Adjustable-frequency invertors and their application to variable-speed drives

If a squirrel-cage induction motor is to be operated efficiently over a wide speed range, it must be supplied from a variable-frequency source whose frequency is adjustable over a range similar to that required for the motor speed. It is now technically possible and economically acceptable to provide power at variable frequency using silicon-controlled-rectifier (or thyristor) invertors. The paper reviews the already well known invertors and concludes that the d.c. link type is preferable for most variable-speed applications. Various methods of artificial commutation are described and applied to invertor circuitry suited to the supply of a squirrel-cage motor. The design flexibility of such an invertor is discussed with reference to component values and losses. Various methods of obtaining the necessary variable voltage are presented. Three different types of control circuits and their influence on drive characteristics are given. Finally, the characteristics of the invertor and motor forming a variable-speed drive are described, and applications are considered where speed, temperature, waterproofing and electrical maintenance are a limitation.