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From: berman@me.ri.cmu.edu (Steven Berman)
Newsgroups: sci.electronics
Subject: UPS's good and bad
Message-ID: <1003@me.ri.cmu.edu>
Date: Tue, 11-Aug-87 09:11:09 EDT
Article-I.D.: me.1003
Posted: Tue Aug 11 09:11:09 1987
Date-Received: Thu, 13-Aug-87 06:20:40 EDT
Reply-To: berman@me.ri.cmu.edu (Steven Berman)
Organization: Carnegie-Mellon University, CS/RI
Lines: 57
Keywords: UPS, on/off line.


Regarding a recent query about quality of UPS (uninterruptible power supply)
output:
	I have recently spent some time researching UPS technology of today.
There is a wide range of equipment available, good, bad, and ugly! Beware
that there are major differences in implementation. What follows is a short
summary of several technologies.

OFF-LINE UPS:
	This machine has a feed-through from main power to output
	with some filter caps across it.  When power fails, a switch-over
	circuit connects the battery to an inverter circuit, which supplies
	the output.  An active battery charge circuit keeps the cells
	charged when "off-line".  This is the least expensive UPS and
	by far the worst for computer-related applications. It's plusses
	are 100% off-line efficiency, quiet operation.  Minuses: dirty
	switchover.  Up to 30 msec switch time.  Really intended for
	brute-force loads, that can handle the spikes, and drop-out.

ON-LINE UPS: 
	This is a good middle-of-the-road machine.  It uses a battery
	charge circuit to both keep up the cells AND supply an inverter.
	The inverter is the only source to the output.  This means that
	switch-overs have EXACT wave-form synchronization.  Pluses:
	Near zero switch-over time.  Modest cost, small.  Minuses:
	VERY noisy, inverter tends to squeal at 5-30 KHz. (very annoying)
	Can be destroyed by a dirty power feed-in.  (Real Life example:
	Gasoline Generators feeding UPS: Blew out all the inverter
	transistors after only 100-hours of up-time.) Sola makes these.

FERRO-RESONANT UPS:
	The cream of the crop! Theese machines use a HUGE transformer
	fed from the main to supply the output.  When power fails, the
	batteries kick-in and the enormous L of the transformer maintains
	the synchronization of the sine-wave.  These machines have small
	switch-over times (~5 msec), but the big plus is that perfect
	sine-wave.  They are quiet, since they use the transformer
	when on-line, and are more efficient than the ON-LINE models.
	BEST Power Tech. makes these, and I bought three recently.

Hope this helps,
	I'll gladly provide more details through mail.
	I have a file that compares ~10 companies with cost/KVA.
					~Steve
-- 
------------------------------------------------------------------------------
Steven Berman
Mechanical Engineering-Robotics
The Robotics Institute
also Computer Science Department
Carnegie-Mellon University

ARPA: berman@me.ri.cmu.edu
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SNAIL: 5668 Darlington Rd. Pittsburgh, PA  15217

This does not represent the opinion or policy of my employer...yet.