Evolution of control techniques for industrial drives

The control system of any variable speed electric drive consists mainly of an outer loop for controlling the mechanical output of the drive, (e.g., torque, speed or position) which generates commands to the inner loop, that controls the electromagnetic states of the electric machine (e.g., voltages, currents and magnetic flux) to produce the desired response. The configuration of the inner loop controller depends on the type of the electric motor and the topology of the power converter being used which in turn are dependent on the requirements of the application. DC variable speed drives were the industrial work horses until the 1960s due to the simplicity of their control. Since the late 1960s, advances in semiconductor technology for power as well as control applications, enabled AC variable speed drives to compete with DC drives in various industrial applications. Most present day research and development efforts are aimed at providing higher performance and more reliable AC drives at a lower cost than those of comparable DC drives. This paper provides an overview of the classical and modern control techniques for both DC and AC drives along with their applications.