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Help me find this gremlin

Mike S

Senior Curmudgeon
Engine is IO320 with Nippon Denso 40A alternator modified for external regulator. Regulator is B & C LR3D-14. Modification was done following instructions in Kit Planes article from a few years back.

On the second flight, the field breaker (5A) was popped when the plane was back at the hangar. No idea when it popped. Pushed the breaker back in with the battery still on, and it refused to reset. Turned the battery off, pushed the breaker in and it worked just fine. Turned the battery back on, and the breaker stayed in, so we tried a ground run, all fine until the engine was shut off, then the breaker popped.

Took a couple days off, and thought about the cause and came up with a possibility-----I suspected a loose field wire, or possibly the field wire has an intermittent short to ground.

Today I pushed the breaker in and it reset just fine. I turned the battery on, and powered up the system and shook, wiggled, moved the entire field wire FF including the fast on connectors at the alternator-----breaker stayed in.

Another possibility was trying to reset the breaker with the battery on. I also tried that today and it reset just fine.

Out of ideas at the moment.

What does the VAF brain trust have to say?
 
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A bad breaker?

A faulty breaker would have the same result. Or the wrong size breaker - should be at least 5 amps....
 
I had a lot of time to think on this on my trip to OSH Mike - I got to wondering if the fact that you have the backup SD-8 alternator on all the time might have something to do with creating a transient at shutdown. But I couldn’t recall how you actually have the two alternators wired up to the busses, with the the two batteries and battery masters. I couldn’t up to any conclusions without a drawing - maybe if you can post a sketch it might help with the he’d scratching…..

Paul
 
I modified an alternator for an external reg once in our sailboat diesel. It is not a simple job and easy to make a mistake, like a bad solder joint or the joint to close to a ground point or even mistake which wire is the field. I suggest that you confirm your work inside the alternator. The wires tend to be filled with contaminants and it is very easy to get a cold solder joint that will open intermittently.

Also, some alternators use Neg ground for the field and some use Pos ground. I can't recall which the NDs are from memory. Has it ever worked? Have you seen charging output? If not the field Grd polarity could be the issue.

Popping a breaker at shut down or with the engine not running is a very uncommon failure mode (back in the day, the OEMs had no fuse or fusible like on the field connection between the reg and alt) and this points to something being mis-wired or something being messed up in the conversion process.

Lot of good data on Google. I would find a internet reference for this task and compare it to the Kitplanes instructions that you followed. Sterling makes a specialized charging regulator and there instructions for modifying an INT reg alternator are VERY detailed. Believe you can get it online. It covers all the different ways you can mess it up.

Larry
 
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My guess is excessive field current thru the alt, bad brushes maybe.

I believe the field current is fed through the windings in the case and does not pass through the brushes. I believe the brushes carry the main output current.
 
I did the same modification using the same Kitplanes article and I have a few thoughts

First, the graphic which shows the solder nub you need to attach to is wrong. The red circle is indicating the wrong nub. This caused me some hours of rework before I finally wrung out the correct path with a multimeter.

Second, the internal routing of the wire you add is pretty tight, IIRC. It seems there is a good potential for an internal short.

Hope this helps. I have a year/80 hours on mine and its been rock solid so far.
 
I believe the field current is fed through the windings in the case and does not pass through the brushes. I believe the brushes carry the main output current.

I believe you are correct if you are talking about a 'Generator', an alt has the field spinning.
 
I had a lot of time to think on this on my trip to OSH Mike - I got to wondering if the fact that you have the backup SD-8 alternator on all the time might have something to do with creating a transient at shutdown. But I couldn’t recall how you actually have the two alternators wired up to the busses, with the the two batteries and battery masters. I couldn’t up to any conclusions without a drawing - maybe if you can post a sketch it might help with the he’d scratching…..

Paul

The electrical layout is the same as the RV 10 was, never has an issue with it.

Diode blocks reverse flow from aux battery/buss to main battery/buss. Each alternator feeds its battery/buss directly.

Do you remember if you did the shutdown sequentially -->fuel pumps-->coils-->ECU?? Shed the loads slowly (in the speed of electrons that is) to avoid a voltage spike from shutting off all at once?

The regulator has a built in over voltage function that doesn't like voltage spikes.
 
The electrical layout is the same as the RV 10 was, never has an issue with it.

Diode blocks reverse flow from aux battery/buss to main battery/buss. Each alternator feeds its battery/buss directly.

Do you remember if you did the shutdown sequentially -->fuel pumps-->coils-->ECU?? Shed the loads slowly (in the speed of electrons that is) to avoid a voltage spike from shutting off all at once?

The regulator has a built in over voltage function that doesn't like voltage spikes.

On the shutdown, yes - sequentially, per the procedure.
 
OK, I checked for loose wire/connectors. Nope.

