On 2012-10-07, 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 ... there is the confusion between Watts and VA
(Volt-Amps). Watts are the real power it can deliver, while VA is the
product of the voltage and the current -- which is the *same* as Watts
for resistive loads (like space heaters and incandescent lamps), but
inductive or capacitive loads (like motors and computer power supplies)
have the current lagging or leading the voltage, so you get less power
than you would expect -- and while the cooling requirements of the
inverters are probably more power based, there is also a limitation on
the maximum current which the transistors can pump out, so you need to
know both the VA and the Watts ratings for most things.
And, I'll bet that the 5000 W rating is *instantaneous* (peak)
wattage, and that it could not run that kind of output steady state.
Are these things not actually made for powering the insanely
loud auto sound systems? You might get that kind of peak load every
time the big pulse of bass comes out, but there are quieter periods
between those.
O.K. Going to their page, I find that the 5000 Watts is the
*peak* power, and the steady state power limit is 2500 Watts.
https://www.cobra.com/detail/cpi-2575-2-500-watt-power-inverter.cfm
I've downloaded the manual, and discovered that the output is
not a sine wave, but rather a "modified sine wave" (synthesized from
square waves), which could make some loads quite unhappy.
Also (on page 12) I find:
================================================== ====================
The inverter can delivery 2500 Watts for about 60 minutes. The
inverter must cool for 15 minutes before it can resume operation
at 2500 Watts. Note: The Wattage rating applies to resistive
loads.
================================================== ====================
So -- you can't even get that 2500 Watts full time -- let alone the 5000
Watts.
O.K. the 5000 Watts is the "Surge rating (0.1 second)". Hardly
useful. :-)
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!
Well ... I used to have a generator which was good for at least
that much current at 28 VDC. (It was used in an army Tank to power the
really high power searchlights.)
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?
Not I, at least.
Perhaps you could have the auto motor running a bank of
alternators instead? And yes, you would need multiple batteries in
parallel to handle the load current. To do this, you would have each
alternator maintaining the charge on one battery, and you would have all
the batteries connected to the inverter through diodes, so one low
battery would not hog current from the others.
You also want serious sized wire. They are suggesting #4 AWG
wire for runs up to four feet, and heavier for longer. They are
suggesting #2 AWG which may suggest that is the largest which will fit
the connectors on the back of the inverter.
Gensets, even nat gas ones, seem to be a real pita, and the noise is
considerable, unless you get real pricey.
Given the various limitations of this, I think that it would not
be satisfactory for powering the house -- unless perhaps you had
multiple ones, each run off its own set of alternators and batteries, so
you could minimize the load on each inverter. And check out what you
might have which would not like the output waveform.
Good Luck,
DoN.
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