Induction heating is caused, as the name implies, by induced electric currents in a material. Just as electrical current can produce a magnetic field which ultimately can be used to power a motor, so can a magnetic field be used to produce secondary current in a material. The principle is exactly the same one used in constructing a voltage transformer whereby one AC voltage across the primary windings of a transformer will induce a different AC voltage (higher or lower) across the secondary windings. The voltage ratio of the transformer is solely dependent on the ratio of coil windings due to electromagnetic principles, and the frequency is the same for both AC voltages.
In an induction furnace, the metal to be heated becomes the “secondary” path. In a coreless induction furnace, shunts are used to cover a significant portion of the furnace coil which are used to focus the magnetic field back into the crucible/metal. These shunts contain layers, with insulating barriers.
O’Donnell performed engineering analysis on issues of such induction furnaces undergoing upgrades.