Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!husc6!cmcl2!rutgers!clyde!cbosgd!cwruecmp!neoucom!wtm From: wtm@neoucom.UUCP (Bill Mayhew) Newsgroups: sci.electronics Subject: Re: UPS's that aren't Message-ID: <654@neoucom.UUCP> Date: Tue, 11-Aug-87 10:55:28 EDT Article-I.D.: neoucom.654 Posted: Tue Aug 11 10:55:28 1987 Date-Received: Fri, 14-Aug-87 04:45:10 EDT References: <161@tmsoft.UUCP> <1924@kitty.UUCP> Organization: Northeastern Ohio Universities College of Medicine Lines: 66 Summary: Motor Generator UPS systems 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)