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B&C Alternator Troubleshooting

Toobuilder

Well Known Member
My former RV-8, which served me flawlessly for 11 years, is now giving the new owner fits (of course).

The airplane is equipped with 2 B&C alternators (60 amp and pad driven 40) and two LR3C regulators.

The new failure mode is the primary alternator field breaker (terminal #6 on the regulator) will pop early in the flight. Charging is good up to that point. Attempts at reset result in near immediate pop. Standby alternator will pick up all load when selected.

I ran a separate wire from the breaker, bypassing the "alternator" switch in an attempt to rule out intermittent shorting somewhere in the bundle. No change in behavior.

I also switched the regulators to see if the problem followed. No change in behavior - "new" regulator pops the breaker with same frequency.

New Battery (PC925) seems to have taken a beating during all this and will accept a charge and top off, but has very limited capacity.

Battery cables cleaned and inspected.

Looks like the last suspect is the alternator itself, but not sure how an unregulated unit can drive that behavior.

Any thoughts are greatly appreciated.
 
Almost sounds like an OV sense or loose field connection which can trigger an OV trip.
Try pulling the bus sense wire off pin 3 and jump it to pin 6. (This will eliminate a few items at least)
 
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Walt - the pop of the CB is supposed to be in response to an OV event and subsequent crowbar from the regulator. Its possible that its a very fast transient, but the EFIS does not show anything strange going on. It sits there at 14.1 v and then pops. It does seem to be related to temp however. I can abuse it for half an hour on the first flight of the day without fail, and then there is no recovery until the next day.
 
Sounds like field wire connection to me. I had a long saga where the symptoms were similar years ago. And I was "SURE" the field wire was all good until I ripped the whole thing out, including the connector at the alternator and simply replaced it. Never did find the smoking gun, but that permanently fixed it.
 
Sounds like field wire connection to me. I had a long saga where the symptoms were similar years ago. And I was "SURE" the field wire was all good until I ripped the whole thing out, including the connector at the alternator and simply replaced it. Never did find the smoking gun, but that permanently fixed it.

Agree with Scott, intermittent field can easily cause a spike on the bus and trip the OV circuit. The 60a alt connector would be high on the suspect list. (I prefer the connector on the 40a unit, more reliable.) The reg is quite sensitive/fast reacting to voltage spikes, I’ve seen the lack of a diode on a contactor cause the brkr to pop.
 
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Looks like the last suspect is the alternator itself, but not sure how an unregulated unit can drive that behavior.

Any thoughts are greatly appreciated.

The field circuit in the Alt is a copper winding that generates the magnetic field. The greater the field current, the stronger the magnetic field and the greater the Alt's output. If the Alt is under performing for any reason (i.e. less output at a given field current input), the VR will keep up'ing the field current to force the Alt to increase output, and thus increase the bus voltage. That VR does not have an internal fld limiter, instead it uses the input source CB for that purpose.

Therefore, a sub-optimal alt can cause excessive field current from the VR and if that goes beyond the CB or fuse rating - Pop. The Alt IS regulated, it is just regulated by the external VR and not via it's own mechanism. They dance in harmony.

CB pops at startup (typically the time of greatest alt output) can also be excessive resistance on the voltage sense lead. The resistance drops the voltage that the VR is reading and so it keeps up'ing the fld current to get it back to the set point. Same as above, this can pop the CB or trip the OV protection circuit. In my case, it was the screw on the back of the CB loosening that created the problem. However, the B&C has a separate volt sense input, so probably not coming from the CB in your case. I would disconnect the voltage sense wire from the VR and measure the voltage on it and compare to the bus voltage from another source. If that checks out, next step is the Alt. If the voltage sense input is just bridged from the main power input (not recommended by B&C), then the CB or it's circuit could be causing the resistance.

Third element is the OV protection in the VR. This is hard to separate in diagnosing, as the crowbar trips the input CB when it happens. That said, it is most common to get a high voltage alert when it happens. In my case, I could see the voltage rise, then pop the CB. Can't remember if the B&C crow bar activates from the power input or the voltage sense, but speculate it is the latter.

