The Study Of Direct Torque Control Of IM And Its Digital Realization

Direct Torque Control (DTC) has become one of the most important control methods in induction motor control system just because of its characteristics, such as simple computation, low sensitivity to motor parameter, and good dynamic as well as static characteristics. According to the disadvantages in low speed region , the author has made simulations and analysis, and put forward a strategy to improve the performance on the basis of studying basic theories of direct torque control. The whole digital implementation with hardware and software has been designed in the thesis.The reasons for periodic torque ripples resulting from conventional direct torque control (DTC) are discussed in detail. A new torque predictive control strategy is adopted, which is used to reduce the torque ripples in low speed region and improve the steady performance of system. Simulations show the validity of the strategy. The stator flux error between LP filter and a pure integrator estimator is analyzed. This thesis presents a novel stator flux compensation method in low speed region based on the voltage model with a form of lowpass(LP) filter. The compensation method is described in detail. It is based on regulating the cutoff frequency of the lowpass filter which varies with the frequency of the stator current detected in the low speed region. The method of realization in digital system is presented. The results of the simulation have shown that the drive performances gain improved with the compensation proposed in low speed in induction motor DTC system, which confirms the validity of this compensation method.