SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 4.12 – Commutation and PWM configurations in 120° inverter.
PWM
The PWM inverter controls the magnitude, waveshape, and frequency of the output voltage at
each terminal by adjusting the duty cycle of the gating signal for each leg—it is the familiar
“standard” usage of PWM applied to three legs. Unlike the previous two methods, three-phase
PWM inverters are only found in the 180° variety because the objective is to produce an output
waveform whose average is continuous; it would not make sense to turn off both transistors in a
leg at any time. The objective is to continuously adjust the duty cycle such that the average
voltage follows some commanded value as shown in Figure 4.13. The waveform can be arbitrary
(within limits) but this report only concerns sinusoidal PWM (SPWM). With this restriction we
can then say the objective is to continuously adjust the duty cycle to produce an output voltage
with a fundamental component of the desired amplitude and frequency.
Figure 4.13 – Concept of PWM for one phase leg; not to scale.
It is the three-phase PWM inverter that is the subject of the remainder of this chapter, thus the
120° inverter will not be discussed further. The traditional method to control each leg of the
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