Master contactor question

"Nathan-Annie Dridiger" wrote in message
...
On Nov 1, 5:47 pm, wrote:

If the starter solenoid is at the battery the only time the high
current conductor is live is when cranking - and virtually all the
rest of the loads can be handled by a "main switch".

Except that you still have a substantial live cable coming into the
cockpit, a hazard that can't be disconnected in flight if something
goes wrong. And that substantial cable and its substantial switch
could also end up costing and weighing more than a $25 contactor and
$3 master switch.

A lot of that can be true or false with nearly equal ease.

First, assuming that we are only talking about light single engine aircraft
with the battery mounted in the fusalage, rather than in one of the wings,
then a little depends on whether the battery is at the same end of the
fuselage as the engine. Personally, I would be inclined to protect the
starting circuit with a cartridge fuse as need the battery as practical.

The next question is what loads need to be protected in what manner, and the
only one that really needs a hefty contactor is the starting motor. After
that is the main output of the alternator/generator which sould be greater
than the combination of all loads other than the starter.

Personally, I would be inclined to protect the starting circuit with a
cartridge fuse, sized to protect the CABLE and placed as near the battery as
practical. I would also do the same thing for the cable from the battery to
the master switch/breaker/contactor and would further protect the main
output cable from the alternator/generator with an in-line or cartridge fuse
as near the alternator/generator as practical. Those three fuses should be
sufficient to protect the main cabling from the power sources to the
distribution points, where the breakers could provide the specific circuit
protection.

Protection, and switching, of the field circuit is a great idea--if the type
of regulator in use makes it practical. Otherwise, it might be necessary to
switch the output with a switch or contactor.


The charging circuit is the only part of the wiring that can get
tricky - and there are ways around that too.

Yeah, but you have no control over it if it's wired directly to
the battery as in an automobile. Regulators are known to fail and get
the alternator working overtime (I've had it happen in a boat) and
burn things out. An alternator switch (actually, a regulator controls
switch) and alt output breaker make things much safer.

There are ways to save cost and weight, but the electrical system is
not a good place to do it.

Dan

Very good point, and this thread started with a discussion of a master
contactor that would supposedly draw nearly an amp for its own coil
current--which seemed like an outrageously large power and heat dissipation!

So, it the starter uses the Bendix type inertial engagement mechanism, I
would be inclined to use a remote contactor near the battery and splurge for
a more efficient main contactor sized for the non-starting loads.

Peter