==== excerpts from a conversation I've been having with a fellow whos rear
alternator on his 337 is "jumpy" . . . .
FROM: Robert L. Nuckolls, III, 72770.552
DATE: 10/12/96 1:11 AM
Re: Your 337 Alernator Problem
Paul,
It's been my experience that voltage instabilities of the type you describe sort
of "grow" over time . . . the condition which precipitates the problem is a
combination of little things which make it difficult and frustrating to fix.
Replacement of any one item isn't enough to calm it down. Further, it's seldom
wrapped around the failure or degradation of things like alternators and
regulators (which you've already discovered on your airplane).
A voltage regulator is a servo system . . . it senses bus voltage and adjusts
field voltage to the alternator in an attempt to keep the bus voltage constant
in spite of load, temperatures and engine rpm. A servo system is "closed loop."
For example, suppose you're leaning over a balcony rail with a golf ball hung by
a string and attempting to drop the ball in a glass 10 feet below. Your sensor
is sight, the reaction is motor output from you arm, the result (if the loop is
tightly closed) is a successful caging of the ball in the glass.
Now, lets replace the string with a long rubber band. Further, let us add a 10
knot breeze. Now, the feedback loop is not so tightly closed; hitting the
target is more a matter of chance than design. In your airplane, the voltage
sensed by the regulator has become contaminated with tiny pieces of "rubber
band" in the form of increased resistance at each electrical connection. For
example, every time a wire goes through a connector, three new joints are added
to the wire. 20+ years ago, the joints were pristine and tight, now . . . well
. . . you get the picture. It's sort of an "electrical fungus" that puts
enough degradation in each of lots of joints to add up to too much "rubber band"
for the voltage regulator to work properly.
This is one of the reasons I lean pretty hard on my homebuilders to keep the
parts count down and in particular, minimize the number of joints in wires.
Particularly high current paths and critical control paths - like sense leads to
alternator regulators. An electrical joint count for the airplanes I was
working on at Cessna back in the 60s shows something on the order of 24 joints
in the voltage regulator sense pathway only 8 of which are contained inside ov
relays and regulators. Replacing either regulator or ov relay gets you 4 new
joints out of 24. A visual inspection or even electrical performance testing
(milliohmeter) of any one joint may not show anything of interest but add up 20
years of degradation of ALL the joints and we'll find that the "rubber band"
has become too long for adequate servo control of bus voltage to take place.
I lean toward this conclusion with your airplane because your already tried the
obvious (1) alternator replacement and (2) regulator replacement. Your concerns
about the diodes becoming "leaky" are not a factor here. Silicon semiconductors
are quite stable with age and in this case, they're not part of the voltage
sense path for your rear regulator.
Try this experiment. Make temporary mounting of virtually any 28 volt
alternator regulator under the rear cowl. Attach the "bus" terminal of the
regulator to the alternator's "B" terminal via a 5 amp inline fuseholder.
Attach the "field" terminal of the regulator to the "field" terminal of the
alternator. Make sure the case of the regulator is grounded and mechanically
secure. Disconnect the existing field wire from the alternator and tie it back.
Now, you have a very simple regulator system installed that has an absolute
minimum of joints in the sense wiring. You won't be able to control the thing
---From the cockpit so it will come on line as soon as you fire up the rear
engine.
I think you will find that the rear system is stable and totally free of the
problems you've been fighting.
It's experience with these kinds of situations that have driven modern regulator
designs and system architectures that I recommend for my home-builder clients.
The solution for your airplane isn't simple because you're saddled with an
obligation to maintain configuration control on a certified system with
designed-in problems! Consider replacing ALL joints (the existing wires are
okay . . . wire doesn't age . . just it's connections) between the bus and the
rear alternator regulator (might as well do both front and rear while you're at
it).
This means circuit breakers, alternator field switches, all of the AMP
mate-n-lock connectors (push the pins out before you cut them off . . . the wire
will be longer), terminals, etc. Short of getting and STC or OTFA on a modern
system, you'll have to cut out all the little "rubber bands" that are adding up
to an unmanageable situation for your regulator. Make sure that the regulators
are getting good electrical connection to ground . . . if they depend on case
connections as opposed to actual ground wires, clean the metal under the
regulator and on the regulator base where it touches the airplane. Curing a
problem like this is more like a spring cleaning than an identification and
replacement of a single offending component.
I wish there were a simpler answer but short of fixing the design, this is the
only way I know . . .
Regards,
Bob . . .
AeroElectric Connection
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* Go ahead, make my day . . . *
* Show me where I'm wrong. *
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72770.552@compuserve.com
http://www.aeroelectric.com
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