Performance improvement of direct torque control drives in low speed region

Direct torque control (DTC) drive is well-known due to its fast torque response, robustness and simplicity. But some difficulties arise in low speed region. Using a pure integrator in voltage model flux observer causes problems such as DC drift and saturation of the integrator that should be solved in this region. Voltage drops over inverter switches normally has been ignored in voltage calculations, but in low speed region with low voltage values these voltage distortions should be considered. In addition Back emf voltage is low in this region and so resistive voltage drop has a considerable value compare to back emf voltage. Therefore an accurate stator resistance value is needed for estimating stator flux precisely. Also speed estimation methods that rely on machine model equations are sensitive to parameter variations. Stator resistance value plays an important role and its value has to be known with good precision in order to obtain accurate speed estimation in the low speed region. In this paper a non-linear inverter model is presented for modelling voltage distortions due to inverter switches. Then a modified integrating algorithm with desired performance in low speed region is introduced. Finally a parallel speed and stator resistance scheme is proposed. This scheme is based on sliding mode observer (SMO) and Popov´s hyperstability theory, which guarantees accurate speed estimation in low speed region.