Research On Torque Ripple Minimization In Direct Torque Control

Direct torque control (DTC) has become the mainstream of the current inverter marketbecause of its excellent dynamic and static performance. Currently, the development of DTCinverter has tended to be mature abroad, but China is still in the initial stage, no matureproducts appears. In addition, the traditional DTC is restricted by its defects, such as largetorque ripple at low speed, serious stator current distortion and the variation of switchfrequency. In view of this, the work in this paper is divided into the following major parts:Firstly, the basic math model of an asynchronous motor and inverter control principle areelaborated. Moreover, structure of the DTC system, the flux and torque control principle andswitching vector selection of the round flux DTC are analyzed.Secondly, the causes and harm of the torque ripple in the traditional DTC are analyzed.Two aspects are improved based on traditional DTC model, on the one hand, the six-sectionflux is subdivided into twelve and its switching table are given, on the other hand, a constantswitching frequency controller which is invented by N.R.N Idrist is used to replace the torquehysteresis comparator, and the twelve-section round flux DTC model with constant switchingfrequency controller is established. Experiments are made on the six-section DTC model andthe CSF twelve-section DTC model respectively at high and low speed, the simulation resultsshow that both methods can keep the magnetic track circular, and the new model can improvethe stator current distortion and reduce torque ripple effectively.Thirdly, the hardware circuit of the system are designed according to the simulation.Power board of Tsinghua University motor experimental platform is followed as the mainpower circuit, a new type of DSP floating-point chip TMS320F28335is adopted as thecontrol core, the sampling and signal conditioning circuit are redesigned, and live debuggingconfirms the rationality of the circuit design.Finally, the software processes and each module program are designded. C languageprogramming is used in the CCS3.3environment to achieve the current and voltage sampling,speed detection, calculation and adjustment of the torque and flux, the output of the switchsignal. The hardware and software are joined for debugging, the control aim is accomplishedby changing the speed of the motor according to the frequency and operating parameters innormal.