VectorControl

Vector control, also known as field-oriented control (FOC), is a method for controlling alternating-current (AC) electrical machines by decoupling torque and flux. It achieves near-DC performance for both components by transforming stator currents into a rotating reference frame aligned with the rotor flux, enabling independent control of torque-producing and flux-producing currents.

The technique relies on transforming three-phase motor currents into two orthogonal components, commonly referred to as d and q axes, using Clarke and Park transforms. A motor model and rotor position information are used to compute these components. In practice, a control loop regulates id (flux) and iq (torque) currents, often with an outer loop that provides speed or torque references and inner current loops that run in real time. The resulting commands are sent to a motor inverter via pulse-width modulation. Position sensing from encoders or resolvers is typical, though sensorless versions estimate rotor position from voltages and currents.

Vector control can be applied to several motor types, including induction motors, permanent-magnet synchronous motors, and brushless DC motors. It is widely used in servo drives, industrial motion control, electric vehicles, and HVAC fans, where precise torque control, wide speed ranges, and fast dynamic response are valuable.

Advantages include precise torque control, smooth operation across a wide speed range, fast transient response, and efficient torque production. Challenges involve sensitivity to motor parameter variations, the need for accurate rotor position estimation, increased computational requirements, and the complexity of implementing robust sensorless schemes, especially at very low speeds or with parameter changes.

Historically, field-oriented control concepts emerged in the 1970s and were popularized in practical induction-motor drives in the 1980s by researchers such as Blaschke, Takahashi, and Noguchi. Today, FOC remains a foundational technique in modern electric drives.