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Knuckolls Z-12 Power Grid Questions

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thorn(at)starflight.aero
Guest

Posted: Sat Mar 13, 2010 3:05 pm Post subject: Knuckolls Z-12 Power Grid Questions

First for Bob K.,  I don’t know what the experimental aircraft community would do without all you’ve done to educate and guide us with your books, classes, and ongoing advice on this forum.  Thank you so much.  

   

   

Hello AeroElectric gang,  

   

My wife and I are building a Lancair Legacy  and I’ve been working on wiring diagrams to help with electrical system planning since they help capture the layout of components, wiring, and connectors in the approximate topology of the airplane.    

   

Here is some background for our electrical system design goals and requirements.  For propulsion/engine reliability, we will fly with dual magnetos though we will likely replace one with electronic ignition in the future for improved efficiency and starting.  We are designing for IFR flight.  We’re planning on a Garmin G900X integrated avionics system.  We have an all electric air conditioning system (hot, humid Houston) that sits behind the seats that steady state can pull about 50A continuous.  To support all this we have a 28V system with a primary 100A alternator and an auxiliary 20A alternator.    

   

For the power grid, we’re using Knuckolls’ Z-12 design (main alt, aux alt, one battery) but without the Endurance Bus.  In the event of an alternator failure, I prefer to selectively load shed to reduce power demands based on the current flight situation.  We’ll be ganging up two 12V Odyssey PC-680 17 AHr batteries to serve as one battery.    

   

I have a couple questions I’d appreciate help with…    

   

First, related to the E-Bus deletion, If we should find ourselves flying along and the battery goes dead but the alternators are still working, I want to still have power to the Battery Bus.  So I’ve connected the Main/System Bus to the Battery Bus with a diode in line to keep power from flowing to the Main Bus when the battery contactor switch is off.  Is this the right way to provide this?  Should this failure mode be protected for?  How likely is it with good maintenance practices that the battery would fail in flight?    

   

Second, with a carbon fiber/epoxy composite airframe, what are the options for achieving a “common ground” for the system?   Is it okay to have the ground bus at the firewall, with a thru bolt to connect both sides, and then run a jumper to a ground bus on the instrument panel where everything on the panel is connected?  Does this count as a common ground that will avoid “electrical ground loops”?  

   

Third, I’d like to have a battery ammeter to see it’s charge and discharge status.  I’ve indicated a 100 mV/200A ammeter shunt.  Should it be larger to be able to see really high load cases like engine start?    

   

I’ve attached a draft of our wiring diagram if that helps understand my questions.  Also, I’d appreciate hearing if anyone sees mistakes in it.  

   

Thanks,  

   

Valin Thorn  

Lancair Legacy project  

Houston, Texas  USA

THORN_Legacy_Wiring_Diagram_2010-3-13_144_dpi.jpg

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Mar 13, 2010 7:27 pm Post subject: Knuckolls Z-12 Power Grid Questions

At 05:02 PM 3/13/2010, you wrote:

First for Bob K.,  I don't know what the experimental aircraft 

community would do without all you've done to educate and guide us 

with your books, classes, and ongoing advice on this forum.  Thank you so much.



    You're welcome. I'm pleased that you find the work useful!



   <snip>



Here is some background for our electrical system design goals and 

requirements.  dual magnetos. Garmin G900X.

All electric air conditioning at 50A continuous.  28V system. 100A 

alternator. 20A aux alternator. Two 12V Odyssey PC-680 17 AHr 

batteries in series.



First, related to the E-Bus deletion, If we should find ourselves 

flying along and the battery goes dead . . .

So I've connected the Main/System Bus to the Battery Bus with a diode 

in line to keep power from flowing to the Main Bus when the battery 

contactor switch is off.  Is this the right way to provide this? 

Should this failure mode be protected for?  How likely is it with 

good maintenance practices that the battery would fail in flight?



     Battery failure for devices favored with good

     preventative maintenance is exceedingly rare.

     We generally don't include battery failure as

     a component of a failure mode effects analysis.



Second, with a carbon fiber/epoxy composite airframe, what are the 

options for achieving a "common ground" for the system?   Is it okay 

to have the ground bus at the firewall, with a thru bolt to connect 

both sides, and then run a jumper to a ground bus on the instrument 

panel where everything on the panel is connected?  Does this count as 

a common ground that will avoid "electrical ground loops"?



