Direct Torque and Predictive Control Strategies in Nine-Phase Electric Drives Using Virtual Voltage Vectors

Abstract

One of the main distinctive features of multiphase machines is the appearance of new degrees of freedom ( ${\boldsymbol{x}}$ – ${\boldsymbol{y}}$ voltages/currents) that do not exist in their three-phase counterparts. As a direct consequence, control approaches that apply a single switching state during the sampling period cannot achieve zero average ${\boldsymbol{x}}$ – ${\boldsymbol{y}}$ voltage production. In direct torque control (DTC) this implies that ${\boldsymbol{x}}$ – ${\boldsymbol{y}}$ currents are not regulated, whereas in finite-control-set model predictive control (FCS-MPC) an enhanced ${\boldsymbol{x}}$ – ${\boldsymbol{y}}$ current regulation is feasible only at the expense of disturbing the flux/torque production. Aiming to avoid these shortcomings, this work makes use of the concept of synthetic/virtual voltage vectors (VVs) to nullify/limit the ${\boldsymbol{x}}$ – ${\boldsymbol{y}}$ voltage production in order to improve the current regulation in the secondary planes. Two strategies using two and four virtual voltage vectors (2-VV and 4-VV, respectively) are proposed and compared with the standard case that applies a single switching state. Since standard MPC has the capability to indirectly regulate ${\boldsymbol{x}}$ – ${\boldsymbol{y}}$ currents, the improvements with the inclusion of VVs are expected to be more significant in DTC strategies. Experimental results validate the proposed VVs and confirm the expectations through a detailed performance comparison of standard, 2-VV and 4-VV approaches for DTC and MPC strategies.