Will and other builders,
It seems like the department of redundant redundancy to have a separate
alternator
and master switch. For many it is a matter of "That's the way we've always
done it!" .
The Rotax manual actually implies a switch breaker not just a fuse/circuit
breaker
(aka Off Load Fuse Switch). The Europa manual shows a switch or solenoid
of course as does Bob Knuckles in Aeroelectric Connections (aircraft electrical
bible).
Rational for a Rotax: In the event of a battery problem, the battery has a
switch
we use to control power to and from the alternator and bus via a master
contactor
"kerchunk" solenoid or relay. Others have used a manual marine battery
switch or as I call them "Igor Switches" which can be very heavy. Batteries
normally do not give problems today, but if overcharged, they can. If a cell
shorts, one may detect nasty odors, high amps, and isolating the battery helps
shorten the problem by isolating the battery from the charging and aircraft
bus system. In the event of a imminent crash landing, one should consider
isolating
the battery from the electrical system and cockpit as a spark prevention
measure since we occupy the same space as the fuel tank.
The Alternator system has had a switch for three reasons in my opinion. To
eliminate
the chance of an overcharging alternator which can damage avionics and
over-charge a battery. Or in the event of a battery problem or master solenoid
failure the alternator will still allow electrical power without the battery.
Or the need to disconnect the alternator due to electrical fires/forced landing
situation as above to kill all power. Hence, there is a separate alternator
switch to kill any spark in our fuel filled cockpit from that electrical source
even forward of the firewall. Normally we separate the alternator from the
bus via the C or control voltage bus feed switch or a 30 amp solenoid/relay
controlled by a DC switch.
Why control the C or control voltage from the bus through a switch is, if the C
wire is open, the regulator is shut down. Typically most in the US wired the
alternator in this way. I have a 914 so my R,Band C wires go to a relay, on
the firewall controlled by the cockpit alternator switch to simply remove the
alternator power from the charge circuit on the firewall. (The alternator output
goes through the firewall plug to the 914 Aux Boost Pump in the panel still,
so I can't really kill all power in the panel. UGH!) In the event of a runaway
Ducati regulator, a C wire of higher voltage from the bus, signals the regulator
to decrease the voltage normally. Although highly unlikely in a B&C type
alternator this can happen (but I haven't seen it ever). Kill the C voltage
and you kill the faulty regulator output if the regulator is working. That
said, Duati regulators don't tend to over volt and a crowbar or any type of over
volt protection is not necessary either. However, if using a power management
system for your bus, the bus will never run at full alternator output voltage
going to the bus, therefore, a C wire off one of these PTC controlled bus
outputs will input a C voltage that is lower (nominally a half a volt) than the
actual output from the regulator and the regulator will be forced to charge
at a higher rate. Not bad for a LiFe type battery or even some AGM types, but
could be harder on the poor Ducati in my opinion. In a low amp draw aircraft
a little overcharge (14.5 volts) helps some of the AGM and LiFe batteries charge
better. (Note, supposedly there are two instances of the Schicke GR6 with
LiFe has had a steady "voltage creep" issue in the Remos and Pipistrel climbing
to 15 volts, but not the Ducati). Today's avionics can take up to 30 volts
without worry, it's the battery that normally can't. So I'm not a fan of
crowbars
or experimenting with alternators without switch protection and extensive
testing. Another topic of contention we won't go to.
With a 914, wired via the Rotax manual, one can isolate the battery in the event
of a short or smoking panel and with the battery off, the alternator will supply
power through the aux boost pump switch preventing a flame out. For those
with the 912 series, this is not a problem.
The Ducati type voltage regulator normally fails in a safe mode (dead)
preventing
a potential sparking mess on your firewall from the AC current being supplied
by the engine stator. So very safe from runaway problems.
For a simple 912 with the battery located forward of the firewall, in a
protective
battery box, AND the regulator is wired to the same contactor as the alternator
output (R,B and C connected together) the system would be safe for most
automotive thinking types. One switch, ON or OFF forward of the firewall like
a auto. This system is car like as you in the cockpit have no control over the
electrical system. It is either ON or OFF. Backup batteries in avionics keep
power alive so one must plan how to shut that off quickly in an emergency
also. Another topic we won't go in to.
Normally a split master switch from suppliers has tabs to make the two switches
work in unison. Those of us who are control freaks, cut these tabs and create
a true split master/alternator.
My vote is do a split master in any aircraft so one has modest control over the
electrical system. Especially if the battery is in the baggage bay or essential
power is necessary in the event of a master contactor failure or battery issue.
I can run both battery and alternator or one or the other to isolate a
problem in foreseeable emergencies.
Best Regards,
Bud Yerly
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