Which UPS systems are suitable for A-V?
Sep 1, 1997 12:00 PM, Jim Furman
The ultimate form of protection from AC line hazards is the uninterruptiblepower supply (UPS). Only a UPS can offer immunity from the severest hazardimaginable: complete line failure. The premise is to have your own sourceof power if a blackout strikes. The necessary energy can be stored in aflywheel, a tank of fuel or a battery, depending on the power required. InA-V applications, it is impractical to run an entire SR system or recordingoperation while the utility power is out, so the usual requirement issimply to continue the power to computers, synthesizers, hard disks andrelated equipment long enough to save current data and shut down in anorderly fashion.
A myriad of products are available to power such equipment. Leaving asidethe long-duration, high-current power sources, such as generators, withtheir noise, fumes, flammable fuels, switchover problems and high cost,we'll focus on battery backup. Sealed, maintenance-free batteries arecommonly used; they're clean enough for indoor use and usually replaceable.They form the heart of the three types of UPSs we will survey here.
The first type, the standby supply, does just that: It stands by, off line,until it is needed. When the power goes out, connected equipment isswitched over to the AC created by an inverter circuit connected to thebatteries. Standby UPSs are the lowest cost option, but their performanceis often flawed by several problems that make them inadequate for reliableA-V use. First, a non-zero switchover time exists. During this time, yourequipment will be running off nothing but its power supply capacitors. Ifthis time is less than about 8 ms, the stored capacitor energy may bebarely sufficient to bridge the gap, but do you want to take the chance?Second, the output waveform is often a square or stepped wave because suchwaveforms are cheaper to create than low-distortion sine waves. Butfast-rise-time waveforms contain many more high harmonics than sine waves,and such harmonics can intensify audible or visible noise and may beincompatible with sensitive loads. Finally, standby supplies provide novoltage regulation and therefore no help with brownout conditions becausethey are offline except during a power outage.
The second type, the line-interactive UPS, combines the backup capabilityof the standby supply with additional circuitry to provide voltageregulation. These UPSs may or may not produce an output waveform moreclosely approximating a sine wave. Regulation is accomplished either by useof a ferroresonant transformer or a tap-switching transformer. Because ofthe ferroresonant's greater bulk, large radiated field, mechanical noiseand frequency sensitivity, tap-switchers will provide better results innoise-critical applications, such as audio, as long as the switching isdone electronically, with thyristors rather than relays, at the voltagezero-crossings, to avoid creating switching glitches. However,line-interactive UPSs also suffer from a power dropout during theswitchover to battery operation, and many of the tap-switchers have onlyone or two taps, allowing only crude regulation.
Which UPS systems are suitable?The last type of UPS, the online UPS, provides the greatest degree ofprotection but at a higher cost. True online operation requires adual-conversion topology in which the batteries are continuously chargedwhile utility power is present, and the inverter circuit continuouslyproduces AC power to supply the load. When a blackout occurs, no switchoverdelay occurs because the load is already connected to battery-derivedpower. Only the recharging ceases. Dual conversion also make tight voltageregulation during normal operation possible because the output is alwayssynthesized from a stable battery source. Some online UPSs also include anautomatic bypass mechanism to switch to utility power in the event of anoverload or equipment failure.
When shopping for a UPS, check the specifications and features carefully.The most important differences between models within a given categoryinclude the following:
Power rating, expressed in volt-amperes (VA). Because most electronicequipment with commonly used capacitor-input power supplies has powerfactors of 0.6 to 0.7, the rating in watts will equate to 0.6 to 0.7 timesthe VA rating.
Run time at full load. This limit should be a minimum of five minutes, butconsider your own situation. In a larger facility, where you might be adistance away when an outage occurs and have to grope your way back to thecomputer in the dark, more time may be needed. Run times increase greatlywhen the load is partial, so you can buy a considerable amount of extratime by using a unit with a higher power rating than the actual load.
Basic power condition capability. To provide a complete protection package,UPSs should be equipped with MOVs, LC networks or other devices to absorbspikes and attenuate high-frequency noise. Attenuation is rated indecibels; energy-absorption capacity is rated in joules. More is better.
Waveform distortion, rated in percent THD. Less is better.
Switchover (transfer) time. Ranges from zero for online UPSs to as much as20 ms. The Computer and Business Equipment Manufacturers Associationrecommends this limit not exceed one half-cycle at 60 Hz, or 8.3 ms.
Communications capability can vary from none to sophisticated bundledpower-management software that allows graphical displays of UPS status andremotely controlled or scheduled equipment shutdowns.
Battery replacement may require full shutdown (generally the case withrack-mount UPSs), or batteries may not be hot-swappable if shutdowns cannever be tolerated.