Larry
 
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It sits there at 14.1 v and then pops. It does seem to be related to temp however. I can abuse it for half an hour on the first flight of the day without fail, and then there is no recovery until the next day.

Likely diodes in the ALt going bad. There are three of them, creating a rectifier circuit (this is what converts the alternators native AC output into DC). They are the most common culprit when temp related issues appear. YOu can take it to a shop for testing, but will need to heat it with a heat gun during the test, given the symptoms.
 
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CB?

Have you tried replacing the CB?

CB was my first thought, too. Corroded contacts on a circuit breaker can make a 5 amp CB act like a 2 amp CB. "Heat related" also supports this guess. Maybe measure that that field current. A 6 dollar Harbor Freight mulimeter on the 10 amp scale connected temporarily to the field circuit would give you some good science to follow.

Max field current is most often present with the master on with engine stopped or idle. The circuit breaker should be well able to support that current, so the diode failure idea may be a bit of a reach. BTW, 6 diodes, 2 sets of 3 each.

Rotor field resistance is the main thing that limits field current. Rotor resistance increases with heat, reducing field current and possibly maximum output, but field windings shorting when hot could easily pop a CB. So, that's another good? guess.

Good Luck.

Ron
 
Sounds like field wire connection to me. I had a long saga where the symptoms were similar years ago. And I was "SURE" the field wire was all good until I ripped the whole thing out, including the connector at the alternator and simply replaced it. Never did find the smoking gun, but that permanently fixed it.

I had the same problem. Turned out the brand new (from B&C) field plug (the plug at the end of the field wire that connects to the alternator) was defective. Apparently, the plug was arcing internally, causing the field voltage to exhibit a transient high voltage, which caused transient high voltage from the alternator, which caused the regulator OV to trip, which tripped the alternator breaker (as intended). I replaced the field plug, the problem has been permanently solved.
 
The arcing terminal theory seems to fit the symptoms. Owner is going to replace the breaker and inspect/tighten up the terminal in the plug and look for improvement. If so, we'll re plug and re wire that segment.

Will report back with findings. Thanks all.
 
2nd opinion

Almost sounds like an OV sense or loose field connection which can trigger an OV trip.
Try pulling the bus sense wire off pin 3 and jump it to pin 6. (This will eliminate a few items at least)

I'm going to fall in line with Walt and others that an intermittent in the field circuit is triggering the OV crowbar and popping the breaker.

That the circuit breaker will not hold at all after it pops more likely means that the crowbar is holding a ground on the circuit after the initial pop. If the circuit breaker had opened from a slow modest overload it should hold for a while when re-connected even with just a few seconds of cool down.

So, flaky field wiring, connector/connection as others have mentioned n. Maybe worn brushes/slipring, too.

Ron
 
Fixed!

Short version- the lug at the alternator output terminal failed and there were only a few strands of wire hanging on to carry the full load. This was not immediately apparent because it was still mechanically intact, but masked by heat shrink tube.

Longer version- years ago I built a new alternator cable and incorporated an automotive style fusable link right at the alternator. I carefully soldered the terminal lug to the fusable link wire and sure enough, over the years the solder soaked section of wire immediately adjacent to the lug fatigued and broke mostly through. The outer jacket of the fusable link and the heat shrink kept the whole mess together just enough to provide intermittent power to the airplane.

I think the make/break and high resistance caused all kinds of voltage transients and the regulator interpreted this as an overvoltage event, and crowbarred.

Several hours on the airplane now with no issues. Sometimes its the simplest things!
 
Thanks for the feedback.

Did you repair it the same way that you originally fabricated it? Any recommendations for future checks to avoid a repeat problem? Check every 10yrs lol...
 
Went back to the tried and true crimped terminal and ANL link at the firewall. Future inspections will inclide a "tug" at the terminals rather than just a visual inspection.
 
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