JP6463021B2 - Fixing device - Google Patents

Description

  The present invention relates to a fixing device that heats a toner image on a recording material using a method in which a cylindrical metal member itself generates heat directly by electromagnetic induction current.

  An electrophotographic image forming apparatus fixes toner on a recording material by heating and pressurizing a toner image formed on the recording material. As a fixing method, a ceramic heater method in which a heating element such as a ceramic heater supplies heat to the heating member, or a current is passed through the heating member by a magnetic field generated by an exciting coil, and the heating member itself is generated by Joule heat generated at that time. And an electromagnetic induction system that generates heat. In the electromagnetic induction method, since the heating member itself generates heat, it is considered that this is a fixing method with less heat loss to the part other than the heating member and superior thermal efficiency compared to the ceramic heater heating method.

  FIG. 17 shows an example of an electromagnetic induction fixing device disclosed in Patent Document 1. The fixing device mainly includes a cylindrical heating roll 201, an elastic pressure roll 205, a split core 202, a coil bobbin 204, an excitation coil 203, and a pressure mechanism (not shown). Inside the heating roll 201, the divided cores 202 are arranged at predetermined intervals, and the coil bobbin 204 and the exciting coil 203 are arranged outside the divided core 202.

  The exciting coil 203 is wound in the rotation direction of the heating roll 201, and the split core 202 is magnetized in the axis X direction by passing a current through the exciting coil 203. Due to the electromagnetic induction, a current flows in the heating roll 201 in a direction that hinders the change in the lines of magnetic force generated at this time, and the heating roll 201 generates heat. A fixing nip N is formed between the heating roll 201 and the pressure roll 205 by a pressure mechanism (not shown), and the recording material is nipped and conveyed in the fixing nip N to fix the toner image on the recording material.

JP 2004-341164 A

  However, it has been found that the temperature of the heating roll 201 differs at the position in the axis X direction depending on the frequency of the high-frequency current flowing through the exciting coil 203. Specifically, when the temperature of the heating roll 201 is compared between the center portion and the end portion in the axis X direction, the temperature is higher at the end portion in the axis X direction when the frequency is high, and the axis is set when the frequency is low. The temperature is higher at the center in the X direction.

  Conventionally, the temperature of a specific part of the heating roll 201 is measured, and control is performed so that the entire heating roll area falls within the appropriate temperature range based on the measured temperature. However, if the temperature distribution in the heating roll axis X direction changes depending on the frequency, the conventional heating method may not be able to keep the entire heating roll within the appropriate temperature range. For this reason, there are problems such as uneven glossiness in the direction of the axis X, and toner offset to the heating member 201 side particularly in a region deviated from the appropriate temperature range to the high temperature side.

  In order to solve the above-described problems, the present invention can perform temperature control so that the temperature of the sheet passing area of the rotating body as the heating member is within the appropriate temperature range at any driving frequency within the possible driving frequency range. The purpose is to propose a device.

A typical configuration of the fixing device of the present invention for achieving the above object is as follows.
A cylindrical rotating body having a conductive layer;
A magnetic core material that is inserted through the hollow portion of the rotating body and is long in the generatrix direction of the rotating body;
In the hollow portion, the end portion in a series from the one end side to the other end side along the longitudinal direction of the magnetic core member directly or through other objects on the outer peripheral surface of the magnetic core member, and compared to the central portion in the longitudinal direction. An exciting coil formed by spirally winding a coil wire so that the density is higher,
Fixing in which the conductive layer generates heat by electromagnetic induction by passing an alternating current through the excitation coil via the power supply contact on one end side and the other end side of the excitation coil, and this heat is used to fix the image on the recording material. A device,
The magnetic core material does not form a loop outside the rotating body,
A temperature sensing element for measuring the temperature of the rotating body;
The rotating body when the the most temperature increases position generatrix direction of the rotary member when the largest in the range of frequencies where the frequency can adopt the alternating current, the lowest frequency within the range of frequencies which can be employed The temperature detecting element is arranged at both the position where the temperature is highest in the direction of the bus .

  As described above, according to the present invention, it is possible to know the highest temperature in the generatrix direction of the rotating body within a possible driving frequency range, and as a result, it is possible to prevent gloss unevenness and toner offset.

FIG. 6 is a graph showing the result of measuring the temperature distribution at the axial position in the fixing device of Example 1. 1 is a schematic configuration diagram of an image forming apparatus using a fixing device according to a first exemplary embodiment. Cross-sectional side view of the main part of the fixing device of Example 1 1 is a perspective view of a main part of a fixing device according to a first embodiment. Diagram explaining two types of magnetic cores Illustration of the magnetic field existing near the magnetic core Magnetic equivalent circuit diagram of FIG. Experimental configuration diagram to obtain the relationship between the magnetic flux ratio passing through the outside of the heat generation layer and the conversion efficiency Experimental result diagram of the relationship between the magnetic flux ratio passing through the outside of the heat generation layer and the conversion efficiency The figure which shows the magnetic coupling of the concentric shaft transformer of the shape which wound the primary coil and the secondary coil Diagram showing equivalent circuit Diagram showing the relationship between the position and the equivalent inductance relative to the magnetic core Diagram showing boundary condition between magnetic field and magnetic core existing in uniform magnetic field Diagram showing equivalent circuit considering apparent permeability The figure explaining the influence on the synthetic impedance by the apparent permeability and the number of turns The figure explaining temperature control using two temperature detection elements Perspective view of the main part of a conventional electromagnetic induction type fixing device

Example 1
Embodiments of the present invention will be described below. However, the material, shape, relative position, and the like of the component parts described in the embodiments are not intended to be limited unless otherwise specified.

<Overall Configuration of Image Forming Apparatus Example>
FIG. 2 is a schematic configuration diagram of an image forming apparatus 100 using the fixing device of this embodiment. The image forming apparatus 100 is an electrophotographic laser beam printer.

  Reference numeral 101 denotes a photosensitive drum as an image carrier, which is driven to rotate in a clockwise direction indicated by an arrow at a predetermined process speed (circumferential speed). The photosensitive drum 101 is uniformly charged to a predetermined polarity and potential by the charging roller 102 during its rotation.

  Reference numeral 103 denotes a laser beam scanner as an image exposure unit, which outputs a laser beam 113 that is on / off modulated in response to a digital pixel signal input from an external device such as a computer (not shown), and outputs the photosensitive drum 101. The charged surface is subjected to scanning exposure. By this scanning exposure, the charge of the exposed bright portion on the surface of the photosensitive drum 101 is removed, and an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum 101.

