three phase exciter winding. This is basically like an inside-out automotive alternator. Inside out, because the output windings and rectifiers spin with the rotor. The exciter field (below that) is logically equivalent to an automotive alternator field. Except, being inside out, the exciter field is stationary, eliminating its set of brushes.
The exciter field here is where this CSD-driven generator differs from Boeing's IDG version. Here, it appears that there are a couple of permanent magnets (PM) and some auxiliary windings. These work together with the voltage regulator to vary the total exciter magnetic field. Which varies the output of the exciter windings, which are rectified into dc and fed to the generator field winding. So by varying the exciter field, you indirectly vary the generator field and finally the generator output voltage.
One difference between the CSD system (above) and the IDG system is the presence of the permanent magnets. These allow the generator to 'bootstrap' itself and reduce the ammount of exciter field current that must be added. The IDG generator has no such magnets in its exciter field. Instead, it has yet another small generator with its own set of permanent magnets on the shaft. This produces a small amount of power (at 1200 Hz IIRC) to the Generator Control Unit. The GCU (which incorporates the voltage regulator) uses this to power up its microprocessor and to provide exciter field current. When the generator field relay opens (it opens the exciter field), the generator effectively goes dead. The residual rotor magnetism is far below that which the CSG PM's produce.
Disconnecting the exciter field is done to stop the generator from producing power (short of mechanically disconnecting it from the engine) should there be a fault in the feeders before the GCB, or in the genertor itself. The CSD design (above) may have been judged to be safe due to the small amount of power the still spinning permanent magnets produce. But as things get bigger (the 747 IDGs are rated at 90 kVA each, about 250 Amps) it is important to remove all excitation in the event of a fault.
Starting at the top of the ac generator block, there are the main stator windings. These produce the 115V 400 Hz power. In order to do so, a rotating magnetic field must sweep across them. So on the rotor (just below on the diagram), you see the "generator rotating field" a DC current through this (basically an electromagnet) produces that field. But here's an interesting problem: how do you get DC into the rotating rotor? Back in the old days (and in automotive alternators), they used brushes to feed the dc in from the regulator. But brushes are a maintenance problem, so here they use a neat trick. Just below the aforementioned field, there is a 6 diode rectifier and a three phase exciter winding. This is basically like an inside-out automotive alternator. Inside out, because the output windings and rectifiers spin with the rotor. The exciter field (below that) is logically equivalent to an automotive alternator field. Except, being inside out, the exciter field is stationary, eliminating its set of brushes.
The exciter field here is where this CSD-driven generator differs from Boeing's IDG version. Here, it appears that there are a couple of permanent magnets (PM) and some auxiliary windings. These work together with the voltage regulator to vary the total exciter magnetic field. Which varies the output of the exciter windings, which are rectified into dc and fed to the generator field winding. So by varying the exciter field, you indirectly vary the generator field and finally the generator output voltage.
One difference between the CSD system (above) and the IDG system is the presence of the permanent magnets. These allow the generator to 'bootstrap' itself and reduce the ammount of exciter field current that must be added. The IDG generator has no such magnets in its exciter field. Instead, it has yet another small generator with its own set of permanent magnets on the shaft. This produces a small amount of power (at 1200 Hz IIRC) to the Generator Control Unit. The GCU (which incorporates the voltage regulator) uses this to power up its microprocessor and to provide exciter field current. When the generator field relay opens (it opens the exciter field), the generator effectively goes dead. The residual rotor magnetism is far below that which the CSG PM's produce.
Disconnecting the exciter field is done to stop the generator from producing power (short of mechanically disconnecting it from the engine) should there be a fault in the feeders before the GCB, or in the genertor itself. The CSD design (above) may have been judged to be safe due to the small amount of power the still spinning permanent magnets produce. But as things get bigger (the 747 IDGs are rated at 90 kVA each, about 250 Amps) it is important to remove all excitation in the event of a fault.
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There has been anecdotal evidence that if the IRU's are in alignment and AC power is tripped off, that the IRU's will not always complete alignment. This is why a lot of pilots will turn off their IRU's.
The nosewheel horn will sound for any IRU on Hot Battery only. The same horn is used for loss of equipment cooling in general (not necessarily because the IRUs have lost equipment cooling).
Re the generator. The engine spins, mechanically turning the accessory gearbox to which the generator is attached.
In the generator, there are 6 permanent magnets which induce an AC (alternating) current in an electrical circuit (permanent magnets are used so that no additional power from the airplane is required .. as long as the engine N2 rotor is spinning fast enough)
The electrical circuit goes to the respective Generator Control Unit (in the Equipment Centre). Here, the AC current is rectified to produce DC current. The Field Switches simply interrupt this process (like a light switch).
Normally, the DC is fed back to the generators, where the DC current is converted to aircraft AC power.
How such small magnets and tonnes of coils ultimately produce high voltage and current is a complete mystery to me (I'll leave that to the scientists here)
The nosewheel horn will sound for any IRU on Hot Battery only. The same horn is used for loss of equipment cooling in general (not necessarily because the IRUs have lost equipment cooling).
Re the generator. The engine spins, mechanically turning the accessory gearbox to which the generator is attached.
In the generator, there are 6 permanent magnets which induce an AC (alternating) current in an electrical circuit (permanent magnets are used so that no additional power from the airplane is required .. as long as the engine N2 rotor is spinning fast enough)
The electrical circuit goes to the respective Generator Control Unit (in the Equipment Centre). Here, the AC current is rectified to produce DC current. The Field Switches simply interrupt this process (like a light switch).
Normally, the DC is fed back to the generators, where the DC current is converted to aircraft AC power.
How such small magnets and tonnes of coils ultimately produce high voltage and current is a complete mystery to me (I'll leave that to the scientists here)
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Thanks for all the replies, they are greatly adding to my understanding of the aircraft's electrical (and inertial reference) systems. I feel that I have at least a rudimentary understanding of the IDG's whereas before I only knew that they made 400hz 115v AC power at 90 kva and what the function of the switches on the overhead were.
To take the discussion in a different direction, can anyone elaborate on the power source for the 744 GE CF6-80C2 EEC's both before engine start and after engine start? I'm guessing it's like other airplanes I've flown: FADEC's were powered off standby/essential buses prior to about 10% N2, at which point a dedicated permanent magnet alternator connected to the accessory gearbox would take over.
Thanks for the continued responses, again they are very informative.
-RP8
To take the discussion in a different direction, can anyone elaborate on the power source for the 744 GE CF6-80C2 EEC's both before engine start and after engine start? I'm guessing it's like other airplanes I've flown: FADEC's were powered off standby/essential buses prior to about 10% N2, at which point a dedicated permanent magnet alternator connected to the accessory gearbox would take over.
Thanks for the continued responses, again they are very informative.
-RP8
EE,
If I understand this right (and at the risk of confusing everyone else), the exciter works like an old-fashioned automobile generator. It has a winding rotating in a stationary magnetic field. Except instead of using a commutator to pull DC power off the winding, it rectifies the current on board and uses it to feed the main generator field coil. That is a neat trick.
If I understand this right (and at the risk of confusing everyone else), the exciter works like an old-fashioned automobile generator. It has a winding rotating in a stationary magnetic field. Except instead of using a commutator to pull DC power off the winding, it rectifies the current on board and uses it to feed the main generator field coil. That is a neat trick.
