Patent Publication Number: US-7215919-B2

Title: Fixing apparatus using induction heating

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a divisional of U.S. application Ser. No. 10/872,472, filed Jun. 22, 2004 now U.S. Pat. No. 7,065,315, the entire contents of which is incorporated herein by reference. 
   This application is based upon and claims the benefit of priority from prior Japanese Patent Applications No. 2003-188634, filed Jun. 30, 2003; and No. 2003-389751, filed Nov. 19, 2003, the entire contents of both of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a fixing apparatus for fusing a developer to a transfer material, which is provided in an image forming apparatus using an electrophotographic process to form an image on a transfer material, such as a copier and printer. 
   2. Description of the Related Art 
   An image forming apparatus such as an electric copier has a fixing apparatus for fusing a heated and fused developer image to a paper sheet by applying pressure. 
   As a method of heating a heating member of a fixing apparatus, induction heating is available. Induction heating is a method of generating a magnetic field by applying a predetermined electric power to a coil to generate a magnetic field and generating a predetermined amount of heat in a heating member by Joule heat generated by an eddy current generated by the magnetic field. 
   For example, Jpn. Pat. Appln. KOKAI Publication No. 2001-235962 discloses a fixing apparatus using an induction heating method, in which a coil having an area where a heating member contacts a paper sheet is arranged opposite to a divided heating area, is divided into a predetermined number of coils according to the size of paper, and placed outside the heating member. 
   Further, Jpn. Pat. Appln. KOKAI Publication No. 2000-206813 discloses a fixing apparatus using an induction heating method, which has a plurality of exciting coils, and controls the amount of current supplied to the exciting coils except a first exciting coil according to the amount of current supplied to the first exciting coil. 
   Further, Jpn. Pat. Appln. KOKAI Publication No. 7-295414 discloses a fixing apparatus using an induction heating method, which has a plurality of coils placed outside of a heating member, having an area where a heating member contacts a paper sheet, is arranged opposite to a divided heating area, according to the size of paper heated by the heating member, and a current is supplied independently to the plurality of coils. 
   In the fixing apparatuses using the induction heating method as disclosed by the above three patent publications, a heating member having a very high heating efficiency is heated very quickly, and if it is heated in the state not rotated, the area near the part opposite to the exciting coil is locally heated. 
   Further, Jpn. Pat. Appln. KOKAI Publication No. 2002-40839 discloses a fixing apparatus which has a fusing belt heated by a heating roller heated by an induction heating method, and a detection means which detects movement of the fusing belt in the rotating direction. 
   Further, Jpn. Pat. Appln. KOKAI Publication No. 2002-82549 discloses a fixing apparatus, in which a part of a belt contacting a passing paper sheet heated by a heating member is separated from a part cooperating with a pressing member to supply a predetermined pressure to a paper sheet, and the heating member starts induction heating after the belt is rotated. 
   Among the fixing apparatuses using an induction heating method, the fixing apparatus which uses a plurality of coils for induction heating may have a weak magnetic field strength supplied to the area adjacent to the coil, compared with the magnetic field supplied close to the center of the coil. In this case, the magnetic field intensity varies in the length direction of the heating member, and the heat amount changes depending on the positions of the heating member. 
   Therefore, the distribution of temperatures in the length direction of the heating member becomes nonuniform, and the heat value supplied to the developer on a paper sheet becomes unstable. 
   In a heating roller with a thin metallic layer noticed in recent years, the temperature difference that occurs particularly among a plurality of coils becomes a problem. 
   As for a detection means for detecting an abnormal temperature, it is demanded to detect a temperature at a predetermined position heated locally by an exciting coil. 
   However, in a fixing apparatus which does not contain an exciting coil and a means for detecting an abnormal temperature, because the heating member is filled inside, it is physically difficult to place a temperature detection means at a predetermined position that is locally heated, for example, between a coil and a heating member. 
   Besides, there is a problem in the fixing apparatus which uses a heating member with a filled inside. As an abnormal temperature detection means for detecting an abnormal temperature is arranged close to an exciting coil, a magnetic field is not evenly supplied from the exciting coil to the heating roller, and the temperature is not held uniform in the rotating direction of the heating member. 
   BRIEF SUMMARY OF THE INVENTION 
   According to an aspect of the present invention, there is provided a fixing apparatus comprising: 
   a heating member which is shaped cylindrical and has a conductor to flow an induced current by induction heating; 
   a pressing member which supplies a pressure to the heating member; 
   a first induction heating mechanism which includes a first coil; 
   a second induction heating mechanism which includes at least one of second coils aligned with the first induction heating mechanism in the axial direction of the heating member; 
   wherein the first coil has a part with a different distance to the heating member. 
   According to another aspect of the present invention, there is provided a fixing apparatus comprising: 
   a heating member which is shaped cylindrical and has a conductor to flow an induced current by induction heating; 
   a pressing member which supplies a pressure to the heating member; 
   a first induction heating mechanism which includes a first coil; 
   a second induction heating mechanism which includes at least one of second coils aligned with the first induction heating mechanism in the axial direction of the heating member; 
   wherein the heating member receives the influence of magnetic field generated from the both first and second coils, in the area divided in the direction orthogonal to the axial direction. 
   According to another aspect of the present invention, there is provided a fixing apparatus comprising: 
   a heating member which is shaped cylindrical and has a conductor to flow an induced current by induction heating; 
   a pressing member which supplies a pressure to the heating member; 
   a first induction heating mechanism which includes a first coil; 
   a second induction heating mechanism which includes at least one of second coils arranged at an angle and phase different from those of the first coil of the first induction heating mechanism; 
   wherein the heating member receives the influence of magnetic field generated from the both first and second coils, in the area divided in the direction orthogonal to the axial direction. 
   Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
       FIG. 1  is a schematic diagram explaining a fixing apparatus to which an embodiment of the present invention is applicable; 
       FIG. 2  is a block diagram explaining a configuration of an induction heating control circuit which is applicable to the fixing apparatus shown in  FIG. 1 ; 
       FIG. 3  is a flowchart explaining an example of a method of controlling the fixing apparatus shown in  FIG. 1 ; 
       FIGS. 4A ,  4 B and  4 C are schematic diagrams explaining another example of the induction heating mechanism shown in  FIG. 2 ; 
       FIGS. 5A and 5B  are schematic diagrams explaining examples of arrangement of an abnormal temperature detection mechanism placed close to the induction heating mechanism shown in  FIGS. 4A ,  4 B and  4 C; 
       FIGS. 6A ,  6 B,  6 C and  6 D are schematic diagrams explaining other examples of the induction heating mechanism shown in  FIG. 2 ; 
       FIGS. 7A and 7B  are schematic diagrams explaining still other examples of the induction heating mechanism shown in  FIG. 2 ; 
       FIGS. 8A ,  8 B and  8 C are schematic diagrams explaining still other examples of the induction heating mechanism shown in  FIG. 2 ; 
       FIGS. 9A and 9B  are schematic diagrams explaining an example of the relationship between the induction heating mechanism and abnormal temperature detection mechanism applicable to the fixing apparatus shown in  FIG. 1 ; 
       FIG. 10  is a schematic diagram explaining an example of the abnormal temperature mechanism shown in  FIGS. 9A and 9B ; 
       FIG. 11  is a schematic diagram explaining another example of the relationship between the induction heating mechanism and abnormal temperature detection mechanism applicable to the fixing apparatus shown in  FIG. 1 ; 
       FIGS. 12A ,  12 B and  12 C are schematic diagrams explaining still other examples of relationship between the induction heating mechanism and abnormal temperature detection mechanism applicable to the fixing apparatus shown in  FIG. 1 ; 
       FIGS. 13A ,  13 B and  13 C are schematic diagrams explaining another example of the induction heating mechanism shown in  FIG. 2 ; 
       FIGS. 14A and 14B  are perspective views explaining types of paper sheet passing applicable to the fixing apparatus of the present invention; 
       FIGS. 15A ,  15 B and  15 C are schematic diagrams explaining other examples of the induction heating mechanism shown in  FIG. 2 ; 
       FIG. 16  is a block diagram explaining a configuration of an induction heating control circuit applicable to the fixing apparatuses shown in  FIG. 15A ,  15 B and  15 C; 
       FIGS. 17A ,  17 B and  17 C are schematic diagrams explaining still other examples of the induction heating mechanism shown in  FIG. 2 ; 
       FIG. 18  is a block diagram explaining a configuration of another induction heating control circuit applicable to the fixing apparatus shown in  FIG. 1 ; 
       FIGS. 19A ,  19 B,  19 C and  19 D are circuit diagrams explaining the flow of current in an equivalent circuit of the inverter circuit shown in  FIG. 18 ; and 
       FIGS. 20A and 20B  are reference drawings showing the relationship between the time and the current flowing in the equivalent circuit of the inverter circuit shown in  FIG. 18 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, embodiments of the present invention will be explained in detail with reference to the accompanied drawings. 
     FIG. 1  shows an example of a fixing apparatus of the present invention. 
   As shown in  FIG. 1 , a fixing apparatus has a heating member (a heating roller)  2  which contacts the surface of transferred material or a paper sheet PS adhered with a toner T, and heats the toner T and paper sheet PS, and a pressing member (a pressing roller)  3  which applies a predetermined pressure to the heating roller  2 . 
   The heating roller  2  has a core metal  2   a  or a metallic shaft (with a high rigidity) which is not deformed by a predetermine pressure, a foamed rubber layer (sponge)  2   b  which is arranged sequentially around the core metal  2   a , a conductive metal layer  2   c , a solid rubber layer  2   d , and a mold lubricant layer  2   e . It is preferable that the thickness of the foamed rubber layer (sponge)  2   b  is 5 mm thick, the conductive metal layer  2   c  is 40 μm, the solid rubber layer  2   d  is 200 μm, and the mold release layer  2   e  is 40 μm, respectively. The heating roller  2  is preferably 40 mm in diameter. The conductive metal layer  2   c  is made of conductive material (e.g. nickel, stainless steel, aluminum, copper, composite material of stainless steel and aluminum, or the like). 
   The pressing roller  3  preferably includes a core metal  3   a  or a metallic shaft (with a high rigidity) which is not deformed by a predetermined pressure, a silicone rubber  3   b  provided around the core metal  3   a  and a fluorine rubber  3   c , and has a diameter of 40 mm. 
   The pressing roller  3  applies a predetermined pressure to the heating roller  2  by receiving a pressure from the pressing mechanism  4 . The heating roller  2  contacting the pressing roller  3  with a certain nip width taken therebetween, is rotated in the arrow direction (clockwise or CW) by a driving motor (not shown). As the heating roller  2  is rotated, the pressing roller  3  is rotated in the arrow direction (counterclockwise or CCW). 
   A coil body (an induction heating mechanism)  5 ( 6 ) which supplies a predetermined magnetic field to the conductive metal layer  2   c  of the heating roller  2  is arranged outside the heating roller  2  with a predetermined interval taken to the outer circumference of the roller. 
