Today's Message Index:
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1. 06:24 AM - Re: New Over-Voltage Protection Architecture (user9253)
2. 07:01 AM - Re: New Over-Voltage Protection Architecture (Eric M. Jones)
3. 07:45 AM - Re: Re: New Over-Voltage Protection Architecture (Charlie England)
4. 09:22 AM - Re: New Over-Voltage Protection Architecture (David Josephson)
5. 09:59 AM - Re: Earth-X lithium (D L Josephson)
6. 10:06 AM - Re: Re: New Over-Voltage Protection Architecture (Jeff Luckey)
7. 11:17 AM - Re: Re: New Over-Voltage Protection Architecture (Robert L. Nuckolls, III)
8. 12:00 PM - Re: Re: New Over-Voltage Protection Architecture (Robert L. Nuckolls, III)
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Subject: | Re: New Over-Voltage Protection Architecture |
A voltage regulator is not likely to fail unless overheated. An over-voltage protection
device is less likely to fail because it is usually not subject to excessive
heat. What is more likely to fail is an electrical connection or switch
somewhere between the main bus and the voltage regulator. Good workmanship,
strain relieved terminals, and a dab of grease to prevent corrosion, will all
help. Even if properly installed, switches and connections can corrode over
time. These problems are unlikely to be detected during annual inspections.
Many modern avionics can operate on voltages between 10 and 30, and can withstand
minor voltage excursions. Over-voltage protection is like buying insurance.
Is it worth the premium?
Joe
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=420718#420718
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Subject: | Re: New Over-Voltage Protection Architecture |
I learned to fly in a club with five Cessnas, and the only alternator failure I
was personally involved with was an alternator bracket that broke from fatigue.
(There was one "failure to charge" whose cause I never learned.)
If you look at all the ways an aircraft can fail, I think alternator failures are
low on the list. I have not heard of a NEW (not rebuilt) Nippondenso alternator
that failed at all, save overheating, drowning, old age, etc. This might
be true of other manufacturers as well. Any reports to the contrary?
Rather than trying to protect the electrical system from a failed alternator, perhaps
some effort in determining when an alternator should be replaced would
be a good approach. 1000 hours, engine TBO?
Most electrical parts benefit from not being driven too hard, and being kept cool.
If this is done, count me as unconvinced that OVP is necessary with an internally-regulated
modern alternator.
--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=420721#420721
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Subject: | Re: New Over-Voltage Protection Architecture |
On 3/20/2014 2:11 PM, Robert L. Nuckolls, III wrote:
> <nuckolls.bob@aeroelectric.com>
>
> At 10:58 AM 3/19/2014, you wrote:
>> You might be approaching a situation where "the cure is worse than
>> the disease". You have to be careful when stacking widgets on top of
>> gizmos in pursuit of greater reliability. That approach quite often
>> leads to greater complexity & lower reliability.
>
> Good put.
>
> What is the line of thought that drives the
> notion that simple removal of field voltage from
> a runaway system is inconsistent with our
> assessment of risk?
>
> I'll have to ask around . . . I've been disconnected
> from the field service loop in regulators for some
> years . . . but I don't recall any controllers ever coming
> back to B&C where a regulator fallen from grace
> was blessed with salvation by the ov protection
> system.
>
> I'm not suggesting that such failure rates are zero but
> they ARE quite small. Further the prudent response
> to a competent FMEA dictates that we include such
> protection in spite of those low failure rates.
>
> It's been a long time since I've observed a car
> approaching me at night with lights that were too
> blue/bright demonstrating the fact that a poor
> battery was doing its best to stand off a
> runaway alternator.
>
> I'm also reading anecdotal bits about stock,
> automotive alternators being incorporated onto
> aircraft sans ov protection. It MIGHT be that
> contemporary regulators have achieved 10 to the
> minus 9 or better failure rates that suggest
> the ov protection is no longer necessary/useful.
>
> If anyone runs across such an incident, I'd like
> to hear about it.
>
>
> Bob . . .
Are you asking for incidents of *non* failures with internally regulated
alternators & no protection? I've been running a Denso on an RV-4 Lyc
O320 for about a decade. Not a lot of hours (probably around 400), but
no issues with OV. This is a very simple VFR a/c that only recently
acquired a transponder, so my financial and safety risks have been
pretty low. I'll probably incorporate some type of protection on the -7
I'm building, just because I'll have more money tied up in the panel.
Charlie
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Subject: | Re: New Over-Voltage Protection Architecture |
> What is the line of thought that drives the
> notion that simple removal of field voltage from
> a runaway system is inconsistent with our
> assessment of risk?
Only that removal of field voltage is impossible with a permanent magnet
alternator and inconvenient with an internally regulated one.
