Saturday, May 21, 2011

Hats Not Cats

The electrical system in those airplanes I once flew (someone suggest a nickname for it, eh?) underwent a few changes from version to version. I remember that the first one was the first airplane I flew equipped with generators, not alternators, the functional difference being that a generator will not charge the battery at low rpm, but it can be used to charge an absolutely flat battery (assuming the battery has the capability of holding a charge) while an alternator can charge at low rpm but needs there to be a bit of juice left in the battery to excite the field so it can work. You know what? I've recited that a hundred times, and can continue, but in order to ensure I really know what it means, I shall try to build an airplane electrical system from stone knives and bearskins. Let this be my apology for weeks of soul-searching, arts classes and lolcats.

Electricity is essentially the displacement of charges. Rub dissimilar materials and they may become oppositely charged and cling together. The differential may discharge in the form of a spark. It's as if every neutrally charged molecules were wearing a hat, but when you rub them together the hats all fly up in the air and the molecules with a greater millinery affinity grab more than their share. So when the dust settles you have the molecules of one substance wearing extra hats and the molecules of the other being hatless. This isn't a stable situation, and given the chance to correct it, the ones with extra hats will give up their headgear and the bareheaded ones will grab up stray hats. By this model, electric current would compare to a string of people with hats, all snatching the hat off the person to their left and putting it on, over and over again around in a circle.

A flow of electric current induces a magnetic field around a wire, and likewise movement of a wire through a magnetic field induces electric current in a wire. It's like passing hats creates a breeze, but a breeze itself lifts up people's hats. Can't have one without the other. The hat passing is driven by the power source, such as a battery. In a motor the magnetic field is used to create rotation--doing work with the breeze from the hats. In a generator, a breeze is used to make the hats move.

Rather than using just one wire for this effect, wire is wrapped in layers around a core, giving more electric-magnetic interaction in a more compact area, so making the whole thing neater and more efficient, like arranging your hat-wearing people on tiered benches so the same breeze can lift more hats or to increase the breeze in a small area. I think I'll stop talking about hats now, but if you like hats, then from here on read "movement of hats" for electric current and "breeze" for magnetic field.

In a generator an electromagnet creates a magnetic field around an armature (a spinny thing) wrapped with wire. The spinning movement induces an electric current along the wire. The force to spin the armature comes from the airplane's internal combustion engine, the thing that's driving the propeller. That's right, they use electricity to make a magnet, then they use the magnet to make more electricity. With this much information it seems as though the generator can't produce power unless it already has power, because otherwise what would power the electromagnet, but the trick is that having acted as the core of an electromagnet, the iron contains remanent magnetism. It's not a lot, just enough to induce a small current in the spinning armature, which is enough to increase the magnetic field so that it can generate a greater electric current and so on until it reaches a steady state. You can get more current by turning the engine faster, but you can't get more at low engine speeds. In fact you might not get enough at low engine speed, the weakness mentioned in the first paragraph of this entry. I've just learned from this site (which describes alternators and generators well, albeit without hats) that when you first connect a generator you need to polarize it before starting the engine. That also implies that if I disconnected a generator for some reason and then reconnected it, I could damage something by starting up without polarizing it. Another reason to keep screwdrivers out of the hands of your pilots.

The direction of the electrical current induced in a wire depends on the direction of its movement relative to the direction of the magnetic field. There are wacky things you can do with your fingers to figure out which way that is, but we're not reliving our grade eleven physics classes here, so you can put your fingers down. The point is, if you're spinning the armature, you can see that you'll get a relatively strong current as the wire cuts across the magnetic field, a current weakening to nothing as it moves to be parallel to the field, and then a strengthening current in the opposite direction. That is, it will produce an alternating current. An old airplane like this wants one-way direct current, so to prevent it from flipping back and forth, it's connected to the rest of the electrical system via a split ring device called a commutator, which reverses the connection every half turn. The end result of having both the connection reversed and the current reversed is steady current.

