Computer and Non-Linear Load Applications of Uninterruptible Power Supplies

In the application of U.P.S. to computer loads, the transient performance criteria of some computers has led to the assumption that a fast acting inverter is the ultimate aim of the designer. Fast-acting inverters use either pulse-width modulation or waveform synthesis to eliminate low order harmonics and are typified by a relatively cheap small output filter. In contrast other inverters have large filters and may also have trap circuits to eliminate specific harmonics. It is not always appreciated that the use of a small filter aggregates the problem of harmonic distortion and overloading when the inverter has a non-linear load. Since most computer and industrial loads contain a.c. to d.c. power supplies which are non-linear in their loading characteristics, the effect is prevalent and can be observed as waveform distortion and current pulsing of the inverter output.