Praveen K. Jain
With the power of electronic technology and large-scale integrated circuits, microprocessor-controlled technology, power semiconductor power conversion technology has also been developing rapidly. The 20th century, the second half of the 60s began, power semiconductor devices from the SCR (thyristor general), GTO (gate GTO), BJT (bipolar transistor), MOSFET (metal oxide silicon field effect transistor) , SIT (Static Induction Transistor), SITH (SITH), MGT (MOS transistor to control), MCT (MOS controlled thyristor) to IGBT (Insulated Gate Bipolar Transistor), HVIGBT (high voltage IGBT). Updated every time the device for power conversion technology to inject new vitality into the development. As weak links and strong ties of power, power conversion technology to provide electric power to control the flow and change shape a powerful means of power, the output best suited to their load voltage and current in order to meet the technical requirements to meet the industrial and energy savings. Electric drive power conversion technology is the most important application areas. Electrical transmission range of applications ranging from sophisticated robot, high-precision position control, adjustable up to a large flow pumps, fan speed control drives, power range from a few kilowatts to several megawatts. Power electronic converter and the motor input power as the interface between the equipment, the control motor speed or rotor position to meet the needs of the motor driven by the needs of machinery and equipment. With the theory of AC motor speed control and speed control devices breakthrough (mainly the inverter) to improve performance, the motor speed from the DC generator - motor group speed, thyristor controlled rectifier DC speed regulator the progressive development of VVVF the AC motor and control technology and control as a means of continuous improvement, and by VVVF VVVF (Variable Voltage Variable Frequency) control of the PWM (Pulse Width Modulation) Frequency Control the development of vector control (Vector or Field -Oriented Control), Direct Torque Control (Direct Torque and Flux Control - DTC) Frequency, frequency converter to increase the dynamic and static properties, making the performance of AC motor VVVF greatly enhanced. In high-performance VVVF control system, the speed (position) part of the closed-loop control is essential, typically coaxial with the motor installed mechanical rotor speed (position) sensors, such as the photoelectric encoder, rotary transformer, etc. However, these mechanical rotor speed (position) sensors have mechanical installation, the use of the environment, many applications such as cable connection constraints, the reliability is very much affected. In order to overcome the mechanical rotor speed (position) sensor installed on the various defects in the hardware system to simplify and reduce equipment failure rates, in the vector control, direct torque control based on the Frequency and the development of the non-speed (position) sensor Frequency Control. In recent years, the study field of AC drive has become a new hot-spot issues.
The reason why the development of AC drive system so rapidly, and a number of breakthroughs in key technologies related. They are power semiconductor devices (including the semi-controlled and controlled the whole) of the manufacturing technology, power electronic circuits based on power transformation technology, AC motor control technology and micro-computers and large-scale integrated circuit-based all-digital control technology. To further improve the performance of AC drive system, both at home and abroad of the study is being carried out around the following areas:
1. A new power semiconductor devices and pulse width modulation (PWM) technology
Power semiconductor devices advances, especially the new GTO devices, such as BJT (bipolar transistor), MOSFET (metal oxide silicon field effect transistor), IGBT (Insulated Gate Bipolar Transistor) is a practical, making switching high frequency of PWM technologies. At present, positive high-voltage power semiconductor devices, high power, high-frequency-based, integrated and intelligent direction. Typical power electronic frequency converter voltage type cross - straight - cycloconverter and current-based pay - Direct - cycloconverter and settlement - the three cycloconverter. Current-mode delivery - direct - the middle of Cycloconverter large DC link inductor for energy storage components, reactive power inductor to the buffer will be large, it merits a prominent when the motor is braking (power) state, the only change network side controllable rectifier to the output voltage polarity so that feedback to the DC side of the renewable energy back to the easy exchange of power grids, with a speed control system consisting of four-quadrant operation capability, can be used to the frequent acceleration and deceleration on the dynamic performance requirements stand-alone applications in large-capacity fans, pumps are also energy-saving speed control applications. Voltage pay - straight - the middle of Cycloconverter large DC link capacitors for energy storage components, reactive power to the buffer will be large capacitance. The motor load, the voltage-frequency converter of a voltage source, does not exceed the capacity limit in the circumstances, it can be more than one motor-driven parallel operation. PWM inverter voltage electricity transmission system in the small and medium-sized power of a dominant position. However, the shortcomings of voltage-frequency converter is in the brake motor (power) state, the feedback to the DC side of the renewable energy it is difficult to exchange back to the grid, in order to achieve this part of the energy of the feedback network side can not be non-controllable rectifier diodes or general controllable rectifier, must be reversible converter, such as the use of two anti-parallel rectifier controlled using PWM control of the Self-converter ( "cut-controlled rectifier" or "PWM rectifier"). Network side converter using PWM inverter control is called "dual PWM control inverter," this type of high-performance renewable energy feedback DC converter with continuously adjustable output voltage, input current (network-side current) waveform is sinusoidal fundamental , to maintain power factor of 1 and two-way flow of energy can be characterized by, on behalf of the technical development of a new trend, but the cost of limiting the speed of its development. The usual pay - cycloconverter input harmonic currents have large, low input power factor of the shortcomings can only be used for low-speed (low-frequency) large-capacity transmission speed. To this end, the matrix delivery - came into being cycloconverter. Matrix Settlement - cycloconverter power density, and there is no intermediate DC link, eliminating the need for bulky and expensive energy storage components, it is to achieve input power factor is 1, the input current sinusoidal and four quadrant operation has opened up new avenues .
