Overloading a UPS

[J-Mag]

I have a 750VA UPS and even though Watts = V(A), there is a secondary rating on the UPS for wattage and it states ~400w.

Now I am curious... I have a 510w PC Power and Cooling hooked up right now. IS there going to be any problem if my machine starts to use over 400w under load, or will the UPS just not be able to provide battery power to the machine at that load?

 

[jonnyGURU]

You think you'll put more than a 400W load on that UPS?

If you're using the PC with the power on, don't worry about it. It can be a 200VA and it's not going to matter. It's not going to affect the UPS at all as long as it's getting juice from the wall.

If you're using the PC to the point where you're putting a 400W load on the mains during a power outage YOU ARE A SICK FREAK!!!

If you want to make sure the UPS can handle brown outs and what not or just have enough time to shut-down, just make sure the monitoring software is installed and then unplug it from the wall. I find that if the battery doesn't drop more than 60%, you're golden. If it drops more than 60%, you need a bigger battery.

 

[HTPC Rookie]

This is from the Virginia Biomedical Center's WWW site:

It's the best plain-English explanation I've found of Watts vs. Volt/Amps

Techs are often confused when given the VA (volt-amps) power rating for equipment instead of Watts. The difference is relatively simple, VA takes into account the power factor for reactive loads and is determined by multiplying the Volts and Amps required to power the circuit. For a purely resistive load, Watts and VA are the same. In equipment with some capacitive or inductive reactance in the load, there is a phase shift between the Voltage and Current waveforms (remember this from school?) which is dependant on the amount of reactance. Because of this phase shift, the peak voltage and peak current do not occur at the same time. If the current waveform is at 70% of peak when the voltage waveform peaks, the power consumed will be only 70% of the power supplied. A piece of equipment with a VA rating of 100 and a the power factor of 70% will actually consume only 70 watts of power. However, the power source must be capable of providing a full 100 watts. Reactance in a load can be caused by a number of things, including a lightly loaded motor or power transformer and power supply rectifier/filter circuits. Enough theory, here is what you need to know:

1. If equipment input power is rated in VA, then the power source must provide the same number of Watts as the VA specified.

2. If equipment input power is rated in Watts, then find out what the power factor is and use this formula: VA = Watts ÷ Power Factor (remember that power factor is in percent, so for 70% power factor, use .70 (or 70/100) in the formula)

3. If you cannot find out the power factor, then try using 70%, this is a guesstimate of the minimum power factor for electronic equipment that is used by most UPS manufacturers and should work in nearly all cases.

4. If the power source's output is rated in VA (as are some UPS's) the Wattage is equal to the VA.

article by Mark Freeman
07/20/97

That's why your UPS is rated at only 400 watts real-life.

Your second question: If your machine should pull more than 400 watts from the UPS, the UPS SHOULD go into overload and shut down. I wouldn't count on that as a safety net though (meaning don't push your luck ).
It's likely though that if your box was drawing even close to 400 watts, and you had a power failure, your UPS would run out of steam very quickly.

Check out www.cyberguys.com They've some decent large capacity UPS units for pretty good prices.

 

[jonnyGURU]

I suggest one of these in EVERYBODY'S tool bag: http://www.supermediastore.com/kilwateldet1.html

 

[Ice Czar]

I suggest one of these in EVERYBODY'S tool bag: http://www.supermediastore.com/kilwateldet1.html

agreed

but why not muddy the water some more
(largely for our viewing audience)

http://hardforum.com/showthread.php?t=890308
http://hardforum.com/showthread.php?t=890652

see I have no real idea of the average or worse case draw of the
24port Linksys switch, time warner cable modem, and linksys accesspoint

the definative answer would be to measure that with a watt meter
they are rather cheap and had at most hardware stores
a far fancier one is reviewed here
http://www.systemcooling.com/watts_up_pro-01.html

as far as what the PSU would actually pull thats dependent on both the load on it
(what its doing) and its efficiency at that load and temperature (Id guess it to be less that 100 watts on average and worsecase at 130W VDC
then factoring in AC Conversion and efficiency

well a supply @ 62% efficiency with a 65W VDC works out to about 105W VAC
a good guess for "typical"
a supply a 69% effeciency at 150W would draw 217W VAC not a bad worse case
(supplies have an effciency sweet spot I grabbed both of those from an FSP350 Mike Chin reviewed)

one of the other things to bear in mind is Power Factor Correction
Batteries dont like nonlinear loads


cut and paste
--------------------------------------------------------------------------------------------------------------


Power Supply System Integration Part 2: Battery Backup @ Power Electronics Technology

UPS Nonlinear Loads

Most end users are oblivious to the problems encountered when an online UPS interacts with nonlinear load, such as a switchmode power supply. Fig. 3, on page 42, illustrates the interface between an online UPS and a switchmode power supply. The switchmode supply within the host system consumes its current in high amplitude pulses, and as a result overloads the UPS, distorts its output and degrades in performance. This isn't the case if the SMPS has power factor correction (PFC); and in the United States most SMPS don't have PFC.

Repeated measurements under practical conditions show that in non-PFC SMPS, the current peak to rms ratio reaches a crest factor in the range of 2.5 to 3.5. This high crest factor imposes a difficult load for the UPS.

