Frequency converter is a device that converts industrial frequency power (50Hz or 60Hz) into AC power of various frequencies to realize the variable speed operation of motors, in which the control circuit completes the control of the main circuit, the rectifier circuit converts the AC power into DC power, the DC intermediate circuit smoothes and filters the output of the rectifier circuit, and the inverter circuit reverses the DC power back into AC power. For vector control inverters, which require a lot of calculations, a CPU for torque calculation and some corresponding circuits are sometimes required. Inverter speed regulation is achieved by changing the frequency of the power supply to the stator windings of the motor.
There are many ways to categorize frequency converter, according to the classification of the main circuit working mode, it can be divided into voltage-type frequency converter and current-type frequency converter; according to the classification of the switching mode, it can be divided into PAM-controlled frequency converter, PWM-controlled frequency converter and high-load frequency PWM-controlled frequency converter; according to the classification of the working principle, it can be divided into V/f-controlled frequency converter, rotational frequency control frequency converter and vector control frequency converter, etc.; according to the usage According to the classification of working principle, it can be divided into general-purpose inverter, high-performance special inverter, high-frequency inverter, single-phase inverter and three-phase inverter.
VVVF: change voltage, change frequency CVCF: constant voltage, constant frequency. The AC power supply used in various countries, whether for homes or factories, has a voltage and frequency of 400V/50Hz or 200V/60Hz (50Hz), and so on. Generally, a device that converts alternating current with fixed voltage and frequency into alternating current with variable voltage or frequency is called an "inverter". In order to produce variable voltage and frequency, the device first converts the alternating current of the power supply to direct current (DC).
Inverters used for motor control can change both voltage and frequency.
How Frequency Converters Work
We know that the synchronous speed expression of an AC motor bit:
n = 60 f (1-s)/p (1)
In the formula
n --- the speed of the asynchronous motor;
f --- frequency of the asynchronous motor;
s --- motor rotation rate;
p---number of motor pole pairs.
From equation (1), it can be seen that the rotational speed n is directly proportional to the frequency f, as long as the frequency f can be changed to change the rotational speed of the motor, when the frequency f is changed in the range of 0 to 50Hz, the motor speed adjustment range is very wide. Frequency converter is by changing the motor power supply frequency to realize the speed regulation, is an ideal means of high efficiency and high performance speed regulation.
Frequency converter control mode
Low-voltage general-purpose inverter output voltage is 380-650V, output power is 0.75-400kW, working frequency is 0-400Hz, and its main circuits all use AC-DC-AC circuits. Its control mode has gone through the following four generations.
1U/f=C sinusoidal pulse width modulation (SPWM) control mode
Characterized by a simple control circuit structure, lower cost, mechanical characteristics of the hardness is also better to meet the general transmission of smooth speed requirements, has been widely used in various fields of industry. However, this control method at low frequency, due to the lower output voltage, torque by the stator resistance voltage drop is more significant, so that the output maximum torque is reduced. In addition, its mechanical characteristics are not as hard as DC motor, the dynamic torque capacity and static speed performance are not satisfactory, and the system performance is not high, the control curve will change with the load, the torque response is slow, the motor torque utilization is not high, the low speed due to the stator resistance and the existence of the dead zone effect of the inverter and performance degradation, stability deterioration and so on. Therefore, vector control frequency conversion speed regulation has been studied.
Voltage space vector (SVPWM) control method
It is based on the premise of the overall generation effect of three-phase waveforms, in order to approximate the ideal circular rotating magnetic field trajectory of the motor air gap for the purpose of generating three-phase modulated waveforms at a time, and control in the way of the inner tangent polygon approximation of the circle. It has been improved after practical use, i.e., frequency compensation is introduced, which can eliminate the error of speed control; the magnetic chain amplitude is estimated by feedback, which eliminates the influence of the stator resistance at low speeds; and the output voltage and current are closed-looped to improve the accuracy and stability of the dynamics. However, the control circuit has more links and does not introduce torque regulation, so the system performance is not fundamentally improved.
Vector control (VC) method
The practice of vector control frequency control is to convert the stator currents Ia, Ib, Ic, of the asynchronous motor in the three-phase coordinate system into the AC currents Ia1Ib1 in the two-phase stationary coordinate system through three-phase - two-phase transformation, and then through the rotary transformation according to the rotor magnetic field orientation, which is equivalent to the DC currents Im1, It1 in the synchronous rotating coordinate system (Im1 is equivalent to the (Im1 is equivalent to the excitation current of DC motor; It1 is equivalent to the armature current which is proportional to the torque), and then imitate the control method of DC motor to get the control quantity of DC motor, and realize the control of asynchronous motor after the corresponding inverse transformation of coordinates. In essence, the AC motor is equivalent to a DC motor, and the two components of speed and magnetic field are controlled independently. By controlling the rotor magnetic chain, and then decomposing the stator current to obtain the torque and magnetic field components, through the coordinate transformation, to realize orthogonal or decoupled control. The proposed vector control method is of epoch-making significance. However, in practical applications, due to the rotor magnetic chain is difficult to accurately observe, the system characteristics are greatly affected by the motor parameters, and the vector rotational transformation used in the control process of equivalent DC motor is more complicated, which makes it difficult for the actual control effect to achieve the results of ideal analysis.
Direct torque control (DTC) method
In 1985, Prof. DePenbrock of Ruhr University in Germany first proposed the direct torque control frequency conversion technology. This technology has largely solved the shortcomings of the above vector control, and has been rapidly developed with novel control ideas, concise and clear system structure, and excellent dynamic and static performance. At present, this technology has been successfully applied to high-power AC drives for electric locomotive traction. Direct torque control analyzes the mathematical model of AC motor directly in the stator coordinate system to control the magnetic chain and torque of the motor. It does not need to equate the AC motor to a DC motor, thus eliminating many complex calculations in the vector rotation transformation; it does not need to mimic the control of a DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling.
Matrix AC-AC control method
VVVF inverter, vector control inverter, and direct torque control inverter are all types of AC-DC-AC inverter. Their common disadvantages are low input power factor, high harmonic currents, the need for large energy storage capacitors in the DC circuit, and the regenerative energy cannot be fed back to the grid, i.e., four-quadrant operation is not possible. For this reason, the matrix AC-AC inverter came into being. As the matrix AC-AC inverter eliminates the intermediate DC link, thus eliminating the large size, expensive electrolytic capacitors. It can realize the power factor of l, the input current is sinusoidal and can operate in four quadrants, the power density of the system is large. The technology is not yet mature, but still attracts many scholars to study in depth. Its essence is not to indirectly control the current, magnetic chain and other quantities, but to realize the torque directly as the controlled quantity. The specific methods are:
--Control of the stator magnetic chain to introduce the stator magnetic chain observer to realize the speed sensor-less method;
--Automatic identification (ID) relies on an accurate mathematical model of the motor to automatically identify the motor parameters;
--Calculate the actual value corresponding to the stator impedance, mutual inductance, magnetic saturation factor, inertia, etc. Calculate the actual torque, stator magnetic chain, rotor speed for real-time control;
--Realize Band-Band control to generate PWM signals according to the Band-Band control of magnetic chain and torque to control the inverter switching state.
Matrix AC-AC inverter has fast torque response (＜2ms), high speed accuracy (±2%, no PG feedback), high torque accuracy (＜+3%); it also has high starting torque and high torque accuracy, especially at low speeds (including 0 speeds), it can output 150%～200% torque.