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Running a pure sine wave inverter off a modified sine

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Posted by Rob on March 3, 2004, 10:38 pm
-Concerned about running my fridge off my Xantrex 1750 modified sine

-Got my hands on an old Best 1000W pure sine wave ups (previous owner
didn't want to put out the $$ for new batteries).

 - The Best needs 120v input to activate the ups.

- I put 48v worth of batts inside the Best (batts required for the Best
to run), plug the Best into the Xantrex (for the 120V requirement  to
the UPS) , power the Xantrex from my 12v set-up (Batt/ booster cables
from car to keep 12v Xantrex  powered).

The Xantrex puts out 109V Modified sine wave
The Best puts out 125V pure sine wave

I've tested this set-up with 4 car batts in the Best 1 for the
Xantrex and it seems to work (the fridge loves the Best, even better
the wall plug).

Whew, now my question, is there something I'm missing? I only tested the

setup for a few minuets. Will the Best just rectify the power from the
so that I will primarily be running off the Xantrex?  If this is the
case I can go for
48V worth of cheap batts (only used to trigger he Best).

Or will I be running off the Best (and its batts) and using the Xantrex
run the 48V charger in the Best (which case I should buy the expensive
deep cycle batts).   Thanks for any replys...Rob

Posted by Steve Thomas on March 6, 2004, 5:38 pm
  You have a Best Ferrups right?
  While the Best unit has AC power available, the resonant transformer acts
as a filter to the line power. By feeding it with the rectangular waveform
from the "modified sine" unit, you are asking it to do a lot, but if the
electronics can stand the transients and nothing overheats, you should be
ok. Best had a control circuit programming optiion for running off a backup
generator, and they had a reputation for durabiity.  The Xantrex will be
working into a highly reactive load, and that may cause it some problems if
significant power is drawn. In my opinion you would be better off to use the
inverter built into the Best unit for power, since it was designed for this
service. Also, since the voltage drop in switching devices is more or less
independant of the voltage being switched, an inverter with a higher voltage
battery will be more efficient, all else being equal.
    I also have an old Best Ferrups unit, about 1800 VA, that I want to use
as an inverter. There has got to be a way to start it without having AC
present first, but I have not spent much time on the problem yet. It may be
as simple as a small circuit change in the control electronics. On the other
hand, if the control electronics require the resonant reactive components to
be "ringing" already, then your way of starting it up may be the only way.
   Let us know how it works out.

 Steve Thomas

Posted by Rob on March 7, 2004, 1:05 am
 Hi Steve, tried the setup, and the Best would detect the AC volts, (green light
came on), but it switched to the inverter mode and ran off the batteries inside
(green light for ac power still on)..  It tried to draw off the Xantrex for
about 30 seconds, then quit trying.  Using the Best is preferred, but it
requires 48Vdc. I was hoping to use the Best to rectify the power from the
Xantrex, or at least the Xantrex run the 48V charger inside the Best.

     I did notice with a digi volt meter that the Best did put out 125v while it
was detecting the Xantrex line voltage, then switched to its internal Batts.  So
it worked for 20 - 30 seconds (the green light for line voltage was a little
dull). Got the fridge to run for about an hour on the cheap batts, but cheap or
not I still need a way to recharge.  Last summer the power was out for 3 days
here, and had to toss everything in the fridge.

    Your comment about a control circuit program (how would one program a ups?)
gives me insperation......other than buying a 48Vdc charger ($$$$)/massive amp
hour Batts($$$$) its like a white elephant to me.  If all worked as I had hoped,
I would run the 12V Xantrex off a combo of batts (recharging them off the cars
alt) use this 109Vac to run the much preferred 125Vac from the Best.  I figured
the 1750 watts off the Xantrex would be lots for the 1000 watt Best, alas, as I
like to say, 'there's no substitute for experience'.   So the only use for the
Best during a blackout is to  use it to run a 120V, 12V batt charger, which may
extend the 'silence' for a bit, but since I plan to use the car to keep the
Xantrex going, seems like a waste.  On a positive note, the Xantrex did at least
get the Best to 'start', thetas gotta give me something to go on.   (Yes it is a
Best Ferrups).
    Thanks for the reply, Keep my fingers crossed and something will come to me
I'm sure.


Steve Thomas wrote:

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