    Yes. See figure Z15 View A at:



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z15K1.pdf



Third, I'd like to have a battery ammeter to see it's charge and 

discharge status.  I've indicated a 100 mV/200A ammeter 

shunt.  Should it be larger to be able to see really high load cases 

like engine start?



   Please don't do this. This was instrumentation of choice

   for automobiles and some airplanes for years but needs

   to go away. The readings presented are not useful for

   managing an airplane in flight and are subject to a lot

   of variability in interpretation. This configuration is

   going to be removed from the 'Connection at Rev 13.



   Active notification of Low Voltage, alternator load meters

   and what ever voltmeters are built into your panel instrumentation

   are more than adequate to the task.



   Bob . . .

user9253

Joined: 28 Mar 2008
Posts: 1929
Location: Riley TWP Michigan

Posted: Sun Mar 14, 2010 6:38 am Post subject: Re: Knuckolls Z-12 Power Grid Questions

Suggestions for Lancair Legacy IFR electrical schematic:



Replace the battery current shunt with a voltmeter.



The battery bus is on the wrong side of the firewall.  It is not protected by a fuse.  If the battery bus circuit starts smoking, there is no way to shut it off.



The avionics master contactor coil needs a diode like the other contactors.  A 30amp automotive relay can replace that contactor.  Mount it on the engine side of the firewall so that it can isolate power in case of smoke in the cockpit.

 	  Quote: 	
		  "If we should find ourselves flying along and the battery goes dead but the alternators are still working." 	
   You are more likely to win the lottery.



The wires feeding the Avionics bus are way too big.



When you are flying in the soup and the avionics master contactor circuit fails, the glass panel will go dark.  This could be a life threatening situation.  Here is how to prevent that:



Disconnect the diode from the battery bus and connect it to the avionics bus instead.  Disconnect the avionics contactor supply wire from the System bus and connect it to the battery bus instead.  Now there are two independent current paths to the avionics bus.



The recommendation to shut off avionics while cranking the engine is based on an old wives tale.  Ask Bob.  If you still want to shut off the avionics bus while cranking, put a switch in series with the diode.  But that introduces an unnecessary failure point.



I hope this helps.



Joe

_________________
Joe Gores

thorn(at)starflight.aero
Guest

Posted: Sun Mar 14, 2010 11:54 am Post subject: Knuckolls Z-12 Power Grid Questions

Thanks Bob and Joe.  

   

I’m going to remove the ammeter for the battery and go with a  tap for a voltmeter.  I’ll add a fuse for the line to the battery bus.  Since the battery bus is going to power things like cabin lights, Hobbs meter, and clock it seems like it is best to put it on the back of the instrument panel near the switches, components it will power...?    For future electronic ignition I thought I’d run a tap directly from the battery.  Agree line to avionics bus was too big.  

   

On the avionics contactor relay, I thought the diode was for if it was switching inductive loads to prevent back EMF.  Is a diode still recommended for an avionics bus with resistive loads?  Do I misunderstand the diodes role here?  

   

I hadn’t heard that Bob K. doesn’t see a need to isolate the avionics from the main bus.  I’d guess that with the power quality protection of modern avionics that the transient from the engine starter motor is not an issue these days.  Still, with all that damn expensive electronics I’d rather not test it at each engine start.  It’s also nice to bring it all up and down with one switch.  I agree that I can go with a  lower capacity relay to handle the job.    

   

Thanks for pointing out that the avionics relay is a single point failure.  What do you guys think about getting one fault tolerant by having two avionics bus relays/switches in parallel -– maybe one passing power from the Battery Bus and one from the System Bus?  Are their some other single point failure modes that might need attention?  

   

This is very helpful, thanks.  

   

Valin  

   

   

Valin Thorn  

Lancair Legacy Project  

Houston, Texas USA  

   

   

        [quote][b]

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Mar 14, 2010 2:35 pm Post subject: Knuckolls Z-12 Power Grid Questions

At 02:25 PM 3/14/2010, you wrote:

 	  Quote: 	
		  Thanks Bob and Joe.



