The renewal of power electronic devices has led to the continuous development of power conversion technology. At first, the frequency conversion technology was limited to the frequency conversion and could not be transformed. Since the 1970s, the study of PWM-VVVF speed control has attracted people's attention. In the 1980s, as the core of the frequency conversion technology, the optimization of the PWM mode attracted people's interest and many optimization modes were derived, such as modulation wave longitudinal division, in-phase carrier PWM, phase-shifted carrier PWM, and carrier modulation. Wave simultaneous phase shift PWM technology.
VVVF inverter control is relatively simple, mechanical properties are also good hardness, to meet the general drive of the smooth speed requirements, has been widely used in various fields of industry. However, this control method at low frequencies, due to the small output voltage, is significantly affected by the stator resistance voltage drop, resulting in reduced output torque.
Vector control frequency control method is: the asynchronous motor in the three-phase coordinate system of the stator AC current Ia, Ib, Ic through three-phase - two-phase transformation, equivalent to synchronous rotation of the DC current under the coordinate system Iml, Itl Then, imitate the control method of the direct-current motor, obtain the control quantity of the direct-current motor, realize the control to the asynchronous motor through the corresponding coordinate reverse transformation.
Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system and controls the flux linkage and torque of the motor. It does not need to convert the AC motor into an equivalent DC motor, thus eliminating many of the complex calculations in the vector rotation transformation; it does not need to mimic the DC motor control, nor does it need to simplify the mathematical model of the AC motor for decoupling.
VVVF frequency conversion, vector control frequency conversion, direct torque control converter is a kind of AC-DC-AC frequency conversion. The common disadvantages are the low input power factor, large harmonic currents, large storage capacitors required in the DC loop, and the fact that the regenerative energy cannot be fed back to the grid, ie it cannot be operated in four quadrants. For this reason, matrix-type AC-AC frequency conversion came into being.
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