Analysis on the development process of inverter technology
The renewal of power electronic devices promotes the continuous development of power conversion technology.
At first, the frequency conversion technology was limited to frequency conversion but not voltage conversion.
Starting from 1970s, pulse width modulation voltage conversion (PWM-VVVF)
The research on speed regulation has attracted great attention.
In 1980s, as the core of frequency conversion technology, PWM mode optimization attracted people's strong interest and many optimization modes were obtained, such: modulation wave longitudinal division method, same phase carrier PWM technology, phase shifting carrier PWM technology, carrier modulation wave simultaneous phase shifting PWM technology, etc.
VVVF inverter control is relatively simple, the mechanical properties of hardness is also good, can meet the requirements of smooth speed regulation of general transmission, has been widely used in various fields of industry.
However, this control mode, at low frequencies, due to the small output voltage, is significantly affected by the stator resistance voltage drop, so the output maximum torque is reduced.
The method of vector control variable frequency speed regulation is to pass the stator AC currents Ia, Ib and Ic of the asynchronous motor in the three-phase coordinate system through three phases--
The two-phase transformation is equivalent to the DC current Iml and Itl in the synchronous rotating coordinate system, and 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, realize the control of asynchronous motor.
Direct torque control directly analyzes the mathematical model of AC motor in stator coordinate system to control the flux linkage and torque of the motor.
It does not need to convert AC motor into equivalent DC motor, thus eliminating many complicated calculations in vector rotation transformation;
It does not need to imitate the control of DC motor, nor does it need to simplify the mathematical model of AC motor for decoupling.
VVVF frequency conversion, vector control frequency conversion and direct torque control frequency converter are all AC-Straight-One of the AC frequency conversion.
The common disadvantage is that the input power factor is low, the harmonic current is large, the DC Circuit needs a large energy storage capacitor, and the regenerative energy cannot be fed back to the power grid, that is, the four-quadrant operation cannot be carried out. To this end, matrix intersection-AC frequency conversion came into being.