The frequency converter is a power converter in the motion control system. Today's motion control system includes many technical fields, and the general development trend is: communication drive, high-frequency power changer, digital, intelligent, and networked control.
VFDs are power converters in motion control systems. Today's motion control system includes many technical fields, and the general development trend is: communication drive, high-frequency power changer, digital, intelligent, and networked control. Therefore, VFD, as an important power conversion component of the system, has been rapidly developed by providing controllable high-performance variable voltage and variable frequency communication power supply.
With the application of new power electronic devices and high-performance microprocessors, as well as the development of control technology, VFDs are becoming more and more cost-effective and smaller. However, manufacturers are still making new efforts to improve the reliability of VFDs and complete their further miniaturization, lightweighting, high functionality, multi-function and pollution-free. The function of VFD depends on the influence of harmonics of its output communication voltage on the motor. second, it depends on the harmonic pollution and input power factor of the power grid; Thirdly, it depends on the energy loss (i.e. power) of the body. Here, we take a large number and wide range of AC-DC-AC VFD as an example to illustrate its development trend:
self-switching, modularization, integration and intelligence of the main circuit power switching components; As the switching frequency increases, the switching losses are further reduced.
VFD main circuit topology. For grid-side converters with VFD, 6-pulse converters are usually used for low-voltage and small-capacity equipment, and converters with more than 12 pulses are used for medium-voltage and large-capacity equipment. Load-side converters typically use two-level bridge inverters for low-voltage and small-capacity equipment, while multi-level inverters are used for medium-voltage and large-capacity equipment. Regarding the four-quadrant rotation, in order to complete the feedback of VFD regenerative energy to the grid and save energy, the grid-side converter should be reversible, and at the same time, it presents a dual PWM VFD with bidirectional power flow, and the proper control of the grid-side converter can make the input current close to the sine wave and reduce the pollution to the grid.
Pulse Width Modulation (VFD) can be controlled by sine wave pulse width modulation, PWM control to eliminate specified harmonics, current pin control, and voltage space vector control (flux pin control).
The progress of the frequency conversion control method of communication motor is mainly reflected in the development from scalar control to vector control and direct torque control with high dynamic function, as well as the development of vector control and direct torque control system without speed sensor.
The advancement of microprocessors has made digital manipulation the direction of modern manipulators. The motion control system is a fast system, especially to communicate the high-function control needs of the motor, store various data in real time and quickly process a lot of information. In recent years, major foreign companies have launched DSP (digital signal processor) as the core, with peripheral functional circuits required for motor control. The DSP monolithic motor manipulator is integrated on a single chip, which greatly reduces the price, reduces the size, is compact, easy to use, and improves reliability. Compared with the general single-chip microcomputer, the digital processing capacity of DSP is increased by 10-15 times, which can ensure the superior control function of the system. Digital control simplifies the hardware, and the flexible control algorithm makes the control sensitive, completes the messy control rules, makes the application of modern control theory in the motion control system practical, easy to connect with the upper system for data transmission, facilitates fault diagnosis, strengthens maintenance and monitoring functions, and makes the system intelligent (for example, some VFDs have self-regulating functions).
Communication synchronous motors have become a new star in the field of communication speed regulation, especially permanent magnet synchronous motors. The motor adopts brushless structure, with high power factor and high power, and the rotor speed is strictly synchronized with the power frequency. There are two types of synchronous motor frequency control systems: independent frequency control and active frequency control. The principle of an active frequency-controlled synchronous motor is very similar to that of a DC motor. The power electronics converter replaces the mechanical commutator of the DC motor. For example, when using an AC-DC-AC voltage VFD, it is called a "DC commutatorless motor" or a "brushless DC motor". The traditional speed control system of active variable frequency synchronous motor has a rotor azimuth sensor, but the system without a rotor azimuth sensor is under development. Vector control can also be used for independent frequency conversion of synchronous motors, and its vector control based on rotor magnetic field orientation is simpler than that of asynchronous motors.
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