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Dual alternator single shunt

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rshannon

Joined: 05 Sep 2007
Posts: 62

Posted: Sun Apr 20, 2008 4:04 pm Post subject: Dual alternator single shunt

My Murphy Rebel is going together with a slightly modified dual alternator, single battery Z-12 design. The primary alternator is the Jabiru 3300's integral 20A PM alternator. The second, backup only alternator is a manually switched B&C SD-20S. Except during very brief, momentary, preflight test switching perhaps, the two alternators will probably never be ON simultaneously.

 

Due to various challenges, I'd like to use just one ammeter shunt to read output from both alternators, whichever is in use. However, that would require the shunt to be located downstream of the two independent ANL/fusible links, instead of upstream from the ANL/fusible links as shown in Z-12.

 

It appears to me there would be no significant electrical difference in having a single shunt downstream from the two ANL/fusible's compared to two shunts being upstream, as long as the single downstream shunt is big enough to take the output of both alternators if need be. Is that correct, or am I missing something?

 

Ron



Murphy Rebel / Jabiru 3300

http://n254mr.com

"wiring the panel and FWF"

  [quote][b]

MartinErni(at)aol.com
Guest

Posted: Sun Apr 20, 2008 4:16 pm Post subject: Dual alternator single shunt

Ron,    

     I found it easier to place the shunt next to my  common ground block that goes to both batteries.

 Earnest    

  

  In a message dated 4/20/2008 8:06:31 P.M. Eastern Daylight Time,  rshannon(at)CRUZCOM.COM writes:

  	  Quote: 	
		  My    Murphy Rebel is going together with a slightly modified dual alternator,    single battery Z-12 design. The primary alternator is the Jabiru 3300's    integral 20A PM alternator. The second, backup only alternator is a manually    switched B&C SD-20S. Except during very brief, momentary, preflight test    switching perhaps, the two alternators will probably never be ON    simultaneously.



Due to various challenges, I'd like to use just one    ammeter shunt to read output from both alternators, whichever is in use.    However, that would require the shunt to be located downstream of the two    independent ANL/fusible links, instead of upstream from the ANL/fusible links    as shown in Z-12.



It appears to me there would be no significant    electrical difference in having a single shunt downstream from the two    ANL/fusible's compared to two shunts being upstream, as long as the single    downstream shunt is big enough to take the output of both alternators if need    be. Is that correct, or am I missing something?



Ron



Murphy Rebel    / Jabiru 3300

http://n254mr.com

"wiring the panel and    FWF"

 	  Quote: 	
		  



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  [quote][b]

rshannon

Joined: 05 Sep 2007
Posts: 62

Posted: Sun Apr 20, 2008 4:51 pm Post subject: Dual alternator single shunt

Thanks for the input. Physically, that's roughly where my shunt will be too, on the FW about 8" or less from the FW ground stud, contactors, battery, etc. (Only one battery here.)  I'm just trolling the group wisdom to confirm I'm not creating some unanticipated electrical risk with a single shunt downstream from the two alternators' ANLs.

 

Ron

On Sun, Apr 20, 2008 at 5:09 PM,  <MartinErni(at)aol.com (MartinErni(at)aol.com)> wrote:

[quote]      Ron,    

     I found it easier to place the shunt next to my  common ground block that goes to both batteries.

 Earnest    



[b]

lamphere(at)vabb.com
Guest

Posted: Mon Apr 21, 2008 4:33 am Post subject: Dual alternator single shunt

Will this position of the shunt cause any problems when the  engine starter is engaged?? (as in peg the meter)

  

 Dave

 [quote]   ---

MartinErni(at)aol.com
Guest

Posted: Mon Apr 21, 2008 6:04 am Post subject: Dual alternator single shunt

I put the starter ground on the same side of the shunt as the battery  grounds. All other grounds attach to the other side of the shunt, so the answer  is "no".

 Earnest

  

  In a message dated 4/21/2008 8:36:06 A.M. Eastern Daylight Time,  lamphere(at)vabb.com writes:

 [quote]   Will this position of the shunt cause any problems when the    engine starter is engaged?? (as in peg the meter)

    

   Dave

   [quote]     ---

rshannon

Joined: 05 Sep 2007
Posts: 62

Posted: Mon Apr 21, 2008 9:29 am Post subject: Dual alternator single shunt

Not sure I understand this point, Earnest. Is your shunt wired differently than shown in Z-12, or the same?



Looking at Z-12, basically my proposal for a single shunt instead of two would mean that the downstream side of both ANL/fusible links would be connected to the shunt, instead of to the battery side of the starter contactor. The other side of the shunt would then connect to the battery side of the starter contactor. I don't see meaningful electrical difference between that and the dual shunt circuit in Z-12. (Except obviously, with just one "joint" shunt you can't discriminate between readings for the two different alternators if they were ever run simultaneously, which mine won't be.)

 

Ron

On Mon, Apr 21, 2008 at 7:01 AM,  <MartinErni(at)aol.com (MartinErni(at)aol.com)> wrote:

[quote]      I put the starter ground on the same side of the shunt as the battery  grounds. All other grounds attach to the other side of the shunt, so the answer  is "no".

