From a technical point of view, the control mode of low voltage frequency converter also shows its technical school to a certain extent. Here we analyze the following control modes:
1, sinusoidal pulse width modulation
Sinusoidal pulse width modulation (SPWM) is characterized by simple control circuit structure, low cost, and good mechanical hardness. It can meet the requirements of smooth speed regulation of general transmission, and has been widely used in various fields of industry.
However, at low frequency, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, so that the maximum output torque decreases. In addition, its mechanical properties, after all, there is no direct current motor, and the static and dynamic torque capacity speed control performance is not satisfactory, and the system performance is not high, the control curve changes over load, torque response is slow, motor torque utilization rate is not high, low speed with the stator resistance and the existence of the inverter dead time effect and performance degradation, poor stability.
2. Voltage space vector (SVPWM)
Voltage space vector (SVPWM) is based on the premise of the overall generation effect of three-phase waveform and the purpose of approximating the ideal circular rotating magnetic field trajectory of the motor air gap. It generates three-phase modulation waveform at one time and is controlled by the way that the inner tangent polygon approximates the circle.
After practice, it is improved, that is, frequency compensation is introduced to eliminate the error of speed control; The amplitude of flux is estimated by feedback to eliminate the influence of stator resistance at low speed. The output voltage and current are closed loop to improve dynamic accuracy and stability.
Direct torque control (DTC) mode
Direct torque control (DTC) method The technology to a large extent to solve the shortcomings of the vector control, and with novel control ideas, simple and clear system structure, excellent dynamic and static performance has been rapidly developed. At present, this technology has been successfully applied to the high power AC drive of electric locomotive traction.
Direct torque control (DTC) directly analyzes the mathematical model of ac motor in the stator coordinate system and controls the flux and torque of the motor. It does not need to equivalent ac motor to DC motor, so it saves many complicated calculations in vector rotation transformation. It does not need to imitate the control of dc motor, nor does it need to simplify the mathematical model of AC motor for decoupling.
4, matrix interchange - interchange control mode
Matrix AC - AC control mode VVVF frequency conversion, vector control frequency conversion, direct torque control frequency conversion are one of the ac - DC - AC frequency conversion. Its common disadvantages are low input power factor, large harmonic current, dc circuit needs large energy storage capacitor, and regenerative energy can not be fed back to the grid, that is, it can not operate in the four quadrant. Therefore, matrix AC - AC frequency conversion came into being. Because matrix AC - AC frequency conversion eliminates the intermediate DC link, thus eliminating the large volume and expensive electrolytic capacitor.
Because matrix AC - AC frequency conversion eliminates the intermediate DC link, thus eliminating the large volume and expensive electrolytic capacitor. It can realize the power factor is L, the input current is sinusoidal and can run in four quadrants, the power density of the system is large. The technology is not mature at present, its essence is not indirect control of current, flux, but the torque directly as the controlled quantity to achieve.