Research On The Field-weakening Control Method For Improving The Performance Of Permanent Magnet Synchronous Motor Drives

Permanent magnet synchronous motors (PMSM) are widely used in many electricdrive applications such as electric vehicles and railway vehicles due to their manyattractive characteristics of high efficiency, high power density and particularly wideflied-weakening region. For these applications, to meet the demands of a variety ofworking conditions, a wide speed range of PMSM is required, and also theperformance of system under the determined inverter capacity is important. Due to thedrive system constrained by inverter capacity, when the terminal voltage of PMSMreaches the maximum value with increasing motor speed, the output voltage of currentregulator is saturated, and accordingly the torque, current and speed performance willbe deteriorated. By using field-weakening control method for PMSM, the drivesystems can achieve its constant torque output at low speed and its constant poweroutput at high speed. Therefore, it is great significance to research a field-weakeningcontrol method for PMSM to expend the range of flied-weakening operation andimprove its control performance.The main purpose of this study is to increase its maximum torque outputcapability at field-weakening operation region, and effectively improve the dynamicresponse performance while expending the operating range of the drive system. Inorder to fully utilize the DC-link voltage of inverter, the space vector pulse widthmodulation(SVPWM) for inverter will be extended to nonlinear modulation regionwhile the PMSM is operating in the field-weakening region in this paper. And thedynamic voltage difference output by before and after overmodulation is used toconstruct a voltage feedback loop for flux-weakening control to extend the speedoperation of the PMSM, and improve the torque output capacity under the limitedinverter capacity.At first, the mathematical model, the operation restrictions of PMSM and vectorcontrol system is described in the paper. For improving the performance of controlsystem in constant power region, overmodulation technique is considered to extendthe operating region of the PMSM. Based on analyzing the principle of SVPWM, inorder to improve the utilization of the DC-link voltage of the inverter and achieve thefull-range output voltage for the inverter, dynamic overmodulation method isproposed for weakening control strategy.Secondly, the principle of field weakening control for PMSM is described in this paper, and the optimal current vector control trajectory at full speed range forproducing maximum torque under operation constraints is determined. Then, thetraditional field weakening control method based on DC-link voltage feedback isdetailedly analyzed, in which the DC-link voltage is not fully utilized. For thisdisadvantage, a field-weakening control method based on voltage difference feedbackat overmodulation region for PMSM. By taking the voltage error separately output bybefore and after overmodulation as objective function, the mathematical models aboutthe voltage difference and the d-and-q-axis current is established to optimize thed-and-q-axis current in field-weakening operating region using gradient descentoptimization principle. Concretely, the voltage difference between the referencevoltage updated by current regulator and the saturated voltage output byovermodulation is used for field-weakening control, which modifies the stator currentphase angle. And thus the d-and-q-axis reference current components are redistributedand finally the speed operation range of PMSM is extended. By using the proposedmethod not only torque output capability is enhanced, but also the problem of integralsaturation in current regulator caused by field-weakening operating at high speed forPMSM can be effectively solved, and thus better dynamic performance of the systemcan be achieved.At last, the simulation experiment has been performed by building the systemsimulation model based on Matlab7.5. At the same time, the hardware and softwarecontrol platform for the whole high speed drive system has been fully implemented.With this method, high-time speed field-weakening operation is realized, and the goodparameter robustness and fast response are also guaranteed. Moreover, since theproposed method utilizes the DC-link voltage more efficiently, it makes the motorgenerate higher output torque in field-weakening operation region than theconventional field-weakening control method under the same voltage and currentlimitation. The research has very important application value for enhancing theoperating efficiency and the fuel economy of the electric vehicle.