The low-voltage universal frequency conversion output voltage is 380~650V, the output power is 0.75~400kW, the working frequency is 0~400Hz, and its main circuit adopts AC-DC-AC circuit. Its control method has gone through the following four generations.
Sine pulse width modulation (SPWM) control mode
It is characterized by simple control circuit structure, low cost, and good mechanical hardness, which can meet the smooth speed regulation requirements of general transmission and has been widely used in various fields of the industry. However, at low frequencies, due to the low output voltage, the torque is significantly affected by the voltage drop of the stator resistance, so that the maximum torque of the output is reduced. In addition, its mechanical characteristics are not as hard as DC motor after all, dynamic torque capacity and static speed regulation performance are not satisfactory, and the system performance is not high, the control curve will change with the change of load, the torque response is slow, the motor torque utilization rate is not high, the performance is reduced due to the existence of stator resistance and inverter dead zone effect at low speed, and the stability becomes poor. Therefore, people have developed vector control frequency conversion speed regulation.
Voltage Space Vector (SVPWM) control mode
It is based on the premise of the overall generation effect of the three-phase waveform, and aims to approximate the ideal circular rotating magnetic field trajectory of the motor air gap, generate a three-phase modulated waveform at one time, and control it by approaching the circle by an inscribed polygon. After practical use, it has been improved, that is, frequency compensation is introduced, which can eliminate the error of speed control; The magnitude of the flux is estimated by feedback to eliminate the influence of stator resistance at low speeds. The output voltage and current are closed to improve dynamic accuracy and stability. However, there are many control circuit links, and no torque adjustment is introduced, so the system performance has not been fundamentally improved.
Vector control (VC) mode
The practice of vector control frequency conversion speed regulation is to convert the stator current Ia, Ib, Ic of the asynchronous motor in the three-phase coordinate system, through the three-phase-two-phase transformation, equivalent to the alternating current Ia1Ib1 in the two-phase stationary coordinate system, and then through the rotor magnetic field-oriented rotation transformation, equivalent to the DC current Im1, It1 in 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, find the control quantity of the DC motor, and realize the control of the asynchronous motor after the corresponding coordinate inverse transformation. Its essence is to equivalentize the AC motor as a DC motor, and independently control the two components of speed and magnetic field. By controlling the rotor flux linkage, and then decomposing the stator current, the two components of torque and magnetic field are obtained, and the quadrature or decoupling control is realized by coordinate transformation. The proposal of vector control method is of epoch-making significance. However, in practical applications, because the rotor flux is difficult to observe accurately, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is more complicated, which makes it difficult for the actual control effect to achieve the ideal analysis results.
Direct torque control (DTC) method
In 1985, Professor DePenbrock of the Ruhr University in Germany first proposed direct torque control frequency conversion technology. This technology solves the shortcomings of the above vector control to a large extent, and has developed rapidly with novel control ideas, concise and clear system structure, and excellent dynamic and static performance. This technology has been successfully applied to high-power AC drives traction by electric locomotives. Direct torque control directly analyzes the mathematical model of AC motor under the stator coordinate system, and controls the flux and torque of the motor. It does not require the AC motor to be equivalent to a DC motor, thus eliminating many complex calculations in vector rotation transformation; It does not need to mimic the control of a DC motor, nor does it need to simplify the mathematical model of an AC motor for decoupling.
Matrix AC-AC control mode
VVVF frequency conversion, vector control frequency conversion, and direct torque control frequency conversion are all one of the AC-DC-AC frequency conversion. Its common disadvantages are low input power factor, large harmonic current, large energy storage capacitance required for DC circuits, and regenerative energy cannot be fed back to the grid, that is, four-quadrant operation cannot be carried out. For this reason, the matrix alternating frequency came into being. Because the matrix AC-AC frequency conversion eliminates the intermediate DC link, thereby eliminating the bulky and expensive electrolytic capacitors. It can achieve a power factor of l, an input current of sinusoidal and four-quadrant operation, and a high power density of the system. Although this technology is not yet mature, it still attracts many scholars to study it in depth. Its essence is not indirect control of current, flux linkage and equal amounts, but the torque is directly realized as the controlled quantity. Here's how:
1. Control the stator flux to introduce the stator flux observer to realize the speedless sensor;
2. Automatic identification (ID) relies on accurate motor mathematical models to automatically identify motor parameters;
3. Calculate the actual value corresponding to the stator impedance, mutual inductance, magnetic saturation factor, inertia, etc., calculate the actual torque, stator flux and rotor speed for real-time control;
4. Realize Band-Band control to generate PWM signals according to the Band-Band control of flux and torque to control the switching state of the inverter.
The matrix type AC-AC frequency has fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), and high torque accuracy (<+3%); At the same time, it also has high starting torque and high torque accuracy, especially at low speed (including 0 speed), it can output 150%~200% torque.