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Question about monitoring output in two alternator configu

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nuckolls.bob(at)aeroelect
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Posted: Thu Feb 12, 2015 7:15 pm Post subject: Question about monitoring output in two alternator configu

At 16:05 2015-02-12, you wrote:




Hi everyone.



I have an electrical system with dual alternators (primary and 

backup) and have a question with regards to monitoring their output 

current.  My EFIS has the capability of monitoring each output 

separately, but considering that the alternators are wired such that 

only one can be on at a time, I'm not a big fan of this, since I 

don't want to get used to the view of an alternator output always 

reading zero during normal operation - it desensitizes a person in 

noticing if there ever is a real problem.  (I used to fly a Mooney 

that had a red gear warning light always on when the gear were up, by 

terrible design.  You get really used to seeing a red light always 

on, and then completely ignore it).



   But 'zero' in a standby alternator is significant. What

   is the likelihood of loosing a standby alternator AND

   the main alternator between two pre-flight tests?

Instead, I'd rather have the EFIS just provide a display showing a 

single output of either alternator - whichever is producing 

current.  What seems to me to be a simple way of achieving this would 

be to just tie the output of each alternator to the same side of the 

shunt (the terminal opposite that going to the contactor).  Then, it 

would seem to me as though the shunt would just measure the combined 

output of both alternators, which in effect would be just the output 

of the alternator that's currently active.  But when I asked the EFIS 

manufacturer about this, they implied that this wouldn't be the 

"right way" to do it - instead, I should use a hall sensor and pass 

both wires through it.



But I can't understand why using the shunt in this manner wouldn't 

work (other than the shunt becoming a single point of failure if it 

should blow like a fuse).



   It would . . . but perhaps hooking neither alternator

   to the EFIS is a better plan. Your alternator's output

   is a KNOWN quantity that is, by design, loaded no

   heavier than original design goals. As long as

   your low voltage light is not flashing, all is

   right with the universe . . . irrespective of

   any reading you might observe for the alternator.



   Similarly, should main alternator fail then the

   standby alternator should be loaded no greater

   than its capability to keep the bus at 12.5 to 13.0

   volts. Again, the voltmeter for one bus is your

   primary statement of electrical system health

   for flight ops.



   Attaching both alternators to a single shunt

   forces both output paths to share circuitry.



   If you'd like to have these paths be totally

   independent of each other, be circuit protected

   with a device appropriate to alternator size,

   -OR- (as in Z-12/ have the two alternators

   drive different parts of the system, then

   the two wires through a hall sensor is the

   preferred architecture.

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Feb 12, 2015 7:21 pm Post subject: Question about monitoring output in two alternator configu

Hilarious.  They ship the EFIS a with a shunt, then tell their 

customers that using the shunt isn't the right way to set it 

up.  Customer support by Laurel & Hardy.



   One shunt per alternator is how it was

   done for the last century or so . . . still

   works good. Tying two alternators to the

   same shunt raises questions about the

   loss of independence/redundancy of the

   two systems.



   Bob . . .

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