In today's industrial and home automation fields, three-phase motors have become the preferred power source due to their high efficiency and wide customizability. Whether it's household appliances, cars, or industrial robots, the application of brushless DC (BLDC) and AC motors is ubiquitous. This popular science news article will take you in depth to understand the field oriented control (FOC) technology of three-phase motors, which is an efficient and advanced motor control strategy.
##Introduction to Field Oriented Control (FOC)
FOC is one of the most effective ways to drive electric motors, with the core goal of maintaining the orthogonality of the stator and rotor magnetic fields to generate maximum torque output. FOC achieves the correct direction of the stator magnetic field by continuously monitoring the three-phase time-varying current and modulating each applied phase voltage. Although this process is relatively simple in theory, it becomes complex in practice due to the increasing hardware and software requirements.
Figure 1: Projection of three-phase current using Clark transform (I α and I β), and then projection to linear d using Parker transform, Q rotation reference coordinate system
Then, the d-q axis component of the instruction is converted back into a 3-phase time-varying system to correctly modulate the 3-phase current through PWM control of the inverter switch.
##The working principle of FOC
In FOC, the three-phase current is monitored and projected onto a stationary reference coordinate system, which is then decomposed into torque (q-axis) and field flux (d-axis) components through Clark and Parker transformations. This process makes it possible to directly control torque within a time invariant reference coordinate system, simplifying the control process and reducing bandwidth requirements.
##The role of sensors in FOC
In order to maintain the orthogonality of the stator and rotor magnetic fields, it is necessary to accurately know the magnetic field angle of the rotor. This information can be obtained through position feedback from encoders or rotary transformers (with sensor FOC), or estimated through back electromotive force/flux observer software (without sensor FOC). Encoders are divided into two types: incremental and absolute. The former provides relative angular position and rotation direction information, while the latter provides true angular position information.
##The importance of phase current detection
Accurate measurement of phase current is crucial for maintaining precise torque control in both sensor based and sensorless FOC implementations. Usually, the measurement of phase current is achieved by using a shunt resistor in the inverter stage. Ideally, all three phases of current should be measured simultaneously, but to reduce system costs and power losses, the number of shunt resistors can be reduced.
##Protection function of FOC motor control system
The protection function of the motor control system is crucial to ensure the safe operation of the motor and control system. FOC systems typically include overcurrent protection (OCP), overvoltage protection (OVP), and over temperature protection (OTP). These protection functions can be implemented through hardware, software, or a combination of both to quickly respond and protect the system from potential damage.
##Conclusion
Field Oriented Control (FOC) technology provides an efficient and accurate control method for three-phase motors, suitable for various industrial and commercial applications. By deeply understanding the working principle and implementation requirements of FOC, engineers and technicians can better design and maintain advanced motor control systems, thereby improving the overall performance and reliability of the system. With the continuous progress of technology, FOC will continue to play its important role in the field of motor control.
