What is Offline UPS? Working, Block Diagram & Applications

The offline UPS is also called line preferred UPS.

Block Diagram of Offline UPS

Fig. 1 shows the block diagram of an offline UPS. When mains supply is present, the charger charges the battery. The inverter is off and UPS static switch is off. The load is connected to mains through mains static switch.

Fig. 1: Block diagram of offline UPS.

Working of Offline UPS

The power flow is shown in Fig. 2 (a). When mains supply is not available, then inverter is turned on. An inverter takes power from the battery. The load is connected to inverter output through UPS static switch. The power flow diagram is shown in Fig. 2 (b). The mains static switch is always on and keeps load connected to mains. The mains static switch is turned off when mains is not available. The charger feeds power only to the battery. Hence its power handling capacity is reduced.

(a) When mains is present.

(b) When mains is absent.

Fig. 2: Power flow in offline UPS

Applications of Offline UPS

1. Computers printers, scanners etc.
2. Emergency Power Supplies, EPABX

Specifications of offline UPS

1. Battery float voltage
2. Battery voltage at start of discharge
3. Battery voltage at end of discharge
4. Capacity derating factor
5. Battery voltage during charging
6. Battery charging efficiency
7. VA rating of inverter
8. Wattage of rectifier.
9. AH capacity
10. Backup time.