PWM uninterruptable power supplies

Uninterruptable power supply (UPS) systems have been developed for those electronic installations, especially computers and communication systems, which require a power supply unaffected by mains degradation or failure. This article describes a laboratory prototype UPS which has been designed to demonstrate the application of PWM techniques to UPS systems. The UPS is based on our Pulse Width Modulation (PWM) speed control system for a.c. motors (see Refs. 1 to 6), and is notable for its simple filter requirements and high efficiency. It has been designed in modular form using two modules: a PWM waveform generation module, type VM5000*, and a power supply and current sensing module, type VM5001*. The power ratings are 3 kVA (three-phase output) and 2 kVA (single-phase output), but the basic design can readily be extended to higher powers.