Main alternator is switched in the drawing, one of those progressive toggles allowing BATT or BATT+ALT.

Hey Mike, if you ask B&C why the ALT is switched, let us know too.

My feeling is that you are overthinking this. You have a lot of components here, all of which will take time and effort to bolt to the plane, and each of which could fail in flight.

As you know, there are two schools of thought - backup alternator and backup battery. I just completed an RV-7A and went with the backup battery plan, which seems substantially simpler to me.

I have backups plugged into my Dynon EFIS. If I lose the alternator, I have more than half an hour of power from the main battery, and then an hour more on each of the displays if need be. It's hard to think of a scenario where I couldn't declare an emergency and get onto the ground in half an hour, even in IMC.

You've also got a clearance delivery switch but no avionics master switch. Maybe I'm missing something, but this looks nonstandard from the perspective of a pilot coming from the certified world, and might be confusing for somebody transitioning into your plane. It's unclear on first look if there is additional avionics on some other switch, or if your clearance delivery is another name for the avionics master.

My primary EFIS turns on when I turn on the master because I need it right away before I turn on any other systems.

You show the avionics ground bus being tied to the main bus with 5x 20 AWG wires. I can't see any reason why you don't want both grounds tied tightly together. Why not make them the same bus? Or if they are physically distant, connect with 8AWG (or two 8AWG if you are concerned about redundancy)?

I don't understand the clearance delivery bus being tied to the main bus via a diode and 18 AWG wire. That looks like you have no way to turn off the clearance delivery bus because of the diode. And you have 18 AWG via the diode, but 14 AWG via the clearance delivery solenoid. I would think you'd want the same wire gauge on both, sized to the maximum current the clearance delivery bus would draw.

For your pMag switch, I can speak from experience that it is wise to choose a switch that is difficult to turn off. Pick one where you have to pull it out to flip down, or put a cage over it. I believe I bumped mine off in turbulence during phase 1 testing, which was one step in a failure chain I'm still diagnosing that led to an off-airport landing.

I'm happy to send you my schematics if they'd be useful.

Enjoy!

David

sorry I missed that.. Personally I would change the switch designation to BAT+ALT or something similar in the 3rd position.

So I talked to B&C today. As far as the switch is concerned, B&C sees it as an option for personal preference as long as there is a pullable breaker installed. So the combinations would be: a regular breaker and a switch, a pullable breaker and a switch or just a pullable breaker. The key seems to be...have a way to disable the ALT. They specifically did not recommend a fuse and a switch combo (I forgot to ask them why).

I agree on the labelling...I just haven't come up with the right verbiage yet.

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Re-Done Schematic (click refresh until it loads)

So the original schematic I posted was a variation of a newer version of a Bob knuckolls architecture. After looking at my version Bob recommend I just use the original Z-12 architecture with the clearance delivery feature.

The one I posted originally had a engine bus for fully electrically dependent engines. That's why the AUX ALT was tied in directly to the battery and bypassed the battery contractor. Bob didn't think that I needed that, so the new schematic is almost a straight Z-12.

B&C recommended that I wire it up with two shunts to be able to monitor both alternators. I'm not sure exactly how this works. Bob's Z-12 schematic shows two shunts AND a Hall monitor. I'm going to have to do some more research on that.

My thoughts as a follower of Aeroelectric List / Bob Nuckols
 

My thoughts as a follower of Aeroelectric List / Bob Nuckols:

All wound field externally-regulated alternators stop producing when the field current is removed.

Attaching the backup alternator B lead to the battery is for the smoke in the cockpit scenario to keep a running alternator.

Perhaps the best place for the battery bus is forward of the firewall, and Bob shows one of his * notes on its feed meaning 6" or less. FAR 23.1361 would have 5 A breakers or fuses on wires out of the battery bus but Bob allows 7 A fuses because they are faster than breakers. Similarly, the clearance delivery bus relay should be forward of the firewall; the clearance delivery bus can be aft of the firewall because it is "protected" by the relay.

MIDI fuses ("mini" ANLs) have become available, they are smaller and can be used instead of ANLs. They are faster than ANLs though so take a look at their curves. Bob has blessed MIDIs; the smallet available is 23 A. There are also MANLs (another name for "mini" ANLs) but I personally wonder about the quality because I can't tell who makes them.

B&C regulators use a crew accessible breaker for the field because they have overvoltage protection that is subject to nuisance trips so are worth one try at a reset. When Bob shows a fuse on the field wire there is no OV protection.

Bob, who designed the B&C regulators, says the backup regulator with it's optional hall effect sensor is an unnecessary complication and cost designed for certified aircraft. BTW, if the hall effect sensor is added, it will flash a warning above 20 A despite the "20 A" alternator will deliver 32 A at cruise RPM. You can get the same effect of the alternator coming alive when the voltage drops by having two LR3Cs and turning the voltage down on one of them; I'm thinking 13.8 and 14.4 V. One reason for setting the standby alternator to a lower voltage is in case the battery is run-down you don't want to spend alternator capacity charging the battery versus keeping systems alive.

I believe Z-12 was designed for a certified aircraft. That's why it shows breakers directly on the main bus. Compare to Z-101 and others. (BTW Z-101 shows 12 awg from main bus to main alternator CB where it can be 18 awg.)

Two ammeter shunts are shown on dual alternator Z dwgs but that doesn't mean you have to install both. Voltage is a better indication of whether an alternator is supplying the current you need. Ammeters are good for diagnostics on the ground. Does your EMS support two shunts; maybe not, that's one reason Z-101 shows both B leads passing thru one hall effect sensor.

