A Voltage Space Vector Diagram Formed by Six Concentric Dodecagons for Induction Motor Drives

A new voltage space vector diagram consisting of six concentric dodecagons is proposed in this paper. The space vector diagram is developed for an open-end induction motor drive fed from two three-level neutral point clamped (NPC) inverters with asymmetric isolated dc voltage sources. As already reported in literature, a dodecagonal space vector diagram is advantageous over a hexagonal one, since the 6n +/- 1 (n = odd) harmonics are absent in the phase voltage and the linear modulation range is extended. The present work further improves the existing dodecagonal space vector diagrams by dividing the space vector plane into six concentric dodecagons, while retaining the aforementioned advantages. A high-resolution pulsewidth modulation strategy is developed in this paper, which compared to existing schemes, reduces the harmonic content of the motor phase voltage without resorting to higher switching frequency. A comparative study of output voltage quality produced by the proposed topology and a five-level inverter shows substantial improvement in the present work. At the same time, the present topology does not require any special design of the inverter and uses two popular three-level NPC inverters. Experimental verification on a laboratory prototype indicates the possible application of this concept for high power drives.