   The coil body  5 ( 6 ) generates a predetermined magnetic field when receiving a predetermined current or voltage. By the magnetic field from the coil body  5 ( 6 ), an eddy current is generated in the conductive metal layer  2   c  of the heating roller  2 , and Joule heat is generated. Toner T is fused by the heat from the heating roller  2 , and fixed to a paper sheet PS when a paper sheet PS adhered with the toner T passes through a contacting part (a nip) between the heating roller  2  and pressing roller  3 , and receives a predetermined pressure from the pressing roller  3 . 
   On the circumference of the heating roller  2 , a separation blade  7  for separating the paper sheet PS from the heating roller  3 , and a mold lubricant application unit  8  for applying a mold lubricant (e.g., silicone oil) for preventing offset to the circumference of the heating roller  2  are arranged sequentially in the rotating direction from the contacting position (nip) between the heating roller and pressing roller  3 . At a predetermined position in the length direction of the heating roller  2 , thermistors  9   a  and  9   b  for detecting the temperatures around the circumference of the heating roller  2  are arranged. In this embodiment, two thermistors  9   a  and  9   b  are used, but three or more can be used. 
   In proximity to the coil bodies  5  and  6 , there is provided an abnormal temperature detection mechanism (a thermostat)  10  which cuts off the current or voltage supplied to the coils  5  and  6  when the temperature of the heating roller  2  reaches an abnormal value. 
     FIG. 2  shows a configuration of an induction heating control circuit applicable to the fixing apparatus shown in  FIG. 1 . 
   The induction heating control circuit has a coil current control circuit  200 , a rectifier circuit  25 , a commercial AC current source  26 , an input power monitor  27 , a CPU  28  and thermistors  31  and  32 . The commercial AC current source  26  is a power supply which supplies power to operate the fixing apparatus of the present invention, and is a part of the power supplied to a copier or the like provided with the fixing apparatus. 
   A coil current control circuit  200  includes a coil body  5  which is located at the position opposite to the central area of the heating roller  2  (the area where a paper sheet PS passes frequently), a coil body  61  which is located at the position opposite to one end of the heating roller  2  in the state aligned with the coil body  5  in the axial direction of the heating roller  2 , and a coil body  62  which is aligned with the coil body  5  to face to the other end of the heating roller  2  opposite to the coil body  62 . The coil body  5  includes an exciting coil  5   a , the coil body  61  includes an exciting coil  61   a , and the coil body  62  includes an exciting coil  62   a . The exciting coils  61   a  and  62   a  are connected in series and electrically one coil as shown in  FIG. 2 , and will be explained as a coil body  6  when explaining both coil bodies  61  and  62 . 
   A first resonance circuit includes the exciting coil  5   a  and resonance condenser  21  connected in parallel. A first inverter circuit includes the first resonance circuit and switching element  23  connected in series. 
   A second resonance circuit includes the exciting coil  6   a  and resonance condenser  22  connected in parallel. The exciting coil  6   a  is electrically one coil connected in series to the exciting coils  61   a  and  62   a . A second inverter circuit includes the second resonance circuit and switching element  24  connected in series. As the switching elements  23  and  24 , IGB or MOS-FET usable with high withstand voltage and large current are used. 
   The first and second inverter circuits are supplied with a DC current from the commercial AC current source  26  smoothed by the rectifier circuit  25 . The thermostat  10  and input power monitor  27  for monitoring the input power PI or the product of the current and voltage supplied from the commercial AC power supply  26  are connected between the rectifier circuit  25  and commercial AC power supply  26 . 
   The input power monitor  27  includes a transformer  27   a  which is connected to the commercial AC power supply  26 , and an input power detection circuit  27   b  which detects the input power PI from the transformer  27   a . The input power detection circuit  27   b  is connected to CPU  28 , to which the information of the input power PI detected by the transformer  27   a  is fed back. 
   The CPU  28  is connected to a memory  28   a , a timer  28   b , and IGBT driving circuits  29  and  30 . The IGBT driving circuit  29  is connected to the control terminal of the switching element  23 . The IGBT driving circuit  30  is connected to the control terminal of the switching element  24 . When the IGBT driving circuits  29  and  30  are operated by the CPU  28 , a high-frequency current flows in the exciting coils  5   a ,  61   a  and  62   a , generating a predetermined magnetic field. When the predetermined magnetic field is supplied to the heating roller  2 , an eddy current is generated in the heating roller  2 , and the exciting coil  5   a  and exciting coils  61   a / 62   a  generate heat in the predetermined areas  2 A (the central area) and  2 B (end area) of the heating roller  2 , respectively. The thermistors  31  and  32  for detecting the surface temperature of the heating roller  2  are arranged in proximity to the predetermined areas  2 A and  2 B of the heating roller  2 , respectively. 
   The thermistors  31  and  32  output the detected surface temperature of the heating roller  2  to the CPU  28  as a temperature detection signal (a voltage value). According to the temperature detection signal, the CPU  28  can select the IGBT driving circuits  29  and  30 . For example, when the temperature of the thermistor  31  is lowered by a predetermined degree compared with the temperature of the thermistor  32 , the CPU  28  drives the IGBT driving circuit  29  connected to the exciting coil  5   a  in order to heat the central area  2 A of the heating roller. Conversely, when the temperature of the thermistor  32  is lowered by a predetermined degree compared with the temperature of the thermistor  31 , the CPU  28  drives the IGBT driving circuit  30  connected to the exciting coils  61   a  and  62   a  in order to heat the end area  2 B of the heating roller. Thus, the central area  2 A and end area  2 B of the heating roller  2  are heated alternately. 
   An induction heating control circuit applicable to the fixing apparatus of the present invention is not limited to the above-mentioned configuration. A half bridge type circuit which changes independently the frequencies of the driving voltage supplied to the switching elements  23  and  24 , and a guasi-E class circuit can be used. As a driving circuit connected to the first and second inverter circuits, a circuit using PWM (pulse width modulation) can be used. 
   Next, explanation will be given on an example of a method of operating the fixing apparatus by referring to the circuit shown in  FIG. 2 . 
   First, explanation will be given on a method of heating alternately the central area  2 A and end area  2 B of the heating roller  2 . 
   The CPU  28  instructs the IGBT driving circuits  29  and  30  to supply a current or voltage (hereinafter, described a coil output power, or a product of this current and voltage) alternately to the exciting coils  5   a  and  6   a  at a predetermined ratio (time ratio). For example, assume the time that the IGBT driving circuit  29  supplies power to the exciting coil  5   a  to be 2 and the time that the IGBT driving circuit  30  supplies power to the exciting coil  6   a  to be 1, and set larger the time ratio of heating the central part of the heating roller  2  where a paper sheet P passes more frequently. The IGBT driving circuits  29  and  30  supply a driving voltage as an ON/OFF signal alternately to the control terminals of the switching elements  23  and  24  at the timing and frequency instructed by the CPU  28 . 
   For example, one switching element  23  supplied with the driving voltage turns on, and the other switching element  24  not supplied with the driving voltage turns off. 
   When the switching element  23  is turned on by the IGBT driving circuit  29 , the rectifier circuit  25  supplies the exciting coil  5   a  with a predetermined power corresponding to the frequency of the driving voltage (including a high-frequency current of 20–50 kHz in this embodiment). The exciting coil  5   a  generates a magnetic field corresponding to the supplied power. When this magnetic field is generated, an eddy current flows in the predetermined area  2 A of the heating roller  2  near the exciting coil  5   a , and the heating roller  2  is heated by Joule heat. Similarly, when the switching element  24  is turned on by the driving circuit  30 , the predetermined area  2 B of the heating roller  2  is heated. 
   Since the heating roller  2  is rotated by a driving motor (not shown) when it is heated, the temperature distribution on the surface of the heating roller  2  can be made uniform in the circumferential direction of the predetermined areas  2 A and  2 B of the heating roller  2  near the exciting coils  5   a  and  6   a.    
   Further, the surface of the heating roller  2  can be heated evenly by selectively changing the timing of supplying power to the exciting coil  5   a  located at the central area of the heating roller  2  and the exciting coil  6   a  located at the end area of the heating roller  2 , according to the size of a paper sheet P passing between the heating roller  2  and pressing roller  3 . 
   Concretely, when the longer side of A4 or A3 paper size is passed parallel to the length direction of the heating roller  2 , and when a full-size paper sheet whose one side is the same length as the paper passing area in the length direction of the heating roller  2 , power is supplied at almost the same ratio to the exciting coil  5   b  located at the center of the heating roller  2  and the exciting coil  6   b  located at the end. 
   Conversely, when passing a small size paper sheet such as a postcard, or when passing the smaller side of A4 paper size parallel to the length direction of the heating roller  2 , set the ratio of power supplied to the exciting coil  5   a  located at the center larger than the power supplied to the exciting coil  6   a  located at the end. 
   Further, when changing the maximum value of the coil output power supplied to the exciting coils  5   a  and  6   a  (the product of the current and voltage supplied to the coils  5   a  and  6   a ) according to the operation mode, the coil output power can be changed in a range of 700 W–1500 W by controlling the frequency of the driving voltage supplied to the switching elements  23  and  24  in a range of 20–50 kHz. 
   Next, explanation will be given on another example of operating the fixing apparatus by referring to the circuit shown in  FIG. 2 . 
     FIG. 3  is a flowchart explaining another example of the method of operating the fixing apparatus explained by using  FIG. 2 . 
   In the method of supplying power to the exciting coils  5   a  and  6   a  described above with reference to  FIG. 2 , while power is being supplied to one exciting coil  5   a , power is not supplied to the other exciting coil  6   a . Now, explanation will be given on a control method for supplying power simultaneously to both exciting coils  5   a  and  6   a.    
   In the example shown in  FIG. 3 , the sum of the center coil output power P ( 5   a ) supplied from the commercial power supply  26  to the exciting coil  5   a  under the control of CPU  28  and the end coil output power P ( 6   a ) supplied to the exciting coil  6   a , or the total coil output power P ( 5   a + 6   a ) is assumed to be 900 W. Namely, a predetermined total coil output power P ( 5   a + 6   a ) is assigned to the exciting coils  5   a  and  6   a  at a predetermined ratio, and supplied at the same time. In this time, the temperature TC of the predetermined area  2 A of the heating roller  2  (near the exciting coil  5   a  located at the center of the heating roller  2 ) is detected by the thermistor  31 , and compared with the standby temperature TS (e.g. 160° C.) at which toner can be fixed to a paper sheet set in the CPU  28  when passing through there (S 1 ). 
   When the temperature TC detected by the thermistor  31  is lower than the standby temperature TS (S 1 —YES), the temperature TC detected by the thermistor  31  is further compared with the temperature TE of the predetermined position  2 B of the heating roller detected by the thermistor  32  (near the exciting coil  6   a  located at the end area of the heating roller  2 ) (S 2 ). When the temperature TC detected by the thermistor  31  is higher than the standby temperature TS (S 1 —NO), step S 1  is finished. 
