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Dual Alternators

ChrisMallory

Active Member
I'm looking for a schematic, or directions on how you hooked up a backup alternator.
Did you use two switches and manually control which is providing power?
Is there some way to hook up an automatic switchover?
Do you have both alternators running all the time inso the main bus?

Thanks for the help,
Chris
 
There are probably as many different ways as there are builders -:) But one idea I saw was to set the standby alternator’s voltage to 13.9, while the primary is set to 14.3 volts; both connected. In normal flight, #2 just coasts. But you get near instant change over if #1 fails.
 
This is how mine is wired. B&C standby the regulator starts output once the bus voltage drops to 13.x which would happen if the main alt quits
 
I have my backup on a separate switch. I check it at run up by turning it on and turning the main off. I like to switch it manually or you would not know if the main ever quit. I've had two main failures (external regulators) and just switched the backup on when I saw the voltage drop. I've since changed to a B&C regulator and no problems since.
 
And that is the recommended approach from B&C. Just be sure to include the light or signal from the backup alternator's regulator in some way that lets you know the switch over has occurred and it's time to look into what is wrong with the primary.

This works very well. I've had live switch overs to the backup in flight twice now. It's also probably a good idea to test the set up periodically by pulling the primary field breaker (or by turning the field off with the master switch half if you have that configuration) and see that the backup picks up and the backup light comes on. Often that will happen anyway typically at idle after a start as the primary at low RPM doesn't quite get to the regulation voltage.
 
Personally I don't like the automatic switchover, I want to KNOW if the primary fails, so I have separate field switches for both and test them each runup. With modern EFIS equipment I have both undervolt and overamp-discharge alarms that will come at me from my Dynon screens, as well as various complaints from the other equipment in my panel. It will be near impossible for me to miss the fact that my primary has taken a vacation, and in fact I have had exactly that occur once to me (Plane Power, replaced by B&C). Upon the death of the primary alternator, I kill the field for the primary and activate the field for the secondary.

During cross country flights (which I do a lot of) I typically shut down the primary and operate on the backup for an hour or so per leg, to make sure they both get plenty of exercise and prove they are still funtioning well. If one gets sick, I want to know about it in advance of needing it.
 
Personally I don't like the automatic switchover, I want to KNOW if the primary fails, so I have separate field switches for both and test them each runup.

I think your thoughts and approach are still compatible with automatic switch over - as long as you have an indicator light/alert that the backup has taken over. The B&C backup regulator provides for that. And the test for both is simply turn off the main alternator field switch and check that the alternate is charging and the switch over light comes on. Can force it to run in flight that way as well. The best of both worlds.

BTW, as long as the backup test passes on start up, I doubt that "exercise" is useful, if not actually using up some life expectancy.
 
Alternator Switch

I used this once before and planning it on my RV-14. A single, 3 position locking switch labeled ALTERNATOR PRI, OFF, STBY. I too like the idea of positive control of the alternators. :cool:
 
Hey Chris,
Check out aeroelectric.com and the aero electric list (forum). Bob Nuckolls has some well thought out example designs - including dual alternator options.
If I remember correctly their figure z101 is a basic dual alt design.

http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z101B.pdf

You should request from the list a link to the most current version in case this is not it.
Peter

PS should be back at Covey Trails in the RV6 in a week or two.
 
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Here is a screenshot of my schematic. Mostly it came directly from Bob K. schematics. I have dual B&C alternators and regulators.

I have dual switches, but the switchover will be automatic as others have said. The two regulators are set at different regulation voltages so the backup alternator doesn't kick-in normally. When the voltage drops then it starts to operate.

I don't have all of this configured yet but presumably a low voltage detect will result in a warning indication and at that point I would see there is no current output out of the primary alternator and I would shutdown the primary alternator and possibly go to a limited set of avionics.
 

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The writing in your attachment is fuzzy (at least for me) for some reason.........
 
Great timing....

By coincidence, this morning I was going to install my stby alternator regulator and start the wiring.

All B&C alternators and regulators. Plan is to use the automatic switching feature of the standby regulator with indicator light. If the light is on, I know the standby is operating, and switch off the primary. That's all pretty much covered by B&C's manuals.

So any thoughts on Standby Alternator connection to Main Bus or only to E Bus? Mr. Nuckolls Z-12 shows connection to Main bus. I'm thinking B Lead and regulator to just the E Bus, thus creating it's own system, separated from the main bus by a schottky diode. Lots of paths to this end.
Am I missing anything here?

