Improvement of low-speed operation performance of DTC for three-level inverter-fed multi-phase synchronous motor

This paper proposes a improved direct torque control scheme for three-level inverter-fed multi-phase synchronous motor on the foundation of modeling of 12-phase synchronous motor with four V-connected 3-phase symmetrical windings displaced in turn by 15°. In the proposed scheme, a space-vector-based regulator appears in DTC drives. So we can control the torque and flux linkage by composed space vectors in each control period and improve low-speed operation performance of DTC. Furthermore, quantity of vector in 12-phase three-level inverter is huge. This scheme optimizes the selection of the vectors and simplified the control process. The modeling and test results confirm that both torque and flux linkage ripples are greatly reduced when compared with those of the basic DTC, while the switching frequency of the modified DTC is almost fixed for different load torque and speed.