Electric and magnetic conditions inside an induction-heated workpiece

THE PURPOSE of this paper is to improve the general understanding of induction heating through a familiarity with what happens electrically and magnetically inside a solid bar being heated. AIEE Standard No. 541 defines induction heating as the heating of a nominally conducting material in a varying magnetic field due to its internal losses. This paper reports the results of a study of how these losses occur, and describes the vector current densities and flux densities inside the material being heated. The AIEE definition is sufficiently general to include not only intentional induction heating as practiced industrially for hardening, forge heating, brazing, and other applications, but also the eddy current and hysteresis heating which appear as core losses in motors and transformers. This paper deals with the case of a long, round, solid bar of homogeneous conducting material located coaxially inside a solenoidal coil carrying sinusoidal alternating current. This is a configuration used frequently in industrial induction heating installations. The same principles apply to more complicated arrangements. The results given here apply over the entire range of sizes and frequencies used in industrial induction heating, as well as to the lower frequencies and smaller dimensions employed to avoid eddy currents and reduce core losses in electromagnetic equipment.