Direct Torque Control Schemes for Dual Three-Phase PMSM Considering Unbalanced DC-Link Voltages

As a dual three-phase permanent magnet synchronous motor (DTP-PMSM) is fed by two DC voltage source inverters (VSIs), the DC-link voltages may become unbalanced. The conventional direct torque control (DTC) schemes under unbalanced DC-link voltages generate large current harmonics and unbalanced stator currents in two sets of windings, which reduce the efficiency and maximum output power of DTP-PMSM. To solve these problems, DTC schemes considering unbalanced DC-link voltages are proposed in this study to improve the performance of DTP-PMSM. First, the variation law of voltage vectors decomposed into the $\bm{\alpha} \text{-} \bm{\beta}$ and x-y subspaces is analyzed under unbalanced DC-link voltages. Then, two or three sets of new virtual vectors synthesized by three nonzero vectors are developed to retain the voltage in the x-y subspace at zero. Moreover, to extend the applicable range, two DTC schemes switch the virtual vector sets with changes in the DC-link voltage ratio (u_dc1/u_dc2). Switching points are theoretically analyzed. Subsequently, their performances are analyzed, including DC-link voltage utilization and ripple current. Finally, experiments are performed to verify the feasibility of the proposed schemes in steady-state and dynamic conditions.