310 Electrical Power Systems Technology
the heat production of this process is extremely rapid. The area of the met-
al that is actually heated can be controlled by the size and position of the
heating coils of the induction heater. This type of control is difficult to ac-
complish by other methods. Induction furnaces use the induction-heating
principle.
By varying the frequency of the voltage applied to the induction
heater windings, it is possible to vary the depth of heat penetration into
the heated metal. At higher frequencies the heat produced by the induced
current from the heating coils will not penetrate as deeply, because of the
so-called “skin effect.” Thus, heat will penetrate more deeply at lower fre-
quencies. When heat must be localized onto the surface of a material only-
for example, for surface hardening of a metal—higher frequencies are
used. The cost of higher-frequency induction heaters is greater, because
more complex oscillator circuits are required to produce these frequen-
cies.
Dielectric (Capacitive) Heating
Induction heating can only be used with conductive materials. There-
fore, some other method must be used to heat nonconductive materials.
Such a method is illustrated in Figure 12-3 and is referred to as dielectric
or capacitive heating. Nonconductors may be heated by placing them in an
electrostatic field, created between two metal electrodes that are supplied
by a high-frequency AC source. The material to be heated becomes the
dielectric or insulation of a capacitive device. The metal electrodes consti-
tute the two plates.
When high-frequency AC is applied to a dielectric heating assembly,
the changing nature of the applied AC causes the internal atomic struc-
ture of the dielectric material to become distorted. As the frequency of
the AC increases, the amount of internal atomic distortion also increases.
Figure 12-3. Capacitive heating principle