Research On Direct Torque Control Methods Of Decreasing Torque Ripples For Permanent Magnet Synchronous Motors

Permanent magnet synchronous motors (PMSM) have been widely used in the industrial field due to no excitation current, their high efficiency and high power density. Direct torque control (DTC) has become a high-performance AC adjustable speed method due to its simple structure and fast response. The new development of PMSM DTC is described in this paper. But PMSM DTC systems have disadvantages of variable stator resistance, high flux and torque ripples in the low speed, inconstant switching frequency in inverters. In order to solve these problems, improved strategies are presented and tested through simulation and experiments. These important conclusions are as follows:Firstly, in order to solve the problem of stator resistance variations which deteriorate the performance of DTC, a stator resistance on-line estimation method which uses fuzzy control according to errors between actual current and reference current was proposed. Effects of stator resistance variation on flux and torque in DTC have been analyzed in detail and uncompensated stator resistance deteriorates system performance. Simulation results show that stator resistance variations result in the errors in torque and flux position calculations which deteriorate the performance of DTC. The stator resistance on-line estimation method can accurately estimate the stator resistance and decrease the errors of torque and flux position calculation caused by stator resistance variations.Secondly, in order to solve the problems of flux increments asymmetry and large torque ripples in the PMSM DTC, a novel DTC combined sectors subdivision with duty ratio control of PMSM was proposed. Trajectories and ripples of flux and torque in the traditional DTC are analyzed in detail. Flux and torque increments asymmetry in the sector borderlines is concluded and increments of flux and torque are proportion to the sampling period and voltage vector amplitude in a sampling period. A novel DTC strategy of PMSM is proposed. In this method six sectors are subdivided into eighteen sectors and the duty ratio is computed according to torque errors, i.e. active voltage vectors amplitude. Simulation and experiment results show that this method can effectively decrease the flux asymmetry, improve the flux trajectories and decrease torque ripples.Thirdly, in order to solve the problems of large torque ripples and inconstant inverter switching frequency in the PMSM DTC, a novel DTC method based on space vector modulation (SVM) of PMSM was proposed. Based on the PMSM mathematical models in the coordinates of stator flux linkage, voltage components in the stator flux linkage axes are determined by errors of flux, torque and speed. Coordinate transformations and SVM method are used to control flux and torque. In this method flux and torque hysteresis controllers in the traditional DTC are not used and SVM method is used, which can effectively decrease flux and torque ripples and make the inverter have constant switching frequency. Simulation and experiment results show validity and correction.Fourthly, in order to solve the problem of torque ripples in the traditional DTC, an improved DTC method based on discrter space vector modulation (DSVM) was proposed and three improved DTC methods are compared. Based on the analysis of torque control equations effects of voltage vectors on torque are concluded at different speed. An improved DTC strategy based on discrete vectors modulation is proposed and voltage vectors which have the same direction and different amplitudes compared with basic vectors are used in this method. Traditional hysteresis controllers are improved in this method. Simulation and experiment results show that this control method is simple to realize and has the same dynamic performance as traditional DTC. The method restrains torque ripples and noises. Through comparing to three improved DTC methods, the improved DTC based on SVM has the best performance in decreasing torque ripples.Finally, a speed estimation method which injects fluctuating high frequency signals into inverters to generate PWM was used to realize the speed sensorless close-loop control of PMSM. The rotating high frequency signal injection and the fluctuating high frequency signal injection that can be used to estimate the motor speed are deduced through principles and tested through simulations. The rotating high frequency signal injection needs the saliency of PMSM but the fluctuating high frequency signal injection only needs the saliency of PMSM under high frequency signals. So the fluctuating high frequency signal injection has wide applications but increase the motor ripples. The fluctuating high frequency signal injection method to estimate speed is proved successful through experiments. The experimental results prove that this method is suitable to the low speed operation of motors and has less sensitivity to the variations of machine parameters.