Adjustable speed drives with square-wave input current: a cost effective step in development to improve their performance

As the excellent performance on the motor side is now a generally available feature of adjustable speed drives (ASDs), issues like improving the interaction with the grid or the robustness of the drive against unbalanced voltage supply becomes an important research field. PWM rectifiers with sinusoidal input current, which fully solve these issues, are commercially available but they are expensive and therefore not widely used. Active front-end topologies with non-sinusoidal input current that offer a better tradeoff between grid side performance and cost are an interesting alternative. This paper investigates the impact of having an ASD with a square-wave input current on both the grid and motor side performance. An interesting topology results if the traditional DC-link choke in a diode-frontend stage ASD is replaced with a low installed power current source (electronic inductor). The design aspects, control method and its protection are presented. It is proven by simulations that it provides not only square-wave input current over a wide range of power grid conditions, but additionally stabilizes the DC-link voltage which enables operation of the motor at rated power.