| 摘要 |
There are many situations requiring temperature regulation in heating elements which the prior art feedback control systems are not capable of handling adequately. In order to provide a temperature regulation in a narrow range around the Curie temperature, the heating element of the present invention is consisted of a substrate (35) or core (7) (25) of a nonmagnetic material having high thermal and electrical conductivity, clad with a surface layer (9) (27, 37) of a ferromagnetic material of relatively low electrical conductivity. When the heating element is energized by an electrical source (3) of high frequency alternating current, the skin effect initially confines current flow principally to the surface layer of ferromagnetic material. As temperature rises into the region of the Curie temperature of the ferromagnetic material, however, the decline in magnetic permeability of the ferromagnetic material causes a significant lessening of the skin effect, permitting migration of current into the high conductivity non-magnetic core, thereby simultaneously enlarging the cross-sectional area of the current flow path and expanding it into the highly conductive material; the resistance of the heating element becomes less due to both causes. By selecting the proper frequency for energization, by regulating the source to produce constant current, and by selecting dimensions and material parameters for the heating element, temperature regulation in a narrow range around the Curie temperature of the ferromagnetic material can be produced, despite considerable fluctuations in thermal load. |
