Induction heat fusing device

An induction heating device for printers, copiers and the like includes a magnetic coil assembly and a heated metal body that form a closed magnetic circuit. To compensate for the effects of heat radiation at the ends of the heated metal body, a greater amount of heat is generated at the ends of the body than at the center thereof. In one approach, the amount of heat is varied by changing physical parameters of the magnetic circuit, such as the spacing of the coil assemblies from the heated body, the relative sizes of the cores, and/or the relative magnetic permeability of the cores. In another approach, the electrical connection of the coils of multiple assemblies are arranged such that the coils in the center of the heated body are in parallel, and the coils at the ends of the body are in series with the parallel central coils.

FIELD OF THE INVENTION 
The present invention relates to a fusing device of the type used in 
electrophotographic copying machines, printers, facsimile machines and so 
forth, and more particularly to a fusing device that utilizes induction 
heating to fuse a toner image to a recording medium. 
BACKGROUND OF THE INVENTION 
Electrophotographic copying machines and the like include a fusing device 
that fuses a toner image transferred onto a recording medium, such as a 
sheet of a recording paper or a transfer material. This fusing device 
typically comprises a fusing roller that thermally fuses toner on a sheet 
and a pressing roller that presses against the fusing roller to pinch and 
hold the sheet. The fusing roller is formed in a cylindrical shape, and a 
heat generating body is retained on the center core of this fusing roller 
by a retaining means. The heat generating body may be a halogen lamp, for 
example, and generates heat by means of a fixed voltage applied thereto. 
Because this heat generating body is positioned at the center core of the 
fusing roller, heat generated from the heat generating body is uniformly 
radiated onto the inner wall of the fusing roller, creating a uniform 
temperature distribution in the circumferential direction on the outer 
wall of the fusing roller. The temperature of the outer wall of the fusing 
roller is heated to a temperature suitable for fusing, e.g., 150.degree. 
to 200.degree. C. In this state, the fusing roller and pressing roller 
rotate in directions opposite to each other while making contact, to hold 
the sheet which has toner adhered to it. The toner on the sheet dissolves 
at the contact portion (hereinafter referred to as the nip portion) 
between the fusing roller and pressing roller by means of the heat of the 
fusing roller, and is fused to the sheet by the pressure exerted from both 
rollers. After the toner adheres, the sheet is fed by a paper delivery 
roller following the rotation of the fusing roller and pressing roller and 
is then fed out to a paper delivery tray. 
In a fusing device provided with a heat generating body comprised of a 
halogen lamp, for example, a comparatively long amount of time is required 
after the power supply is turned ON before the temperature of the fusing 
roller reaches a temperature suitable for fusing. Problems exist such as 
operators not being able to use the copying machine and being forced to 
wait a long time during that warm-up period. In contrast to those 
problems, when the heating capacity of the fusing roller is increased for 
the purpose of reducing the waiting time to improve the operability for 
the users, problems such as increases in the power consumption of the 
fusing device arise, which work against reductions in energy consumption. 
Therefore, in order to increase the value of products such as copying 
machines, the objective of concurrently reducing energy consumption (lower 
power consumption) of the fusing devices and improving the operability for 
users (quick prints) have attracted more and more attention as an 
important topic. According to this trend, there is a growing demand for 
reducing not only the toner fusing temperature and the thermal capacity of 
the fusing roller, but also improving the electricity-to-heat conversion 
efficiency. 
An induction heat fusing device has been proposed in Japanese Laid-open 
Patent Application Sho 59-33788 as a device to satisfy these requirements. 
As shown in FIGS. 18A and 18B, this induction heat fusing device has a 
spirally wound coil 2 concentrically arranged inside a fusing roller 1 
comprised of a metal conductor. A high-frequency current flows through the 
coil 2 in proximity to an inner surface of the fusing roller 1. A 
high-frequency magnetic field resulting from this current flow causes an 
induction eddy current in the fusing roller 1, with the skin resistance of 
the fusing roller causing joule heat generation to occur in the fusing 
roller 1. 
This induction heating method has various advantages compared to other 
heating methods. The first advantage is quicker temperature increases and 
less heat generation and heat transfer to portions of the device other 
than the fusing roller, compared to indirect heating by means of near 
infrared heat generation of a halogen lamp. Further, there is no loss 
corresponding to light leakage of the halogen lamp. The second advantage 
is better heat generation efficiency due to the characteristic skin effect 
of electromagnetic induction. In addition, the fusing device has greater 
reliability over an extended period of time, in comparison to surface 
heating devices having a solid resistance heat generating body on the 
surface of the fusing roller, which require sliding contacts that are 
subject to wear due to friction. 
