Some induction practitioners have heard about simultaneous dual frequency gear hardening. Coil current waveform comprises two appreciably different frequencies making effect of two single frequency inverters working on the same coil at the same time. Exact frequency selection and required powers depend on tooth geometry. Lower frequency (i.e, 3-10kHz) helps autenitize the roots of the teeth and higher frequency (70-400kHz) helps austenitize the teeth flanks and tips.
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