Two positive-side IGBTs (IGBTp) and two negative-side IGBTs (IGBTn) are switched alternately. The switching sequences and PWM control signals are prepared manually with a personal computer in advance and fed to the memory of a pulse generator (PG). Then, the inverter is operated according to the pulse signals from the PG in an open loop and produces a quasi-sinusoidal current waveform with a frequency of 50/60 Hz and an amplitude of lO A. The switching frequency of the inverter was 1