UPS as a stand alone inverter? [RE-wrenches]

[Robert Warren]

Doug,
 I did a similar thing with two Best UPS's at Rocky Mountain Institute 
about 5 years ago. But I used PV to directly charge the batteries, not 
another inverter, which I would not recomnend. I removed the internal 
batteries of both a 3.1 KVA and a 5.3 KVA unit and hooked the DC battery 
bus of both to a huge external Absolyte battery bank. Then I added a 
capicitor on the battery circuit as a filter for the PV PWM controller, 
and the batteries are float charged by a 2 kW PV array. This arrangement 
was designed to provide about 2 days backup for a network of 30 
computers.
 But you are wrong about one thing--the Best inverter (Ferrups model 
UPS) is not on all the time. That is the beauty of this UPS. It is 
normally off and only comes on during a brown-out or blackout. it takes 
2 to 4 ms for the inverter to switch on. However, as they have a big 
ferroressonant transformer between the inverter and the load, the 
collapsing magnetic field in the transformer holds up the load VA for a 
good 20 ms so that at the computer, you don't see any power flicker 
whatsoever. 
 In my 12 years as a Best service rep, I think these things are pretty 
much bullet proof. I rearely had to change out boards, except to upgrade 
or change voltage configurations. By contrast, a lot of other UPS's I 
serviced had a much higher failure rate. The only thing is, for a 
PV-UPS, you have about a 10 to 12% loss due to heat and magnetic 
saturation of the ferro transformer. If you can live with that, it is 
worth looking into. However, this efficiency is if you are at 70 to 90% 
of capacity with your load. If you have a load rated at a lot less than 
this, the efficiency goes to hell, as you are on the low end of the 
output efficiency curve.
   Best got bought out by Exide, which got bought out by Invensys. They 
are now made by Exide, I think, but I don't know if they still  still 
make all models. I think they rebranded it into the Powerware line, 
which blurs the technology line of a different type of UPS, which is 
Exide's main seller.
 Robert Warren 

Doug Pratt wrote:

> Has any one tried using a high end sinewave (online, zero transfer time) 
> UPS as a standalone inverter?
> 
> Bill Loesch
> 
> 
> I've got some limited experience. Back in the mid-90s Real Goods took a 
> pair of Best UPS units, one at 3kva, one at 7kva, that we were already 
> using for on-site critical backup, and by adding some separate robust 
> battery chargers (Trace SW-series inverters) and some funky 
> non-auto-starting generators, we made ourselves a couple of pretty 
> robust UPS systems. The Best inverters are always on. Normally they 
> recharge simultaneously from utility power, but they were damn fussy 
> about generator power. Rather than fight them, we just did battery 
> charging thru the SWs. We never had a utility outage longer than a day 
> or so. So all I can say is that short term, they presented no problems.
> More battery capacity would have helped them greatly. The Best inverters
> certainly held up well. Ten years later we're still using the 3k unit. I 
> suspect the efficiency really sucks, but efficiency is less of a concern 
> with UPS units than rock-solid reliability.
> 
> Doug Pratt  
> 



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