With the continuous maturity of low-voltage frequency converter technology, the application of low-voltage frequency conversion determines its different classifications. From a technical point of view, the control method of the low-voltage inverter also shows its technical genre to a certain extent. Here, we MICNO have analyzed the following control methods:
Sinusoidal pulse width modulation is characterized by simple control circuit structure, low cost, and good mechanical properties and hardness, which can meet the smooth speed regulation requirements of general transmission, and has been widely used in various fields of industry. However, when this control method is at low frequency, due to the low output voltage, the torque is significantly affected by the voltage drop of the stator resistance, which reduces the maximum output torque. In addition, its mechanical characteristics are not as hard as DC motors after all, and its dynamic torque capability and static speed regulation performance are not satisfactory, and the performance of the sinusoidal pulse width modulation system of the low-voltage inverter is not high, and the control curve will change with the change of the load. The torque response is slow, the torque utilization rate of the motor is not high, the performance is degraded due to the existence of the stator resistance and the dead zone effect of the inverter at low speed, and the stability is deteriorated. Therefore, people have developed vector control frequency conversion speed regulation. But this kind of control method is also one of the control methods commonly used by frequency converters at present. It is also one of the control methods most used by domestic brands.
The voltage space vector is based on the premise of the overall generation effect of the three-phase waveform, and aims at approaching the ideal circular rotating magnetic field trajectory of the motor air gap. It generates a three-phase modulation waveform at one time, and controls it in a way that an inscribed polygon approximates a circle. The voltage space vector control of low-voltage inverters has been improved after practical use, that is, the introduction of frequency compensation can eliminate the error of speed control; the amplitude of flux linkage can be estimated through feedback to eliminate the influence of stator resistance at low speed; the output voltage and current closed-loop , to improve the dynamic accuracy and stability.
Direct torque control method: This technology solves the above-mentioned shortcomings of vector control to a large extent, and has developed rapidly with novel control ideas, simple and clear system structure, and excellent dynamic and static performance. At present, this technology has been successfully applied to the high-power AC transmission of electric locomotive traction. The direct torque control of the low-voltage frequency converter directly analyzes the mathematical model of the AC motor in the stator coordinate system, and controls the flux linkage and torque of the motor. It does not need to equate the AC motor to a DC motor, thus saving many complicated calculations in the vector rotation transformation; it does not need to simulate the control of the DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling.
Low-voltage frequency converter matrix AC control mode frequency conversion, vector control frequency conversion, direct torque control frequency conversion are all AC-DC-AC frequency conversion. Their common disadvantages are low input power factor, large harmonic current, large energy storage capacitors for DC circuits, and regenerative energy cannot be fed back to the grid, that is, four-quadrant operation cannot be performed. For this reason, matrix AC-AC frequency conversion came into being. Because the matrix AC-AC frequency conversion eliminates the intermediate DC link, the bulky and expensive electrolytic capacitors are omitted. It can realize the power factor is l, the input current is sinusoidal and can run in four quadrants, and the power density of the system is high. The technology is not yet mature, and its essence is not to indirectly control the current, flux linkage, etc., but to realize the torque directly as the controlled quantity.