You might be surprised how low the frequency needs to be for melting even relatively small crucible loads. My 100kW furnace runs at frequencies ranging from 2.2 kHz to 3.25 kHz for crucibles of about
3 - 12 kgs load in iron. My low power experimental unit that I made last year ran at 20kHz for a 500 gram load. The resonant frequency is of course very dependant on the charge in the crucible. Remember that standard graphite crucibles conduct electricity so will heat up when empty and waste energy. You should use an alumina one.AWEM