The inverter is a device that converts the power frequency power supply (50Hz or 60Hz) into AC power supply of various frequencies to realize the variable speed operation of the motor, in which the control circuit completes the control of the main circuit, the rectifier circuit converts the alternating current into direct current, the DC intermediate circuit smooths the output of the rectifier circuit, and the inverter circuit reverses the direct current into alternating current.
For inverters that require a lot of calculations, such as vector control frequency converters, sometimes a CPU for torque calculations and some corresponding circuits are also required. Frequency conversion speed regulation is to achieve the purpose of speed regulation by changing the frequency of power supply from the stator winding of the motor.
Classification of frequency converters
There are several ways to classify frequency converters:
According to the classification of the working mode of the main circuit, it can be divided into voltage type inverter and current type inverter;
According to the classification of switching methods, it can be divided into PAM control inverter, PWM control inverter and high carrier frequency PWM control inverter;
According to the classification of working principle, it can be divided into V/f control inverter, slip frequency control inverter and vector control inverter, etc.;
According to the classification of use, it can be divided into general-purpose inverter, high-performance special inverter, high-frequency inverter, single-phase inverter and three-phase inverter.
How the inverter works
We know that the synchronous speed expression of the AC motor bits:
n＝60 f（1——s）/p （1）
where
n - the speed of the asynchronous motor;
f - the frequency of the asynchronous motor;
s - motor slip rate;
p - the number of pole pairs of the motor.
It can be seen from equation (1) that the speed n is proportional to the frequency f, as long as the frequency f is changed, the speed of the motor can be changed, and when the frequency f changes in the range of 0-50Hz, the speed adjustment range of the motor is very wide. The inverter is to realize speed regulation by changing the frequency of the motor power supply, which is an ideal speed regulation method with high rate and high performance.
Inverter control mode
The output voltage of low-voltage general frequency conversion is 380-650V, the output power is 0.75-400kW, and the working frequency is 0-400Hz. Its control has gone through the following four generations.
1. 1U/f=C sinusoidal pulse width modulation (SPWM) control mode
It is characterized by simple structure of control circuit, low cost, good hardness of mechanical characteristics, and can meet the smooth speed regulation requirements of general transmission, and has been widely used in various fields of industry. However, at low frequency, due to the low output voltage, the torque is significantly affected by the voltage drop of the stator resistance, so that the output torque is reduced.
In addition, its mechanical characteristics are not as hard as DC motors, dynamic torque ability 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 motor torque utilization rate is not high, and the performance decreases due to the existence of stator resistance and inverter dead zone effect at low speed, and the stability deteriorates. 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 the purpose is to approximate the ideal circular rotating magnetic field trajectory of the motor air gap, and the three-phase modulation waveform is generated at one time, and the inscribed polygon is controlled by approximating the circle.
After practical use, it has been improved, that is, the introduction of frequency compensation, which can eliminate the error of speed control; The flux amplitude is estimated through feedback, and the influence of stator resistance at low speed is eliminated. The output voltage and current are closed to improve the accuracy and stability of the dynamics. 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 method of vector control frequency conversion speed regulation is to convert the stator current Ia, Ib, and Ic of the asynchronous motor in the three-phase coordinate system into the alternating current Ia1Ib1 in the two-phase stationary coordinate system through the three-phase two-phase transformation, and then through the directional rotation transformation according to the rotor magnetic field, it is equivalent to the direct current Im1 and It1 under 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 control method of the DC motor, 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 to equivalence the AC motor to the DC motor, and the two components of speed and magnetic field are independently controlled. By controlling the rotor flux, and then decomposing the stator current, the two components of torque and magnetic field are obtained, and the orthogonal or decoupled control is realized through coordinate transformation. The vector control method is of 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 control process of the equivalent DC motor is complex, which makes it difficult to achieve the ideal analysis results for the actual control effect.
4. Direct torque control (DTC) mode
In 1985, Professor DePenbrock of the Ruhr University in Germany first proposed direct torque control frequency conversion technology. To a large extent, this technology solves the shortcomings of the above-mentioned 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 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 and torque of the motor. It does not need to equate an AC motor to a DC motor, thus eliminating many complex calculations in vector rotation transformations; It does not need to mimic the control of a DC motor, nor does it need to simplify the mathematical model of an AC motor for decoupling.
5. Matrix traffic-traffic control mode
VVVF frequency conversion, vector control frequency conversion, and direct torque control frequency conversion are all one of the AC-DC-AC frequency conversion. Its common disadvantages are that the input power factor is low, the harmonic current is large, the DC circuit requires a large energy storage capacitor, and the regenerative energy cannot be fed back to the power grid, that is, it cannot carry out four-quadrant operation.
For this reason, matrix alternating frequency-alternating frequency came into being. Because the matrix AC-AC frequency conversion eliminates the intermediate DC link, the large and expensive electrolytic capacitor is eliminated. It can achieve a power factor of L, an input current of sinusoidal and four-quadrant operation, and a high power density of the system. Although the technology is not yet mature, it still attracts many scholars for in-depth research. Its essence is not to indirectly control the current and flux equals, but to realize the torque directly as the controlled quantity.
--Here's how:
The stator flux is controlled by introducing a stator flux observer to realize the speed sensorless mode;
Automatic identification (ID) relies on the first-class motor mathematical model to automatically identify motor parameters;
The actual value is calculated and the actual torque, stator flux and rotor speed are calculated for real-time control corresponding to stator impedance, mutual inductance, magnetic saturation factor, inertia, etc.;
Realize Band-Band control, generate PWM signals according to the Band-Band control of flux and torque, and control the switching state of the inverter.
The matrix AC-AC frequency conversion has fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), and 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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