What does an uninterruptible power supply  mean?

A UPS, short for uninterruptible power supply, is an energy storage device designed to provide stable, uninterrupted power to critical equipment. It's primarily used to prevent equipment downtime or data loss caused by utility power outages or fluctuations. Its core functions include charging stored energy when the utility power is available and seamlessly switching to battery power during a power outage, ensuring continuous operation of loads such as computers and servers.

How is an UPS constructed?

The most common type is a battery-based UPS. Therefore, most UPSs have a battery that is charged from the utility power. UPS batteries typically have a minimum power rating of 300 VA (volt-amperes), but can range up to several hundred VA. In this case, the power rating depends primarily on the load capacity of the power converter. The battery capacity determines the maximum bridging time, which can range from seconds to hours depending on needs and objectives. If power and bridging time are extremely demanding, a generator set is used to charge the battery.

Charging occurs via a so-called rectifier on the system's input side. This converts AC power from the grid into DC power. If the regular power source fails, the battery takes over the energy supply. An inverter on the output side then converts the DC power from the battery back into AC power.

Key Features:

Uninterruptibility: A built-in inverter converts DC power (from the battery) to AC power, typically in less than 10 milliseconds, ensuring equipment does not shut down.

Power Purification: An online UPS also filters out mains power fluctuations (such as overvoltage and undervoltage), providing a clean and stable output.

Limited Backup Time: Typically 5-7 minutes (at full load), primarily for emergency data preservation and shutdown, but can be extended with an external battery pack.

Different Classes of Uninterruptible Power Supplies

What are the different classes of uninterruptible power supplies?

Uninterruptible power supplies are based on different technologies and are therefore classified into different categories. The appropriate class for the application depends on the specific protection requirements. Uninterruptible power supplies offer the following levels of protection:

Category 1: Online Uninterruptible Power Supply

Category 2: Line-Interactive or Mains-Interactive Uninterruptible Power Supply

Category 3: Offline Uninterruptible Power Supply

Online or double-conversion systems (Category 1) are particularly suitable for backing up business-critical applications. Although more expensive than the other two technologies, they require no switchover time during a power outage. Therefore, online uninterruptible power supplies continuously supply AC voltage to connected equipment, compensating for power failures, undervoltage and overvoltage conditions, frequency fluctuations, or noise. Double conversion is used here.

The mains voltage is continuously converted to DC voltage, which then flows through an inverter via a battery. In this way, AC voltage is continuously generated and supplied to user equipment. Therefore, current flows continuously through the uninterruptible power supply. The only drawback is that due to the continuous operation, these devices have a relatively short lifespan and low efficiency, at most 90%.

Low-cost offline or backup systems (Category 3) are primarily suitable for personal computers. This is because they only protect against complete power outages and brief voltage fluctuations. If the voltage drops below a certain value, the system switches to battery operation. This step can result in a delay of up to ten milliseconds, which connected devices must handle without losing any functionality. Because these devices don't operate continuously but only charge their batteries during normal operation, they offer a long lifespan and high efficiency of up to 95%.

Common Application Scenarios:

Critical Equipment Protection: Ensures uninterrupted operation during power outages for data center servers, medical equipment, or financial systems.

Home and Office: Provides short-term backup for desktop computers and routers to prevent work loss.

Compared to outdoor power supplies, UPSs focus on uninterrupted power supply in fixed environments, rather than portable mobile applications.