Dual-Inverter Topology for Single-Phase Supplied Drive Systems without Electrolytic Capacitor

Single-phase supplied variable speed drive (VSD) systems are widely used in industrial applications and typically feature a two-stage design with a power factor corrected (PFC) boost rectifier and a three-phase voltage source inverter (VSI). However, the electrolytic DC-link capacitor, which is needed to cope with the twice grid frequency power pulsation, and the required boost inductor are unfavourable in terms of reliability, volume, cost, and complexity. Therefore, the proposed concept employs a dual-inverter topology with a three-phase open-end winding (OEW) machine, avoiding high-frequency inductors, and controls the system such that the power pulsation is buffered in the inertia of the drive train. Accordingly, the DC-link capacitance can be reduced drastically, enabling an electrolytic capacitor-less system, featuring a higher power density and an increased lifetime. This paper presents the operating principle and the corresponding closed-loop control structure, to achieve PFC operation, DC-link voltage balancing and average speed control. Detailed analysis reveals that the machine voltage can be selected independently of the grid peak voltage in contrast to existing concepts. The converter performance is evaluated based on simple performance indices with respect to the machine voltage. In the context of a 7.5kW compressor application for railway brakes with a wide input voltage range, a semiconductor loss reduction of 30% can be obtained compared to a state-of-the-art approach, further reducing the converter volume. Finally, the proper operation is verified with a closed-loop circuit simulation.