Newsgroups: comp.periphs
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!quimby
From: quimby@madoka.its.rpi.edu (Tom Stewart)
Subject: Re: UPSes
Message-ID: <XV-=}##@rpi.edu>
Keywords: UPS
Nntp-Posting-Host: madoka.its.rpi.edu
References: <1991Mar19.200553.13176@athena.cs.uga.edu>
Date: 20 Mar 91 07:04:12 GMT
Lines: 49


Here's the wonderfull electrical stuff:
  
UPS's are, or should be, rated in VA.  A VA is a VoltAmpere, and is
simply the AC RMS voltage multiplied by the AC RMS current.  You 
probably have been led to believe that a Watt is a VoltAmpere, and
this is mostly, but not quite, true.  A Watt is only a VA into a 
resistive load -- this is because with a complex load the voltage 
and current aren't in phase, so multiplying the scaler quantities
doesn't represent the actual power at all.  The electrical term for
this difference is called _power factor_, and in a purely resistive
load the power factor is 1, and W=VA.  For a complex load, the power
factor is always less than 1, and W=VA(power_factor).  The purpose of
power factor is to reduce the need for electricians to do vector math.
  
Here's what it means to you:
  
A UPS is limited by VA, not Watts, so if a UPS manufactor gives Watts you
should assume worst case, which is that they really mean VA.  Computer
equipment is either rated in VA, A, or Watts.  Use the VA rating if
possible.  If they give A multiply by 120 to get VA.  If they give Watts
divide by .7 or .8 to get an estimated VA.  (This is ballpark for a
typical switching power supply.  Note that the VA value is higher than
the Watts value.)  Add up all of the VA's, multiply by your favorite
safety margin, and pick yourself a UPS.  

Unless you've got either a very screwy power system, or a screwy
computer, you really don't need full time power conditioning.  The 
protection you want is from brownouts, blackouts/dropouts, and spikes,
which any good switching UPS will do.  If your power is bad enough
that you need something else -- say you're running from a diesel 
generator and power is very random -- you want a true on-line UPS.
Following the "fewer parts is better" rule suggests a switch mode
UPS with surge suppression for every "normal" installation.

My vote for a UPS would be an American Power, maybe a 450 or 520.  They're
small, light, and very well built, have excellent surge supression, and
have the fastest switch times I've seen.  They're also reasonably 
inexpensive.
  
Hope this helps
  
Quimby
  
(mailer disfunctional, replies to: quimby@mts.rpi.edu, quimby@rpitsmts.bitnet)
disclaimer:  I have no connection with American Power, any UPS/SPS
             manufactor, or Hallmark greeting cards.
  

