3-phase Field Oriented Control (FOC)

Field-oriented control (FOC), or vector control, is a technique for variable frequency control of the stator in a three phase AC induction motor drive using two orthogonal components. One defines the magnetic flux generated by the stator, while the other corresponds to the torque as determined by the speed of the motor determined by the rotor position.

We offer the entire range of power semiconductors and ICs including discrete IGBTs and power MOSFETs as well as power modules and intelligent power modules (IPM), high-voltage gate drivers and powerful STM32 microcontrollers needed to implement high-efficiency variable-frequency drive (VFD) motor control.

To help reduce and simplify the design cycle, we offer a complete ecosystem of hardware – evaluation boards and reference designs – as well as firmware and software libraries.

• Advantages of Field-Oriented Control

  • These typically use proportional-integral (PI) controllers where the current components are compared to reference values, rather than using pulse width modulation (PWM). This allows electric motors to operate smoothly over the full speed range and generate full torque at zero speed. Another benefit of field-oriented control is that it can deliver fast acceleration and deceleration of the motor, giving more accurate control in high performance motors.

    As the space vector control algorithms used for FOC are implemented more efficiently and low-cost microcontrollers have more processing power, the technique can be used for lower performance induction motor drives. As the performance of the controllers increases, the technique is expected to replace scalar volts/hertz control algorithms.

• Direct and Indirect Field-Oriented Control

  • Conventional direct field-oriented control (DFOC) algorithms provide more precision for torque control than scalar schemes, but require sensors for the speed control of the rotor and the magnetic flux to provide the data for the FOC algorithms. They also face challenges in the dynamic response and the dependence on measuring the parameters in the motor.

    Instead an indirect field-oriented control (IFOC) method estimates the phase angle of the rotor magnetic field flux, eliminating the need for additional sensors but adding to the complexity and the computation time of the control system.

• Sensorless field-oriented control

  • Replacing the sensors entirely in an FOC motor controller reduces the cost and increases the reliability of an AC induction motor, but also increases the complexity and cost of the controller. To replace the sensor, the information on the rotor speed is extracted from the voltages and currents in the stator windings via the motor terminals. This is then fed back into the current control of the motor.

    The dynamic performance and steady-state speed can be determined accurately at low speeds using the parasitic effects. Profiling tools can be used to determine the performance of particular system designs to build a model that can be used for sensorless motor control.

• Applications

  • FOC can be used across AC induction motors and brushless (BLDC) motors to improve the control and accuracy of the motors for a wide range of applications from pumps and fans to conveyors, particularly in industrial automation where long-term reliability is essential.

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