Abstract：The three-level multi-phase drive system combines the advantages of multi-phase motors and three-level inverters. However, midpoint voltage imbalance and excessive zero-sequence current exist. Midpoint voltage imbalance can lead to output voltage distortion, while excessive zero-sequence current can cause distortion of the stator current and increased losses. Direct torque control (DTC) requires minimal dependence on motor parameters, with a simple algorithm and strong adaptability to load changes. However, research based on DTC theory is still relatively rare. The symmetrical six-phase permanent magnet synchronous motor (PMSM) has high efficiency, a fast response rate among multi-phase motors, and superior fault tolerance. Therefore, this paper proposes a DTC strategy for a symmetrical six-phase PMSM powered by a T-type three-level inverter to balance midpoint voltage and suppress zero-sequence current. Firstly, by using a spatial vector projection method to remove the redundant switching states in the switching state, the switching states are reduced from 729 to 189. Secondly, based on the three-phase PMSM-DTC structure powered by the two-level inverter, the scheme synthesizes four voltage vectors to meet the control requirements on the six-phase axis using the simplified switching state. In addition, by combining flux linkage and torque hysteresis comparator with simple sector division, the axis of the optimal voltage vector is determined, and the midpoint voltage and zero-sequence current control plane are constructed. The sector where the system is located in the control plane is judged. According to the polarity of the winding phase current iy of the optimal voltage vector, the optimal voltage vector selection is realized, and the optimal switch vector table of DTC is further constructed. Steady-state comparative experiments are conducted. Compared to the traditional approach, the proposed method maintains a balanced midpoint voltage, effectively suppresses zero-sequence current to approximately 0 A, and significantly sinusoidalize phase current. Steady-state running experiments are conducted at different speeds and loads to investigate the steady-state performance of the motor. The results show that zero-sequence current is effectively suppressed to approximately 0 A, and the midpoint voltage remains balanced. Speed step and sudden load change experiments are performed to validate the dynamic performance of the motor. The results demonstrate that the zero-sequence current can still be well controlled around 0 A, even during speed steps and sudden load changes. Although the midpoint voltage may fluctuate slightly, it can be quickly adjusted to a balanced state. The following conclusions can be drawn. This paper proposes a DTC strategy for a symmetrical six-phase PMSM powered by a T-type three-level inverter, addressing the control of |ψs| and Te, midpoint voltage balance, and zero-sequence current suppression.
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2024, Vol. 39 
