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Sizing a UPS

Discussion in 'Electrical Engineering' started by Tom Del Rosso, Sep 28, 2004.

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  1. I measured the current on the AC cord to a file server with a clamp (on
    an extension cord that I broke the wires out of) and a Fluke 73.

    Does it matter that the 73 is not true RMS? I thought the wave would be
    nearly a sine.

    Since UPS's are rated in VA, not watts, is measuring the current alone
    good enough? Does that make PF irrelevant?

    The peak current I measured while excersizing the server was 1.4 A.
    That's only 168 VA, but if that's true then UPS's must be rated for the
    load that would run them down in 30 seconds.

    I'm thinking that this was just a useless measurement, but not sure why.
  2. Not even close.
    Yes it was. Size the UPS based on the rating of the server power supply or
    supplies. Add up the rating of all of the equipment you will attach to the
    UPS and that will give you a good idea of what you need. This does NOT
    apply if you want to connect the following to the UPS: motors, printers
    (particularly laser), or highly inductive loads. It does work if all you
    are connecting are computers, monitors, and routers.

    Charles Perry P.E.
  3. repatch

    repatch Guest

    Unfortunately yes, by a large margin.
    Nowhere near, all switching power supplies do bad things to their power
    feeds, some worse then others.
    Nope, since you are only seeing a rough average of current usage, you
    aren't seeing the peaks at all.
    Useless? I wouldn't go that far, it did give you a rough (perhaps
    extrememly rough, depending on how "non sine" the wave was) idea of what
    average current is being drawn.

    Basically use the ratings on the power supplies of the server, add them
    up, and then add an additional percentage, how much is up to you, I'd say
    50% as a minimum.

    Also remember that all UPSs basically have two ratings, a rating of the
    inverter used, and a Wh rating, which tells you how long it can hold out.
    Adding up the wattages will tell you what kind of inverter your UPS has to
    have, the Wh rating is up to you in how long you want it to run after
    power has failed.

    Also, remember that the rating of a UPS is the absolute max it can supply,
    so if your device only draws 100W normally, but does peak at 1000W for a
    very short while (like laser printers) then you do need an inverter that
    can do that 1000W.

    The power line is very "forgiving" of loads requiring short bursts of
    power way beyond what they normally use, UPSs are not. TTYL
  4. Thanks. How do you convert the rating of those devices in watts to the
    UPS rating in VA? If the current is not even close to a sine then it
    seems optimistic to expect V and A to be in phase, right?
  5. Unfortunately none of the UPS's I've seen mention anything about Wh.
    Can I calculate it from the Ah of the batteries, at least roughly?
  6. Actually the typical computer power supply has a displacement power factor
    (phase angle) of close to unity. That has nothing to do with the fact that
    the waveshape is not a perfect sine wave.

    The server power supply should have an amp rating. Use this, along with
    rated voltage, to calculate the VA.

    As for run time; it should be stated on the UPS documentation (so much time
    at full load). It is true that you will get longer run times for lesser
    load, but use the rating at full load as a worst case. As the UPS ages
    (actually the batteries) your run time will decrease.

    Charles Perry P.E.
  7. operator jay

    operator jay Guest

    Is this the load your UPS will carry? One server? No monitor, no printer,
    no nothing ever? Also please tell me how you calculated the 30 seconds
    number - I'm interested in what figures you (we) have to work from. In
    fact it sounds like you may already have a UPS. If so, let's take this from
    the other way around. Tell us what UPS you have and all the ratings /
    nameplate data for it and tell us your load and we'll tell you are thoughts.
    I can't even believe I just typed 'are thoughts'. Our thoughts. Also, if
    there is some specific problem you are experiencing, it's best to tell us
  8. repatch

    repatch Guest

    Yes, the Ah * V of the batteries will give you a rough idea of actual Wh
    available, remember though that there is a loss with the inverter, so
    derate what value you get, I'd say to about 75%. TTYL
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