   When the temperature TE of the end area is higher than the temperature TC of the central area of the heating roller  2  (S 2 —NO), whether the difference between the temperatures TC and TE is less than a first predetermined temperature, 5° C. for example, is judged (S 3 ). When the difference between the temperatures TC and TE is less than 5° C., the exciting coils  5   a  and  6   a  are supplied with power of the same value (same ratio) (S 3 —YES). Namely, the total coil output current 900 W is assigned at a ratio of 5:5, and power of 450 W is supplied to the exciting coil (center coil)  5   a  and exciting coil (end coil)  6   a , respectively. 
   Conversely, when the difference between the temperatures TC and TE is larger than 5° C. (S 3 —NO), whether the difference between the temperatures TC and TE is less than a second predetermined temperature (e.g. 10° C.) is judged (S 4 ). When the difference between the temperatures TC and TE is less than 10° C., set the ratio of power supplied to the center coil  5   a  larger than the ratio of power supplied to the end coil  6   a  (S 4 —YES). Namely, the total coil output current 900 W is assigned at a ratio of 5:4, and power of 500 W is supplied to the center coil  5   a  and 400 W is supplied to the end coil  6   a.    
   Conversely, when the difference between the temperatures TC and TE is larger than 10° C. (S 4 —NO), set the ratio of power supplied to the center coil  5   a  larger than the ratio of power supplied to the end coil  6   a . Namely, the total coil output current 900 W is assigned at a ration of 2:1, and power of 600 W is supplied to the center coil  5   a  and 300 W is supplied to the end coil  6   a.    
   Returning to step S 2 , when the temperature TC of the central area of the heating roller  2  is higher than the temperature TE of the end area (S 2 —YES), whether the difference between the temperatures TC and TE is less than a first predetermined temperature (e.g. 5° C.) is judged (S 5 ) as in step 3. When the temperature difference is less than 5° C., power of the same value is supplied to the center coil  5   a  and end coil  6   a  (S 5 —YES). Namely, the total coil output current 900 W is assigned at a ration of 5:5, and power of 450 W is supplied to the center coil  5   a  and end coil  6   a.    
   When the temperature difference between TC and TE is larger than 5° C. (S 5 —NO), whether the difference between the temperatures TC and TE is less than a second predetermined temperature (e.g. 10° C.) is judged (S 5 ) as in step 4 (S 6 ). When the temperature difference is less than 10° C., set the ratio of power supplied to the center coil  6   a  larger than the ratio of power supplied to the end coil  5   a . Namely, the total coil output current 900 W is assigned at a ration of 5:4, and power of 500 W is supplied to the end coil  6   a  and 400 W is supplied to the center coil  5   a.    
   Conversely, when the difference between the temperatures TC and TE is larger than 10° C. (S 6 —NO), set the ratio of power supplied to the end coil  6   a  larger than the ratio of power supplied to the center coil  5   a . Namely, the total coil output current 900 W is assigned at a ration of 2:1, and power of 600 W is supplied to the end coil  6   a  and 300 W is supplied to the center coil  5   a.    
   As for a value of the total coil output power P ( 5   a + 6   a ) set in step 1, a predetermined value is selected according to the operation modes of the fixing apparatus. For example, 1200 W is set for warm-up mode, 900 W is set for paper passing mode to pass a paper sheet P between the heating roller  2  and pressing roller  3 , and 700 W is set for ready mode, respectively. 
   Therefore, even if a temperature difference occurs in the length direction of the heating roller  2 , the total power supplied to the center coil  5   a  and end coil  6   a  is not changed, and electric power can be used efficiently for induction heating. 
   In this control method, the electric power supplied to the coil corresponding to the lower temperature, out of TE and TC at the end area and center area of the heating roller  2 , is larger than the power supplied to the coil of a higher temperature, and the power is supplied to reduce the temperature difference between the coils, maintaining constant temperature distribution in the length direction of the heating roller  2 . 
   The ratio of power supplied to the center coil  5   a  and end coil  6   a , and the first and second predetermined temperatures are saved in the memory  28   a  connected to the CPU  28 , and set optionally. 
   Next, explanation will be given on an example of a coil body applicable to the coil bodies  5 ,  61  and  62  shown in  FIG. 2 , with reference to  FIGS. 4A–4C .  FIG. 4B  is a schematic perspective view seen from the arrow P direction of  FIG. 4A .  FIG. 4C  is a schematic perspective view seen from the arrow Q direction of  FIG. 4A . 
   As shown in  FIGS. 4B and 4C , a coil body  105  which heats the central area (the area to pass a paper sheet PS frequently) of the heating roller  2 , and a coil body  106  (including  161  and  162 ) which heats both end areas of the heating roller  2  are arranged linearly in the axial direction outside the heating roller  2 . The coil body  105  has an exciting coil  105   a , and a magnetic core  105   b  holding the exciting coil  105   a . The coil body  161  has an exciting coil  161   a , and a magnetic core  161   b  holding the exciting coil  161   a . The coil body  162  has an exciting coil  162   a , and a magnetic core  162   b  holding the exciting coil  162   a.    
   The exciting coil  105   a  has edges  115 CE and  125 CE formed at a predetermined angle θ 1  at both ends, that is, the edge  115 CE located in the side of the joint W 11  of the exciting coil, and the edge  125 CE located in the side of the joint W 12 . The exciting coil  105   a  is not limited to trapezoidal as shown in  FIG. 4B . A parallelogramatic coil is permitted. 
   The exciting coils  161   a  and  162   a  are arranged with the centers aligned (at the same angle and phase). The exciting coil  161   a  has an edge  161 CE formed at a predetermined angle θ 1  in the side of the joint W 11  of the exciting coil. The exciting coil  162   a  has an edge  162 CE formed at a predetermined angle θ 1  in the side of the joint W 12  of the exciting coil. 
   As shown in  FIG. 4B , in the state that the opposite ends (folded parts) of the exciting coils  105   a  and  161   a  are arranged in parallel, the acute angle (θ 1 ) of the edge  115 CE of the joint W 11  forms an alternate angle to the acute angle (θ 1 ) of the edge  161 CE, with respect to the conductors of the exciting coils  105   a  and  161   a  extending parallel to the axial direction of the heating roller  2 . Similarly, the acute angle of the edge  125 CE of the joint W 12  is at an alternate angle with the acute angle of the edge  162 CE, with respect to the conductor extending parallel to the axial directions of the exciting coil  105   a  and  162   a.    
   In other words, the exciting coil  105   a  includes the parallel wire part consisting of the wire extending parallel to the axial direction of the heating roller  2 , and the folded wire part connecting one parallel wire part to the other parallel wire part arranged opposite to each other on both sides of an imaginary axis. The folded wire part crosses the parallel wire part at a predetermined angle. Namely, the folded wire part includes first and second linear parts that are the not-parallel sides of the trapezoidal exciting coil  105   a.    
   The exciting coil  161   a  has a third linear part that is formed corresponding to one folded wire part (the first linear part) of the exciting coil  105  adjacent to one end. The exciting coil  162   a  has a fourth linear part that is formed corresponding to the other folded wire part (the second linear part) of the exciting coil  105   a  adjacent to one end. 
   Therefore, when a predetermined electric power is supplied, the exciting coils  105   a  and  161   a  can supply a magnetic field generated by both exciting coils to the area divided in the direction orthogonal to the axial direction on the outer circumference of the heating roller  2 , that is, the joint W 11 . Similarly, when a predetermined electric power is supplied, the exciting coils  105   a  and  162   a  can supply a magnetic field generated by both exciting coils to the area divided in the direction orthogonal to the axial direction on the outer circumference of the heating roller  2 , that is, the joint W 12 . 
   In other words, when a predetermined coil output power is supplied to the exciting coils  105   a ,  161   a  and  162   a , the heating roller  2  has the joint W 11  where a predetermined magnetic field is supplied from both exciting coils  105   a  and  161   a , and the joint W 12  where a predetermined magnetic field is supplied from both exciting coils  105   a  and  162   a , in the area divided in the direction orthogonal to the axial direction. 
   Therefore, since the areas with magnetic fields supplied from the adjacent exciting coils are overlapped in the coil joints W 11  and W 12 , a temperature drop can be prevented, and the temperature distribution in the length direction of the heating roller can be made uniform. 
   The angle θ 1  is set to a determined value by evaluating the temperature based on the result of using the fixing apparatus (when passing a paper sheet P). The angle is 70° in this example. 
   Therefore, even in the fixing apparatus having two heating rollers  2  that cannot contain an exciting coil as shown in  FIG. 11 , the space to arrange an exciting coil (the area occupied by the exciting coils against the heating roller  2 ) can be limited to a predetermined area divided in the axial direction of the heating roller  2  where the exciting coils are aligned as a single line. 
   Next, the magnetic cores  105   b ,  161   b  and  162   b  will be explained. 
   The magnetic core  105   b  is at least shaped to cover a window (space)  105 D of the exciting coil  105   b . The part covering the window  105 D of the magnetic core  105   b  is thick compared with the part covering the coil conductor and fitted into the space surrounded by the coil conductor, as shown in  FIG. 4A . Similarly, in the magnetic cores  161   b  and  162   b , the space surrounded by the coil conductor or the part fitted into the window is thick compared with the part arranged on the wires of the exciting coils  161   a  and  162   a.    
   By using the magnetic cores shaped as described above, the magnetic fields generated by the exciting coils  105   a ,  161   a  and  162   a  from the supplied electric power can be supplied efficiently to the heating roller  2 . 
   Next, explanation will be given on an example of the relationship between the coil bodies  105 ,  161 ,  162  and thermostat  110  explained in  FIGS. 4A–4C  with reference to  FIGS. 5A and 5B . 
   As shown in  FIG. 5A , a thermostat  110  is placed between the coil bodies  105  and  161 , and a thermostat  111  is placed between the coil bodies  105  and  162 . The thermostats  110  and  111  are configured to detect the surface temperature of the heating roller  2 , and cut off the electric power supplied to the exciting coils  105   a ,  161   a  and  162   a  when the detected temperature reaches an abnormal value. 
   In details, the distance between the exciting coils  161   a  and  105   a  is L 1 , and the distance between the exciting coils  162   a  and  105   a  is L 2 . The thermostats  110  and  111  are preferably arranged close to the exciting coil. 
   Therefore, the thermostats  110  and  111  are arranged near the area where the magnetic field between the heating roller  2  and the wire of the exciting coil  105   a  is supplied continuously, for example, and can detect the temperature of the heating roller  2  by thermal conduction with a faster response. Therefore, even if the heating roller  2  is stopped and heated locally, the thermostats can detect an abnormal temperature rise in the area where the temperature rises to the highest value. 
   Further, in a fixing apparatus which has two heating rollers  2  and cannot contain an exciting coil, for example, the space to place an abnormal temperature detection mechanism (the area occupied by the exciting coils arranged outside the heating roller  2 ) includes the area to place the exciting coils  105   a ,  161   a  and  162   a , and the unit can be made compact. 