Recently, low fusing temperature toners, which melt at a temperature in the 
range of 110.degree.-130.degree. C., have become available, which provide 
for lower power consumption and quicker machine warm-up. In addition, the 
cost of inverter circuit switching devices in residential high-frequency 
power supplies has been reduced, making it possible to realize induction 
heat fusing devices having the foregoing desirable characteristics. 
In order to achieve uniform fusing performance in the direction of the axis 
of rotation (lengthwise direction) of the fusing roller in an induction 
heat fusing device, it is necessary to make the temperature distribution 
in the direction of the axis of rotation of the fusing roller almost 
uniform. However, compared to the center portion, the temperature at both 
ends of the fusing roller is lower because of the influence of heat 
radiation. Thus, it is generally necessary to make the quantity of heat 
generated at both ends of the fusing roller higher than the center 
portion. 
Reduced temperature by means of heat radiation at both ends of the fusing 
roller is addressed in the induction heat fusing device disclosed in 
Japanese Laid-open Patent Application Sho 59-33788 mentioned above. As 
shown in FIGS. 19A to 19C, the coil 2 at both ends of the roller is wound 
denser than the center portion, so that the quantity of heat generated at 
both ends of the roller is higher than the center portion and the 
temperature distribution in the direction of the axis of rotation of the 
fusing roller 1 is almost uniform. 
However, because the winding density of the coil 2 changes midway in the 
lengthwise direction in this arrangement, there is a problem in the 
ability to mass-produce the coils 2, making it difficult to reduce the 
cost of the coils. 
As further shown in FIGS. 18A and 18B, the winding direction of the coil 2 
is identical to the peripheral direction of the fusing roller 1 and, since 
the generated magnetic flux and fusing roller 1 are in parallel, there is 
a problem of leakage of the magnetic flux from both ends, thereby 
decreasing the heat generating efficiency. 
SUMMARY OF THE INVENTION 
The present invention is directed to a device that solves the foregoing 
problems which accompany the conventional fuser technology. The object of 
this invention is to provide an induction heat fusing device with a 
reduced amount of magnetic flux leakage and good heat generation 
efficiency that adjusts the distribution of the quantity of heat generated 
without causing the winding density of the coil to change, thereby 
allowing the temperature distribution of a fusing roller or metal plate to 
be almost uniform in the lengthwise direction, in addition to providing a 
better ability to mass-produce coils. 
In order to achieve these objects, the induction heat fusing device of the 
present invention comprises an induction heat fusing device having a 
hollow metal roller or a metal heating plate that is a heated metal body, 
a core arranged at a right angle to the heated metal body and a coil wound 
on the core so the winding generates a magnetic flux in a direction at a 
right angle to the heated metal body. The induction heat fusing device 
further changes the distribution of the quantity of heat generated in the 
lengthwise direction of the heated metal body by varying one or more 
parameters of the magnetic circuit which generates the magnetic field 
across the heated metal body, to thereby vary the amount of heat that is 
generated. 
Because the induction heat fusing device of the present invention causes a 
magnetic flux to be generated in a direction at a right angle to the 
heated metal body, leakage of the magnetic flux at both ends of the heated 
metal body in the lengthwise direction is small, resulting in a higher 
heat generation efficiency. 
In one embodiment of the invention, the distance between the core of the 
magnetic circuit and the heated metal body is varied. If the distance 
between the core and the heated metal body is reduced, the magnetic 
coupling becomes stronger, thereby increasing the quantity of heat 
generated. If the distance is increased, the magnetic coupling becomes 
weaker, thereby decreasing the quantity of heat generated. Therefore, even 
if the winding density of the coil is not changed, the distribution of the 
quantity of heat generated in the lengthwise direction of the heated metal 
body can be adjusted as desired by changing the distance between the 
heated metal body and the core, resulting in an enhanced ability to 
mass-produce coils. 
Because both ends of the heated metal body in the lengthwise direction are 
easily influenced by heat radiation, with the temperature becoming lower 
compared to the center portion, the distance between the heated metal body 
and the core at the edge portion in the lengthwise direction of the heated 
metal body can be made smaller than the distance between the heated metal 
body and the core at the center portion. When structured in this way, the 
temperature distribution in the lengthwise direction of the heated metal 
body can be made almost uniform, considering the effects of heat 
radiation, thereby making it possible to achieve uniform fusing 
characteristics in the lengthwise direction of the heated metal body. 
