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Just a note to all the Lancairians,
Finally found it -- brushes were worn in the alternator to < 1/2
their normal length, causing "brush hop" -- solution $19.95 for new
brushes (even bought a spare set). I guess the old adage, "When
there's a problem, think horses not zebras". I know, I know; this is
is only for us N.A. folks ;-)
Tried all of the normal things: bench tested with VR twice, soldering
all terminals, checking grounds, replacing some suspect crimps,
replacing a circuit breaker, bypassing the ALT position on the Master
switch, etc. Nothing helped and it just kept getting worse. Was just
about to the point of hooking up an oscilloscope to the VR/alternator
combo (with a 200' foot extension cord) to find out what was starting
the failure mode.
The generator/alternator person stated this was the first time he'd
ever seen the kind of symptoms for having bad brushes. Normal symptoms
is the alternator working correctly on light loads but blowing the
field breaker with heavier output loading -- my issue was just the
opposite! Guess with 20/20 hindsight, the failure makes sense. Excess
vibration being transmitted to to the brushes at specific RPMs caused
the brushes to hop away from the commutator just long enough to let the
output voltage drop. Then when the brush again made contact, the
alternator output spiked to over 16.1V for >50msec, causing the VR's
SCR OVP to trigger, popping the Field circuit breaker. Since even
B&C Specialties could not tell me the time constant for the
VR/alternator closed loop system, the differing vibration frequency
from certain engine RPMs would cause a long enough hop.
Thanks again to everyone who had some thoughts on this. Also to Ben at
Gilgen in Gilroy, CA for his perserverence and specialized test
equipment and finally to Tim at B&C Specialties in Missouri for
providing invaluable information on how the LR3B internals were
actually working and timings.
jim...
N1222K, LNC2, 597.6 hrs
Thanks Jeff, really appreciate the inputs -- I'll check it. Had the
original battery in since April 2000 and finally replaced it in
September 2005.
Thought I had the problem licked when I did the following about 2 weeks
ago:
- pulled out the alternator & VR, had them tested as a pair on
a bench for 15 minutes at 60A load with no problems,
- thought I found a nicked wire (on the field wire between the VR
and Alternator side of the split Master switch),
- cleaned up some minor dissimilar metal corrosion (on VR bolts and
ground screw from alternator into the engine crankcase),
- re-routed a wire bundle, and
- then flew for 10+ hours with no issues.
Then went to Baja and it started again. Just barely made it back to
home base. Now the field circuit breaker trips 12 times in just going
once around the pattern. Right now it seems to be heavily weighted
towards vibration sensitivity since it's now occuring when the oil temp
is just barely 100F and cylinder head temps are 250+ with no heat
soaking required.
We're using a B&C Specialty voltage regulator (VR), LR3B, with a
built-in OVP (65A SCR that is triggered with a 50msec
pulse/intermittent). This then shorts the alternator field lead to
ground, tripping the alternator field breaker. The alternator is a 90A
Bosch, AL9971N replacing the original alternator in late 2001 (the
original aviation grade alternator went bad in the first 100 hours or
so).
Talked with the voltage regulator designer at B&C and his input was
that the OVP is most likely tripping the field breaker. This can be
caused by a very large number of things:
- a very fast open circuit intermittent in either the field or B
lead (+12V output lead from the alternator);
- a poor ground (a real possibility in our fiberglass birds);
- bad crimp in any one of the wires; a bad master switch;
- misaligned commutator brushes in the alternator;
- etc.
As a note, the previous time I put it together I found some mild
dissimilar metal corrosion underneath the washers bolting the VR to the
backside of the firewall (a fine, whitish opaque powder in some places).
I thought the next steps would be (before shotgunning it by replacing
the alternator & VR):
- Solder ALL the crimp joints in the field and ground system,
- jumper the Alternator side of the split Master switch to be ON
all the time,
- add a separate ground wire from the VR unit directly to ground,
i.e. not relying on the firewall bolts to get the good ground), and
- have the alternator person open the alternator and see if the
commutator brushes are not misaligned or too short)
I'm going to try and do that all tonight then test fly it Thursday. If
that doesn't improve things, then get out the ol' shotgun...
I guess the good news is that now it's happening often enough that it
should be much easier to find ;-)
Any other ideas are definitely welcome.
jim...
N1222K
LNC2
VTAILJEFF@aol.com
wrote:
Jack,
If you have not found the problem I would try the battery.
Sounds like a problem I had with a bad cell in the battery on my IVP.
It would crank the engine fine but about 5 minutes after takeoff it
would cause the field cb to trip. I went through troubleshooting
everything you mention here and found it was a bad cell in the battery.
Regards,
Jeff Edwards
LIVP
Marvin Kaye wrote:
Posted for "Mark Ravinski" <mjrav@comcast.net>:
Jim,
Maybe your field breaker has mechanical issues. Try swapping that
out.
Mark Ravinski
N360KB
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