Battery, Generator, Alternator...

[vart]

Hello Guys,

Recently I have been flying the AT-3 R100. It has a Bombardier Rotax 912S engine as some of the very light aircraft have, like the Tecnam and the DV20 Katana.
My question is "WHY the AT3 has both a generator AND an alternator?" Why not just an alternator?
Why do I need to switch ON the alternator before engine start?
Tip: The engine can start with only the BAT ON.
I tried to find the answer in the POH, in forums, in the Rotax website and others but with no luck.

Any help will be greatly appreciated!

[Uplinker]

vart,

I don't know the answers to the specific questions you're asking, but if an alternator is rotating, it should always be connected to a battery. This is because an alternator needs a voltage reference to prevent it's output voltage going too high and blowing all the diodes in it's diode bridge that converts the 3 phase AC to 'ripple' DC.

If an alternator spins without a voltage reference, residual magnetism in the exciter winding can generate enough voltage in the output windings to cause the above.

[vart]

Thanks for trying to answer my question but I posted the question a bit wrong my bad... And the question is "Why do I need to switch ON the generator before engine start?" The alternator is switched ON after engine start according to the checklist.

I started the engine without the generator switched ON (BAT ON only) and the engine started normally. So I am really confused on what is the purpose of the generator on that aircraft.

[riverrock83]

I don't know anything about that aircraft, but an alternator requires in initial charge before it can start working. Normally that will come from the battery. However it does mean that if it is turned on, and the engine isn't turned on, it is eating up some of the battery's output. Therefore to get maximum power from the battery at startup, an alternator is traditionally left until the engine is started.
A generator doesn't need an initial charge so doesn't have that limitation.
However a generator isn't as efficient and is likely to require more maintenance.

No idea why that aircraft has both.

See a short description here: The differences between a Alternator and a Generator | Ask a Flight Instructor (http://www.askacfi.com/559/generator-vs-an-alternator.htm)

[vart]

Thanks for the replies guys.
It looks like this is one of those questions which stay unanswered :bored:
The more I try to find the answer the more I come to the conclusion that
the designer had some free space on the panel so he placed a switch and a
bulb and named it generator :ugh:

[Uplinker]

I am guessing, but maybe the generator on your aircraft is a back-up stand alone DC source used to run vital standby instruments in the event of a major electrical failure of the alternator and battery?

Or does your aircraft have electronic displays only, which if the normal DC power failed would leave you with nothing - hence the need for a back-up DC source?

I would need to see the aircraft's electrical schematic to confirm this - can you print it here?

[crippen]

if I am allowed to butt in

An alternator always supplies A.C. If a diode pack is fitted,this changes the A.C. to D.C. Does the 'plane require any A.C. supplies?

[vart]

The AT3 is a very light aircraft used mainly for training. So.. no electronic displays, standby instruments etc. Just the six primary instruments of which the AI, TC and DI are electric.

[bingofuel]

Is it possible the generator is part of the basic engine design, whereas the alternator is a bolt on ancillary, fitted when required depending on other extras installed on the aircraft?


Check the max output of the generator, possibly insufficient for the particular installation so extra generating capacity is required.

[Capt Pit Bull]

by any chance, is the generator also the starter motor?

[barrel]

What do you have to switch on, alternator or generator?

If it's the first one, an alternator has to be disconnected when not used because it discharges the battery. In cars it is made when you turn the key before starter receives energy from the battery, that's similar.

If you have to switch on generator, it could be to let energy reach it because it is an starter/generator, it moves the engine in the starting and then it is disconnected and used as a DC generator.

I don't know why it's not made automatically.

[vart]

No idea... It doesn't say anything in the manual about generator being the starter.

I have to switch on the generator before engine start.

[italia458]

It's possible that the alternator doesn't have a diode bridge (bridge rectifier) to convert the AC to DC (or any rectifier for that matter). I assume it uses the alternator to power AC items and the generator to power DC items in the plane.

It doesn't make sense why they wouldn't just add a diode bridge to the alternator. Like some people have said it's possible the starter is also the generator and therefore you don't need a diode bridge. Still a little weird.

[rudderrudderrat]

Hi vart,
And the question is "Why do I need to switch ON the generator before engine start?"
If you read:http://www.rotax-aircraft-engines.com/portaldata/5/dokus/d04670.pdf,
7.4 Electric System: "Two independent charging coils, located on the generator stator, supply one ignition circuit each."

Your generator appears to be an essential part of the ignition circuit in order to keep the capacitors fully charged.
The Alternator is an optional extra for high load electrics.

[Uplinker]

OK. I've had a look at the Rotax manual.

From what I can gather; the engine fitted to your aircraft has two sources of electrical power. The manual says they are both alternators. One; the 'internal generator' is fitted to supply power and triggering for the engine ignition, and an engine rev-counter. This alternator can supply 20 amps and the text says it has an external diode bridge to convert the 3 phase AC to DC. I should imagine that this "internal generator" is 'self exciting', i.e. needs no battery to operate. In other words, it's a bit like a magneto, and makes the engine electrically self contained.

The circuit around the "internal generator" also includes switches to cut the ignition circuits. It is these that would need to be switched on in order for the engine to start, and conversly how the engine is stopped - by switching them off. For aircraft which just need engine power to drive a propellor, this is all that is required. (e.g.powered hang gliders?)

If an aircraft has instrumentation which needs powering, and perhaps an electric starter; a battery and a second, "external alternator" can be fitted. This is similar (but smaller - only 40 amp) to the unit found on a car and has an internal diode bridge to convert the AC to DC for any circuits 'external' to the engine.

That's as much as I can glean. I hope it answers your question, and of course I stand to be corrected by any Rotax experts.

U

[Uplinker]

Don't mention it.

U