 Earnest



[b]

MartinErni(at)aol.com
Guest

Posted: Mon Apr 21, 2008 2:54 pm Post subject: Dual alternator single shunt

I don't have z-12 handy so I couldn't say. Basically my starter ground  bypasses the shunt. 

  

 In a message dated 4/21/2008 1:31:38 P.M. Eastern Daylight Time,  rshannon(at)CRUZCOM.COM writes:

   	  Quote: 	
		  Not sure    I understand this point, Earnest. Is your shunt wired differently than shown    in Z-12, or the same?



Looking at Z-12, basically my proposal for a    single shunt instead of two would mean that the downstream side of both    ANL/fusible links would be connected to the shunt, instead of to the battery    side of the starter contactor. The other side of the shunt would then connect    to the battery side of the starter contactor. I don't see meaningful    electrical difference between that and the dual shunt circuit in Z-12. (Except    obviously, with just one "joint" shunt you can't discriminate between readings    for the two different alternators if they were ever run simultaneously, which    mine won't be.)



Ron

   On Mon, Apr 21, 2008 at 7:01 AM, <MartinErni(at)aol.com (MartinErni(at)aol.com)> wrote:

    	  Quote: 	
		            I put the starter ground on the same side of the shunt as the battery      grounds. All other grounds attach to the other side of the shunt, so the      answer is "no".

     Earnest







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.matronics.com/">http://forums.matronics.com

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  [quote][b]

nuckolls.bob(at)cox.net
Guest

Posted: Mon Apr 21, 2008 5:36 pm Post subject: Dual alternator single shunt

At 04:57 PM 4/20/2008 -0700, you wrote:



 	  Quote: 	
		  My Murphy Rebel is going together with a slightly modified dual 

alternator, single battery Z-12 design. The primary alternator is the 

Jabiru 3300's integral 20A PM alternator. The second, backup only 

alternator is a manually switched B&C SD-20S. Except during very brief, 

momentary, preflight test switching perhaps, the two alternators will 

probably never be ON simultaneously.



Due to various challenges, I'd like to use just one ammeter shunt to read 

output from both alternators, whichever is in use. However, that would 

require the shunt to be located downstream of the two independent 

ANL/fusible links, instead of upstream from the ANL/fusible links as shown 

in Z-12.


	


   That' fine . . . .

 	  Quote: 	
		  It appears to me there would be no significant electrical difference in 

having a single shunt downstream from the two ANL/fusible's compared to 

two shunts being upstream, as long as the single downstream shunt is big 

enough to take the output of both alternators if need be. Is that correct, 

or am I missing something?


	


   That's correct.



   Bob . . .

nuckolls.bob(at)cox.net
Guest

Posted: Mon Apr 21, 2008 5:42 pm Post subject: Dual alternator single shunt

At 08:35 AM 4/21/2008 -0400, you wrote:



 	  Quote: 	
		  Will this position of the shunt cause any problems when the engine starter 

is engaged?? (as in peg the meter)


	


   No . . .

        Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

nuckolls.bob(at)cox.net
Guest

Posted: Mon Apr 21, 2008 5:44 pm Post subject: Dual alternator single shunt

At 10:01 AM 4/21/2008 -0400, you wrote:



 	  Quote: 	
		  I put the starter ground on the same side of the shunt as the battery 

grounds. All other grounds attach to the other side of the shunt, so the 

answer is "no".

Earnest


	


   ???? I think the question was about having two alternator

   output leads share a common shunt where they feed the bus.

   Contrary to a suggestion in the 'Connection about ground-side

   shunts, it's not a good idea and will be eliminated in

   Revision 12.



   Bob . . .

MartinErni(at)aol.com
Guest

Posted: Mon Apr 21, 2008 5:57 pm Post subject: Dual alternator single shunt

Which is not a good idea "ground-side shunts or 2 alt leads sharing a  common shunt"? Could you elaborate?

 Earnest

  

  In a message dated 4/21/2008 9:45:45 P.M. Eastern Daylight Time,  nuckolls.bob(at)cox.net writes:

  	  Quote: 	
		  -->    AeroElectric-List message posted by: "Robert L. Nuckolls, III"    <nuckolls.bob(at)cox.net>



At 10:01 AM 4/21/2008 -0400, you    wrote:



 	  Quote: 	
		  I put the starter ground on the same side of the shunt as    the battery 

grounds. All other grounds attach to the other side of the    shunt, so the 

answer is "no".

Earnest


	


   ????    I think the question was about having two alternator

   output    leads share a common shunt where they feed the bus.

   Contrary    to a suggestion in the 'Connection about ground-side

   shunts,    it's not a good idea and will be eliminated in

   Revision    12.



   Bob . .  = Use  lities  y       -->                 - MATRONICS WEB FORUMS  nbsp;               - List Contribution Web Site  ;                             =========================



	




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  [quote][b]

lamphere(at)vabb.com
Guest

Posted: Mon Apr 21, 2008 6:57 pm Post subject: Dual alternator single shunt

I just have to ask ....



