Outback inverter actual ratings [RE-wrenches]

Christopher Freitas --- OutBack Power cfreitas at outbackpower.com
Tue May 1 12:12:33 PDT 2007


Todd - 

The OutBack VFX3648 is able to sell and invert 3600 VA (watts) at 25C 
ambient.  The 3000 Watt rating is at 40C ambient.  

There are numerous reasons you might be seeing lower performance - here 
are some of them:

Grid Distortion - if your grid is not really clean than the added 
distortion caused by it will cause the inverter to run hotter.  The 
UL1741 tests are done with a clean grid - A grid with 5% THD can cause 
as much as 10% higher temperatures from my measurements.  

Higher Ambient Temperatures - for each degree C (1.8 degree F) above 25C 
/ 77F the inverter will reduce its output by about 40 watts - this is 
why the unit is rated at 3000 Watts at 40C of course.  Some 
installations can restrict the air around the unit as well.  The DCC 
"valve cover" is not the best design - which is why replaced that 
mechanical designer.  

Altitude - the lower air density has an effect on the performance of 
electronics.  I beleive you are at 5000 or so feet elevation - at 1000 
meters / 3000 feet the performance is reduced 5% - at 1500 meters / 5000 
feet it is reduced by 10%.  This is on top of the other issues listed.  

So - it is pretty simple to imagine that out "3600" watt unit might have 
a hard time doing 3000 watts in your particular application.  No 
surprise - just physics.  

All other inverters are affected the same way - remember the "sunny 
breeze" issue with the SMA2500?  Some have there nominal rating listed 
at higher ambients (such as 40 or 50C) which can be a good thing - but 
as pointed out - the PV arrays performance is drastically affected as 
well at these high temps so its not a real problem in most cases.  

As far as out internal temperatures - these are very typical 
temperatures for a power electronic device.  The transformers we use are 
actually rated for 200C - UL does not allow us to go over 150C on the 
outside windings - so we have considerable margin left in them.   

As far as your other questions - here are the answers I can provide 
right now - I will follow up on the others:

> 2) will it throttle back first or just shut down?

It will throttle back at first but can still shutdown under some extreme 
conditions

> 3) will it still pass grid AC to the back up loads when it has shut down 
> on over temp?

The transfer switch will remain closed to pass through power to the AC 
loads from the grid.  

> 4) what is more efficient, one unit running at 96% capacity or two 
> running at 48% capacity?

Two units running at 1/2 power would be more efficient and more reliable 
as well.  

Let me know if you have any more questions.

Christopher



Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com  
www.outbackpower.com
Arlington WA USA
Tel 360 435 6030  


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