Inverter control method: low-voltage general frequency conversion output voltage is 380-650V, output power is 0.75-400kW, operating frequency is 0-400Hz, and its main circuit adopts AC-DC-AC circuit. Its control methods have gone through the following five generations.
1U/f=C Sine Pulse Width Modulation (SPWM) Manipulation Method:
It is characterized by simple control circuit structure, low cost, good hardness of mechanical characteristics, can meet the requirements of smooth speed regulation of general transmission, and has been widely used in various fields of the industry. However, this control method is at low frequency, because the output voltage is low, and the torque is significantly affected by the voltage drop of the stator resistance, so that the output torque is reduced.
In addition, its mechanical characteristics are not as hard as DC motors, dynamic torque and static speed regulation functions are not perfect, and the system function is not high, the control curve will change with the change of load, the torque echo is slow, the motor torque utilization rate is not high, and the function decreases due to the existence of stator resistance and inverter dead zone effect at low speed, and the stability deteriorates. Therefore, people have studied vector control frequency conversion speed regulation.
Voltage Space Vector (SVPWM) Manipulation Method:
It is based on the condition of the whole generation of the three-phase waveform, with the intention of imminenting the ideal circular rotating magnetic field orbit of the motor air gap, and generating the three-phase modulation waveform at one time, and controlling it by the method of inscribed polygon imminent circle.
After practical application, it has been improved, that is, the introduction of frequency compensation, which can eliminate the error of speed control; After the reaction to estimate the flux amplitude, the influence of stator resistance at low speed is eliminated. The output voltage and current are closed to improve the accuracy and stability of the dynamics. However, there are many control circuit links, and there is no torque adjustment, so the system function has not been fundamentally improved.
Vector Manipulation (VC) Method:
The method of vector control frequency conversion speed regulation is to convert the stator current Ia, Ib, and Ic of the asynchronous motor in the three-phase coordinate system through three-phase - two-phase change, which is equivalent to the communication current Ia1Ib1 under the two-phase stop coordinate system, and then rotate and change according to the rotor magnetic field, which is equivalent to the DC current Im1 and It1 under the synchronous rotation coordinate system (Im1 is equivalent to the excitation current of the DC motor; IT1 is equivalent to the armature current proportional to the torque), and then imitate the control method of the DC motor, obtain the control quantity of the DC motor, and complete the control of the asynchronous motor after the corresponding coordinate reversal.
Its essence is to equivalence the communication motor to the DC motor, and the speed and magnetic field weight are controlled independently. After manipulating the rotor flux, and then differentiating the stator current to obtain the two weights of torque and magnetic field, the coordinates are changed to complete the orthogonal or decoupling control. The vector manipulation method is of epoch-making significance. However, in practical application, because the rotor flux is difficult to observe accurately, the characteristics of the system are greatly affected by the motor parameters, and the vector rotation change used in the control process of the equivalent DC motor is more messy, which makes it difficult to achieve the ideal analysis results of the control function in practice.
Direct Torque Manipulation (DTC) Method:
In 1985, Professor DePenbrock of the Ruhr University in Germany first proposed the direct torque control frequency conversion technology. This skill solves the shortcomings of the above-mentioned vector manipulation to a large extent, and has been rapidly developed with novel manipulation ideas, concise and clear architecture, and excellent dynamic and static functions.
Now, this skill has been successfully applied to the high-power communication transmission of electric locomotive traction. Direct torque manipulationDirectly analyze the mathematical model of the communication motor under the stator coordinate system, and control the flux and torque of the motor. It does not require the communication motor to be equivalent to a DC motor, so it eliminates a lot of messy calculations in vector rotation reversal; It does not require the operation of a DC motor to mimic it, nor does it require a mathematical model that simplifies the communication motor for decoupling.
Matrix inter-traffic control method:
VVVF frequency conversion, vector control frequency conversion, and direct torque control frequency conversion are all one of the AC-DC-AC frequency conversion. One of the disadvantages is that the input power factor is low, the harmonic current is large, the DC circuit requires a large energy storage capacitor, and the regenerative energy cannot be reflected back to the power grid, that is, it cannot carry out four-quadrant operation.
For this reason, matrix alternating frequency-alternating frequency came into being. Because the matrix AC-AC frequency conversion eliminates the intermediate DC link, thus eliminating the large and expensive electrolytic capacitors. It can complete the power factor of L, the input current is sinusoidal and can operate in four quadrants, and the power density of the system is large. Although the skill is not sophisticated now, it still attracts many scholars to study it in depth. Its essence is not to indirectly manipulate the current and flux equals, but to directly use the torque as the quantity to be manipulated.
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