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Two completely isolated busses

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allthegooduseridsaregone(
Guest

Posted: Sat Dec 12, 2009 12:06 pm Post subject: Two completely isolated busses

I have what may be a somewhat unique situation, flying behind a Jabiru 5100

with its dual-alternator option.  Each alternator is identical, integral to

the engine, permanent-magnet type, externally regulated, producing 25A Max

and 20A continuous.  With "everything on", including pitot heat and landing

lights, etc., I'll need about 35A, exceeding the supply of just one

alternator.  So I'll need to have them BOTH online for normal operations.



The thing is, I don't know of any way to wire two alternators to charge a

single battery safely.  I think that this is a no-no.



So what I've been thinking is to use two smaller (identical) batteries, with

one alternator connected to each, independently.  I'd have two completely

independent power busses, and arrange my avionics redundancy so that I could

fly with EITHER of them active.  Of course I'd have them both on normally,

powering all of my stuff.  But I could still make it home with just one side

or the other. 



The only time that the two busses could OPTIONALLY be connected together, by

a third battery contactor, would be if I disconnected one of the alternators

.. tho' not sure how this would be accomplished for these permanent-magnet

alternators, short of adding still two MORE battery contactors (5

total?!?!).  I'd do this to supply extra cranking juice on cold mornings,

for example.



Of course I'd need two ammeters, two low-voltage warnings, two ... you name

it.  Two completely independent, isolated, and identical busses.



Does this make sense?



Thanks,

   Greg

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Dec 12, 2009 1:09 pm Post subject: Two completely isolated busses

At 02:03 PM 12/12/2009, you wrote:

 	  Quote: 	
		  

<allthegooduseridsaregone(at)hotmail.com>



I have what may be a somewhat unique situation, flying behind a Jabiru 5100

with its dual-alternator option.  Each alternator is identical, integral to

the engine, permanent-magnet type, externally regulated, producing 25A Max

and 20A continuous.  With "everything on", including pitot heat and landing

lights, etc., I'll need about 35A, exceeding the supply of just one

alternator.  So I'll need to have them BOTH online for normal operations.



The thing is, I don't know of any way to wire two alternators to charge a

single battery safely.  I think that this is a no-no.


	


   It CAN be done. I have done it. But it's not easy or

   inexpensiver.



 	  Quote: 	
		  So what I've been thinking is to use two smaller (identical) batteries, with

one alternator connected to each, independently.  I'd have two completely

independent power busses, and arrange my avionics redundancy so that I could

fly with EITHER of them active.  Of course I'd have them both on normally,

powering all of my stuff.  But I could still make it home with just one side

or the other.



The only time that the two busses could OPTIONALLY be connected together, by

a third battery contactor, would be if I disconnected one of the alternators

... tho' not sure how this would be accomplished for these permanent-magnet

alternators, short of adding still two MORE battery contactors (5

total?!?!).  I'd do this to supply extra cranking juice on cold mornings,

for example.



Of course I'd need two ammeters, two low-voltage warnings, two ... you name

it.  Two completely independent, isolated, and identical busses.


	


    See chapter 17 of the AeroElectric Connection and

    particularly the discussion and notes on Figure Z-14

    in the back of the book and available from the website

    at:



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



   and



http://www.aeroelectric.com/R12A/AppZ_12A4.pdf



   What you propose is entirely feasible . . . in fact

   recommended for the situation you've been given.

        Bob . . .

                    ////

                   (o o)

    ===========o00o=(_)=o00o=========

    < Go ahead, make my day . . .   >

    < show me where I'm wrong.      >

    =================================

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allthegooduseridsaregone(
Guest

Posted: Sun Dec 13, 2009 9:20 am Post subject: Two completely isolated busses

Thanks for the quick response, Bob.  OK, so it looks like I'll go with the split bus.  But I have a follow-on question/concern regarding the Z14 diagram's introduction:  

   

"In normal operations, the crossfeed contactor is left open and the two systems operate independently of each other. Should one alternator fail, the crossfeed contactor may be used for the failed system to  

borrow power from the working system."  

   

Yes, that's how I see it working, as described initially.  But wouldn't it be REALLY BAD to crossfeed when both alternators are still WORKING?    

   

Well, I don't see in Z14N anywhere that closing the crossfeed will automatically disable either alternator.  Shouldn't there at least be a manual means (e.g. split master/alt switch) to turn off the field of alternator A or B before closing the crossfeed?  As currently shown in Z14N, turning off the master switch on one side would turn off the alternator field (good) but also turn off the battery contactor that is in series with the cross-feed (bad).  

   

In MY case, with permanent-magnet alternators to turn off, I have no field wire of course so I'll use a relay in series with each alternator-to-regulator, as shown in Rotax Z16M.  (I notice that you still haven't updated the Jabiru Z14 diagram to move that relay to the somewhat better alternator-to-regulator position rather than the regulator-to-battery position.)  

   

I don't much like the fact that I'll have to remember to turn off a "failed" alternator before closing the crossfeed.  What bad things would happen (and how quickly) if I forgot to do so, so that both of my permanent-magnet alternators were connected to the crossfed busses?   

   

Thanks again,   

    Greg  

      [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Dec 13, 2009 10:37 am Post subject: Two completely isolated busses

At 11:16 AM 12/13/2009, you wrote:

  	  Quote: 	
		  Thanks for the quick response, Bob.  OK, so it looks like I'll go with the split bus.  But I have a follow-on question/concern regarding the Z14 diagram's introduction:

  

 "In normal operations, the crossfeed contactor is left open and the two systems operate independently of each other. Should one alternator fail, the crossfeed contactor may be used for the failed system to

 borrow power from the working system."

  

 Yes, that's how I see it working, as described initially.  But wouldn't it be REALLY BAD to crossfeed when both alternators are still WORKING?  	


   No . . . the alternator with the higher set-point

   will hog the load. When it's capability limits are

   reached, the bus voltage sags and the relaxed alternator

   wakes up to carry the difference.



   Now, this assumes that the dynamics of the two

   regulators are not antagonistic to paralleled

   operation . . . I've seen regulators designed

   for slow response but tight voltage control go into

   a see-saw oscillation when paralleled.

  

   All of my regulator designs have gone for

   stability under all conditions as opposed to

   very tight voltage regulation. Further, regulators

   for PM alternators are also fast response, loose

   regulation so you're not going to see this phenomenon

   pop up.

  	  Quote: 	
		   

 Well, I don't see in Z14N anywhere that closing the crossfeed will automatically disable either alternator.  Shouldn't there at least be a manual means (e.g. split master/alt switch) to turn off the field of alternator A or B before closing the crossfeed?  As currently shown in Z14N, turning off the master switch on one side would turn off the alternator field (good) but also turn off the battery contactor that is in series with the cross-feed (bad). 	


   Nope. No risks



 

        Bob . . .

                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

    =================================



     [quote][b]

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