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is the calculation right ?????

Discussion in 'Electronic Repair' started by hurry, May 6, 2006.

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  1. hurry

    hurry Guest

    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 .
     
  2. Ralph Mowery

    Ralph Mowery Guest

    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.
     
  3. Bob

    Bob Guest

    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.
     
  4. 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:
    | http://www.apcc.com/products/runtime_for_extendedruntime.cfm?upsfamily=17
    | http://www.apcc.com/products/runtime_for_extendedruntime.cfm
    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:
    | http://www.the-gadgeteer.com/review/kill_a_watt_electric_usage_monitor_review
    They're not exactly a precision instrument, but are useful in making
    basic electrical measurements (including power factor).
     
  5. hurry

    hurry Guest

    Thnk you. had a good insight .

    bye,
    hurry.
     
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