🚀 go-pugleaf

Somebody around here had some bucks and bought a 50 KVA UPS system
that runs our 3 Vaxes, Phone System, and Fire Alarm.  With
batteries, it is about the size of three refrigerators!  I think it
cost in the neighborhood of 40-60K dollars (ouch!!).

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
(well, really three phase alternator) output and not switched.  The
disadvantage of the mot-gen type of UPS is that eventually the
bearings will have to go out, then somebody will have to descend
into the inner sanctum of steam pipes in the basement where the UPS
is and  patch around it.  The mean time before failure (MTBF) on
mot-gen units is usually quoted to be around five years.  This
seems reasonable, since the mechanical part is a sealed unit.

An alternative UPS system is a static sine-wave unit.  In a static
unit, the incomming AC is fed through diodes to the batteries.  An
inverter circuit then powers the load continuously from the
batteries, even when normal line voltage is present.  Static UPSs
work well, but don't provide 100% isolation from incomming
transients.  The switching transistors or SCRs in the inverter
circuit are a sensitive area that can be damaged by transients.
MTBF ratings are quoted at 1 to 5 years.  Power delivered to the
load is usually pretty good since there is no switch-over when the
mains power is lost.

The least desirable type of UPS usues a transfer relay or SCR bank
to cut in the inverter only when mains power is lost.  The
advantage of such a unit is that the inverter is only operated when
required, thus potentially lengthening its life.  The disadvantage
is that nasty switching glitches can result.  Also, the inverter in
this type of UPS might be designed less robustly, since it is not
expected to be in use continuously.  MTBF ratings vary widely, but
usually fall into the 1 to 5 year category.

Maintenance interval can vary too.  With transfer type and mot-gen
UPSs, the batteries can be serviced without taking the UPS offline.
Static UPSs must be shut down to service the batteries (at least,
as a general rule).  Types of batteries vary.  Wet cell car-battery
type units usually require attention every 6 months, and
replacement of the batteries every 5 to 10 years.  Gel electrolyte
batteries are available too.  Usually gel cells require only
periodic inspection.  Some manufacturers quite optimisticly claim
that their gel batteries last up to 20 years.  I'm not so sure I
believe that claim.

The rule of thumb is that you get what you pay for in a UPS.  I've
been using a little 200 watt Questa UPS on my XT compatible at home
for about the last 6 months and it kept going through several black
outs.  Occasionally, the noise pulse from turning on a lamp will
make the UPS kick in for a few seconds, even when the AC is good,
but that seems to be no big deal.  I bought my UPS used, and was
surprised to find out the list price was $400 when I looked it up
in a catalog.  Seems like a lot of $$$ for a seemingly simple
device.  One nice feature of the Questa UPS is that it has
terminals in the back so that a large storage battery can be
attached for extended back up power.  The little interal battery is
only good for about 5 to 15 min. depending on the load applied.


  --Bill
(wtm@neoucom.UUCP)