The following pages are meant for those who wish to use thyristors. The details of the physics of semiconductor materials or the design of thyristors themselves are unnecessary here. Still, a general description of the device may help to avoid pitfalls during electric circuit design. The thyristor is the internationally recognized name for a particular semi­ conductor device. The name is derived from Greek, the first part meaning switch and the second part an association with the transistor family. It has a trading name, viz. SCR (silicon controlled rectifier) and got this name principally because it is a silicon device and used as a rectifier that can be controlled. It forms a group together with the electromagnetic relay, the thyratron and the mercury arc rectifier as a controlled switch. The advantages and disadvantages of the thyristor become apparent in describing the device and its range of application. However, the present general interest, development, and use of thyristor indicate that its many advantages make it superior to other devices for many cases. Control of rotating electric machines is a major interest of the author so that in this book, the applications of thyristor are towards this end. Thyristors are used so much in connection with the control of machines that it is worthwhile to go into some details of both the electric drive to be controlled and the possible thyristor control units. This book is primarily concerned with the application of thyristors in the control of electric motors. The first chapter gives an overall picture of electric drive control and the part that power electronics plays. Chapter Two describes the physical electronics sufficient for the reader to feel for the thyristor as a device. The next three chapters deal exclusively with the thyristor as a