   The distance L 1  and L 2  is set to a value not causing a temperature difference in the axial direction of the heating roller  2 . In details, the distance L 1  is set to a value that the difference between the magnetic field supplied from the exciting coils  105   a / 161   a  and the magnetic field supplied from the exciting coil  105  or the center of the exciting coil  161   a  becomes minimum or zero in the joint W 11  of the magnetic coils, when a predetermined electric power is supplied to the exciting coils  105   a  and  161   a . Similarly, the distance L 2  is set to a value that the difference between the magnetic field supplied from the exciting coils  105   a / 162   a  and the magnetic field supplied from the exciting coil  105  or the center of the exciting coil  162   a  becomes minimum or zero in the joint W 12  of the magnetic coil, when a predetermined electric power is supplied to the exciting coils  105   a  and  162   a.    
   Therefore, since the areas where the magnetic field is supplied from the adjacent exciting coils is overlapped in the coil joints W 11  and W 12 , a temperature drop can be prevented, and the temperature distribution in the length direction of the heating roller  2  can be made uniform. 
   Magnetic field shielding materials  110 A and  111 A may be provided in the thermostats  110  and  111 , respectively. The magnetic field shielding materials  111 A and  111 A prevent supply of the magnetic field to the thermostats  110  and  111  from the surrounding magnetic coils, for example. With the magnetic field shielding material  110 A, the thermostat  110  is prevented from being influenced by the magnetic field from the exciting coil  105   a , and a malfunction such as failure to detect a correct temperature caused by the temperature increase by induction heating (inductive current) can be prevented. The magnetic field shielding material  110 A is preferably shaped to cover the surface of the thermostat  110  facing to the exciting coil, except the part where the thermostat  110  faces to the outer circumference of the heating roller  2 , as shown in  FIG. 5B  with respect to thermostat  10  and magnetic shielding material  10 A. 
   Next, explanation will be given on another example different from the coil bodies  5  and  6  shown in  FIG. 2  with reference to  FIGS. 6A–6D .  FIG. 6A  is a schematic perspective view seen from the arrow P direction of  FIG. 6C .  FIG. 6B  is a schematic perspective view seen from the arrow Q direction of  FIG. 6C . 
   As shown in  FIGS. 6A and 6B , a coil body  205  which heats the central area of the heating roller  2 , and a coil body  206  which heats both end areas of the heating roller  2  are provided outside the heating roller  2 . The coil body  206  includes a coil body  261  which heats one end of the heating roller  2 , and a coil body  262  which heats the other end of the heating roller  2 . The coil bodies  261  and  262  are connected in series, and formed electrically as one coil. 
   The coil bodies  205 ,  261  and  262  have exciting coils  205   a ,  261   a  and  262   a  whose at least one end is inclined to the opposite side to the heating roller  2 , and magnetic cores  205   b ,  261   b  and  262   b  which hold the exciting coils  205   a ,  261   a  and  262   a , respectively. 
   The exciting coils  205   a ,  261   a  and  262   a  have the largeness that the adjacent coils are overlapped in the joint of exciting coils, when they are arranged linearly outside the heating roller  2 . 
   Thus, one ends  215 CE and  261 CE of the adjacent exciting coils  205   a  and  261   a  are folded not to contact each other in the folding line part located at the boundary of the central part and end part of the exciting coil, and raised 90° toward the opposite side (the magnetic core side) of the heating roller  2 . Similarly, the other ends  225 CE and  262 CE of the adjacent exciting coils  205   a  and  262   a  are folded not to contact each other in the bending line part located at the boundary of the central part and end part of the exciting coil, and raised 90° toward the opposite side (the magnetic core side) of the heating roller  2 . 
   In other words, the interval between the heating roller  2  and the central part of the exciting coil  205   a  is narrow, compared with the interval between the heating roller  2  and the end  215 CE of the exciting coil  205   a  adjacent to the exciting coil  261   a , and the interval between the heating roller  2  and the end  225 CE of the exciting coil  205   a  adjacent to the exciting coil  262   a . The interval between the heating roller  2  and the central part of the exciting coil  261   a  is narrow, compared with the interval between the heating roller  2  and the end  261 CE of the exciting coil  261   a  adjacent to the exciting coil  205   a . The interval between the heating roller  2  and the central part of the exciting coil  262   a  is narrow, compared with the interval between the heating roller  2  and the end  262 CE of the exciting coil  262   a  adjacent to the exciting coil  205   a.    
   The folding line part is preferably located on the innermost and turned wire among the wires of the exciting coil. In the folding line part, the end of the exciting coil can be inclined easily. Further, since the part not inclined or the central part of the exciting coil consisting of the wire parallel to the axial direction of the heating roller  2  is arranged with a certain interval taken to the heating roller  2 , a uniform magnetic field in the axial direction of the heating roller  2  can be obtained. 
   The end of the exciting coil may be inclined more outside the folding line than the innermost and turned wire. This suppresses the height of the exciting coil from the outer circumference of the heating roller  2 , and the unit can be made compact. 
   The adjacent exciting coils  205   a  and  261   a  are arranged outside the heating roller  2  close to each other, so that the ends  215 CE and  261 CE of the folded side do not contact each other and the coil centers are aligned (at the same angle and phase). Similarly, the adjacent exciting coils  205   a  and  262   a  are arranged outside the heating roller  2  close to each other, so that the coil centers are aligned. As the magnetic cores  205   b ,  261   b  and  262   b  of each coil body become close to one another, and the magnetic flux density (a intensity of magnetic flux) between the coils can be increased. 
   A magnetic field shielding plate  65  may be placed in the joints W 21  and W 22  of the exciting coil, as shown in  FIG. 6D . By using the magnetic field shielding plate  65 , when electric power is supplied simultaneously to all exciting coils  205   a ,  261   a  and  262   a , a change in the magnetic field caused by the mutual induction occurring between the ends  215 CE and  261 CE or between the ends  225 CE and  262 CE of each exciting coil is prevented, and a temperature fluctuation in the axial direction of the heating roller  2  caused by the change in the magnetic field can be suppressed. 
   In this embodiment, by raising the coil ends ( 215 CE,  225 CE,  261 CE and  262 CE) where the intensity of the magnetic field generated when electric power is supplied is weak to the opposite side of the heating roller  2 , the magnetic field from the coil centers ( 205 CC,  261 CC and  262 CC) where the magnetic flux density is stronger can be supplied to the heating roller  2 . The coil ends  215 CE and  225  CE are generically referred to as  205 CE in  FIG. 6B . Therefore, in the heating roller  2  where the coil centers generating a uniform magnetic field are faced close to each other, the temperature distribution in the length direction becomes uniform. 
   Next, explanation will be given on still another example different from the coil bodies  5  and  6  shown in  FIG. 2 , with reference to  FIGS. 7A and 7B . 
   As shown in  FIGS. 7A and 7B , a coil body  305  which heats the central part of the heating roller  2 , and a coil body  306  which heats both ends of the heating roller  2  are provided outside the heating roller  2 . The coil body  306  includes a coil body  361  which heats one end of the heating roller  2 , and a coil body  362  which heats the other end of the heating roller  2 . The coil bodies  361  and  362  are connected in series, and formed electrically as one coil. 
   The coil body  305  has an exciting coil  305   a , and a magnetic core  305   b  for holding the exciting coil  305   a . The coil body  361  has an exciting coil  361   a  with one end adjacent to the coil body  305  and inclined to the opposite side to the heating roller  2 , and a magnetic core  361   b  for holding the exciting coil  361   a . The coil body  362  has an exciting coil  362   a  with one end adjacent to the coil body  305  and inclined to the opposite side to the heating roller  2 , and a magnetic core  362   b  for holding the exciting coil  362   a.    
   In other words, the exciting coil  305   a  does not have an inclined part, and has an interval of distance Y 31  to the heating roller  2  at the center and both ends. The exciting coil  361   a  has an interval of distance Y 31  between the central part and the heating roller  2 , and has an interval of distance Y 32  between the end  361 CE adjacent to the excitation coil  305   a  and the heating roller  2 . The exciting coil  362   a  has an interval of distance Y 31  between the central part and the heating roller  2 , and has an interval of distance Y 32  between the end  362 CE adjacent to the exciting coil  305   a  and the heating roller  2 . The distance Y 31  is small compared with the distance Y 32 . Namely, only the ends of the exciting coils  361   a  and  362   a  are adjacent to the exciting coil  305   a  are separated from the heating roller  2 , and both ends of the exciting coil  305   a  are placed between the heating roller  2  and the ends  361 CE and  362 CE of the exciting coils  361   a  and  362   a.    
   The exciting coils  305   a ,  361   a  and  362   a  have s size such that the adjacent coils are overlapped in the joints W 31  and W 32  of the exciting coils, like the exciting coils  205   a ,  261   a  and  262   a.    
   The exciting coils  305   a ,  361   a  and  362   a  are aligned (at the same angle and phase) outside the heating roller  2 . In this time, one end  361 CE of the exciting coil  361   a  is bent toward the opposite side (the magnetic core side) of the heating roller  2 , not to make contact with one end  315 CE of the adjacent exciting coil  305   a . Similarly, one end  362 CE of the exciting coil  362   a  is bent toward the opposite side (the magnetic core side) of the heating roller  2 , not to make contact with the other end  325 CE of the adjacent exciting coil  305   a.    
   Namely, one end  361 CE of the exciting coil  361   a  and one end  362 CE of the exciting coil  362   a  are bent to the upper side in an imaginary line X 1  on the paper surface (refer to  FIG. 7B ). Then, the front end of the exciting coil  361  is bent to the right side in an imaginary line X 2  on the paper surface, and front end of the exciting coil  362  is bent to the left side in an imaginary line X 2  on the paper surface. Therefore, the exciting coils  361  and  362  are overlapped in the state not contacting the exciting coil  305   a.    
   Thus, on the outer circumference, the exciting coil joints W 31  and W 32  receives the influence of a predetermined magnetic field supplied from both adjacent exciting coils  305   a  and  261   a  (or the adjacent exciting coils  305   a  and  362   a ), in the area divided in the direction orthogonal to the axial direction. 
   Next, explanation will be given on still another example different from the coil bodies  5  and  6  shown in  FIG. 2 , with reference to  FIGS. 8A and 8C .  FIG. 8A  is a schematic perspective view seen from the arrow Q direction of  FIG. 8B . 
   As shown in  FIG. 8A , a coil body  405  which heats the central area of the heating roller  2 , and a coil body  406  which heats both end areas of the heating roller  2  are provided outside the heating roller  2 . The coil body  406  includes a coil body  461  which heats one end area of the heating roller  2 , and a coil body  462  which heats the other end area of the heating roller  2 . The coil bodies  461  and  462  are connected in series, and formed electrically as one coil. 
   The coil bodies  405 ,  461  and  462  have exciting coils  405   a ,  461   a  and  462   a  with at least one end inclined toward the heating roller  2 , and magnetic cores  405   b ,  461   b  and  462   b  for holding the exciting coils  405   a ,  461   a  and  462   a , respectively. In other words, in the exciting coil  405   a , the interval between the coil end  405 CE and the heating roller  2  is small, compared with the interval between the coil center  405 CC and the heating roller  2 . In the exciting coils  461   a  and  462   a , the interval between the heating roller  2  and the coil ends  461 CE and  462 CE of the side adjacent to the exciting coil  405   a  is small, compared with the interval between the heating roller  2  and the coil centers  461 CC and  462 CC. 
