Nearly all systems in power
electronics rely on feedback control systems for their operation. This chapter
presents the basic analog analysis of such systems because, in this author’s
opinion, it offers a more intuitive understanding of their behavior than can be
obtained from modern control theory with digital techniques.

Many systems require nested
control loops to control several variables. One example is a DC motor drive
that must have a very fast current control loop to limit the armature current
but also requires a voltage loop for speed control.

The voltage control cannot
override the current loop, but it will set the required current so long as it
is within the limits set by the current loop. In short, the voltage or speed
loop commands the current that is required to satisfy the voltage, but the current
loop sets the current limit.

Both loops must be
unconditionally stable. Figure 4.9 shows a typical system.

The armature current is regulated
by feedback from a current shunt and isolator amplifier. The frequency of such
a current regulator using SCRs can have a crossover as high as 1000
radians/sec, but 500 radians/sec is easier to handle and less critical on
feedback.

If the current loop is set up for
500 radians/sec, the voltage loop must, generally, crossover at a decade lower
in frequency, 50 radians/sec, for stability on a 50- or 60-Hz system.

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