748 BASIC CONTROL SYSTEMS
is essential to every engineer involved in the understanding of the dynamic behavior of various
systems.
This chapter introduces different types of control systems, and some elementary methods for
studying their behavior. Three classes of control systems are presented:- Power semiconductor-controlled drives, in which the electrical input to a motor is adjusted
to control performance. - Feedback control systems, in which a measure of the actual performance has to be known
in order to effect control. - Digital control systems, in which a digital processor becomes an essential element of the
system, and the resulting processed output forms the basis for system control.
Many of the concepts and techniques used may be similar to those already developed earlier
in the book. Indeed, any discussion of control methodology integrates much of the material
on circuits, electronic devices, and electromechanical energy-conversion devices. Such control
techniques are also employed in business, ecological, and social systems, as well as in problem
areas related to inventory control, economic models, health-care delivery systems, and urban
planning.16.1 POWER SEMICONDUCTOR-CONTROLLED DRIVES
Power electronics deals with the applications of solid-state electronics for the control and
conversion of electric power. Conversion techniques require switching power semiconductor
devices on and off. The development of solid-state motor drive packages has progressed to the
point at which they can be used to solve practically any power-control problem. This section
describes fundamentals common to all electric drives: dc drives fed by controlled rectifiers and
choppers; squirrel-cage induction motor drives controlled by ac voltage controllers, inverters,
and cycloconverters; slip-power-controlled wound-rotor induction motor drives; and inverter-
controlled and cycloconverter-controlled synchronous motor drives, including brushless dc and
ac motor drives. Even though the detailed study of such power electronic circuits and components
would require a book in itself, some familiarity becomes important to an understanding of modern
motor applications. This section is only a very modest introduction.
The essential components of an electric drive controlled by a power semiconductor converter
are shown in the block diagram of Figure 16.1.1. The converter regulates the flow of power from
the source to the motor in such a way that the motor speed–torque and speed–current characteristics
become compatible with the load requirements. The low-voltage control unit, which may consist
of integrated transistorized circuits or a microprocessor, is electrically isolated from the converter-
motor circuit and controls the converter. The sensing unit, required for closed-loop operation or
protection, or both, is used to sense the power circuit’s electrical parameters, such as converter
current, voltage, and motor speed. The command signal forms an input to the control unit, adjusting
the operating point of the drive. The complete electric drive system shown in Figure 16.1.1 must
be treated as an integrated system.
A motor operates in two modes—motoring and braking. Supporting its motion, it converts
electric energy to mechanical energy while motoring. In braking, while opposing the motion, it
works as a generator converting mechanical energy to electric energy, which is consumed in some
part of the circuit. The motor can provide motoring and braking operations in both forward and
reverse directions. Figure 16.1.2 illustrates the four-quadrant operation of drives. The continuous
as well as the transient torque and power limitations of a drive in the four quadrants of operation