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From: ssm@munsell.UUCP (Steve McLafferty)
Newsgroups: sci.electronics
Subject: Re: UPS's that aren't
Message-ID: <1182@weber.munsell.UUCP>
Date: Thu, 13-Aug-87 13:05:38 EDT
Article-I.D.: weber.1182
Posted: Thu Aug 13 13:05:38 1987
Date-Received: Sat, 15-Aug-87 11:37:20 EDT
References: <161@tmsoft.UUCP> <1924@kitty.UUCP> <654@neoucom.UUCP> <1932@kitty.UUCP>
Reply-To: ssm@weber.UUCP (Steve McLafferty)
Distribution: na
Organization: Eikonix Corp., Bedford, MA
Lines: 51

In article <1932@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>In article <654@neoucom.UUCP>, wtm@neoucom.UUCP (Bill Mayhew) writes:
>> In our case, the administrator opted for a motor/generator type
>> UPS.  The motor/generator takes advantage of the intertia of the
>> mot-gen armature to keep power going while the switch over from
>> mains power to batteries takes place.  There is also never any
>> glitch on the load since it is always connected to the generator
>
>	I once saw an interesting variety of a UPS that was installed
>in a remote pipeline pumping station to maintain control power and
>emergency lighting.
>	This UPS consisted of an AC motor operated from the line, an
>AC generator which powered the load, a large enclosed flywheel, all
>of which was connected to a 4-cylinder propane-fueled engine.  The
>engine turned over continuously with the AC motor and generator, but
>was not running since the ignition and fuel were turned off.
>	When power failed, the ignition and fuel were turned on, and
>the engine would start and be up to load within a cycle time since
>it was already running at governor speed.  The flywheel provided
>sufficient inertia that only negligible degredation of output power
>occured during the power loss transition.
>
The UPSs described in the above two articles seem to make a clean
transition from main power to backup power.  My question is:
How do you go back from backup power to main power once the main power
has been restored?  Presumably there is a relay at the main power
input to the UPS that opens when main power is lost, so that the power
from your UPS goes only to the desired load, not back into the main
power grid.  (Otherwise your UPS might be supplying backup power to
your neighbor's computer :) .)

Once the main power is restored, you will want to close that relay to
let the main power back in.  The problem is that your UPS frequency
may have drifted slightly off from the 60Hz frequency of the main power 
grid, so that when you close the relay the two power sources might be 180
degrees out of phase.  The resulting current surge could damage the relay 
and the generator in the UPS, and possibly the equipment the UPS was 
supposed to protect.

Did the UPSs described above make the switch back to main power
automatically, or was operator intervention involved?  (Such as the
system involving lights between the two sources of power to show the
phase difference, where the relay is closed when the lights are off.)


Steven McLafferty                      "If I lose my mind,
Eikonix Corp                            I hope some one
Bedford, Mass                           will turn it in"
(617) 663-2115 x468                        -George Carlin

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