What Are the Control Methods of the Variable Frequency Drive?

The output voltage of low-voltage universal frequency conversion is 220-460V, the output power is 0.75-400kW, and the working frequency is 0-5000Hz. Its main circuit adopts AC-DC-AC circuit. Its control method has gone through the following four generations.

1. U/f=C sinusoidal pulse width modulation (SPWM) control method

It 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 the dynamic torque capability and static speed regulation performance are not satisfactory, and the system performance is not high, the control curve will change with the load change, the torque response is slow, and the motor turns The torque utilization rate is not high, and the performance decreases due to the existence of the stator resistance and the dead zone effect of the inverter at low speed, and the stability becomes poor. Therefore, people have developed vector control frequency conversion speed regulation.

2. Voltage space vector (SVPWM) control mode

It 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.

After practical use, it has been improved, that is, the introduction of frequency compensation can eliminate the error of speed control; the magnitude of flux linkage can be estimated through feedback to eliminate the influence of stator resistance at low speed; the output voltage and current are closed-loop to improve dynamic accuracy and stability. However, there are many links in the control circuit, and no torque adjustment is introduced, so the system performance has not been fundamentally improved.

3. Vector control (VC) mode

The practice of vector control frequency conversion speed regulation is to convert the stator currents Ia, Ib, and Ic of the asynchronous motor in the three-phase coordinate system into an equivalent AC current Ia1Ib1 in the two-phase stationary coordinate system through three-phase-two-phase conversion, and then through According to the orientation rotation transformation of the rotor magnetic field, it is equivalent to the DC current Im1 and It1 in 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 DC motor In the control method, 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 that the AC motor is equivalent to a DC motor, and the two components of speed and magnetic field are independently controlled. By controlling the rotor flux linkage, and then decomposing the stator current to obtain the two components of torque and magnetic field, the coordinate transformation can realize the quadrature or decoupling control. The proposal of the vector control method has epoch-making significance. However, in practical applications, due to the difficulty of accurately observing the rotor flux linkage, 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, making it difficult for the actual control effect to achieve ideal analysis. result.

4. Direct torque control (DTC) mode

In 1985, Professor DePenbrock of Ruhr University in Germany proposed the direct torque control frequency conversion technology for the first time. This technology largely solves the above-mentioned shortcomings of vector control, 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. Direct torque control 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.

5. Matrix cross-hand control mode

VVVF frequency conversion, vector control frequency conversion, and 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. Although the technology is not yet mature, it still attracts many scholars to do in-depth research. Its essence is not to indirectly control the current, flux linkage, etc., but to realize the torque directly as the controlled quantity.