   The exciting coils  405   a ,  461   a  and  462   a  have a size such that the adjacent coils are not overlapped, in the joints W 41  and W 42  of the exciting coils, when they are aligned outside the heating roller  2 . One end  405 CE of the adjacent exciting coil  405   a  and one end  461 CE of the exciting coil  461   a  are bent toward the heating roller  2  to come closer to the outer circumference of the heating roller  2 , compared with the coil center which can supply a uniform magnetic field in the length direction of the heating roller  2 . 
   Namely, the centers  405 CC,  461 CC and  462 CC of the respective exciting coils are arranged with an interval Y 1  taken to the surface of the heating roller  2 . Conversely, the ends  405 CE,  461 CE and  462 CE of the respective exciting coils have an interval Y 2  shorter than Y 1  in the space to the surface of the heating roller  2 . 
   Similarly, one end  405 CE of the adjacent exciting coil  405   a  and one end  462 CE of the exciting coil  462   a  are bent toward the heating roller  2  to have an interval Y 2  shorter than Y 1  in the space to the surface of the heating roller  2 . 
   Thus, the ends  405 CE,  461 CE and  462 CE of the exciting coils with the shorter distance to the heating roller  2  consumes less magnetic field supplied to the heating roller  2 , compared with the centers  405 CC,  461 CC and  462 CC with the longer distance to the heating roller  2 . Therefore, the supplied magnetic field can be made uniform in the length direction of the heating roller  2 , and a temperature drop in the coil joints W 41  and W 42  can be improved. 
   Further, as shown in  FIG. 8A , the ends of the exciting coils  461   a  and  462   a  located at the ends of the length direction of the heating roller  2  may be bent toward the heating roller  2 , like the coil ends  461 CE and  462 CE on the opposite side. This prevents a temperature drop caused by a heat escape at the ends of the heating roller  2 . 
   The ends  405 CE,  461 CE and  462 CE of the exciting coils  405   a ,  461   a  and  462   a  may be bent in the direction of the outer circumference of the heating roller  2 , to have a predetermined curvature along the outer circumference of the heating roller  2 , as shown in  FIG. 8C . 
   The bent part is located more close to the heating roller  2  than the not-bent part, and the consumption of the magnetic field supplied to the heating roller  2  can be reduced. 
   Further, the exciting coils  5   a ,  61   a  and  62   a  shown in  FIG. 2 , the exciting coils  105   a ,  161   a  and  162   a  shown in  FIG. 4B , the exciting coils  205   a ,  261   a  and  262   a  shown in  FIG. 6A , and the exciting coils  305   a ,  361   a  and  362   a  shown in  FIG. 7A  may be bent in the direction of the outer circumference of the heating roller  2  all over the axial direction of the heating roller  2  of the coil, to have a predetermined curvature along the outer circumference of the heating roller  2 , as shown in  FIG. 8C . This makes it possible to supply a magnetic field to the heating roller  2  more efficiently. 
   Next, explanation will be given on the arrangement of the thermostat  10  in a fixing apparatus having a plurality of exciting coils arranged linearly outside the heating roller  2 , as explained above. 
     FIGS. 9A and 9B  show an example of arrangement of the thermostat  10 .  FIG. 9A  is a view seen from the arrow P direction of  FIG. 1 .  FIG. 9B  is a view seen from the arrow Q direction of  FIG. 1 . 
   As shown in  FIGS. 9A and 9B , a coil body  505  is placed at the position opposite to the central area of the heating roller  2 . Coil bodies  561  and  562  are placed at the position opposite to both ends of the heating roller  2 , in the state arranged linearly with the coil body  505  in the axial direction of the heating roller  2 . The coil bodies  561  and  562  are connected in series, and formed electrically as one coil. 
   The coil body  505  has an exciting coil  505   a  whose wire is wound around an imaginary axis and shaped to be a predetermined form (e.g. doughnut-shaped), and a magnetic core  505   b  placed on the wire of the exciting coil  505   a . At the center of the exciting coil  505   a  including the imaginary axis, a space (hereinafter referred to as a window)  505   c  with no wire is formed. In the window  505   c , the magnetic core  505  does not exist. Namely, the magnetic coil  505   a  includes one parallel wire part consisting of a parallel extending wire on which the magnetic core  505   b  is placed, and a folded wire part which connects the other parallel wire part placed opposite to one parallel wire part on the opposite side of the imaginary axis (window  505   c ). 
   Similarly, the coil bodies  561  and  562  have exciting coils  561   a  and  562   a  whose wire is wound around an imaginary axis and shaped to be a predetermined form (e.g. doughnut-shaped), and magnetic cores  561   b  and  562   b  placed on the wires of the exciting coils  561   a  and  562   a , respectively. At the centers of the exciting coils  561   a  and  562   a  including the imaginary axis, spaces (windows)  561   c  and  562   c  with no wire are formed. Namely, the magnetic coil  561   a  and  562   a  include one parallel wire part consisting of a parallel extending wire on which the magnetic cores  561   b  and  562   b  are placed, and a folded wire part which connects the other parallel wire part placed opposite to one parallel wire part on the other side of the imaginary axis (windows  561   c  and  562   c ). The magnetic cores  561   b  and  562   b  can be arranged on the parallel wire part except the windows  561   c  and  562   c , like the magnetic core  505   b.    
   As a wire of the exciting coils  505   a ,  561   a  and  562   a , use a litz wire with insulated surface and made by binding a plurality of wires. The exciting coils  505   a ,  561   a  and  562   a  formed by the litz wire can generate a magnetic field effectively even if an alternating current is supplied. This embodiment uses a litz wire insulated by using heat-resistant polyamide and formed by binding  16  copper wires of 0.5 mm in diameter. 
   The number of turns of the wires of the exciting coils  505   a ,  561   a  and  562   a  can be reduced by providing magnetic cores  505   b ,  561   b  and  562   b . The coil bodies  505 ,  561  and  562  formed as explained above can generate a magnetic flux intensively and heat locally a predetermined area of the heating roller  2 . 
   The exciting coils  505   a ,  561   a  and  562   a  are arranged so that the imaginary axes are crossed vertically to the outer circumference of the heating roller  2 . On the outer circumference of the heating roller  2 , there are areas  2 - 5   a ,  2 - 61   a  and  2 - 62   a  opposite to the exciting coils  505   a ,  561   a  and  562   a  (hereinafter, referred to as a coil area), and areas  2 - 5   c ,  2 - 61   c  and  2 - 62   c  (hereinafter, referred to as a window area) corresponding to the windows  505   c ,  561   c  and  562   c  with no wires and surrounded by wires. Therefore, when viewing the heating roller from the direction shown in  FIG. 9A , no wires are arranged on the window areas  2 - 5   c ,  2 - 61   c  and  2 - 62   c , and the surface of the heating roller  2  is seen. 
   In the window area  2 - 5   c , an abnormal temperature detection mechanism (a thermostat)  510  is provided not contacting the heating roller  2 , which detects the temperature of the heating roller  2 , and when the detected temperature reaches an abnormal value, cuts off the power supplied to the exciting coils  505   a ,  561   a  and  562   a . The abnormal temperature is a temperature higher than a temperature range demanded for fusing (a normal temperature), and is defined as an upper limit temperature at which other members mounted in the fixing apparatus malfunction, or the heating roller  2  and pressing roller  3  are stopped, and a current is abnormally and continuously supplied to the exciting coil. 
   Therefore, the thermostat  510  can detect the heat generated from the window area  2 – 5   c  of the heating roller  2  heated by the magnetic field supplied from the surrounding exciting coil  505   a . The heat of the coil area  2 - 5   a  generated by the magnetic field supplied from the exciting coil  505   a  is transmitted to the window area  2 - 5   c . Thus, even if the heating roller  2  is stopped, and the coil area  2 - 5   a  is locally heated to an abnormal temperature, the thermostat can detect the temperature close to the value in the coil area  2 - 5   a  where the temperature rises to the highest. 
   When detecting the abnormal temperature, the thermostat  510  cuts off the power supplied to the exciting coils  505   a ,  561   a  and  562   a.    
   Further, as shown in  FIG. 10 , the thermostat  510  may have a magnetic field shielding material  510 A to prevent supply of a magnetic field from the surrounding exciting coil  505   a . With the magnetic field shielding material  510 A, for example, the thermostat  510  is prevented from being influenced by the magnetic field from the exciting coil  505   a , and a malfunction such as failure to detect a correct temperature caused by the temperature increase by induction heating (inductive current) can be prevented. 
   By placing the thermostat  510  in the coil window, the space for the abnormal temperature detection mechanism is shared by the exciting coil, and the space around the outside of the heating roller  2  can be used effectively. 
   In the example shown in  FIGS. 9A ,  9 B and  10 , the thermostat  510  is placed in the window  505   c  of the exciting coil  505   a , but the present invention is not limited to this configuration. It is permitted to place the thermostat in one of the window  561   c  of the exciting coil  561   a  and window  562   c  of the exciting coil  562   a . It is also permitted to place two thermostats in the windows  505   c  and  561   c  or the windows  505   c  and  562   c.    
   Next, explanation will be give on an example of different arrangement of the thermostat  10 .  FIG. 11  is a view seen from the arrow P direction of  FIG. 1 . Detailed explanation of the same configurations as those shown in  FIGS. 9A and 9B  is omitted. 
   As shown in  FIG. 11 , a coil body  605  which heats the central area of the heating roller  2 , and coil bodies  661  and  662  which heat both end areas of the heating roller  2  are arranged linearly in the axial direction outside the heating roller  2 . 
   The coil body  605  has an exciting coil  505   a  whose wire is wound around an imaginary axis and shaped to be a predetermined form, and a magnetic core  605   b  which is placed on the wire of the exciting coil  505   a  and covers the window  505   c.    
   Similarly, the coil bodies  661  and  662  have exciting coils  561   a  and  562   a  whose wire is wound around an imaginary axis and shaped to be a predetermined form, and magnetic cores  661   b  and  662   b  which are placed on the wires of the exciting coils  561   a  and  562   a , and cover the windows  561   c  and  562   c  respectively. 
   When the imaginary axes of the exciting coils  505   a ,  561   a  and  562   a  are arranged to cross vertically to the outer circumference of the heating roller  2 , an area through which the surface of the heating roller  2  is seen is formed in a predetermined area of the joint area W 61  between the exciting coils  505   a  and  561   a . At a predetermined position of the joint area W 61 , a thermostat  610  is placed, which detects the temperature of the heating roller  2  and cuts off the power supplied to the exciting coils  505   a ,  561   a  and  562   a  when the detected temperature reaches an abnormal value. 
