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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