  A developing device 104 supplies developer (toner) to the surface of the photosensitive drum 101 from the developing roller 104a, and the electrostatic latent image on the surface of the photosensitive drum 101 is sequentially developed as a toner image which is a transferable image. Is done. Reference numeral 105 denotes a feeding cassette on which the recording material 114 is loaded and stored. The feeding roller 106 is driven based on the feeding start signal, and the recording material 114 in the feeding cassette 105 is separated and fed one by one. Then, the toner is introduced at a predetermined timing into a transfer portion 108T that is a contact nip portion with the transfer roller 108 that is rotated by contact with the photosensitive drum 101 via the registration roller pair 107.

  That is, the conveyance of the recording material 114 is controlled by the registration roller 107 so that the leading edge of the toner image on the photosensitive drum 101 and the leading edge of the recording material 114 reach the transfer portion 108T at the same time. Thereafter, the recording material 114 is conveyed while sandwiching the transfer portion 108T, and during that time, a transfer voltage (transfer bias) controlled to a predetermined level is applied to the transfer roller 108 from a transfer bias application power source (not shown). A transfer bias having a polarity opposite to that of the toner is applied to the transfer roller 108, and the toner image on the surface side of the photosensitive drum 101 is electrostatically transferred onto the surface of the recording material 114 at the transfer portion 108T.

  The recording material 114 after the transfer is separated from the surface of the photosensitive drum 101, passes through the conveyance guide 109, and is introduced into a fixing device 115 as a heating device. The fixing device 115 receives heat fixing processing of the toner image 21 (FIG. 3). On the other hand, the surface of the photosensitive drum 101 after transfer of the toner image to the recording material 114 is cleaned by the cleaning device 110 after removal of transfer residual toner, paper dust, and the like, and is repeatedly used for image formation. The recording material 114 that has passed through the fixing device 115 is discharged from the discharge port 111 onto the discharge tray 112.

<Fixing device>
In this embodiment, the fixing device 115 is an electromagnetic induction heating type device. FIG. 3 is a cross-sectional side view of the main part of the fixing device 115 of this example. The pressure roller 8 as a pressure rotator includes a cored bar 8a, a heat-resistant / elastic material layer 8b such as silicone rubber, fluororubber, and fluororesin formed and coated concentrically around the cored bar. The release layer 8c is provided on the surface layer.

  For example, the release layer 8c can be selected from a material having good release properties and heat resistance such as fluororesin, silicone resin, fluorosilicone rubber, fluororubber, silicone rubber, PFA, PTFE, and FEP. The elastic layer 8b is preferably made of silicone rubber, fluorine rubber, fluorosilicone rubber, or the like, which has good heat resistance and good thermal conductivity.

  Reference numeral 1 denotes a fixing roller (fixing sleeve) which is a cylindrical rotating body having a conductive layer. The fixing roller 1 is a heating member that generates heat by electromagnetic induction and fixes an image on a recording material. In this embodiment, the fixing roller 1 includes a cylindrical heat generating layer (conductive layer) 1a as a base layer, and an elastic layer 1b and a release layer 1c sequentially stacked on the outer peripheral surface thereof. The heat generating layer 1a is a conductive metal member that is an electromagnetic induction heat generating member. The heat generating layer 1a is pressed in the direction of the pressure roller 8 by a pressing means (not shown) with a total pressure of about 100N to 500N. Thus, a fixing nip N having a predetermined width is formed between the fixing roller 1 and the pressure roller 8 in the recording material conveyance direction.

  In this embodiment, the pressure roller 8 is rotationally driven in the counterclockwise direction indicated by an arrow by a driving means (not shown), and the fixing roller 1 is driven in the clockwise direction indicated by the arrow by a frictional force with the pressure roller 8 in the fixing nip N. Then rotate.

  In this embodiment, the heat generating layer 1a employs SUS having a thickness of 1.0 mm. The thickness of the heat generating layer 1a is determined to be an optimum value depending on the rigidity against bending during pressurization and the amount of heat generated by the heat generating layer 1a. The relationship between the thickness of the heat generating layer 1a and the amount of heat generated will be described later.

Elastic layer 1b and releasing layer 1c is preferably a material having excellent heat resistance similarly to the elastic layer 8b of the pressure roller 8. In the configuration of this embodiment, a silicone rubber having a thickness of 2.0 mm and a JIS-A hardness of 15 ° to 25 ° is employed as the elastic layer 1b, and a PFA tube having a thickness of 50 μm is employed as the release layer 1c .
FIG. 4 is a perspective view of a main part of the fixing device 115. Inside the fixing roller 1, a magnetic core 2 as a magnetic core material is arranged in the rotation axis direction (the generatrix direction of the fixing roller 1) to penetrate (insert) the hollow portion of the fixing roller 1 by a fixing means (not shown). Has been. That is, the magnetic core 2 that is long in the generatrix direction of the fixing roller 1 is inserted into the hollow portion of the fixing roller 1. The magnetic core 2 has a shape that does not form a loop outside the fixing roller 1, that is, an end shape, and forms an open magnetic path in which a part of the magnetic path is broken.

  The exciting coil 3 is formed by spirally winding a normal single conducting wire around the magnetic core 2 in the hollow portion of the fixing roller 1. That is, the exciting coil 3 is wound around the magnetic core directly or via another object such as a bobbin in a direction intersecting the generatrix direction outside the magnetic core in the hollow portion. Therefore, when a high-frequency current (alternating current, alternating current) is supplied to the exciting coil 3 by the high-frequency converter 16 or the like via the power supply contact portions 3a and 3b, a magnetic flux is generated in a direction parallel to the busbar direction of the fixing roller 1. Can do.

  In this embodiment, the number of turns is 70. Although details will be described later, the exciting coil 3 is formed by winding the coil so that the density at the end portion is higher than that at the longitudinal center portion. Insulation between the exciting coil 3 and the heat generating layer 1a is achieved by the outer insulating member 4.

  The exciting coil 3 is wound in a direction crossing the generatrix direction of the fixing roller 1 inside the fixing roller 1 as a rotating body. Therefore, when a high-frequency current (alternating current, alternating current) is supplied to the exciting coil 3 by the high-frequency converter 16 or the like via the power supply contact portions 3a and 3b, a magnetic flux is generated in a direction parallel to the busbar direction of the fixing roller 1. Can do. The magnetic core 2 functions as a member that guides a magnetic force line (magnetic flux) generated by the alternating magnetic field generated by the exciting coil 3 to the inside of the fixing roller 1 to form a path (magnetic path) of the magnetic force line.