In another embodiment, the induction heat fusing device of the present 
invention comprises a hollow metal roller or a metal heating plate that is 
a heated metal body, a plurality of cores arranged at right angles to the 
heated metal body and a coil wound on each core to generate a magnetic 
flux in a direction at a right angle to the heated metal body. The 
induction heat fusing device further changes the distribution of the 
quantity of heat generated in the lengthwise direction of the heated metal 
body by arranging cores with different magnetic permeabilities in the 
lengthwise direction of the heated metal body. Because the induction heat 
fusing device of the present invention causes a magnetic flux to be 
generated in a direction at a right angle to the heated metal body, 
leakage of the magnetic flux at the end of the heated metal body in the 
lengthwise direction is small, resulting in a higher heat generation 
efficiency. 
Furthermore, because the generated magnetic flux increases as the magnetic 
permeability of the cores increases, if the magnetic permeability of the 
cores is made larger, the magnetic flux intertwining with the heated metal 
body increases and the quantity of heat generated grows larger. If the 
magnetic permeability of the cores is made smaller, the magnetic flux 
intertwining with the heated metal body decreases and the quantity of heat 
generated grows smaller. Therefore, even if the winding density of the 
coil is not changed, the distribution of the quantity of heat generated in 
the lengthwise direction of the heated metal body can be adjusted as 
desired by changing the magnetic permeability of each core arranged in the 
lengthwise direction of the heated metal body, resulting in an excellent 
ability to mass-produce coils. 
Because the end of the heated metal body in the lengthwise direction is 
easily influenced by heat radiation with the temperature being lower than 
the center portion, the magnetic permeability of the cores at the edge 
portion in the lengthwise direction of the heated metal body can be made 
larger than the magnetic permeability of the cores at the center portion. 
When arranged in this way, the temperature distribution in the lengthwise 
direction of the heated metal body can be made almost uniform, considering 
the effects of heat radiation, thereby making it possible to achieve 
uniform fusing characteristics in the lengthwise direction of the heated 
metal body. 
In accordance with another embodiment of the invention, the sizes of the 
cores which are used to generate the magnetic field are varied. As the 
size of the core increases, the magnetic field strength increases, even 
though the number of windings remains the same. Therefore, by utilizing 
larger cores at the ends of the heated metal body, relative to the cores 
at the center of the body, a larger amount of heat can be generated at the 
ends of the body while maintaining a uniform number of windings per core. 
Thus, the dual objectives of enhanced ability to mass-produce coils and 
uniform fusing temperature are achieved. 
In accordance with another embodiment of the present invention, a plurality 
of coils for inductively heating the fusing roller are connected to each 
other through a combination of a parallel connection and a series 
connection of the coils. With this arrangement, the amount of current 
which flows through each coil of the plurality of coils differs between 
the portion of the parallel connection and the portion of the series 
connection, and consequently the induction current generated in the fusing 
roller varies. By means of this arrangement, the heat distribution of the 
fusing roller is made uniform by appropriately combining the parallel 
connection with the series connection of the coils.

DETAILED DESCRIPTION 
FIG. 1 is a cross-section of an induction heat fusing device that can 
employ the present invention, and FIG. 2 is a perspective view of the 
fusing roller and the pressing roller shown in FIG. 1. 
As shown in FIG. 1, an induction heat fusing device incorporated in devices 
such as printers has a heat roller, or more precisely, a fusing roller 10 
provided such that it can rotate in the direction of arrow a, and a 
pressing roller 11 that presses against the fusing roller 10 and is driven 
to rotate following the rotation of the fusing roller 10. The fusing 
roller 10 is a hollow pipe of a conductive material, and inside this pipe 
a plurality of coil assemblies 12 are arranged which cause an induction 
current (eddy current) to be generated in the fusing roller 10. Each coil 
assembly 12 is retained in a holder 24 and comprises a holder unit 13. The 
fusing roller 10 itself is what generates heat by means of the induction 
current and this fusing roller 10 comprises the heated metal body of the 
fusing device. 
The fusing roller 10 has a sliding bearing portion formed on both ends and 
is mounted on a fusing unit frame (not shown in the figure) to freely 
rotate. The fusing roller 10 has a drive gear (not shown in the figure) 
fixed to one end and is driven to rotate by a drive source (not shown in 
the figure) such as a motor which is connected to this drive gear. The 
holder unit 13 maintains a gap at a fixed dimension with the inner 
peripheral surface of the fusing roller 10 and is housed inside the fusing 
roller 10. This holder unit 13 is fixed to the fusing unit frame and does 
not rotate. 
