1.2 A Matter of Perspective

One way to understand how FOC (sometimes referred to as vector control) works is to form an image of the coordinate reference transformation process. Visualizing an AC motor operation from the perspective of the stator, a sinusoidal input current applied to the stator can be observed. This time variant signal generates a rotating magnetic flux. The speed of the rotor is a function of the rotating flux vector. From a stationary perspective, the stator currents and the rotating flux vector look like AC quantities.

The spinning rotor is moving at the same speed as the rotating flux vector generated by the stator currents. Viewing the motor from the perspective of the rotor during steady state conditions, the stator currents look like constant values, and the rotating flux vector is stationary.

The objective is to control the stator currents to obtain the desired rotating flux vector components (which cannot be measured directly). With coordinate reference transformation, the stator currents can be controlled like DC values using standard control loops.