This is the first part of a series of three articles on the speed control of squirrel-cage motors with pulse-width modulation (PWM) frequency converters. The squirrel-cage motor is the most widely used type of electric motor in industry today. Its excellent features can be used to great effect in variable-speed drive applications by the insertion of a frequency converter between the motor and the supply. The authors' company has been one of the pioneers in the development of such converters and, with the experience of several thousand installations worldwide, is well qualified to present this informative series.