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Dual alternator setup and master contactor failure.

kaa

Well Known Member
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/Adobe_Architecture_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.
 
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...
 
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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.
 
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/UsefulDocs/AeroElectric-List_FAQ.pdf

And so I won't lose that document I put it here https://drive.google.com/file/d/1IKASzTtuTEgLHuL-CG8MWQz5LLbC-DDc/view?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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I found this http://www.homebuiltexperimental.com/UsefulDocs/AeroElectric-List_FAQ.pdf

And so I won't lose it I put it here https://drive.google.com/file/d/1IKASzTtuTEgLHuL-CG8MWQz5LLbC-DDc/view?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.

A lot has happened since January 2003 and apparently even Bob's opinions have changed, be careful about taking an excerpt from an archive as gospel.
 
Kaa,

Do I have this correct? Your concern is that the master relay fails taking the battery out of the system. The resulting undamped alternator voltage may? go over 16 volts and activate the overvoltage protection and shut down the alternator. The backup alternator will not come on as there is now no voltage in the system to excite the field circuit (battery out and main alternator out).

I now have a B&C alternator with it’s external regulator but previously had an internally regulated alternator with built in OV protection which I inadvertently operated with the Master switch off. I can’t recall the voltage but the alternator kept working and the only noticeable difference was some background noise in the headset.

I do have an alternate feed to the buss. It comes directly off the battery through a 50 amp fuse (to protect the wire) to a CB toggle switch to the buss. The only apparent downside is that the fuse will blow if you accidentally try to start the engine through the feed circuit :eek:.

Fin
9A
 
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Thanks everyone for comments!

Yes, this was my concern. Is there any downside to connecting the backup field alternator to the e-bus, and its output to the battery side of master contactor? Like this:
uc
 
Thanks everyone for comments!

Yes, this was my concern. Is there any downside to connecting the backup field alternator to the e-bus, and its output to the battery side of master contactor? Like this:
uc

Could drain the battery through the B lead when the engine is not running if there is a bad diode in the alternator?

Fin 9A
 
How close are you cutting it on your E-bus load, to alternator capacity? If you can tolerate adding the ~2A load of the master contactor, why not re-arrange things so the 'main' bus feeds your essentials, and the 'E' bus becomes the 'load shed' bus?

For a little history, the reason the E bus came to exist was to eliminate *everything* non-essential, including even the master contactor, to stretch battery-only operation as long as possible. When the 1st vacuum pad mounted alternator came out, it was the little 8A PM model, so electron management was still an issue requiring an E bus.

Now if you have 20A-30A available from the backup alt, you might be able to keep the 'main' bus alive with the contactor and just 'load shed' the non essential stuff that would be on the (for lack of a better term) 'non-E' bus. If your total load was close to the backup alt's capacity, you could eliminate the E bus completely.

My build has the luxury of 2 identical alts. I won't need to shed load (either alt can tote the full load), so I have no E bus. I do have an 'airframe bus' and an 'engine bus' for my electron dependent engine (fed from both the battery and from the airframe bus and airframe bus).

Charlie
 
Thanks everyone for comments!

Yes, this was my concern. Is there any downside to connecting the backup field alternator to the e-bus, and its output to the battery side of master contactor? Like this:
uc

Why not connect both the alternator field and output to the e-bus? If the master relay fails, close the alternate feed switch and turn on the back-up alternator. But you will not have a way to power to the main bus with failed main relay.
 
Yes, it is possible to have the master contractor fail. I?ve had it happen to one of my customers.
On my airplane I run a separate properly-rated switch directly from the battery through a 20 amp fuse to power the avionics, and i labeled it emergency power.


So far I?ve never needed to use it, but it is there for the very scenario you describe.

I do replace my aircraft master solenoid at 1000 hours. Probably overkill, but for $25 it is piece of mind for me on an aircraft I need to be reliable.

Vic
 
If you're interested in a better quality contactor I went with the GigaVac MX11, about 4x the price of the cole hearse unit though.
 
Why have a master contactor at all?

I got rid of my master contactor and replaced it with a >full load rated marine battery switch. These things are effectively indestructible, and don't draw any load. I have a 2-battery/1 alternator set up and there are switches for that, too.
Andy
 
I?ve used Blue Sea 9012 solenoids for the master relay function on the last two projects. Very good quality, draws 3.6 amps when changing state but only 0.13 amp for holding current. One of these on each battery. If you shop around you can find these for under $100.

I have four 30 amp relays that feed power from each battery to the panel, a primary on each battery to feed half the panel, and an alternate on each battery (as in left battery feeding the right side of the panel, etc.). The 30 amp relays draw around 100ma.

Carl
 
I got rid of my master contactor and replaced it with a >full load rated marine battery switch. These things are effectively indestructible, and don't draw any load. I have a 2-battery/1 alternator set up and there are switches for that, too.
Andy

Is the switch you used, located very close to the battery?

