Asynchronous Motor Direct Torque Control System To Improve Low-speed Performance

Direct Torque Control (DTC) of induction motors, which has been developed in the recent decade, is a powerful control method for motor drives. Following the Field-Oriented Control technique, the quick development of DTC makes itself a new high performance AC driving technique paralleling to the Field-Oriented Control. Now it has been an active field. Some problems of it are discussed in this thesis.Based on the integration of the phase voltages, conventional DTC scheme is very sensitive to stator resistance variations during low-speed operations. In such situations an incorrect stator resistance value causes a bad estimation of the stator flux and consequently poor control performances. A new scheme is presented in this thesis, which is based on an estimation of the torque and the air-gap flux fully independent from stator resistance variations. First, this scheme estimates the third harmonic component of the air-gap flux from the stator third harmonic voltage; then estimates the air-gap flux from the third harmonic component of the air-gap flux and uses the air-gap flux in the DTC system. Being unaffected by stator resistance, the system is relatively simple and allows to perform near-zero speed operations, and the performance is still good. In addition, in this scheme, information of moto speed is not required, so a simple and lost-cost position sensorless control schem can be easily achieved. Simulation results are shown to confirm the consistence and correctness of the proposed technique.