Uninterruptible power supplies

Uninterruptible power supplies

An off-line uninterruptible power supply (UPS) backs up the supply to electrical loads from energy stored in batteries beforehand. The external battery is rated at 240 V (20 × 12 V) or 360 V (30 × 12 V). The charger delivers (10…30) A. A UPS is needed where the mains supply voltage falls outside its permitted limits. It keeps the load powered without interruption and protects computers and other industrial and domestic equipment rated at 220/380 V AC, 50 Hz, up to 50 kVA for three-phase loads and up to 17 kVA per phase for single-phase loads. An off-line UPS with a fast static thyristor switch, a built-in charger and an external battery gives:

• a short transfer time;
• minimal conversion losses;
• low heat output;
• silent operation;
• the ability to run for long periods — set by the capacity of the external battery — without starting an external diesel generator.

The UPS has an output to start an external generator set automatically; a version with an automatic contactor bypass circuit is available. When running from the batteries it supplies the load with a three-phase 50 Hz sinusoidal AC voltage. The UPS monitors the state of the battery and the mains and shows the input and output states.