Alternating current electrically resistive heating element having intrinsic temperature control
First Claim
1. An alternating-current electrically resistive heating element electrically coupled to a source of high frequency electric power, said heating element having an electrical resistance which, at least over a certain range of temperatures, declines with increasing temperature, and comprises:
- an electrically conductive non-magnetic substrate member of high thermal and high electrically conductive material and having over at least a portion of the surface thereof, a generally thin layer of a magnetic material having, below its Curie temperature, a maximum relative permeability greater than 1 and above its Curie temperature a minimum relative permeability of substantially 1, whereby when said heating element is electrically coupled to said source of high frequency electric power, an alternating current flows at said high frequency, causing Joule heating of said element, said current being principally confined by said maximum permeability to said generally thin magnetic layer in accordance with the effect at temperatures below the Curie temperature of said magnetic layer, said current spreading into said non-magnetic member as temperature rises to approach said Curie temperature and said relative permeability declines.
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Accused Products
Abstract
The heating element consists of a substrate or core of a non-magnetic material having high thermal and electrical conductivity, clad with a surface layer of a ferromagnetic material of relatively low electrical conductivity. When the heating element is energized by a source 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.
476 Citations
11 Claims
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1. An alternating-current electrically resistive heating element electrically coupled to a source of high frequency electric power, said heating element having an electrical resistance which, at least over a certain range of temperatures, declines with increasing temperature, and comprises:
an electrically conductive non-magnetic substrate member of high thermal and high electrically conductive material and having over at least a portion of the surface thereof, a generally thin layer of a magnetic material having, below its Curie temperature, a maximum relative permeability greater than 1 and above its Curie temperature a minimum relative permeability of substantially 1, whereby when said heating element is electrically coupled to said source of high frequency electric power, an alternating current flows at said high frequency, causing Joule heating of said element, said current being principally confined by said maximum permeability to said generally thin magnetic layer in accordance with the effect at temperatures below the Curie temperature of said magnetic layer, said current spreading into said non-magnetic member as temperature rises to approach said Curie temperature and said relative permeability declines. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
Specification