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nuckolls.bob(at)aeroelect
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 Posted: Sun Aug 16, 2015 8:20 am Post subject: Newbie with PM alternator feed question |
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| Quote: |
(1) Is dynamo current "self limiting" a characteristic of all 3 phase PM type alternators? (dynamos?) Would gladly eliminate those three 20 amp c/b's if not required! |
the PM alternators will self-destruct if presented
with a hard short on their windings generated by
failure of the rectifier=regulator. Use in-line
fuses. One for the single phase machines, two for
the three phase machines. These need no be crew-
accessible.
| Quote: | | (2) Was planning on a 40A ANL type fuse for the 10 ga wire from the relay to the primary bus. |
What is the rated output of your altenrator?
An inline maxi-fuse of next-step higher value
would be fine and a lot less bulky.
| Quote: | | (3) Was not planning on fuse/circuit protection from the battery to the primary bus. |
Most people do not . . . including hundreds of
thousands of TC aircraft.
Bob . . . [quote][b]
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nuckolls.bob(at)aeroelect
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| Posted: Sun Aug 16, 2015 8:26 am Post subject: Newbie with PM alternator feed question |
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At 05:59 PM 8/10/2015, you wrote:
| Quote: | --> AeroElectric-List message posted by: "blues750" <den_beaulieu(at)yahoo.com>
Follow on questions...
Having trouble reconciling the tinyurl link wiring that you posted versus the wiring diagram for the B&C OVPM package (505-500) I planned on using. Appear to be only one relay to control on/off of the alternator via the capacitor and R/R versus your sketch with two relays for two of three phases. Also, why the need to fuse each phase of the alternator prior to the R/R as depicted in the B&C diagram? I feel like I'm missing parts of the big picture [Embarassed] |
OV protection is a separate issue from
the protection of wires. The single
pole relay in series with the DC output
of a R/R ALWAYS works to protect the
ship's systems. However, that relay ALSO
serves the legacy purpose of offering any
time, any conditions, ON-OFF control
of the engine driven power source from
the pilot's position.
If one embraces the goal of generating
a MAX COLD power condition for ALT OFF
then one is encouraged to open the power
source as far upstream as practical.
Hence, relay(s) in the AC winding leads.
You need to break TWO leads of a 3-phase
machine to bring it to heel. A two-pole
relay would work . . . but inexpensive,
automotive relays in 2-poles at that current
are more rare and expensive that using
two, robust single-pole devices to do
the job.
Two relays was a cost-of-ownership . . .
not a functional design decision.
Bob . . . [quote][b]
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user9253
Joined: 28 Mar 2008
Posts: 1922
Location: Riley TWP Michigan
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| Posted: Sun Aug 16, 2015 7:04 pm Post subject: Re: Newbie with PM alternator feed question |
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| Quote: | 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?
Thanks, Joe
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_________________
Joe Gores |
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nuckolls.bob(at)aeroelect
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| Posted: Mon Aug 17, 2015 4:49 am Post subject: Newbie with PM alternator feed question |
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At 10:04 PM 8/16/2015, you wrote:
| Quote: | --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
> 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 . . . [quote][b]
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blues750
Joined: 06 Jun 2015
Posts: 56
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| Posted: Sat Aug 22, 2015 6:24 pm Post subject: Re: Newbie with PM alternator feed question |
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[quote="nuckolls.bob(at)aeroelect"] | Quote: |
(1) Is dynamo current "self limiting" a characteristic of all 3 phase PM type alternators? (dynamos?) Would gladly eliminate those three 20 amp c/b's if not required! |
the PM alternators will self-destruct if presented
with a hard short on their windings generated by
failure of the rectifier=regulator. Use in-line
fuses. One for the single phase machines, two for
the three phase machines. These need no be crew-
accessible.
Do you mean these "need to be" or "need not be" crew accessible? Why?
or why not?
| Quote: | | (2) Was planning on a 40A ANL type fuse for the 10 ga wire from the relay to the primary bus. |
What is the rated output of your altenrator?
