>My experience has been that most of the hardware old enough to use an
>external regulator is also less reliable. Note that I said 'my
>experience' and 'most.' I went through 4 externally regulated
>alternators & at least that many regulators in about 4 months before
>switching to an internally regulated alt. almost 5 years ago, with not a
>single glitch since. None of the above failures were of the o/v nature &
>none damaged any avionics.
Agreed . . . and MOST of the externally regulated alternator
hardware out there is specific to aircraft and designed/certified
20-30 years ago. Check the service difficulty reports using keyword
"alternator" and then filter for single engine aircraft . . .
Everything that might be deduced in the way of alternator failures
happens every month in a TC aircraft . . . "casting broke, thru-bolts
stripped, brushes worn out, windings burned, bearings siezed, diodes
shorted, etc. etc." FBO's love 'em . . . regulated job security well
into the new century.
On the other hand, when you take a brand new, Nipon-Dienso, 40 or
60 amp alternator, disassemble for modification to run external
regulation, balance the rotor to about 10x tighter specs than
they come out of the factory and re-assemble with due care, you
end up with an alternator that runs well for a very long time.
I have first hand knowledge of 2,000+ such alternators sold over
the last 8 years . . . not one has returned for wear-out or
repair. It's the difference between a 1990's product and a
1960's product.
>I don't use o/v protection, but if I did, it would probably be the very
>simple zener/fuse arrangement. I'm sure Bob's solid state system is more
>sophisticated & works better, but as the guy used to write in BYTE
>Magazine, 'Better is the enemy of good enough.'
The zener fuse combo was certified onto early American and subsequently
Grumman-American aircraft. It's a sort of poor-man's crowbar ov
protection scheme. When I first heard of it, I was skeptical.
It has been about 15 years since I brass-boarded this system onto
an alternator-battery system in the lab. Here's what I found.
Proper operation of the system depended heavily on two things.
(1) A fuse (fast acting) had to be used upstream of the zener
and (2) the zener had to be a 1W glass encapsulated device -
p/n 1N4745.
It works like this: In an ov condition, the zener tries its
best to keep the bus voltage from rising above 16 volts. In so
doing, internal disipation rises well above the diode's 1W
rating and it commits electronic suicide by becoming a dead
short. The resulting short opens the fuse and corrals the runaway
alternator.
Over the years, folk who did not understand the "balance of
power" implicit the this design made well meaning-changes to
this scheme with the unintended consequences of degrading
performance or even making the system ineffectual. Common
errors include:
(1) Replace pesky fuse with a real circuit breaker: Opening
times for breakers vs. fuses is 10x to 50x longer. The slow
response of the breaker stresses the zener to explosive
destruction. The altenrator runaway continues unabated.
(2) Substituted any ol zener with the number "1N4745"
printed on it: Plastic parts were unable to withstand
the rapid onset of heat dissipation and explosive
destruction of the zener results. The runaway continues
unabated.
(3) Substitute a really husky 16 volt zener for the itty-
bitty 1W device. This change was often combined with
a change from fuse to circuit breaker. The general idea
was to make the protection scheme "reusable" . . . no
fuses -or- zeners to replace: The general effect of this
modification was to push the time-constant for tripping
OV protection out by hundreds of milliseconds to perhaps
several seconds. Contemporary OV protection is designed
to react to a step from 14-20 volts on the bus in 50
milliseconds or less.
>My personal feeling about external regulators & o/v protection is this:
>Once I reach a certain (hard to define) comfort level about the
>reliability of a system, I'd rather not add failure modes. Over-voltage
>type failures in self-regulated alternators seem to be so rare that you
>hear about every one. Generator/regulator & alt/regulator failures are
>so common that they are treated like dry vac pump failures, you know
>they are going to happen sometime in the near future.
No argument about comfort levels . . . I'll suggest that
the greatest body of experience with aircraft alternators
comes from the world of certified aircraft. My best
recommendation is to see what's happening with true
state-of-the art designs and fabrication techniques.
There's a mistaken perception that the certified aircraft
world is benefiting from the advance of technologies
in all respects . . . I'll suggest it happens only in
area of things you bolt into holes on the instrument
panel. Stuff under the cowl has evolved very slowly if
at all in 50 years or so that have passed since
the first generator was bolted to a single engine
airplane. If you want to know what a modern alternator
can really do for you, you'll have to limit your
observations to the real leading edge of aviation
technologies . . . check out the flight line at
OSH.
Bob . . .
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( Knowing about a thing is different than )
( understanding it. One can know a lot )
( still understand knothing. )
( C.F. Kettering )
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http://www.aeroelectric.com
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