Q. Explain 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 acmotor 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. The converter regulates the flow of power from the source to themotor in such away that themotor 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 amicroprocessor, is electrically isolated fromthe 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, andmotor 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 must be treated as an integrated system.