A three-phase supply system is described which has particular application for single to three-phase converters and variable speed three-phase a.c. motor controllers. This is achieved by using a solid state six element voltage-source inverter (20) to achieve and control the dual conversion of voltage and frequency. The solid state unit is used in the single to three-phase converter and in a three-phase to three-phase converter and consists of six power switches (34), (gate turn-off switches (GTOs)) which invert the fixed d.c. voltage derived from a recti­fied single-phase or three-phase a.c. mains (12,82) to provide a required symmetrical three-phase output supply (22). The use of solid state elements enables the unit to be controlled from a processing unit (54) which can be programmed and set so that the output voltage and frequency is carefully controlled to meet supply requirements for both the three-phase power supply and to achieve variable speed a.c. motor control.