Neutral-point voltage control of the three-level inverter in permanent magnet synchronous motor based direct-drive belt conveyor

Combined with the operation condition of belt conveyor and the running state of low speed direct-drive permanent magnet synchronous motor (PMSM), the reason for the high fluctuation of the neutral-point voltage (NPV) in the three-level inverter with the conventional method is analyzed. An improved NPV control strategy based on self-optimization of the switching sequence is proposed to ensure that the NPV could be completely controlled with the minimal number of switching times. A test platform with a 800 kW low speed PMSM is built, where the proposed NPV control strategy is implemented on a digital processor using the programming language C. Compared with the conventional method, the experimental results show that the NPV fluctuation of the proposed method can be reduced by a ratio of 46.3% to 89.5% at the least cost of the switching loss increase and the low-frequency harmonics in the output current can also be mitigated.