Field-oriented control (FOC): what it is and why it makes drives better

Published September 25, 2026 · Persystems GmbH, Regensburg, Germany

Why control at all?

A permanent-magnet synchronous motor – whether a “BLDC” outrunner on a drone or a PMSM in a vehicle – produces torque efficiently only when the stator field stands at the right angle to the rotor field: ideally exactly 90 electrical degrees ahead. But the rotor keeps turning. The inverter’s job is therefore to set the three phase currents so that this angle is right at every instant – at every speed, through every load step.

How well a drive does that decides three things you notice immediately in operation: efficiency (heat in the motor and the electronics), smoothness (vibration, noise) and the accuracy with which a requested torque is actually delivered.

Block commutation: the simple way

Classic ESCs use block commutation, often called six-step or trapezoidal control. Two of the three phases carry current, the third is left open, and the pattern switches every 60 electrical degrees. The rotor position is usually derived from the back-EMF of the unpowered phase – an elegant, very inexpensive solution.

The price: the current vector jumps in 60-degree steps, the torque pulsates in time with the commutation, and the currents are not sinusoidal. That means extra losses in motor and electronics, audible noise and a torque that can only be set roughly. On small, fast-spinning drone motors it hardly shows; on large propellers, heavy-lift drones or traction drives it becomes the limiting factor.

Field-oriented control: controlling the current where it acts

Field-oriented control (FOC, also called vector control) takes a different approach. The three measured phase currents are transformed mathematically into a coordinate system that rotates with the rotor. In this d/q system the current has only two components: I_q produces torque, I_d affects the flux. In steady state both are DC quantities – and DC quantities can be controlled very precisely with simple PI controllers.

The controllers output the desired voltages U_d and U_q, which are transformed back into the stator frame and translated by space-vector modulation into the switching pattern of the six transistors. The result: sinusoidal phase currents, a current vector that continuously follows the rotor at the right angle, and a torque that tracks its setpoint without pulsation.

On top of this usually sits a speed or torque controller that provides the current setpoint I_q. That is exactly how the INV-60V-120A is built: speed and torque control with an inner I_d/I_q current control loop.

What FOC delivers in practice

Lower losses. Sinusoidal currents without commutation steps mean fewer harmonics, lower iron and copper losses in the motor and less stress on the power stage. For a drive that runs continuously near its thermal limit, this decides the continuous power actually available.

Smooth torque. No torque ripple at the commutation frequency – the drive runs quieter and smoother, bearings and structure are excited less. For propellers that means less vibration in the aircraft, for vehicles a gentler start.

Precise torque limit. Because I_q corresponds directly to torque, the maximum torque can be limited exactly – important for current limiting, overload protection and reproducible behavior on the test bench.

Full control at every speed. FOC works from standstill to maximum speed with the same control concept, including controlled start-up under load.

What FOC requires

Compared with block commutation, FOC needs two things: measured phase currents and the rotor position. The inverter measures the currents directly in the power stage. The rotor position comes either from a sensor – a sin/cos or incremental encoder – or sensorlessly from the motor model and the back-EMF. The INV-60V-120A supports both.

FOC also needs computing power: transformations, current controllers and modulation run at the inverter’s switching frequency. Modern microcontrollers handle that easily; what matters is that the firmware implements the control cleanly and that the drive is commissioned with the right motor parameters. This is exactly where a physically exact simulation model helps: with OverDrive, the current control of an inverter can be designed and tested on the model before the first test-bench run.

Field-oriented drives at Persystems

The INV-60V-120A is a 48 V inverter with field-oriented control: 6 kW continuous from 108 grams, rotor position sensorless or via sin/cos and incremental encoders, setpoints via CAN, PWM/PPM or analog. Over PersyCAN it reports the control quantities I_d/I_q and U_d/U_q as telemetry – you can watch the control loop at work. And with OverDrive we compute the same control in the model beforehand, physically exact and faster than real time.