Research On High And Low Speed Control Strategy For High Power DTC System

Direct torque control is a common motor control technology.Because of its simple control structure,fast torque response speed,and strong robustness to motor parameter changes,it is very suitable for High-power motor control system used in electric locomotives.The switching frequency must be controlled to a low level because the switching losses cause a lot of heat.The main content of this paper is the control strategy of high-power motor direct torque control in high and low speed zones under the limit of low switching frequency.The thesis analyzes the asynchronous motor mathematical model,the basic principle of traditional direct torque control,the output voltage vector of the two-level voltage-type inverter,and its influence on the stator flux linkage and electromagnetic torque of the motor firstly.At the same time,this paper studies the structure of direct torque control system under the control of circular,hexagonal and octagonal flux linkages,the division and judgment of sectors,and the selection of voltage vector for each sector.Analysis and simulation studies have been made on the switching strategies between different flux linkages.For the problem of stator flux setting in high-speed flux weakening zone of high-power motor,this paper analyzes the torque changing rate and torque hysteresis output at different speeds in the flux weakening zone,and proposes a self-optimized stator flux linkage setting Strategy and simulation in MATLAB.The simulation shows that the weak magnetic strategy can make the stator flux closer to the optimal value of the flux linkage than the traditional weakening strategy based on speed,which improves the motor load capacity.At the same time,for the problem of torque ripple in the low speed region,the duty ratio control was introduced.Four common duty ratio calculation methods were compared.At the same time,a simplified duty ratio calculation method was proposed and verified by simulation.Through simulation,it is verified that the duty ratio scheme in this paper can effectively improve torque ripple and reduce current harmonics.In this thesis,the traction inverter for electric locomotive and its 1.2MW asynchronous motor were used in the experimental platform.The control system of this paper was designed based on DSP and FPGA.The main program of direct torque control was realized on FPGA.Finally,the voltage and current waveforms of the flux linkage changing,load carrying,new weak magnetic strategy,and low-frequency duty ratio strategy experiments are given in this paper.Experiments show that different flux linkage modes can be switched smoothly,and the new flux-weakening strategy can effectively improve the motor load carrying capacity.The duty ratio control proposed in this paper in low speed zone can effectively suppress torque ripple and improve current waveform.