PURPOSE: To prevent inverter from failure by comparing the full-wave rectified waveform from the output current of an inverter for induction heating with the mean current waveform, generating pulses at the timing the order of the sizes of the two is inverted, and thereby carrying out operation so that the output voltage of the inverter has an advanced phase always from the output current.
CONSTITUTION: The full-wave rectified waveform I from the inverter output current I0 given by a full-wave rectifier 12 and the mean current waveform I' obtained by passing this waveform I through a filter 31 are fed to a comparator 32, which compares the order of the sizes of the two. The output rectangular wave C0 from this comparator 32 is fed to a pulse shaper circuit 33, and the timing is sensed at which the full-wave rectified waveform I decreases to become smaller than the mean current waveform I', when pulse of γL shall be emitted. The γL pulse signal thus obtained and a frequency command value f* given by a current adjuster 15 are summarized by an adding device 34, and the sum obtained is fed to a V/F circuit 16 to cause generation of frequency pulses (f), and thereby the inverter output voltage V0 can always be kept in advanced phase to the output current I0.
NOMURA TOSHIHIRO
FUJII MASAAKI
SHIRAISHI HIROTAKA