Generators are old-fashioned and more troublesome for maintenance, so the manufacturer allows you to replace the ones in this airplane with alternators. An alternator works on the same principle as the generator except that in the alternator, the wire windings in which the current will be induced are on the outside, and hold still, while the electromagnet that creates the magnetic field spins on the inside, connected to electrical power via a slip ring, which is simpler than a commutator. AC is converted to DC with a diode. A voltage regulator increases the current to electromagnet so that the alternator produces sufficient power for the aircraft electrical services, even at low rpm. That requires the alternator to draw power from the battery in order to get going, but it's worth it, because the alternator can then charge the battery immediately after start. A generator may be still discharging during low power taxi.

The POH copy I have here claims the aircraft has a 12V 33 ampere-hour battery and two 12V 50 A generators, one on each engine, but who knows if the airplane it belonged to still has generators. If it does, each generator has a voltage regulator and a there is a paralleling circuit to divide the load between the generators. If only one engine is running, a reverse current relay cuts its generator out of the circuit. Also, typically an alternator produces a higher voltage than the battery, so that it can charge the battery. I'm winding down now, so I'll see if I can write about voltage regulators, RCRs and paralleling circuits later.

Hyperphysics is an excellent site for this kind of thing, but I'm not too impressed with their 3D field diagrams.


A Squared said...

Also, typically an alternator produces a higher voltage than the battery, so that it can charge the battery.

Umm, so do the generators, that's the only way a battery can get charged, is by a higher voltage. Or maybe I'm not following your meaning.

A Squared said...

"AC is converted to DC with a diode."

Not *a* diode, an arrangement of diodes called a rectifier. A simple rectifier for a dipole device would have 4 diodes

fatfred said...

Forget all of those silly technical terms, amp, volt, and watt as they are not needed.
The smoke theory of electrical repair will carry you through any problems you may encounter.
The way this works is all electrical components contain smoke. If the smoke comes out then the part is no longer of use and you need another one.

Aviatrix said...

A Squared: re your first comment. That was my comment on the fact that the airplane comes standard with a 12V battery and 12V generators. I'm used to the 24V battery and 28V alternators, so I was looking squinty-eyed at that purported set-up.

A Squared said...

"12 volt" generators are actually 14 volt generators. Is that what you're getting at?

Anoynmous said...

If the aircraft is what I think it is, I suggest "Kokopelli" as a nickname. He's a Native American flute player.

A Squared said...

Kokopelli Hah! That's a good one.

If there's light twin with Hershey bar wings and a steel tube frame other then the Kokopelli, I can't think of it.

zb said...

Awesome how you explain generators. I admit I hadn't really thought about how the counter-acting field is generated on start-up, maybe even throughout my entire time at college.

A side-note: A while ago, you had a post about potentiometers. For some reason, my link to the great stories of poor Kur Killapot hasn't made it through to appearing in the comments. I'll try again, because I think you might like this very special application note despite being part of a huge, scanned pdf document with odd page numbering (..., 18, 19, 2, 20, 21, ... or something). Here it is, inside The Potentiometer Handbook by Bourns (approx. 33 MB) Be sure to check chapter nine, and also check out the cool picture of a V/STOL aircraft.

zb said...

Awesome how you explain generators. I admit I hadn't really thought about how the counter-acting field is generated on start-up, maybe even throughout my entire time at college.

A side-note: A while ago, you had a post about potentiometers. For some reason, my link to the great stories of poor Kur Killapot hasn't made it through to appearing in the comments. I tried again, because I think you might like it. Again, it appeared for some minutes, and then obviously got deleted. I really can't think of a reason why it would be offensive or considered spam. Any idea why this happens?

mattheww50 said...

Generators and Alternators are significantly different creatures.
Most Alternators will in fact self excite, but often not to levels high enough to be useful in an aircraft. Both Generators and Alternators require voltage regulators to control the output voltage.

I've seen aircraft alternators equipped with a dry cell battery to make sure they can in fact provide power when the main battery is dead. There also exist Permanent magnet alternators, which need no external battery supply to operate. They tend to be heavier than standard Alternators, and I am not sure you really want a big magnetic in relatively close proximity to the navigation equipment.

There are other important differences. Generators have brushes, which make and break contacts with the rotor windings seveal times per rotation. Alternators have slip rings, which make continuous contact.

