A low cost approach to improve the performance of an adjustable speed drive (ASD) under voltage sags and short-term power interruptions

Voltage sags are a common occurrence in industrial power distribution systems. Although a typical sag may last only 5-20 cycles, a voltage magnitude lower than 20% of its rated value can trip an adjustable speed AC drive (ASD). Such a nuisance tripping of a continuous industrial process can be very costly. In this paper, a low cost approach to improve the performance of an ASD under voltage sag and short term interruptions is presented. The approach consists of a low cost modification (addition of three diodes and an inductor) to the front end diode rectifier topology. This modification along with the dynamic-braking IGBT (Q_db) control (a standard component in a ASD) is shown to provide ride-through capability for voltage sags. Further, it is shown that with the addition of a diode and battery, the ride-through capability can also be extended to short-term power interruptions. The IGBT Q_db is suitably controlled in the event of a sag to maintain rated DC-link voltage in a closed loop, thus avoiding any nuisance tripping or momentary speed fluctuations. A 460 V, 10 hp commercially available ASD is modified with the proposed approach. Analysis, design and simulation results are discussed. Experimental results illustrating the performance of the ASD with the proposed ride-through topology for a wide range of voltage sag conditions are presented