Uninterruptible power supplies (UPS) are systems designed to ensure the continuity of critical loads in the event of a power outage. There are different types and working principles. Here are the basic working principles of uninterruptible power supplies:

1. Basic Components
Battery or Accumulator: Used to store energy. It feeds the load during a power outage.
Rectifier: Converts AC electrical current to DC. It is also used to charge batteries.
Inverter: Converts DC back to AC and feeds the load.
Control System: Monitors and manages the operating status of the UPS, ensuring that the system is automatically activated when necessary.
2. Working Principles
a. Online UPS
Continuous Operation: AC power from the electrical network first passes through the rectifier and is converted to DC. This DC energy both charges the batteries and feeds the load directly through the inverter.
Uninterruptible Power: When the power from the mains is cut off, the system automatically continues to be supplied by the batteries. This provides uninterrupted power to the load.
b. Offline (Standby) UPS
Standby Mode: Under normal conditions, the load is directly supplied by the mains. When the power from the mains is stable, the rectifier is not in operation.
Emergency Activation: When the power from the mains is cut off, the control system switches on the batteries. This occurs within a few milliseconds and the load is supplied by the batteries via the inverter.
c. Line-Interactive UPS
Voltage Regulation: This type of UPS includes both a rectifier and an inverter. Normally, the load is supplied by the mains, while the system automatically switches on in case of over or under voltage.
Faster Response Time: A line-interactive UPS protects the load by regulating short-term power fluctuations.
3. Charge and Discharge Cycles
Charge: The UPS charges the batteries when the mains power is restored. It usually provides DC current via a rectifier.
Discharge: During a power outage, the inverter converts DC energy from the batteries to AC to power the load.

4. Automatic Switchover Time
Switchover Time: Offline and line-interactive UPS systems usually come into operation within a few milliseconds. This time can be important for critical applications.

5. Efficiency and Management
Power Management: Modern UPS systems have advanced control systems to increase energy efficiency. Integration with smart grids is possible.
Uninterruptible power supplies are vital for ensuring energy continuity in data processing centers, hospitals, telecommunications and other critical systems.