PURPOSE: To make it unnecessary to process the minimum pulse width, thus simplify the control circuit, by getting pulse modulation signal in comparison of modulated triangular-wave signal in the wave form of which is included the information on the decided minimum pulse width required for commutation with the standard triangular-wave signal.
CONSTITUTION: Instruction pulse generator 28 gives signals ISU∼ISZ for instructing the ignition term to the inverter controller based on frequency instructing pulse f 0 ' sent from the instructor. The generator 28 is formed by pulse modulator 26 outputting frequency signal 6f 0 six times larger than the standard frequency f 0 and pulse modulation signal PM and by wave-form associator 27 for processing the signal. Counter 30 gives output of standard triangular-wave signal SO 1/6 times smaller than the frequency f 0 based on the instructing pulse f 0 '. ROM33 outputs the content of address designated by modulated wave-form address signal SMD as modulated triangular wave signal SM having the range of maximum and minimum levels larger than the minimum pulse width decided by the condition of commutation of inverter, while pulse modulation signal PM on 0 or 1 level being sent out from comparator 34.