Zhang Wenhao 1, Cui Liancheng 2, Mu Xiaojing 3
(School of Automation and Electronic Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042, China)
Study of Asynchronous Motor Direct Torque Control Based on MATLAB/Simulink
Zhang Wenhao, Cui Liancheng, Mu Xiaojing
College of Automation and Electronic Engineering, Qingdao University of Science & Technology, Qingdao 266042, China
Abstract: This paper introduces the control principle of three-phase asynchronous motor direct torque control based on the asynchronous motor mathematical model established in this paper. The whole and different components simulation models of three-phase asynchronous motor direct torque control system are established based on simulation platform MATLAB/Simulink. The simulation result shows that the motor speed can be tracked quickly by means of this control method with higher dynamic and static performance, meanwhile, motor flux and torque ripple can be lowered effectively. The steady state performance of the AC speed-regulating system is greatly improved.
Key Words: asynchronous motor; direct torque control; simulation platform; flux
0 Introduction
Direct torque control (DTC) is a novel variable frequency speed control technology developed after vector control. It was first proposed in the 1980s by German scholar M. Depenbrock and Japanese scholar I. Takahashi for asynchronous motors. In the 1990s, scholars Zhong. L, Rahman MF, and Hu YW proposed the theory of direct torque control for permanent magnet synchronous motors. It employs space vector analysis to directly calculate and control the torque and flux linkage of the AC motor in the stator coordinate system. Using stator field orientation, it generates pulse width signals through discrete two-point control (Band-Band control) to directly and optimally control the switching state of the inverter, thereby achieving high dynamic torque performance.
DTC (Direct Current Traction Control) boasts advantages such as simple control structure, rapid dynamic torque response, low dependence on motor parameters, and good robustness to changes in motor parameters. It is currently widely used in asynchronous motors and permanent magnet synchronous motors, playing a significant role in industrial production, including household appliances, the automotive industry, and electric locomotive traction.
Based on the analysis of the mathematical model of a three-phase asynchronous motor, this paper introduces the control principle of a direct torque control system for a three-phase asynchronous motor. A comprehensive simulation model of the direct torque control system and its individual components are established using the MATLAB/Simulink simulation platform. Simulation results show that this control method can effectively achieve rapid tracking of motor speed. The system exhibits high dynamic and static performance, effectively reduces motor flux and torque ripple, and improves the steady-state performance of the AC speed control system.
- Mathematical model of asynchronous motor
An asynchronous motor is a high-order, nonlinear, and strongly coupled multivariable system. Therefore, when analyzing the mathematical model of an asynchronous motor, the following assumptions are usually made:
(1) Ignoring spatial harmonics, assuming that the three-phase windings are symmetrical, the generated air gap magnetic field is sinusoidally distributed.
(2) Ignore magnetic circuit saturation phenomenon.
(3) Core loss is not considered.
(4) The effects of frequency and temperature changes on the winding are not considered.
An asynchronous motor is described in an orthogonal stator coordinate system using spatial vector analysis. The mathematical model of the motor in the stator coordinate system consists of voltage equations, flux linkage equations, torque equations, and motion equations.
The voltage equation is:
2. Direct Torque Control (DTC) Principle of Asynchronous Motors
The Direct Torque Control (DTC) method uses space vector analysis to directly analyze the mathematical model of the AC motor in the stator stationary coordinate system, constructs the algorithm model of torque and flux linkage, calculates and controls the torque of the AC motor, generates PWM signals with the help of hysteresis controller (Bang-Bang control), and directly controls the switching state of the inverter through the switching meter to obtain high dynamic torque performance.
The basic principle is to make full use of the switching characteristics of the voltage-source inverter, and make the stator flux trajectory approach a circle by continuously switching the voltage state. The slip frequency is changed by the interleaving of the zero voltage vector to control the motor torque and its rate of change, so that the flux and torque of the AC motor can change rapidly as required.
The direct torque control (DTC) system for asynchronous motors consists of an inverter, a three-phase asynchronous motor, flux linkage estimation, torque estimation, rotor position estimation, switching meters, a PI controller, and a hysteresis comparator. The control system uses the error between the given motor speed and the actual speed as the torque input signal via the PI controller. Simultaneously, based on the detected three-phase current and voltage values of the motor, the system calculates the magnitude of the motor's flux linkage and torque using flux linkage and torque models, respectively, and calculates the rotor position, the error between the given flux linkage and torque and the actual values. Finally, based on these states, the system selects the inverter's switching voltage vector, enabling the motor to adjust its output torque according to control requirements, ultimately achieving speed regulation. The system block diagram is shown in Figure 2-1 below.
3. Construction of Simulation Model for Asynchronous Motor DTC Control System
Based on the established mathematical model of the asynchronous motor and the principle of direct torque control, a comprehensive simulation model of the asynchronous motor direct torque control system was built using Simulink in the MATLAB environment. The overall simulation model and the simulation models of each component are as follows:
4. Simulation Results and Analysis
Using the simulation model established in MATLAB/Simulink, a simulation was performed in MATLAB R2010a. The system parameters were set as follows:
The simulated waveforms of motor speed, electromagnetic torque, stator current, and stator flux vector are shown in Figures (a), (b), (c), and (d), respectively.
Simulation results show that the motor starts quickly, and the actual speed changes rapidly without overshoot. It can quickly track large changes in the given speed, demonstrating that direct torque control indeed has excellent dynamic and static performance. Stator current harmonics have minimal impact. Except during the magnetic field establishment process at startup, the stator flux linkage trajectory remains approximately circular during both static and dynamic processes, and the stator values remain essentially constant, effectively reducing flux linkage pulsation when motor parameters change. After applying load torque, the motor's torque dynamic response is fast with minimal pulsation, improving the steady-state performance of the AC speed control system.
5. Conclusion
Direct torque control (DTC) is a novel, high-performance AC speed control technology developed in the mid-1980s following vector transformation control. With the increasingly widespread application of DTC, continuous technological improvements are necessary. This paper proposes corresponding control methods for the stator flux linkage amplitude and motor torque, the main controlled objects, to achieve an approximately circular stator flux linkage trajectory, resulting in rapid torque response, reduced motor harmonics, noise, and torque ripple, and high-performance torque response. Direct control of the circular flux linkage is implemented on the MATLAB/Simulink simulation platform, verifying the excellent performance of this new control method. Simulation results show that the DTC control system can effectively achieve rapid motor speed tracking, effectively reduce motor flux linkage and torque ripple, provide rapid dynamic response, have low dependence on motor parameters, and exhibit good dynamic and static performance.
About the author:
Zhang Wenhao (1988-) Male, Qingdao University of Science and Technology, Master's degree, major: Control Theory and Control Engineering
Cui Liancheng (1986-) Male, Qingdao University of Science and Technology, Master's degree, major: Control Theory and Control Engineering
Mu Xiaojing (1986-) Female, Qingdao University of Science and Technology, Master's degree, major: Control Theory and Control Engineering
Mailing Address: No. 53 Zhengzhou Road, Sifang District, Qingdao, Shandong Province
Contact numbers: 15854201463, 15192073615
Email: [email protected]
