Abstract: Vector control is a high-performance AC motor control method with advantages such as low control torque and low current harmonic content. This paper introduces the design of a vector control-based variable frequency speed control system using TI's TMS320F2812 as the core, describes the composition and design characteristics of each hardware functional unit, and presents the design scheme and software/hardware design methods.
Keywords: asynchronous motor; vector control; current regulation
DSP-based vector control of asynchronous electromotor
Ma Zhaoxun Yang Guowei
(Harbin Jiuzhou Electric Co.,LTD. Haerbin 150081 Heilongjiang)
Abstract: Vector control is a high performance AC motor contro method. With the advantage of small control torque, small current harmonics and so on. This paper introduce a system design that based on vector control VVVF to TI's TMS320F2812 as the core, description of the hardware functional unit of the structure and design features, and gives the design proposal and design methods of hardware and software.
Key words: asynchronous electromotor; vector control; current Regulator
1. Introduction
In recent years, with the rapid development of power electronics and the emergence of various new high-power semiconductor devices, such as IGBTs, power electronic devices are developing towards larger capacity, higher frequency, easier drive, lower loss, and intelligent modularity, which has spurred the rapid development of AC drives. AC drives have entered a stage of competition with DC drives and are showing a trend of replacing them. Compared to DC motors, AC motors are simpler in structure, cheaper, and more reliable in operation. The rapid development of AC motor control theory, microcomputer technology, and especially power electronics technology has greatly promoted the development of high-performance AC variable frequency speed control technology, which is widely used in various fields of production and daily life. Digital Signal Processors (DSPs) are a rapidly developing type of microprocessor with a special structure that integrates microelectronics, digital signal processing, and computer technology. The internal structure of a DSP adopts a Harvard architecture with separate program and data buses, has a dedicated hardware multiplier, and uses pipelined operation, so it can quickly implement various digital signal processing algorithms and is suitable for real-time control.
2. Asynchronous Motor Vector Control Principle
Vector control achieves decoupled control of motor torque and flux by transforming the motor's current, voltage, and flux linkage into a synchronous coordinate system, thus enabling rapid torque response and high-efficiency operation. This method controls both the amplitude and phase of the AC quantities simultaneously, hence the name vector control. Vector control mimics the mechanism of a DC motor's commutator maintaining perpendicularity between the flux and the armature magnetomotive force, allowing the control performance of AC motors to reach the level of DC motors.
Asynchronous motor vector control can be categorized into rotor field-oriented, stator field-oriented, and air-gap field-oriented vector control based on the orientation of the coordinate system used. While stator field-oriented vector control does not completely decouple flux and torque, decoupling can be achieved through feedforward compensation. Because it directly controls the stator flux, it maximizes the utilization of bus voltage and inverter current output, making it more suitable for control under weak magnetic field conditions. Air-gap field-oriented vector control utilizes the air-gap flux linkage, which can be directly measured by sensors, and the saturation level of the motor flux matches the air-gap flux, making it better suited for handling saturation effects. However, its flux and torque decoupling is also incomplete, making control slightly more complex. Rotor field-oriented vector control achieves the most complete decoupling of excitation current and torque current, and is therefore widely used in the vector control of asynchronous motors.
3. Hardware control circuit section
The control chip of this system uses TI's latest TMS320F2812 digital signal processor, a dedicated DSP chip for applications in motors and other motion control fields. The entire system adopts an AC-DC-AC converter circuit. The main circuit consists of an inverter circuit composed of a rectifier bridge, a filter circuit, and intelligent modules. The control circuit is based on the TMS320C2812 DSP chip, forming a fully functional control circuit. The current detection circuit consists of a Hall sensor circuit connected to the output of the inverter bridge, and the phase voltage detection circuit is also a Hall sensor circuit. The interface circuit mainly refers to the keyboard, display circuit, and memory expansion circuit. In designing the hardware circuit of this system, modular chips are used to reduce complex peripheral circuits. From the motor model, it can be derived and compensated using a feedforward method, while the disturbance of the speed regulator requires load torque observation for compensation; the performance indicators of the current regulator can be determined after the switching frequency and motor parameters are determined, while the speed regulator may have different requirements for different applications. Therefore, the current regulator of general products is designed to be built-in, while only the speed regulator can be manually adjusted by the user according to different application requirements.
4. Current regulation in vector control
In asynchronous motors, both electromagnetic torque and magnetic field are controlled by the stator current. Therefore, the control effect of the stator current directly affects the performance of the speed control system. Since current regulation is essentially the same as flux and torque regulation, these two regulation components are often combined and not strictly distinguished. In actual control systems, simplification or combination is often required depending on the specific requirements, and the task of the current regulator is to select the correct voltage vector.
The fundamental reason for choosing stator current as the control variable is that, during field orientation, the electromagnetic torque and flux, after decoupling, are directly controlled by the torque and flux components of the stator current. By controlling the stator current, torque and flux can be effectively controlled. Furthermore, the current regulator can be considered, in a sense, as having the characteristics of an ideal current source. The dynamic behavior of the motor's stator side due to resistance, inductance, or back EMF can be disregarded, thus reducing the order of the control system and the complexity of the control loop.
5. Control System Software Design
In this system, all control of the induction motor is implemented through software using the TMS320C2812 DSP chip. The software program consists of a main program and a timer underflow interrupt subroutine. The main program initializes the system, converts the externally input frequency adjustment ratio into angular frequency, and determines the amplitude of the reference voltage based on the U/f curve. The interrupt subroutine calculates the comparison values of the three compare registers for the next PWM cycle in each PWM cycle and stores them in the compare registers.
6. Conclusion
This paper takes an asynchronous motor as the controlled object and applies vector control technology to the speed control system of the asynchronous motor. Vector control is a significant improvement over the previous V/F control, especially with the addition of an encoder for speed feedback. It can achieve torque control and greatly improves low-speed performance. General-purpose frequency converters usually use SPWM control, which generates SPWM waves through real-time calculation. Real-time calculation places very high demands on the controller's computing speed, and DSP is the most cost-effective controller to meet this requirement. The designed controller has a fast response speed to motor speed, strong anti-disturbance capability, and achieves satisfactory control results.
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