The lowdown on motor control: Fundamentals

As I pointed out in my earlier article, motors are all around us, and an important part of many system designs. Field oriented control (FOC) is a high-performance technique for motor control that is becoming increasingly attractive for all kinds of applications. It's worthwhile, therefore, for developers to have a basic understanding of FOC.

Let's start by briefly comparing FOC to the more traditional trapezoidal commutation on brushless DC (BLDC) motors. But first, let's address the misnomer that BLDC motors are DC motors. They are in fact AC motors. If you look at the voltages and currents on a BLDC motor, they alternate back and forth just like any other AC motor.

So, which works better on BLDC motors—FOC or commutated control? This is a more complicated question to answer than you might think.

If torque ripple or audible noise is a concern, then the shape of the back-EMF waveform becomes very important. Most BLDC motors in fact have a sinusoidal back-EMF waveform. If you employ trapezoidal commutation on a motor like this, torque ripple will occur which is synchronous to the commutation process, as illustrated in figure 1. To achieve low-torque ripple with trapezoidal commutation (especially at lower speeds), the back-EMF waveform should be trapezoidal in shape. But this back-EMF shape is harder to achieve in motor design than sinusoidal back-EMF since the airgap flux would need to be held constant. You can reduce the torque ripple on motors with sinusoidal back-EMF by driving them with sinusoidal currents, like those produced by FOC.

Another design consideration is efficiency. In most cases, a BLDC motor with sinusoidal back-EMF will run more efficiently with FOC than with trapezoidal commutation. This is because the higher harmonics in the trapezoidal current waveform do not have corresponding equals in the back-EMF waveform, and therefore don't contribute to airgap power. Instead, they simply increase the heat in the motor windings. On the other hand, trapezoidal commutated control at full speed can be achieved without PWM, which significantly reduces switching losses in the inverter. So with FOC, the motor usually runs more efficiently; but with trapezoidal commutation at full speed, the inverter runs more efficiently. Which is best depends on your particular motor and inverter.

Bus utilization is another concern. The fundamental harmonic of the voltage waveform resulting from trapezoidal commutation is larger than the highest voltage sinewave possible with FOC, which allows the motor to achieve higher speeds. You can increase the FOC voltage using modulation enhancements such as third harmonic injection and space vector modulation, increasing the FOC voltage amplitude 15% by dynamically shifting the common-mode voltage of the output waveforms. But this is still not equal to the fundamental voltage harmonic with trapezoidal commutation. More advanced FOC systems (such as InstaSPIN motor control technology from Texas Instruments) allow you to over-modulate the voltage waveform until it gradually morphs into a trapezoidal form. This increases the modulation index by 33% compared to sinusoidal modulation alone.