Performance Improvement of DTC for Induction Motor with 12-Sector Methodology

Direct Torque Control (DTC) is an advanced computation speed control method and is possibly the best among the techniques for variable frequency drive. Because it does not need axes transformation and voltage decoupling blocks. DTC provides direct selection of voltage vectors according to the error obtained and allows direct and independent control of flux linkage and electromagnetic torque by the selection of optimum inverter switching modes. Based on the estimates of flux position and instantaneous errors in torque and stator flux magnitude, a voltage vector is selected to restrict the torque and the flux errors, within the torque and flux hysteresis bands. In the conventional DTC the circular locus is divided into 6 sectors and a total of 8 voltage vectors are used. However, the discrete inverter switching vectors cannot always generate exact stator voltage required to obtain the demanded electromagnetic torque and stator flux linkages. This results in production of ripples in the flux as well as torque. In this proposed work, neutral point clamped inverter is combined with 12 sector methodology. At present, the researchers have used all the 27 voltage vectors available in the 12 sector methodology. But in this proposed work the new switching table is constructed by using only 12 voltage vectors which includes large, medium and zero voltage vectors. The usage of 12 voltage vector gives satisfactory performance as compared to usage of all the 27 voltage vectors. The performance of the proposed work is also compared with classical DTC and 12 sector methodology with 27 voltage vectors. The simulation is performed in MATLAB / SIMULINK environment.