Nomex donned. Open for comments on the latter.
 
So any thoughts on Standby Alternator connection to Main Bus or only to E Bus? Mr. Nuckolls Z-12 shows connection to Main bus. I'm thinking B Lead and regulator to just the E Bus, thus creating it's own system, separated from the main bus by a schottky diode. Lots of paths to this end.
Am I missing anything here?

Nomex donned. Open for comments on the latter.

Dealers choice on that one. Myself, I have sufficient output on both alternators to handle the entire ships load in night IFR - and have tested that. So in that case I have no need to power the E-buss with one of them - they both feed through individual B-lead fuses (to protect against internal shorts) then to a common feed on my main buss. I do have an E-buss, it's hot across the battery for only the critical items to keep the prop turning in the very remote case of dual alternator failure.
 
?

So Greg, if I’m reading correctly, this sounds like Z-12. Only difference I’m thinking about is Stdby connects only to a reduced load E Bus.
Yes?
 
Sorry about the image. I think its just the lack of resolution.

Mine is mostly like the Z13/20K, Dual Bus (Main and Endurance).
 
So Greg, if I’m reading correctly, this sounds like Z-12. Only difference I’m thinking about is Stdby connects only to a reduced load E Bus.
Yes?

I couldn't tell you. I didn't reference Nuckolls drawings and honestly don't think I've ever seen one. I've been working on electrical items since my dad (a 30-year IBM engineer) taught me schematics back in junior high, it just makes sense to me and I rolled my own.

EDIT - I looked up Nuckolls Z-12, and yes I'm pretty much like that (with regard to the alternators) except I don't use the alt fail lights (I am relying on digital low-volt and high-amp) alarms, and I only have one shunt for amperage, placed downstream of my starter solenoid, and upstream of all my loads+alternators - this way the alternators pull all the load plus charge the battery, and I monitor amps in/out of the battery.
 
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Is there a current limiting device (not a fuse or circuit breaker) that limits how much current can be transferred between busses?

Say you have a 30 amp alternator on the vacuum pad suppling bus B

Buss B normal load is 15 amps.

Your current limiter would only allow 15 amps to transfer to other bus A. Bus B always gets its 15 amps.
 
Is there a current limiting device (not a fuse or circuit breaker) that limits how much current can be transferred between busses?
Current is determined by the load. Loads take what they want. Each load should have a fuse in series with it. If you want to limit the current, then don't turn too many loads on.
 
Ed, In the EE world we would say this is "by design". That means you design your system such that it can't be a problem. So if you have an alternator that can only put out 30 amps you would make sure that the bus that it drives can only have a total of a 30 amp load. Some folks may have a larger possible load but their plan is to "shed" loads to reduce the required current to the amount the alternator can supply.

The way these things normally work is that the primary alternator is sized to handle the full current of of your entire electrical system. The backup or standby alternator assumes a fallback to a more limited set of items on your endurance bus. So normally your primary alternator is supplying current for your main bus as well as the endurance bus. If it goes belly up then the backup alternator supplies the limited set of devices on your endurance bus.
 
Is there a current limiting device (not a fuse or circuit breaker) that limits how much current can be transferred between busses?

Say you have a 30 amp alternator on the vacuum pad suppling bus B

Buss B normal load is 15 amps.

Your current limiter would only allow 15 amps to transfer to other bus A. Bus B always gets its 15 amps.

Doing that would be interesting - you would need a Darlington-type power transistor driven by shunt feedback - likely more trouble and troublesome than you want to play with.
 
Quote:
Originally Posted by emsvitil View Post
Is there a current limiting device (not a fuse or circuit breaker) that limits how much current can be transferred between busses?

Say you have a 30 amp alternator on the vacuum pad suppling bus B

Buss B normal load is 15 amps.

Your current limiter would only allow 15 amps to transfer to other bus A. Bus B always gets its 15 amps.
Doing that would be interesting - you would need a Darlington-type power transistor driven by shunt feedback - likely more trouble and troublesome than you want to play with.

Actually I think you are envisioning all this incorrectly. If you source alternator can supply 30 amps and your load is only 15 amps there is nothing to limit. The alternator will only supply 15 amps.

Now if you had a 15 amp alternator and you had a load that required 30 amps you could technically by some means limit the current to 15 amps. The result would be that the voltage would have to drop for this to occur. Do a search on Ohm Law. The reason the voltage would have to drop is the load is a fixed resistance to get half the current you would need to reduce the voltage by half as well. Ohms Law -> V(voltage) = I (current) * R (resistance).
 
