The concept of braking
This refers to the flow of electrical energy from the motor side to the inverter side (or power supply side), at which point the motor speed is higher than the synchronous speed. The energy of a load is divided into kinetic energy and potential energy . Kinetic energy (determined by speed and weight) accumulates as an object moves; when the kinetic energy decreases to zero, the object comes to a stop. Mechanical braking devices convert the object's kinetic energy into frictional energy, which is then dissipated. For inverters, if the output frequency decreases, the motor speed will decrease accordingly, resulting in a braking process. The power generated by braking is returned to the inverter side, where it can be dissipated by heating the resistors. When used for lifting loads , energy (potential energy) also returns to the inverter (or power supply) side during descent for braking. This method is called "regenerative braking," and it can be applied to inverter braking. If the power generated during deceleration is not dissipated by heat but instead returned to the inverter's power supply side, it is called "power return regeneration." In practice, this application requires an "energy feedback unit" option.
How to improve braking performance?
To dissipate regenerative power through heat dissipation, a braking resistor needs to be installed on the inverter side. Improving braking capability cannot be achieved by simply increasing the inverter's capacity. Please select options such as a "braking resistor," "braking unit," or "power regeneration converter" to improve the inverter's braking capacity. AC variable frequency speed control technology is a comprehensive technology integrating strong and weak currents and electromechanical systems. It handles both the conversion of massive electrical energy (rectification and inversion) and the collection, transformation, and transmission of information. Therefore, its common technologies are inevitably divided into two main parts: power and control. The former addresses technical issues related to high voltage and high current, as well as the application of new power electronic devices. The latter addresses the hardware and software development issues (currently mainly based on fully digital control technology) of control strategies and intelligent control strategies (based on modern control theory).
Its main development directions are as follows.
(1) Achieving high-level control. Control strategies based on motor and mechanical models include vector control, magnetic field control, direct torque control, and mechanical torsional vibration compensation; control strategies based on modern theories include sliding mode variable structure technology, model reference adaptive technology, nonlinear decoupling using differential geometry theory, robust observer, optimal control technology under certain indexes, and inverse Nyquist array design method; control strategies based on intelligent control concepts include fuzzy control, neural networks, expert systems, and various self-optimization and self-diagnosis technologies.
(2) Developing converters for clean energy. A clean energy converter is one with a power factor of 1 and as low a harmonic component as possible on both the grid and load sides to reduce pollution to the power grid and torque ripple in the motor. For small and medium-capacity converters, PWM control with increased switching frequency is effective. For large-capacity converters, the circuit structure and control method can be modified at conventional switching frequencies to achieve clean energy conversion.
(3) Reducing device size. Compact converters require a high degree of integration of power and control components, including intelligent power modules, compact optocouplers, high-frequency switching power supplies, and small-volume transformers, reactors, and capacitors made with new electrical materials. Changes in power device cooling methods (such as water cooling, evaporative cooling, and heat pipes) are also effective in reducing device size.
(4) High-speed digital control. Digital control modules based on 32-bit high-speed microprocessors have sufficient capability to implement various control algorithms. The introduction of the Windows operating system allows for free design, and graphical programming control technology has also made great progress.
(5) Simulation and Computer-Aided Design (CAD) Technology. The introduction of motor simulators, load simulators, and various CAD software has provided strong support for the design and testing of frequency converters.
The main research and development projects are as follows.
(1) A transmission device powered by a high-power AC-AC frequency converter with digital control.
(2) The application of high-power load converter current type inverter power supply transmission equipment in pumped storage power stations, large wind turbines and pumps.
(3) Promotion and application of voltage-type GTO inverters on railway locomotives.
(4) Expand the functions and improve the performance of drive equipment powered by voltage-type IGBT and IGCT inverters. For example, it can operate in four quadrants, with self-measurement and self-setting of electrode parameters, automatic compensation for changes in motor parameters, and sensorless vector control and direct torque control.
(5) Research on energy-saving speed regulation of high-voltage motors for fans and pumps. As is well known, fans and pumps save a lot of electricity after switching to speed regulation transmission. In particular, high-voltage motors have a large capacity and the energy-saving effect is more significant. Researching economical and reasonable speed regulation methods for high-voltage motors is a major issue at present.
The main research contents and key technologies are as follows.
(1) High voltage and high current technology: dynamic and static voltage equalization technology (3-inch thyristors in series in 6kV and 10kV circuits, with a static and dynamic voltage equalization coefficient greater than 0.9 ); current equalization technology, current equalization technology of high-power thyristors in parallel, with a current equalization coefficient greater than 0.85 ); surge absorption technology (in 10kV and 6kV circuits); optical control and electromagnetic triggering technology (electric/optical, optical/electrical conversion technology); heat conduction and heat dissipation technology (mainly solving technologies with good heat conduction and heat dissipation and large current output, such as heat pipe heat dissipation technology); high voltage and high current system protection technology (anti-high current electromagnetic force structure, insulation design); equivalent load simulation technology.
(2) Application technologies of new power electronic devices: turn-off drive technology; dual PWM inverter technology; cyclic converter/current-type AC-DC-AC.
(CC/CSI0) converter technology (12-pulse frequency conversion technology); synchronous machine AC excitation variable speed operation technology; soft-switching PWM converter technology.
(3) Fully digital automatic control technology: parameter self-setting technology; process self-optimization technology; fault self-diagnosis technology; object self-identification technology.
(4) Modern control technologies: multivariable decoupling control technology; vector control and direct torque control technology; adaptive technology.
