Generalized DTC Strategy for Multilevel Inverter Fed IPMSMs with Constant Inverter Switching Frequency and Reduced Torque Ripples

This paper proposes a generalized direct torque control (DTC) strategy for multilevel inverter fed DTC (MLI-DTC) drives. By using a simple voltage vector decomposition technique, the proposed method is compatible with inverters possessing any number of voltage levels. Consequently, voltage vectors are applied in a similar way as in the case of a two-level inverter fed DTC. A constant inverter switching frequency is imposed in the proposed method by using a torque regulator in place of the traditional torque hysteresis controller, which enables the application of two voltage vectors within one switching cycle. This also contributes to reduce the torque and flux ripples. Detailed design guidelines for the proposed torque regulator are provided. Experimental results obtained with a three-level neutral point clamped inverter show that the proposed MLI-DTC drive operates with low, constant switching frequency while alleviating torque and flux ripples under all operating conditions. Comparative study with a prior art and parametric sensitivity analysis are also presented to verify the effectiveness and robustness of the proposed MLI-DTC method.