If you use a shunt that "sees" all current flowing from the battery and the 

starter draws 200a, doesn't that hit the 50ma ammeter (if using a 40a shunt) 

with 250ma??  Doesn't seem like that would do the ammeter any good...



I must be missing something about this... sorry..



Dave



---

rshannon

Joined: 05 Sep 2007
Posts: 62

Posted: Mon Apr 21, 2008 7:38 pm Post subject: Dual alternator single shunt

Take a look at Z-12. As is the case in most all of Bob's schematics (I think) the shunt is in series with the alternator output, which connects to the upstream side of the starter contactor. The starter motor current goes through the starter contactor. That heavy current does not split off to the side, through the alternator. In that position the shunt senses the output of the alternator.

 

Ron

On Mon, Apr 21, 2008 at 7:58 PM, David & Elaine Lamphere <lamphere(at)vabb.com (lamphere(at)vabb.com)> wrote:

[quote] --> AeroElectric-List message posted by: "David & Elaine Lamphere" <lamphere(at)vabb.com (lamphere(at)vabb.com)>

 

 I just have to ask ....

 

 If you use a shunt that "sees" all current flowing from the battery and the starter draws 200a, doesn't that hit the 50ma ammeter (if using a 40a shunt) with 250ma??  Doesn't seem like that would do the ammeter any good...

  

 I must be missing something about this... sorry..

 

 Dave

 [b]

nuckolls.bob(at)cox.net
Guest

Posted: Tue Apr 22, 2008 6:35 am Post subject: Dual alternator single shunt

At 08:35 PM 4/21/2008 -0700, you wrote:

 	  Quote: 	
		  Take a look at Z-12. As is the case in most all of Bob's schematics (I 

think) the shunt is in series with the alternator output, which connects 

to the upstream side of the starter contactor. The starter motor current 

goes through the starter contactor. That heavy current does not split off 

to the side, through the alternator. In that position the shunt senses the 

output of the alternator.

Ron

On Mon, Apr 21, 2008 at 7:58 PM, David & Elaine Lamphere 

<<mailto:lamphere(at)vabb.com>lamphere(at)vabb.com> wrote:

>

><<mailto:lamphere(at)vabb.com>lamphere(at)vabb.com>

>

>I just have to ask ....

>

>If you use a shunt that "sees" all current flowing from the battery and 

>the starter draws 200a, doesn't that hit the 50ma ammeter (if using a 40a 

>shunt) with 250ma??  Doesn't seem like that would do the ammeter any good...

>

>I must be missing something about this... sorry..

>

>Dave

   Ron, that is correct. In the instrumentation chapter of the Connection

   I suggested that one MIGHT install a battery ammeter in series

   with the battery as long as the equipment (shunt size for wattage

   dissipation) and the associated instrument was not adversely

   affected by the starter inrush currents during cranking. This

   was a bit of a brain fart that wound up in print and I should have

   fixed it years ago. In the TC experimental instrumentation business,

   we stick all kinds of shunts about anywhere there is a curiosity

   about current flows . . . including in series with batteries. But

   to consider this particular architecture as the normal operating

   system for an airplane is exceedingly inelegant.

   If one is interested in knowing currents during normal flight

   then having a way to observe alternator loading is about as

   useful as anything one might choose to do . . . hence later

   recommendations in the Z-figures that alternator loadmeters

   be a part of ship's instrumentation.

   I'm working on an e-book version of the 'Connection along

   with an overhaul of several chapters. R12 will include

   a rewrite of the instrumentation chapter which will

   (among other things) delete the suggestion of a battery

   ammeter shunt. In fact, the whole idea of a battery ammeter

   will be discussed for the purposes of illuminating history

   but it will NOT be recommended for new design.

   Bob . . .

lamphere(at)vabb.com
Guest

Posted: Tue Apr 22, 2008 7:36 am Post subject: Dual alternator single shunt

Ron, Bob,



Please understand that I was and still am in agreement with the location of 

the shunt as shown in the schematic you mentioned. While my electrical 

system will be simpler, that is the route I am taking.



It just sounded like the individual that started this thread was 

contemplating a singular shunt for the whole system (where the starter 

current would go through the shunt). Thanks for making clear the details.



Dave



---

nuckolls.bob(at)cox.net
Guest

Posted: Tue Apr 22, 2008 7:59 am Post subject: Dual alternator single shunt

At 11:37 AM 4/22/2008 -0400, you wrote:



 	  Quote: 	
		  

<lamphere(at)vabb.com>



Ron, Bob,



Please understand that I was and still am in agreement with the location 

of the shunt as shown in the schematic you mentioned. While my electrical 

system will be simpler, that is the route I am taking.



It just sounded like the individual that started this thread was 

contemplating a singular shunt for the whole system (where the starter 

current would go through the shunt). Thanks for making clear the details.



Dave


	


    My reply was intended to address all the conversation about shunts

    wherein one thread was talking about load-meters and the other was

    getting battery-ammeters stirred into the same conversation. Don't

    recall exactly what was said by whom but there was no intention

    of "standing anyone against the wall."



    Bob . . .

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