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Single shunt for two alternators?

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N601RT

Joined: 25 Feb 2006
Posts: 31
Location: Hillsboro, Oregon

Posted: Wed Sep 15, 2010 10:40 am Post subject: Single shunt for two alternators?

Bob and Gang,



I'm building an RV-7A using with an Advanced Flight System (AFS) Advanced Deck 4500s. The 4500s supports a single shunt and has the option for a hall effect transducer to read the output of a second alternator. AFS recommends mounting the hall effect transducer on the cabin side of the firewall and that means bringing the B-lead of one alternator through the firewall. 



I'm basically using Z-12 with a Plane Power 60A alternator for my primary alternator and a SD-20 for my aux alternator with a B&C stand-by regulator. I'm not planning the current sensor for the B&C stand-by regulator show on Z-12.



I'm considering using a single shunt that has both alternator outputs connected to one side and the other side of the shunt connected to a ANL60 which would connect to the starter contactor. I would turn on the SD-20 to verify it was operational as part of preflight before turning on the PP-60A alternator. Above idle, the two alternators should not be delivering power at the same time. I think this would allow me to monitor the output of both alternators AND allow me to eliminate the cabin side hall effect transducer.



I'd like help understanding the downside to sharing a single shunt that I may not be seeing.



Regards,



Roy

Flying N601RT: 2003 CH601HDS, nose gear, Rotax 912ULS, All electric, IFR equipped, 1475hrs

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jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Thu Sep 16, 2010 7:16 am Post subject: Re: Single shunt for two alternators?

 	  Quote: 	
		   AFS recommends mounting the hall effect transducer on the cabin side of the firewall and that means bringing the B-lead of one alternator through the firewal 	




Hi Roy,



FWIW, I'm also using an AFS 4500 in my Glasair and have a two alternator set up. When I inquired at AFS about the HE sensor location, Rob told me that people were mounting them in the engine compartment to avoid having the B leads come into the cabin. I was shopping for other HE sensors at the time because the AFS one wouldn't accomodate my two #4 AWG B leads. Two other brands, Tamura and LEM both had the same operating temp limitation of 80 deg C (176 F). Above that temp the accuracy falls off, which to my mind is not a big deal because it still tells me what I want to know, i.e. that my alternator of choice is/isn't producing power.

After checking with AFS re the non-OEM HE sensor and getting back a detailed testing protocol to see if it would work, I decided to use AFS' HE sensor (on its own tidy little PCB w/capacitors and 5' tefzel pigtail) and downsize my B leads to fit in its hole, rather than risk frying my EFIS by rolling my own HE sensor.



John

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Sep 16, 2010 8:10 am Post subject: Single shunt for two alternators?

At 01:40 PM 9/15/2010, you wrote:




Bob and Gang,



I'm building an RV-7A using with an Advanced Flight System (AFS) 

Advanced Deck 4500s. The 4500s supports a single shunt and has the 

option for a hall effect transducer to read the output of a second 

alternator. AFS recommends mounting the hall effect transducer on the 

cabin side of the firewall and that means bringing the B-lead of one 

alternator through the firewall.



I'm basically using Z-12 with a Plane Power 60A alternator for my 

primary alternator and a SD-20 for my aux alternator with a B&C 

stand-by regulator. I'm not planning the current sensor for the B&C 

stand-by regulator show on Z-12.



I'm considering using a single shunt that has both alternator outputs 

connected to one side and the other side of the shunt connected to a 

ANL60 which would connect to the starter contactor. I would turn on 

the SD-20 to verify it was operational as part of preflight before 

turning on the PP-60A alternator. Above idle, the two alternators 

should not be delivering power at the same time. I think this would 

allow me to monitor the output of both alternators AND allow me to 

eliminate the cabin side hall effect transducer.



I'd like help understanding the downside to sharing a single shunt 

that I may not be seeing.

   You can put the sensor on the forward side of the firewall

   and run BOTH b-leads through the single sensor. The display

   will then read TOTAL output for the two but since the SD-20

   only comes alive when the bus voltage depresses (main alternator

   off line) then your instrumentation will only display one

   alternator at a time.



   Bob . . .

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user9253

Joined: 28 Mar 2008
Posts: 1926
Location: Riley TWP Michigan

Posted: Thu Sep 16, 2010 8:18 am Post subject: Re: Single shunt for two alternators?

Connecting both alternators to the shunt is the same as connecting the alternators in parallel.  Both alternators should not be turned on at the same time.

  If one of the alternators shorts out, the fuse will blow from excessive battery current.  Shunts have also been known to fail.  Either one of these failure modes will open the circuit to both alternators.  Sharing components between two circuits is like putting all of one's eggs in one basket.

  Dynon has a good diagram of 3 possible ammeter shunt locations on page 3-12:

http://www.dynonavionics.com/downloads/Install_Guides/FlightDEK-D180%20Installation%20Guide.pdf

Location "C" eliminates bringing the alternator B lead into the cockpit (for a hall effect transducer) and will show current used by the aircraft except for battery charging current or main contactor coil current.

Joe

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_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Sep 17, 2010 8:06 pm Post subject: Single shunt for two alternators?

 	  Quote: 	
		  Bob and Gang,



I'm building an RV-7A using with an Advanced Flight System (AFS) 

Advanced Deck 4500s. The 4500s supports a single shunt and has the 

option for a hall effect transducer to read the output of a second 

alternator. AFS recommends mounting the hall effect transducer on 

the cabin side of the firewall and that means bringing the B-lead of 

one alternator through the firewall.


	


    My bad. I thought we were talking ONE current

    measurement channel using a hall sensor. If

    you can accommodate BOTH a shunt for

    one alternator and a hall sensor for a second

    alternator then there's no problem for

    potential ambiguity of the display . . . I presume

    each alternator has its own display.



    The hall sensors generally live quite well

    under the cowl. We do it all the time on the

    TC aircraft. I can't imagine that AFS would

    choose an extra-ordinarily fragile device.



   Bob . . .

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