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How UPS work

Knowledge Base

How does a UPS work?

1. The functions of UPS
An UPS can perform:

  • Power down protection
  • Under-voltage and over-voltage protection
  • Waveform distortion correction
  • Frequency stabilization
  • Voltage regulation
  • Normal mode noise rejection
  • Common mode noise rejection
  • Surge protection
  • Transient response protection
  • Power supply monitoring

2. What are the main categories of UPS?

There are basically three different types of devices by topology, which are offline, line interactive and on line ones.

Offline UPS

Power is usually derived directly from the power line, until power fails. After power failure, a battery powered inverter turns on to continue supplying power. Batteries are charged, as necessary, when line power is available. This type of supply is sometimes called an "backup" UPS.
The quality and effectiveness of this class of devices varies considerably; however, they are generally quite a bit cheaper than online UPS. The time required for the inverter to come on line, typically called the transfer time, varies by unit.
Since appliances connected to the supply are basically connected directly from the power line, offline UPS provide relatively poor protection from line noise, frequency fluctuation, line spikes, and brownouts.

Line-interactive UPS
The line-interactive UPS uses a totally different design than any type of offline UPS. In this type of unit, the separate battery charger, inverter and source selection switch have all been replaced by a combination inverter/converter, which both charges the battery and converts its energy to AC for the output as required. AC line power is still the primary power source, and the battery is the secondary. When the line power is operating, the inverter/converter charges the battery; when the power fails, it operates in reverse.
The main advantage of this design is that the inverter/converter unit is always connected to the output, powering the equipment. This design allows for faster response to a power failure than a offline UPS. The inverter/converter is also normally fitted with circuitry to filter out noise and spikes, and to regulate the power output, providing additional power during brownouts and curtailing output during surges.

Online UPS
What we call as "true" UPS systems, those power supplies that continuously operate from an inverter. Obviously, there is no transfer time, and these supplies generally provide the best isolation from power line problems. The disadvantages to these devices are increased cost, increased power consumption, and increased heat generation.