Flexible Series/Parallel AC–DC–AC Motor Drive System

This paper presents a three-phase ac-dc-ac drive system to cope with applications in which high performance and reliability are required with reduced dc-link and switch blocking voltages. The subsystem at the grid-side can be implemented into two alternatives, i.e., topologies G1 and G2. While G1 presents a sole three-phase transformer connected in series between the three-phase grid and converter Ag, whose secondary is connected to the converter Bg, G2 has a three-phase transformer with two taps at the secondary. The transformers in G1 and G2 process part and total system power, respectively. The subsystem at the machine-side is composed of the machine and two converters that allow seriesand parallel-modes for the three-phase machine. The interesting characteristic of the system is to adapt its machine-side configuration as a function of the speed. The proposed configurations guarantee low harmonic distortion for both high speed (high voltage) and low speed (low voltage) operations with a higher modulation index. In addition, the system allows the machine´s operation with high current at low speed, which is interesting for applications with constant mechanical load. A comprehensive model of the proposed systems and modulation strategies are presented. Simulated and experimental results validate the theoretical expectations.