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In article <1003@me.ri.cmu.edu>, berman@me.ri.cmu.edu (Steven Berman) writes:
> 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.

	A well-designed UPS has a "reverse transfer" feature using a
solid-state switch that switches output power load back to the AC line
in the event of inverter failure and/or output load overcurrent.

> 	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)

	Admittedly they can be noisy, but with 60 Hz/120 Hz hum - which
is much more tolerable than a high-frequency inverter whine.
 
> 	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.)

	I don't see this being a problem on a well-designed UPS.  The
battery charger circuit is going to "float" the battery with a small
net positive current flow above demand.  As a result, the float charging
circuitry will be both current and voltage limited so that any
AC line input overvoltage will either be harmless or will trip an
overcurrent device (on the charger circuit only) before harm can occur.

	A sign of a well-designed UPS is one which has panel metering
and failure alarms.  Some of the best designed UPS systems which work
on the above principles are made by Elgar and Lorain Power Products,
and I am admittedly partial to both vendors.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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