Direct Torque Control With Reduced Switching Losses for Asymmetric Multilevel Inverter Fed Induction Motor Drives

In this paper a high performance torque and flux control strategy for multilevel inverter fed induction motor drives is presented. The control method is based on Direct Torque Control operating principles. The stator voltage vector reference is computed from the stator flux error imposed by the torque controller. This voltage reference is then generated using an asymmetric cascaded H-bridge multilevel inverter. This inverter provides nearly sinusoidal voltages with very low distortion, using less switching devices. Due to the small dv/dt's, torque ripple is greatly reduced. In addition, the asymmetric multilevel inverter can generate a high and fixed switching frequency output voltages with less switching losses, since only the small power cells of the inverter operate at high switching rate. Therefore a high performance and also efficient torque and flux controller is obtained, enabling a DTC solution for multilevel inverter powered motor drives.