  The temperature detection of the fixing roller 1 is performed by the thermistors 9 and 10 as temperature detection elements. As shown in FIGS. 3 and 4, the thermistors 9 and 10 are arranged at the longitudinal center and the fixing roller facing position at the end on the side where the recording material 114 is conveyed to the fixing device 115. The drive frequency of the high-frequency converter 16 is 20 kHz to 35 kHz, and the control unit 15 controls the output voltage so that the detected temperature of the thermistor 9 or the thermistor 10 becomes the target temperature (about 150 ° C. to 200 ° C.). Specific positions of the thermistors 9 and 10 which are the features of the present invention will be described later.

<Heat generation principle>
A heating mechanism of the fixing device of this embodiment will be described with reference to FIG. Magnetic field lines generated by passing an alternating current through the coil 3 pass through the inside of the magnetic core 2 inside the cylindrical heat generating layer 1 of the fixing roller 1 in the direction of the generatrix of the heat generating layer 1a (direction from S to N). From one end (N) of the core 2 to the outside of the heat generating layer, it returns to the other end (S) of the magnetic core 2. As a result, an induced electromotive force is generated in the heat generating layer 1a to generate a magnetic force line in a direction that hinders increase / decrease in magnetic flux passing through the inside of the heat generating layer 1a in the generatrix direction of the heat generating layer 1a, and current is induced in the circumferential direction of the heat generating layer 1a. . The heat generating layer 1a generates heat by Joule heat generated by the induced current.

  The heat of the heat generating layer 1a is transmitted to the elastic layer 1b and the release layer 1c, the entire fixing roller 1 is heated, and the recording material 114 introduced into the fixing nip N is heated to fix the unfixed toner image 21. .

  The magnitude of the induced electromotive force V generated in the heat generating layer 1a is proportional to the amount of change in magnetic flux per unit time (Δφ / Δt) passing through the inside of the heat generating layer 1a and the number of turns of the coil from the following equation (1). To do.

V = −N (Δφ / Δt) (1)
<Relationship between ratio of magnetic flux passing outside heat generation layer and power conversion efficiency>
By the way, the magnetic core 2 in FIG. 5A has a shape that does not form a loop but has an end. The magnetic lines of force in the fixing device in which the magnetic core 2 forms a loop outside the heat generating layer 1a as shown in FIG. 5B is guided by the magnetic core 3 and exits from the inside of the heat generating layer 1a to the inside. Return.

  However, in the case where the magnetic core 2 has an end portion as in this embodiment, there is nothing that induces the lines of magnetic force emitted from the end portion of the magnetic core 2. Therefore, the path (N to S) in which the magnetic lines of force exiting one end of the magnetic core 2 return to the other end of the magnetic core 2 is an outer route passing outside the heat generating layer 1a, an inner route passing inside the heat generating layer 1a, Any of these may pass. Hereinafter, the route from N to S of the magnetic core 2 through the outside of the heat generating layer 1a is referred to as an outer route, and the route from N to S of the magnetic core 2 through the inside of the heat generating layer 1a is referred to as an inner route.

  The ratio of the magnetic force lines passing through the outer route out of the magnetic force lines coming out from one end of the magnetic core 2 has a correlation with the power consumed by the heat generation of the heat generating layer 1a (power conversion efficiency) of the power supplied to the coil 3. It is an important parameter. As the ratio of the magnetic field lines passing through the outer route increases, the ratio of power consumed by the heat generation of the heat generating layer 1a (power conversion efficiency) among the power input to the coil 3 increases.

  The reason is the same as the principle that the power conversion efficiency increases when the number of magnetic fluxes passing through the primary and secondary windings of the transformer is the same. That is, in this embodiment, the closer the number of magnetic fluxes passing through the inside of the magnetic core 2 and the number of magnetic fluxes passing through the outer route, the higher the power conversion efficiency, and the high-frequency current passed through the coil 3 is converted into the heating layer 1a. Therefore, electromagnetic induction can be efficiently performed as a circular current.

  This is because the magnetic field lines from S to N in the core 2 in FIG. 5A and the magnetic field lines passing through the inner route are opposite in direction, so that the entire inside of the heat generating layer 1a including the magnetic core 2 is viewed. These magnetic field lines will cancel each other. As a result, the number of magnetic lines of force (magnetic flux) passing through the entire inside of the heat generating layer 1a from S to N is reduced, and the amount of change in magnetic flux per unit time is reduced. When the amount of change in magnetic flux per unit time decreases, the induced electromotive force generated in the heat generating layer 1a decreases, and the heat generation amount of the heat generating layer decreases.

  From the foregoing, it is important for the fixing device of this embodiment to manage the ratio of the magnetic field lines passing through the outer route in order to obtain the necessary power conversion efficiency.

<Indicator indicating the ratio of magnetic flux passing outside the heat generation layer>
Therefore, the ratio of the magnetic force lines passing through the outer route in the fixing device 115 is expressed using an index called permeance. First, the concept of a general magnetic circuit will be described. A circuit of a magnetic path through which magnetic lines of force pass is called a magnetic circuit with respect to an electric circuit. When calculating the magnetic flux in the magnetic circuit, it can be performed in accordance with the calculation of the electric circuit current. Ohm's law for electrical circuits can be applied to magnetic circuits. When the magnetic flux corresponding to the electric circuit current is φ, the magnetomotive force corresponding to the electromotive force is V, and the magnetic resistance corresponding to the electric resistance is R, the following equation (2) is satisfied.

φ = V / R (2)
However, here, in order to explain the principle more easily, a permeance P that is the reciprocal of the magnetic resistance R will be used. When the permeance P is used, the above equation (2) can be expressed as the following equation (3).

φ = V × P (3)
Further, this permeance P can be expressed by the following equation (4), where B is the length of the magnetic path, S is the cross-sectional area of the magnetic path, and μ is the magnetic permeability of the magnetic path.

P = μS / B (4)
The permeance P is proportional to the cross-sectional area S and the magnetic permeability μ, and is inversely proportional to the length B of the magnetic path.

