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ceengland7(at)gmail.com
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 Posted: Thu Jan 30, 2014 10:27 am Post subject: Copper Bar (Heresy Alert!) |
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On 1/30/2014 10:32 AM, Robert L. Nuckolls, III wrote:
| Quote: |
<nuckolls.bob(at)aeroelectric.com>
> Another consideration is that copper oxidizes very quickly so you
> need to make sure that the connections are very clean when you
> assemble. In industry, copper bussbars are commonly plated w/ either
> silver or tin to protect against poor connection quality due to
> oxidation.
If the joined materials are in good contact with
each other and made up with sufficient force, joint
integrity is assured in spite of any corrosion visible
outside the joint. So FLAT, clean parts bolted together
with non-trivial force is the key . . .
This is one of the reasons why I have not been a
strong proponent of mashing copper round things
into copper flat things for use as bus bars . . .
it's hard to get them really flat around the bolt
holes.
If you can start with sheet materials, then you are
75% of the way along the path to gas-tight Nirvana.
> Brass is another material commonly used in electrical busses &
> devices and it does not oxidize as quickly and is less expensive. I
> just made some bussbars from .050 brass (they were not in the starter
> circuit) and the brass was easy to work with & cheaper than the
> equivalent copper.
A reader pointed out here on the List that brass
(depending on alloy) has a resistance 2 to 3x
that of copper. So to get the same temperature
rise on a brass bar, it needs to be at least 2x
more 'meat' between the studs.
Just to put it into perspective, a 4AWG
wire is 250 micro-ohms per foot. 200A
would give us a voltage drop on the order
of 50mV/foot. A 4" length of 4AWG
between studs would drop 1/3 that or 16
mV.
In this experiment I conducted on the bench
a few years back
http://tinyurl.com/k9v6mnt
a strip of brass shim stock .75" wide and
.005" thick offered a 40F rise at 20A of
current flow and a 58mV drop. Consider a
brass bar .75 x .05 (10x thicker) at 200A
would offer about the same performance.
We'd have to be 3 times thicker yet or .15"
to bring voltage drop into the same neighborhood
as 4AWG copper.
A copper bar could be 1/2 that thickness
and offer the about the same performance.
4awg has a cross section of 0.03 square
inches. So a copper strip .062 thick
needs to be 0.03/0.63 or about 0.50"
wide. You can't drill 0.32 holes in
strip that narrow so a 0.75" wide x
0.062" copper is a good choice for
this application.
Bob . . .
You know, aluminum is only slightly less conductive than gold; certainly
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as good as or better than brass. Anyone building an aluminum aircraft
will likely have quite a bit of 6061 alloy scrap & 'cutoffs' lying
around. If the same care in prep & installation is taken with an
aluminum bus bar that is taken with wing ribs, skins, etc, there's no
reason for fatigue to be an issue. A little dielectric compound on the
joint (I'm talking about a screwed/bolted joint, not a spring tab in an
electrical outlet), and it's no more likely to corrode than a wing rib.
As a point of reference, if you live in a house in the USA, your
electrical service wire (street to meter to mains breaker) is almost
certainly aluminum.
Flame suit ready,
Charlie
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nuckolls.bob(at)aeroelect
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| Posted: Thu Jan 30, 2014 12:29 pm Post subject: Copper Bar (Heresy Alert!) |
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You know, aluminum is only slightly less conductive than gold;
certainly as good as or better than brass. Anyone building an
aluminum aircraft will likely have quite a bit of 6061 alloy scrap &
'cutoffs' lying around.
True. 6061 is on the same order of resistivity
as brass . . . so an 0.13 or thicker aluminum
sheet or bar stock would offer an electrically
equivalent bus bar.
If the same care in prep & installation is taken with an aluminum
bus bar that is taken with wing ribs, skins, etc, there's no reason
for fatigue to be an issue. A little dielectric compound on the joint
(I'm talking about a screwed/bolted joint, not a spring tab in an
electrical outlet), and it's no more likely to corrode than a wing rib.
The aluminums found in most shops tend to be
harder than the copper. It's the malleable nature
of copper than makes it more 'terminal friendly'
for achieving gas-tight joints under the make-up
forces of threaded fasteners.
As a point of reference, if you live in a house in the USA, your
electrical service wire (street to meter to mains breaker) is almost
certainly aluminum.
I think the alloy of those wires moves toward
the dead-soft spectrum of alloys with a volume
resistivity on the order of 2.6x10^-8 as opposed to
4.2 to 5.8x10^-8 for the structural alloys.
I'm not aware of any parts on a metal project
that would call for 0.125" material . . . but
you're absolutely correct. There's no
electrical reason to prefer brass over
aluminum . . . but I'll suggest there
are reasons both electrical and mechanical
to prefer copper over both.
If I were fabricating an aluminum bus bar,
I think I would cut and drill in the 'hard'
state for easy machining then anneal the
finished part with techniques described
all over the 'nets metal fabrication forums.
Seal clean joints as you've suggested. Works
good and lasts a long time.
Bob . . .
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