There are two basic types of static UPS on the market in today. They are: line interactive and true on-line (double conversion). A static UPS system usually consists of a battery to provide continuous source of electrical power; a rectifier/charger to maintain battery charge and to provide input to inverter when utility power is available; an inverter to provide power to load during normal operation; a static switch to transfer load automatically and without disturbance between inverter and utility power; a manual switch to bypass the static switch for maintenance; input and output isolation transformers and filters to provide appropriate isolation and disturbance attenuation; and monitors, sensors, and control circuits.

Battery power is a group of electro-chemical cells interconnected to supply a nominal voltage of direct current (DC) power to a suitable connected electrical load. The number of cells connected in series determines the nominal voltage rating of the battery is the basic factor that determines the discharge capacity rating of the entire battery. The combination of the charger and the battery becomes a system when it is connected to an electrical load. The load is the equipment that draws DC power from the charger and/or battery. The charger-battery load combination is most efficient only when all of the components are properly matched to each other. The charger converts alternating current (AC) power into DC power that is compatible with the battery’s voltage and current characteristics.The charger is the converter section of the UPS which charges the batteries and supplies DC to the inverter. 



When the charger and battery are connected permanently to each other and to the load, and the charger regulates the voltage supplied to the load and the battery, the system is known as a floating battery system. The battery in such systems is mounted normally on a rack or racks housed inside a building or enclosure. The load in this case is the inverter. The inverter provides three primary functions in the UPS. Inversion is the changing of the DC power to AC power composed of a sine wave free from harmful harmonic distortion [typically 5% total harmonic distortion (THD) or less]. The inverter also provides regulation of the AC voltage to a tolerance level acceptable to the load, typically +2 percent of the nominal voltage. 



PWM inverters synthesize a sinusoidal output waveform from a constant height, variable width high frequency pulse stream. The PWM inverter configuration is referred to as an active regulator. Voltage and current feedback loops gather downstream information and deliver it to a microprocessor based control circuit through a series of amplifiers. The bridge circuit in modern PWM technology normally uses insulated gate bipolar transistors (IGBT) for power switching, although, silicon controlled rectifiers (SCR) could be used in high capacity, lower frequency applications.The control circuit adjusts the transistor conduction time based on the information received from the voltage and current feedback loops to maintain output regulation.
The driver network is a parallel power or darlington transistor array. The positive and negative pulse train is then coupled from primary to secondary of a linear output transformer. A small (line-commutated) LC filter with harmonic traps removes the odd order harmonics from the pulse train to create a sine wave. PWM inverters offer many advantages when used in computer room applications, including high efficiency due to the use of IGBTs and a linear output transformer, small physical size, low cost, and rapid response to transient load changes.

- US Army Corp of Engineers
- Labrijn Automotive and Industrial Products
- Powermatic Co. Ltd.

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