The core technique in the vector control of VFD-VE is FOC (Field Oriented Control), namely the so-called magnetic field oriented control or magnetic vector control. More and more motor drives adopt magnetic field oriented control due to that it enables the motor to work under the best torque at any time with higher efficiency and better dynamic reaction. The very accurate variable speed control and lower torque ripples offer a stable and consistent motor running in the start, operation and stop of the motor.

The basic control principle of FOC is that the 3-phase stator side flowing current combines into an equivalent compound current vector. Its rotation angle speed is the angle frequency ω of the input power. Through the coordinate conversion technique, the current vector is mapped onto the 2-axis rotation coordinate. Provided that this 2-axis coordinate rotates by the angle speed ω as well, the current vector in this coordinate can be regarded as static. In other words, the current vector in this coordinate is a DC value, which can make the motor torque in proportional to the current. However, the rotor flux of the motor has to be overlapped with the d axis and the d axis component of the current vector has to be remained as a fixed value. Assume all the criteria mentioned above are true, the torque of the AC motor will be in proportional with the stator current. Therefore, controlling the vector of the stator current can be as easy and precise as controlling the DC motor.