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Posted By Jesse-M,
Dec 6, 2008 at 3:48 AM
How long would a 50 watt motor run on a 1000 watt Uninterupted Power Sorce
Insufficient data... The 1kw UPS spec is not how much power it "holds" - it is how much current draw it can support w/o putting out smoke signals or popping fuses, how long it can support it is a seperate question, with the size of the load being a factor... Remember that the intent for most UPS units is NOT to keep the system attached to it up until the power comes back, but only for long enough to allow a graceful shutdown, or to allow the backup generator to kick in. A small unit might carry 1KW for only 5-15 minutes. Something for a data center might go for 30 minutes. This will mostly depend on the size of the internal batteries.
Other factors will include the age of the unit, as the runtimes go down as the batteries get old (note that the batteries in most units CAN be replaced, despite the usual "No user servicable parts inside" labels), the efficiency of the inverter, how much charge is on the batteries, and so on.
Assuming that you have a fairly standard size UPS, I would guess 1-2 hours probably, maybe longer.
Another factor to consider is that if you will be switching the fan on and off, is that many UPS units can't start up in DC mode - IOW if you turn the load off, the UPS may well shut down and not be able to start back up again. (and this probably won't be documented anywhere easy to find, as the "Use model" is that once you've shut down you don't start up until the AC is restored...)
If I were looking for a backup system I'd consider getting a low power inverter, a trickle charger, and a Deep Cycle battery as something that would give me the longest runtime for the least $$$
Thanks for the info. I like the power inverter idea, but how could i make that setup work if im not at home when the power loss happens ?
It can be a bit of a challenge, but it is doable, and is essentially the way some of the high end UPS systems work - you set it up so the fan is ALWAYS running on the inverter which is connected to the battery, and use a "smart" battery charger that has an output slightly greater than the inverter draw.
The net result is a slight efficency loss from the two AC / DC conversions, but your fan is going to run off the battery charger if it's got AC, and the battery if it doesn't... The battery gets charged by the difference between the charger output and the inverter draw.
I think I'm following you! Doesn't sound to hard! What do you think about this idea? http://www.butkus.org/ups/ups.htm
The guy has the right idea in general, but some stuff in the article is seriously wrong... I've found UPS units with everything from a single 6 volt battery to four 12 volt batteries, some of his wiring theories are a bit strange, and things along that line... It can work, with a few cautionary notes, and I'd want a more reliable source than that article...
1. I've heard if you try to put a really big battery on a UPS, and run it down flat, you can over cook the charging circuit getting it back up to full charge - not sure how true that is though. I would tend to want to use a UPS carcass that was from a larger "server grade" unit as opposed to a cheap consumer unit on that basis though.
2. I would not try to do a series / parallel battery bank to get more juice, as that can also be tricky to get all the batteries to charge evenly. Stick with duplicating the existing setup, (however you can substitute one 12V battery for two series wired 6V batteries) except use heavier wire between the batteries.
3. Do pay attention to design safety, a marine battery does contain sulfuric acid (not hydrochloric!) and will give off potentially explosive hydrogen gas - make sure your batteries are safely contained and vented to put any vapors where they won't do damage.
4. If you don't have a pretty good grasp of electrical / electronics wiring, this is not the worlds safest project...