FIG. 6A shows a magnetic core 2 having a radius a ₁ [m], a length B [m], and a relative magnetic permeability μ ₁ inside the heat generating layer 1 a, and a coil 3 having a helical axis as a generatrix of the heat generating layer 1 a. It is the figure showing what was wound N [times] so that it might become substantially parallel to a direction. Here, the heat generating layer 1a is a conductor having a length B [m], an inner diameter a2 [m], an outer diameter a3 [m], and a relative permeability μ2. The vacuum permeability inside and outside the heat generating layer 1a is μ0 [H / m]. A magnetic flux generated per unit length of the magnetic core 2 when a current I [A] is passed through the coil 3 is defined as φc (x).

  FIG. 6B is a cross-sectional view perpendicular to the longitudinal direction of the magnetic core 2. The arrows in the figure represent magnetic flux parallel to the longitudinal direction of the magnetic core 2 that passes through the inside of the magnetic core 2, the inside of the heat generating layer 1 a, and the outside of the heat generating layer 1 a when the current I flows through the coil 3. . The magnetic flux passing through the inside of the magnetic core 2 is φc (= φc (x)), the magnetic flux passing through the inside of the heat generating layer 1a (the region between the heat generating layer 1a and the magnetic core 2) is φa_in, and the magnetic flux passing through the heat generating layer itself is φs. A magnetic flux passing outside the heat generating layer is defined as φa_out.

  FIG. 7A shows a magnetic equivalent circuit of a space including the core 2, the coil 3, and the heat generating layer 1a per unit length shown in FIG. The magnetomotive force generated by the magnetic flux φc passing through the magnetic core 2 is Vm, the permeance of the magnetic core 2 is Pc, the permeance inside the heat generating layer 1a is Pa_in, the permeance inside the heat generating layer 1a is Ps, and the permeance outside the heat generating layer 1a is Let Pa_out.

  Here, when Pc is sufficiently larger than Pa_in and Ps, the magnetic flux that passes through the inside of the magnetic core 2 and exits from one end of the magnetic core 2 passes through any one of φa_in, φs, and φa_out. 2 is considered to return to the other end. Therefore, the following relational expression (5) is established.

φc = φa_in + φs + Pa_out (5)
Also, φc, φa_in, φs, and φa_out are expressed by the following equations (6) to (9), respectively.

φc = Pc × Wm (6)
φs = Ps × Wm (7)
φa_in = Pa_in × Wm (8)
φa_out = Pa_out × Wm (9)
Therefore, when (6) to (9) are substituted into the equation (5), Pa_out is expressed as the following equation (10).

Pa_out = Pc−Pa_in−Ps (10)
From FIG. 6B, when the cross-sectional area of the magnetic core 2 is Sc, the cross-sectional area inside the heat generating layer 1a is Sa_in, and the cross-sectional area of the heat generating layer 1a itself is Ss, the permeance is “permeability × cross-sectional area”. Can be represented. The unit is [H · m].

Pc = μ ₁ S _c = μ ₁ × πa ₁ ² (11)
Pa_in = μ ₀ Sa_in = μ ₀ × π (a ₂ ² −a ₁ ² ) (12)
Ps = μ ₂ Ss = μ ₂ × π (a ₃ ² −a ₂ ² ) (13)
By substituting these (11) to (13) into equation (10), Pa_out can be expressed by equation (14).

Pa_out = μ ₁ × πa ₁ ² −μ ₀ × π (a ₂ ² −a ₁ ² ) −μ ₂ × π (a ₃ ² −a ₂ ² )
(14)
By using the above formula (14), Pa_out / Pc, which is the ratio of the lines of magnetic force passing outside the heat generating layer 1a, can be calculated.

  Instead of the permeance P, a magnetic resistance R may be used. When discussing using the magnetic resistance R, since the magnetic resistance R is simply the reciprocal of the permeance P, the magnetic resistance R per unit length can be expressed by “1 / (permeability × cross-sectional area)”. The unit is “1 / (H · m)”.

  Table 1 shows specific calculation results using parameters of the apparatus of the example.

The magnetic core 2 is made of ferrite (relative magnetic permeability 1800), has a diameter of 14 [mm], and a cross-sectional area of 1.5 × 10 ⁻⁴ [m ² ]. The heat generating layer 1a is made of SUS (relative magnetic permeability 1.0), has a diameter of 24 [mm], a thickness of 1 [mm], and a cross-sectional area of 7.2 × 10 ⁻⁵ [m ² ].

  The cross-sectional area of the region between the heat generating layer 1a and the magnetic core 2 is calculated by subtracting the cross-sectional area of the magnetic core from the cross-sectional area of the hollow portion inside the heat generating layer having a diameter of 24 [mm]. The elastic layer 1b and the surface layer 1c are provided outside the heat generating layer 1a and do not contribute to heat generation. Therefore, in the magnetic circuit model for calculating the permeance, it can be regarded as an air layer outside the heat generating layer, and therefore it is not necessary to take into account. From Table 1, Pc, Pa_in, and Ps have the following values.

Pc = 3.5 × 10 ⁻⁷ [H · m]
Pa_in = 2.8 × 10 ⁻¹⁰ [H · m]
Ps = 9.1 × 10 ⁻¹¹ [H · m]
Using these values, Pa_out / Pc can be calculated from the following equation (15).

Pa_out / Pc = (Pc−Pa_in−Ps) / Pc
= 0.999 (99.9%) (15)
From the above, it was shown that the percentage of magnetic field lines passing through the outer route can be expressed using permeance or magnetoresistance.

<Conversion efficiency of power required for the device>
Next, the power conversion efficiency required for the fixing device of this embodiment will be described. For example, when the power conversion efficiency is 80%, the remaining 20% of the power is converted into heat energy by a coil or core other than the heat generation layer and consumed. When power conversion efficiency is low, things that should not generate heat, such as magnetic cores and coils, generate heat, and it may be necessary to take measures to cool them.

  Therefore, the power conversion efficiency is evaluated by changing the ratio of the magnetic flux passing through the outer route of the heat generating layer 1a. FIG. 8 is a diagram illustrating an experimental apparatus used in a measurement experiment of power conversion efficiency. The metal sheet 1S is an aluminum sheet having a width of 230 mm, a length of 600 mm, and a thickness of 20 μm. The metal sheet 1S is rolled into a cylindrical shape so as to surround the magnetic core 2 and the coil 3, and is made conductive by a thick line 1ST portion to form a heat generating layer.

