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jesse(at)saintaviation.co
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 Posted: Fri Feb 01, 2008 8:34 am Post subject: Ammeter Location |
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This brings up another issue. Where are people putting their ammeter
shunts? You can measure how much is coming out of or going into the
battery, how much the alternator is putting out, or how much your
system is drawing, depending on where you put it.
I have been putting it between the starter solenoid and the bus, and I
connect the alternator to the bus side, so basically it is reading
what is happening to the battery, whether charging or discharging,
except, of course, when cranking. Now with the VP, since it tells me
how much power the system is using at any given time, I have put it
between the alternator and the system, so I know how much the
alternator is putting out.
do not archive
Jesse Saint
Saint Aviation, Inc.
jesse(at)saintaviation.com
Cell: 352-427-0285
Fax: 815-377-3694
On Feb 1, 2008, at 10:19 AM, Tim Olson wrote:
| Quote: |
Yes, that's true too. So basically:
1. You pick your alternator to fit your load. Plan for
perhaps 15-20% over capacity as a minimum, so that
you never work your alternator at 100%
2. You pick your wire size that will handle the load that
your alternator can provide, again, err'ing on
the high side so if you have a 70A alternator, make
sure your wire can handle > 69.999A as a minimum...and
you can use larger wire if you wish.
3. You pick your fuse to protect your wire and blow if too
much current is flowing either into or out of the
alternator lead. (Shorts can happen anywhere) But,
you can always choose to put a smaller fuse in, if
all the rest is kept in spec. You may be wasting
the possible capacity (i.e. you're paying for a 70A
alternator but you're giving up some capacity if you
plan it as a 60A system) but there is nothing wrong
with looking at your loads and putting a safe undersized
fuse in there.
You may not KNOW what your actual common max operating draw will
turn out to be, to within an amp or two, until you first fly, but
at that point, if you want to protect against drawing far in
excess of what is required, you can put in an appropriately
sized fuse.
In my case, I have a 70A alternator, chosen for it's lower turning
speed, and cooler operation, and plenty of capacity beyond my
requirements. I sized the wire to handle 70A from the alternator,
and used #2 back to the battery from the solenoid. Then, I stuck
in a 60A fuse. After running the plane, I find that my norm is
to pull maybe 30-36A, and 40-42A with everything on. Yeah,
during a low battery event I may pull a little more for a couple
minutes, tapering off quickly as it charges. But, 60A would be
a huge increase in current, so if I saw 60A, I would start
wondering what the heck was going wrong. Add in the slow blow
time to blow that 60A ANL, and the fact that it can handle
surges beyond 60A for a period of time, and there's no reason for
me to put greater than a 60A fuse in there. I could even go
with less if I wanted. In 320+ hours, I haven't had a nuisance
trip of any fuse or breaker in the plane, except for my HID's
initially....because you need to size the fuse on those to handle
the startup inrush current that tapers off.
So you're right, you want to protect the wire, but you can
always undersize your fuse to a reasonable point that it would
blow if you had a current draw that was indicative of a
serious issue. You could wire your LED lights with 12GA if
you really wanted to....but you still would want a small fuse
on them, because if your LED's are drawing 10A, something
is wrong.
Tim Olson - RV-10 N104CD - Flying
do not archive
linn Walters wrote:
>
> >
> Tim Olson wrote:
>>
>>
>> That's exactly my reasoning on using a 60A fuse (despite my 70A
>> alternator). The fact is, if I find myself drawing 60A off
>> my alternator, something is not right at all. None of my
>> avionics or equipment can draw that much, so my battery or
>> wires are shorted or something. You don't size your fuse by
>> the alternator, you size it to the max load you care to see
>> before you blow.
> Not really true. The size of the breaker is there to protect the
> wire, which could cause a fire if it gets a large escess of
> current. The size of the wire is dictated by the load. True of
> small wires and big ones too.
> Linn
> do not archive
>> A 60A ANL fuse won't blow until well over
>> 60A anyway, so if that baby blows, I sure as heck don't
>> want to push a reset button in flight and figure it out.
>> That fuse/breaker is a very big problem indicator.
>> Tim
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wcurtis(at)nerv10.com
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| Posted: Fri Feb 01, 2008 9:27 am Post subject: Ammeter Location |
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The ammeter shunt in the path from the battery to the buss is known as Battery Lead mode. This would be my preferred way to install the ammeter as it allows you to see how much current you are consuming when the alternator is OFF LINE. When the alternator is functioning normally, it shows how much current is being used to charge the battery. This mode is typically used in Cessnas.
The other way of wiring the ammeter is in Alternator Lead mode. That is, it is in the path between the alternator and the buss. This mode will show how much current is being produced by the alternator. In an alternator failure it would just read zero. This is mode typically used in Pipers.
The stand alone ammeter instruments allows either mode but some EFIS restrict you to one mode or another. I know AFS for example only supports Alternator Lead mode. Dynon supports both modes, I'm not sure what Chelton, or GRT, supports.
William
http://wcurtis.nerv10.com/
"Be who you are and say what you feel because those who mind don't matter and those who matter don't mind."
-- Dr. Suess
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