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1. 05:49 AM - Re: Re: Newbie with PM alternator feed question (Robert L. Nuckolls, III)
2. 07:36 AM - Re: electrical system planning (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Newbie with PM alternator feed question |
At 10:04 PM 8/16/2015, you wrote:
>
>
> > the PM alternators will self-destruct if presented
> > with a hard short on their windings generated by
> > failure of the rectifier=regulator.
>
>Bob,
>What self destructs, the winding or the magnets or both? I am
>guessing the magnets only but do not know. Was I wrong to state
>that all alternators are self current limiting?
This windings are at-risk. Browse through
the various forums that feature engines with
built in PM alternators, Jabiru in particular
and you'll read builder stories of woe over
smelly wires that don't look so good either . . .
http://tinyurl.com/obudcwn
The term 'current limited' is not necessarily
applicable throughout the full spectrum
of load conditions. I have a bench supply that
is electronically current limited and will
not self destruct for any load condition down
to a dead short. But that kind of current limiting
is seldom found in the wild.
With respect to generators vs. alternators, the
physics that drive performance and stresses to
least robust components have prompted us to
assert that 'inherent current limiting' in
the physics of alternators makes the I-limit
control relay common to generator controllers
unnecessary . . . but you can burn windings in
EITHER machine when you venture outside the
condition of what one might call 'mild overloading'.
I was appalled at my recent discovery of a
kit or recommendation by the Jabiru factory that
bi-filar (two wires in parallel) windings of
their 3300 alternator be rewired in series to
'produce more output at lower rpm'. A very sad
demonstration of . . . well . . . 'nuf said . . .
http://tinyurl.com/obudcwn
Yes, available VOLTAGE goes up by 2x for any
given rpm . . . but available current drops by
half . . . and loading on individual wires
doubles. Given the strong positive temperature
coefficient of resistance for copper, risk
for destructive overheat goes up by more than
half because the wires dissipate MORE energy
as their operating temperature RISES. It's
a positive feedback effect that hastens the
march to self-destruction.
I show fuses in these leads to do what fuses
do best . . . protect wires from the hard-falut
condition with would arise from shorted semi-
conductors in the rectifier/regulator. But if
those same wires are not properly sized and
cooled to meet design goals they can
probably be 'smoked' at current levels far
below those expected in the hard-fault condition.
In the mean time, the magnets are happily spinning
in total ignorance of the havoc they can sow
upon their copper brothers just a few millimeters
away.
Bob . . .
Message 2
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| Subject: | Re: electrical system planning |
A t 11:29 PM 8/16/2015, you wrote:
>Thanks Bob.
>The 5A figures are the estimates I got from the
>manufacturer for average consumption. I haven't
>measured it myself. The breaker size is 10A.
When you get this airplane flying, I'd REALLY
like to get some data off it that describes
ENERGY measurements . . . but 5A probably
works for now.
>I filled out a spreadsheet with the load
>analysis for each stage here:=C2
<https://goo.gl/0FPFIQ>https://goo.gl/0FPFIQ
>I'm not entirely sure I have everything assigned to the right bus.
>
>I estimate the plane will hold 5.5 hours of
>fuel, so I guess that's what my endurance goal should be.
No guessing here . . . that's a number YOU pick
based on how you intend to use the airplane. If
you plan on frequent departures from airports
often clagged in and surrounded by mountains, then
the idea of endurance to carry you to demonstrably
lower-risk haven is a part of solid reasoning. But
if that comfortable termination of flight is
never more than say 2 hours away, then designing
and maintained that level of endurance is not
unreasonable.
The main thing is to know what the endurance numbers
are, preventative maintenance to insure those
numbers and risks go down markedly.
>That's over 70 amp hours just to keep the engine
>running, so it seems a backup alternator is a better bet than batteries.
Bingo!
A second engine driven power source is ALWAYS the
best cost-weight-performance alternative to ANY
chemical system.
>One question I have is if my endurance goal
>should be 5.5 hours with pitot heat on. I don't
>plan on tons of ifr flight, and I kind of doubt
>my brain could handle 5 hours of instrument
>flight. So it seems like lower endurance for ifr cruise could be
acceptable.
Probably so . . .
>
>If I do want to keep the heater on with the
>backup alternator I need a little more than 20
>amps. I'm a little confused looking at the
>alternators here. The SD-20 is described as a
>20/30A alternator. What does that mean. I saw
>some mention that the vacuum pad on a
>continental spins faster, does that mean it's
>30A on a continental and 20A on a lycoming?
I am not sure of the details . . . I've not had occasion
to touch the phycics of that product in about 20
years but I do understand that under presently
demonstrated conditions, its output can be that
high.
>The plane power FS14 is just listed as 30A, so I
>assume it can actually put out 30A on a lycoming.
Pitot heat was never a really good idea on light
aircraft . . . yeah, it MAY have saved the bacon
for a hand-full of pilots in years gone by but
far more airplanes belabored with ice have gone down
in spite of knowing exactly what their airspeed
was before they hit the rocks.
Having 'little chippers' to deal with some forms
of ice (heated windshield patches, boots, pitot
heat) have offered too many pilots a false sense
of capability to deal with mother nature's vagaries.
Modern weather observation and prediction systems
are so capable that go-no go decision to launch
is far wiser than any notion of being able to 'deal
with a little ice' . . . same thing with those
little yellow, orange and red areas on the radar . . .
good reasons to just stay on the ground or take
another route.
My preference for conducting the FMEA says
keeping the tube warm is at the bottom of the
list with respect to other appliances on board.
Just for grins, noodle through the plan-c for dealing
with loss of airspeed indication. What do you KNOW
about handling qualities, changes of trim and
power that speak to impending stall? If that
airspeed value becomes suspect for what you believe
is high-probability of ice, I'll suggest that
plans to seek warmer environs far outweighs
any value of knowing exactly what your airspeed
unless you think that climbing is the path to
salvation . . . which is very seldom the case.
IAS as a warning for impending stall is
based on a CLEAN wing. I got a really big
lesson in fouled aerodynamics when a landing
in Hutchinson KS ended with the airplane plopping
down hard in the flare while yours truly was
patting himself on the butt for having kept
the needles centered for over fifteen minutes
in the clouds, outbound through procedure turns
and then all the way back to the threshold.
My instructor was grinning ear to ear and said,
"Okay Nuckolls . . . while you were glued to the
glass, your wings were taking on new shapes with
unpredictable performance. None of that C-172-
full-stall-flare stuff in instrument conditions.
Fly it like a Grumman . . . carrier landings are
called for unless you are CERTAIN that your wings
are golden."
You need to make up your own mind but it seems
to me that a Z-12 system with a 15 a.h. SVLA
battery would provide a comfortable reduction
in risks . . . especially if you consider pitot
heat to be more cosmetic than practical.
Bob . . .
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