nuckolls.bob(at)aeroelect
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 Posted: Sun Dec 07, 2008 9:47 am Post subject: Dancing ammeter and alternator drop outs |
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At 09:28 AM 12/7/2008, you wrote:
| Quote: | --> AeroElectric-List message posted by: "Gautier, Thomas N" <thomas.n.gautier(at)jpl.nasa.gov>
Bob,
Thanks for your reply. I'm sorry I was not more specific. |
Not a problem . . .
| Quote: | The current symptom of the airplane is that the alternator will shut
down and produce no current. The load meter goes to zero and the voltage
drops to the battery voltage, a little over 12v on my flight. Turning the
alternator half of the split master off and then back on restores alternator
output, the load meter goes to 70A briefly and then starts dancing. I
carelessly did not observe the voltage with the alternator working. After
ten or 15 minutes the alternator output will again go to zero. Cycling the
alternator half of the master will again restore the load meter indication.
The 5A alternator field breaker does not pop during this process. I am
unsure how long I left the alternator off line before cycling the split
master. |
These split-rocker switches are close cousins to the S700 series
Carling toggle switches offered by B&C and described here:
http://aeroelectric.com/Mfgr_Data/Switches/Carling_G-series.pdf
http://aeroelectric.com/Pictures/Switches/Carling_Cutaway.jpg
http://aeroelectric.com/Pictures/Switches/Split_Rocker_Front.jpg
http://aeroelectric.com/Pictures/Switches/Split_Rocker_Rear.jpg
I used to have a plastic bag in my desk drawer that contained
two split-rockers removed from TC aircraft that were plagued
with some form of alternator recalcitrance . . . teardown inspection
showed that the contacts on the alternator were badly degraded
due to combinations of corrosion exacerbated by time, and current
draw thorough the switch's rising contact resistance. Next
time I put my hands on them, I'll get them photographed for the
rogue's gallery of switch failures.
| Quote: | I did search the archives for 'dancing & ammeter' and I understand the
issue of excessive resistance in the voltage sense/field current wire. Looks
to me like this plane has that problem. |
It's quite possible . . .
| Quote: | You seem to be saying that the
alternator drop out I describe is not related to the dancing ammeter symptom
but that the drop outs are probably caused by a loose connection. But why
would cycling the alternator half of the master reliably restore operation
if there was a loose connection? |
You're very close to answering your own question with
a high probability of accuracy . . . what gets "wiggled"
when you cycle the switch?
| Quote: | | Are there any more tests you would suggest? |
Sure, take a voltmeter along and do the divide/conquer study.
Connect a test lead at half-way between bus and alternator
field terminal. See if you can get the problem to repeat.
If symptom repeats and voltage disappears during alternator
misbehavior, move test point toward the bus 1/4 of the way
and test again. If you're feeling really confident about the
switch, just probe the downstream side of the switch.
Now, if there's alternator ammeter wiggle when things are
working "normally", then perhaps you can get the folks to
do the bus-to-regulator refurbishment cited in the aforementioned
articles . . . most likely, this will fix both the dancing
ammeter -AND- the alternator drop out. If you do replace the
switch for any reason, I'd like to have the carcass.
Bob . . .
[quote][b]
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