A variable-speed squirrel-cage motor has been a desirable aim since the general acceptance of a.c. supplies. The paper contains a description of a suitable and practically proved system of variable-frequency control for this and other purposes provided by a self-contained rotating frequency convertor, called the Speedmaster. The theory and application of this frequency convertor and some variants to a.c. variable-speed single- and multi-motor drives, utilizing different types of a.c. motors, are presented. Selected examples of actual equipments are included.