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From: don@zl2tnm.gp.govt.nz (Don Stokes)
Newsgroups: sci.electronics
Subject: UPS
Message-ID: <7TuPm1w162w@zl2tnm.gp.govt.nz>
Date: 22 Jul 90 05:01:33 GMT
Organization: Me?  Organised?
Lines: 79

>Subject: UPS (Uninterruptable Power Supplies)... kinda long.
>From: shenkin@cunixf.cc.columbia.edu (Peter S. Shenkin)
>
>I'm trying to sort through the maze of vendors' claims in order to pick
>a UPS to protect a Silicon Graphics Personal Iris (PI).  I am hoping that
>someone out there more knowledgable than I can help me out.  I will summarize
>to the above newsgroups.
>
>Now, I gather there are "on-line" units, and "standby" units;  standby units
>are cheaper, and also seem to come in lower power ratings.  Elgar sells
>a 1.5kVA standby unit for $1875, and then jumps to a 3kVA on-line unit
>for $6145.  A standby unit kicks in only when the power goes down, which
>has two implications:  (1) there is a time-lag during which you don't have
>power.  Elgar quotes 4ms for their 1.5kVA unit.  (2) you get no (or little)
>surge protection and power conditioning from the unit.  The on-line units
>are, theoretically at least, supplying power all the time, and are said to
>give good power conditioning.
>
>So my FIRST QUESTION is:  for the intended use, is this a red herring?  I have
>been assuming I need an on-line unit, but is a 4ms delay likely to give
>problems?  (I note that using 60 Hz power, 4 ms is a bit less than 1/4 of
>a cycle.)  In fact, I probably want a unit with larger capacity anyway, but
>the answer to this question would still be good to know, and it comes up
>again, just below.

The question has to asked: what happens before the battery cuts in, ie
how are you going to ride the mess that could well precede power going
out altogether.  It's also going to depend a lot on the capabilities of
the PI's power supply, as it has to handle any spikes and other
transients.  IMHO, a standby UPS is going to give you the best
protection, as there is no "kicking in" cycles involved; the rectifier
and inverter are running all the time.  It's your inverter that's
providing 60Hz power, not the supply -- the supply can do anything it
likes; as long as it's there often enough to keep your battery charged
you have clean 60Hz power.

>So my SECOND QUESTION is whether the ballast route to making a unit "on-line"
>as "good" as the inverter-always-on route?  That is, perhaps instead of
>a two way classification (standby and on-line), we should have a three-way
>classification:

Maybe; depends a lot on the quality of the unit.  It still isn't a true
online UPS; what you've got in the "balasted standby" is a power
conditioner of sorts with a battery, and must be treated as such.

>My THIRD QUESTION is:  can you clarify my confusion about UL-544 and low
>leakage current?  (a) What does it mean, and (b) Do I need it?

Probably leakage across the rectifier or inverter, meaning the UPS
consumes a little power whether it is under load or not.  I don't know.

>QUESTION FOUR:  why do you care whether or not the UPS turns itself off, once
>it's safely brought the machine down?

Probably not much of an issue on a small system, but on a big one, once
you have pulled power out from under a system, you want to bring it back
up again in a controlled fashion.  The reason for this is that a startup
loads can be *much* higher than normal loads; some devices, eg large
disk drives, can pull something like ten times the normal running load.
Just loading power supply capacitors can pull surprisingly large
currents for a short time.  An uncontrolled startup load on the
VAXcluster at GPO stands a pretty good chance of tripping the power
conditioner out (we don't have a UPS; 100kVA UPSs cost *real* money).

>QUESTION FIVE:  Do you have personal experience with any of these power
>supplies, or with their competitors?  If so, I'd appreciate it if you'd share
>your experiences, likes, dislikes.

Not personally; I have done a lot of talking turkey to UPS and power
conditioner salesbeings, and find it *very* handy to have some idea
about what you are talking about before starting.  It helps the
salesbeing a lot if you know what you want too.  Remember to take
startup loads into account; usually power conditioning gear is rated for
normal currents and is capable of standing up to a heavy startup load,
but check.  Mistakes in this game tend to be expensive!

Don Stokes, ZL2TNM  /  /                            Home: don@zl2tnm.gp.govt.nz
Systems Programmer /GP/ Government Printing Office  Work:        don@gp.govt.nz
__________________/  /__Wellington, New Zealand_____or:_PSI%(5301)47000028::DON