Today's Message Index:
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1. 03:15 PM - Re: Potential for Overvoltage Issue (Robert L. Nuckolls, III)
2. 03:33 PM - Re: OVM14 MkIII, rev P1 (user9253)
3. 04:49 PM - Re: Re: OVM14 MkIII, rev P1 (Robert L. Nuckolls, III)
4. 04:52 PM - Re: Re: OVM-14 MkIII (Robert L. Nuckolls, III)
5. 04:54 PM - Re: Re: Practical data acquisition (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Potential for Overvoltage Issue |
At 03:39 PM 9/22/2023, you wrote:
>I've been luring on this list for years and this is my first post.
>I've recently completed my plane (Tailwind) and have if flying.
>I used the previous version of the OVM crowbar circuit and I have
>modified an automotive alternator to use an external Ford-type
>voltage regulator.
>All is working well. But, in going through a FMEA in my mind I have
>identified a potential issue: I modified the alternator using the
>method in Kitplanes, November 2019.
>In this method one of the brush terminals is isolated with
>insulating washers and an insulating sleeve. In my case I have
>grounded the other brush to case ground. The issue is that if ever
>the insulating washer or sleeve were breached the B--lead would
>directly feed the field through the brush and an overvoltage would
>result that the crowbar could not shut down. Turning off the master
>switch would disconnect the battery but the run away alternator
>would still be feeding the bus.
>I post this in case anyone else has this setup.
Yes they do . . . along with about every single
engine airplane since 1968 or so.
FMEA also takes into account probability of
any particular event. Many potentially catastrophic
events in aviation are statistically or demonstrably
so small as to be ignored in process of crafting
a plan-b, or redesigning to remove potential for
the fault entirely.
Number one failure of a wiring pathway is
disconnect. Wire breaks, stud nut is works loose,
wire pulls out of terminal crimp, etc. Another
root cause for failure is short to ground due
to compromised wire insulation (chaffing or
mechanical damage). The probability of an
wire becoming disconnected followed by a
low-impedance, re-positioning of that wire
to a robust power source is really TWO
failures happening simultaneously.
Original structural failure followed
by misadventurous 'wandering' of that
same wire to a hazardous source of
power.
This combination of events is so remote
that it has not been treated as potentially
hazardous in hundreds of thousands of airplanes
from Super Cubs to Gulfstream V's
You are to be commended for considering
the scenario and asking the question.
Be assured that the question has been
considered and discounted countless times
before.
Bob . . .
////
(o o)
===========o00o=(_)=o00o========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
Message 2
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| Subject: | Re: OVM14 MkIII, rev P1 |
I think that most people would expect the crowbar overvoltage device to trip
the breaker when the push-to-test button is pressed, regardless of whether
the alternator is running or not, especially during an annual condition inspection
by a mechanic or new owner.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=511479#511479
Message 3
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| Subject: | Re: OVM14 MkIII, rev P1 |
At 05:33 PM 9/23/2023, you wrote:
>
>I think that most people would expect the crowbar overvoltage device to trip
>the breaker when the push-to-test button is pressed, regardless of whether
>the alternator is running or not, especially during an annual
>condition inspection
>by a mechanic or new owner.
The product would be supplied with instructions
calling out a procedure for validating
continued airworthiness. Leave the button
off if you will. Do not label it anything.
At a propitious time verification procedures
say, join the two wires together. Start
the engine leaving alternator OFF. Field
breaker should remain closed. Turn alternator
ON. Breaker should open. Shut down engine.
Disconnect the two wires and secure for
service.
The procedure not only verifies gross
functionality but makes a good ball-
park check of calibration. A push
button precludes inadvertently leaving
them together . . . but you'd know
about it early on when the alternator
fails to stay on line . . . so it's
pretty much fail-passive either
way.
Bob . . .
////
(o o)
===========o00o=(_)=o00o========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
Message 4
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| Subject: | Re: OVM-14 MkIII |
>I looked at the proposed circuit, but didn=99t
>study it, and in not studying, didn=99t grok it
>' I can look at the simplest of scchematics for
>an eternity and still not get it.=C2 Not pairing
>the words with the proposed schematic, I
>imagined one light, multiple functions (I seem
>to remember a device (LR3x, maybe?) =C2 that could
>set a lamp ON for over-voltage, blinking for low voltage).
Does it really do that? All the data I have
here says that light is ALWAYS flashed and
ONLY in response to a detected low voltage.
Due to the nature of LR3 crowbar ov protection,
an OV event is followed milliseconds later
by a LV event. There would not be any time
to sense, display and perceive an OV condition.
Bob . . .
////
(o o)
===========o00o=(_)=o00o=======
=
< Go ahead, make my day . . . >
< show me where I'm wrong. >
========================
========
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
Message 5
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| Subject: | Re: Practical data acquisition |
>>
>> . .
>Thanks Bob. But let me start a new thread (subject) when I've pulled
>the cowling and taken some pictures.
>
>Finn
Found the Kitplanes piece. I understand at least
some of your concerns. I have a lengthy reply
in process. Those DIY open loop current sensors
are problematic. Economical manufactured
alternatives are available.
Watch this space.
Bob . . .
////
(o o)
===========o00o=(_)=o00o========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
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