Manufacturers specify UPS capacity in volt-amperes, and set the UPS current limit with resistive loading. This affects the user in two ways: first, the actual power (in watts) that the online UPS can deliver is 75% to 80% of its VA rating. Second, the UPS cannot support a nonlinear load like a SMPS to full power capacity. In fact, it can support an SMPS only rated to 30% to 40% of its capacity. A 1kVA online UPS will only be suitable for backing up a system energized by a 300W to 400W (non-PFC) switchmode power supply. Attempts to draw higher power by the SMPS will severely distort the output of the UPS and may result in shutdown. Some UPSs support nonlinear loads to various degrees, but the above rule is safe to use as a guideline.

The whole idea of generating a high-purity sinewave within the UPS, and then rectifying it within the SMPS is wasteful. A UPS may be deployed to run non-electronic loads, such as lights or motors. However, this is a minor portion of the market for online UPS. Many applications back up computers and telecom devices, as well as instrumentation systems used for critical applications. In all of these applications, the immediate load for the UPS is the SMPS within the systems. Peak current and inrush current are associated with this load at startup. This current of 40A to 80A per SMPS will cause the UPS to shutdown due to overloading, or the UPS must transfer the load to utility to draw its inrush.

Im "upgrading" the importance of active PFC in purchasing, as it does seem to have a great deal to do with not abusing and effectively employing the capacity of an online UPS (and by extention Id assume a line interactive UPS \ SPS when its on batteries, a far more common UPS scheme)


a bit more on PFC
PFC Decoded
and on UPSs
Power Conditioning 101
DIY UPS all @ dansdata

--------------------------------------------------------------------------------------------------------------------------------

Id vote for research,
1 get a watt meter and measure the worse case for all the equipment involved
2. see if the supply is PFC or not
3. select a UPS based on that (adding additional capacity for a nonlinar load)

On the bright side that PCP&C has the best PFC your likely to find, on the not so bright side that hurts efficiency and mike chin hasnt done that supply but lee garbutt and jonnyGURU have

http://www.pcper.com/article.php?aid=-270&type=expert&pid=8
http://www.slcentral.com/pc-power-and-cooling-turbo-cool-510-express/

what the actual effciency and real VAC draw at your typical or probable load would be however....
as jonny said a watt meter will work wonders compared to dead reckoning

 

[J-Mag]

I suggest one of these in EVERYBODY'S tool bag: http://www.supermediastore.com/kilwateldet1.html

Well actually I have essentially the same thing: A Seasonic Power Angel
http://www.newegg.com/app/viewProductDesc.asp?description=99-887-001&depa=0

And I have hooked it up to a powerstrip that was feeding my machine and I was pulling around 360W at full load (however, I only had one 6800GT at the time and I wasn't overclocking it, now I use two at 420 1100). This reading included my Monitor b/c I had it hooked up to the same power strip.

I am assuming that the second GT is going to push me over 400w, not to mention I have been adding drives since my new PSU seems to handle these extra drives without an issue. And currently I am running 4 SATA 7200rpm disks, 2 Opticals, and 4 IDE disks...

I am not really worried about running full load during a power failure, but more worried about what would happen to the power feed from my UPS when I go over 400w while the main outlet is still functional.

Edit: I mean 360watt draw at full load w/ one gt

 

[Ice Czar]

I am not really worried about running full load during a power failure, but more worried about what would happen to the power feed from my UPS when I go over 400w while the main outlet is still functional.

hmmm...
now thats a good question
at some point it will think there is a short
that point might be documented

 

[J-Mag]

at some point it will think there is a short
that point might be documented

What would happen if the UPS shorted? I am thinking it would just cut power off to all components... (wondering if it would cut power off to both thte batter side and the surge side)

Point documented?

Actually now that I think about it: I have MY PC, Monitor, Wireless headphones, Speakers, Subwoofer, Phone Charger, Camera Base Station, and a Whispure Aire Filter all attached to the same UPS... Now the first 4 are on the battery side and the last four are on the Surge Side... The total has to be running over 400W, especially with the Sub and Air Filter on high...

 

[jonnyGURU]

I am not really worried about running full load during a power failure, but more worried about what would happen to the power feed from my UPS when I go over 400w while the main outlet is still functional.

On my test bench I've got one of those inexpensive 1000VA's. If I hook three PC's up to it, and the 17" monitor that's always hooked up to it, the overload LED's are all lit up. I'll go ahead and hook up a fourth and I'm pulling about 800W through it at IDLE, but everything works fine.

Mind you, if I pull the plug on the battery all four PC's immediately go down because I've overloaded the UPS, but while everything is getting power from the mains, nothing unusual happens.

 

[Ice Czar]

if the UPS shorted it would depend on which side
on the source AC side hopefully it would kick over to batteries and alert you
on the AC supplied side it would hopefully shutdown


point = at some VAC amperage draw from a powered device the UPS will assume it has shorted and likely a fuse or MOV will kick in

what I dont know is the typical surge suppression scheme of a common SPS
but Im now looking for it

AC input -> fuse -> MOVs connected across input -> line filter -> rectifier -> ....

MOV's are "metal oxide varistors" - they're sort of like a zener diode, but bidirectional. They act as an open circuit across the AC line until the voltage reaches a certain point, then when that happens they basically short out the line. So If a power surge occurs, the MOVs will conduct at the higher surge voltage and blow the fuse.

"Surge supressor" power bars just contain a bunch of MOVs. Decent ones have a circuit breaker before the MOVs which will trip with the high current, but cheap ones just rely on your house's circuit breaker to trip.

There's no super-extra-high-advanced technology inside Monster power strips or anything. This stuff has been around since the 60's.