Megmeet inverter - what are the control methods of the inverter?

Sinusoidal Pulse Width Modulation (SPWM) control mode

It is characterized by simple structure of control circuit, low cost, good hardness of mechanical characteristics, and can meet the smooth speed regulation requirements of general transmission, and has been widely used in various fields of industry. However, in this control mode, at low frequency, due to the low output voltage, the torque is significantly affected by the voltage drop of the stator resistance, so that the maximum output torque is reduced. In addition, its mechanical characteristics are not as hard as DC motors, dynamic torque ability 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, and the performance decreases due to the existence of stator resistance and inverter dead zone effect at low speed, and the stability deteriorates. 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 the purpose is to approximate the ideal circular rotating magnetic field trajectory of the motor air gap, and the three-phase modulation waveform is generated at one time, and the inscribed polygon is controlled by approximating the circle. After practical use, it has been improved, that is, the introduction of frequency compensation, which can eliminate the error of speed control; The flux amplitude is estimated through feedback, and 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 links in the control circuit, and no torque adjustment is introduced, so the system performance has not been fundamentally improved.

Vector control (VC) mode

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 into the alternating current Ia1Ib1 in the two-phase stationary coordinate system through the three-phase two-phase transformation, and then through the directional rotation transformation according to the rotor magnetic field, it is equivalent to the direct current Im1 and It1 under the synchronous rotating 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 imitates the control method of the DC motor, the control quantity of the DC motor is obtained, and the control of the asynchronous motor is realized through the corresponding coordinate inverse transformation. Its essence is to equivalence the AC motor to the DC motor, and the two components of speed and magnetic field are independently controlled. By controlling the rotor flux, and then decomposing the stator current, the two components of torque and magnetic field are obtained, and the orthogonal or decoupled control is realized through coordinate transformation. The vector control method is of epoch-making significance. However, in practical application, 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 control process of the equivalent DC motor is complex, which makes it difficult to achieve the ideal analysis results for the actual control effect.

Direct torque control (DTC) method

In 1985, Professor DePenbrock of the Ruhr University in Germany first proposed direct torque control frequency conversion technology. To a large extent, this technology solves the shortcomings of the above-mentioned vector control, and has been rapidly developed with novel control ideas, concise and clear system structure, and excellent dynamic and static performance. This technology has been successfully applied to the high-power AC drive for electric locomotive traction. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system, and controls the flux and torque of the motor. It does not need to equate an AC motor to a DC motor, thus eliminating many complex calculations in vector rotation transformations; 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 alternating control

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 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 fed 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, the large and expensive electrolytic capacitor is eliminated. 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 the technology is not yet mature, it still attracts many scholars for in-depth research. Its essence is not to indirectly control the current and flux equals, but to realize the torque directly as the controlled quantity. Here's how:

1. The stator flux is controlled by introducing a stator flux observer to realize the non-speed sensor mode;

2. Automatic identification (ID) relies on the accurate mathematical model of the motor to automatically identify the motor parameters;

3. Calculate the actual value corresponding to the stator impedance, mutual inductance, magnetic saturation factor, inertia, etc., and calculate the actual torque, stator flux, and rotor speed for real-time control;

4. Realize Band-Band control, generate PWM signal according to the Band-Band control of flux and torque, and control the switching state of the inverter.

The matrix AC-AC frequency conversion 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.

Megmeet inverter - what are the control methods of the inverter?

Megmeet inverter - what are the control methods of the inverter?

Sections viewed

Contact

working hours