SOLUTION

Choice of Frequency for Induction Heating

Induction heating is used to heat metals for different purposes such as melting, forging, annealing, hardening, brazing, and soldering. The choice of frequency depends on the factors described below:

Size of the object

For a given current and frequency in the inductor coil, the flux density in the charge remains constant. Therefore, the induced emf is proportional to the square of the radius of the charge. The increase in radius increases the length and cross-sectional area of the eddy current path, proportionately. Thus the resistance offered to the path of flow of eddy current remains constant. In order to achieve the same amount of heat for a given material, the supply frequency should, therefore, be reduced with the increase in the size of the charge.

Permeability

The flux density in magnetic materials is high. Therefore, to produce the same amount of heat in magnetic materials compared to non-magnetic materials, the frequency required will be less.

Depth of penetration

Hardening of metals requires a very high frequency for shallow depths of penetration.

Resistivity of material

The frequency required is less for a higher resistivity of the material.

Thermal conductivity

The greater the thermal conductivity of the material, the higher will be the frequency needed as the heat gets conducted quickly.