SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

ut because they are the best basis for understanding the more advanced PWM techniques such as
SVM.
The simplest method is to use a controlled front-end converter to control the voltage and switch
(commutate) the transistors in a certain pattern to generate a fixed-shape output voltage waveform
that varies in frequency. In both 120° and 180° inverters the simplest pattern involves six steps
per electrical cycle and results in rectangular output voltage waveforms; this is called a six-step
squarewave inverter. The second method uses the same six steps but eliminates the front-end
converter by integrating the voltage control with the commutation; this is called a six-step PWM
(or squarewave PWM) inverter. The third method does not use commutation. Instead, pulse width
modulation is used to control directly the magnitude, waveshape, and frequency of the output
voltage; this is called a PWM inverter. (There is a fourth method of control but since it is usually
associated with SVM its presentation is delayed until that section.) 29
Six-Step Squarewave
Both 120° and 180° inverters can be operated as a “six-step squarewave” inverter in which the
output voltage is rectangular. In both inverters the voltage is controlled by the front-end converter
and the rate of transistor commutation determines the frequency of the output. The gating and
pole voltages are shown in Figure 4.9. (The full voltage waveforms are given in Figure 4.10 and
Figure 4.11.) Because the 180° inverter always has three switches closed the neutral voltage will
never be zero and therefore the line-neutral voltage will never be the same as the pole voltage. In
the 120° inverter it appears that the line-neutral voltage is always the same as the pole voltage.
However, this is true only for a resistive load [37, p.379]. The flat-top segments of the pole
voltages will always be as shown since the transistors are ON. Contrarily, the zero-voltage
segments of the pole voltage will not always be zero because current can flow through the diodes
antiparallel to the transistors. This causes the neutral voltage to be nonzero, which causes the lineneutral
voltages to deviate from the pole voltages. In addition, when the load is a motor it will
have a bEMF that influences the pole and line-neutral voltages during these segments.
29 In addition to these methods there are numerous “advanced” techniques for both overall inverter control
and for control of the PWM specifically, such as on-line optimization techniques, selective harmonic
elimination (SHE), delta modulation, sigma-delta modulation, random pulse width modulation (RPWM),
sliding mode control (SMC), fuzzy methods, and innumerable artificial intelligence schemes (artificial
neural networks, fuzzy-neural nets, and combinations of neutral control with most every other control
method). For the most part each of these is a technique that exists to improve some specific aspect of
operation or control and a comparison of the techniques would be a work in itself. Many are complicated
enough or do not provide quantifiable benefits so they are still academic curiosities that are not
implemented in industry; as such, they deserve no mention here.
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