Permanent Magnet Synchronous Motor High Efficiency Direct Torque Control Research Based On SVPWM In 60° Coordinate System

Due to the characters of simple structure, high power density, high efficiency for permanent magnet synchronous motor(PMSM), it has been widely used in the military, industry, agriculture and many other occasions. It is of important economic and social significance to study the high performance control strategy of PMSM. There are two main control schemes for PMSM, which namely “Field Oriented Control(FOC)” and “Direct Torque Control(DTC)” respectively. Electrical torque is as direct control object for DTC which results fast torque response. And the voltage model DTC is essentially a speed sensorless control method. Therefore, PMSM DTC quickly became a hot topic in the field of motor since it was proposed in 1997. This paper focuses on PMSM DTC and the main object is surface-mounted PMSM. Theoretical analysis, simulation and experimental verification for PMSM DTC are done in the paper. It is hoped to further promote the application of DTC in PMSM’s drive. The main research issues of this article are as follows:The structure and control methods of PMSM are reviewed. The content of PMSM DTC is very extensive. This article has made a more comprehensive summary on the system architecture of PMSM DTC witch including two major categories, namely switching table mode and space vector modulation mode.Observation of stator flux is an important part of PMSM DTC. There are two usually used stator flux observation methods, namely voltage model and current model. The principle and the differences between the two methods are analyzed in the paper. The problems of initial value and error accumulation exist in the pure integrator. The solution for the problems is that the pure integrator can be replaced by a low-pass filter(LPF). But it is showed in the article that the LPF would introduce amplitude and phase error in the stator flux observation. It is necessary to compensate the error through manipulating the input signal. The simulation and experimental results demonstrate the validity of the compensation method and illustrate that using of voltage model can achieve accurate observation of the stator flux.For more time-consuming problem of SVPWM algorithm in orthogonal coordinate system, the characteristics of 60o coordinate system are analyzed in this paper and a novel SVPWM and its over-modulation algorithm based on 60o coordinate system is proposed. The new algorithm effectively saves the execution of CPU time, which fits well with the advantage of fast torque response in DTC. The using of new algorithm would be beneficial to the application of SVPWM in DTC. The simulation and experiment results confirm the algorithm in this paper.The reference stator flux amplitude is generally constant in traditional PMSM DTC. However, the operating condition of the motor is varied, which results in reduced efficiency of the motor. Based on the analysis of the stator flux vector and the current vector relations, a non-constant flux amplitude reference setting method is proposed from the view of stator current optimal control. The new method can make the motor efficiency improved. Simulation and experiments confirm the correctness of the algorithm.The velocity of the motor is limited while the DC-link voltage of the inverter is a fixed value. The principle of field weakening to increase the speed of motor and how to achieve field weakening in PMSM DTC is analyzed in the article. In order to improve the utilization of DC-link voltage and enhance the ability of the inverter output, the over-modulation strategy based on 60o coordinate system is considered in the field weakening control. The field weakening setting process is discussed in the paper when considering the over-modulation. The paper compares the performances of motor while field weakening before and after over-modulation. The results confirm that over-modulation algorithm could improve the motor efficiency in the field weakening control.An experimental platform for PMSM DTC is designed based on TI’s digital processor and Mitsubishi’s intelligent power module. The related experimental results confirm the reliability of the platform.