REFLECTOR: Using the starter supply(+) wire as a charge wire

[Terry Miles]

Hi,

 

Thanks Dave.  That's my thought too.  S-t!  I wish you were here.  .bring
your fish wire.you're welcome any time.  

 

Scott, you have gleaned one of the better and more comprehensive extracts
from the somewhat enormous library of Nuckols stuff out there.  As an aside,
I recommend that anyone who uses or refers to his main text book also be
sure to read his Freq Ask Questions piece which is a pdf file you can access
off his web site.  It runs 300 pages but has a lot of good concise info
restated from several perspectives so you can get a grip on some of this
more complicated stuff.  Thanks again to Scott for what he has pulled out
for us.  Nuckols book can get awfully wordy, esoteric and off topic
sometimes, or so it seems to me and if you follow the aeroelectric list it
gets to be time consuming and very hard to follow sometimes.  

 

Joe, I looked again at that set-resetting fuse, and while I would never
suggest it is not a good idea, I am choosing against it because of the
automatic reset.  If your v-reg goes haywire there is an o'volt protection
in the B&C stuff the pulls it off line for keeps.  If it does other weirdo
things, then the battery somehow gets into the picture and the amp load will
go up and charge wire opens.  In either event I would not myself attempt to
re-set it in flight if I thought either one of these issues might be at
play.  I know that there are transients and other glitches and re-sets are
always possible, but I will consider it lost for the balance of the flight.
But that's just me, and my take on things.coupled with the fact I have two
55 amp hour batts which I can draw from to get back on the ground.  

 

To me, some of this protection philosophy that Scott has raised has to do
with knowing clearly why a given circuit protection device would trigger.
My 2awg in this application has a dual purpose, so it is really 2 circuits.
Nuckols quotes the rule book below which says if the circuit is starter
only, no protection is required, and further that "fat" wires are often not
protected.  So that 200+ amp starter circuit in my airplane is not going to
be protected as I see things today.  The 2nd circuit, for this dual use #2,
is the charge wire.  It is protected by me having control of the alternator
field sw in the cockpit as a dedicated single purpose on/off, and a low
voltage warning on my panel, and an over voltage protection device that will
kill alternator output.  Of the two wire segments involved:  The b-lead back
to starter lug will get a current limiter, the #2 from that point back to
the master contactor output lug is unprotected, and then from the output lug
of the master contactor via a #8 into the main bus will go thru a 60 amp
breaker on the main bus. 

 

Let's raise another last topic and that's the cockpit starter button.  It
will take main bus current thru a 7.5/10 amp breaker up to the starter
button and then out via a dedicated #14 to the starter motor coil that
closes the big contactors and throws the pinion gear into the fly wheel.
The starter button is rated at 30 amps dc, and I will stick a 1N4002 diode
(not sure of the actual number) from the starter motor coil (+) with the
other end on the starter case ground.  No way is that switch a danger to the
cockpit despite the monstrous draw that it controls.  

 

Terry

 

 

 

 

 

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of David Scharfenberg
Sent: Monday, January 29, 2007 11:33 PM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Using the starter supply(+) wire as a charge wire

 

Hi

My theory is that I can turn off the battery contactor (in the nose, near
the batteries) if the #2 wire should short to ground somewhere between the
battery and the starter.

Dave Scharfenberg


On Jan 29, 2007, at 10:37 AM, Scott Derrick wrote:

Terry,

This conversation about protecting the "big" wire that runs to the rear of
the plane to power the starter and to provide for the alternators charge
circuit got me interested in possibly protecting it with a big fuse or
breaker.  200 Amp or so... Starters draw a huge current.   

I'm not talking about protection for the alternator, this being the short
#10/8 or so wire that connects the alternators charge circuit(B lead) to the
supply side of the main starter relay(big wire from main relay to starter
relay).  That small wire should have a fuse or resettable breaker. I think a
fuse would be the right choice.

I searched the Aero-Electric list and found this... Its not for a canard but
it is about a battery lead from the rear of a tractor style airplane to the
firewall.  Similar application..

