SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 4.11 – Ideal voltage waveforms for 120° six-step squarewave inverter with resistive load.
Six-Step PWM
The six-step PWM technique is essentially the same as the six-step squarewave except the voltage
regulation is performed by the transistors using pulse width modulation. The commutation can be
performed using the top transistors and the PWM can be performed using the bottom transistors.
Or the opposite could be done. In addition to practical differences in implementation, these
choices affect which quadrants the inverter can operate in [118]. PWM could be used on both top
and bottom transistors, resulting in synchronous rectification, which affects current ripple. The
basic varieties are shown in Figure 4.12 for a 120° inverter. Note that the PWM exists to maintain
an average voltage—the duty cycle is essentially constant over a commutation period. If the duty
cycle were 0% in Figure 4.12 it is clear that only one phase would be connected to the bus at any
time thus the load voltage would be zero. If the duty cycle were 100%, during any 60°
commutation period two phases would be connected across the bus (transistors full-on).
Therefore the six-step PWM is the same as six-step squarewave mode when the duty cycle
reaches 100%. It seems that the six-step PWM method is used primarily for trapezoidal BPMS
motor control and the 120° inverter is the de facto implementation. A 180° variety of the six-step
PWM inverter was at one time common in variable speed induction motor drives (sometimes it
was called block modulation) but is no longer in common usage [78, p.19]. Information on 120°
PWM in six-step commutation is presented in [117], [118].
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