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Yet another question on shunt location...

Reflex

Well Known Member
After reading enough material on where to put the shunt(s)to get me fully confused, I've decided that there is no agreed upon location. My aircraft configuration is:

  • Primary Alternator/external regulator
  • Back-up Alternator/external regulator
  • Manual switch over upon failure of alternator(s)
My thinking is to place the shunt on the battery rather than on the B-lead(s). Here's why:

Set Primary alternator to 14.2V
Set Back-up alternator to 13.8V

Won't the voltage will tell me if I have a failure? Ie: if below 14.2V the primary alt has failed, if below 13.8V back-up alt has failed. At that point I'd like to know what is being consumed from the battery.

Is this line of thinking sound?

One other question, why would I want to know the output of my alternator(s)? Seems if I have proper voltage, they are operational.

Thanks,

Fred
 
Yes but

Yes , if everything is going as expected, the voltage should tell you. But when the information will be the most important is when things are not going per plan. Then is not the time to be doing mental gymnastics to figure out what is wrong. JMHO

I have s simple system so in fact I have mine on the battery. Your system is a bit more complex, so It might warrant a different setup
 
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One other question, why would I want to know the output of my alternator(s)? Seems if I have proper voltage, they are operational.


All valid approaches, no wrong choice in my mind.

I would also suggest thinking about it in the context of a pilot not as familar with the plane as they should be. Either clear annunciation or a placard. Alt1 amps / Alt2 amps makes it pretty clear, if relying on voltage a reminder placard of the voltages could make sense.

Derek
 
I see little value to knowing how much current is going to the battery. It only shows you how hungry the batteries are without giving a clear view on it's state of charge. Knowing how much current is coming out of the alternator will tell you how much current your ship is consuming. This is substantially more valuable information. Not going to list them all here - search.

Voltage will only tell you if an alternator stopped working or is sub optimal. Much better to know current AND voltage for in air diagnostics.
 
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I put mine to read current that is going into the busses. If alt. fails you will see it in voltage as you mention. In the air is a terrible time to trouble shoot just about anything electrical. I'm then getting on the ground to figure out the problem. The only purpose of the backup alt. is so that I can get get home instead of being stranded somewhere.
 
I Agree

After reading enough material on where to put the shunt(s)to get me fully confused, I've decided that there is no agreed upon location. My aircraft configuration is:

  • Primary Alternator/external regulator
  • Back-up Alternator/external regulator
  • Manual switch over upon failure of alternator(s)
My thinking is to place the shunt on the battery rather than on the B-lead(s). Here's why:

Set Primary alternator to 14.2V
Set Back-up alternator to 13.8V

Won't the voltage will tell me if I have a failure? Ie: if below 14.2V the primary alt has failed, if below 13.8V back-up alt has failed. At that point I'd like to know what is being consumed from the battery.

Is this line of thinking sound?

One other question, why would I want to know the output of my alternator(s)? Seems if I have proper voltage, they are operational.

Thanks,

Fred


I agree, the voltage reading tells you the source of your electrical power: primary alternator, standby alternator, or battery. This information gives shape to one aspect of your mission profile: fuel limited or battery limited.

Here's some of my thought process about shunts (YMMV):

Since I was focused on schematic z-12 (dual alternator, single battery) which shows a shunt on each b-lead and subsequently realizing my intended EFIS has only 1 shunt input, forced me to think long and hard about where that shunt should go.

Obviously, measuring one of the b-leads (presumably primary) I'd lose amperage information just when I'm more intently interested. Since the b-leads tie together on a single post, putting a shunt next to that and before further distribution of current is like reading either b-lead.

In the micro-miniscule chance that both alternators have failed, measuring any b-lead deprives me of amperage information just when it is most important. I then realized that in this failed alternator configuration and by putting the shunt next to the battery (my preference is the negative side), I am effectively reading the overall current draw whether I'm on the main+e busses or just the e-bus.

In other words, I agree with your line of thinking.

In this configuration, a normal reading of 14.2V/0 amps is enough information for me. Most days, I couldn't care less whether the ship is consuming 22.8 or 47.3 amps. A table can be built of various voltage/amperage combinations and their meanings.

If more current measurement locations are desired, I believe an EFIS accepts Hall-effect sensors as inputs (details I've not worked out). While I have not settled on one, the most likely location for the first one of these is the main bus. Unfortunately, Hall-effect sensors cost several times more than a shunt.
 
After reading enough material on where to put the shunt(s)to get me fully confused, I've decided that there is no agreed upon location. My aircraft configuration is:

  • Primary Alternator/external regulator
  • Back-up Alternator/external regulator
  • Manual switch over upon failure of alternator(s)
My thinking is to place the shunt on the battery rather than on the B-lead(s). Here's why:

Set Primary alternator to 14.2V
Set Back-up alternator to 13.8V

Won't the voltage will tell me if I have a failure? Ie: if below 14.2V the primary alt has failed, if below 13.8V back-up alt has failed. At that point I'd like to know what is being consumed from the battery.

Is this line of thinking sound?

One other question, why would I want to know the output of my alternator(s)? Seems if I have proper voltage, they are operational.

Thanks,

Fred

Agree with your sound thinking.

I have shunt in the battery-to-system there for two reasons: 1. The amp shunt will tell me what all the system demands are. 2. When an alternator fails, then I can still monitor ampere demand to manage to a specific number.

And - yes- -you can determine the alternator health by its voltage. I use voltage to assess the alternator health, it is the primary indicator of health. One caveat though . . you could use a discrete input to capture the 2nd alternator voltage if needed, I use a discrete for monitoring a backup battery voltage for drain.

Not sure why you desire a manual switch for the alternator though, a simple alarm light can be used to indicate the failure/backup active condition.
 
SNIP
Not sure why you desire a manual switch for the alternator though, a simple alarm light can be used to indicate the failure/backup active condition.

+1
Assuming you are using the B&C backup alternator, or the nice Monkworkz generator on your vacuum pad there is no reason to have a manual switch.

If the primary alternator fails:
- With the B&C the yellow light that comes with the backup alternator VR comes on telling you the primary alternator failed
- With the Monkworkz it sends a signal when it is on to an “Aux Alt” light I programmed on the SkyView EMS display.

Thoughts on shunts:
- Three RVs and never installed one, and do not find a need to have it. During testing I know exactly what power draw on the plane so having a constant measure of that adds no new information - that and I consider voltage a better indication of operation and early indication of problems. I have no problem with people who do but I just dislike the clunky thing on the firewall - along with the big terminals. If you must have a shunt I suggest getting a current sensor from Grand Rapids: https://grtavionics.com/product-category/eis-probes/current-sensors/
- If you use the Monkworkx generator it has a built in current shunt. All you do is wire it to your EMS.

Carl
 
The shunt should NOT carry starter motor current. That rules out connecting the shunt to the negative battery terminal. Not only could the shunt be damaged by starter current, but the added resistance will slow down the engine cranking speed. Carl's advice is good about using a hall effect sensor which only costs $25 more than a shunt. The vast majority of electrical problems are due to bad connections. Eliminating the shunt eliminates 4 connections: 2 crimps and 2 bolted ring terminals that can corrode or loosen or short to ground.
 
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