nuckollsr(at)cox.net
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 Posted: Sun Jun 10, 2007 7:10 pm Post subject: Alternator Load-Dump |
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At 11:40 AM 6/10/2007 +1000, you wrote:
| Quote: |
<peterjfharris(at)bigpond.com>
Bob,
I don't know how you have the patience to suffer that kind of uninformed
rhetoric when it is aimed at your totally generous and valuable work.
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A wise man suggested to me some years ago that I have absolute
and total control over what some folks would find insulting. He observed
that good critical review is factual, logical and worthy of considered
attention. Tossed cabbages representing illogical thought, erroneous
ideas or vindictive intent are the tools of those who simply can not
or choose not to behave honorably. It's perfectly okay to ignore them.
It's also okay to offer what you can as teacher to correct errors. But
to expend emotional capital on such individuals is a wasted effort. I.e,
don't
give anyone permission to insult you. I KNOW that the gentleman who
considered me to be a bully is simply wrong and if he is interested,
I'd be pleased to correct his erroneous assumption. But if he chooses
not to, I'm not going to loose any sleep over it . . . nor will I bother
to expend any emotional capital on the matter.
It's truly amazing how calming this philosophy can be.
| Quote: |
I have a couple more also uninformed questions re the operation of the crow
bar ie when it trips as in Z-21A and Z-25 are we not interrupting the B lead
and likely to cause load dumping.? If the PM alternator/regulator is runaway
and the crowbar tripped would we expect that energy to cook what is left of
the regulator?
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No. Your query focused on PM alternators and the same principles
apply but let's explore the skinny on load-dump in the general
case for wound-field alternators.
Suppose you're given a rope to hold that has a string of 1# weights
attached to it. You're asked to hold your arm out horizontal
and maintain it as close to horizontal as you can no matter
what. Suppose someone then cuts away the bottom weight. The force
necessary to hold the horizontal position is suddenly reduced by
1 pound. It takes a bit for your senses to detect the reduction
in weight so your presently applied force momentarily lifts the
remaining weights until you've hand time to adjust to a new,
lighter force and restore your arm to horizontal.
Now, suppose we cut away 5 of the weights. It's intuitively
obvious that the momentary upward transient during this adjustment
would be larger than the demonstrated excursion for the 1 pound
adjustment.
Translating this useful analogy to an alternator (or any other
servo controlled power generation source), let's assume
a 60A alternator is loaded to 10A. This requires so many amps
of field flux and rpm to support the load at the desired setpoint
of say 14.2 volts. Now, let's suddenly reduce the load to zero
and it's easy to see how the regulator (deliberately damped for
smooth response) will take a few milliseconds to sense the rise
in output voltage and adjust the field current as need to restore
order. The little "bump" in output voltage is the effect of a
10A load dump.
Now, load the same alternator to 60 amps and repeat the experiment.
It's intuitively obvious that the voltage "bump" in the second
experiment will be substantially greater than the first experiment.
In the piece I posted at . . .
http://aeroelectric.com/articles/Adapting_IR_Alternators_to_Aircraft.pdf
consider the first diagram Z-24. We could establish a design goal
of being able to turn the IR alternator OFF and ON at will just as
the ER alternators and generators have behaved before them. So
let's take the crowbar OV module out.
Note that the act of turning the alternator OFF physically
disconnects the alternator from the ship's load and from
the battery. The transient mitigating qualities of load cited
by George have been isolated. Further, the transient mitigating
qualities of the battery (attributed to "benign" regulator
failure by George) are also gone.
EVERY engine driven power source will demonstrate some "bump"
in output-terminal voltage depending on a combination of alternator
rpm, load at the time the disconnect occurs and most importantly,
the DYNAMICS of the regulator-alternator combination in responding
to a sudden rise in output voltage.
For some combinations, this can be a substantial transient
rising to 40v or more for tens of milliseconds.
Now, it's my best guess that a stock automotive alternator/
regulator of reputable design is capable of withstanding
its own shortcomings with respect to load dump response. However,
if the alternator has been repaired . . . particularly with a
regulator not designed with the same dynamics AND ability to
stand off it's own load dump response, then there is risk that
folks who shut the alternator off under load will experience
exactly the event cited by Van's customers.
What is not well understood by the most vocal critics is that
damage to the alternator was NOT caused by the crowbar OV
protection system but by the transient instability of the
alternator/regulator combination combined with a vulnerability
to products of that instability.
It may very well be that factory stock alternators would not
suffer this indignity . . . don't know. I don't have the
resources to test them all nor will any of the designers
share such data. After all, they would much rather we not
put this product in an airplane at at!
We can now choose to put the crowbar OV module back into
the circuit and the same contactor intended to provide
CONTROL now contributes to the task of OV protection as
well. Critics would have you believe I've suggested that
the IR alternator is somehow "bad" or "inferior". Not so.
They are fine, very reliable products . . . but the failure
rates are not known unless one chooses to restrict their
alternator choices to those devices KNOWN to exhibit
what ever failure rate the installer considers acceptable.
My goal is to take any alternator and install it in an
airplane with CONTROL and OV Protection . . . the very
same design goals we've worked toward since the first generator
and battery went into an airplane. This isn't about who's
good, who's bad, which alternator is better, or who is
willing to fly not having met those design goals. Certainly,
many OBAM aircraft are flying without having achieve those
goals and most owners will suffer no deleterious consequences.
But the risks are not insignificant (ten to the minus 6 failures
per flight hour).
However, depending on a whole raft of indeterminable variables
not the least of which is the pedigree of the particular
alternator, it will require some thoughtful experimentation
and design exemplified by the second schematic in the piece
cited above.
For the Z-figures you cited, the load dump phenomenon for
unhooking a PM alternator under load is still present and
it MIGHT present a hazard to the regulator . . . don't know
and it's unlikely that we'll have an opportunity to craft
the repeatable experiment to find out.
However, whether you're working with and IR alternator installed
per the original Z-24 or the proposed Z-24A or a PM alternator
wired per any of the Z-figures. There's no need to switch
a loaded alternator OFF except for unusual cases that are
controlled by circumstances for which the CONTROL and OV
Protection systems were crafted in the first place. Having
offered that, it's also my belief that your risks are quite
low compared to the situation that started this thread
some years ago when builders DID unload their alternators
and DID experience failures which were NOT a design flaw
in the proposed OV protection system.
My suggestion is that you install as depicted in the Z-figures
and refrain from idle switch-flipping while knowing that
you have the control and protections cited in the design
goals.
Bob . . .
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