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Subject: | Re: Earth-X lithium |
Just as lead-acid electrochemistry is fairly well understood, lithium
cell vendors and types are well known. Every maker of small "aviation"
lithium batteries uses the same LiFePO4 18650 cell type as far as I
know, which is also the same as Tesla uses in their car battery packs,
and the same chemistry used in most smartphones today. There are various
quality levels available, with the leader seeming to be the Panasonic
NCR18650A, which is honestly rated at 2.9 amp-hours at 3.2 volts (more
than 2.9 can be claimed by assuming a lower cutout voltage, which
requires more precise low voltage disconnect to avoid damage.) Packs are
made of strings of 4 cells in series, which is why you see 8-, 12-, 16
etc. cell "batteries."
At last year's CAFE Electric Aircraft Symposium, Eric Darcy from NASA,
who is responsible for designing the lithium batteries for the Space
Station, gave some pointers on testing these ubiquitous cells for
quality (you measure open circuit voltage over a few weeks -- cells that
droop faster than the norm are suspect) and the measures that need to be
taken to assure safe charging and low voltage disconnect. We learned
that quality was primarily related to clean shearing and even winding of
the internal foil materials, with failures mostly due to minute stray
particles of the sheared material causing leakage, or misalignment of
the wound structure allowing the edges to touch.
Ideally each cell is charged separately. Next best is to charge each
pack of 4 cells separately. The trick comes in designing a battery
management system inside the box that manages charge current for
multiple packs and at the very least alerts you if something is wrong. A
battery company that will actually explain how they are doing these
things, and takes the time to explain why a watt-hour of lithium cells
might be more useful than the same watt-hour of lead acid, would be
worth paying attention to.
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Subject: | Re: New Over-Voltage Protection Architecture |
=0A=0A=0A=0A________________________________=0A From: David Josephson <dlj0
4@josephson.com>=0ATo: aeroelectric-list@matronics.com =0ASent: Friday, Mar
ch 21, 2014 9:21 AM=0ASubject: AeroElectric-List: Re: New Over-Voltage Prot
Josephson <dlj04@josephson.com>=0A=0A=0A=0A>- - - What is the line
of thought that drives the=0A>- - - notion that simple removal of fi
eld voltage from=0A>- - - a runaway system is inconsistent with our
=0A>- - - assessment of risk?=0A=0AOnly that removal of field voltag
e is impossible with a permanent magnet =0Aalternator and inconvenient with
an internally regulated one.=0A=0ADavid,=0A=0AI've never had occasion to f
iddle with a PM alternator. =0A=0AI've always thought of a run-away conditi
on as=0Aa regulator failure where it is no longer "regulating" =0Aand apply
ing full field current.=0A=0ACan a PM alternator "run-away"?=0A=0A-Jeff=0A
=========================0A
===================
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Subject: | Re: New Over-Voltage Protection Architecture |
At 11:21 AM 3/21/2014, you wrote:
>
>
>> What is the line of thought that drives the
>> notion that simple removal of field voltage from
>> a runaway system is inconsistent with our
>> assessment of risk?
>
>Only that removal of field voltage is impossible with a permanent
>magnet alternator and inconvenient with an internally regulated one.
That's why all the z-figures that incorporate
pm alternators use a disconnect relay paired with
an ov sensor to effect a disconnect . . .
This unique characteristic of the PM alternator
is what suggested that they be called a 'dynamo'
to separate them out from the herd of engine driven
power sources. B-lead disconnect has also been
suggested in Figure z-24 of the 'Connection's collection
of architectures.
Bob . . .
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Subject: | Re: New Over-Voltage Protection Architecture |
At 08:23 AM 3/21/2014, you wrote:
A voltage regulator is not likely to fail unless overheated. An
over-voltage protection device is less likely to fail because it is
usually not subject to excessive heat. What is more likely to fail
is an electrical connection or switch somewhere between the main bus
and the voltage regulator. Good workmanship, strain relieved
terminals, and a dab of grease to prevent corrosion, will all
help. Even if properly installed, switches and connections can
corrode over time. These problems are unlikely to be detected during
annual inspections.
There are two approaches to system reliability analysis.
One approach assumes NO backup . . . either because such
features are too costly/bulky/heavy/etc or simply not
possible.
If any of these features are critical to comfortable
termination of flight, they get designed and tested
for very high probability of meeting a 'design service life'
that is generally some multiple of 'practical service life'.
For example, flown long enough, the wings on EVERY aluminum
airplane are going to break off. This is because aluminum,
unlike steel, has a stress-to-cycles plot that never goes
'flat'. If you stress and relieve a steel part to maxiumum
ever expected loads 10,000,000 times without breaking
it, the part is considered 'golden' and will last forever.
