This book is intended to explain the technical principles involved in the many AC variable speed drive systems available today. It deals with all the DC link inverter and direct AC to AC converter systems that are in commercial use. The principles of AC motors are considered specifically from the variable frequency point of view, and this chapter concentrates on the effects of harmonics. The different types of power semiconductor switches are considered separately from the drive systems in which they are used. A total of seven separate and technically different drive systems are considered in such a way that their principles can be fully understood and their performance capabilities explained. Square wave and pulse width modulated DC link inverter systems, cycloconverters and slip power recovery drives are all included in this comprehensive book. This book has been written so that it can be understood by general engineers, not just by experts in the field. It should therefore be of great use to any engineer involved with variable speed drives in any capacity. It should also be of interest to university and college electrical engineering departments and students.