The magnetic core 2 is a ferrite having a relative permeability of 1800 and a saturation magnetic flux density of 500 mT, and has a cylindrical shape with a cross-sectional area of 26 mm ² and a length of 230 mm. The magnetic core 2 is arranged in the approximate center of the cylinder of the aluminum sheet 1S by fixing means (not shown). A coil 3 is spirally wound around the magnetic core 2 with 25 turns. When the end of the metal sheet 1S is drawn in the direction of the arrow 1SZ, the diameter 1SD of the heat generating layer can be adjusted in the range of 18 to 191 mm.

  FIG. 9 is a graph in which the horizontal axis represents the ratio [%] of the magnetic flux passing through the outer route of the heat generating layer, and the vertical axis represents the power conversion efficiency at a frequency of 21 kHz. The power conversion efficiency rapidly increases after plot P1 in the graph of FIG. 9 and exceeds 70%, and in the range R1 indicated by the arrow, the power conversion efficiency is maintained at 70% or more. In the vicinity of P3, the power conversion efficiency rapidly increases again, and is 80% or more in the range R2. In the range R3 after P4, the power conversion efficiency is stable at a high value of 94% or more. The reason why the power conversion efficiency has begun to rise rapidly is that the circulating current has efficiently started to flow through the heat generating layer.

  Table 2 below shows the results of actually designing and evaluating the configuration corresponding to P1 to P4 in FIG. 9 as a fixing device.

(Fixing device P1)
In this configuration, the cross-sectional area of the magnetic core is 26.5 mm ² (5.75 mm × 4.5 mm), the heat generating layer has a diameter of 143.2 mm, and the ratio of the magnetic flux passing through the outer route is 64%. The power conversion efficiency obtained by the impedance analyzer of this apparatus was 54.4%. The power conversion efficiency is a parameter indicating the amount of power input to the fixing device that contributes to the heat generation of the heat generating layer. Therefore, other than that, it becomes a loss, and the loss becomes a heat generation of the coil and the magnetic core.

  In the case of this configuration, the coil temperature may exceed 200 ° C. even when 900 W is input for several seconds at startup. Considering that the heat resistance temperature of the coil insulator is in the latter half of 200 ° C., and that the Curie point of the magnetic core of ferrite is usually about 200 ° C. to 250 ° C., the loss of 45% keeps members such as the excitation coil below the heat resistance temperature. It will be difficult to keep on. Further, when the temperature of the magnetic core exceeds the Curie point, the inductance of the coil is abruptly reduced, resulting in load fluctuation.

  Since about 45% of the power supplied to the fixing device is not used for heat generation of the heat generating layer, it is necessary to supply about 1636 W of power to supply 900 W of power to the heat generating layer. This is a power source that consumes 16.36 A at 100 V input. There is a possibility of exceeding the allowable current that can be input from the commercial AC attachment plug. Therefore, there is a possibility that the fixing device P1 having a power conversion efficiency of 54.4% may have insufficient power to be supplied to the fixing device.

(Fixing device P2)
In this configuration, the cross-sectional area of the magnetic core is the same as P1, the diameter of the heat generation layer is 127.3 mm, and the ratio of the magnetic flux passing through the outer route is 71.2%. The power conversion efficiency obtained by the impedance analyzer of this apparatus is 70.8%. Depending on the specifications of the fixing device, the temperature rise of the coil and the core may be a problem.

  If the fixing device having this configuration is a high-spec device capable of printing at 60 sheets / min, the rotation speed of the heat generating layer is 330 mm / sec, and the temperature of the heat generating layer needs to be maintained at 180 ° C. If the temperature of the heat generating layer is maintained at 180 ° C., the temperature of the magnetic core may exceed 240 ° C. in 20 seconds.

  Since the Curie temperature of the ferrite used as the magnetic core 2 is usually about 200 ° C. to 250 ° C., the ferrite exceeds the Curie temperature, the magnetic core permeability rapidly decreases, and magnetic field lines can be appropriately induced in the magnetic core. It may not be possible. As a result, it may be difficult to induce a circulating current to generate heat in the heat generating layer.

  Therefore, if the fixing device having the range R1 of the magnetic flux passing through the outer route is the above-mentioned high-spec device, it is desirable to provide a cooling means to lower the temperature of the ferrite core. As the cooling means, an air cooling fan, water cooling, a heat radiating plate, a heat radiating fin, a heat pipe, a Beltier element, or the like can be used. Of course, if this configuration does not require such high specifications, the cooling means is unnecessary.

(Fixing device P3)
In this configuration, the cross-sectional area of the magnetic core is the same as P1, and the diameter of the heat generating layer is 63.7 mm. The power conversion efficiency required by the impedance analyzer of this apparatus is 83.9%. Although heat is constantly generated in the magnetic core and the coil, the cooling means is not at a necessary level.

  If the fixing device having this configuration is a high-spec device capable of printing at 60 sheets / min, the rotation speed of the heat generating layer is 330 mm / sec, and the surface temperature of the heat generating layer may be maintained at 180 ° C., but the magnetic core ( The temperature of the ferrite does not rise above 220 ° C. Therefore, in this configuration, when the fixing device has the above-mentioned high specifications, it is desirable to use ferrite having a Curie temperature of 220 ° C. or higher.

  From the foregoing, it is desirable that the fixing device having the configuration in which the ratio of the magnetic flux passing through the outer route is in the range R2 is optimized for heat-resistant design such as ferrite when used at high specifications. On the other hand, such a heat-resistant design is not necessary when high specifications are not required for the fixing device.

(Fixing device P4)
In this configuration, the cross-sectional area of the magnetic core is the same as P1, and the diameter of the cylindrical body is 47.7 mm. The power conversion efficiency required by the impedance analyzer in this apparatus is 94.7%. Even if the fixing device of this configuration is a high-spec device capable of performing a printing operation of 60 sheets / min (the rotation speed of the heat generating layer is 330 mm / sec) and the surface temperature of the heat generating layer is maintained at 180 ° C., the exciting coil Neither coils nor coils reach 180 ° C or higher. Therefore, a cooling means for cooling the magnetic core, the coil and the like and a special heat resistant design are unnecessary.

  As described above, in the range R3 in which the ratio of the magnetic flux passing through the outer route is 94.7% or more, the power conversion efficiency is 94.7% or more, and the power conversion efficiency is sufficiently high. Therefore, no cooling means is required even when used as a further high-spec fixing device.