Aluminum has no such feature . . . it eventually fails at
any stress loading . . . the cycles may be very high but
there is no flat spot on the s/n curve to failure. Hence
you see a totally different approach for the design setting
service limits for qualification of structural parts on
airplanes. Approaches that have continuously evolved over
the years particularly in response to incidents like this . . .
http://www.aloha.net/~icarus/243a.jpg
Emacs!
An alternative technique for making design decisions
can be adopted for items that are not immediately
catastrophic. The generally categorized by criticality
level not unlike software under DO-178 . . .
(a) Catastrophic - Failure may cause multiple fatalities, usually
with loss of the airplane.
(b) Hazardous - Failure has a large negative impact on safety or
performance, or reduces the ability of the crew to operate the
aircraft due to physical distress or a higher workload, or causes
serious or fatal injuries among the passengers.
(c) Major - Failure significantly reduces the safety margin or
significantly increases crew workload. May result in passenger
discomfort (or even minor injuries).
(d) Minor - Failure slightly reduces the safety margin or slightly
increases crew workload. Examples might include causing passenger
inconvenience or a routine flight plan change.
(e) No Effect - Failure has no impact on safety, aircraft operation,
or crew workload.
For OBAM aircraft, we're free to tighten up our spectrum
of criticality level. Depending on the design of our
plan-B, we can generally drop to 3 categories . . .
(a) Engine stops and we're going to descend . . . NOW
(b) Some appliance goes dark and finding our way to
comfortable landing . . . preferably at airport
of intended destination . . . is at risk.
(c) Some appliance goes dark but while convenient,
is not critical to continued flight, navigation,
approach to landing and parking the airplane.
These three categories focus on system components
like fuel pumps, ECM, nav receivers, orientation
aids, panel lighting, electronic ignition, comm,
and perhaps xponder. The builder/pilot has to
decide what order of important fits their particular
project and the environment in which they intend to fly.
Then an architecture and hardware compliment needs
to be crafted such that no single failure takes down
more than one accessory and all really useful or
critical accessories have a plan-B.
The elegant solution minimizes weight, cost, volume,
parts-count and pilot work loads.
It takes an appreciation of the thoughts outlined
above to understand the rationale for careful
consideration before ov protection is no longer
in your plans.
Recall that MTBF numbers say NOTHING about the
behavior of any single part. If one strives for
predictable behavior, then you venture into the
world of 'established reliability' accessories
where a great deal of money has been spend to design,
test, manufacture and perhaps even screen finished
goods to weed out infant mortality . . .
The easiest way to deal with SYSTEM reliability
with internally regulated alternators is to ASSUME
it will fail and install a plan-b . . . as long as
that addition is not a significant cost/weight
adder. OV protection adds little overhead. Second
alternators add some weight and up-front costs
but minimizes cost of ownership. Second batteries
are probably cheaper up front but add more weight
and perpetual cost of ownership burdens for
preventative maintenance and r/r costs common
to an expendable commodity.
Don't forget one super-significant feature of
the engine driven power source: It's an inexhaustible
source of energy . . . at potentially high voltages
(100+ volts). The alternator is unique in this
regard. The battery can deliver a lot of current
but its energy reservoir is limited. The alternator
is current limited but for all practical purposes,
it's output voltage and total energy is not limited.
These features are foundations on which you build
your personal failure mode effects analysis.
An analysis that could not care less about anecdotal
reliability narratives and builds a foundation of
system reliability on logic assembly of simple-ideas
irrespective of any perceptions of probability.
Many modern avionics can operate on voltages between 10 and 30, and
can withstand minor voltage excursions. Over-voltage protection is
like buying insurance. Is it worth the premium?
OV protection is not about minor excursions. It's about
unleashing the flame thrower first against your battery
(by the way, if it's a tiny lithium super-cranker,
it will toss in the towel much faster than your 18 or
24Ah SVLA brick) and then upon system accessories.
Just a few steps from where I sit right now is a lab
where these features and effects have been studied
for years. It's not a concept to be dismissed lightly.
A runaway alternator is the acme of electrical disasters
on about any DC power system. If you're willing to buy
into the anecdotal 'never heard of it happening' . . . then
ask the supplier of your alternator/regulator combination
if he'll guarantee replacement of electro-whizzies
on your panel if his gizmo fails?
This is why we elected to bring the B&c alternator
controllers to market as a TRIO of accessories that
offered a very favorable FMEA. The thought was that
if the customer wanted to go a different route, they're
certainly free to do so . . . but not with our product.
Proceed with both caution and confidence borne out of
lessons learned . . .
Bob . . .
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