Research On The Key Issuses In Direct Torque Control System For Permanet Magnet Synchrounous Motor

Permanent magnet synchronous motor has been widely applied in various AC drive systems for itsmerits of high efficiency, high power density and high torque/inertia ratio and the research ofpermanent magnet synchronous motor drive system with high performance deserves importanteconomy and society value.At present, there are mainly two control schemes in high performancedrive systems: one is vector control or called field orientation control, and the other is direct torquecontrol(DTC).As known, the essence of motor control is the control of motor torque.In vector controlscheme, the torque control is indirectly achieved by the control of d-axis and q axis current,in thisprocess,the accurate flux linkage orientation is needed.And DTC method considers the motor andinverter as a whole,and controls the torque directly by applying the optimal voltage vectors. DTC waspresented and applied in induction motor firstly and the DTC method for sinusoidal permanentmagnet synchronous motor was not presented until1997which was introduced by Nanjing Universityof Aeronautics and Astronautics and University of New South Wales. DTC has been the hot researchissuse since it was introduced.The main content of this thesis is about direct torque control ofpermanent magnet synchronous motor.Both of the sinusoidal and trapezoidal motors areresearched.The theoretical analysis, simulation and experiments are taken to solve the key issues indirect torque control of the two motors which helps to laying the foundation for further developmentand application of DTC.The main research issues are as follows:The research of permanent magnet synchronous motor at home and aboard is reviewed.It ispointed that the width and depth of DTC research on sinusoidal motor have been a new level,manysubjects are included and the content is very rich, and they can be roughly divided into three subjects:the research about torque ripple reduction; the research about sesorless operation;the control of statorflux linkage and so on.But for the trapezoidal motor,there is some specificity with DTC for thenon-sinusoidal voltage and current,and the turn-off phase.And the research of DTC of trapezoidalmotor has been the hot research issuses.The models of sinusoidal permanent magnet synchronous motor under different coordinate systemsand the traditional direct torque control basic theory are introduced,and the implementation procseesof traditional DTC with different stator winding connection styles are presented.With respect to theproblem of parameter determination,taking a sinusoidal permanent magnet synchronous motor forinstance, a parameter determination method for DTC is introduced detailedly.And the parametervariation under different situations is analyzed and the results are verified by the experiments.Theproposed parameter determination method is helpful for the normal DTC application.The characteristics of traditional DTC for sinusoidal motor under digital control are analyzedincluding the stator flux linkage variation,torque angle variation and the torque variation.The resultsshow that: the lag control of stator flux linkage exits in digital congrol system;the influence of rotorflux linkage positon variation can not be neglected under high speed operation;and the control ofmotor torque by voltage vectors shows the positive and negative asymmetry; the maintenance strengthof zero voltage vector to the torque depends on the motor speed.On the base of analysis of traditionalDTC,a torque regulator design method is proposed, and low torque ripple and short starting time can be achieved with this method,simulation and experiment results validate the regulator designmethod.The torque regulator design method provides the guidance for normal torque regulator designin DTC.Taking the non-salient pole sinusoidal permanent magnet synchronous motor for example,therelationship between stator flux linkage d-axis and q-axis components and motor torque.A stator fluxlinkage adaptive direct torque control is presented, in this method,the stator flux linkage referenceadaptive control is achieved,meanwhile, it also meets the requirement of load torque,and high powerfactor is gained.Simulation and experiment results verified the effectiveness and validity.It can be seen from sinusoidal permanent magnet synchronous motor traditional DTC that itsimplementation is still affected by induction motor DTC,in which flux linkage and torque close loopcontrol method is adopted.In fact, the amplitude of stator flux linkage and torque angle arecontrollable variables while the rotor flux linkage is constant in sinusoidal permanent magnetsynchronous motor,therefore,the implementation of DTC is not unique.After analyzing andcomparing different methods,a DTC method without flux linkage close loop control is presented.Themain feature of optimal direct torque control is that the rapid torque response is achieved bycontrolling the stator flux linkage q-axis component directly,and the optimal voltage vector is selectedaccoding to the torque requirement only,in this method,the flux linkage control loop iseliminated.The optimal control of torque is achieved by making using of the characteristic of constantrotor flux linkage in sinusoidal motor.This method provides the reference for further research in DTC.The direct torque control method of trapezoidal permanent magnet synchronous motor has its ownspecificity for the non-sinusoidal voltage and current,turn-off pahse.The torque variation is analyzedfrom the normal torque expression of trapezoidal motor, and the results show that the amplitude offlux linkage vector which is induced from rotor flux linkage is not constant,it is the function of rotorpositon.And for a certain rotor position,the torque variation is only decided by the variation of statorflux linkage q-axis component.Therefore,the direct torque control of trapezoidal motor can beachieved by choosing the voltage vector which the q-axis component is optimal according to therequirement of torque control,and there is no need to observe the stator flux linkage.And the currentlimitation is achieved by control the motor torque under120degree conduction mode.Theeffectiveness and validity of the proposed DTC method without flux linkage observation is verified bythe simulation and experiment results.The permanent magnet synchronous motor general experiment platform is designed based on TIhigh performance digital controller, and the stability is verified by different experiments mentionedabove.