   Similarly, an area through which the surface of the heating roller  2  is seen is formed in the joint area W 62  between the exciting coils  505   a  and  562   a . A thermostat  611  can be placed at a predetermined position in the joint area W 62 . The thermostats  610  and  611  are preferably placed close to the exciting coil. 
   Therefore, the thermostats  610  and  611  can detect the temperature of the heating roller  2  by thermal conduction at a faster response speed. Because the thermostats  610  and  611  are placed in proximity to the area where the magnetic field between the heating roller  2  and the wire of the exciting coil  505   a  is continuously supplied, and an appropriate response speed is ensured. Therefore, even if the heating roller  2  is stopped and locally heated, the thermostat can detect an abnormal temperature rise in the area of the outer circumference of the heating roller  2  where the temperature rises to the highest. 
   By providing two thermostats  610  and  611 , even if one of them fails and does not function, the other detects an abnormal temperature. Of course, even only one thermostat can detect an abnormal temperature rise in the area of the outer circumference of the heating roller  2  where the temperature rises to the highest. 
   It is also possible to provide a magnetic field shielding material  610 A in the thermostat  610  to prevent supply of magnetic field from the exciting coils  505   a  and  561   a , and to provide a magnetic field shielding material  611 A in the thermostat  611  to prevent supply of magnetic field from the exciting coils  505   a  and  562   a.    
     FIGS. 12A ,  12 B and  12 C show still another example of the thermostat  10 .  FIG. 12A  is a schematic diagram showing the relationship between the heating roller  2  and abnormal temperature detection mechanism.  FIG. 12B  is a view seen from the arrow P direction of  FIG. 12A .  FIG. 12C  is a view seen from the arrow Q direction of  FIG. 12A . Detailed explanation of the same configurations as those shown in  FIGS. 9A and 9B  is omitted. 
   As shown in  FIG. 12A , a coil body  605  which heats the central area of the heating roller  2 , and coil bodies  661  and  662  which heat both end areas of the heating roller  2  are provided outside the heating roller  2 . 
   The coil body  661  which heats one end area of the heating roller  2  is arranged opposite to a part of the area R 1  where a paper sheet PS passes in the length direction of the heating roller  2 , and a part of area R 2  (area not to pass paper) where a paper sheet PS is not passed in the length direction of the heating roller  2 . The coil body  662  which heats the other end area is arranged opposite to a part of the area R 1  where a paper sheet PS passes in the length direction of the heating roller  2 , and a part of the area R 3  (area not to pass paper) where a paper sheet PS is not passed in the length direction of the heating roller  2 . 
   The coil body  605  has an exciting coil  505   a  and a magnetic core  605   b . The coil bodies  661  and  662  have exciting coils  561   a / 562   a  and magnetic cores  661   b / 662   b , respectively. 
   Between the exciting coil  562   a  and the area R 3  not to pass paper, a part of a heat pipe type abnormal temperature detection mechanism  710  is provided closely or contacted. 
   The abnormal temperature detection mechanism  710  has a first conductive member  711  which is provided close to or contacting the outer circumference of the heating roller  2  between the outer circumference of the heating roller  2  and the wire of the exciting coil  562   a , a heat pipe  712  which transmits the heat from the first conductive member  711  to a position separated from the heating roller  2 , a second conductive member  713  which conducts the heat from the heat pipe  712 , and an abnormal temperature detector  714  which detects the temperature of the second conductive member  713  and cuts off the power supplied to the exciting coils  505   a ,  561   a  and  562   a  when the detected temperature reaches an abnormal value. 
   The first conductive member  711  is composed of material with a high thermal conductivity (e.g. material including copper, aluminum, silver or the like). The first conductive member  711  may include material that is hard to heat by the induction heating to heat the heating roller  2 , or material having deep penetration depth of the magnetic flux generated from the exciting coil used for the induction heating. Therefore, most magnetic flux from the excited coil passes through the first conductive member  711 , and the first conductive member  711  is not heated. 
   The second conductive member  713  is composed of material (e.g. materials including copper, aluminum, silver or the like) with a very high thermal conductivity and not heated by the magnetic field supplied from the exciting coil  562   a.    
   The first conductive member  711 , heat pipe  712  and second conductive member  713  can be made in one body. 
   When a predetermined electric power is supplied to the exciting coils  505   a ,  561   a  and  562   a  and the heating roller  2  is heated, the first conductive member  711  is heated to the temperature almost equal to the surface temperature of the heating roller  2  by the radiation heat from the heating roller  2 . The second conductive member  713  on which the abnormal temperature detector  714  is placed at a predetermined placeable position, is held at the temperature of the first conductive member  711  by the thermal conduction using the heat pipe  712 . Thus, the abnormal temperature detector  714  supplied with the radiation heat from the second conductive member  713  can detect the temperature of the first conductive member  711 , that is, the temperature almost equal to the outer circumference of the heating roller  2  even at a position separated from the heating roller  2 . 
   Therefore, the abnormal temperature detector  714  is not necessarily placed near the heating roller  2 , and the mounting positions of the abnormal temperature detector  714  and exciting coils  505   a ,  561   a  and  562   a  are not limited. 
   When the heating roller  2  is heated to an abnormal value, the conductive member  711  placed between the heating roller  2  and exciting coil  562   a  can detect the temperature of the heating roller  2  at a faster response speed. This temperature is conducted through the heat pipe  712  and second conductive member  713 , and detected by the abnormal temperature detector  714 . The abnormal temperature detector  714  detects the abnormal temperature, and cuts off the power supplied to the exciting coils  505   a ,  561   a  and  562   a.    
   Therefore, even if the heating roller  2  is stopped, the abnormal temperature can detect the temperature almost equal to the temperature of the first conductive member  711  located close to the heating roller  2  at a fast response speed, and the heating roller is prevented from being locally heated. 
   By contacting the first conductive member  711  with the heating roller  2 , the first conductive member  711  can detect the surface temperature of the heating roller  2  at a faster response speed. 
   The mounting position of the first conductive member  711  is not limited to the place described above. It may be placed in the area R 1  where a paper sheet PS passes. In this case, it is preferable not to bring the first conductive member  711  into contact with the outer circumference of the heating roller  2 . 
   When the response speed of the abnormal temperature detector  714  is delayed caused by a delay in the thermal conduction, set a temperature lower than the abnormal temperature of the heating roller  2  as an abnormal temperature at which the abnormal temperature detector  714  cuts off the power supplied to the exciting coils  505   a ,  561   a  and  562   a  when the heating roller  2  reaches the abnormal temperature. 
   Next, explanation will be given on another example of the fixing apparatus shown in  FIG. 1 . 
   The fixing apparatus shown in  FIG. 13A  has coil bodies  805  and  806  different from those of the fixing apparatus shown in  FIG. 1 .  FIG. 13B  is a schematic perspective view seen from the arrow P direction of  FIG. 13A .  FIG. 13C  is a schematic perspective view seen from the arrow Q direction of  FIG. 13A . 
   As shown in  FIGS. 13B and 13C , the coil body  805  is placed at the position opposite to the central area of the heating roller  2 , and the coil body  806  is placed at the position opposite to both end areas of the heating roller  2 . The coil body  806  includes a coil body  861  located at one end of the heating roller  2 , and a coil body  862  located at the other end of the heating roller  2 . The coil bodies  861  and  862  are connected in series, and formed electrically as one coil. 
   The coil body  805  is placed outside the heating roller  2  at the angle and phase different from those of the adjacent coil bodies  861  and  862 . The coil bodies  805 ,  861  and  862  placed with different angles and phases show the state that the angel θ 2  formed by virtual lines  5 L and  6 L which connect the axis of the heating roller  2  to the centers of the coil bodies  805 ,  861  and  862  is larger than 0° in the state viewed from the axial direction of the heating roller  2 , as shown in  FIG. 13A .  FIG. 13A  shows the state that the angle θ 2  is 90°. The angle θ 2  may be a range where the coil bodies  805 ,  861  and  862  do not contact each other. 
   The coil body  805  has an exciting coil  805   a , and a magnetic core  805   b  for holding the exciting coil  805   a . The coil bodies  861  and  862  have exciting coils  861   a  and  862   a , and magnetic cores  861   b  and  862   b  for holding the exciting coils  861   a  and  862   a , respectively. The number of turns of the wires of the exciting coils  805   a ,  861   a  and  862   a  can be reduced by providing magnetic cores  805   b ,  861   b  and  862   b . The coil bodies  805 ,  861  and  862  whose wires are wound around an imaginary axis and shaped to be a predetermined form (e.g. doughnut-shaped) as sown in  FIG. 13B  can heat locally a predetermined area of the heating roller  2  by the magnetic flux generated intensively. 
   The exciting coils  805   a ,  861   a  and  862   a  have a size such that the adjacent coils are overlapped in the joints W 81  and W 82  of exciting coils, when they are arranged linearly outside the heating roller  2 . The exciting coils  861   a  and  862   a  are arranged so that the centers of the coils are aligned outside the heating roller (at the same angle and phase), and the exciting coil  805   a  is arranged at a different angle and phase so that the end part  805 CE at both ends is not overlapped with the end parts  861 CE and  862 CE of the exciting coils  861   a  and  862   a.    
   Therefore, the heating roller  2  has the joint W 81  of the exciting coils where a predetermined magnetic field is supplied from both exciting coils  805   a  and  861   a  and the joint W 82  of the exciting coils where a predetermined magnetic field is supplied from both exciting coils  805   a  and  862   a , in the area divided in the direction orthogonal to the axial direction, when an electric power is supplied to the exciting coils  805   a ,  861   a  and  862   a.    
   The size of the coil bodies  805 ,  861  and  862  is set to a predetermined value by evaluating the temperature based on the result of using the fixing apparatus (when passing a paper sheet PS). In this example, when the coils are aligned outside the heating roller  2 , the lengths L 81  and L 82  where the adjacent coils are overlapped are 10 mm, respectively. 
   Therefore, even if the conventional doughnut-shaped coil is used as an excited mechanism, a temperature drop in the joints W 81  and W 82  of the coils can be prevented. 
   As seen from  FIGS. 13B and 13C , in this embodiment, the coils  805   a ,  861   a  and  862   a  include the center parts  805 CC,  861 CC and  862 CC composed of the electric wire wound flat along the outer circumference of the heating roller  2 , and the end parts  805 CE,  861 CE and  862 CE composed of the electric wire bent outward with an equally distributed curvature. This improves a drawback that while the center part of the coil can supply the heating roller  2  with a magnetic field of a predetermined direction, the direction of the magnetic field supplied is not constant at the end part of the coil, and the magnetic flux density is uneven and the surface temperature of the heating roller  2  is uneven. 
   The embodiment explained above explains a fixing apparatus of the type that the area R 11  to pass the center of paper in the area R of the heating roller  2  to which can a paper sheet is set at the center of the heating roller, and marginal areas R 12  and R 13  are set on both sides of the area R 11 , as shown in  FIG. 14A . The area R 11  is determined according to the shorter side length of A4 paper and small size paper such as a postcard, and is the area where a paper sheet passes frequently in the heating roller  2 . The marginal areas R 12  and R 13  are the areas to pass large size paper such as A4 and A3, where a paper sheet passes less frequent than the area R 11 . 