My understanding is you don't have to limit the current coming out of the backup alternator, it will already limit itself. Nothing to worry about. If you keep all your loads on, then the battery will supply the rest of the needed current and your battery will eventually drain.
So in that case it would help to have the shunt measuring what goes in and out of the battery. You turn stuff off until you are net zero or slightly loading the battery. If you don't have a shunt at the battery, you have to compare what the generator makes with the total current used (which a VPX can give you).
 
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My thinking process.....

1. You have 2 alternators, utilize them; don't do a primary / backup.
2. Alternators last longer if they aren't run at full capacity, try not to run above 1/2 capacity or so.
3. Vacuum pad alternators have less capacity
4. Primary alternator A / bus A can handle all the load if it has too
5. Alternator B / bus B is essential bus on vacuum pad. Lower capacity, can't handle everything.

If you lose Alternator A, Alternator B's priority is bus B. Bus B gets any amps it needs and normal voltage. Any excess capacity can go to Bus A. Don't let bus A lower the voltage on Bus B under any circumstance.

You can do an automatic crossfeed, but bus A can't take out bus B because you're limiting the amount of crossfeed.
 
My thinking process.....
Don't let bus A lower the voltage on Bus B under any circumstance.

If you have old light bulbs, you can regulate the amount of amps drawn by lowering the voltage but anything nowadays has internal switching power supplies, meaning if you lower the voltage the opposite happens, they draw more current to satisfy their need for Watts. So there is no way to "limit the current" that goes from bus B to bus A.
 
My thinking process.....
Don't let bus A lower the voltage on Bus B under any circumstance.

If you have old light bulbs, you can regulate the amount of amps drawn by lowering the voltage but anything nowadays has internal switching power supplies, meaning if you lower the voltage the opposite happens, they draw more current to satisfy their need for Watts. So there is no way to "limit the current" that goes from bus B to bus A.
You need to turn off devices to lower the current.
 
My thinking process.....

1. You have 2 alternators, utilize them; don't do a primary / backup.
2. Alternators last longer if they aren't run at full capacity, try not to run above 1/2 capacity or so.
3. Vacuum pad alternators have less capacity
4. Primary alternator A / bus A can handle all the load if it has too
5. Alternator B / bus B is essential bus on vacuum pad. Lower capacity, can't handle everything.

If you lose Alternator A, Alternator B's priority is bus B. Bus B gets any amps it needs and normal voltage. Any excess capacity can go to Bus A. Don't let bus A lower the voltage on Bus B under any circumstance.

You can do an automatic crossfeed, but bus A can't take out bus B because you're limiting the amount of crossfeed.

I suggest that any real 20 amp or so standby alternator will run everything in your plane. For example, with pitot heat off the total load in my plane (full IFR planned wiht dual SkyView EFIS and associated modules, a GTN-650 and a Comm #2) is 10 amps, perhaps a little more if I leave the LED landing lights and strobes on.

The 60 amp primary alternator does a much better job of topping off the battery after engine start, but the function of the standby alternator is to keep the panel up, not as the only power source.

Here we have the opportunity to make life simple. No isolation diodes, not extra switching between valorous busses and such - all not helpful while also doing the primary job of flying the plane. Follow the instructions on the 20 amp B&C vacuum pad alternator and voltage regulator and all this becomes automatic.

You know if the standby alternator is carrying the load as:
- Buss voltage will fall from your normal ~14.1vdc to the standby alternator output setting of ~13.5vdc.
- The nice little yellow light that comes with the standby alternaor voltage regulator will come on.

Carl
 
My attempt at ASAP (As Simple As Possible) with redundancy for my -10.

Components:
1 Bus - Bussman 15401-2-0-1-0A fuse panel
1 Main Battery - EarthEx ETX900-VNT
2 Standby Batteries - TCW IBBS 3aH
1 Main Alternator & V/R - B&C BC 460H & LR3C-14
1 Standby Alternator - Monkworkz MZ-30L

Controls:
1 Main Battery switch
1 Standby Battery switch
1 Main Alternator C/B
1 Backup Alternator switch

Indicators:
Bus Volts
IBBS Volts
Bus Amps
Main Battery Amps
Standby Alt Amps


Concept:
* Single bus reduces the number of relays and switches to a minimum.
* Fuses are simple, light, cheap and easy to install.
* Bussman fuse panel is sealed with aft exiting wires. Mounted under panel on a hinge so that it can be swung down for service.
* Earthex is light, powerful and safe when installed according to the manual.
* IBBS serves as an Emergency Bus using 2nd power input on Garmin LRU's.
* IBBS keeps bus voltage normal when starting engine.
* GAD 27 keep alive circuit is used when starting engine.
* Main Alternator is sized to carry all systems and max battery charging load.
* Standby Alternator is small, lightweight and easy to install. Will power essential items indefinitely.

