The current density of the alternating current in the coil conductor and the eddy current in the metal billet is not uniformly distributed on the cross-section, and the maximum current density appears on the surface of the cross-section and decays to the center with the regularity of exponential function, which is called skin effect.
The reason for skin effect: When alternating voltage U is applied to both ends of the metal billet, an alternating electric field is established inside the metal billet. As a result of electromagnetic induction, the alternating magnetic field formed by the current in the billet produces an opposite direction
The ring effect is favorable for heating the outer surface of the cylindrical blank. This is also why there is no need to add a magnet when the blank is heated in the solenoid sensor. However, for inner hole heating or plane heating, the ring effect is unfavorable to heating. In this case, magnetic flux can be used to change the state of magnetic field lines and drive the current from the inside to the outside.
It is divided into the billet end effect and the induction coil ends the effect. The skin effect describes the magnetic field distribution on the cross-section of the metal billet while the end effect shows the magnetic field distribution on the end of the billet and induction coil. It will affect the power distribution along the billet axis and the distribution of the heating temperature of the billet. For the non-magnetic billet in a longitudinal magnetic field, the end effect of the billet increases the absorbed power of the billet. For magnetic billets, the end effect increases or decreases the absorbed power depending on the radius of the billet, material characteristics, frequency, and magnetic field strength. Induction heating is a comprehensive application of the above four effects. The effect of the inductor coil system is the ring effect, while the effect of the billet system is the skin effect. The proximity effect and end effect are between the two.