A block diagram of UPS system is shown in figure. It essentially consists of four major components: i)Rectifier (or battery charger) ii) Battery bank iii) Inverter vi) Transfer switch
i) Rectifier (or battery charger): Its function is to convert available AC supply at input line into DC supply and then to feed DC power to inverter and also to battery bank to keep it charged. It is designed to handle the total current supplied to inverter and battery bank.
ii)Battery bank: It consists of number of batteries in series. The rating and number of units in the bank depends upon the following factors: Input voltage required by inverter, Back-up time requirement of UPS, Efficiency of inverter and load power. When the line voltage is present, the battery trickle charged to compensate for the slight self-discharge. The battery continuously draws a small amount of current to maintain itself in a fully charged state. When AC input fails, battery supplies DC power to inveter, wherein it is converted into AC and then fed to load. During this period, the battery discharges. On recovery of AC mains supply, the battery charging starts.
iii) Inverter: It is used to convert DC supply available at its input terminals into AC supply. The filter is normally used at the output of inverter to minimize the harmonic distortion. Most of the loads are highly non-linear and inject large harmonic currents into the UPS.
iv) Transfer switch: It is a change over switch. When AC supply is available, the transfer switch connects load to AC supply directly. However, when AC supply from line is not available and load demands for AC supply, the transfer switch can be placed to connect inverter output AC supply to the load. Due to fast action requirement, transfer switch can be implemented by fast acting semiconductor devices.