   The present invention is not limited to the above-mentioned type. The invention is also applicable to a fixing apparatus of the type that the area R 21  which can pass the center of paper in the area R of the heating roller  2  to pass a paper sheet is set aligned with one end of the heating roller  2  in the length direction, and the marginal area R 22  is set adjacent to the area R 21 , as shown in  FIG. 14B . 
   Next, explanation will be given on an example of a fixing apparatus of the type shown in  FIG. 14B . 
   As shown in  FIG. 15A , a fixing apparatus having the areas R 21  and R 22  has a heating roller  2  and a pressing roller  3 , like the fixing apparatus shown in  FIG. 1 . The fixing apparatus further includes a coil body  905  arranged opposite to the area R 21  (the area where a paper sheet P passes frequently) outside the heating roller, and a coil body  906  arranged opposite to the area R 22  outside the heating roller  2 . 
   The coil bodies  905  and  906  show the state that the angle θ 2  formed by virtual lines  5 L and  6 L which connect the axis of the heating roller  2  to the centers of the coil bodies  905  and  906  is larger than 0° in the state viewed from the axial direction of the heating roller  2 , like the coil bodies  805  and  806  explained in  FIG. 13A .  FIG. 15A  shows the relationship between the coil bodies  905  and  906  with the angle θ 2  of 90° and the heating roller  2 .  FIG. 15B  is a schematic perspective view seen from the arrow P direction of  FIG. 15A .  FIG. 15C  is a schematic perspective view seen from the arrow Q direction of  FIG. 15A . 
   The coil body  905  has an exciting coil  905   a , and a magnetic core  905   b  for holding the exciting coil  905   a . The coil body  906  has an exciting coil  906   a , and a magnetic core  906   b  for holding the exciting coil  906   a.    
   The exciting coils  905   a  and  906   a  have the largeness that the adjacent coils are overlapped in the joint W 91  of the exciting coils, when they are arranged linearly outside the heating roller  2 . The exciting coil  905   a  is arranged at a different angle and phase so that the end part  905 CE is not overlapped with the end part  906 CE of the exciting coil  906   a.    
   Therefore, the heating roller  2  has the joint W 91  of the exciting coils where a predetermined magnetic field is supplied from both exciting coils  905   a  and  906   a , in the area divided in the direction orthogonal to the axial direction, when electric power is supplied to the exciting coils  905   a  and  906   a.    
   The size of the coil bodies  905  and  906  is set to a predetermined value by evaluating the temperature based on the result of using the fixing apparatus (when passing a paper sheet PS). In this example, when the coils are aligned outside the heating roller  2 , the length L 91  where the adjacent coils are overlapped is 0 mm. 
   Next, explanation will be given on the configuration of an electric circuit applicable to the fixing apparatus, and a method of operating the fixing apparatus, by referring to  FIG. 16 . This electric circuit has a coil current control circuit  300 , and has the same configuration as the circuit shown in  FIG. 2 , except the coil current control circuit  200 , and detailed explanation is omitted. 
   The coil current control circuit  300  has an exciting coil  905   a  which supplies a magnetic field to the area R 21  of the heating roller  2 , and an exciting coil  906   a  which supplies a magnetic field to the area R 22  of the heating roller  2 . Namely, one end of the exciting coil  905   a  is aligned with one end of the heating roller  2  in the length direction, so that the coil faces to the area R 21  to pass the center of paper in the area R of the heating roller  2  which can pass a paper sheet, and the exciting coil  906   a  is placed adjacent to the exciting coil  905   a.    
   The exciting coil  905   a  is connected in parallel with the resonance condenser  21 , and is connected in series with the switching element  23 . The exciting coil  906   a  is connected in parallel with the resonance condenser  22 , and is connected in series with the switching element  24 . 
   The same method as the method of operating the fixing apparatus explained by using  FIGS. 2 and 3  is applicable to this fixing apparatus. 
   Namely, a method of supplying electric power alternately to the exciting coils  905   a  and  906   a  at a predetermined ratio (a time ratio), and a method of supplying predetermined electric power simultaneously to the exciting coils  905   a  and  906   a , as shown in  FIG. 3 , can be applied. 
   Therefore, as explained above, by using any method, it is possible to make uniform the intensity of the magnetic field supplied from the exciting coils  905   a / 906   a  and the temperature distribution in the length direction of the heating roller  2 . 
   The above-mentioned embodiment explains a fixing apparatus of the type that the foamed rubber  2   b  is provided inside the heating roller  2 , as shown in  FIG. 1 . The present invention is not limited to this type. The fixing apparatuses shown in  FIGS. 17A–17C  are also permitted. 
   As shown in  FIG. 17A , this fixing apparatus has a heating member  1002 , a coil body  1005  provided inside the heating member  1002 , and a coil body  1006  provided outside the heating member  1002 . The other components of this fixing apparatus are the same as those of the fixing apparatus shown in  FIG. 1 , and detailed explanation is omitted. 
   The heating member (heating roller)  1002  is an endless belt which is made of conductive material such as nickel, stainless steel, copper, aluminum, stainless steel and aluminum alloy, and iron, and shaped cylindrical with a predetermined circumference, and has a predetermined hardness, and is kept in a predetermined form by an external force. 
   Like the coil bodies  805  and  806  explained in  FIG. 13   a , the coil bodies  1005  and  1006  show the state that the angle θ 2  formed by virtual lines  5 L and  6 L which connect the axis of the heating roller  2  to the centers of the coil bodies  1005  and  1006  is larger than 90° in the state viewed from the axial direction of the heating roller  2 . 
   The coil body  1006  includes coil bodies  1061  and  1062  connected in series, and is formed electrically as one coil. The angle θ 2  is not limited to this value, and may be 0°. 
     FIG. 17B  is a schematic perspective view seen from the arrow P direction of  FIG. 17A .  FIG. 17C  is a schematic perspective view seen from the arrow Q of  FIG. 17A . 
   The coil body  1005  has an exciting coil  1005   a , and a magnetic core  1005   b  holding the excitation coil  1005   a . The coil bodies  1061  and  1062  have exciting coils  1061   a  and  1062   a , and magnetic cores  1061   b  and  1062   b  holding the exciting coils  1061   a  and  1062   a.    
   The exciting coils  1005   a ,  1061   a  and  1062   a  have a size such that the adjacent coils are overlapped, in the joints W 101  and W 102  of the exciting coils, when they are aligned outside the heating roller  2 . 
   The exciting coils  1061   a  and  1062   a  are arranged outside the heating roller  2 , so that the centers of the coils are aligned (at the same angle and phase). The exciting coil  1005  is arranged inside the heating roller  2 , so as to overlap with the exciting coils  1061   a  and  1062   a  in the joints W 101  and W 102 . 
   Therefore, when electric power is supplied to the exciting coils  1005   a ,  1061   a  and  1062   a , the heating roller  2  has a joint W 101  of the exciting coils where a predetermined magnetic field is supplied from both exciting coils  1005   a  and  1061   a , and a joint W 102  of the exciting coils where a predetermined magnetic field is supplied from both exciting coils  1005   a  and  1062   a.    
   The size of the coil bodies  1005   a ,  1061   a  and  1062   a  is set to a predetermined value by evaluating the temperature based on the result of using the fixing apparatus (when passing a paper sheet P). In this example, when the coils are aligned outside the heating roller  2 , the lengths L 101  and L 102  where the adjacent coils are overlapped are 10 mm, respectively. 
   The fixing apparatus explained above, the exciting coils provided in the fixing apparatus and the method of controlling the fixing apparatus can be combined optionally. 
   Next, explanation will be given on a modification of the induction heating control circuit sown in  FIG. 2 . 
   As shown in  FIG. 18 , the core metal  2   a  of the heating roller  2  is provided with a rotation detection mechanism  33  which can detect rotation of the heating roller  2 . 
   The rotation detection mechanism  33  detects rotation of the heating roller  2  by detecting that a pulse plate (FG plate)  33   a  fixed to the core metal  2   a  (shaft) or the like of the heating roller  2  is rotated together with the heating roller  2 , by a photo-sensor  33   b  fixed to a predetermined position of the fixing apparatus  1 . The rotation detection mechanism  33  is not limited to this configuration. It is also permitted to detect rotation of the heating roller  2  by detecting a marking at the predetermined position on the outer circumference of the heating roller  2  by using an optical detection means or the like. As explained above, by using a rotation detection mechanism, the heating roller  2  is prevented from being heated to an abnormal temperature, and the safety of the fixing apparatus is improved. 
   The rotation detection mechanism  33  is connected to the input terminal of an AND circuit  34  whose input terminal is connected to the IGBT driving circuit  29 , and the input terminal of an AND circuit  35  whose output terminal is connected to the IGBT circuit  30 . The input terminals of the AND circuits  34  and  35  are connected to the CPU  28 . The IGBT circuits  29  and  30  are part of a coil current control system  400 . 
   Therefore, the AND circuit  34  outputs a signal (hereinafter, referred to as a driving signal) to drive the IGBT driving circuit  29 , when receiving a rotation detection signal from the rotation detection mechanism  33 , and an instruction signal (hereinafter, referred to as an excitation control signal) to drive the IGBT driving circuit  29  from the CPU  28 . Receiving the driving signal, the IGBT driving circuit  29  turns on the switching element  23 , and supplies a predetermined electric power to the exciting coil  5   a.    
   Similarly, the AND circuit  35  outputs a driving signal to drive the IGBT driving circuit  30 , when receiving a rotation detection signal from the rotation detection mechanism  33 , and an excitation control signal to drive the IGBT driving circuit  30  from the CPU  28 . Receiving the driving signal, the IGBT driving circuit  30  turns on the switching element  24 , and supplies a predetermined electric power to the exciting coils  61   a  and  62   a.    
   Namely, electric power is supplied to the exciting coils  5   a ,  61   a  and  62   a  while the heating roller  2  is rotating, and not supplied when the heating roller  2  is stopping. 
   Therefore, even if a trouble should occur in the CPU  28  or thermistors  9   a  and  9   b , the heating roller  2  is not heated by the exciting coils  5   a  and  6   a  as long as it is not rotated. This prevents the outer circumference of the heating roller  2  from being heated locally to an abnormal temperature, and the safety of the fixing apparatus  1  is remarkably increased over those currently in use. 
   To increase the safety furthermore, it is permitted to provide a temperature detection mechanism (a thermistor)  36  which detects the temperature of the pressing roller  3 , at a predetermined position in proximity to the outer circumference of the pressing roller  3 . When the rotation detection signal and excitation control signal are applied to one of the AND circuits  34  and  35 , the temperature of the pressing roller  3  is increased in a predetermined range by the thermal conduction from the rotating heating roller  2 . Namely, based on the temperature information of the pressing roller  3  from the thermistor  36 , it can be determined that the heating roller  2  is rotating when the temperature of the pressing roller  3  output from the thermistor  36  is increased to a predetermined range, and the heating roller is not rotating when the temperature of the heating roller  3  is not increased. 
