5.1.1. Field-oriented control (FOC): an overview¶
Field-oriented control (FOC), also known as vector control, is a technique of controlling the stator magnetic field of a motor relative to a rotating reference frame of electrical excitation. It can achieve smooth torque, high torque capability, high efficiency, and high bandwidth.
FOC can be used by several types of motors: permanent-magnet synchronous motors, synchronous reluctance motors, and induction motors. The reference frame is chosen to decouple the components of the stator field into a flux-producing component and a torque-producing component, which can be managed separately. In permanent-magnet synchronous motors and synchronous reluctance motors, a reference frame is used that rotates synchronously with the rotor, aligned along the axis of rotor magnetic flux. In induction motors, the reference frame rotates at the required synchronous speed needed to achieve a desired slip.
FOC does have more stringent signal sensing and conditioning requirements than six-step control, requiring the following:
phase current measurements, to estimate the stator field vector
an estimate of rotor angle, usually within 5 – 10 electrical degrees (higher pole count motors require better mechanical accuracy)
Computational requirements of FOC vary, but are generally higher than six-step. Currents and voltages are translated between reference frames by matrix multiplications, effectively a few calculations of the form \(ax + by\) or \(ax + by + cz\). If the rotor angle is not sensed directly — with resolver or quadrature encoder — it can be estimated from a sensorless estimator through calculations based on current and voltage measurements.
The benefits of high-performance torque control are often worth the costs, even in applications with relatively slow dynamics, such as pumps and fans.