Vector control (motor)

Vector control, also known as field-oriented control (FOC), is a variable-frequency drive (VFD) control technique used to control AC induction and synchronous motors. It controls the motor by separately controlling the magnitude and angle (the space vector) of the motor current, resulting in control analogous to that of a separately excited DC motor. This enables independent control of torque and flux, providing superior dynamic performance and higher efficiency compared to simpler scalar control methods like V/Hz control.

The fundamental principle of vector control is to decompose the stator currents into two orthogonal components: a torque-producing component and a flux-producing component. This decomposition is typically achieved through coordinate transformations, such as the Clarke and Park transforms, which project the three-phase stator currents onto a rotating reference frame.

Vector control systems typically employ a closed-loop architecture with current and speed feedback. The controller regulates the motor currents to their desired reference values, which are derived from the speed command and the motor model. Pulse-width modulation (PWM) is used to generate the appropriate voltage waveforms to drive the motor.

There are two main types of vector control: direct torque control (DTC) and field-oriented control (FOC).

• Direct Torque Control (DTC): DTC directly controls the stator flux linkage and torque of the motor. It selects the optimal voltage vector to apply to the motor based on the error between the reference and actual values of torque and flux. DTC offers advantages such as fast torque response and simplicity of implementation. However, it can suffer from torque ripple and variable switching frequency.

• Field-Oriented Control (FOC): FOC controls the motor current in a synchronously rotating reference frame aligned with the rotor flux. This allows for independent control of the torque and flux components of the current. FOC generally offers better torque smoothness and lower harmonic distortion compared to DTC but requires more complex calculations and accurate motor parameters.

Vector control requires precise knowledge of motor parameters, such as stator resistance, rotor resistance, stator inductance, and rotor inductance. Accurate parameter identification is crucial for achieving optimal performance. Parameter variations due to temperature changes or manufacturing tolerances can degrade the performance of the vector control system. Parameter estimation techniques are often employed to compensate for these variations.

Applications of vector control include:

• High-performance drives in industrial automation
• Electric vehicles (EVs) and hybrid electric vehicles (HEVs)
• Robotics
• Machine tools
• Wind turbines
• Pumps and fans

Advantages of vector control include:

• Precise torque and speed control
• High dynamic performance
• Improved efficiency
• Ability to operate at low speeds with full torque
• Reduced harmonic distortion

Disadvantages of vector control include:

• Higher complexity compared to scalar control
• Requires accurate motor parameters
• More computationally intensive