They are essentially the same thing, but the alternator is an engineering advancement over the generator. A DC generator's output power is determined by the rate at which it is turned. In other words, if you are sitting on the taxiway waiting to take off, you are producing less power than you will in flight. Your battery can actually drain if you don't turn enough RPMs.
The AC alternator can produce about the same amount of output power over the spectrum of available RPMs. That AC power is then converted to DC through a series of diodes to charge the systems in your aircraft. Think of these as one-way check valves which allow electrical flow in one direction only. When the alternator is working properly, a certain amount of the charge is usually "reserved" for charging the battery.
I'm a pilot, not an electrician. So that's about the best I can do for you. Perhaps someone else can expound on this.
Simply put, a generator creates direct current (DC) and an alternator creates alternating current (AC). A generator consists of a fixed magnet and rotating coils of wire within the magnetic field. The DC current is then regulated and sent to the voltage buss. A disadvantage of a DC generator is its dependency on a narrow rotational speed range to maintain a consistant voltage. The alternator (or AC generator) consists of a rotating magnet within fixed coils of wire. This type of generator requires a rectifier to make DC from AC but is relatively free from the speed limitations suffered by the DC generator. Hope this helps.
The way it was explained to me is like this (I am also a pilot- not an electrician so please correct me if I'm wrong) Newer Aircraft use alternators to supply electrical power over generators for the above mentioned reasons concerning rpm's.
The alternators produce AC (alternating current-hence the name alternator) Yet there is no way to ground an aircraft so the power needs to be converted to DC (direct current) DC current flows in one direction you either use at the end or it dissipates and is lost. It also requires no ground-it travels direct.
To use DC current in something like your wall socket in your house would be extremely wasteful, so AC is used. It will provide power on demand, but inorder to use AC it has to be grounded (somewhere in every building a large metal spike is stuck into the ground which provides the ground) Its not very practical to fly around with a cable trailling along behind us connected to a metal spike in the ground, so converting the AC power (more stable power to produce) to DC (power that can actually be used by the plane) is required. Your car works the same way.
Kind of confusing!
AC power can definitely be used in aircraft. I am not an engineer either, but the distinction as to whether or not something uses AC or DC power is based loosely on the type of load that it draws. Generally speaking higher load items use AC power and lower load items use DC. Lots of exceptions but generally true. Beyond that the distinction is basically what the consumer was designed to use. Whether you have alternators or generators on your airplane is a function of what's required to meet all the needs of the electrical system. My airplane uses 3 DC generators, 2 AC Alternators, two inverters (DC to AC), and one TRU (AC to DC). Seems complicated but in practice it actually works well.
I am a pilot. I have a degree in how airplanes are built and fixed basically. I get my A&P in a week. I have recently been employed in an electrical engineering position (actually, my contract ended just today).
The difference is in what spins and what is stationary. A generator has a coil of wire that spins inside a magnet. An Alternator has a stationary coil, but a spinning magnet.
But here's where that AC/DC stuff comes in (hehe). The coil of wire is what produces the current, due to the relative movement between the coil and the magnet (the rotation due to the engine). Now, because the magnet has both north and south poles, basically the current induced on the coil is going one direction for half of a turn, and the other way for the other half. This is basic AC electricity. But we'd generally prefer to use DC in light airplanes, so...
Because a generator has a rotating coil, they use a system of commutators, rings and brushes such that, say, the left brush is always picking up the positive side of the coil, and the other is picking up the negative. But that commutator IS acting as a rectifier. It is taking the alternating current applied to the coil, and changing it to DC before any wire leaves the generator.
The alternator does basically the same thing, but because the coil is stationary, the output of the coil is AC and a mechanical rectifier such as the communator, ring and brush system on a generator won't work. So, the output is put through what amounts to a simple bridge rectifier (two wires go in, AC, two wires come out, DC) to produce that DC current we all know and love.
So that's a quick and dirty way of saying: the alternator is a next generation generator. They are lighter to produce the same power, and they are FAR more reliable due to the fact that they rectify the current using diodes (non moving electrical parts) as opposed to a communator and brush which is constantly rubbing against one another.
As to the use of DC versus AC... Basically AC is far more efficient to use because it can be efficiently transformed (by what is commonly referred to as a TRU) to different voltages and currents (why your house uses it, that and politics). But, DC is simpler to use because it doesn't have all of these cycles and such. If you've got two engines producing AC, the generators have to be put in phase with one another because as we all remember from physics, when a high wave and a low wave collide, they cancel one another out! Also, You can't store AC in batteries.
