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RV-12iS EMS and generator operation

kshunz

I'm New Here
We have two new RV-12iS aircraft coming soon to our flying club. I've never flown one, but as the Ops Officer I need to understand them.

I believe there's a switch on the upper console marked "EMS Backup Battery". As far as I can tell, there's no backup battery; this switch allows the main aircraft battery to power the ignition in case both generators fail. Do I have that right?

If Generator A fails, Generator B powers the EMS but the rest of the airplane is running off the main battery. If the battery voltage drops too low to run the fuel pumps, the engine stops. Correct?

The POH advises to turn off non-essential electrical equipment with a generator failure. I wonder if that ought to include one of the fuel pumps.

If both generators fail, I assume the engine stops until you flip on the EMS Backup Battery switch and then restart the engine.

These will each have a G3X display, and I understand there's an actual backup battery for that. But if aircraft power is lost, the fuel pumps stop and the engine stops. So this backup battery is just to keep the EFIS running during the final glide? This makes me wonder whether there's something I don't understand.

Can someone who knows this system confirm that I'm on the right track?
 
Pilots should never takeoff with a weak battery. The battery should be load tested annually.

I installed new PC680 last year (RV-12 w/ 912ULS) and it lasted about 8 months. On the day it failed, engine started as usual followed by 30-minute flight with shutdown. No cranking for restart. Odyssey replaced battery under warranty. Everything looks good....until it doesn't....
 
https://youtu.be/xo2JTX2Rwk8

Here’s a great explanation, with wonderful graphics.
Yes, a 915, but I believe this is identical to the 912.

Great video, and if it applies to the 912iS then I think it points out an error in my understanding. It shows the fuel pumps being supplied directly by the generators. With a failure of either generator, you might run the main battery dead but the pumps (and engine) will continue to run. The POH procedures and the EFIS backup battery provision make a lot more sense with this insight.

Thanks!
Scott
 
There is a backup battery. See the attached schematic.

-Jerald
 

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You seem to have a good understanding of how the system works.

If Generator A fails, Generator B powers the EMS but the rest of the airplane is running off the main battery. If the battery voltage drops too low to run the fuel pumps, the engine stops. Correct?
The pumps are not powered how you'd probably normally think. Think of them as part of the ECU/EMS. They would continue to be powered by Gen B.

The POH advises to turn off non-essential electrical equipment with a generator failure. I wonder if that ought to include one of the fuel pumps.
Definitely leave both running. They're being powered by Gen B (which is now dedicated to running the engine only at this point, after Gen A died). Everything else in the aircraft is now running off battery -- so kill things like landing lights, nav lights, USB power, etc. as safety permits.

If both generators fail, I assume the engine stops until you flip on the EMS Backup Battery switch and then restart the engine.
Exactly. Only at this point, would I even think about turning off one fuel pump -- and that would be in some ridiculous circumstance where the next suitable field is far away and I'm trying to conserve as much power as possible to make sure I can get there.

But if aircraft power is lost, the fuel pumps stop and the engine stops. So this backup battery is just to keep the EFIS running during the final glide? This makes me wonder whether there's something I don't understand.
If Gen A, Gen B dies, and you activate the EMS Battery Backup switch and restart the engine... Eventually your main battery will die. You'll have the EFIS battery to rely on for the rest of the way down.

You should realize too, that there are other ways in which the EFIS battery may be useful. Suppose there is a short on the circuit which is powering "EFIS" (PFD, MFD, AHARS, Magetometer).. that would take out all the displays. In that case, the EFIS battery could be used to power the PFD/AHARS using a separate power line that wouldn't be shorted.

I also need to mention here that I brought a wiring bug up to the attention of Support that hopefully will be fixed through a SB soon (my proposed fix to them was a very simple 1 hour job, so nobody panic!). In this last case I mention, the EFIS battery actually fails to work at all.
 
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Welcome to VAF Scott. I think you will find that the 12iS is a great little airplane. Where is your flying club?
 