  Further, in the region R3, even if the ratio of the magnetic flux external to the metal sheet varies, there is little change in the power conversion efficiency. This means that a highly flexible member such as a film is employed as the heat generating layer, and high power conversion efficiency can be stably maintained even if the distance from the magnetic core varies somewhat.

  From the foregoing, it can be seen that the fixing device of this embodiment needs to have a ratio of magnetic flux passing through the outer route of 72% or more in order to satisfy at least the necessary power conversion efficiency. The numerical value in Table 2 is 71.2% or more, but is 72% in consideration of measurement errors and the like.

<Relationship of permeance or magnetoresistance to be satisfied by the device>
The ratio of the magnetic flux passing through the outer route of the heat generating layer 1a is 72% or more, because the sum of the permeance of the heat generating layer 1a and the permeance inside the heat generating layer 1a (the region between the heat generating layer and the magnetic core) is magnetic. This is equivalent to 28% or less of the core 2 permeance. Therefore, one of the characteristic configurations of this embodiment is that the following equation (16) is satisfied when the permeance of the magnetic core 2 is Pc, the permeance inside the heat generating layer 1a is Pa_in, and the permeance Ps of the heat generating layer. It is to be.

0.28 × Pc ≧ Ps + Pa_in (16)
Further, when the permeance relational expression is replaced with a magnetic resistance, the following expression (17) is obtained.

0.28 × (1 / Rc) ≧ (1 / Rs) + (1 / Ra_in) (≡1 / Rsa)
0.28 × Rsa ≧ Rc (17)
Here, the combined magnetoresistance Rsa of Rs and Ra is defined as the following equation (18).

Rsa = (Ra_in × Rs) / (Ra_in + Rs) (18)
Rc: Magnetoresistance of the magnetic core Rs: Magnetoresistance of the heat generation layer Ra_in: Magnetoresistance of the region between the heat generation layer and the magnetic core Rsa: Composite magnetoresistance of Rs and Ra Fix the above-mentioned relational expression of permeance or magnetoresistance It is desirable that the cross section in the direction perpendicular to the generatrix direction of the cylindrical rotating body is satisfied throughout the maximum conveyance area of the recording material of the apparatus.

  Similarly, in the fixing device in the range R2 of this embodiment, the ratio of the magnetic flux passing through the outer route of the heat generating layer is 92% or more. The numerical value in Table 2 is 91.7% or more, but is 92% in consideration of measurement error and the like. The ratio of the magnetic flux passing through the outer route of the conductive layer 1a is 92% or more indicates that the sum of the permeance of the conductive layer 1a and the permeance of the inner side of the conductive layer (region between the conductive layer and the magnetic core) is the magnetic core. This is equivalent to 8% or less of the permeance. Therefore, the permeance relational expression is the following expression (19).

0.08 × Pc ≧ Ps + Pa_in (19)
When the permeance relational expression is converted into a magnetic resistance relational expression, the following expression (20) is obtained.

0.08 × Rsa ≧ Rc (20)
Further, in the fixing device in the range R3 of this embodiment, the ratio of the magnetic flux passing through the outer route of the heat generating layer is 95% or more. To be exact, it is 94.7% or more from Table 2, but 95% considering the measurement error. The ratio of the magnetic flux passing through the outer route of the conductive layer 1a is 95% or more because the sum of the permeance of the conductive layer 1a and the permeance of the inner side of the conductive layer 1a (the region between the conductive layer and the magnetic core) is magnetic. This is equivalent to 5% or less of the core permeance. The permeance relational expression is the following expression (21).

0.05 × Pc ≧ Ps + Pa_in (21)
When the permeance relational expression (21) is converted into a magnetic resistance relational expression, the following expression (22) is obtained.

0.05 × Rsa ≧ Rc (22)
In the configuration of this example, Pc, Ps, and Pa_in are as shown in Table 1, and it can be seen that the above equation (21) is satisfied. Therefore, it can be seen that power can be efficiently supplied to the heat generating layer.

  The above is the basic heat generation principle. Next, the fact that there is a heat generation distribution depending on the longitudinal position and the reason why it occurs will be described.

<Equivalent circuit>
As described with reference to (a) of FIG. 5, an induced current flows in the entire circumferential direction of the heat generating layer 1 a so as to cancel the alternating magnetic field formed by the current flowing in the exciting coil 2. As shown in FIG. 10, the physical model for inducing this current is equivalent to magnetic coupling of a concentric shaft transformer having a shape in which a primary coil 81 indicated by a solid line and a secondary coil 82 indicated by a dotted line are wound.

  The secondary coil 82 forms a circuit and has a resistor 83. The alternating voltage generated from the high frequency converter 16 generates a high frequency current in the primary coil 81, and as a result, an induced electromotive force is applied to the secondary coil 82 and is consumed as heat by the resistor 83. Here, the secondary coil 82 and the resistor 83 model Joule heat generated in the heat generating layer 1a.

  An equivalent circuit of the model diagram shown in FIG. 10 is shown in FIG. L1 is the inductance of the primary coil 81 in FIG. 10, L2 is the inductance of the secondary coil 82 in FIG. 10, M is the mutual inductance of the primary coil 81 and the secondary coil 82, and R is the resistor 83. This circuit diagram (a) can be equivalently converted to (b) in FIG. In order to consider a simplified model, it is assumed that the mutual inductance M is sufficiently large and L1≈L2≈M. In this case, since (L1-M) and (L2-M) are sufficiently small, the circuit can be approximated from (b) in FIG. 11 to (c) in FIG.

Therefore, FIG. 11C is considered as an equivalent circuit approximated to the configuration of the present invention shown in FIG. Here, the resistance will be described. In the state of FIG. 11A, the secondary-side impedance is the electric resistance R in the circulation direction of the heat generating layer 1a. In the transformer, when viewed from the primary side, the impedance on the secondary side becomes an equivalent resistance R ′ that is N ² (N is the transformer turns ratio) times.

Here, the turns ratio N of the transformer is such that the number of turns of the heating coil 1a is 1 with respect to the number of turns of the primary coil = the number of turns of the exciting coil in the heating layer 1a (70 turns in this embodiment). Therefore, it can be considered that the transformer turns ratio N = 70. Therefore, it can be considered that R ′ = N ² R = 70 ² R, and the equivalent resistance R shown in FIG. 11C increases as the number of turns increases. The combined impedance X of the mutual inductances M and R ′ is as shown in Expression (23).

