gerry.vandyk(at)eastlink.
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 Posted: Fri Dec 29, 2023 7:12 am Post subject: Re[2]: Tailwind Electrical System Drawing and Power Analysi |
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Hi Peter, thanks for your thoughts.
Why do you want to switch the SD-8 alternator?
because it is shown on the original Z-13/8 drawing. I'm working from rev T 1/07/14. Is there a later revision with this switch removed?
Why switch battery power to the bus twice?
As I mentioned, I want to start the engine on the E-bus. I need to supply power to the starter, but leave the main bus un-powered, in order to check the function of SD-8. Supplying starting power through the E-bus seems impractical.
Use an externally regulated main alternator to avoid having to switch the alternator output.
Yeah, I'm hamstringing myself a little bit here. I bought the AL 12-E160 alt several years ago, I don't have a spare couple thousand dollars to buy another. I'll have to think about it better, but on the face of it, having a separate regulator doesn't seem to change how the alternator is controlled. Mr. Nuckolls has mentioned on many occasions that we want to have definite control of the alternator output with a switch in the cockpit. Z12/8 shows this switch in the center of the drawing.
Provide a method to connect the battery directly to the bus. (I presume your're talking about the main bus here.)
Perhaps I'm being foolish here. I'm placing an absolute requirement to check the operation of the backup alternator at the beginning of every flight. My life will depend on it should the main alternator fail. With the main bus and main alternator connected directly to the battery contactor they will be running at startup. As yet I don't see a way to check the backup alternator operation with the main's running. I'd like to do so without turning half the panel on-off-on during startup.
Do not use a diode in the feed to the essential bus.
This diode is called for by B&C on their installation drawing 504-500, and they provide it in the kit. What would be the reason to remove it?
There are a number of services connected directly to the battery just waiting to flatten it…
Quite right, and this duplicates what's shown on Z13-8. They are a couple of conveniences, loading the baggage compartment, charging a phone etc, without having to turn on the battery master and it's relatively large current draw. In building my panel, all switches will be down=off, and they're all in two rows so visually obvious when left on. I'll have to have a quick scan of the panel when leaving the airplane, and if I flatten the battery by mistake, my fault.
If the voltage of the SD8 is set (say) 0.2v lower than the main alternator it will not produce power if the main is on line. If the main fails it takes up the load.
I presume you are making another case for removing the backup alt switch. Again, it is called for on Z13/8, but your point is well taken.
For my liking you have too many relays between the battery and the bus bar.
I absolutely agree, I haven't yet come up with an elegant solution to check the backup alternator without cycling the avionics.
If you use an externally regulated main alternator then initially only switch on the SD8 on start up. Then power up the main alternator regulator and bring that on line.
I think we've covered this already. In order to start the engine I need to switch on the battery contactor, which would also bring the main bus and main alt online. I could move the starter wire to the always-hot side of the battery contactor, so the mains are still inop, but with that wire permanently hot, and 2 gauge wire at that, it violates the idea of making the electrical system as inert as possible in a forced landing. Again, I don't see the difference between an externally or internally regulated alternator. The red wire to lead 1 on the alt connector is the equivalent of the A wire on an external regulator, the jumper is the same as one to the S wire, and the F wire is internal.
Peter, writing this is making me think about separating the main alt control to a separate switch. Let me think that through a bit and I'll report back later.
Install a switch that powers up the essential (or main) bus directly from the battery, if the switch is a double pole use the second pole to disable the starter.
Interesting, I'll think that out as well.
The large diode in the feed line to the essential bus will always drop the voltage by an amount. The benefit is auto load shedding but do you need that with 2 alternators? If you really want to shed the main bus use a large switch and ensure there is a path to the essential bus directly from the battery. I used to have a feed diode and would not use one again.
Yes, I believe it is necessary. 8 amps will not power the entire panel, so the load shedding is still valid. Incidentally, I discovered on the SD-8 literature that a Lycoming will need to run at 2500 rpm to get the full 8 amps out of the SD-8. it's looking like about 6.8 amps at 2300 rpm. Your suggestion will mean both main and E-bus need to be switched on for normal operations. Bob has made note on many of his drawings that "all buses are on when the main is on". I've had that in mind when drawing this up. Having the E-bus switched on for normal ops means the backup alternator is always running, though as you've suggested it can be set lower than the main, and loaf along in the background. I need to get my head around this idea.
A diode is very beneficial on the starter contactor.
That's a whoops, should have added it to the drawing, I'll get it on there.
Having a low voltage waring that is independent of the EFIS is useful. It is possible you will get immune to EFIS warnings…
Yes, I do agree here. I was hoping the 'alt out light' function of the main alternator would be a 'low voltage monitor' as B&C does on their newer AVC regulator. After exchanging a couple of emails with Tim at Plane Power, it indeed only sees the output within the alternator, nothing outside. I've already got the Dynon EMS with all the engine sensors, so it makes some sense to use what I've got. My thinking is if it becomes annoying I can add the separate voltage monitoring system after the fact. Tim at PP did mention that he regularly sees OBAM guys using the EFIS for voltage monitoring, and all too often they've got parameters set way too stringent, and get many nuisance warnings. As yet I haven't investigated what parameters and settings I can work with, so the external LV warning is always on the table.
Thanks very much for the thoughts Peter. It's very useful to think through the questions and comments.
Cheers
Gerry
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