Study notes on asynchronous motor vector control

Introduction: This article systematically summarizes asynchronous motor vector control and comprehensively analyzes the similarities and differences of various implementation methods. Through this summary, you can have a deeper understanding of FOC.

1. Introduction

According to statistics, about 60% of my country's electricity consumption is consumed by electric motors, and most of them are used to drive asynchronous motors. Asynchronous motors have simple structures, high reliability, and are easy to maintain. They can adapt to various complex environments and are currently widely used drive devices in industrial sites. With the development of technologies such as power electronic devices and digital processors, frequency conversion control technology has become the main technical means to improve the operating efficiency and transmission performance of motors.

In the past few decades, due to the improvement of the performance and efficiency of the AC speed control system, its application field and application range have become more and more extensive. High-performance asynchronous motor speed control system can not only meet the demand for power saving and improve energy efficiency, but also adapt to the process requirements of industrial production and improve the level of automation in my country. At present, frequency converters have penetrated into all walks of life, and their main application purpose is energy saving and process control needs. For energy-saving speed control occasions with general performance requirements such as fans and water pumps, a simple variable voltage and frequency conversion (VVVF) can meet the needs. However, many industrial applications have strict requirements on the control accuracy and response time of speed and torque, such as electric traction in the transportation industry, steel rolling systems in the metallurgical industry, and elevator drives in the construction industry. As modern industrial applications have higher and higher requirements for the performance and control accuracy of the speed control system, these requirements make it difficult for the frequency conversion control system to simply meet them by improving the performance of hardware equipment, and they need to be considered and solved from the perspective of control. Therefore, it is very necessary to study more advanced control solutions based on traditional control strategies.

The design of a high-performance speed control system can be regarded as solving an optimization problem. Usually, it may include the following key optimization objectives:

• Fast dynamic response and minimal steady-state tracking error (no beats);

• Excellent operating efficiency to save energy;

• Smaller current THD to meet relevant regulatory requirements; (SVPWM)

• Electromagnetic radiation and electromagnetic compatibility issues to meet regulatory requirements;

• Common-mode voltage suppression to improve system safety and operating life;

• Ability to meet the above requirements within the entire speed regulation range;

2. Working Principle of Vector Control

Vector control (FOC, Field Oriented Control) decomposes the stator current into excitation component and torque component under the premise of rotor magnetic field orientation, and then uses PI regulator to achieve independent regulation of the two, and finally uses pulse modulation (SVPWM, Space Vector Pulse Width Modulation) to synthesize the reference voltage vector. Vector control is generally known as field-oriented control technology internationally, that is, a control method that uses the direction of the motor's own magnetic field vector as the reference direction of the coordinate axis and the direction of coordinate transformation to control the magnitude and direction of the motor current.

FOC can achieve good dynamic and static performance and has been widely used in small and medium power occasions, but its performance is seriously dependent on the setting of the regulator parameters. Due to the many defects of the traditional linear PI regulator plus feedforward decoupling structure, especially when the system switching frequency is low or the motor speed is high, the system cannot even run stably. To solve this problem, many scholars at home and abroad use accurate complex vector mathematical models including system control delay to design complex vector current regulators, but there is still room for further improvement of regulator parameters based on continuous domain design. Considering the discrete characteristics of actual digital control systems, existing literature directly designs the current inner loop regulator in the discrete domain to ensure that the system has good stability margin and dynamic characteristics. In vector control, the inverter link is only regarded as a gain system. This structure of independent separation of the upper control algorithm and the underlying PWM makes the overall performance of the system have room for further optimization. This is because different PWM strategies correspond to different steady-state performance and inverter switching losses. Due to the mutual coupling between multiple control objectives of the system, it is difficult to significantly improve the performance of the system by simply optimizing the PWM level. Therefore, if the impact of different switching state combinations of the inverter on the overall system performance is considered in the upper-level control algorithm, the optimal control performance can be obtained in a larger feasible space.

The vector control oriented by the rotor magnetic field is the control method commonly used in the current high-performance variable frequency speed regulation system. The SVPWM (space vector pulse width modulation) technology is its core technology. Its goal is to make the controlled motor obtain a circular rotating magnetic field. The specific implementation method is to correctly control the on and off states of the power devices in the inverter, and the magnetic flux vector obtained thereby is used to track the ideal circular magnetic flux trajectory. Compared with PWM technology, the use of SVPWM technology can increase the DC voltage utilization rate by 15%, reduce the switching loss by 30%, significantly reduce the stator current harmonics, and is easier to implement digitally.

Figure 1 Vector control block diagram

2.1 Magnetic field orientation

2.1.1 Magnetic flux orientation method

Comprehensively comparing these three flux orientation methods, only the rotor flux orientation can achieve complete decoupling of the excitation component and torque component of the stator current without adding a decoupler. However, the observation of the rotor flux is greatly affected by the rotor parameters, and whether the accurate orientation of the rotor flux can be guaranteed becomes the difficulty in the application of this method. The stator flux orientation method can simplify the flux observer model and is suitable for a wide range of weak magnetic speed regulation, but it requires an additional decoupler for decoupling, and the control is more complicated. Since the air gap flux can reflect the degree of motor flux saturation, the air gap flux orientation method can be used to solve the motor magnetic saturation problem. Therefore, this paper adopts rotor magnetic field oriented vector control.

The d-axis coincides with the rotor flux vector, so:

2.1.2 Indirect and direct targeting

According to the rotor flux orientation method, it can be divided into indirect orientation and direct orientation.

(1) Indirect targeting

Figure 2 Block diagram of indirect rotor flux oriented vector control system

The indirect orientation vector control actually uses the flux open-loop control method, because the indirect magnetic field orientation does not use the flux model to actually calculate the amplitude and phase of the rotor flux, but uses the given value to calculate indirectly. When the given value of the rotor flux synchronous angular frequency deviates from the actual value, it will cause inaccurate flux orientation, causing a certain degree of coupling between the excitation component and the torque component of the stator current, reducing the dynamic performance of the system. Especially for higher-power motors, when in a weak magnetic operation state, if the slip angular frequency is not compensated in time, it may cause oscillation of torque and flux. At the same time, the indirect magnetic field orientation is greatly affected by the rotor time parameter. When this parameter fluctuates during operation, the problem of inaccurate flux orientation will also occur.

(2) Direct orientation

The direct vector control system needs to provide the actual phase of the rotor flux.

The amplitude of the rotor flux is also essential for the feedback and torque control. Since it is difficult to detect the flux directly, the flux observer is usually constructed by using easy-to-detect signals such as motor speed, stator current or stator voltage.

Figure 3 Block diagram of direct rotor flux oriented vector control system

At present, voltage-type inverters are widely used. The coupling of the dq-axis components of the stator voltage and current is not conducive to the regulation of the excitation and torque of the asynchronous motor. At this time, the coupling can be eliminated by superimposing a feedforward decoupling term on the given stator voltage. The structural block diagram of the decoupling compensation term is shown in Figure 4.

Figure 3 Decoupling compensation structure diagram

2.2 Learning of each module of vector control system

2.2.1 Mathematical model of asynchronous motor

1.1 Mathematical model of induction motor

With stator flux and rotor flux as state variables, the dynamic equation of the induction motor in a stationary coordinate system can be expressed as follows (the state variables can also be a combination of stator current and rotor flux, or electron current and stator flux):