Except for old tube units, the incoming ac is going to get rectified to dc, then sent through 4 IGBTS to turn it to high frequency ac, then a transformer to drive the work coil and the capacitor.
You will need to investigate what it has for filter caps and what current they are rated for, as well as the diodes used in the rectifier. It should work fine with a 50% derate like any other vfd can do.
Whether or not its continuous duty matters if the diodes can handle the current. You can also run it from a rotary phase converter, if under load, you verify the third generated phase is delivering a substantial amount of current compared to the other 2 lines.
if there isn't much filtering on the dc side of the rectifier, you're going to get a 120hz ripple in the metal you're melting. this may also cause the unit not to work properly, as it constantly changes the frequency based on current flowing in the coil. 120hz ripple from the rectifier may not be anticipated in the control circuitry and it may not be able to deliver full output power.
there is a chance that the incoming power is sent through some other circuitry to boost the voltage. in which case it will need 3 phase to work properly unless you can bypass that circuitry and run it off a transformer and rectifier to deliver the voltage it needs. 480volts 3 phase could be transformed into 800-1200 volts dc for example, so as to draw 99% power factor from the ac line.