Checked for chaffed wire that could short to ground. Nope.

Looked for loose wires, FOD, anything else behind panel. Nope.

Test ran engine again and 3 seconds or so after shutdown it popped.

Test ran again, shut off field switch before shutdown and no pop.

Ran tests a couple more times and it is now staying in like it should.

I hate spontaneous remission of electrical issues.
 
Trust me, problems don’t miraculously fix themselves, it will be back:eek:

Yep, that is what exactly concerns me when I can no longer duplicate the failure.

The popped breaker is only the symptom, I still have not found the problem.

I do have a good lead on something I need to check, Charlie England (RV7Charlie) and I have been discussing this and he thinks the voltage drop from the alternator output with the engine running down to buss voltage when the engine shuts off may be driving the regulator to try bringing the buss voltage back up. The regulator sends more and more current to the no longer spinning alternator until the breaker maxes out and pops.

I did try shutting off the field switch prior to engine shut down and no popped breaker. Now I need to actually measure amp flow into the alternator at the field connections to see if it goes up (over the 5a the breaker is rated at) when the engine is shut off.

This is why we do test flying on a new home built.
 
Well, just to close the loop on this-----all indications are that Charlie nailed it.

Yesterday I put an amp meter in line with the field supply.

Running current a little over 2.5 just after start, dropped to under 2 in a minute or two.

I shut off the engine with the field switch still on----amp meter started climbing as soon as the engine was shut off. Last number I saw was 3.8 then the breaker popped.

Tried it again but shut off the field switch before engine shut off----amp went to 0 and stayed there.

Guess the voltage regulator is smarter than I am.

Charlie too.

Take care, fly safe out there.
 
Have used

I replaced my alternator when I rebuilt my engine. (bigger pistons) So I have probably the same alternator sitting on the shelf if you want to swap out for testing. Yours for the price of the shipping. PM me. Randy
 
Just a guess - but if B&C recommended a 5 amp fuse/CB for its regulator, I'd think that regulator cannot put out more than 4.5 amps. Which leaves the CB itself. Perhaps it's tripping at 4.4 amps, especially when it's warm. Is the breaker mechanically tied to a metal panel (heat sink)? And is it in a particularly warm spot? For the price of a CB I'd be tempted to just try a new CB.
 
Just a guess - but if B&C recommended a 5 amp fuse/CB for its regulator, I'd think that regulator cannot put out more than 4.5 amps. Which leaves the CB itself. Perhaps it's tripping at 4.4 amps, especially when it's warm. Is the breaker mechanically tied to a metal panel (heat sink)? And is it in a particularly warm spot? For the price of a CB I'd be tempted to just try a new CB.

Metal panel, yes. Overly warm spot---no.

I agree that it could be a weak CB, but it is going to be a bear to switch out.

Will keep that in my bag of tricks if the problem occurs again.

As for now, a minor change to the shut down procedures takes care of things.
 
I replaced my alternator when I rebuilt my engine. (bigger pistons) So I have probably the same alternator sitting on the shelf if you want to swap out for testing. Yours for the price of the shipping. PM me. Randy

Randy, appreciate the offer, but nothing points to the alternator as the problem.

How many hours on your alt, is it set up for external regulator?
 
alternator

The alternator is approximately 5-7 years old. It is not set up for an external regulator.
 
---amp meter started climbing as soon as the engine was shut off. Last number I saw was 3.8 then the breaker popped.

Are you not concerned that your 5A breaker is tripping at 3.8 amps? If I remember the charts correctly, it should take close to an hour @ 5.5 amps to trip a 5A thermal breaker. Yours it tripping within a few seconds at 3.8.

Larry
 
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Are you not concerned that your 5A breaker is tripping at 3.8 amps? If I remember the charts correctly, it should take close to an hour @ 5.5 amps to trip a 5A thermal breaker. Yours it tripping within a few seconds at 3.8.

Larry

The meter was rising rapidly, not even displaying all numbers. It is possible it was over 5 when the breaker popped.

Kinda like watching the cents column when you fill up with gas.
 
Bob Nuckolls has posted that max alternator field current is around 3 amps.
Search this archive for "3 amps".
http://www.matronics.com/digest/aeroelectric-list/Digest.AeroElectric-List.2016-08-21.html
For testing purposes, parallel the 5 amp breaker with a 10 amp fuse. Then measure the field current with engine off.
Having that info will help with future decisions.

This is correct. The normal field resistance is high enough that the current will never exceed the 5A of the breaker. Think about it - sitting on the ground with the master and field on and engine off, the regulator will max out the filed current trying to charge the battery, which won't because the engine isn't running, but the breaker normally never trips. Measure your field circuit resistance to ground with the field switch off. It should not be less than 14V/5A=~3 ohms. Less will trip the breaker. Typical is 6 or 7 ohms. Can check on the B&C site troubleshooting tips. There's likely a problem with the alternator field/wire, or possibly a weak breaker is my guess. The parallel fuse is a good test. If you can pull your breaker, or after tripped, leave it and parallel with a 5A fuse.
 
