nuckolls.bob(at)aeroelect
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 Posted: Thu Jul 24, 2014 6:31 am Post subject: one-wire alternators- the subject that just won't die |
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At 02:58 PM 7/23/2014, you wrote:
Hi Bob,
Please forgive me, but I need a refresher on why one-wire (internally regulated) alternators are no longer recommended for a/c; even banned from 'the Book'.
I've never 'banned' anything from the book. But in the
interest of brevity and shortest path to demonstrated
success at low risk levels, I have focused AeroElectric
efforts on a limited range of ideas and products that
embrace the best-we-know-how-to-do . . .
Far too many over-excited people have morphed my
recommendations into warnings: "DON'T DO THAT lest
you fall out of the sky an die!" I have never
worked on or read about an accident that had
root cause in failure of electrical parts to perform
as advertised . . . EVERY case I'm aware of had roots
in human failings to understand limits imposed by
poor craftsmanship or operation of those parts.
I do remember (scratch that; know from experience) that if the B-lead contactor opens while the alternator is under load, there can be a 'load dump' (high voltage spike) issue that can kill the electronics (the regulator) in the alternator. Makes sense.
Only on certain alternators that fall short of the-
best-we-know-how-to-do. Consider this excerpt from
the chapter 3 of the 'Connection
http://tinyurl.com/nchenmr
[img]cid:.0[/img]
But if the only way the B-lead contactor is allowed to open is due to an overvoltage fault, does it matter? Â At that point, it's a given that the regulator is already toast, and the entire alternator will be swapped out to fix it. Given the very high reliability of modern automotive alternators, and that the only time the B-lead contactor would need to open under load is during a true OV fault event, what's the compelling reason to keep the 'not recommended' philosophy?
Not a 'big deal' as you have deduced. The 'problem'
is rooted in legacy cockpit protocols that do
not prohibit turning off a loaded alternator.
You can do it in any TC aircraft without risk
of damage. You can do it in any aircraft fitted
with an alternator of the quality cited in the
narrative about MPA. But unlike the cookie-cutter
conformity control offered by the TC aircraft
world, we have no way to KNOW what quality of
alternator was being offered by Van's . . .
it seems that one or more of his customers
experienced a untimely demise of his alternator
by switching it on and off with the b-lead
contactor.
I've had a couple of different career paths involving running a soldering iron & modifying stuff (both consumer and industrial electronics maintenance), so I'm not 'afraid' to dig into an alternator to modify it for external regulation. But I'm of the 'if it ain't broke, don't fix it' school.
By all means, dig in. Photograph every step.
Show us what you found and how you dealt with
it . . . you're not 'fixing' it, you're adapting
it to higher design goals.
I'd like to be educated on what I'm overlooking in terms of risk, failure modes (fire?), etc.
If it's reasonable to bring the one-wire alternator (with contactor style OV protection)back into the mix, the next question will be about integrating a backup dynamo style alternator without spending close to half an Aviation Unit for the hardware. 
Yeah . . . but. The same project economics are in
play here as for the dummy-load conversation we had
earlier this week. The $parts$ are cheap but the
$time$ is significant. Further, there are $risks$
for not being aware of all the design subtleties.
Not risks of life, limb and airplane . . . but of
future $time$ expended to 'fix' something overlooked.
You can purchase an ND cored reman for a very reasonable
price. But check installation dimensions so that you
can use a commonly available bracket. You may need to
find a new pulley. Doing the 'mod' is straightforward
and low risk.
The PM alternator is a bit harder. You need either a pulley
on the engine and brackets to hold the alternator . . .
or the machining resources to craft a pad-friendly spline drive.
I think the SD-8 is pretty attractively priced given
the effort it would take to DIY.
Bob . . .
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