What is an Off-line UPS?
Offline UPS is also called as Standby UPS system as the inverter is in standby mode until the mains power supply fails.They are not expensive and recommended for home offices. Here, the primary source is the filtered AC mains (shown in solid path in figure 1). When the power breakage occurs, the transfer switch will select the backup source (shown in dashed path in figure 1). Thus we can clearly see that the stand by system will start working only when there is any failure in mains. In this system, the AC voltage is first rectified and stored in the storage battery connected to the rectifier.
When power breakage occurs, this DC voltage is converted to AC voltage by means of inverter and given to the load connected to it. This is the least expensive UPS system and it provides surge protection in addition to back up. The transfer time can be about 25 milliseconds which can be related to the time taken by the UPS system to detect the utility voltage that is lost. The block diagram is shown below.
An offline UPS provides battery power to equipment when the mains power supply falls below a pre-determined limit. Battery will usually last ten minutes.
Offline UPS units are often referred to as standby systems, as the inverter is in standby mode until the mains power supply fails. They are inexpensive and recommended for home offices. Offline technology should be avoided for applications where there is frequent power disturbance.