AC POWER SUPPLY

UPS

Basic Operating Principle of UPS

1. Standby Off-Line UPS:
   The standby a.c. power supply for critical loads, where normal a.c. power supply is not available, is known as UPS. The critical loads are normally supplied from a.c. main supply and the rectifier maintains full charge of the battery. If the a.c. supply fails, load is switched to the output of the inverter, which then takes over the main supply. The solid state switch normally takes 4-5 msec for the switchover. The inverter runs only during the time supply failure. 
2. Standby On-Line UPS:
   The schematic arrangement is shown in Fig.1a.
   
   In the alternate configuration, shown in Fig.1b, the inverter operates continuously and is connected to the critical load. In case of inverter failure, the load is switched to the main supply. In this case, conditions of static switches are reversed, however. In case of power failure, the battery supplies the inverter.

Figure 1: UPS configuration

The stand-by battery is normally either nickel-cadmium or lead-acid type. However, the former is preferred due to long life and its ability to withstand overheating and discharging. The cost of former is quite high in comparison, however. An alternative arrangement of an UPS system is shown in Fig.2. It consists of a battery, inverter and static switch. In case of power failure, the battery supplies the inverter. When the main supply is on, the inverter operates as a rectifier and charges the battery.

In this arrangement the inverter has to operate at the fundamental output frequency. Consequently, the high frequency capability of the inverter is not utilized in reducing the size of the transformer. Similar to the d.c. power supplies, the a.c. power supplies can be categorized into- Switched-mode, Resonant and Bidirectional a.c. power supplies.

Figure 2: Arrangement of UPS system.

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