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Dual alternator setup and master contactor failure.
Hi,
I'm planning an upgrade to an all-glass setup, so I've been studying electrical systems in general and AeroElectric connection in general. I think I'll go with Z-12 (dual alternator, single battery setup). A diagram can be found here: http://www.aeroelectric.com/PPS/Adob..._Pdfs/Z12M.pdf It seems robust enough, but I have a question the answer to which I couldn't find. What if the master contactor gets disconnected in flight? It can happen due to a failure in the contactor itself, or in any wiring around it. In this case both alternators get disconnected from the battery. Many people say that the alternators will still work with some ripple in the output voltage. But it is also plausible to think that OV protection will take both alternators out without any possibility of restarting them. It seems like this failure can defeat dual-alternator setup with a single failure around master contactor. Also with automatic alternator failover setup (where backup alternator does not kick in until bus voltage drops below a certain voltage) is it possible that ripple amplitude will be large enough to trigger backup alternator? What would the consequences of this be? Assuming alternators keep working after master is disconnected, everything is fine, but how do I test it without potentially frying a lot of expensive avionics? |
I'd suggest that the better place to ask the question is at the source; the designer monitors that list almost every day. But as a FWIW, I think that a perusal of the B&C Specialty page will show the same products are available with a PMA sticker for a bit more money. Odds are good that earning certification would have involved answering the questions about interaction between the primary & backup systems.
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I wouldn?t think boh alternators would be in use at the same time but I don?t have Z12 in front of me at the moment. Are you planning to split the load between two alternators running simulataniously, or is one alternator a backup? My second alternator is a backup and not on until I turn it on.
Bevan 2 alternators, 1 battery RV7A |
Good luck asking the list about that. I noticed the same contactor issue and when I asked I got some rather conflicting information. Contractors are simultaneously extremely robust and not going to fail but also will corrode significantly and measure like turning them upside down and drilling weep holes is a near requirement...
From discussions on the list alternators will work without a battery but their voltage can wander a bit with varying loads. If the alternator is heavily loaded and then disconnected from the battery it can cause a voltage spike, though unsure how bad. I've wondered myself if you could still charge the battery through an alt bus feed (ebus or main), but then your switch/relay has to be able to handle the current.. but also the alts won't be seeing the battery voltage just the bus if it's through a diode? Hmm... Ugh, now I'm back to thinking I need a secondary "alt feed" contactor... |
Both alternators are not used at the same time (I linked the Z12 schematics in my original post). However the B&C backup alternator control will turn on backup alternator field every time bug voltage drops below 13V. If primary alternator voltage oscillations are large enough it can happen at least once. Maybe turning backup alternator will stabilize the bus voltage, but maybe not?
I'm mostly interested in a scenario where both alternators are taken down by their OV protection (first the primary one, then backup). Actually backup alternator may not even have a chance to go online depending on how fast OV protection shuts down the primary. I wonder how to test what happens without potentially exposing electronics to harmful voltages (in case OV fails to catch the spike in time). |
I don?t think planning for multiple failures on the same flight is a thing. My theory is to make everything as robust as I can. Check and maintain everything just as well. If something goes south in flight, I will deal with it and divert (or possibly carry on to destination depending on the nature of the failure), and repair once one the ground. One back up plan for each scenario is about as good as I?m gonna get with this little non-commercial airplane. If I was to make backups for the backups, it would be extra heavy and take way longer for little additional benefit.
The main alternator is the work horse. The backup alternator will be activated by me within a minute of main alternator failure keeping the battery (limited electron reservoir) in reserve for landing loads (landing lights etc). That coupled with load shedding will get me to my destination without breaking a sweat. YMMV |
But my point is that in Z-12 master contactor failure can take out both alternators via OV protection, leaving just the battery. This is not catastrophic, but not ideal either.
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Has anyone contacted Bob about these configurations?
I ask because he designed them just as or before modern EFIS displays were coming out. Now that they all have battery backup options, the complexity of two alternators/batteries may no longer be necessary. |
Aeroelectric List FAQs
I found these FAQ's put together by Louie Holt of Florida http://www.homebuiltexperimental.com...c-List_FAQ.pdf
And so I won't lose that document I put it here https://drive.google.com/file/d/1IKA...ew?usp=sharing I learned a lot by searching for all instances of "Z-12" and "Z-14". It's all worth reading but to zero in on this thread search for "Loss of Batt Contactor in Fig Z-12" I wish I knew what Z-14A is. The FAQs linked above are rev 01/2003. The latest Aeroelectric Connection has different revs or no rev listed depending on section; Appendix Z, where the electrical diagrams are, is rev 12A3 of 03/10/2009. Look for the free download at http://www.aeroelectric.com/Catalog/pub/pub.html |
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