Aircraft Motors & Generators

[dream747]

Was reading through some materials regarding aircraft generators and motors with the principles of operation based on electromagnatisem. It baffles me a little about where does the source of power come from for generators and motors and aircraft? For example, a current is needed to run through the coils of wires in a magnetic field in order to create a force and turn the armature hence the shaft. Where would this source of electrical current come from, when other the hand mechanical force is required to turn an armature in a magnetic field to produce electrical current in the wires.

It seems that both complements each other in power generating to power the systems of the aircraft but there has to be some source to start off this chain?

Thaks for any inputs!

[SNS3Guppy]

The battery, or any other electrical source, supplied through a field circuit.

[Blacksheep]

Many of the larger aircraft generators have an integrated permanent magnet generator for self excitation; the output of this PMG is fed via the voltage regulator to the main field windings. The generators are mechanically driven from the engine auxiliary gearbox along with other services such as the hydraulic pumps. In large aircraft, alternating current generators are driven at a constant speed and thus constant frequency, through a constant speed drive unit which may be either a seperate unit or built into the generator as an integrated drive.

[stankou]

you can have mechanical drive ( i.e. starter generator, ac gen) or electrical power from battery, or GPU.

[WOTME?]

BAC 1-11 had a constant speed drive & starter(CSDS)but I wouldn't go there if I was you.

[Blacksheep]

Then there was that 'Labinal' contraption - confusing, but it worked. :uhoh:

The chap is asking a very basic question so I think we'd better keep it simple. The generators are engine driven and excitation is either from a battery or from a built-in permanent magnet generator.

[dream747]

Thanks for the replies everyone. In addition to this, for hydraulics, why would the fluid level when the system is pressurised drop and then fluctuate given the fact that hydraulic fluid is near incompressible?

[Bobcanfixit]

Lots of reasons.

some internal bypass (wanted or not)
Higher components (Rudder PCU etc) return fluid to Res over time.
Pumps on bypass fluid via Case Drain (cooling etc) back to res.
Pumps on will charge Accumulators (might take 2-5 lt up).

The list goes on.

[ArthurR]

Blacksheep... not always, If I remember rightly, we always had a few problems with the Labinal system on the Gnats..
But yes it was a little complicated

[Salusa]

Aircraft electrics always seemed like modern sorcery to me.

Highly recommend this book:

Aircraft Electrical Systems, E.H.J Pallett

· ISBN-10: 0582988195
· ISBN-13: 978-0582988194

[dream747]

Thanks for that recommendation. Is the book easy to understand most importantly?:)

I'm quite bad at electricity as well... after repeated readings I still don't completely understand why certain applications require AC and others DC since apparently AC has more advantages than DC itself. It's rather easy to convert DC to AC and vice versa anyway.

One thing I'm quite interested in also are the buses in aircraft. It's pretty amazing how the system can balance the electrical loads supplied by the generators and provide them to the aircraft systems and having so much redundancies and protection circuits and systems in itself.

I understand that in DC generators and circuits it's very important for all the generators to share the electrical loads of the aircraft if not the generators would be damaged by recirculating current from those which output are higher. What about AC generator circuits?

Anyway, what aircraft systems, generally, require AC or DC respectively?

Excuse me for so many doubts I have... thought this could be the best place to get some answers!

Thanks!

[Beeline]

Too keep it simple, if the aircraft is on the ground without an external power source like any other device around the home it requires a battery to get it started, these are only available in D.C.

28v D.C is the standard and this can be used to start other electrical sources that can supply A.C power e.g the Auxillary Power Unit. D.C current is used by aircraft computers, solenoids within system components and also some motors when battery power is the only available source (Very Bad day!!)

Large aircraft have A.C over three phases, when connected in various ways this can supply a handy two voltages a phase/line voltage 115/200 volts respectively. 400hz is used to the conventional domestic 50/60Hz supply to keep size and weight down on most components.

On some aircraft the power sources from each engine generator can be parallelled on a syncronous Bus, the 747 is an example of this. Most twins such as a 767 have bus tie breakers and load control units that open and close to allow other sources to feed its respective bus if a genny drops off-line.

Parallelling can be acheived by clever circuitry reading the power output of each genny, modern aircraft are goverened by an electronic master Generator Control Unit that talks to its slave GCUs changing an excitation field thus output This keeps them within a certain tolerance and prevents a reverse voltage that can burn out windings and cause all sorts of mischief!

I suggest the Eismin book (ISBN0-07-113286-4) is an excellent read, you can get into the nitty gritty and really see the theory of the wiggly amps in A.C and D.C, I have not even scratched the surface!!

Bline

[glad rag]

And just to add spice some of the latest designs run altenators frequency wild with rectification (if required) carried on/at the consumer units...........:}

[dream747]

Thanks guys for the replies. Certainly cleared some doubts here! One last thing, how exactly do self-excited circuits work? They self-excite (by introducing current into the armature/wires to create the field) from an external source, or an internal source?

Thanks!