Design and calculation of induction heating coils

WHEN ALTERNATING CURRENT flows in the coil turns of a typical induction heating load, such as shown in Fig. 1, three components of magnetic flux are established, i.e. (1) in the workpiece, (2) in the air gap, and (3) in the copper of the coil turns. This is characteristic of any induction heating load regardless of shape, size, or frequency, and a general equation can be written for total flux linking the coil turns in terms of the magnetizing force H
∗∗∗0
established at the surface of the work. The rate of change of total flux determines the voltage E
c
which must be applied to oppose the counter electromotive force and establish the field intensity H
∗∗∗0
. The current which flows in the coil consists of two components: the one proportional to and in phase with H
∗∗∗0
, and the other proportional to and in phase with the product of total flux Φ
∗∗∗0
and the reluctance of the external flux path. The first is the current which would flow if the external reluctance were negligible (long coil), and its ampere-turns are reflected exactly in the induced current in the workpiece. The second simply supplies the external magnetic drop and its ampere-turns are not reflected in the workpiece.