I’m going to remove the ammeter for the battery 

and go with a  tap for a voltmeter.  I’ll add a 

fuse for the line to the battery bus.  Since the 

battery bus is going to power things like cabin 

lights, Hobbs meter, and clock it seems like it 

is best to put it on the back of the instrument 

panel near the switches, components it will power...?


	


   Then it's not a battery bus but an always hot bus without

   benefit of a battery contactor. The rule of thumb for always hot

   wires is 6" or less in length and/or fused not greater that 7A

   or breakered greater that 5A.  This is why the battery busses

   are shown on short feeders and have small fuses EXCEPT where

   augmented by co-located relays.



   We've got lots of battery busses in TC aircraft and many

   do indeed have a goodly number of little bitty wires that

   fan out to other places in the airplane.



 	  Quote: 	
		    For future electronic ignition I thought I’d 

 run a tap directly from the battery.  Agree line to avionics bus was too big.


	


   Power it with its own fuse/breaker at the REAL

   battery bus.

 	  Quote: 	
		  

On the avionics contactor relay, I thought the 

diode was for if it was switching inductive 

loads to prevent back EMF.  Is a diode still 

recommended for an avionics bus with resistive 

loads?  Do I misunderstand the diodes role here?


	


   Diodes are check valves for electron flow in a host

   of applications which include trapping off inductive

   spikes from contactor coils and STEERING of power from

   some source to some load while preventing back-flow of

   power along the same path. A search of the website for

   "diode" yields a collection of discussions on various

   uses for diodes.

 	  Quote: 	
		  

I hadn’t heard that Bob K. doesn’t see a need to 

isolate the avionics from the main bus.  I’d 

guess that with the power quality protection of 

modern avionics that the transient from the 

engine starter motor is not an issue these 

days.  Still, with all that damn expensive 

electronics I’d rather not test it at each 

engine start.  It’s also nice to bring it all up 

and down with one switch.  I agree that I can go 

with a  lower capacity relay to handle the job.


	




   Check out this article in particular:



http://www.aeroelectric.com/articles/avmaster.pdf



   Go to the website at http://aeroelectric.com

   and do a search on avionics + master + switch

   for 15 hits on articles that discuss this topic.

 	  Quote: 	
		  

Thanks for pointing out that the avionics relay 

is a single point failure.  What do you guys 

think about getting one fault tolerant by having 

two avionics bus relays/switches in parallel -– 

maybe one passing power from the Battery Bus and 

one from the System Bus?  Are their some other 

single point failure modes that might need attention?


	


   Have you reviewed chapter 17 in the 'Connection

   concerning system reliability along with the notes

   that accompany the Z-figures? The philosophy and

   features for failure tolerance in each example is

   discussed.



   The e-bus was crafted to provide a failure-tolerant

   source for pieces of equipment particularly handy

   for extended flight in spite of single failures of

   hardware.



   There is risk for "cherry picking" features of any

   z-figure. Those things are all in there for a reason.

   May I suggest you 'back up' and consider your z-figure

   of choice (in this case Z-12) and explore how your

   design goals differ from what's illustrated.



   Bob . . .

thorn(at)starflight.aero
Guest

Posted: Sun Mar 14, 2010 5:51 pm Post subject: Knuckolls Z-12 Power Grid Questions

Thanks Bob,



I read your excellent Avionics Master Switch (AMS) article and re-read

AeroElectric Connection's chapter 17.  Since we've only recently progressed

to the point in our Lancair Legacy project to where the electrical system

design details have to be decided, I've only recently fully engaged in the

subject.  I was out of town on business when you came through Houston with

your class so my wife attended without me (but I'm doing the electrical...?)

So, I missed the previous debates, discussions on the AMS subject.  I think

you've convinced me that we don't need an AMS.  



I need to check on how you turn on a G900X integrated avionics system -- it

may be the equivalent of an AMS anyway...  I'll check Monday.



With the addition of a 43 lb electric A/C system in our little Lancair

Legacy, we're working to trim some weight out.  We've saved 17 lbs with an

MT prop vs. the Hartzell.  If it were not for the weight concern I'd

probably opt for the dual alternator/dual battery system (or if I were going

dual electronic ignition).   It seems that the Z-12 power grid is an

excellent, reliable architecture for us.  I'm not yet convinced that the

Essential/Endurance Bus adds that much value.  Though, if I found myself IFR

with electrical problems (aka panic:)) the idea of flipping a couple

switches to reconfigure would probably look like genius...    It seems,

though, that it might improve reliability more for Z-12 to have dual

parallel battery contactors.  



