Guide to field-oriented control: Step 1

Field-oriented control (FOC) is perhaps the most important invention in the motor world since the AC induction motor. It is more than just a set of vector equations; it is a treatise on how all AC motors fundamentally work. By its basic definition, FOC simply means controlling the orientation of a motor's field(s) to achieve a desired result. From that definition, all motors do field-oriented control in one form or another. Even brush DC motors do FOC; they just accomplish it mechanically by using brushes and a commutator.

Recall that vector control was born out of the desire to control AC motors as we can brush DC motors. Controlling the torque on a brush DC motor is fairly easy to understand, so let's start with that. Remember, to control torque, we must control the motor's current. Control of current can be divided into four discrete steps as shown in figure 1.

1. Measure the current already flowing in the motor. There are many ways to accomplish this, such as using a strategically placed shunt resistor, or some form of inline current sensor. But the goal is the same. If we want to control the current, we first have to measure it.

2. Compare the measured current to the desired current, and generate an error signal.

3. Amplify the error signal to create a correction voltage. To regulate the current, we either increase or decrease the voltage on the motor's terminals. Due to the frequency poles in a typical closed-loop current controlled motor winding, this amplifier often takes the form of a P-I controller.

4. Modulate the correction voltage on to the motor terminals. This is usually done with some type of power switching technique, such as PWM.

When viewed as discrete operations, these four steps represent how you would accomplish torque control on a microcontroller (as an Interrupt Service Routine), where they would be executed thousands, or perhaps tens of thousands of times per second in a sequential fashion.

On a brush DC motor this process only controls the magnitude of the motor current. The brushes and commutator on the DC motor mechanically perform field orientation. They keep the rotor current vector always oriented by 90° with respect to the stator flux vector, to achieve maximum torque per amp (MTPA).

If you understand the process outlined in figure 1, then you are well on your way to understanding field-oriented control. In fact, these same four steps are also used to do FOC on an AC motor. The main difference is that, since AC motors do not have brushes and a commutator, the process must also include the orientation part of the job.

Now, let's go through the FOC process in the context of these four steps. The process is simpler to understand if we first consider a three-phase permanent magnet motor, so let's start with that. Later, we can extrapolate the technique to induction machines. Fasten your seat belts...here we go!