Modern AC drive technology

Author: Chief Editor: Hu Chongyue
Publisher:
Publish Date: 2003-01-01
Features: This book aims to introduce modern AC speed regulation technology, focusing on practical application techniques while also considering future development trends. The book consists of 9 chapters. Chapter 1 introduces an overview of domestic and international AC speed regulation technology, development trends, AC speed regulation methods, and major application areas. Chapter 2 covers the working principles, main parameters, driving circuits, and protection techniques of various power electronic devices. Chapters 3 to 5 respectively introduce various AC-DC-AC inverters, AC-AC inverters, and the variable frequency speed regulation systems they form. Chapter 6 explains the basic principles of vector control and vector control techniques for asynchronous and synchronous motors. Chapters 7 to 9 respectively introduce the basic principles, characteristics, control methods, and design calculation examples of different types of speed regulation systems for various motors. This book serves as a reference for technical personnel engaged in electrical drive, electrical automation, and electromechanical integration research, development, production, operation, and maintenance. It is also suitable for teachers, graduate students, and senior students in relevant fields of higher education.
Excerpt: With the continuous emergence of new power electronic devices, variable frequency technology has experienced rapid development. After the square wave inverter composed of ordinary thyristors was replaced by a pulse width modulation (PWM) inverter made of fully controlled high-frequency switching devices, SPWM inverters and their dedicated chips became widely used. Flux-tracking PWM inverters use different switching modes to generate actual flux in the motor, approximating the stator flux trajectory—the ideal flux circle—by determining the inverter's switching state using the spatial voltage vector method to form PWM waveforms. Due to their simple control and ease of digitization, they are showing a trend of replacing traditional SPWM inverters. Current-tracking PWM inverters are voltage-source inverters of current control type, combining the advantages of both voltage and current-controlled inverters. Hysteretic current-tracking PWM inverters are particularly valued for their fast dynamic current response and ease of implementation. Currently, with the increase in device switching frequency and the optimization of control modes to eliminate specified harmonics, the output waveform of PWM inverters has become very close to a sine wave. However, on the grid side, although uncontrolled rectifiers have replaced phase-controlled rectifiers, making the fundamental power factor (displacement factor) close to 1, the large current harmonic components still result in a low total power factor. Eliminating grid harmonic pollution and improving the power factor have become unavoidable challenges in variable frequency technology. To address this, the research on PWM rectifier technology and the development of new unity power factor converters have attracted widespread attention abroad. The further increase in PWM inverter operating frequency is limited by switching losses, especially for high-power inverters, where the operating frequency is constrained by switching losses rather than the switching speed of the devices. A resonant inverter, a new type of soft-switching inverter developed in recent years, uses resonant technology to enable power switches to transition between states at zero voltage or zero current, resulting in nearly zero switching losses. This improves efficiency, reduces size and weight, lowers costs, and holds great promise as a variable frequency inverter.
As variable frequency technology continues to evolve rapidly, AC motor control technology has made breakthroughs. Compared to DC motors, controlling torque in AC motors, which are multivariable, strongly coupled, and nonlinear systems, is much more challenging. The vector control theory proposed in the early 1970s solved the torque control problem for AC motors by applying coordinate transformation to convert a three-phase system into a two-phase system and then using a synchronous rotation transformation oriented to the rotor magnetic field to achieve decoupling between the stator current excitation component and the torque component, thereby enabling separate control of the motor's flux and current. This allows a three-phase asynchronous motor to be controlled as if it were a DC motor, achieving the same excellent static and dynamic performance as DC speed regulation systems and ushering in an era where AC speed regulation competes with DC speed regulation. Direct torque control (DTC), proposed in the mid-1980s, is another torque control method that treats the motor and inverter as a single unit. It uses spatial voltage vector analysis to calculate flux and torque in the stator coordinate system and directly controls torque by determining the switching state of the flux-tracking PWM inverter. Therefore, it eliminates the need for stator current decoupling and avoids the complex calculations of vector transformation, resulting in a simple control structure that is easy to digitize. It is currently receiving attention from scholars worldwide. In the past decade, scholars have focused on sensorless control systems, using easily measurable physical quantities such as stator voltage and current to estimate speed and replace speed sensors. The key lies in obtaining speed information in real time while ensuring high control accuracy and meeting real-time control requirements. Speed estimation methods include calculating motor speed based on mathematical models, as well as more commonly used model reference adaptive control and extended Kalman filtering. Sensorless control technology eliminates the need for detection hardware and avoids the environmental adaptability, installation, and maintenance issues associated with sensors, improving system reliability and reducing costs, thus sparking widespread interest. The introduction of microprocessors into control systems has accelerated the transition from analog to digital control. Digital technology enables the implementation of complex vector control, greatly simplifying hardware, reducing costs, and improving control accuracy. The realization of self-diagnostic and self-tuning functions further enhances system reliability, saving significant labor and time while making operation and maintenance more convenient. The increasing computing speed of microprocessors and the expansion of memory capacity will further promote the replacement of analog control systems with digital ones, making digitization the direction of control technology.