Working principle and control mode of frequency converter

Update:20 May 2020

Frequency converter control mode
The output voltage of low-voltage general inverter 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.
Sinusoidal pulse width modulation (SPWM) control mode with 2.1u / F = C
It is characterized by simple control circuit structure, low cost, good mechanical properties and hardness, which can meet the requirements of smooth speed regulation of general drive, and has been widely used in various fields of the industry. However, when the control mode is at low frequency, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, so that the maximum output torque is reduced. In addition, its mechanical characteristics are not as hard as DC motor, its 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 utilization rate of motor torque is not high, the performance will decline due to the existence of stator resistance and inverter dead zone effect at low speed, and the stability will become poor. Therefore, people have developed vector control variable frequency speed regulation.
2.2 SVPWM control mode
It is based on the overall generation effect of the three-phase waveform, with the purpose of approaching the ideal circular rotating magnetic field track of the air gap of the motor, generating three-phase modulation waveform at one time, and controlling in the way of the inscribed polygon approaching the circle. After practical application, it has been improved, that is, introducing frequency compensation can eliminate the error of speed control; estimating the magnitude of flux through feedback to eliminate the influence of stator resistance at low speed; closing the output voltage and current to improve the dynamic accuracy and stability. But there are many control circuit links, and there is no torque regulation, so the system performance has not been fundamentally improved.
2.3 vector control (VC) mode
The method of vector control variable frequency speed regulation is that the stator current IA, IB, IC of asynchronous motor in three-phase coordinate system is equivalent to the AC current ia1ib1 in two-phase static coordinate system through three-phase to two-phase transformation, and then the DC current IM1 and it1 (IM1 is equivalent to the excitation current of DC motor; it1 phase) in synchronous rotation coordinate system are equivalent through the directional rotation transformation of rotor magnetic field When the armature current is proportional to the torque), then the control quantity of the DC motor is obtained by imitating the control method of the DC motor. After the corresponding coordinate inverse transformation, the control of the asynchronous motor is realized. The essence of this method is that the AC motor is equivalent to the DC motor, and the speed and magnetic field are controlled independently. By controlling the rotor flux, and then decomposing the stator current, two components of torque and magnetic field are obtained. Through coordinate transformation, orthogonal or decoupling control is realized. 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, making the actual control effect difficult to achieve the ideal analysis results.
2.4 direct torque control (DTC) mode
In 1985, Professor depenbrock of Ruhr University in Germany first proposed the direct torque control frequency conversion technology. To a great extent, this technology solves the shortcomings of the above vector control, and develops rapidly with novel control ideas, simple and clear system structure, excellent dynamic and static performance. At present, the technology has been successfully applied to the high-power AC drive 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 and torque of the motor. It does not need to be equivalent to DC motor, so it saves many complex calculations in vector rotation transformation; it does not need to imitate the control of DC motor, and it does not need to simplify the mathematical model of AC motor for decoupling.
2.5 matrix intersection control mode
VVVF frequency conversion, vector control frequency conversion and direct torque control frequency conversion are all one of AC-DC-AC frequency conversion. Its common disadvantages are low input power factor, large harmonic current, large energy storage capacity required by DC circuit, and regenerative energy can not be fed back to the grid, that is, it can not operate in four quadrants. For this reason, matrix AC-AC converter came into being. Because the matrix AC-AC Variable Frequency eliminates the intermediate DC link, the large volume and expensive electrolytic capacitor is saved. It can realize the power factor of L, the input current of sine and four quadrant operation, the power density of the system is large. Although the technology is not mature at present, it still attracts many scholars to study deeply. In fact, the quality is not the indirect control of current, flux and so on, but the torque is directly realized as the controlled quantity. The specific methods are as follows:
——The control stator flux is introduced into the stator flux observer to realize the speed sensorless mode;
——Automatic identification (ID) relies on the accurate mathematical model of the motor to identify the motor parameters automatically;
——The actual value corresponds to the stator impedance, mutual inductance, magnetic saturation factor, inertia, etc. to calculate the actual torque, stator flux and rotor speed for real-time control;
——To realize the band band control, PWM signals are generated according to the band band control of flux and torque, and the switch state of the inverter is controlled.
Matrix AC-AC frequency has fast torque response (< 2ms), high speed accuracy (± 2%, no PG feedback), 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

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