Overview

S. Leeb

A modern actively controlled motor drive is a system that combines a power-electronic circuit and a motor. The electronics in the drive system control the behavior of the mechanical shaft of the motor. For example, the electronics might be configured to provide the possibility of directly varying the shaft speed or torque based on a low-power, signal level command input. The earliest variable-speed drives avoided the need for (the unavailable) highly capable power electronic devices by cascading motors and generators, i.e., an M-G set. In an M-G set, a prime mover, such as a steam turbine or an induction motor running from a fixed frequency ac source, would be used to turn the shaft of a generator. The generator s terminal voltage could be controlled through a relatively low-power field or control winding. The generator s variable terminal voltage could then be used to alter the speed of a motor powered by the generator.

An M-G set involves a significant investment in rotating electric machines. For very high-power applications, cascades of electric machines, e.g., a Ward-Leonard style system1, are still occasionally used or maintained. The advent of high-performance power-electronic components, however, has generally made it possible to develop electrical terminal drives that can directly control the flow of power through a motor from an electrical source to the machine shaft. Power-electronic circuits permit the construction of compact and highly efficient motor drive systems that are increasingly used in applications from the milliwatt to the megawatt range, including variable fan...