Electrical Power Systems Technology

Heating Systems 309

is produced in magnetic materials when they are exposed to an alternat-

ing current (AC) field. In the example shown, current is induced in the

material heated by electromagnetic induction. This is brought about by

the application of an AC to the heating coil. The material to be heated

must be a conductor in order for current to be induced. Ordinarily, a high-

frequency AC source in the range of 100-500 kHz is used to produce a high

heat output. This high heat output is due to greater amounts of induced

voltage.

As the magnetic field created by the high-frequency AC source

moves across the material to be heated, the induced voltage causes eddy

currents (circulating currents) to flow in the material. Heat results because

of the resistance of the material to the flow of the eddy currents. The heat

is produced rapidly by this method, which is an advantage.

The major application of the induction-heating process is in metal-

working industries, for such processes as hardening, soldering, melting,

and annealing of metals. Compared to that of other methods of heating

Figure 12-1. Resistance heating principle

Figure 12-2. Induction heating principle