An a.c. motor is controlled by the output of a three phase inverter that generates signals with the required phase differences. The inverter has transistor switching stages controlled by a microprocessor operating with an EPROM memory programme and receiving pulses from a counter coupled to a clock circuit. The microprocessor sets the counter value and the counter cycles until the zero state is obtained. When the clock is in a zero state an interrupt is generated to transfer the transistor switching states into a buffer (25) and the inverter generates the specified output.
