A modified direct torque control using an adaptive flux observer

Direct torque control drive (DTC) is well-known due to its fast torque response, simplicity and robustness. But some difficulties arise during steady state and also low speed operation. Stator flux estimation is a crucial issue in a DTC drive, especially in low speed region. An open loop voltage model observer does not work properly due to existence of an open loop integrator and stator resistance variation. In this paper an adaptive flux observer is presented which has the advantage of removing the open loop integration and providing a mechanism to include stator resistance estimation. A precise estimation of stator resistance helps the system to have a desired performance in low speed region. Voltage drops over inverter switches normally had been ignored in voltage calculations, but in low speed region with low voltage values these voltage distortions should be considered. A non-linear inverter model is presented for modeling the voltage distortions. Also for eliminating torque, flux and current pulsations during steady state, instead of classical DTC method a combined DTC-SVM method is used. This method removes the pulsations while maintains the basic advantages of DTC drive.