What do you think about that?



Thanks again,



Valin



BTW, I understand now what you mean by Battery Bus...  It's by the battery

  I'm reworking that architecture.

--

trigo(at)mail.telepac.pt
Guest

Posted: Sun Mar 14, 2010 6:40 pm Post subject: Knuckolls Z-12 Power Grid Questions

Hi Valin  

   

I liked very much your wiring diagram, so when you finish re-doing it, please post it again.  

   

Thanks in advance  

Carlos  

   

   

[quote] --

user9253

Joined: 28 Mar 2008
Posts: 1929
Location: Riley TWP Michigan

Posted: Mon Mar 15, 2010 3:25 pm Post subject: Re: Knuckolls Z-12 Power Grid Questions

 	  Quote: 	
		  "On the avionics contactor relay, I thought the diode was for if it was switching inductive loads to prevent back EMF. Is a diode still recommended for an avionics bus with resistive loads? Do I misunderstand the diodes role here?" 	


A relay has two (or more) circuits.  The coil is one circuit and the relay contacts are in another circuit.  The two circuits can be completely isolated from each other.  The purpose of a diode connected across a relay (or contactor) coil is to protect the controlling switch from high voltage spark generated by the relay coil when it is shut off by the switch.  That high voltage has nothing to do with the load on the relay contacts, whether that load is inductive or resistive.  In fact, a relay coil will produce a high voltage when it is shut off, even if nothing at all is connected to the relay contacts.  The bigger the coil, the bigger the spark.  A relay coil will not make as big of a spark as a contactor coil.  Connecting a diode across a relay coil will prolong the life of the switch that controls it.  The diode arrow should point towards positive.



 	  Quote: 	
		  "Thanks for pointing out that the avionics relay is a single point failure. What do you guys think about getting one fault tolerant by having two avionics bus relays/switches in parallel -â€“ maybe one passing power from the Battery Bus and one from the System Bus?" 	


Yes, that will work.  What you are proposing is similar to the E-Bus.  And it is similar to what I proposed in my previous post, although I might not have explained it clearly.  Even if you do use two relays, the diode must still be used to prevent high current from flowing from the avionics bus (AKA E-Bus) to the system bus.



 	  Quote: 	
		  "I'm not yet convinced that the Essential/Endurance Bus adds that much value." 	
The important feature of an E-Bus is that is has two independent current paths.  It would be easy to turn your avionics bus into an E-Bus by adding the alternate feed path from the battery bus and a diode to prevent back-feeding the system bus.



 	  Quote: 	
		  "It seems, though, that it might improve reliability more for Z-12 to have dual parallel battery contactors." 	


Yes, that will work, but will cost more and weigh more than using a relay and diode along with an E-Bus that will accomplish the same thing.



It can be satisfying to design one's own electrical system.  However, there could be failure modes that you might not be aware of.  The big advantage of using one of Bob's drawings is that they have been proven over time and scrutinized by many eyes.  Any bugs have been worked out.  Your schematic looks great in colors.  After a couple of minor changes, it will be similar to Z-11.

E-Bus.jpg

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_________________
Joe Gores

thorn(at)starflight.aero
Guest

Posted: Wed Mar 17, 2010 3:05 am Post subject: Knuckolls Z-12 Power Grid Questions

Thanks Joe.Â  Bob and you have convinced me that the Z-12 with the E-Bus set up this way is optimized.Â  BTW, my AeroElectric Connection Rev 11 book doesn't have a Z-11 design in it.Â  Guess yours is older...?Â    

   

The process of thinking through the logic behind the architecture, with yours and Bob's help, has been very helpful.Â    

   

Thanks again,  

   

Valin  

   

   

Â   

   

--

user9253

Joined: 28 Mar 2008
Posts: 1929
Location: Riley TWP Michigan

Posted: Wed Mar 17, 2010 4:21 am Post subject: Re: Knuckolls Z-12 Power Grid Questions

You are welcome, Valin

You can get an update to your book here: http://www.matronics.com/aeroelectric/R12A/AppZ_12A4.pdf

It includes Z-11.

Joe

_________________
Joe Gores

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