Mike, thinking about your gremlin some more, it all makes sense for either a weak breaker (somewhat unlikely) or a field resistance problem. When the engine is running, the field current is only what is necessary for charging to the setpoint voltage, and with a good reasonably charged battery, is some mid nominal current. When you shut down and the electric is still on, the alternator stops charging, but the regulator doesn't know the alternator isn't turning anymore, so it maxes out the field current trying to charge. (for the electronic geeks, the TO-3 series pass transistor on top of the B&C regulator is full on, pretty much connecting the field directly to the battery, through the breaker and switch). Normally that isn't enough current to trip the breaker.

So, I would suggest, with care, shut down, cowl off, remove the field connector at the alternator and connect the alternator field input directly to the battery, through a 10A or 20A fuse and a series multimeter in 20A mode, and see how much current the field draws in static position. If it is greater than 5A (or 3A for that matter), you have a partially shorted field or incorrect mod. If it is an acceptable current, you have a field wiring problem. This requires some alligator clip leads, and care not to accidentally short the battery + to ground in the process.

Reinhard Metz
 
So, I would suggest, with care, shut down, cowl off, remove the field connector at the alternator and connect the alternator field input directly to the battery, through a 10A or 20A fuse and a series multimeter in 20A mode, and see how much current the field draws in static position. If it is greater than 5A (or 3A for that matter), you have a partially shorted field or incorrect mod. If it is an acceptable current, you have a field wiring problem.

Reinhard Metz

Reinhard, thanks for the info.

My multimeter only has a 10a capacity, will give your suggestion a try with that using a 10a fuse in line.

I will report back with the results.
 
Measure your field circuit resistance to ground with the field switch off.

O.K.got the measurement for you.

Field wire removed from the alternator, gave me a reading of 11.1 ohms. This was from the blade connectors to the case.
 
So, I would suggest, with care, shut down, cowl off, remove the field connector at the alternator and connect the alternator field input directly to the battery, through a 10A or 20A fuse and a series multimeter in 20A mode, and see how much current the field draws in static position.

I just did this test, got 3.2A reading.

The battery may have been a bit low, as I have been doing a lot of limit setting on the EIS and EFIS units------and have had a few folks who wanted to see the panel lit up. I have no idea if this could have caused an error in the amp draw or not??
 
At least the alt and reg is ok . . .

I just did this test, got 3.2A reading.

The battery may have been a bit low, as I have been doing a lot of limit setting on the EIS and EFIS units------and have had a few folks who wanted to see the panel lit up. I have no idea if this could have caused an error in the amp draw or not??

A spot on test, Mike, and since it has the alternator isolated confirms your regulator can go dead short and it won't exceed the breaker rating. I might suspect that the breaker is misrated since it seemed to pop at a lower amps than the name plate printed on the side, not the button. thinking out loud, I thought breakers had a higher amp pop than a fuse, so maybe a 5 amp fuse as a test instead of the breaker (as a test)? Unless you have an o-scope to measure instantaneous volts/amps. How about Paul the-tool-man? Regardless: Sounds like the breaker is the next point of elimination now?

You don't have some led tied to the field that might short and do this? Surely not as you know better.

You mentioned the crowbar for voltage. That sounds like an attractive red herring and could short the field, but why would it think a voltage was too high? Can that be measured too (regulator test)? My adjustable power supply does not go higher than 15.00 VDC.

The wiring diagram could be the same but components are different. A wild thinking stimulator for a future elimination/diagnostic step.

Sorry - nothing definitive. Happy hunting!!
 
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Test ran engine again and 3 seconds or so after shutdown it popped.

Test ran again, shut off field switch before shutdown and no pop.

Ran tests a couple more times and it is now staying in like it should.

From the above and your measurements: OK, so your static field current is just about what it should be and after shut down should not be able to trip the 5A breaker. With the shut down sequence of shutting off the field switch, breaker trips become impossible, as the field circuit is now open. So the remaining (likely) possibilities are a weak breaker, or a transient that causes the over voltage crowbar to trip the breaker. So, some thoughts on how to determine which it is. One thing would be to parallel the 5A breaker with another one, and use the engine shut down sequence without turning off the field switch first, which has in the past resulted in the breaker trip. If you have a weak breaker, then neither breaker will trip. If it is a transient, then the regulator crowbar will easily trip both breakers. And then, it would be back to trying to figure out how the transient happens, which I would need to see a schematic of the regulator/alternator/batteries etc. total wiring.

Reinhard
 
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