How Many Types Can Single-Phase to Three-Phase VFDs Be Divided Into?

The single-phase to three-phase inverter is a speed regulating device that can adjust the output voltage and frequency. Simply put, it is to convert alternating current into direct current, and then use electronic components to switch the direct current into alternating current. In terms of function alone, there is not much difference from the inverter.

Generally, a single-phase to three-phase frequency converter with a large power uses a thyristor, and an adjustable frequency device is provided to make the frequency adjustable within a certain range, which is used to control the number of revolutions of the motor so that the number of revolutions is within a certain range. Internally adjustable.

The control mode of single-phase to three-phase inverter includes U/f coordinated control, slip frequency control, vector control and direct torque control.

(1) U/f control

U/f control is to change the frequency of the power supply to adjust the speed, while ensuring that the magnetic flux of the motor remains unchanged. Common types of inverters basically adopt this control method. The structure of the U/f control frequency converter is very simple, but this frequency converter adopts an open-loop control method, which cannot achieve high control performance. In addition, at low frequencies, torque compensation must be performed to change the low frequency torque characteristics.

(2) Slip frequency control

Slip frequency control is a control method of direct torque control. Based on U/f control, the output frequency of the inverter can be adjusted according to the power frequency corresponding to the actual speed of the asynchronous motor and the expected torque, so that the motor can have a corresponding output torque. In this control mode, a speed sensor needs to be installed in the control system, and sometimes a current feedback is added to control the frequency and current. Therefore, it is a closed-loop control method, which can make the inverter have good stability and good response characteristics to fast acceleration and deceleration and load changes.

(3) Vector control

Vector control is to control the magnitude and phase of the motor stator current through the vector coordinate circuit, so as to control the excitation current and torque current of the motor in the coordinate axis system, and then achieve the purpose of controlling the motor torque. By controlling the action sequence and time of each vector and the action time of the zero vector, various PWM waves can be formed to achieve various control purposes. For example, a pulse width modulated wave with the fewest switching times is formed to reduce switching loss. The vector control methods currently used in frequency converters mainly include the vector control method based on slip frequency control and the vector control method without speed sensor.

The steady-state characteristics of vector control based on slip frequency are consistent with those of slip frequency control, but the phase of the motor stator current needs to be controlled by coordinate transformation to meet certain conditions, thereby eliminating the torque-current transition fluctuation. Therefore, compared with the slip frequency control method, the slip frequency based vector control method can greatly improve the output characteristics. However, this control method is a closed-loop control method, and a speed sensor needs to be installed on the motor, so its application range is limited.

The speed sensorless vector control is to control the excitation current and torque current respectively through coordinate transformation processing, and then control the excitation current and torque current by controlling the voltage and current on the stator winding of the motor, so as to identify the speed. This control method has a wide speed range, large starting torque, reliable operation, and convenient operation, but the calculation is complicated, and usually requires a special processor for calculation. Therefore, the real-time performance is not ideal, and the control accuracy is affected by the calculation accuracy.

(4) Direct torque control

Direct torque control (DTC) uses the concept of space vector coordinates to analyze the mathematical model of the AC motor in the stator coordinate system, control the flux linkage and torque of the motor, and achieve the purpose of observing the stator flux linkage by detecting the stator resistance. Therefore, complex transformation calculations such as vector control are omitted, the system is intuitive and simple, and the calculation speed and accuracy are improved compared with vector control. Even in the open loop state, it can output 100% of the rated torque and has a load balancing function for multiple drives.

With the development of the economy, small businesses and factories have sprung up like mushrooms, and the importance of the function of converting AC power into DC power of AC inverters is self-evident for these factories. Therefore, AC frequency conversion speed regulation technology is an important development direction of modern power transmission technology. With the development of power electronics technology, AC frequency conversion technology has gradually matured from theory to practice. AC frequency conversion speed regulation has gradually replaced traditional speed regulation systems such as slip speed regulation, pole-changing speed regulation, and DC speed regulation, and is more and more widely used in metallurgy, textiles, printing and dyeing, range hoods, production lines and building water supply, air conditioning, etc. field.