On Sat, 6 Oct 2012 23:51:42 -0400, "Existential Angst"
wrote:

Awl --

In discussing using inverters+car alternator as a backup power supply, and
observing some hefty cobra 5,000 W inverters ($310, google shopping), I was
wondering if these can be made even heftier with stock parts? I wonder if
these large ratings in cobra units aren't like Sears hp....

Well, they kinda are rated in "Sears HP" because nobody ever allows
for start surges on motorized items - like your Refrigerator and
Freezer. The inverter might have enough power to /run/ both at once
along with a few lights and things, BUT it'll take pretty much the
entire inverter output to get either one *started* - the start surge
on an induction motor is usually around 10X the running current, if
not more.

Look up the Locked Rotor Amps if it's listed on the nameplate, that's
the Start Current for the first quarter-second when the motor is at a
dead stop, then the current ramps down as the motor spins up.

It seems like the bottleneck in this strategy as a house backup power supply
is the car alternator. 5,000 W would require about a 500 Amp alternator!
But mebbe a regular car alternator + a bank of batteries could handle all
the peak loads/draws, as long as they didn't last forever. Anyone here ever
do this?

For one, running a 5KW inverter to backup the house will require a
large battery bank to feed it with a low enough impedance to toss a
500A surge toward the inverter. You can use an ultra-low-impedance
battery like a Gates Cyclon or Optima Spiracell, but while the design
can supply huge surges it doesn't have the Amp-Hours of capacity to
back it up for long.

Good old Tank Cells would be better for that - put up a rack in the
garage and load it up with about a half-ton string of cells, and
you're good to go. Get the Lead-Calcium ones, and they'll go 15 or 20
years between replacements with proper care.

You don't really need a 500A alternator to charge the battery and/or
pass-through to the inverter, but it has to be large enough to both
satisfy the constant loads and have enough excess to get the battery
mostly recharged before the next big start surge.

Gensets, even nat gas ones, seem to be a real pita, and the noise is
considerable, unless you get real pricey.

But a genset is the only practical long-term alternate supply -
whether it's putting out 120/240V to run the load, or large quantities
of 12V or 24V or 48V DC to recharge the battery string. Even with
that inverter and battery bank, you still need to recharge the
batteries or your backup won't take you very far.

And it is possible to totally silence a genset if you start out with
one that's intrinsically quiet to begin with - water cooled engine at
a slower operating speed - 1800 RPM is a lot easier to damp than 3600
RPM, but you need to over-rate a gasoline engine running at the lower
speed.

Then it's a matter of building an enclosure that keeps the noise in,
and developing a labyrinth with acoustic damping to let cooling air in
and out of said enclosure without letting the noise out.

The "Movie Generators" they bring out that are almost totally silent
wouldn't cost that much more than a standard 60KW to 200KW unit if
they were mass-produced - half the cost is they're all built custom.

-- Bruce --