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: 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.