Electric heating – Inductive heating – With heat exchange
Reexamination Certificate
2003-04-24
2004-11-16
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
With heat exchange
C219S670000, C219S674000, C399S328000, C399S330000
Reexamination Certificate
active
06818871
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved induction heating roller unit, and improved fixing device and image forming apparatus incorporating the same.
2. The Basic Construction of the Prior Art
Conventionally, heating rollers which incorporate a halogen lamp as a heat source have been used to thermally fix a toner image. However, heating rollers of this type are disadvantageously inefficient and need a large amount of power. Thus, in order to eliminate the disadvantage, heating rollers involving induction heating are under development.
In Japanese Patent Laid-Open No. 2000-215974, there is described an exciting coil which is disposed near a heating roller made of a magnetic material, which is a body to be heated, and causes an induced current in the heating roller, the exciting coil being formed by deforming a two-dimensionally coiled wire to fit to the curved surface of the body to be heated, and magnetic cores shaped to fit to the curved surface of the exciting coil being disposed at both longitudinal ends of the exciting coil and on the side opposite to the body to be heated (prior art 1).
Besides, in Japanese Patent Laid-Open No. 2000-215971, there is described an induction heating apparatus which comprises a heating rotator that generates heat in an electromagnetic induction manner, that is, a heating roller and magnetic flux generating means disposed inside the heating rotator, and heats a body to be heated by a high frequency induced magnetic flux generated by the magnetic flux generating means causing the heating rotator to generate heat in an electromagnetic induction manner, in which the magnetic flux generating means includes a core made of a magnetic material and an electromagnetic conversion coil wound around the core, and the magnetic material core has a core section around which the electromagnetic coil is wound and magnetic flux guiding core sections for concentrating the magnetic flux from the core section to one region of the heating rotator that are disposed opposite to each other with a magnetic space gap interposed between tips thereof (prior art 2).
The prior arts 1 and 2 are both based on a heating scheme involving an eddy current loss (referred to as “eddy current loss scheme”; hereinafter), which is adopted in IH rice cookers having been commercially practical. Here, the frequency of the high frequency wave in the eddy current loss scheme approximately ranges from 20 to 100 kHz.
On the other hand, In Japanese Patent Laid-Open No. 59-33787, there is described a high frequency induction heating roller comprising a cylindrical roller body made of a conductive material, that is, a heating roller, a cylindrical bobbin disposed in the roller body concentrically, and an induction coil wound around the bobbin in a spiral manner which causes an induced current in the roller body for heating when it is energized (prior art 3).
In the prior art 3, the cylindrical roller body serves as a secondary coil of a closed circuit, and the induction coil serves as a primary coil, whereby a transformer coupling occurs between them to induce a secondary voltage in the secondary coil, that is, the cylindrical roller body. Then, the secondary voltage produces a secondary current flowing through the closed circuit of the secondary coil, whereby the cylindrical roller body generates heat. Thus, the prior art 3 adopts a heating scheme based on heat generation on the secondary side (referred to as “transformer scheme”, hereinafter). The transformer scheme has advantages in that the fixing device has a simple structure compared to the prior arts 1 and 2, because it is higher in efficiency due to its stronger magnetic coupling than the eddy current loss scheme and it can heat the entire heating roller. In addition, if the operating frequency thereof is set at 100 kHz or higher, more preferably, at 1 MHz or higher, the quality factor Q can be increased to provide a high power transfer efficiency. Thus, the total efficiency of heating can be increased, so that power saving can be achieved. Furthermore, the transformer scheme has an advantage in that the fixing device has a simple structure compared to that for the eddy current loss scheme. Furthermore, the heat capacity can be significantly reduced compared to the heating roller for the eddy current loss scheme. Therefore, the transformer scheme is highly suitable for speedup of thermal fixing.
The inventors have already devised a transformer coupling type in which a hollow heating roller coreless-transformer-coupled with an induction coil and rotatably supported thereon has a secondary coil formed into a closed circuit in which the secondary-side resistance is approximately equal to the secondary reactance, whereby the efficiency of power transfer from the induction coil to the heating roller is increased and thus, the heating roller can be efficiently heated. This invention is Japanese Patent Application No. 2001-016335. This invention has facilitated power saving in induction heating of the heating roller and speedup of thermal fixing.
Disadvantage of Prior Art
Furthermore, the inventors have conducted researches on the appropriate distance between the heating roller and the induction coil disposed in the heating roller. As a result, the inventors have found that the efficiency of transfer of high frequency power from the induction coil to the heating roller is closely related to the distance between the heating roller and the induction coil, and in particular, if the distance is equal to or longer than a predetermined value, a high transfer efficiency of high frequency power can be achieved. This finding has lead to the present invention.
SUMMARY OF THE INVENTION
Object of the Invention
An object of the present invention is to provide an induction heating device arranged to transfer a high frequency power from an induction coil to a heating roller with a high efficiency, and fixing device and image forming apparatus incorporating the same.
Characterized Construction of the Invention
An induction heating roller unit according to the invention is characterized in that it comprises an induction coil that has an outer diameter more than 0.7 times that of a heating roller, is disposed inside the heating roller, produces a magnetic field and induces a current in the heating roller to cause the heating roller to generate heat, the current being induced by interlinkage of the magnetic field and the heating roller.
Here, “the induction coil has an outer diameter more than 0.7 times that of the heating roller” means that the outer diameter of the finished induction coil is 70% or more of the outer diameter of a main heat generating part of the heating roller, that is, a part serving as a secondary coil. As the outer diameter of the induction coil becomes closer to that of the heating roller, the magnetic coupling between the induction coil and the heating roller becomes stronger, and therefore, high frequency electrical energy becomes transferred from the induction coil to the heating roller with a higher efficiency. Surprisingly, researches by the inventors have proved that the power transfer efficiency is significantly changed at the outer diameter ratio of 70%. Specifically, if the outer diameter ratio is less than 70%, the efficiency of high frequency electrical power transfer from the induction coil to the heating roller is extremely reduced. To the contrary, according to the invention, since the outer diameter ratio is set to 70% or higher, the efficiency of high frequency electrical power transfer from the induction coil to the heating roller is significantly increased. Within the range of the outer diameter ratio of 70% or higher, the transfer efficiency of high frequency electrical energy increases little by little, while being kept at a high value.
Preferably, the outer diameter of the heating roller which can be practically accommodated in a fixing device in an image forming apparatus is selected from a range from 20 mm to 60 mm. Firstly, this is because a relatively high
Ogasawara Takayuki
Tanaka Takaaki
Yokozeki Ichiro
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Harison Toshiba Lighting Corp.
Leung Philip H.
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