If not, and the battery cable passes through any structure (such as the firewall) to get to your switch, you have a rather large current capable wire connected to your battery that you are not able to de-energize if that portion of the wire path became shorted to ground.

This is the reason master relays are typically used.... that the battery is often located quite a distance from the main power bus or other devices (starter motor) and having one gives the pilot the control to de-energize all heavy cable down stream from the battery with the exception of the short cable that links the battery to the relay.
 
This thread has me thinking. I'm planning dual batteries with single alternator when/if I upgrade to full IFR. My question is this. With dual batteries and each one with it's own contactor that then join up next at the starter solenoid.... if one or the other contacter failed, how would you know? Seems everything would keep right on purring along. If the OTHER contactor or battery failed it would then become obvious.
 
This thread has me thinking. I'm planning dual batteries with single alternator when/if I upgrade to full IFR. My question is this. With dual batteries and each one with it's own contactor that then join up next at the starter solenoid.... if one or the other contacter failed, how would you know? Seems everything would keep right on purring along. If the OTHER contactor or battery failed it would then become obvious.

If one of the master solenoids failed in flight, and you took the step to have voltage monitored from each battery on your EMS, the battery with the failed master would be disconnected from the alternator. This will become evident with that battery voltage dropping from the normal alternator output voltage. This assumes you are not cross connecting the batteries in some other way - which I strongly recommend not doing as this could create a path for one fault to take out both batteries. This leads to powering your panel via stand alone relays connected to each battery instead of a common buss off the master solenoids. I have only non-vital loads off this common buss - like pitot heat, landing lights, nav/strobes, starter circuit and such.

Carl
 
Thanks everyone for comments. Looks like I'll have enough capacity of the backup alternator to merge both buses. I also realized that I already have an essentially backup contactor - right now it's used as a contactor for GPU connection. My plan now is to use it as a redundant master connector by connecting battery to it and wiring a switch to turn it on from the cockpit. This way I can avoid having an essential bus and high current switch in the cockpit. I haven't seen this arrangement before though, any thoughts on why it may not work?

Schematics is here: https://drive.google.com/uc?export=view&id=1IhYHdHQH50fWUpo0H0Zc790CtxwnM7HR
 
Konstantin,

Your power distribution scheme achieves two things over a standard design:
- Mitigates the loss of alternator risk
- Mitigates the loss of a single master solenoid risk

It still leaves the less likely, but more severe outcome risks. Look at a fault on the main buss - your immediate action will be to open the master solenoid, but the fault will reappear when you shut the second solenoid. You are left with no panel. Another example using your specific design is a high resistance contact on the battery side of the (left) master. You now have no path to power your main buss.

If you insist on a single battery design, recommend you take the step of powering your panel via two avionics relays or switches to the battery, half of the panel on each. That way a fault on one avionics buss will not take down the other (as in EFIS #1, COMM #1, NAV #1 on one buss, EFIS #2, COMM #2 and NAV #2 on the other buss). Doing so you can eliminate the second master solenoid. Such 30amp relays are cheap.

Keep all your non-IFR critical loads on the main buss from the now single master.

Carl
 
I get the desire to have redundant redundancy, but how far should it be taken? Assuming proper downstream circuit protection (fuses/CBs on the bus), what fault could take down an entire bus? It feels like we're getting into the realm of carrying a spare wing spar....
 
Thanks.

What kind of bus fault are we talking about here? The only thing I can think of is that the main power wire from the master solenoid to the bus fails somehow, or the fuse block fails. How likely is that? These are all beefy wires and bolted connections.

I'll also have a battery in the D5, btw.
 
What can take down a single main buss?

- The feed wire to the common buss fatigues off at one of the connections.
- A high resistance common contact in the buss supply line, or a high resistance contact that ends up melting, or arcs shut. The latter happened to a neighbor?s Mooney gear motor resulting in a gear up landing after flying off an hour of fuel over the Everglades, in the dark with a dark panel. Luckily the cell phone worked to clear the runway for the belly landing.
- The 1960 approach of using a bare copper bar to tie in all the circuit breakers - then you find the screw that you lost behind the panel or that vibrated out of something during turbulence and it will of course short out the nice bare copper bar to the panel (ground). The smoke coming from behind the panel will be the tip off to open the master solenoid.

Point - I do not trust any single power or ground connection, be it a master solenoid or master solenoid terminals, either battery terminal or a feed wire to a buss. I offer these are low probability risks, but they result in severe consequences. Considering is it simple to avoid these and other single failure pitfalls I ask why not?

And the kicker - most people have dual everything these days in the panel. Why not dual and separate power to each side so that the worst case is you loose half the panel? This can be done with just a little wire and a couple of $5 relays.

Note - no extra wing spar required or desired.

Carl
 
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