The five most commonly used methods for frequency converter to control motor are as follows:
The low-voltage universal frequency conversion output voltage is 380~650V, the output power is 0.75~400kW, and the working frequency is 0~400Hz. Its main circuit adopts AC-DC-AC circuit. Its control mode has experienced the following four generations.
1. Sinusoidal pulse width modulation (SPWM) control mode with 1U/f=C
It is characterized by simple control circuit structure, low cost, good mechanical property hardness, and can meet the requirements of smooth speed regulation of general transmission. It has been widely used in various fields of the industry.
However, in low frequency, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, which makes the maximum output torque reduced.
In addition, its mechanical characteristics are not as hard as DC motor after all, and its dynamic torque capability and static speed regulation performance are not satisfactory. Moreover, its system performance is not high, and its control curve will change with the change of load. Its torque response is slow, and its motor torque utilization is not high. At low speed, its performance and stability will decline due to the existence of stator resistance and inverter dead time effect. Therefore, people have developed vector control variable frequency speed regulation.
2. Voltage space vector (SVPWM) control mode
On the premise of the overall generation effect of three-phase waveforms, it aims to approximate the ideal circular rotating magnetic field trajectory of the motor air gap, generates three-phase modulation waveforms at a time, and controls them in the way that the inscribed polygon approximates the circle.
After practice, it has been improved, that is, frequency compensation is introduced to eliminate the error of speed control; The flux amplitude is estimated by feedback to eliminate the influence of stator resistance at low speed; The output voltage and current are closed loop to improve the dynamic accuracy and stability. However, there are many control circuits and no torque regulation is introduced, so the system performance has not been fundamentally improved.
3. Vector control (VC) mode
The method of vector controlled variable frequency speed regulation is to make the stator currents Ia, Ib, Ic of asynchronous motor in three-phase coordinate system equivalent to the AC current Ia1Ib1 in two-phase static coordinate system through three-phase two-phase transformation, and then equivalent to the DC current Im1, It1 (Im1 is equivalent to the excitation current of DC motor; It1 is equivalent to the armature current proportional to the torque) in synchronous rotating coordinate system through directional rotation transformation according to the rotor magnetic field, Then the control method of DC motor is simulated to obtain the control quantity of DC motor, and the control of asynchronous motor is realized through the corresponding inverse transformation of coordinates.
Its essence is that the AC motor is equivalent to the DC motor, and the speed and magnetic field are independently controlled. The torque and magnetic field are obtained by controlling the rotor flux, and then decomposing the stator current. The orthogonal or decoupling control is realized through coordinate transformation. The vector control method has 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 equivalent DC motor control process is more complex, the actual control effect is difficult to achieve the ideal analysis result.
4. DTC mode
In 1985, Professor DePenbrock of Ruhr University in Germany first proposed the frequency conversion technology of direct torque control. This technology has solved the shortcomings of vector control to a large extent, and has been rapidly developed with novel control ideas, concise system structure, and excellent dynamic and static performance.
At present, the technology has been successfully applied to high-power AC drive of electric locomotive traction. Direct torque control 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 equate AC motor with DC motor, so many complicated calculations in vector rotation transformation are omitted; 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.
5. Matrix AC-AC control mode
VVVF frequency conversion, vector control frequency conversion and direct torque control frequency conversion are all among AC-DC-AC frequency conversion. Its common disadvantages are low input power factor, large harmonic current, large energy storage capacitor required by DC circuit, and regenerative energy can not be fed back to the power grid, that is, it can not operate in four quadrants.
Therefore, matrix AC-AC frequency conversion came into being. As the matrix AC AC frequency eliminates the intermediate DC link, the electrolytic capacitor with large volume and expensive price is saved. It can realize that the power factor is l, the input current is sinusoidal, and it can operate in four quadrants. The power density of the system is large. Although the technology is not yet mature, it still attracts many scholars to study it in depth. Its essence is not to control the current and flux indirectly, but to realize the torque directly as the controlled quantity.
Source: Kongde Motor