A toner support member onto which is transferred a toner image that has not 
yet been fused, such as a sheet 14, is fed from the left side as indicated 
by arrow b in FIG. 1 and sent to the nip portion between the fusing roller 
10 and the pressing roller 11. While the heat of the fusing roller and the 
pressure exerted from both rollers 10, 11 are being applied, the sheet 14 
is fed through the nip portion. The toner is fused by this action and a 
fused toner image is formed on the sheet 14. The sheet 14 that has passed 
through the nip portion naturally separates from the fusing roller 10 by 
means of the curvature of the fusing roller itself 10 or, as shown in FIG. 
1, is forcibly separated from the fusing roller 10 by means of either a 
separation claw 15 or separation guides provided such that the leading 
edge portion makes contact with the surface of the fusing roller 10 and 
then the sheet is fed in a direction to the right in FIG. 1. This sheet 14 
is fed by a paper delivery roller (not shown in the figure) and delivered 
to a paper delivery tray. 
A temperature sensor 16 that detects the temperature of the fusing roller 
10 is provided above the fusing roller 10. This temperature sensor 16 is 
located on the side of the fusing roller 10 opposite a coil 22, and 
presses against the surface of the fusing roller 10. The temperature 
sensor 16 comprises, for example, a thermistor. While the temperature of 
the fusing roller 10 is detected by this thermistor 16, the flow of 
electricity to the coil 22 is regulated to ensure an optimum temperature 
of the fusing roller. 
A thermostat 17 is further provided above the fusing roller 10 as a safety 
mechanism when the temperature rises abnormally. This thermostat 17 
presses against the surface of the fusing roller 10 and when the 
temperature reaches a previously set value, its contacts open to cut off 
the flow of electricity to the coil 22. This prevents the temperature of 
the fusing roller 10 from reaching more than a fixed value. 
The fusing roller 10 is formed from a conductive member such as iron, a 
stainless steel alloy tube, nickel, a carbon steel tube or an aluminum 
alloy tube. Preferably, the material which is used to form the fusing 
roller also has magnetic properties. The outer peripheral surface of this 
member is coated with a fluororesin and a heat resistant separation layer 
is formed on the surface. The pressing roller 11 comprises a silicon 
rubber layer 19, which forms a surface separation heat resistant rubber 
layer, on the periphery of a shaft core 18. The sliding bearing and 
separation claw 15 are formed from a heat resistant slidable engineered 
plastic. 
As shown in FIG. 3, a coil assembly 12 has a square-shaped bobbin 20 that 
forms a through-hole 20a in the center portion. A copper wire 21 is wound 
around this bobbin 20 several times in one direction to form a coil 22. A 
core 23 is inserted in the through-hole 20a of the bobbin 20 at a 
right-angle to the copper wire 21 of the coil 22. The bobbin 20 can be 
formed from, for example, a ceramic or heat resistant insulating 
engineered plastic and, for the coil 22, it is preferable to use a single 
or a Litz wire having a fusing layer and insulating layer on the surface. 
The core 23 is comprised, for example, of a ferrite core or a laminated 
layer core. 
FIG. 4 is a perspective view of the coils 22 and the cores 23 inside the 
fusing roller 10. In this embodiment, four coil assemblies 12 are arranged 
to generate a magnetic flux in a direction at a right angle to the 
direction of the axis of rotation, that is the lengthwise direction of the 
fusing roller 10. The assemblies are also arranged so the copper wire 21 
wound around the bobbin 20 lies along a plane parallel to the axis of 
rotation of the fusing roller 10 or, in other words, in a direction so the 
core 23 is at a right angle to the axis of rotation. 
Further, as shown in FIGS. 1 and 5, in this embodiment, the plurality of 
coil assemblies 12 are arranged in the holder 24 so that they are lined up 
in the direction of the shaft of the fusing roller 10, so their cores 23 
are parallel to the feed direction of the sheet 14 and the coil 22 of each 
assembly is opposite the pressing roller 11. This holder 24 is formed from 
a heat resistant insulating engineered plastic and, as shown in FIG. 5, is 
shaped with a plurality of open holes penetrating the periphery of the 
cylinder from the top to the bottom and from the left to the right. The 
holder is further provided with protruding portions 25 at both ends to 
secure it to the fusing unit frame. The coil assemblies 12 can be 
incorporated in the holder 24, for example, by inserting bobbins 20 into 
holes provided around the holder 24 in the right-left direction and 
thereafter inserting the cores 23 into holes provided in the top-bottom 
direction. The plurality of coils 22 are connected in series inside the 
holder 24 and a lead wire 26 (FIG. 5) is drawn through both ends of the 
holder 24 to allow electrical current to flow to these coils 22. The 
holder unit 13 has an external diameter slightly smaller than the inner 
diameter of the fusing roller 10, to form a gap with the inner wall of the 
fusing roller 10. 