  Here, ω = 2πf, and f is the drive frequency. From equation (23), it can be seen that the combined impedance X depends on the turn ratio N, the electrical resistance R in the circumferential direction of the heat generating layer 1a, the drive frequency f, and the mutual inductance M. In a general transformer, the mutual inductance M depends on the magnetic permeability μ. In the configuration of the open magnetic path of the present embodiment, the behavior in which the magnetic permeability μ changes in the longitudinal position is shown. Therefore, the synthetic impedance X differs at the longitudinal position. Details will be described below.

<Apparent permeability>
FIG. 12 illustrates the relationship between the longitudinal position of the magnetic core 2 and the equivalent inductance. When a coil 141 having a diameter of 30 mm (N = 5 turns) is passed through the magnetic core 2 and an equivalent inductance L (frequency 35 kHz) from both ends of the coil is measured at each position in the longitudinal direction using an impedance analyzer, The distribution shape is a mountain shape shown in FIG. The equivalent inductance L is attenuated to less than half of the center at the end. L follows the following equation (24).

L = μN ² S / l (24)
Here, μ is the magnetic permeability of the magnetic core, N is the number of turns of the coil, l is the length of the coil, and S is the cross-sectional area of the coil. Since the shape of the coil 141 is not changed, S, N, and l are not changed in this experiment. Therefore, a change in the equivalent inductance L means that the magnetic permeability μ has changed. Hereinafter, μ that changes depending on the position is referred to as “apparent magnetic permeability”. To summarize the above, FIG. 12 shows that the apparent permeability changes depending on the longitudinal position of the magnetic core.

  FIG. 13 shows a magnetic flux when a uniform magnetic field H exists in the air and the magnetic core 201 is disposed therein. The magnetic core 201 forms an open magnetic path having boundary surfaces NPＮ and SP⊥ perpendicular to the magnetic field H. As shown in FIG. 13, the magnetic core end portion 201E has a lower magnetic flux density than the magnetic core central portion 201C. This is due to the boundary condition between air and magnetic core. Since the magnetic flux density is continuous at the boundary surfaces NP⊥ and SP⊥ perpendicular to the magnetic field lines, the magnetic core end portion 201E in contact with air has a lower magnetic flux density than the magnetic core central portion 201C.

  In a uniform magnetic field H, the magnetic flux density B in the space follows the following formula (25) in a magnetic field region where the magnetization of the object is substantially proportional to the external magnetic field.

B = μH (25)
That is, the magnetic flux density B changes due to the continuity of the magnetic flux density at the boundary surfaces NP⊥ and SP⊥ perpendicular to the magnetic field lines, and as a result, the apparent permeability μ changes. When creating a magnetic path, there are a closed magnetic path that connects the magnetic paths themselves with a loop, and an open magnetic path that breaks the magnetic path by making it an open end, but the above phenomenon occurs only in the open magnetic path .

<Heat distribution>
Since the apparent permeability μ changes depending on the position, the equivalent circuit of FIG. 11A is as shown in FIG. The apparent permeability μ is divided into N parts in consideration of a small width. As in FIG. 11, FIG. 14 is changed to (b) in FIG. 14 by performing equivalent exchange and approximation. Since the division here is performed with a uniform width, the resistors R all have the same value. Since (b) in FIG. 14 is a series circuit of each combined impedance, it can be written as (c) in FIG.

As described above, the combined impedance X at each position can be handled independently, and the heat generation distribution in the longitudinal direction can be considered by considering the combined impedance X at each position.
For the sake of simplicity, the heat generation distribution in the longitudinal direction will be considered by comparing the combined impedance at two points, the center and the end. The combined impedance Xc at the central portion in the longitudinal direction is expressed as Equation (26), and the combined impedance Xe at the end portion in the longitudinal direction is expressed as Equation (27).

FIG. 15 schematically shows how the combined impedances Xc and Xe change with frequency. When the exciting coil 3 is wound at equal intervals in the longitudinal direction, Nc = Ne (= N). Therefore, when the frequency is increased to infinity, the combined impedances Xc and Xe converge to the same value N ² R. As described above, the apparent permeability μc at the center and the apparent permeability μe at the end have a relationship of μc> μe. As a result, the mutual inductance also has a relationship of Mc> Me. Therefore, Xc> Xe holds at any frequency.

  In the configuration of the present embodiment, a frequency within the range of 20 kHz to 35 kHz is used, but the difference between the combined impedances Xc and Xe becomes larger when the frequency is reduced within this frequency range. This means that the ratio of the calorific value Qc at the center and the calorific value Qe at the end varies with the frequency. Further, when the frequency is reduced, the heat generation amount Qe at the end portion is reduced with respect to the heat generation amount Qc at the center portion. Thus, when fixing a short sheet in the longitudinal direction, excessive heat generation outside the sheet passing area can be suppressed by lowering the frequency.

  On the other hand, when fixing a sheet that is long in the longitudinal direction, the amount of heat generated at the central portion in the longitudinal direction and the end portion in the longitudinal direction can be made the same by using a high frequency. However, in consideration of heat dissipation at the end in the longitudinal direction, the amount of heat generated at the end in the longitudinal direction needs to be slightly higher than that at the center in the longitudinal direction. Therefore, in this embodiment, the interval between the exciting coils 2 is shortened at the end in the longitudinal direction. Then, the combined impedance changes from Xe to Xe ′ shown in FIG.

  Reducing the interval between the exciting coils 3 at a certain longitudinal position is the same as increasing the turn ratio N at that longitudinal position, and the saturation value increases when the frequency is increased to infinity. As a result, the overall impedance increases as shown in FIG. As a result, the calorific value increases.

  The specific interval between the exciting coils 3 was determined such that the central portion was 5 mm and the end portion was 2 mm, and the interval between them was determined by linear interpolation of both. That is, in the embodiment, Qc> Qe or Qc <Qe is satisfied by adjusting the frequency.

<Measurement results>
FIG. 1 shows the measurement result of the surface temperature distribution of the fixing roller 1 in the axis X direction. This measurement was performed using TVS-8500 manufactured by NEC Avio. The drive frequencies were 20 kHz, 25 kHz, 30 kHz, and 35 kHz, respectively.

