Vector control, also known as Field-Oriented Control, is an advanced method for controlling AC motors. It works by decomposing the stator current into two orthogonal vector components, allowing independent control of motor flux and torque, similar to DC motor control.
The key principle of vector control is decomposing the stator current vector into two orthogonal components. The d-axis component controls the magnetic flux, while the q-axis component controls the torque. This decomposition allows independent control of these two fundamental motor parameters, similar to how a DC motor naturally separates field and armature currents.
Vector control relies on coordinate transformations to convert three-phase currents into a two-axis rotating reference frame. The Clarke transform converts abc coordinates to stationary alpha-beta coordinates. Then the Park transform converts to the dq rotating frame that aligns with the rotor flux. In this rotating frame, AC quantities become DC-like, enabling precise control.
The vector control system consists of cascaded control loops. The outer speed controller generates the torque current reference Iq-star. The flux current reference Id-star is typically set to zero for maximum efficiency. Two current controllers regulate Id and Iq independently. The outputs are transformed back to three-phase voltages through inverse Park and Clarke transforms, then applied to the motor via PWM inverter. Position and current feedback complete the control loops.
Vector control offers significant advantages over traditional V/f control. It provides precise torque control at all speeds, including zero speed, with fast dynamic response and high efficiency. The torque-speed characteristic shows constant torque capability across the entire speed range, unlike V/f control which has poor low-speed performance. These advantages make vector control ideal for demanding applications like electric vehicles, industrial servo drives, robotics, and high-performance motor control systems where precision and efficiency are critical.