introduction
PWM control has been increasingly widely used in AC motor variable frequency speed regulation. The classic sinusoidal pulse width modulation (SPWM) primarily aims to make the inverter output voltage as close to a sine wave as possible, maximizing the fundamental component and minimizing harmonic components of the PWM voltage wave. However, this method is only an approximation, and its ability to suppress harmonics is limited. Voltage space vector pulse width modulation (SVPWM), on the other hand, treats the inverter and motor as a single unit, controlling the motor to obtain a circular rotating magnetic field with constant amplitude. It can significantly reduce the harmonic components of the inverter output voltage and the harmonic losses of the motor, thus reducing torque ripple. This paper establishes a simulation model based on the principles of vector control and SVPWM modulation, and analyzes the key issues and simulation results.
Establishment of system simulation model
Vector control model based on SVPWM
Figure 1 shows the block diagram of the position servo control system. This system uses Clarke and Park transformations to convert the detected three-phase stator current into DC components id and iq in a synchronous rotating coordinate system as current feedback. The deviation between the given position and the position feedback value is processed by a P regulator, and the output serves as the speed input for speed control. The deviation between the position loop output and the feedback speed is processed by a PI regulator, and the output serves as the current q-axis component for torque control, along with the current d-axis component calculated through transformation. The deviation from the current feedback value is processed by the PI regulator, and the outputs are voltage components vq, vd, vq, vd in the synchronous rotating coordinate system, respectively. These components are then converted into voltage components vα and vβ in a two-phase stationary coordinate system via an inverse Park transformation. Finally, SVPWM technology is used to generate PWM control signals to control the inverter.
Based on the principle of SVPWM vector control, a simulation model of the system was built in MATLAB/Simulink, as shown in Figure 2. The entire simulation model mainly consists of several parts, including the motor body module, inverter module, SVPWM generation module, vector transformation module, and rotor flux linkage position observation module. To make the simulation model closer to the actual system, the motor and inverter models in the simulation model are models from the simpowersystems module in MATLAB/Simulink, which are equivalent to the hardware parts in the actual system. The other models are subsystems built using various basic modules in Simulink and encapsulated using encapsulation technology, which can be implemented in software in the actual system.
Position, speed and current PI regulator
The system has four PI controllers: a position PI controller, a speed PI controller, a torque current PI controller, and a magnetizing current PI controller. The outputs of these four PI controllers all require limiting. The position controller's output is limited to the maximum motor speed, the speed controller's output is limited to the maximum motor torque, and the two current PI controllers are limited to the maximum voltage of the voltage space vector. Furthermore, the settings of these four PI controller parameters are crucial to system stability and are also a challenge during system simulation and debugging. Because the four PI controllers are interconnected, a change in the parameter of any one controller will cause system instability. Experience in debugging these four PI controller parameters is very important. First, determine the approximate range of the four PI controller parameters based on experience; then, adjust them gradually within this range. Generally, the outer loop (position loop) PI parameters have a more significant impact on the system, so the general order of PI parameter adjustment is: outer loop (position) first, then inner loop (current loop), and proportional coefficient first, then integral coefficient.
In the simulation experiment, through repeated debugging, the final pi parameters are as follows:
Position p adjuster kp=20;
Speed pi regulator kp=10, ki=5;
Torque current pi regulator kp=200, ki=70;
The excitation current pi regulator has kp=200 and ki=70.
svpwm generation module
The SVPWM generation module is a key part of this simulation system. This module takes the components vα and vβ of the voltage vector in the two-phase stationary coordinate system as inputs and internally provides the switching period tpwm signal. Internally, it determines the voltage vector interval, generates x, y, and z based on the input, and then calculates the power device conduction time. Finally, the SVPWM pulse signal generated by the interval signal and the conduction time controls the inverter's operating mode. The structure is shown in Figure 3.
Simulation Results and Analysis
The simulation parameters are as follows: rated voltage of 400V, frequency of 50Hz, rr=1.395ω, rs=1.405ω, l1r=0.005839h, l1s=0.005839h, lm=0.1722h, number of pole pairs p=2, rated power pn=4kW, and moment of inertia j=0.0131kg·m2. The simulation results are shown in Figure 4.
Conclusion
Simulation results show that the system has good dynamic tracking performance under load and can quickly reach a stable operating state, which also proves the feasibility of the position servo vector control system designed in this paper.
