In 1967, Finland's Vasa Control Systems Co., Ltd. created the world's first frequency inverter, which is known as the originator of frequency inverters, thus creating a precedent for commercial frequency inverters in the world. With the continuous development of microelectronics technology, the technology is also constantly upgrading and iterating, and has mainly gone through the following three stages of development:

The first stage: constant voltage frequency ratio V/f control technology starting in the 1960s.

(1) Constant voltage-frequency ratio V/f control: The output voltage of the frequency inverter is changed in proportion to the output frequency through the voltage-to-frequency inverter, that is, v/f = constant.

(2) Technical features: high cost performance, constant output torque, that is, constant flux control, but the accuracy of speed control is not high. It is suitable for energy saving purposes and occasions with low speed accuracy requirements. It is the most common frequency conversion technology used by low-end frequency inverter at present. 

The second stage: variable frequency vector control technology starting in the 1970s.

(1) Vector control: The control method of simulating a DC motor is used to control an AC motor through an electronic calculation circuit inside the frequency inverter.

(3) Characteristics of vector control: 

    a. Control the amplitude and phase of the current at the same time, and this control method can also be set through software; 

    b. Can be controlled from zero speed, with large low-frequency torque and wide speed regulation range; 

    c. The torque can be controlled accurately; 

    d. The system has fast response speed and high speed control accuracy.

The third stage: direct torque control technology starting in the 1980s.

(1) Direct torque control: Using the analysis method of space vector and stator magnetic field orientation, the mathematical model of the asynchronous motor is directly analyzed in the stator coordinate system, and the flux linkage and torque of the asynchronous motor are calculated and controlled using a discrete two-point formula. The regulator compares the torque detection value with the torque given value to limit the torque fluctuation within a certain slip range. It is a kind of closed loop control.

(2) Features of direct torque control:

   a. It can be controlled from zero speed, with a wide speed range, and the output torque is still maintained at 0 HZ; 

   b. The torque can be precisely controlled; 

   c. The system has fast response speed and high precision of speed control.