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andrew.d.zachar(at)gmail.
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 Posted: Thu Jul 29, 2010 10:21 am Post subject: Pondering dual alternator, dual bus, but single battery setu |
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Ladies and Gentlemen,
I am just finishing up my emp kit, but am having a lot of fun researching and pondering some of the electron-shuffling related questions for the distant future. I've read the 'Connection once through (I realize I have 5 more times through before stuff will start settling in for good), but wanted to throw something out their for constructive criticism. I have not had enough time to really think through all of its implications yet, but I wanted to run it by everyone to see if there are any large gaps in judgement or understanding. I'll bold my primary questions toward the end of the email.
A little background...
1) I like the avionics master switch, but intend on dual feeding the avionics bus per the single point of failure concern.
2) I am not planning on a vacuum system, so the prospect of a backup 8A or 20A alternator is hard to turn down.
3) Aircraft is an IFR, glass, RV-7, and weight will be an issue (I realize the image I've linked below is very close to Z-14, but I can't make the leap to dual battery, dual alternator.)
4) I have not laid out all of the load (current) requirements for now, I'm just being philosophical about layout concept.
This is my proposed setup, simplified greatly. Two busses, two alternators, but single battery.
Here's a link to an image of my visio file: http://n999za.files.wordpress.com/2010/07/n999za-electrical-concept.jpg
If anyone wants the visio file, write me back and I'll email it to you directly.
(No making fun of any erroneous electrical symbols...I am a mechanical engineer.)
While I will not be enjoying the quick load-shed capability that a true endurance bus affords, it will get me an Avionics switch (I know, Mr. Nuckolls, it will only take me 4 seconds to turn on and off my avionics equipment, but I still can't seem to let go!), and I believe that with the right cockpit indications, I can load shed with switches and breakers pretty quickly. (Avionics bus OFF, Main bus OFF, turn off all switches and pull all breakers. Then Main bus ON, no smoke? Okay, let's slowly bring back essential systems. Avionics ON, no smoke? Good. Turn on essential avionics. Let's find somewhere to land...or maybe I'm comfortable continuing in good Wx...)
My big questions:
1) Can I have a main alternator providing power on the main bus side and an aux alternator providing power on the avionics side, but both buses are connected through the battery?
2) Even with the bus tie open, doesn't this really turn into one big bus through the battery?
3) Will one of of the alternators really be supporting all of the loads if the voltage regulator setpoints are slightly different? Should I leave the aux alt switch OFF for normal operations, or keep it on with a low voltage regulator setpoint, and let it automatically "kick in" upon failure of the main alternator?
4) Right now I have the main alternator powering the bus and battery through the main bus contactor, but the aux alternator is connected to the battery through the hot battery bus. If I use a 20A aux alternator, will I need to go through the avionics contactor instead of through the hot batter bus?
I realize my (perhaps unrequited) love of an AMS and interest in a vacuum pad alternator has pushed me into something very close to Z-14, so maybe I should just thinking about adding a second battery, however, I am not planning on an electric ignition only ignition system, so that level of redundancy might be overkill for me. (Planning regular mags, but with the g3i system I've been eyeing for a little while...).
I have been comfortable flying in single-battery, single alternator installations in the soup for many years. Maybe I should just put everything on a single bus, have a backup alternator with a setpoint just below the main alternator, and be okay with everything going though a single battery contactor.
Any initial reactions or suggestions?
--
Andrew Zachar
andrew.d.zachar(at)gmail.com (andrew.d.zachar(at)gmail.com)
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longg(at)pjm.com
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| Posted: Thu Jul 29, 2010 11:34 am Post subject: Pondering dual alternator, dual bus, but single battery setu |
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Andrew,
Don't make yourself crazy, buy Vertical Power's setup and add two
alternators. If you are really power hungry, use the B & C 20 amp job on
the pad. I'll tell you unless you are running air-condition and want to
stay cool when you blackout and are on backup you don't need the 20 amp
job.
Since you are building a new airplane with all the fancy-shmancy
electronic stuff, calculate your total draw when running the bare
essentials. VP will to that for you - how nice of them. If your panel is
worth what you'll pay for the technology, it won't draw more than 8 amps
(the whole thing). Use LED for everything except your sneakers and stay
away from radios that hum and double as hand warmers when you turn them
on.
In an RV-7, the handheld you are going to take a long for a backup runs
on batteries and will do most of your thinking.
Also, don't mention Avionics Switch here - you'll have bad dreams 
Enjoy the adventure,
Glenn E. Long
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