But even if your AC bus powers something, more than likely it is rectified internally into DC, because digital electronics pretty much all rely on a steady DC current, and unless we're talking about BIG motors, DC is more than good enough.
Hey Dan, you really seem to know what youre talking about.
Let me ask you if there is any truth to my original post about not being able to ground an aircraft and that requires the use of DC power.
The reason I ask is that I got kind of beat up by an instructor in a stage check over this question about 6 months ago. He explained it almost word for word as I did above.
I see the guy regularly and would gain no small pleasure from knocking him off his high horse a little by proving he doesn't know everything!
Thanks, I stayed at a Holiday Inn Express last night.
Actually, when I read your post talking about being unable to ground and stuff--I thought to myself "whattheheckishetalkinbout"
You can't really mix AC and DC together, because the AC waveform would be constantly messing with that supposed-to-be constant DC voltage. So that is why we rectify AC, so that we can have DC (because DC is a less efficeint, but much simpler form of electrical power)...
Think about that basic, basic circuit in any electronics textbook, a battery, a resister, and two wires to connect them. In DC, the current always flows that single direction. But in AC, the current direction is CONSTANTLY changing, one moment it goes one way, the next it reverses. But the circuit will work equally effectively in either case, with the same exact components.
A ground is really just a shortcut. When something is grounded, it means that somebody decided to be lazy for whatever reason and use a conductive structure to serve as a wire. In that basic circuit I talked about above, add a bunch of resistors in paralell. Now, even though there are many different loads, ALL of the current must still flow through the same place in two places in the circuit. Into, and out of the battery. But let's say that one resistor is in the nose of an airplane, and oen is in the tail, one is in a wing... Raise your hand if you want to run a wire both to and from each of those resistors to the battery. I am lazy at heart, so I would say to myself: "hey, aluminum can conduct electricity, why not pretend that the body of the airplane is just one big, huge wire?" So, I just connect that end of the battery to the airframe, and I connect one of the leads from every resistor to the frame as well. Basically, we've still got to have one of the wires running to everything (and this wire is what we will use to turn it on and off), but with careful design, we can make it so that EVERY component in the airplane will use the same return path to the battery. This is what the term grounded refers to on an airplane. It means it is connected to the airframe.
Look at your starter, you will see a big cable coming from a realy to it (and that relay will be connected to the battery), and probably another cable that runs directly to the frame of the airplane. That is the cable that grounds it out. Look at a nav light, it will have one wire going into it (the power supply), but we all know that without some sort of return path, nothing will work, so we use the airframe as a return path.
As to that thing about the ground in your house--that is connected to that odd pin on your standard Edison socket. That is there to protect you by always providing a path for electricity to go to the earth (which, in the grand scheme of things is where it would always rather go). This way, if you were to have, say, a drill that was shorted out internally (a wire coming from one of the hot connections, that touches the case), and you touch that case, without that third prong ground, the electricity sees your body as the easiest way to get back to the earth (as opposed to running through the drill motor and travelling all the way back along the power lines to the power company). So, instead of allowing it to travel through your body, they provide a very low resistance path to the ground from the case. Hooray, I don't get shocked!!!
Yes, your car works the same way as an airplane (unless you've got some of those freaky british cars that have positive grounds--wierd). The power in your house is not, in any way, connected to that pole stuck in the ground. That pole has a wire running to all of those third prongs, to the case of every outlet, to the case of every circuit breaker panel, etc... If you've got current running through it, you've got a problem.
As to that DC power travelling one way, using or it being lost... There always has to be a return path. If you've got that circuit with the resistor I talked about above, and you unhooked a terminal from the in side of the battery, you would still have an electrical potential at all points in the system, it just wouldn't be doing anything because it couldn't flow.
Hope that makes sense. I've covered about a weeks worth of class in a few hundred words...
Hey Dan! thanks that explaination clears up a lot of my confusion concerning the subject. I had thought a lot about that instructors little speal on AC/DC power and wondered how accurate it really was....
He may not of taught me much about aircraft electrical power that day but in the end he did teach me something about being an instructor......if you give your student BS answers or explainations you will lose any respect they have for you!
Generators typically start producing power ~1,000 rpm, higher if the generators are old and worn.
Second, generators don't require excitation voltage, meaning you could 'jump start' an airplane with completely dead batteries and the generators would produce power and charge the system.
Alternators need excitation voltage, some amount of battery voltage to polarize the field winding. This is probably the only benefit generators have over alternators.
Remember the generator operated light on your bicycle ? the light output varied with wheel speed.