Why Use the Field-Oriented Control (FOC) Algorithm?

Semiconductor Integrated Circuits (ICs)
, ,
1

The core value of using the Field-Oriented Control (FOC) algorithm lies in realizing the independent adjustment of torque and magnetic flux through coordinate transformation + decoupling control. The specific reasoning can be derived from the functions of the block diagram modules as follows:

Core Logical Framework of Sensorless Field-Oriented Control (FOC) for PMSM

image
image690×391 26.3 KB

(Image source: Microchip)

1. Decoupling Control: Converting Coupled Three-Phase Currents into Independent Torque/Magnetic Flux Components

The core of the FOC block diagram lies in the Clarke transformation (3-phase stationary coordinate system → 2-phase stationary coordinate system) and Park transformation (2-phase stationary coordinate system → 2-phase synchronous rotating coordinate system). The essence of these two transformations is mathematical decoupling:

  • The three-phase stator currents (Ia, Ib, Ic) are converted into two-phase stationary currents (Iα, Iβ) via the Clarke transformation;
  • Then, through the Park transformation, they are converted into the torque component Iq and magnetic flux component Id (consistent with the direction of the rotor magnetic field) in the synchronous rotating coordinate system.

Since Iq and Id are DC components in the rotating coordinate system, they can be adjusted separately by independent PI controllers (similar to the control of armature current and excitation current in DC motors), which completely solves the problem of “torque and magnetic flux coupling” in three-phase AC motors.

For the principles of Clarke transformation and Park transformation, please refer to the following posts:

2. Precise Regulation: Independent PI Control of Torque and Magnetic Flux

The independent PI controllers for Iq and Id in the block diagram are the keys to FOC realizing precise control:

  • Iq Loop (Torque Loop): Directly controls the output torque of the motor, enabling extremely fast torque response (e.g., when the load changes suddenly, Iq can be adjusted to the target value within 10 ms);
  • Id Loop (Magnetic Flux Loop): Controls the angle between the stator magnetic field and the rotor magnetic field (maintaining 90° at all times), maximizing the “torque/current ratio” (i.e., higher output torque under the same current) while avoiding magnetic flux saturation.

3. Sinusoidal Output: Converting Decoupled Components Back to Three-Phase Drive Signals

After PI regulation, Iq and Id are converted back to the two-phase stationary coordinate system (Iα, Iβ) via the inverse Park transformation, and then sinusoidal three-phase voltage signals are generated through SVPWM (Space Vector PWM) to drive the inverter to output stable three-phase currents:

  • Compared with square-wave control, sinusoidal current can eliminate torque ripple (fluctuation < 1%) and reduce motor vibration and noise;
  • Compared with scalar control (V/F control), the sinusoidal output of FOC allows the motor to maintain high efficiency (efficiency > 95%) over the full speed range (5%～100% rated speed).

4. Dynamic Response and Multi-Mode Adaptation

The FOC block diagram also embodies the three-loop cascade control of “speed loop - torque loop - current loop”:

  • The outermost speed loop outputs torque commands based on the target speed (e.g., given by a potentiometer);
  • The middle torque loop (Iq loop) converts the torque command into a current command;
  • The innermost current loop (Iq, Id loops) directly controls the input current of the motor.

This structure enables FOC to support multi-scenario requirements such as torque mode, speed mode, and position mode, with extremely fast dynamic response (e.g., the torque step response when an elevator starts).

Summary: The Irreplaceability of the FOC Algorithm

FOC is the only algorithm that can realize torque and magnetic flux decoupling + precise PI regulation + sinusoidal output, which makes it far superior to traditional control methods (such as scalar control and square-wave control) in terms of performance (torque accuracy, speed regulation range), efficiency (energy saving), and reliability (low vibration, low noise). Therefore, FOC has become the preferred solution for mid-to-high-end PMSM control.

Related Products

Related Application Notes

More Content Related to FOC

Why Use a DSP to Control a Three-Phase Permanent Magnet Synchronous Motor (PMSM)?
3 Motor Control Techs Comparison: FOC, V/f Control, Trapezoidal Six-Step Control (BLDC)
なぜフィールドオリエンテッド制御（FOC）を使うのでしょうか？