Principles of Induction Heating
Induction heating has been used for decades to heat
various metals. It began in the 1920’s and saw a rapid
expansion during World War 2 [1]. It has been used as a heat
treating process for metallurgy processes, including case
hardening for many years and more recently by the cooking
industry for heating food.
Heating a metal by induction heating begins with a
high−frequency power converter that generates a high ac
current in an inductor, which in turn generates a
high−frequency magnetic field. In the case of induction
heating, a magnetic pan is placed in close proximity to the
heating coil and absorbs most of the energy in the field.
There are two effects that contribute to the heating of the
pan. Eddy current losses in the metal covert part of the
energy into heat. This occurs in any metal pan. In addition,
hysteresis losses occur in magnetic materials which improve
the efficiency of the heating.
Figure 1 shows the series impedance of an aluminum and
cast iron pan over a frequency range of 1 kHz to 1 MHz. It
can be seen that the equivalent resistance of the cast iron pan
is much higher than that of the aluminum pan. This is
because the aluminum pan only has eddy current losses;
however, the cast iron pan experiences both eddy current and
hysteresis losses due to its magnetic properties.