Solid-state ac variable-frequency drives offer a new generation of high-performance, maintenance-free drive systems. The desire to use the rugged ac motor as a high-performance, variable-speed device goes back many years; however, its practical implementation had to wait three-quarters of a century for development of the silicon controlled rectifier. This paper opens with a brief review of the history of variable-speed dc and ac drive systems. Then the fundamentals of solid-state ac variable-speed drives are presented with emphasis on future technical and economic trends. The ac system is evaluated as an economic solution to providing basic functional drive requirements. The characteristics are compared with traditional high-performance dc systems. Finally, an attempt is made to describe the solid-state ac variable-speed drive of the future.
