The Research Of Sensorless Direct Torque Control System Based On The New Stator Flux Estimator

Direct torque control (DTC) technique is based on stator field orientation, thus the estimated accuracy of stator flux linkage influences the performance and stable operation of DTC control system directly. Although many schemes based on stator flux estimator are proposed, the key problem (the dynamic performance in low speed) hasn't been solved. Until recently the stator flux estimator is the focus of the research all over the world.For solving the key problem of the stator flux estimator, the dynamic and static performance of tradition Low-Pass Filter (LPF) is deeply analyzed. The research results show the estimated error of stator frequency and direct current drift influence the dynamic and static performance of tradition LPF, while the ripple of stator frequency will also influence the dynamic and static performance of tradition LPF. The quantitative relationships of the flux linkage estimated accuracy and the error signal between Modified Low-Pass Filter (NMLPF) and Programmable Cascaded Low-pass Filter (MPCLPF) are derived. By theoretical analysis and simulation results the conclusions as follow are obtained: the ripple of stator frequency will greatly influence the dynamic and static performance of tradition LPF in low speed. Tradition LPF has dynamic error in theory. The prime reason that tradition LPF has dynamic error is the calculation sequence of LPF and compensation.A new stator flux estimator design method was proposed which adopts compensation before LPF. New Modified Low-Pass Filter (NMLPF) and Modified Programmable Cascaded Low-pass Filter (MPCLPF) were designed. Theoretical analysis, simulation and experimental results show the influence of the ripple of stator frequency to the stator flux estimated accuracy and the dynamic error in tradition LPF was diminished by the new stator flux estimator. By which the dynamic performance of stator flux estimator, especially the low speed performance and the low speed range of operation, is improved greatly. It is remarkably that the content of current harmonic in very low speed is quite low. The robustness of NMLPF to the stator resistance error is improved by increasing the cutoff frequency of Low-Pass Filter while the increased cutoff frequency of Low-Pass Filter has little influence to the dynamic performance of NMLPF. The new stator flux estimator only needs to exchange the sequence of the compensator and LPF in flux estimator, so the tradition designs and optimizations of LPF are also suitable for it. Also a new undisturbed start method is proposed against the start problem of Programmable Cascaded Low-pass Filter.The accurate estimation of flux linkage is also the key of rotor speed estimation. The dynamic and static performance of Sensorless DTC system which adopted the new stator flux estimator is the second main researched content. Theoretical analysis, simulation and experimental results show Sensorless DTC system based on the new stator flux estimator and dynamic equation has excellent dynamic and static performance, simple algorithm and a good prospect in industry. Based on the NMLPF, the modified model reference adaptive speed estimation approach solves the ideal integrator problem in model reference adaptive method based on the rotor flux; The NMLPF also preferably solves the influence of stator resistance error to the performance of system in low speed and dose not need to identify stator resistance parameter on line due to its good robustness to stator resistance error.