Voltage type PWM inverter with high-performance AC drive system in the application of the increasingly widespread, PWM technology more and more in depth. PWM power semiconductor devices using high-frequency opening and turn-off, the DC voltage into law by certain changes in the width of the voltage pulse sequence in order to achieve the inverter, transformer and effectively control and eliminate harmonics. PWM techniques can be divided into three categories: sinusoidal PWM, optimized PWM and random PWM. Sinusoidal PWM including voltage, current and magnetic flux of the sinusoidal PWM for the objectives of the program. General sinusoidal PWM switching frequency of power devices with improved performance will be very good, so in the small and medium-power AC drive system has been widely used. However, large-capacity power conversion devices, the high switching frequency will lead to large switching losses, and high-power devices such as GTO's switching frequency can not be done is still very high, in this case, the optimized PWM technique is in line with the needs of devices. Specific harmonic elimination (Selected Harmonic Elimination PWM - SHE PWM), the efficiency of the optimal PWM and PWM minimum torque ripple are optimized PWM technology areas. Ordinary PWM inverter output current often contains a large power device switching frequency and associated harmonic components caused by harmonic current role in the motor torque ripple, the stator will have a vibration motor electromagnetic noise issue, and its intensity the scope and frequency of torque pulsation depends on the size and frequency of alternating. If the electromagnetic noise in the sensitive human ear frequency range, it will cause damage to the hearing. IF the larger number of harmonic current is also easily lead to motor mechanical resonance, resulting in reduced stability of the system. To solve this problem, a method is to increase the switching frequency power devices, but this method makes switching losses will increase; Another method is to randomly change the power devices on-location and switching frequency, so that the output voltage of inverter harmonic components uniformly distributed in a wide range of frequency bands, thereby inhibiting certain harmonic components of larger amplitude to achieve the suppression of electromagnetic noise and the purpose of mechanical resonance, which is random PWM technique.
2. The application of vector control technology, the Direct Torque Control technology and modern control theory
AC drive system of the AC motor is a multivariable, nonlinear, strong coupling, time-varying object, VVVF control is a steady-state equation from the study of motor control characteristics, the effect of dynamic control is not ideal. 70 In the early 20th century, proposed to transform the vector to study the dynamics of AC motor control process, not only to control the amplitude of the variables, but also to control its phase, in order to achieve the exchange of motor flux and torque decoupling to a high-performance AC drive system step by step practical use. High dynamic performance of current vector control inverter has been successfully used in rolling mill main drive, electric locomotive traction systems and CNC machine tools. In addition, in order to solve the system and control the complexity of the conflict between the precision and made a number of new control methods, such as direct torque control, voltage-oriented control. In particular, with the microprocessor-controlled technology, modern control theory in a variety of control methods have been applied, such as the quadratic performance index of optimal control and adjust the double-analog control system can improve the dynamic performance of sliding mode (Sliding mode) variable structure control system to enhance robustness, state observer and Kalman filter can not be measured can be the state information, adaptive control can improve the overall performance of the system. In addition, the intelligent control techniques such as fuzzy control, neural networks also began to be used to control AC Drive transmission in order to improve control accuracy and robustness.
3. The wider use of microelectronics technology
With the development of microelectronic technology, digital control processor computing capacity and reliability have been greatly improved, which makes all-digital control system to replace the previous analog control system possible. AC drive system is currently suitable for single-chip microprocessors have, digital signal processors (Digital Signal Processor - DSP), ASIC (Application Specific Integrated Circuit - ASIC) and so on. Among them, the form of high-performance computer architecture the use of ultra-high-speed buffer memory, multi-bus architecture, pipeline structure and multi-processor structure. The core of real-time control algorithm to complete, power-driven devices, as well as signals arising from the monitoring system, the protection functions can be achieve through the microprocessor for the control of AC drive system to provide a great deal of flexibility, and the controller of a high degree of standardization of hardware circuit low cost, makes the microprocessor component of the all-digital control system to achieve a high ratio of performance to price.
4. To develop new motor and non-mechanical sensor technology
AC drive system of the development of the motor body is also put forward higher requirements. Motor design and modeling study of a new content, such as three-dimensional eddy current field calculation, consider the frequency of rotor movement and the external power supply system of the joint solution of equations, damping motor windings and the rational design of the cage, such as fault detection. In order to more detailed analysis of motor internal processes, such as winding or short-circuit issues such as broken rotor bars, multi-loop theory came into being. With the era of the 20th century, 80 permanent magnet materials, especially the development of NdFeB permanent magnet, permanent magnet synchronous motor (Permanent-Magnet Synchronous Motor - PMSM) research increasingly popular and in-depth, because without this kind of motor excitation current, operating efficiency , power factor and high power density, which was in the AC drive system in an increasingly wide range of applications. In addition, the switching variable reluctance theory SRM (Switched Reluctance Motor - SRM) the rapid development of Switched Reluctance Motor stepping motor with the response similar to the rotor position in Canada after the closed-loop testing can effectively solve the out-of-step problems, can easily start, speed or control, the torque characteristics of the fine is particularly suited to high static torque applications.
In the exchange of high-performance speed control drive system, the rotor speed (position) are often closed-loop control is essential. In order to achieve speed (position) feedback control to be used or rotary optical encoder, such as transformers and electric motors installed coaxial machine speed (position) sensors to achieve rotor speed and position detection. However, there are mechanical sensor installation, and maintenance of cables and so on, reducing the reliability of the system. In this regard, many scholars to carry out the speed (position) sensor control technology, namely the use of detected power of the motor end round (such as electrical voltage, current), to estimate the rotor speed, position, the motor can also be observed within The flux, torque, etc., which constitute a non-speed (position) sensor high-performance AC drive system. The technology without the motor rotor and the plane seat to install mechanical sensors, with lower costs and maintenance costs, free from restrictions on the use of the environment, etc., will become the exchange of electric drive technology development inevitable trend.