If you excite the rotor winding (most do), the current the slip rings handle is quite modest compared to the full Alternator output. The result is generally the slip rings will last as long as the Alternator. Those who are older may remember that the brushes on generators had to be replaced.They quite literally wore away.

IN larger aircraft, the AC output of the Alternator (400hz) dramtically (about 87%) reduces the iron content required in devices like transformers and motors, so 400Hz AC is huge weight saver

Lastly, you get can get considerably more power per unit weight and unit volume from an Alternator than from a generator.

Brewster said...

Fascinating... here I've been following you for several years (mostly in stealth/lurk mode), also back-read all your archives that I could get to, and not until today (stone knives and bearskins) have I realized that you are a Trekker!

If I have misinterpreted your use of the phrase, I apologize. If, on the other hand, I'm blind and stating what the rest of your faithful readership already knew, then let the slings and arrows of outrageous fortune commence.

(I am certain that, unlike me, you are not of the generation to be called a "Trekkie").

And btw, a great read as usual.

Aviatrix said...

Brewster: You make no mistake. I am astonished that I've managed to sufficiently subtle to only now alert you.

Sarah said...

Brewster, I'm not seeing the Trek clue in this post, but Aviatrix has previously shown such proclivity. What did I miss? Some technobabble?

Anoynmous Kokopelli is brilliant. The nose art opportunity would be excellent, which makes up for it not being a perfect match.

Magnetism and electricity is what got me interested in technology and physics as a child. As I write this, I'm playing with some neodymium toys. How mysterious... any sufficiently advanced technology is indistinguishable from magic. Just ask the Insane Clown Posse. (Or much better, ask Feynman ).

Brewster said...

Sarah, "I shall try to build an airplane electrical system from stone knives and bearskins" is an artful adaptation of an exchange in the 1967 classic episode "The City on the Edge of Forever," arguably the best original episode.

'Trix, my wife would tell you that subtelty is a quality absolutely wasted on me... and I apologize for hijacking the comments.

Sarah said...

Ah, of course, Brewster, thank you. The City on the Edge of Forever was wonderful, even if the whole episode p'ed off Harlan Ellison. But then, most things do.

zb said...

Sorry for cluttering the comments with a double-post-slash-complaint (see above). It's just that the first one appeared, then vanished, then appeared again. Feel free to take the double one (and this one) down again, if you like.

BTW, I am just now enjoying car talk on NPR. Not just do car talk and cockpit conversation sound really alike, it's also that if there ever was a really cool and witty radio show about how planes work and can be fixed, I think it should be hosted by no one else than our great Aviatrix.

Aviatrix said...

Thank you, good name. I am going to call it Koko, making it fit Kokopelli, the Hershey (chocolate) bar wing, and another reason that works for me.

You're not hijacking the comments, merely diverting them, and diversions are welcome here. Especially Star Trek-related ones.

Aluwings said...

Re: "Generators and Alternators are significantly different creatures."

I love how aviation includes folks with all sorts of interests, inclinations, skills and backgrounds. Which is very good.

A few years ago someone figured out that in order for pilots to effectively fly an aircraft they could quite safely adopt a simpler view. Namely: "Generators and Alternators are basically the same thing. They make electricity so the rest of the aircraft can function."

The airlines call it Need To Know, and it sure cuts down on training time. But it's always fun for some of us who like to know more to have a great teacher like Aviatrix and her commenters to explain the mysteries - often with some rather original analogies.

Thanks all.

Aviatrix said...

The need to know aspects of generators versus alternators are that

* if the battery is dead and I can hand-prop the generator-equipped airplane, I can charge the battery en route, but the alternator won't charge the battery: I have to get a jump.

* if I run the generator-equipped bird too long on the ground I may lose electrical services, but the alternator should have pretty much recharged the battery by the time I'm through the run-up.

cockney steve said...

HMMM.- Every alternator I've come across so far, has sufficient residual magnetism to produce a self-excitation of the field...once ths starts generating,the regulator should allow the battery in-circuit.

A slight diversion on what's already been stated- Stators are wound to give 3-phase output. Usual failure-mode is a phase going down due to a failed winding or diode-set. You'll still get a reduced output at reduced voltage, probably enough to run the basics needed to get you down safely.