Normal Operation:
* IBBS switch is turned ON first. Operational check to ensure expected units power up and voltage is good.
* Battery switch is turned ON next. Remaining systems power up and IBBS goes into standby mode. Battery voltage checked.
* Main Alternator C/B (field power) is normally in. No switch. Can be pulled if alternator fails without popping C/B. V/R has internal crowbar overvoltage protection required for EarthEx battery.
* Standby Alternator switch remains off unless needed.
* Engine started. The IBBS batteries and GAD 27 Keep Alive circuit provide adequate power if bus voltage drops during start. This eliminates the need for a separate Avionics bus and single point failure switch.
* Bus voltage is checked to confirm Main Alternator is working.
* Battery Amperage is checked. EarthEx requires high amps (~20a) to charge after a start. This drops to 0 quickly and is monitored during taxi.

Non-Normal operation:
* If Main Alt fails (bus low volt annunciation) the Main Battery carries the load. Electrical system is monitored. Non-essential items are depowered (the A/C fan draws 12 amps). Main Alt C/B - pulled. Standby Alt switch - ON. Standby Alt amps monitored to ensure remaining below capacity. (If the demand is higher than 30a, the battery will discharge as needed.)

Annually during Condition Inspection the Battery capacity (see operator manuals) and Standby Alternator operation are checked.
 
I looked at a lot of drawings and ways of doing this, I think you can break this down into 3 fundamentally different solutions:

1. Dual alternators always online and try to make sure that one is at a higher voltage than other so that it bares the load.

2. Primary alternator with a secondary alternator that can be brought online with an aux alternator switch. Typically another switch is needed to route power such as an e-bus alternate feed. This is the Z-13/8 design.

3. Dual electrical systems. Both alternators power independent electrical systems with some sort of cross feed solution. This is the Z-14 design.

Given that many modern avionics systems have dual power inputs, and because only a separate electrical system can survive something like a short to ground, I have opted for separate systems, but without any cross feeds.

Attached is my latest draft electrical system drawing and switch setup.

From left to right, row one:

Master (this connects my main battery to the main bus using a contractor)

Aux Master (this connects my tiny 4Ah lifepo4 battery to the Aux bus and would power on most of my avionics through their secondary power wires as well as the CPI-2 through the secondary power wires)

IGN 1 (this ties the CPI-2 to the main bus, so it will no longer be in fault and ready to go)

IGN 2 (this ties the CPI-2 to the main bus, so it will no longer be in fault and ready to go)

From left to right, row two:

Keyed start switch (runs the starter only)

Aux ALT (after the engine is running it brings the AUX alternator online)

Avionics (connects the avionics to the main bus through their primary power wires)

IGN TEST (shorts the mag switch wires on CPI-2 to do a mag check)

The idea is to run through the switches in order and have the following without the pilot really needing to understand.

Avionics start on aux battery to test it works, provide isolated power during engine start, and ready to show oil pressure.

Don’t add the aux alternator to the aux bus until the engine is running.

Don’t swing avionics to main bus until the engine is running.

Cut coils to IGN for mag check (power off won’t work since they would swing to aux bus)

Shutdown isn’t critical since I’m not worried about spikes and brownouts during shutdown, but normal operation would be bottom row in reverse order, mixture lean, then top row in reverse order.

This would:
Isolate the avionics from the main bus so they are on the aux bus
Remove the aux alternator so avionics are on aux battery
Shutdown engine
Power down CPI-2
Disconnect aux battery from aux bus
Disconnect main battery from main bus
 

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Part of my design decisions are to provide very reliable power for key systems, but also provide startup power to engine monitoring, limit the number of switches and decisions the pilot must make, very fast failover, minimal deviation from certified norms, taking advantage of systems with dual input, all while eliminating stuff I don't need like backup battery systems (IBBS).

If one wasn't using a electric ignition they could omit the ignition related switches and simply use:

Master, Aux master, keyed/start/mag switch, Aux ALT, Avionics.

Pretty simple.
 
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