Alternator field switch: If it's not there and the engine is not running the field will draw current running down the battery and heating the alternator. I don't know exactly but maybe it will draw 2 A for the backup alternator and 4 A for the primary alternator.

Bob has renamed the endurance bus to clearance delivery on Z-101. Endurance bus is a term for systems with one alternator or with a small, 8 A, backup alternator. Bob want's to be able to energize the clearance delivery bus without closing the battery contactor which draws significant current (15 ohm coil). There are battery contactors that draw 1/4 or less the current of a legacy contactor but Bob has not adopted them.

Bob is in the one battery two alternator camp and is working on Z-101 as potential successor to Z-12 and Z-14 for OBAM and with EFI+I in mind.

Good comments John.

Mike's latest diagram moves the ALT B regulator's field, sense, and light connections to the main bus. I don't understand why.

Normal operation has ALT B ready to begin generating if main bus voltage falls below its regulator set point. So far so good.

Ok, enter the "classic smoke in the cockpit" response; open the master contactor. Given the progressive BATT-ALT master switch, the main bus is no longer connected to the battery and the primary alternator's field is open, so it also stops producing power. Main bus voltage drops. So how does that affect the ALT B system? Does it come on line in the very short time period while main bus voltage is falling to zero? Or lacking a field supply, does it just not come on line?

Note if it comes on line, it supplies power to the main bus...which is odd, given that the master contactor was opened for a reason.

Sure seems like the entire ALT B system should be a branch of the battery bus. It would still feed the main bus following ALT A failure, assuming normal ops, i.e. a closed master contactor. With an open contactor, it would be truly independent, powering the battery bus and avionics, all ballasted by the battery.

In which case an ALT-B on the battery bus would need a field switch. As an aside, it would not matter if that switch was on or off in normal flight. If on, ALT B would come on line automatically following ALT A failure or an opened contactor. If off, the pilot could fire it up at any time, as it always has a field power supply.

So is the thought that when there is smoke in the cockpit the AUX ALT stays on during the smoke event or everything off and then bring AUX ALT back on through the battery bus?

So when we think of the battery bus being forward of the firewall, do we mean the fuse block also. (sorry if this is a dumb question)


B&C told me that I didn't need the Hall sensor on the back-up regulator and the cost is the same for the LR3 and SB1B...but really, Bob seems to think a generic Ford regulator would be just fine. I guess it just the flip of a switch instead of the AUX ALT coming on automatically.


Totally missed that Z-12 had breakers instead of fuses...I couldn't see the forest for the trees :confused:
I'm into fuses where appropriate.

My GSE 24 appears to support two shunts but really all I care about is that the AUX ALT comes on line and I have some sort of a indication. The less stuff the better. (say's the man who's stuffing his panel with gadgets :rolleyes:


This I did not know...very important info.

I followed Bob N's Z-12 when I redid the schematic. I understand that with smoke in the cockpit opening the master would kill all energy production. Are we thinking that during a smoke event it would be acceptable to leave the AUX ALT online?

I'm just guessing but I think that opening the master would nix any chance that the AUX ALT coming on line (if tied the the main bus)

Sounds like a question for B&C...Monday. Aren't most AUX ALT tied to the main bus?

Totally agree, I was influenced by Bob N. when he suggested I just used Z-12...which this new design basically is.

So with the AUX ALT totally on the battery bus:

• During smoke event, master and AUX ALT field switches OFF
  
• At shutdown, master OFF and AUX ALT field switch OFF

I'm not sure I understand.
If the AUX ALT was tied to the battery bus AND the AUX ALT field switch was off/closed, isn't it true that the AUX ALT could not come on line if the main ALT failed? No energy to the AUX ALT field means no excitement, ergo no juice or have I just confused myself :confused:

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Thanks
Michael

Strongly recommend sticking with the B&C regulators - especially the SB regulator. When stuff happens having the SB alternator automatically pick up the load is well worth it. Less pilot action to fix something is preferred.

Current shunts are a left over from when cars had ammeters instead of voltmeters, but running fat wires to a 1940 instrument In the panel is not a great idea - so we have shunts. Back in the day it was cheap to build and ammeter and voltmeters were expensive. A voltmeter provides better system health information than the ammeter. Having both is a very marginal gain in information - for me it does not justify installing a shunt.

If you want to know current loads, do that with your bench power supply on the ground. The data will not change (or if it does the thing you are testing most likely already failed).

Carl

Like this Mike:



I'd put the battery bus on the cockpit side of the firewall, as shown by the purple line. There are only two high current wires passing through the firewall, one for each bus. If either high current wire or bus shorts to ground, the fault pops the respective ANL. Everything downstream of the battery bus and main bus has its own fault protection at the bus, so it's become real hard to get smoke in the cockpit...but if you do, sure, the safest choice is to switch off everything but the mags, find an airport, and sort it out on the ground.

If you build with physical separation between the main bus and battery bus systems (mostly separate firewall pass-throughs and wire routes), there's a pretty good chance you could inspect, then fly home on one of them.

Assume no shorts and popped ANLs. I'd venture an open contactor, contactor control wire, or master switch is the next most probable failure. With an open contactor, the primary alternator should stay live. If it didn't, ALT B (with switch on) would go live, charging the battery and powering the avionics.

I forgot to add it to the switch panel illustration, but there would be one more breaker for the ALT-B field. Operation is dead simple; normal flight is breakers in, all switches UP.

And I'm with Carl on current shunts. I find ammeters pretty useless in flight.