FIG. 6 is an explanatory drawing to describe the heating principle of the 
fusing roller 10 in an induction heat fusing device in which this 
invention is applied. As shown in the figure, when a high-frequency 
electric current (from a few kHz to tens of kHz) flows through the coil 
22, a magnetic flux is generated from the core 23 at a right angle to the 
lengthwise shaft direction of the fusing roller 10, following Ampere's 
"right-hand rule." This magnetic flux is also a high-frequency magnetic 
flux. 
The magnetic flux that reaches the fusing roller 10 of the conductor curves 
along the fusing roller 10, becoming a magnetic flux that passes through 
the inside of the circular peripheral surface of the fusing roller 10 at a 
rate dependent on the relative magnetic permeability of the conductor. The 
density of the magnetic flux concentrated at the peripheral surface of the 
fusing roller 10 is largest at the portions opposite the coil 22. 
Following Lentz's Rule, the action of this concentrated magnetic flux 
generates a vortex-shaped induction current in the fusing roller 10 within 
the inner wall, which causes a counter magnetic flux to be generated with 
a direction opposite to this magnetic flux, which inhibits the original 
magnetic flux from the coil assembly. Because this induction current is 
converted to joule heat by means of the skin resistance of the fusing 
roller 10, the fusing roller 10 generates heat. 
In this composition, the magnetic flux density within the peripheral 
surface at points P, R of the fusing roller 10 is highest, and in contrast 
to this, is lower at points Q, S. Consequently, because the induction 
current density tends to change in a like manner as well, the heat 
generation of the fusing roller 10 may not be uniform around the 
peripheral surface, with heat being locally higher at the portions 
surrounded by the two dot-dash lines. If these portions where the heat is 
locally higher are shown in FIG. 1, they are equivalent to the top region 
and the bottom region of the fusing roller 10. Therefore, at least one 
portion of the nip portion and one side of the heat generation will be 
overlapping. Furthermore, the thermistor 16 makes contact with the other 
locally heated region and the thermostat 17 is also arranged to make 
contact with, or be in proximity to, the region. The mounting location of 
the thermistor 16 can be either above or below the fusing roller 10. In 
the embodiment shown in the figure, the thermistor is mounted on the 
outside above the roller. Further, if the thermistor 16 is a small, 
compact type, is can be mounted on the inside above or below the fusing 
roller 10. 
As shown in FIGS. 7A and 7B in this embodiment, the core 23 of each coil 
assembly 12 is arranged in a direction at a right angle to the fusing 
roller 10 and the coil 22 is arranged such that it is wound about an axis 
that is at a right angle to the axis of rotation of the fusing roller 10. 
Thereby, the direction of the generated magnetic flux is in a direction at 
a right angle to the fusing roller 10, resulting in the fusing roller 10 
and the core 23 creating a closed magnetic circuit. Because of this, there 
is either no leakage of magnetic flux from both ends of the fusing roller 
10, or it is small, thereby increasing the heat generating efficiency. 
As illustrated in FIG. 7B, the pressing roller 11 can be located in the 
dotted line position in which it is positioned opposite one of the ends of 
the core 23, for the reasons indicated previously. However, in situations 
where the heat is generally uniform around the circumference of the fusing 
roller 10, the pressing roller can alternatively be located at the solid 
line position, opposite the coil 22. 
FIG. 8 is a block diagram of the circuit through which the high-frequency 
current flows to the induction heating coil 22, to control the temperature 
of the fusing roller 10. Since it is necessary for a high-frequency 
electrical current to flow in an induction heat fusing device in order to 
increase the heat generating efficiency, means are taken to smoothly 
rectify the alternating current of a commercial power supply and invert it 
to produce a high frequency. That is, the high-frequency electrical 
current is generated by rectifying alternating current of a commercial 
power supply 35 by means of a rectification circuit 36 and inverting it to 
a high frequency using an inverter circuit 37. 
The thermostat 17 functions as a safety device if the inversion action 
between the commercial power supply 35 and the circuit begins to operate 
out-of-control. The electrical current flowing to the induction heating 
coil 22 is supplied through the thermostat 17 that is pressing against the 
surface of the fusing roller 10, and if the surface temperature of the 
fusing roller 10 reaches a previously set abnormal temperature value, the 
electrical circuit is opened by the thermostat 17. A control circuit 38 
comprised of a microprocessor and memory carries out temperature control 
based on the electric potential of the thermistor 16, while monitoring the 
temperature of the fusing roller 10. The control circuit 38 carries out 
the temperature control by outputting an ON/OFF signal to a drive circuit 
40 inside the inverter circuit 37. The inverter circuit 37 carries out 
frequency conversion on direct current from the rectification circuit 36, 
converting it to a high-frequency electrical current, and supplies it to 
the coil 22. 
