Melting of a cold insert in a electric furnace without liquid heel.
An important factor is the penetration depth into the material to be molten in the induced current in the NF induction furnace (skin effect). For rational energy transmission, the diameter of the workpiece has to be coordinated with the penetration depth. The optimum workpiece size is approx. 4 times the value of the penetration depth. Consequently, NF furnaces require a minimum diameter of 300 mm (according to F. Neumann: 80 mm penetration depth at 50 Hz; 80 x 4 = 320 mm) and respectively large insert pieces (starter block) have to be used to achieve efficient coupling.
With increasing frequency, the penetration depth is decreased so that coupling of small sizes already occurs. This also enables cold starting of MF furnaces.