All digital controlled three-phase UPS without inverter-transformer

A fully digital-controlled three-phase UPS (Uninterruptible Power System) is developed which does not have an inverter-transformer. To remove the transformer, a different type of main circuit from the conventional UPS is adopted. One phase of the input AC is regarded as the common voltage of the main circuit. A DC voltage stabilizer solves problems caused when this transformer-less circuit is applied to the UPS. a digital controller with two RISC (Reduced Instruction Set Computer) processing micro-computers to operate the circuit is also proposed. Experiments show the THD (Total Harmonic Distortion) of the input current is less than 5%, and that of the output voltage is less than 3.6% under non-linear load.