Direct torque control of an induction motor using a three-level inverter and fuzzy logic

Multilevel inverters, such as three-level inverters, are an emerging technology and are commonly applied to high power motion drives instead of the standard two-level voltage source inverters (VSI). In three-level inverters the blocking voltage of each switch is half of the DC bus voltage, lower dV/dt is generated and the output voltage and current have lower harmonic distortion. In this paper the application of the direct torque control (DTC) strategy is studied for this particular topology. In the three-level inverter the voltage selection possibilities are enhanced since more inverter states are available. Two different control methods have been designed. The first is based on the conventional DTC scheme adapted for a three level inverter. The second is based on the application of a fuzzy logic controller to the DTC scheme to select the appropriate switching state, instead of applying the conventional DTC table. In the simulation results, the novel system based on fuzzy logic shows reduction in torque ripple and harmonic distortion in the stator currents and voltages. Both features are important for applications in motion control where smooth movement and high efficiency are necessary.