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nuckollsr(at)cox.net
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 Posted: Tue Aug 22, 2006 10:47 pm Post subject: B-lead Contactors (was MOV's) |
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At 09:47 PM 8/22/2006 -0700, you wrote:
| Quote: |
I disagree that opening time is not significant as related to an aircraft
application.
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I didn't say that opening time (contact spreading velocity) was
insignificant.
Obviously, the slower moving contact keeps an arc established longer.
What I
said was that the benefits of any other coil suppression technique were
not significantly better than the simple diode with respect to arc duration
in the experiments I conducted. I.e, no compelling reason to take all
the diodes
out and put in bi-directional transorbs or transorb-diode networks.
| Quote: | I have done dozens of repeatable tests where the contact opening rate was
the subject and have demonstrated the duration of the contact opening arc
is extended and the arc intensity is much greater, specifically if you are
looking at a big Contactor opening an inductor or generator like an
alternator. This when the common diode is used across the coil.
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You use "extended" and "intensity" and "greater" which are
non quantified. How much? And how does your experience illuminate
the tasks before our readers on this List?
| Quote: | The issue of "opening time" is very important as the contact suppressor
Transorb must absorb the energy from the alternator load dump over 50 ms
vs. 10 ms (typical) and this means the transorb must be 5X bigger in heat
rating. (The need for a contact suppressor is below a ways)
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Opps! Stuck your foot in it sir. If you install a transorb as
in cases 7/8 then the transorb is isolated from an alternator
load dump by the accompanying diode. If you install per cases
5/6 then yes, alternator load dump is impressed across the
transorb in the zener mode . . . but if this technique is no
better than a simple diode for coil suppression, why do it?
Use a plain vanilla diode and the "problem" goes away.
| Quote: | The specific test set up is the contactor has either the diode or the
bi-directional transorb across the coil and there is a uni-directional
transorb clamping the alternator load dump on the alternator side of the
contacts. The alternator is either properly running or has been failed to
produce full output with no voltage control.
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Now were far-afield from the original discussion. You've injected
a feature (alternator load dump) that is a whole new consideration
having nothing to do with what I was attempting to explain earlier this
evening.
| Quote: | Perhaps not clearly stated above, but the testing was done several years
ago to simulate internal regulator alternators where the "B" lead was
opened to protect the system (Or the pilot simply opened the "B" lead on a
healthy alternator for what ever reason including the false triggering of
a OV crow bar circuit. From the point of the opening of the relay coil to
the end of the time the relay contacts are fully open the Transorb
clipping the OV was tasked with clamping the voltage.
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Fine. But what does that have to do with spike suppression
on relay coils?
| Quote: | The point is that with out the transorb to clamp the alternator output,
the opening arc voltage jumped to over 100V and the open contacts
continued to arc after they were fully open (this happens regardless of
any OV crowbar present as the crow bar is on the other side of the open
contacts). The Transorb on the alternator side clips the arc voltage
during the opening process and only after the contacts were fully open and
unable to restart an arc did the transorb fail short and then open. Thus
the time from relay coil power off to final contact open determines the
power rating of the transorb and using a diode across the coil added 40+
ms to the total time which is typically 5 times longer that the transorb
must keep the alternator voltage in line if the transorb was across the
coil. Further the duration and intensity of the arc was increased
significantly. If the transorb was not rated high enough it failed during
the process and the arc continued after the contacts were fully open. For
example 50 amps for 50 ms is a lot of power for the common transorb and
they fail short and then quickly fail open.
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Yes, but you're illuminating a separate issue. If the loads
on a relay's contacts are particularly antagonistic such as
during the load dump phenomenon on a b-lead contactor, then one has
two possibilities. Increase ratings of the contactor or limit
the load dump intensity or a combination of both. But this
is a special case that is only slightly related to the original
discussion. Further, the use of Transorbs on the b-lead contactor
in lieu of diodes is not the ultimate solution.
Further, getting the b-lead opened is only part of the task.
How do you propose to bring a runaway alternator to heel . . .
or is the plan to let it self-destruct?
| Quote: | Its well known that relays that are required to break DC voltages above
the 40-50V range must use a arc quencher circuit or they usually will
simply never stop arcing. Thus something is required to keep the arc
voltage under say 35V until the contacts are well open where the contact
gap is big enough to prevent a restart of the arc.
|
| Quote: | My conclusion to the above tests is opening a failed internally regulated
alternator where the failure was unregulated high voltage output is not as
simple as some would believe and the "B" lead contactor (if not special
design) has the potential to continue to arc after any type of OV
protection has opened the contactor.
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Absolutely . . . and I think there's a rational way
to mitigate this. If current negotiations for an alternator
drive stand work out, I'll be able to get the energy data
you claimed to have but could/would not share some years
ago. And guess what? As soon as I have the results of the
tests, they will be published here and on my website.
You see, I do intend to service my duty as teacher.
Bob . . .
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paulm(at)olypen.com
Guest
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| Posted: Wed Aug 23, 2006 6:11 am Post subject: B-lead Contactors (was MOV's) |
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I did change the subject to point out a case where the diode as a coil
suppressor was a bad idea as it extends the opening time 5x approx.
You did state as far as the relay and its drive there was no difference and
usually the delay in opening was OK and I agree. But there are cases where
its not OK and I stated one used in many aircraft.
I did not quantify the exact results of my tests in my post as that is not
important to the main point that a simple "B" lead contactor may fail to
properly work. The intensity of the arc etc is not important, the fact the
arc continues AFTER the contacts are fully open IS.
There is a lot more testing required for any interested person in the design
of a protection circuit for an internally regulated (very rare) failure of
the internal regulator when the failure mode is full power out.
As there are thousands of owners out there flying with internal regulated
alternators I was trying to point out that the solution once suggested on
this list (at one time the simple "B" lead contactor) is not always a
reliable one. I had hoped that by now you had looked into a solution. Given
the large numbers of aircraft so equipped it would, in my opinion have a
high priority. You know simply saying to not use internal regulated
alternators, just based on your word alone is not sufficient when major
manufacturers/suppliers of experimental aircraft parts etc say otherwise.
My point was to be an informer of a problem and yes it does relate to the
Transorb in a way as I know of NO case where a transorb will not be as good
as a diode. I just pointed out where a diode was NOT as good. My conclusion
was you cannot go wrong with a transorb (and you can with a diode) and so
far you do not seem to have case that disputes that conclusion.
Personally I have changed my mind about internally regulated alternators
after my tests of several years ago. I did and do not have what I consider a
good solution to the problem other that the Kilovac contactor which will
cleanly break a runaway alternator safely. I did mention on this list
recently how to convert a ND alternator to external regulation with a simple
mod to the regulator. I offered to post an article on the subject but only a
couple showed interest and that was not enough for me to spend the time on
it.
Sorry I seem to be unable to state my point without your misunderstanding
it.
Paul
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