Field-oriented control
How the inverter decides what current to command, tens of thousands of times a second.
- Explain field-oriented control at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive system
Overview
Underneath the switching pattern sits a control algorithm that decides, moment to moment, exactly what current each phase should carry. Field-oriented control (FOC) is the industry-standard way to do that with independent, DC-like control of torque and flux.
In depth
Rotating into a friendlier frame
FOC mathematically transforms the three time-varying phase currents into two constant quantities in a reference frame that rotates with the rotor: a torque-producing current (Iq) and a flux-producing current (Id). Controlling two DC-like quantities with standard PI controllers is far simpler than trying to regulate three interacting sine waves directly.
- 1Measure
Sample phase currents and rotor position at the PWM carrier rate.
- 2Transform
Convert measured currents into the rotor-aligned Id/Iq frame.
- 3Control
PI loops compare Id/Iq to their targets and compute a voltage command.
- 4Inverse transform
Convert the voltage command back to three-phase and hand it to the PWM stage.
Key takeaways
- FOC controls torque and flux as two independent, DC-like quantities.
- Clarke and Park transforms move between the physical and rotor-aligned reference frames.
- The choice of Id sets the torque-per-amp and field-weakening behaviour of the drive.
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