In the inverter circuit 37, when the control signal generated from the 
control circuit 38 turns ON, at first, the drive circuit 40 activates the 
switching device 41 and a voltage is applied to an LC resonant circuit 
comprised of the induction heating coil 22 and a resonance condenser 44. 
This action causes an electrical current to flow in the induction heating 
coil 22. The switch-on time is determined by a timer circuit 47. The timer 
circuit 47 turns on the switching device 41 to obtain a stable heating 
output only during a fixed time that is determined by an RC charge and 
discharge property using a voltage regulator 46. When the set time is 
reached, a signal is sent to the drive circuit 40 to turn the switching 
device 41 OFF. When switching device 41 turns off, a resonant electrical 
current flows between the induction heating coil 22 and the resonance 
condenser 44. Thereafter, when a voltage detection circuit 43 detects that 
the drain voltage on the induction heating coil 22 side of the switching 
device 41 drops close to 0 V by means of the resonance, a signal is sent 
to the drive circuit 40 to turn on the switching device 41 again. 
Basically, the switch-off time is determined by the voltage detection 
circuit 43. By repeating this switching cycle, a high-frequency electrical 
current flows to the induction heating coil 22. 
Furthermore, a DC power supply 45 for the circuit is provided, which is a 
simple stabilized power supply to supply a direct current to the timer 
circuit 47, drive circuit 40, voltage detection circuit 43 and the voltage 
regulator 46. 
As stated above, because the temperature at both ends of the fusing roller 
10 are lower, compared to the center portion, due to the influence of heat 
radiation, if the quantity of heat generated at both ends of the fusing 
roller 10 is not higher compared to the center portion, the temperature 
distribution in the lengthwise direction of the fusing roller 10 is not 
uniform, making it difficult to achieve uniform fusing performance in the 
lengthwise direction. To compensate for this effect, in one embodiment of 
the invention shown in FIGS. 9A and 9B, the distribution of the quantity 
of heat generated in the lengthwise direction of the fusing roller 10 is 
varied by changing the distance between the fusing roller 10 and the core 
23. The distance between the fusing roller 10 and the core 23 at the edge 
portion in the lengthwise direction of the fusing roller 10 is smaller 
than the distance at the center portion. In other words, the air gap D1 
between the cores 23 at the two outside coil assemblies 12 and the inner 
surface of the fusing roller 10 is smaller than the air gap D2 at the two 
inside coil assemblies 12. In this embodiment, the variation in distance 
is achieved by making the cores on the outside larger than the cores on 
the interior. 
If the distance between the fusing roller 10 and the core 23 is made 
smaller, the magnetic coupling becomes stronger, thereby increasing the 
quantity of heat generated. If the distance is made larger, the magnetic 
coupling becomes weaker, thereby decreasing the quantity of heat 
generated. Therefore, even if the winding density of the coil 22 is not 
changed, the distribution of the quantity of heat generated in the 
lengthwise direction of the fusing roller 10 can be adjusted as desired by 
only changing the distance between the fusing roller 10 and the core 23. 
Further, because the coil 22 is constructed using a uniform winding 
density, the ability to mass-produce coils is improved, allowing 
reductions in the cost of the coils 22. 
Because the air gap D1 at the two outside coil assemblies 12 is smaller 
than the air gap D2 at the two inside coil assemblies 12, the distribution 
of the quantity of heat generated in the lengthwise direction of the 
fusing roller 10 is as shown in FIG. 9C. Assuming this type of 
distribution of the quantity of heat, even though the edge portion in the 
lengthwise direction of the fusing roller 10 is easily influenced by heat 
radiation, the temperature distribution in the lengthwise direction of the 
fusing roller 10 can be made almost uniform, thereby making it possible to 
achieve uniform fusing characteristics in the lengthwise direction of the 
fusing roller 10. 
FIGS. 10A-10C, respectively, comprise a transverse view of the fusing 
roller in a second embodiment of the invention, a cross-section along line 
B--B of FIG. 10A, and a heat generation distribution drawing. To the 
extent that a fusing roller 10 and a plurality of coil assemblies 12 are 
used, this second embodiment is similar to the above-mentioned first 
embodiment. However, the second embodiment is different in the fact that 
it continuously changes the quantity of heat generated, while the first 
embodiment changes the quantity of heat in a stepwise manner. 