I have a dumb question prompted by a question I was asked by a friend: the 912iS engine's electrical system is powered by 2 generators correct? It sounds like it is possible to install external alternators if desired based upon this video:

https://www.youtube.com/watch?v=vgtq289Wj_Q

I was also wondering if there's a diagram that shows where on the 912iS the generators are located. Google so far has failed me.

When I answered my friend's question they pushed back with, "no new engines are using generators any longer, yours has to have an alternator..."
 
See the Rotax 912iS Heavy MM, chapter 24-20-00 which you can download from rotax-owner.com.:)
 
I'm curious... Since generators can operate in 'reverse,' acting as motors and therefore as generator-starters, why does the engine have a conventional starter motor?
 
Rotax has alternator built into field-windings on back of engine case. Not the same as Tesla. My Prius however, doesn't have starter motor. Main drive motor (dynamic brake) back-drives the Internal Combustion Engine (ICE) to start it. ICE starts / stops dozens of times when driving....
 
So Lockwood just told me that they are dynamos feeding AC into combined rectifier/voltage regulators. But when I google dynamo it says they produce DC, not AC. :confused:

The article below says magnetos (AC), dynamos (DC), and alternators (AC) are all types of generators. If our engines are producing AC, then the conclusion is that they must use either magnetos or alternators. Lockwood said there are no commutators, which would mean they must be magnetos... My suspicion is that they are dynamos feeding DC to the rectifier-regulators, and the Lockwood tech just got that detail wrong. If that's the case, then why use a rectifier at all? To protect against reverse polarity?

https://www.linkedin.com/pulse/diff...siekierzyński?trk=public_profile_article_view

(Sorry if this is thread hijacking, but I hope it's pertinent)
 
I have a dumb question prompted by a question I was asked by a friend: the 912iS engine's electrical system is powered by 2 generators correct? It sounds like it is possible to install external alternators if desired based upon this video:

Yes, See the Rotax Installation Manual, Chapter 24-00-00, Page 17. Page 33 has a schematic of how you'd integrate it with the rest of your aircraft. Its belt-driven and completely separate from the Fuse Box.. so you can think of it as an alternator in a traditional aircraft engine.

I think people use the terms generator and alternator a little loosely. See here for an explanation. Someone smarter than me will have to say if the Rotax internal "generators" are indeed true "generators". All I know is that they output AC and the external rectifiers mounted on the Fuse Box convert it to DC.
 

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As rcarsey states, the terms get used loosely. This article attempts to explain the terms, https://en.wikipedia.org/wiki/Electric_generator

Rotax 912, 914 and 912iS use similar power generating systems. They consist of multiple stationary coils mounted to the engine and magnets mounted on the crankshaft that rotate within the coils. This produces AC voltage. The AC voltage is delivered to an external rectifier/regulator where it is converted to DC.

The coil assemblies are limited in the amount of current (amps) they can create and the rectifier/regulator is also limited in the amount of current (amps) it can produce/control.

The output of the coils varies with the rpm of the magnets past the coils, which is why a Rotax at idle produces very little electrical energy. It starts to produce significant energy above about 2500 rpm.

Hope this helps,

Jim Butcher
 
So much reading today, but very interesting and enlightening.

Here's my current understanding:

Generator: Any device that converts mechanical (or chemical) energy into electrical energy. https://en.wikipedia.org/wiki/Electric_generator

Dynamo: A generator that produces pulsing direct current through the use of a commutator. https://en.wikipedia.org/wiki/Dynamo

Alternator: A generator that produces alternating current. https://en.wikipedia.org/wiki/Alternator

Magneto: (permanent magnet synchronous generator) An alternator that uses rotating permanent magnets (ilo field coils). https://en.wikipedia.org/wiki/Magneto


So it seems to me that our engine uses internal magentos, which are a type of alternator, which in turn are a type of generator. So everyone is right! (Except the dynamo guy)

Yes, no? Certainly makes sense to me.

P.S. After Rob's post, I went and looked at the wires coming into the voltage regulator from the 'generator,' and there are three, so 3-phase AC, yeah?
 
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