An inline maxi-fuse of next-step higher value
would be fine and a lot less bulky.
30 Amp alternator, and I like the idea of the inline maxi-fuse versus the ANL because of the reduced size. I thought ANL type was preferred because of the longer response time and thus helped to avoid potential nuisance trips?
Bob . . .
Thanks Bob! A real treat to be able to discuss topics with subject matter experts! My cluebag continues to fill...
Dave
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nuckolls.bob(at)aeroelect
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| Posted: Sun Aug 23, 2015 5:42 am Post subject: Newbie with PM alternator feed question |
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At 09:24 PM 8/22/2015, you wrote:
--> AeroElectric-List message posted by: "blues750" <den_beaulieu(at)yahoo.com>
[quote="nuckolls.bob(at)aeroelect"]
>
> (1) Is dynamo current "self limiting" a characteristic of all 3 phase PM type alternators? (dynamos?) Would gladly eliminate those three 20 amp c/b's if not required!
the PM alternators will self-destruct if presented
with a hard short on their windings generated by
failure of the rectifier=regulator. Use in-line
fuses. One for the single phase machines, two for
the three phase machines. These need no be crew-
accessible.
Do you mean these "need to be" or "need not be" crew accessible? Why?
or why not?
Do the Failure Mode Effects Analysis: What
kind of failure would put these windings
at risk? What is the probability that system
functionality will be restored by resetting
crew accessible circuit protection after
suffering such a failure?
Suggest you review the Fuses vs. Breakers
discussions on the website . . .
http://tinyurl.com/nmwyak5
. . . I think there's a pretty good case
to be made for having NO crew accessible
circuit protection for a constellation of
reasons including.
(1) Likelihood of restoring functionality
after any righteous trip is zero. The fuse
blew because something was broke . . .
(2) Unlike the circuit protection in the
TC aircraft, any discovery of a nuisance-trip
condition in an OBAM aircraft can be quickly
FIXED . . .
(3) Crew accessible circuit protection has
little up-side. It takes up panel space and
it's a potential distraction from being pilot-
in-the-air and saving mechanic-on-the-ground
activities for later.
(4) Paying homage to the federally mandated
prohibition for unreachable protection of
'critical' hardware calls for doing your FMEA
and making minimizing single-systems critical
to comfortable termination of flight.
In cases where this is simply
not possible . . . like a one and only-one
fuel injection system, then that system needs
to be crafted with critical-item reliability
like propeller bolts . . . ten to the minus
bazillion failures per flight hour. Then,
having its breaker/fuse accessible in the
cockpit becomes immaterial.
> (2) Was planning on a 40A ANL type fuse for the 10 ga wire from the relay to the primary bus.
ANL's and their cousins are members of the
genus "current limiter" . . . exceedingly
robust and designed to protect fat-feeders
from battery-fed, 1000A+ style faults.
There are few instances in our size airplanes
where the ANL-type devices are really well
placed. The vast majority of protection goals
are handily met with fast-fuses sized to avoid
nuisance trips. I prefer the ANL-style
device for automotive-style alternator b-leads
because of their bolt-on style mounting and
wire-termination that makes them a bit more
convenient than any of the fuse holders but
a well-considered fuse installation is never
a 'bad' idea.
What is the rated output of your altenrator?
An inline maxi-fuse of next-step higher value
would be fine and a lot less bulky.
30 Amp alternator, and I like the idea of the inline maxi-fuse versus the ANL because of the reduced size. I thought ANL type was preferred because of the longer response time and thus helped to avoid potential nuisance trips?
It's true that current limiters simply do
not nuisance trip but in this case, the
maxi-fuse seems the more elegant solution.
Thanks Bob! A real treat to be able to discuss topics with subject matter experts! My cluebag continues to fill...
You're most welcome my friend . . . [quote][b]
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