Actually, that POH on the Cirrus website is for the conventional 6-pack
SR-22. The Entegra-equipped SR-22 replaces the instrumentation on the
essential bus with the PFD.
The Cirrus system was designed to require no immediate pilot action to keep
the
essential power system up and bare-bones IFR capability functioning
in case of an alternator failure, and indeed that's what it does - if
alternator
2 fails, alternator 1 instantly feeds the essential bus, and the pilot can
at the earliest convenience load shed on the main bus until the
"alternator overcurrent" flashy light goes out; the batteries will handle
the
overload for that time. If alt 1 fails, the PFD, basic comm/nav and
autopilot
are still up, but the rest of the plane goes dead. If both alts fail, both
batteries
feed both buses. This sort of automatic backup was apparently required by
the FAA for all-electric IFR operation, particularly with a PFD.
It does depend heavily on the isolation diodes, and it appears that alt 2 is
the most heavily loaded of the two. What's also not obvious is that those
diodes
are kept reverse-biased by alt2's regulation voltage being higher than
alt1's.
We could concoct the same thing with a crossfeed switch and some clever
load sensing, add some more flexibility, and remove a potential failure
source;
but we'd add others. Yup, it's a compromise.
Incidentally, the Cirrus system has a neat warning system: one big red "low
volts",
and a yellow for each alternator. The low volts is hooked to the essential
bus, and
indicates that both alternators have failed and the ship is on battery
power.
The yellow control circuit uses a current sensor in the alternator(s) output
lead, and
flashes if charging current demands are too high; if the alt fails it glows
continuously.
A quirk of this system is that alt2 doesn't come on line 'til about
1800-2000 RPM
(because of the high regulation voltage),
so it really isn't a 2-alternator system until you're at cruise power.
SS
----- Original Message -----
From: "Fred Fillinger" <fillinger@ameritech.net>
Subject: Re: Low-hold-current contactors and All-electric
panel
> Approach with caution though! That diode between system #1 and #2
> needs to be rated for the max load on system #2, to include charging
> its battery plus whatever may have caused the system to fail. The amp
> ratings on high-current diodes may be for 1/2-wave AC, and she'll
> smoke under DC, requiring considerable derating and likely
> heat-sinking in any case. This is especially critical where, as on
> the Cirrus, one puts the "essential" items on the secondary
> alternator. Lose the secondary system, then cook the interconnecting
> diode, and there's no switch to manually connect the essential stuff
> to a system #1 that's still working! As in life, everything's a
> compromise.
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