Abstract：The five-phase permanent magnet synchronous motor (PMSM) has received attention in electric vehicles and marine transportation due to its more control degrees of freedom, fast response, and high torque density. The most popular control strategies are field-oriented control (FOC) and direct torque control (DTC) strategies. Compared with FOC, a switching table and two hysteresis controllers are applied in DTC to control torque and stator flux linkage. Thus, DTC has the advantages of simple structure, high torque dynamic performance, and fast dynamic response. It also has disadvantages, such as large torque and flux linkage fluctuations. Deadbeat-based DTC (DB-DTC) can solve this problem by discretizing the mathematical model to obtain the voltage vector, which can reduce the fluctuations of torque and stator flux linkage within one sampling period. However, the amplitude of reference flux linkage is usually constant in traditional DTC strategy, resulting in inefficient operation of five-phase PMSM under both healthy and open-circuit fault conditions. This paper proposes a novel DTC based on virtual square wave injection (VSWI-DTC) strategy, which can detect the optimal stator flux linkage amplitude online under healthy and open-circuit fault conditions, achieve the minimum copper loss, improve the operational efficiency, and maintain good dynamic performance. Firstly, the model of five-phase PMSM is derived in the rotor-flux reference frame under healthy and open-circuit fault conditions, and the relationship between stator current and stator flux linkage under constant torque is deduced. Secondly, the optimal stator flux linkage detector is designed. The virtual square wave signal is injected into the stator flux linkage amplitude, which is compensated according to current amplitude variations under constant torque. Thus, online detection of the optimal stator flux linkage amplitude can be realized. Finally, combined with DB-DTC, the torque and stator flux linkage fluctuations can be reduced. The experimental results demonstrate that VSWI-DTC has good steady-state and dynamic performance. The proposed strategy requires about 0.1 s to detect the optimal stator flux linkage amplitude, while the DTC based on virtual sinusoidal signal injection (VSSI-DTC) takes about 1 s. Most importantly, the optimal stator flux linkage amplitudes obtained by the two strategies are almost the same. In addition, the optimal stator flux linkage amplitude is almost the same under healthy and open-circuit fault conditions, while the torque reference remains invariable. When the torque reference steps, the amplitude of stator flux linkage can accurately and quickly track its reference. Therefore, the following conclusions can be drawn. (1) The proposed VSWI-DTC strategy can quickly detect the optimal stator flux linkage amplitude online without filters under healthy and open-circuit fault conditions. (2) The proposed VSWI-DTC strategy has similar dynamic performance under healthy and open-circuit fault conditions. Additionally, the torque fluctuations caused by open-circuit faults can be restrained. (3) No additional torque fluctuation and loss exist because the square wave signal injected into the stator flux linkage is virtual.
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2024, Vol. 39 
