A method and apparatus for controlling an inverter circuit to convert dc voltage to variable amplitude and frequency ac voltage use firing pattern tables which are stored and selected in accordance with a required voltage level for the ac voltage. The firing pattern tables are accessed by writable timers which both address the firing pattern tables and define the times between readout of consecutive entires of the firing pattern tables. If requirements of the ac voltage being produced are changed, a new firing pattern table is selected at the end of the currently selected pattern table. Further, as the required frequency of the ac voltage being produced changes during the readout of a selected firing pattern table, the times written into the writable timers are changed accordingly to vary the frequency of the ac signal on the fly. Angle tables corresponding to the pattern tables are stored and accessed by a control processor to determine the times written into the timers. The angle tables are stored in terms of angles between transitions of the pattern tables and one or more angle tables are provided for each pattern table. The amplitude of the ac voltage being produced is determined by pattern table selection and angle table section, if more than one angle table is provided for a given pattern table, and the frequency of the ac voltage being produced is determined by the time signals generated by the processor.