How does a uninterruptible power supply work?

Author:BSLBATT    Publish Time: 2021-03-04

Uninterruptible power supplies (UPS) and battery back up systems are commonly used in applications where a mains power failure could be catastrophic, such as hospital operating theatres or intensive care units, computer equipment, production systems, alarm and signalling equipment. In the event of a failure of the mains power input, some large facilities, particularly hospitals and data centres, must have local energy sources to continue to power the loads. In the wake of the 2020 epidemic, more and more small and medium-sized organisations are also realising the reliability and necessity of UPS. But how does a UPS work? How does it power our system? We hope that this article has been helpful.


A UPS consists of three basic building blocks. These are:

  1. The battery charger or rectifier (a.c. to d.c. converter), to convert the incoming mains supply to d.c. in order to charge the battery. The d.c. output can also be used as the energy source of the inverter in normal operation.

  2.The battery to provide the energy required in the case of the failure of the mains supply.

  3.The inverter (d.c. to a.c. converter), to convert the d.c. supply of the rectifier or the battery to the required a.c. output.


The UPS can be online or offline. Both systems use a DC-link inverter with battery pack and charger. For off-line systems, the power is supplied directly from the AC supply during normal operation. In the unlikely event of a power failure, the transfer switch disconnects the power line and connects the inverter to the load. When the mains power is restored, the load is reconnected to the power line.

off-line system

For on-line systems, the rectifier-inverter combination provides the load power from the AC supply during normal operation. If the mains supply fails, the battery automatically supplies the DC link to the inverter with no time delay. In the event of a fault in the rectifier-inverter system, a transfer switch can be used to switch the load to the AC supply.

The purpose of a UPS is to provide the load with emergency power in the event of a fault being sensed in the input power supply. They differ from emergency power systems or standby generators in that they provide near-instantaneous protection against power interruptions through the use of batteries.

The battery itself usually has a short running time (around 5 to 20 minutes), but it is enough to save all the valuable data/processes you have completed, shut everything down properly or resolve the problem that caused the power failure.

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