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UPS for high load short time

Discussion in 'Electrical Engineering' started by kunt, Mar 20, 2008.

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  1. kunt

    kunt Guest

    Hi there,
    we have a server room in a place that is backed up by a diesel generator
    but does not have an UPS, so that the electricity can go down for about
    1 minute before the generator can be fully working.

    We a lot of new servers now which can draw up to 12KW, tri-phase (but it
    could probably be brought to single-phase 220V if needed).

    We would like to buy an UPS but we have space and money constraints. All
    the UPSs I see at APC are both expensive and also have a low maximum
    load, while they have an unnededly long running time for us.
    We would like an UPS which can provide 12KW for just 1 minute, possibly
    small and cheap.
    Where should we look?

    Thank you
     
  2. James Sweet

    James Sweet Guest


    There's no getting around the need for a large, expensive UPS. Where you can
    skimp a little is battery capacity, but batteries are cheap compared to the
    cost of the rest of the unit. Extending run time is inexpensive and easy,
    increasing peak capacity is relatively expensive and difficult.
     
  3. Guest

    The big cost of a 12KW UPS is going to be in the transformers and switching
    circuitry. There is just no way it's going to be "cheap." To have enough
    battery capacity to keep up a 12KW load you'll be forced to have more running
    time than you think you'll need. That's a physical limitation of how
    fast you can draw current from a battery. No way around it.

    Bill Ranck
    Blacksburg, Va.
     
  4. Guest

    | Hi there,
    | we have a server room in a place that is backed up by a diesel generator
    | but does not have an UPS, so that the electricity can go down for about
    | 1 minute before the generator can be fully working.
    |
    | We a lot of new servers now which can draw up to 12KW, tri-phase (but it
    | could probably be brought to single-phase 220V if needed).
    |
    | We would like to buy an UPS but we have space and money constraints. All
    | the UPSs I see at APC are both expensive and also have a low maximum
    | load, while they have an unnededly long running time for us.
    | We would like an UPS which can provide 12KW for just 1 minute, possibly
    | small and cheap.

    It won't be all that much cheaper. The electronics still has to be the
    same because it is load based. You need less battery capacity, but the
    market tends to want more capacity, not less, especially in these mid
    sized units. Additionally, a smaller battery would not tend to have the
    current delivery capability to do more power in less time.
     
  5. kunt

    kunt Guest

    Why, what could be the problem?

    Thanks everybody
     
  6. Guest

    |
    | kunt wrote:
    |>
    |> newsey wrote:
    |> > You need to be careful when selecting a UPS to work with a generator.
    |> > Be sure to mention this to the UPS sales guy.
    |>
    |> Why, what could be the problem?
    |
    |
    | If the UPS is expecting a stable 60 Hz (or 50 Hz) and the generator
    | is off by a fraction of a cycle, it is considered unstable, and the UPS
    | doesn't switch over.

    The "double conversion continuous online" UPS types generally do not have
    this problem. These are the ones that convert the incoming AC power to DC,
    parallel it with the battery (under control of a circuit that manages how
    the battery is charged, etc), and invert the DC back to AC (usually with a
    sine wave inversion, although a cheap square wave version might exist).
    It's possible to make such a UPS picky about the input frequency. But ones
    I have looked at (and selected one from) generally support frequencies in
    the range 47-63 Hz. Some will make the output AC match the input frequency
    either exactly, or will select 50 Hz or 60 Hz depending on which is closest
    to the input (by default, though with many you can select the frequency for
    the output in the control panel). The more expensive models (usually of a
    lot higher capacity) tend to have the smarter features.
     
  7. Palindrome

    Palindrome Guest

    I haven't read the entire thread, so someone may have alrady covered this:

    I have worked on several sites where this requirement was met with a
    specially adapted diesel generator set incorporating a very large mass
    flywheel kept spinning by mains power under normal conditions.

    When the mains failed, the slowing mass of the flywheel generated the
    power until the diesel(s) had reached operating speed.

    I have seen some where the flywheel itself was used to get the diesel
    started. Others where compressed air was used.

    It may be my bad luck, but every single one of these sites had problems.
    Usually put down to the stored energy in the flywheel being inadequate.

    The most "exciting" solution I came across was an on-site one. Baffles
    were welded in the U cross-section flywheel and it was filled with
    water, when up to speed. This attempt at increasing the mass of the
    flywheel without replacing it was very ingenious - unfortunately the
    many gallons of water decided to add their energy to the switchboard and
    motor, rather than the baffles, when the mains supply was removed.

    A slight variation was a battery bank linked to an auxiliary motor on
    the drive train. When the electric supply failed, the main ac motor was
    automatically declutched and the auxiliary motor powered-up and clutched
    in. The only problem was the choice of clutches. Each consisted of a
    chamber containing an input and output "fanblade" and filled with oil
    when "clutched in". The main motor chamber was above the auxiliary one
    and the idea was very simple - a big plate was held in place between the
    two chambers (top full, bottom empty) by the incoming supply. On supply
    failure, the plate fell away and the oil dropped into the lower chamber.
    Thus making sure that it wasn't possible for both clutches to be engaged
    simultaneously and prducing something that was extremely simple.

    You can probably guess what happened..
     
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