SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

There are also the harmonics associated with the carrier frequency f tri . These are the carrier
harmonics and their sidebands, which we will lump together and call the switching harmonics.
The spectrum of switching harmonics is governed by the frequency modulation ratio defined by
Equation (4.3) [67], [75]. In this report, “harmonics” is generally used to refer to the baseband
harmonics; switching harmonics are not discussed further.
m
f
tri
f (4.3)
f1
At the most basic level (shown in Figure 4.14) the sine-triangle PWM it is an open-loop scheme;
in other words, it functions as a controlled voltage source. The usual way to make it closed-loop
is to generate the voltage command by closing the loop around current; closed-loop current
control of voltage converters is the most commonly used method [124]. The result is called a
current-regulated PWM (CRPWM) inverter and it functions as a controlled current source. There
are essentially three groups of methods that can be used to make a CRPWM [78, p.322]. The first
method is to use some sort of ramp-comparison controller. The simplest example of this type is
shown in Figure 4.15. When the reference and feedback signals are phase variables, this method
has the disadvantage that the current ripple is fed back into the command [125] and usually use
filtering [162] which would limit the bandwidth and stability [47]. Further the tuning of the
regulator is dependent on the frequency of operation. However, these problems are limited can be
eliminated by executing control in the synchronous reference frame (discussed in a later section).
Figure 4.15 – Ramp-comparison current regulator for one phase.
The second method to make a CRPWM is to use a bang-bang controller with hysteresis (also
called tolerance band controller) [75, p.241], [61, p.243], [80, p.252]. In this method the carrier is
eliminated and the switching signals are generated directly by feeding the current error to a
comparator with hysteresis, as shown in Figure 4.16. The advantages of this method are that it is
simple, it inherently compensates for dead-time distortion, it has excellent disturbance rejection,
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