is the calculation right ?????

hi, I was trying to run my table fun on an UPS (660 VA, with full load time

10 mins). My table fan has a rating of 80VA. On searching on net for calculation I came up with the following figure:

time of run = 660/80 * 1.5 * 10 = 123.75 mins

The figure 1.5, to be applied if ( UPS rating / FAN rating ) = 660/80 (this case) is grtr than 3 else multiplying factor = 1.3, This was from one of the pages, can someone reason out this.

I am not from an electrical background I was able to come up with this figure after some googling.

but I find my UPS supporting my fan on an average of 1/2 hr. Can someone verify this and tell me wat am I missing .

Reply to
hurry
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Some batteries discharge at differant rates depending on the load. Usually the smaller the load the longer the discharge curve. That is if you draw the full 660 VA the battery will last 10 minuits or a factor of 1. If you only use say 66 va the supply should last 100 minuits, but it may last 1.5 times longer than this because the current load is much less than its maximum ratings.

Even without the multiplying factor it should run 660/80 x 10 or 82.5 minuits.

The batteries may be old and weak.

Reply to
Ralph Mowery

How old is the UPS (battery age)? Where is the 660 VA for 10 minutes listed? Some of these are a little on the optomistic side. 660 VA for 10 minutes would need somewhere around the capacity of two 7 amp hour, 12 volt batteries. (7ah * 12volts * 2= ~168 watt hours) * 6 (the 10 minute run time) = 1008 watts for 10 minutes. This is reasonable considering conversion inefficencies, and heat generation in the batteries due to fast discharge. I picked 7 ah batteries for the calcs because that's what's common for that size UPS. The runtime should be longer for smaller loads because of the battery discharge rate. As in, it might do 100 wats for much longer than an hour.

Reply to
Bob

"hurry" hath wroth:

Make and model of UPS please? Many UPS manufactories lie as to their runtime. The usual problem is that they can't seem to agree on exactly when to declare the runtime over. Is it when the battery alarm declares that the battery needs a charge? Or is it when the UPS quits altogether? Does one shutoff the load when the battery is discharged to 70% of capacity (to not destroy the battery by discharging to zero), or do they just run the battery into the ground. Hard to tell what the 10 minutes really means as each vendor seems to have their own definition and tests.

There are also some oddities. Note that the runtime at half load is far more than twice the runtime at full load. For example, see: |

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|
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for some APC UPS's. At full load (300w), the BK-500 will go for 2 minutes. At half load (150w), the BK-500 will go for 14 minutes. I'm not sure I could supply a suitable physics explanation of why batteries deliver less power near full load. You cannot use a linear calculation to calculate UPS runtime.

A fan is a motor which is an inductive load. The UPS runtime measurements were probably tested with a purely resistive load. Unlike the AC power meter, which works on a power factor corrected billing, the typical UPS has no power factor correction on the load. It works on volts times amps with the worst case numbers. No power factor correction. My guess(tm) is that the 1.5 is someones attempt at PF correction or perhaps preserving the battery at some level above where the UPS just craps out. Hard to tell.

Sounds like old batteries. Runtime drops as the batteries get old. Try the same test with an incandescent light bulb of known wattage.

You might wanna invest in one of these: |

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They're not exactly a precision instrument, but are useful in making basic electrical measurements (including power factor).

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Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
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Reply to
Jeff Liebermann

Thnk you. had a good insight .

bye, hurry.

Jeff Liebermann wrote:

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Reply to
hurry

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