Electromagnetic induction image heating device and image forming apparatus

Abstract

An excitation coil is arranged so as to be opposed to a rotatable heat generating roller of a conductive material, and on a rear side of the excitation coil, a core of a magnetic material is provided. The core is composed of a central core that is formed continuously in a rotation axis direction of the heat generating roller, and a plurality of U-shaped cores arranged at a distance from each other in that direction. A high-frequency current is applied to the excitation coil so that the heat generating roller generates heat by electromagnetic induction. An additional coil is wound around the U-shaped core. Both ends of the additional coil are connected to a switching unit. When the switching unit is brought to a connected state, under an induction current generated in the additional coil, a magnetic flux in a direction in which a magnetic flux of the excitation coil is cancelled out is generated, so that heat generation of the heat generating roller can be suppressed. The switching unit is switched over according to a width of a paper sheet to be passed and a temperature distribution in the rotation axis direction. Thus, a uniform temperature distribution of the heat generating roller in the rotation axis direction can be maintained.