   Because of the above reason, the CPU  28  is set to output the excitation control signal to one of the AND circuits  34  and  35  only when the temperature of the pressing roller  3  is increased to a predetermined range. Thus, even in the case of a malfunction that the rotation detection signal is applied to the AND circuit  31  or  32 , the excitation control signal is not outputted and electric power is not supplied to the exciting coils  5   a ,  61   a  and  62   a , though the heating roller  2  is not rotating. 
   Thus, even if a trouble occurs in the rotation detection mechanism  33  or thermistors  9   a  and  9   b , the heating roller  2  is not heated by the exciting coils  5   a  and  6   a  if it is not rotating. Therefore, the outer circumference of the heating roller  2  is prevented from being heated locally to an abnormal temperature. 
   Further, the rotation detection mechanism  33  may detect the rotation speed of the heating roller  2 . By feeding back the detection result, the CPU  28  can maintain the rotation speed of the heating roller  2  at a constant level. Therefore, an appropriate image is formed on a paper sheet passing between the heating roller  2  and the pressing roller  3 . 
   As explained in  FIG. 2 , the first inverter circuit includes the condenser  21  and switching element  23 , and supplies electric power to the exciting coil  5   a . The second inverter circuit includes the condenser  22  and switching element  24 , and supplies electric power to the exciting coils  61   a  and  62   a . The first and second inverter circuits are connected to the IGBT driving circuits  29  and  30 , respectively. The first and second inverter circuits form a self-excited oscillator which utilizes the resonance by the exciting coils and condensers, and supplies a high-frequency current efficiently to the exciting coils. 
   Next, the operation of the self-exited oscillator will be explained. 
     FIGS. 19A ,  19 B,  19 C and  19 D show an equivalent circuit EC of the first inverter circuit, and are circuit diagrams explaining the current flowing in the equivalent circuit EC.  FIGS. 20A and 20B  are reference drawings showing the relationship between the time and current flowing in the equivalent circuit EC of the first inverter circuit. 
   As shown in  FIG. 20A , a current having a predetermined frequency corresponding to the ON time (O-P time) of the switching element  23  turned on/off by the CPU  28  flows in the equivalent circuit EC of the first inverter circuit. One period of this frequency is time O-S. 
   As shown in  FIG. 19A , after the time O-P, a current from a power supply PW flows in the turned-on switching element  23  and exciting coil  5   a , as indicated by the arrow A. When the switching element  23  turns off, the current flowing in the switching element  23  flows in the resonance condenser  21  as indicated by the arrow B in  FIG. 19B , and the resonance condenser  21  is charged in the time P-Q. 
   The charged resonance condenser  21  starts discharging, as shown in  FIG. 19C . A reverse current as indicated by the arrow C flows in the discharged resonance condenser  21 , and after the time Q-R, the voltage becomes zero. But, this reverse current cannot stop at once, and flows into the diode  23   a  of the switching element  23 , and flows for the time R-S as indicated by the arrow D in  FIG. 19D . 
   When the switching element  23  turns on again, a predetermined current flows in the exciting coil  5   a . By repeating this period O-S, the heating roller  2  is supplied with a predetermined magnetic field, and heated. The value X 1  of the current flowing in the exciting coil  5   a  in the time P is a peak current value. This peak current value X 1  can be calculated by feeding back the input power PI monitored by the input power monitor  27  explained before to the CPU  28 . This input power PI is determined based on the thermal output (W) of the heating roller  2  heated by the magnetic field supplied from the exciting coil  5   a.    
   The thermal output of the heating roller  2  is the heat energy generated when the heating roller  2  flowing an eddy current is heated by the magnetic field generated corresponding to the predetermined current value flowing in the exciting coil  5   a , and is defined by the energy obtained by subtracting a predetermined energy consumed by the induction heating from the input power PI monitored by the input power monitor  27 , for example. 
   Therefore, the temperature of the heating roller  2  based on the energy generated when the heating roller  2  is heated can be detected by monitoring the input power PI and calculating the peak current value X 1  of the exciting coil  5   a.    
   The exciting coil  5   a  and heating roller  2  have a predetermined magnetic characteristic (magnetic coupling), and the peak current value X 1 , frequency and voltage of the current supplied to the exciting coil  5   a  are determined by this magnetic characteristic. This magnetic characteristic is initially determined by the permeability and resistivity of the heating roller  2 , the number of turns (windings) of the exciting coil  5   a  and the position of the magnetic core  5   b.    
   However, when the heating roller  2  is heated to an abnormal temperature, the exciting coil  5   a  is heated to a predetermined temperature by the radiant heat from the heating roller  2 , and the magnetic characteristic of the heated exciting coil  5   a  is changed to the characteristic different from that before heated. Namely, this magnetic characteristic has temperature dependability. 
     FIG. 20B  shows the relationship between the time and the current flowing in the exciting coil  5   a  having the changed magnetic characteristic.  FIGS. 20A and 20B  show the relationship between the time and the current flowing in the exciting coil  5   a  when the thermal output of the heating roller  2  is set to 900 W. 
   As shown in  FIG. 20B  a current with a peak current value X 2  larger than the peak current value X 1  flows in the exciting coil  5   a  whose magnetic characteristic has been changed as a result of an abnormal temperature rise in that the heating roller  2 . The frequency of this current changes also to 1 period O′-S′ longer than the period O-S, that is, the frequency is decreased. 
   Therefore, the CPU  28  judges that the heating roller  2  is increased to an abnormal temperature based on the value set according to the frequency value of the predetermined current supplied to the exciting coil  5   a , when the input power PI fed back from the input power monitor  27 , or the peak current value is not maintained in a predetermined range. This set value (threshold value) is the frequency and peak current value having a predetermined range according to the magnetic characteristics of the exciting coil  5   a  and heating roller  2 , and is stored in the memory of the CPU  28 . For example, when the thermal output of the heating roller  2  is 700 W, the peak value of the current flowing in the exciting coil  5   a  is 55 A, and the frequency is 26 kHz. When the thermal output is 900 W, the peak current value is 60 A, and the frequency is 23 kHz. When the thermal output is 1200 W, the peak current value is 65 A, and the frequency is 21 kHz. 
   For example, when the set thermal output of the heating roller  2  is 900 W and the temperature of the heating roller  2  is about 150°, as shown in  FIG. 20A , the peak value X 1  of the current flowing in the exciting coil  5   a  is 50 A. But, when the heating roller  2  is heated to an abnormal temperature (e.g. 300°) and the magnetic characteristic is changed, a current with a peak value X 2  of 60 A flows in the exciting coil  5   a . By calculating the change in the peak current value from the input power PI fed back from the input power monitor  27 , the CPU  28  detects that the frequency shown in  FIG. 20B  is decreased, that is, the thermal output supplied to the heating roller  2  is increased. 
   When detecting the increased thermal output supplied to the heating roller  2 , the CPU  28  stops the power supplied to the exciting coil  5   a.    
   As shown in  FIGS. 20A and 20B , the frequency of the current supplied to the exciting coil  5   a  can be calculated by detecting the ON time of the switching element  23 . This frequency is defined by the value of the ON time of the switching element  23  plus the resonance time of the resonance condenser  21 . 
   The CPU  28  is connected to a timer  28   b  for detecting the ON time of the switching element  23  ( FIG. 2 ). The memory  28   a  connected to the CPU  28  stores the threshold value to set the ON time of the switching element  23  and resonance time of the resonance condenser  21  according to the magnetic characteristics of the exciting coil  5   a  and magnetic core  5   b.    
   Therefore, the CPU  28  compares the detected ON time of the switching element  23  with the preset threshold value, and judges that the frequency is decreased when the ON time is longer than the threshold value. When this frequency is lowered below the set value explained above, for example, the CPU  28  cuts off the power supplied to the exciting coil  5   a  to increase the temperature of the heating roller  2  to an abnormal value. 
   Therefore, even if the heating roller  2  is stopped, electric power to increase the temperature of the heating roller  2  to an abnormal value is not supplied to the exciting coil  5   a , and the heating roller  2  is prevented from being heated locally. 
   Further, it is possible to detect an error in the heating roller  2  without using an abnormal temperature detection mechanism, and the safety of the fixing apparatus  1  is remarkably increased over those currently in use. 
   Of course, a similar method of detecting an abnormal temperature can be applied to the exciting coils  61   a  and  62   a.    
   This embodiment can also use the method of detecting an abnormal temperature which utilizes the changes in the magnetic characteristics of the magnetic cores  5   b ,  61   b  and  62   b  holding the exciting coils  5   a ,  61   a  and  62   a , respectively, when heated by the radiant heat of the heating roller  2 . 
   The magnetic cores  5   b ,  61   b  and  62   b  are composed of materials whose magnetic characteristic is saturated and changed after passing a predetermined Curie point when the heating roller  2  is heated to an abnormal temperature. 
   This Curie point is preferably a temperature value a little higher than the normal temperature ranges of the magnetic cores  5   b ,  61   b  and  62   b , by evaluating the temperatures of the magnetic cores  5   b ,  61   b  and  62   b  when exceeding the normal temperature range, so that the Curie point is not exceeded while the heating roller  2  is heated within a preset normal temperature range. 
   When the temperature of the heating roller  2  exceeds the preset normal temperature range, the temperatures of magnetic cores  5   b ,  61   b  and  62   b  exceed the Curie point, cause magnetic saturation, and then the magnetic characteristics are changed from the magnetic characteristics of the exciting coils  5   a ,  61   a  and  62   a . Thus, the current value (peak value) flowing in the exciting coils  5   a ,  61   a  and  62   a  is changed. This change in the current value is detected by the CPU  28  by comparing the input power PI fed back from the input power monitor  27  with the set value as explained above. 
   When detecting the changes in the magnetic characteristics of the exciting coil  5   a  and magnetic core  5   b  caused by the changes in the current value, the CPU  28  stops the power supplied to the exciting coil  5   a.    
   Therefore, even if the heating roller  2  is stopped, the exciting coil  5   a  is not supplied with electric power to increase the temperature of the heating roller to an abnormal value, and the heating roller  2  is prevented from being heated locally. 
   An error in the heating roller  2  can be detected without using an abnormal temperature detection mechanism, and the safety of the fixing apparatus  1  is remarkably increased over those currently in use. 
   The fixing apparatus explained above, the exciting coil provided in the fixing apparatus and the method of controlling the fixing apparatus can be combined optionally. 
   As explained above, the fixing apparatus of the present invention makes the temperature distribution uniform in the length direction of the heating roller, and provides a good image, by preventing a temperature drop at the joints of the heating rollers. 
   As explained above, the fixing apparatus of the present invention makes the temperature distribution uniform in the length direction of the heating roller, by preventing a temperature drop at the joints of the coils of the heating rollers, thereby providing a good image. 
   The fixing apparatus of the present invention makes the temperature distribution uniform in the length direction of the heating roller, by preventing a temperature drop at the joints of the coils of the heating roller, providing a good image. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.