As shown in FIG. 10A, the surfaces of the top and bottom edges of each core 
23 in the figure are inclined surfaces, making the air gap gradually 
smaller from the center portion of the fusing roller 10 toward the edge 
portion. When constructed this way, the distribution of the quantity of 
heat generated in the lengthwise direction of the fusing roller 10 is as 
shown in FIG. 10C. Thus, the quantity of heat generated in the lengthwise 
direction of the fusing roller 10 continuously changes. 
In the second embodiment as well, since the magnetic flux is generated in a 
direction at a right angle to the fusing roller 10, the leakage of 
magnetic flux from both ends of the fusing roller 10 is small, and the 
heat generating efficiency is increased. The temperature distribution in 
the lengthwise direction of the fusing roller 10 can be made almost 
uniform, thereby making it possible to further achieve uniform fusing 
characteristics in the lengthwise direction of the fusing roller 10. 
FIGS. 11A-11C, respectively, comprise a transverse view of the fusing 
roller in a third embodiment of the invention, a cross-section along line 
B--B of FIG. 11A and a heat generation distribution drawing. Due to the 
fact that a fusing roller 10 is used and the quantity of heat generated is 
continuously changed, the third embodiment is similar to the second 
embodiment. However, the third embodiment is different with respect to the 
fact that it uses one comparatively long coil assembly 12, while the 
second embodiment uses four comparatively short coil assemblies 12. 
As shown in FIG. 11A, the surfaces of the top and bottom edge of one 
comparatively long core 23 in the figure are inclined surfaces, making the 
air gap gradually smaller from the center portion of the fusing roller 10 
toward the edge portion. When constructed this way, the distribution of 
the quantity of heat generated in the lengthwise direction of the fusing 
roller 10 is as shown in FIG. 11C. In the third embodiment as well, the 
heat generating efficiency is increased and uniform fusing characteristics 
in the lengthwise direction of the fusing roller 10 can be achieved. 
Induction heat fusing devices can alternatively use a fusing belt that is 
comprised of a conductive member, such as metal, and that carries out the 
transfer while making contact with the recording paper, in place of the 
fusing roller 10. An induction heating coil arranged opposite the fusing 
belt causes joule heat generation in the fusing belt itself. This fusing 
belt is equivalent to a metal heating plate. 
An arrangement for a fusing device which utilizes a fusing belt is shown in 
FIGS. 20 and 21. Referring thereto, a fusing belt 30 which includes a 
layer of conductive material, such as metal, is wound about a pair of 
rollers 35 and 36. The roller 35 is a drive roller which is connected to a 
suitable drive mechanism (not shown), and drives the belt in the direction 
of the arrow illustrated in FIG. 21. A pressing roller 37 is located 
opposite the roller 36 and in contact with the fusing belt 30, to form a 
nip through which a sheet of paper, or other record medium carrying an 
unfused toner image, passes. The sheet of paper is guided into the nip by 
a paper guide 38 shown in FIG. 21. 
An inductive heating coil assembly 33 is located within the interior of the 
belt, between the two rollers 35 and 36. This coil assembly causes the 
belt 30 to be heated as it advances towards the nip formed by the rollers 
36 and 37. As the paper passes through the nip the image is fused onto it, 
as depicted in FIG. 20. A cleaning roller 39 is located above the belt 30, 
and functions to remove any toner particles which may have adhered to the 
belt. 
As shown in FIGS. 12A-12B, in an induction heat fusing device that uses a 
fusing belt 30, the core 33 of each coil assembly 34 is arranged at a 
right angle to the fusing belt 30 and the coil 32 is arranged such that it 
is wound around a central shaft that is disposed at a right angle to the 
fusing belt 30. In like manner to the previous embodiments, the direction 
of the generated magnetic flux is in a direction at a right angle to the 
fusing belt 30, resulting in the fusing belt 30 and the core 33 creating a 
closed magnetic circuit. Because of this, there is either no leakage of 
magnetic flux the edges of the fusing belt 30, or it is small, thereby 
increasing the heat generating efficiency. 
FIGS. 13A-13C, respectively, comprise a transverse view of the main parts 
of the fusing belt in a fourth embodiment of the invention, a 
cross-section along line B--B of FIG. 13A, and a heat generation 
distribution drawing. This fourth embodiment is similar to the first 
embodiment except for the fact that a fusing belt 30 is used in place of 
the fusing roller 10. 
As shown in FIG. 13A, the air gap D1 at the two outside coil assemblies 34 
is smaller than the air gap D2 at the two inside coil assemblies 34. When 
constructed this way, the distribution of the quantity of heat generated 
in the crosswise direction of the fusing belt 30 is as shown in FIG. 13C. 
