Field-oriented control (FOC)
The algorithm that treats a spinning AC motor almost like a simple DC motor to control.
- Explain field-oriented control (foc) at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive unit
Overview
Field-oriented control is the mathematical trick that makes controlling a three-phase AC motor as clean as controlling a DC motor, by rotating the frame of reference to match the rotor.
In depth
The Park and Clarke transforms
FOC converts the three time-varying phase currents into two constant (in steady state) quantities in a rotating reference frame locked to the rotor: Id, aligned with the magnetic flux, and Iq, aligned perpendicular to it and directly responsible for torque. The Clarke transform converts three-phase to a two-axis stationary frame; the Park transform then rotates that frame to align with the rotor's instantaneous position.
Torque comes from the magnet-flux term (λ_m × I_q) plus a reluctance term from any saliency (L_d ≠ L_q) — the basis for exploiting reluctance torque in IPM motors.
Key takeaways
- FOC transforms three AC phase currents into two DC-like quantities, Id and Iq, in a rotor-aligned frame.
- Torque becomes directly proportional to Iq, decoupled from the flux-producing Id.
- Accurate, continuous rotor position feedback is essential to FOC.
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