  FIG. 1 shows a comparison of temperature distributions when power is supplied by the high-frequency converter 16 and the temperature of the thermistor 9 reaches a certain temperature. Within the range of the driving frequency employed by the fixing device 115, there are three places where the fixing roller 1 can take the maximum temperature in the axis X direction of the fixing roller 1. When the drive frequency is low, the temperature of the central portion 9x in the axis X direction is the highest, and when the drive frequency is increased to 35 kHz, the temperature is highest at the position of the end portion 10x in the axis X direction. At 35 kHz, a temperature peak also exists at the position in the axis X direction position 11x.

  11x is a 10x point-symmetrical position with respect to the center position of the fixing roller 1, and can be the highest temperature even at the 11x position. The reason why the temperature difference can be made between the positions 11x and 10x in the axis X direction is the uneven shape of the magnetic core 2 and the uneven winding position of the exciting coil 3. However, since the temperature difference is small, the thermistor 10 may be arranged at any of the axial X direction positions 10x and 11x.

  Also, the thermistors 9 and 10 are practically not problematic even if they are placed at a temperature lower by T ° C. than the highest temperature. In the configuration of the present embodiment, there is no problem even if the thermistors 9 and 10 are arranged at a location where T = 5 ° C. Therefore, the thermistors 9 and 10 need only be within the ranges 9y and 10y, and are not limited to the positions 9x and 10x.

  Hereinafter, a temperature control example of the heating roller 1 when the thermistors 9 and 10 are used will be described. FIG. 16 shows a temperature distribution with respect to the position in the longitudinal direction of the heating roller 1. The temperature distribution is shown when the frequency is high and the temperature at the end in the longitudinal direction is higher than the center. From the viewpoint of temperature unevenness and offset, the temperature of the heating roller 1 needs to be within a range lower than the temperature T1 and higher than the temperature T2.

  When the temperature control is performed at the position 9x, even if the temperature is within the appropriate temperature range at the position 9x, the temperature of the heating roller 1 may be higher than the temperature T1 at the end in the longitudinal direction and may be out of the proper temperature range. (Temperature distribution (1)).

  Therefore, in the configuration of this embodiment, the thermistor 10 arranged at the position 10x also measures the temperature of the heating roller 1 at the same time. The set temperature at the position 9x is lowered and adjusted so that the temperature of the heating roller 1 at the position 10x is lower than the temperature T1 and the temperature of the heating roller 1 at the position 9x is higher than the temperature T2 ( Temperature distribution (2)).

  As described above, in the fixing device 115 of this embodiment, the thermistor 9 is arranged at the position 9x in the axial X direction where the temperature is highest when the driving frequency is 20 kHz. Further, the thermistor 10 is arranged at the position 10x in the axial X direction where the temperature is highest when the driving frequency is 35 kHz. By disposing the thermistor in this way, the temperature of the fixing roller 1 in the paper passing area can be kept within the appropriate temperature range within the frequency range to be employed. As a result, image defects such as uneven gloss and offset can be suppressed.

  The above fixing device configuration is summarized as follows.

  1) It has a cylindrical rotating body (fixing roller) 1 having a conductive layer 1a. The magnetic core member 2 is inserted in the hollow portion of the rotating body 1 and is long in the generatrix direction of the rotating body 1. In the hollow portion, the exciting coil 3 is wound around the magnetic core material 2 directly or via another object in a direction intersecting the generatrix direction outside the magnetic core material 2. The fixing device fixes the image 21 to the recording material 114 by causing the conductive layer 1 a to generate heat by electromagnetic induction by passing an alternating current through the exciting coil 3.

  The magnetic core material 3 does not form a loop outside the rotating body 1. Temperature sensing elements 9 and 10 for measuring the temperature of the rotating body 1 are provided. The temperature detection elements 9 and 10 are arranged so that the temperature of the portion where the temperature of the rotating body 1 is maximum in the bus line direction of the rotating body 1 can be measured in the frequency range that can be adopted as the frequency of the alternating current.

  2) In addition, when the frequency of the alternating current is the highest value in the frequency range that can be adopted, the temperature detection element is arranged at the highest temperature position in the bus bar direction of the rotating body 1.

  3) Further, when the frequency of the alternating current is the smallest value in the frequency range that can be adopted, the temperature detecting element is arranged at a position where the temperature is highest in the direction of the bus of the rotating body 1.

  4) Further, the position where the temperature is highest in the direction of the bus of the rotating body 1 when the frequency of the alternating current is the highest in the frequency range that can be adopted is defined as a position A Position B is the position where the temperature is highest in the direction of the bus of the rotating body when the frequency is the lowest in the range of frequencies that can be adopted. Temperature sensing elements are arranged at both position A and position B.

  Here, in addition to fixing the unfixed toner image as a fixed image, the fixing device improves the glossiness by re-heating and pressurizing the toner image assumed to be fixed on the recording material or once heated and fixed. A device (also called a fixing device in this case) is also included.

  The cylindrical rotating body 1 having the conductive layer 1a can be in the form of a hard or flexible hollow roller or a pipe, or is suspended and stretched between a plurality of stretching members. It is also possible to use a flexible endless belt.

  DESCRIPTION OF SYMBOLS 1 ... Fixing roller, 2 ... Magnetic core, 3 ... Excitation coil, 8 ... Pressure roller, 9, 10 ... Thermistor, 15 ... Control part, 16 ... High frequency inverter, 100 ... Image forming apparatus 114 Recording material 115 Fixing device

Claims (2)

A cylindrical rotating body having a conductive layer;
A magnetic core material that is inserted through the hollow portion of the rotating body and is long in the generatrix direction of the rotating body;
In the hollow portion, the end portion in a series from the one end side to the other end side along the longitudinal direction of the magnetic core member directly or through other objects on the outer peripheral surface of the magnetic core member, and compared to the central portion in the longitudinal direction. An exciting coil formed by spirally winding a coil wire so that the density is higher,
Fixing in which the conductive layer generates heat by electromagnetic induction by passing an alternating current through the excitation coil via the power supply contact on one end side and the other end side of the excitation coil, and this heat is used to fix the image on the recording material. A device,
The magnetic core material does not form a loop outside the rotating body,
A temperature sensing element for measuring the temperature of the rotating body;
The rotating body when the the most temperature increases position generatrix direction of the rotary member when the largest in the range of frequencies where the frequency can adopt the alternating current, the lowest frequency within the range of frequencies which can be employed The fixing device is characterized in that the temperature detecting element is arranged at both the position where the temperature is highest in the direction of the bus . The fixing device according to claim 1 , further comprising an insulating member outside the exciting coil.