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james(at)etravel.org
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 Posted: Sun Apr 04, 2010 4:58 am Post subject: Alternator protection ANL-40 rating |
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Bob,
Interesting what you said about the B-lead protection.
I had thought that my (planned) ANL-40 is going to protect the B-lead
wire from the battery. It's interesting to read about this
over-current scenario that could end up blowing the ANL-40 in a
"normal" situation, i.e. something designed to protect has introduced
a new failure mode.
However, I read on the B&C Specialty web site that the ANL-40 can
handle 80% more current than its rated capacity on a continuous basis.
It sounds as though this is going to stand anything extra the
alternator can put out. Is this assertion correct?
Thanks in anticipation,
James
On Sat, Apr 3, 2010 at 5:07 AM, Robert L. Nuckolls, III
<nuckolls.bob(at)aeroelectric.com> wrote:
| Quote: |
<nuckolls.bob(at)aeroelectric.com>
At 10:14 PM 4/2/2010, you wrote:
>
>
>
> James,
>
> The 5 amp breaker in the line controlling the regulator is for overvoltage
> protection, assuming you are using a B&C or PlanePower regulator for an
> externally regulated alternator or are using a PlanePower internally
> regulated alternator with built in OV protection (or an OVM-14 module).
>
> Your overcurrent protection would be in the form of a big breaker or fuse
> (40+ amps) on the B lead (output) of the alternator.
Very close except that alternators don't require over-current
protection like their older cousins, the generator.
Alternators are magnetically limited in their ability
to deliver current . . . so as the load on an alternator
goes up, there comes a time about 10-20% over nameplate
rating where the critter wont deliver any more and the
output sags.
Maximum output from the alternator happens when the
machine is cold. On rare occasions (cold morning,
jump start dead battery, battery is relatively
new and will accept lots of recharge current)
one can get a nuisance trip of the 60A breaker
in an airplane fitted with a 60A alternator.
This is because total ship's electrical loads
plus battery recharge current will be what ever
the alternator will deliver . . . which may be
greater than the 60A breaker rating on the
panel.
This happened to me once . . . the second of
only two times I've had a breaker open in flight.
This is why we select b-lead protection well above the
name-plate rating for the alternator so that the
breaker doesn't nuisance trip. It's also why I
call the 60A breaker on most Cessnas and Pipers
the "breaker designed to nuisance trip".
In any case, the b-lead breaker is to protect
the rest of the system if you get shorted diodes
in the alternator (very rare). The fusible link
in most cares serves the same purpose.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Sun Apr 04, 2010 5:51 am Post subject: Alternator protection ANL-40 rating |
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At 07:55 AM 4/4/2010, you wrote:
| Quote: |
Bob,
Interesting what you said about the B-lead protection.
I had thought that my (planned) ANL-40 is going to protect the B-lead
wire from the battery. It's interesting to read about this
over-current scenario that could end up blowing the ANL-40 in a
"normal" situation, i.e. something designed to protect has introduced
a new failure mode.
However, I read on the B&C Specialty web site that the ANL-40 can
handle 80% more current than its rated capacity on a continuous basis.
It sounds as though this is going to stand anything extra the
alternator can put out. Is this assertion correct?
|
Correct. One COULD use a fuse or breaker in the b-lead.
They just need to be sized such that the alternator would
NEVER open the breaker even when temporarily "overloaded".
ANL limiters use the same schematic symbol as a fuse
because they ARE a very robust, one-time, melting element
protector, i.e. "fuse". But the differences in response
time for the ANL limiter and the ATC fuse are huge. Circuit
breakers fall in between.
A fundamental of power distribution system architecture
is to install over-current protection devices (1) to
prevent a wire from becoming overheated to the point
of becoming hazardous and (2) isolate the faulted branch
of the system without propagating the failure to other
branches.
This means that as you move from the load to the
source of power, the circuit protective devices must
become increasingly robust as you move toward the
source. This idea is common to all reliable, failure
tolerant power distribution systems. For example:
A fuse inside a toaster is much less robust (faster
acting) than the breaker in a home's power distribution
box. The breaker in the box is much less robust than
the protection for a transformer on the pole which
powers multiple houses. As one moves upstream toward
the power source, the relative robustness of each
protective device must be sized to allow operation
of any single protective device in the system without
tripping any protection upstream. This prevents a
short in a toaster from turning out lights in the
whole neighborhood.
A design goal for the crowbar ov protection system
is to place the responding circuit breaker in reach
of the pilot. IF your airplane is fitted with legacy
panel mounted bus-bars and acres-of-breakers, then
integration of the crowbar-ov protection system is
no big deal. If you're using remotely mounted fuse
blocks, then there's got to be a piece of wire that
runs from the bus (fuse block feed terminal) to the
circuit breaker on the panel. Legacy design practice
and common sense tells us to protect that wire with
some device that is more robust than the breaker,
hence the fusible link . . . which is a little
brother to an ANL limiter.
Compare the operating characteristics of the
ATC plastic fuse with ANL limiters . . .
http://aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ATC_Specs.pdf
http://aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ANL_Specs.pdf
and a typical miniature aircraft circuit breaker.
http://aeroelectric.com/Mfgr_Data/Breakers/Eaton/Eaton_4200.pdf
Under mild overload (2x rating) a fuse will be expected
to operate in about 50-100 mS, the breaker in 1-2 seconds
and an ANL never.
These differences in robustness suggest their proper
position in a power distribution system where they might
be in series with each other. Getting them out of
order can produce some unhappy results in a system that
is not failure tolerant.
Bob . . .
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