induction working pricipals

The basic setup is an AC power supply that provides electricity with low voltage but very high current and high frequency. The workpiece to heat is placed inside an air coil driven by the power supply, usually in combination with a resonant tank capacitor to increase the reactive power. The alternating magnetic field induces eddy currents in the workpiece.

The frequency of the inductive current determines the depth that the induced eddy currents penetrate into the workpiece. In the simplest case of a solid round bar, the induced current decreases exponentially from the surface. An "effective" depth of the current-carrying layers can be derived as {\displaystyle d=5000{\sqrt {\frac {\rho }{\mu f}}}}, where {\displaystyle d} is the depth in centimeters, {\displaystyle \rho } is the resistivity of the workpiece in ohm-centimeters, {\displaystyle \mu } is the dimensionless relative magnetic permeability of the workpiece, and {\displaystyle f} is the frequency of the ac field in Hz.^[3] The equivalent resistance of the workpiece and thus the efficiency is a function of the workpiece diameter {\displaystyle a} over the reference depth {\displaystyle d}, increasing rapidly up to about {\displaystyle a/d=4}.^[4] Since the workpiece diameter is fixed by the application, the value of {\displaystyle a/d} is determined by the reference depth. Decreasing the reference depth requires increasing the frequency. Since the cost of induction power supplies increase with frequency, supplies are often optimized to achieve a critical frequency at which {\displaystyle a/d=4}. If operated below critical frequency, heating efficiency is reduced because eddy currents from either side of the workpiece impinge upon one another and cancel out. Increasing the frequency beyond the critical frequency creates minimal further improvement in heating efficiency, although it is used in applications that seek to heat treat only the surface of the workpiece.

Relative depth varies with temperature because the resistivities and permeability vary with temperature. For steel, the relative permeability drops to 1 above the Curie temperature. Thus the reference depth can vary with temperature by a factor of 2-3 for nonmagnetic conductors, and by as much as 20 for magnetic steels.