Generators versus Alternators

[Pugh]

The Diamond DA-20-C1 Eclipse I fly has a generator instead of an alternator. I thought that most GA planes these days use alternators instead because they don't need a service program and they are lighter. This is a brand new plane, so what gives?

 

[TR4A]

Diamond Aircraft says they come with alternators.

ELECTRICAL / AVIONICS SYSTEM
12V, 40A automotive style alternator

http://www.diamondair.com/contentc/Eclipse.htm

 

[avbug]

What service program is required for a generator? None, specifically. What service program is required for an alternator? None, specifically. Refer to the particular item for AD's or manufacturer maintenance publications...but if it's being operated Part 91, it's an on-condition item regardless of weather it's a generator or alternator.

From your perspective, you wouldn't know the difference in the cockpit. An alternator puts out stable voltages at lower RPM's, whereas a generator output increases with RPM and may be unstable or unreliable at lower end minimum values. Alternators are generally lighter, but usually not designed for the large capacities that generators can handle, for various reasons. Today, most light airplanes use alternators (not all), and most large aircraft with high electrical demands use generators. (Also, in many cases for weight issues, as the generator first functions as the starter during engine start, and is called a starter-generator).

Often in aircraft literature, the electrical generating device is referred to as a generator (correctly so), regardless of weather it's a "generator" or an "alternator." This may be confusing you. It's generating DC current output,and that's all you need to care about. The alternator generates alternating current which is filtered through a rectifier (usually internal diodes) to allow only a DC output...but the result is the same to you, the user. You get DC power to run your electrical system.

Pilot handbooks do sometimes confuse the terms, and correctly refer to the electrical generating device as a generator, even if it's an alternator. If you want specifics on the device, go look on the airplane to see what you've got, and review the aircraft maintenance publications. Some light airplanes are type certificated with a generator, but outfitted via STC at a later date with an alternator (Cessna 140, for example). This mostly applies to older aircraft.

For a quick check, visit your airplane on the line and look. If it's got a clunky black solid looking tube with a few small square holes at the back, it's probably a generator. If it looks like a little silver bird cage with lots of long holes and wires and spinning things inside, you're probably looking at an automotive type alternator.

 

[gern_blanston]

Yep. It's an A/C (Alternating Current) generator, rectified to give you D/C.

 

[Pugh]

Thanks for the replies. From the POH it says "the 40 amp generator is attached to the engine near the propeller hub." Also, the master switch in the cockpit it labelled GEN/BAT, and there is a red warning light that says "GEN" if the generator is not on.

 

[avbug]

It is indeed a generator...but an alternator and a "generator" are both generators. Remember that POH's and AFM's are dumbed down and watered down...all you need to know is that you have something up front generating electricity. There's nothing you can do about it, you can't fix it, you can't adjust it, you can only monitor it. It generates electricity, and therefore is your generator. Weather it's producing alternating current that's filtered (rectified) for DC, or producing straight DC, is beside the point, with respect to your POH.

I believe that's the point of the statements in your POH, and the labeling on the panel.

As you brought up the warning light, what is it telling you? Low voltage? Switch not on? Overvoltage? Reverse current? Could be any of those things...but from the pilot perspective, either it's working, or it's not. Right?

 

[Pugh]

You're right, in the end it's still electricity, but my curiousity was spurred when I was told that most GA made today planes use alternators. As far as the warning light is concerned, it is only on when the GEN switch is in the off position.

 

[EagleRJ]

As you brought up the warning light, what is it telling you? Low voltage? Switch not on? Overvoltage? Reverse current? Could be any of those things...but from the pilot perspective, either it's working, or it's not. Right?

Understanding exactly what your aircraft's warning lights are telling you is vital to successful diagnosis of malfunctions. The same goes for understanding exactly what you are testing when you push a 'Press to Test' button. The same light label might be telling you different things in two different aircraft, and knowing what the problem is might be important if you need to fix it airborne (like needing to restore a source of electric power besides the battery).
I believe most alternator warning lights in GA aircraft are simple undervolt indications. Since system voltage will be just over 12V on battery power and just over 14V with a generator or alternator operating, it's an easy way to tell that the electrical generation has stopped.

Another important difference between generators and alternators is the point from which the power is drawn. Generators produce their power in the rotating coil (the rotor), which needs to be transported onto the case with brushes that ride on a segmented slip ring (called a commutator). Since the brushes need to transmit the full amperage of the generator, the generator's reliability is not as high as an alternator.
An alternator excites the field in the rotor with power brought in from the non-rotating coil (the stator) through brushes riding on a slip ring. The primary output comes from the three fields in the stator, which is then converted to DC in the voltage rectifier. Since the only power being transmitted through brushes and slip rings is the power needed to energize the rotor, alternators last a lot longer.

 

[TR4A]

One of main reasons for an alternator in recip aircraft is they will produce power at low RPM's which a generator doesn't. They maybe labling it as generator but it is an alternator.