>
>Bob and everyone else ...
>When considering a ground fault, what is best used to protect a #4 cable
>run, from the rear battery, in an RV, while powering a bus?
>
>What is best used to protect this same #4 cable, when it is in parallel
with
>the front battery and charging current is flowing to the rear batt?
>Thanks ...
>Jerry Grimmonpre'

Fat wires in light aircraft are not, as a rule, "protected".
If you study the wiring diagrams for light airplanes going
back to the first generator and battery installations
(mid 40's), things like fuses and circuit breakers do not
appear in the fat feeders. Let's see what the certified guys
design to by reading from FAR23.

Sec. 23.1357 Circuit protective devices.

First, keep in mind that I quote this document only as a means
of illustrating thought processes for crafting the most trouble
free system . . . and in no way am I suggesting that these
words be treated as a "requirement" that should flow down into
the OBAM aircraft community.

(a) Protective devices, such as fuses or circuit breakers, must be
installed in all electrical circuits other than--

(1) Main circuits of starter motors used during starting only; and

(2) Circuits in which no hazard is presented by their omission.

I can tell you that these paragraphs describe the portion of the
architecture you are asking about. Fat feeders in light aircraft
are seldom if ever involved in a hard fault situation because of
the extra care we take in securing such wires and thoughtfulness
for installation to keep them clear of moving parts that might
pose a threat.

When a fat feeder suffers compromised insulation, it's generally
against thin sheet metal and the fault tends to be "self clearing",
i.e., it take a lot of force designed to drive the faulted conductor
to ground such that the conductor is now at-risk for catastrophic
failure due to high currents. Relatively light, passing contacts
tend to burn open with little or no effect on the rest of the system.

(b) A protective device for a circuit essential to flight safety may not be
used to protect any other circuit.

Common sense. A fuse or breaker for every accessory. Don't stack
multiple accessories on a single breaker/fuse whether "essential"
or not. Of course, this begs for a definition of "essential" which
is a topic for a who chapter. The government position on aviation
is to build ever more goodies into the essential category . . .

As I've described in Chapter 17 . . . it's up to YOU to decide
what's "essential" based on your understanding of personal
and mechanical limits based on proposed missions.


(c) Each resettable circuit protective device ("trip free" device in which
the tripping mechanism cannot be overridden by the operating control) must
be
designed so that--

(1) A manual operation is required to restore service after tripping; and

(2) If an overload or circuit fault exists, the device will open the
circuit regardless of the position of the operating control.

Common sense.

(d) If the ability to reset a circuit breaker or replace a fuse is
essential to safety in flight, that circuit breaker or fuse must be so
located and identified that it can be readily reset or replaced in flight.

Yup, good thing to do. Now, I've suggested for years that there's
no good reason for ANY single piece of equipment to become so
"essential" that it qualifies for special treatment of access to
fuses or breakers. There are hundreds more things that cause
an accessory to fail that DOES NOT open a fuse compared to
failures that DO open a fuse. Focusing on the ability to restore
a fuse or breaker circuit is non-productive. These are
a tiny proportion of all failures and likelihood of getting
a system back by replacing the fuse is nil.

It stands to reason then that if any one SYSTEM is so
desirable, then there had better be a backup SYSTEM.
Once this condition is achieved, there is no reason for
making ANY fuse or breaker accessible in flight.

(e) For fuses identified as replaceable in flight--

(1) There must be one spare of each rating or 50 percent
spare fuses of each rating, whichever is greater; and

(2) The spare fuse(s) must be readily accessible to any
required pilot.

As thoughtful designers and users of OBAM aircraft, we're
able to craft flight systems that completely negate any
reason to observe this requirement . . . we can design
so that there is no need to reach any fuse/breaker because
there are no singular, "essential" systems likely to be
resurrected by replacing a fuse or resetting a breaker.

This is the general answer on circuit protection . . . the
short answer to your specific question is that experience
and common sense have shown that there is no value in
adding "protection" to long battery feeders (other than
the locally situated battery contactor). This includes
the generally smaller but still quite robust feeder from
the cranking circuit to the main bus.

Bob. . .




-- 

-
The only security of all is in a free press. The force of public opinion
cannot be resisted when permitted freely to be expressed. The agitation it
produces must be submitted to. It is necessary, to keep the waters pure.

Thomas Jefferson to Lafayette, 1823. ME 15:491 

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