In this fourth embodiment, because the magnetic flux is generated in a 
direction at a right angle to the fusing belt 30, the leakage of magnetic 
flux from both ends of the fusing belt 30, as viewed in the crosswise 
direction, is small and the heat generating efficiency is increased. The 
temperature distribution in the crosswise direction of the fusing belt 30 
is made almost uniform, thereby making it possible to further achieve 
uniform fusing characteristics in the crosswise direction of the fusing 
belt 30. 
FIGS. 14A-14C are views of the main parts of the fusing belt in a fifth 
embodiment of the invention. This fifth embodiment is similar to the 
second embodiment, except for the fact that a fusing belt 30 is used in 
place of the fusing roller 10. As shown in FIG. 14A, the surface of the 
bottom edge of each core 33 in the figure is an inclined surface, making 
the air gap gradually smaller from the center portion of the fusing belt 
30 toward the longitudinal edges. When constructed this way, the 
distribution of the quantity of heat generated in the crosswise direction 
of the fusing belt 30 is as shown in FIG. 14C. In this embodiment, the 
quantity of heat generated in the crosswise direction of the fusing belt 
30 changes continuously. In the fifth embodiment as well, the heat 
generating efficiency is increased and uniform fusing characteristics in 
the crosswise direction of the fusing belt 30 can be achieved. 
FIGS. 15A-15C are views of a sixth embodiment of the invention. This sixth 
embodiment is similar to the third embodiment, except for the fact that a 
fusing belt 30 is used in place of the fusing roller 10. As shown in FIG. 
15A, the surface of the bottom edge of one comparatively long core 33 in 
the figure is an inclined surface, making the air gap gradually smaller 
from the center portion of the fusing belt 30 toward the longitudinal 
edges. When constructed this way, the distribution of the quantity of heat 
generated in the crosswise direction of the fusing belt 30 is as shown in 
FIG. 15C. In the sixth embodiment, therefore, the heat generating 
efficiency is increased and uniform fusing characteristics in the 
crosswise direction of the fusing belt 30 can be achieved. 
FIGS. 16A-16C are views of the main parts of a seventh embodiment of the 
invention. This seventh embodiment uses multiple coil assemblies 34, e.g., 
four coil assemblies, each of which has the same composition. As shown in 
FIG. 16A, each coil assembly 34 is arranged to be inclined relative to the 
fusing belt 30 so the air gap gradually becomes smaller from the center 
portion of the fusing belt 30 toward the longitudinal edges. When 
constructed this way, the distribution of the quantity of heat generated 
in the crosswise direction of the fusing belt 30 is as shown in FIG. 16C. 
In this embodiment, the quantity of heat generated in the crosswise 
direction of the fusing belt 30 changes continuously. The heat generating 
efficiency is increased and uniform fusing characteristics in the 
crosswise direction of the fusing belt 30 can be achieved. 
FIGS. 17A-17C are views of the fusing roller in an eighth embodiment of the 
invention. This eighth embodiment is an embodiment in which the 
distribution of the quantity of heat changes in the lengthwise direction 
of the fusing roller 10 by varying the magnetic permeability of the cores 
23. Specifically, the magnetic permeability of the cores 23 at the edge 
portion in the lengthwise direction of the fusing roller 10 is larger than 
the magnetic permeability of the cores 23 at the center portion. In the 
example in the figure, four coil assemblies 12 are arranged in a line, and 
the shape of the cores 23 and the coil 22 at each individual coil assembly 
12, as well as the winding direction, are all the same. In addition, the 
wiring connection is such that the same electrical current flows to all 
the coils 22. However, the magnetic permeability of the cores 23 at the 
two outside coil assemblies 12, from among the four coil assemblies 12, is 
larger than the magnetic permeability of the cores 23 at the two inside 
coil assemblies 12, as explained below. 
Because the generated magnetic flux increases as the magnetic permeability 
of the cores 23 increases, if the magnetic permeability of the cores 23 is 
made larger, the magnetic flux intertwining with the fusing roller 10 
increases, and the quantity of inductively generated heat increases. 
Conversely, if the magnetic permeability of the cores 23 is made smaller, 
the magnetic flux intertwining with the fusing roller 10 decreases and the 
quantity of inductively generated heat becomes smaller. Therefore, even if 
the winding density of the coil is not changed, the distribution of the 
quantity of heat generated in the lengthwise direction of the fusing 
roller 10 can be adjusted as desired by only changing the magnetic 
permeability of each individual core 23 along the lengthwise direction of 
the fusing roller 10. Further, the coils 22 can be manufactured with a 
uniform winding density, thereby enhancing the ability to efficiently 
mass-produce the coils 22. 
Since the magnetic permeability of the cores at the two outside coil 
assemblies 12 is larger than the magnetic permeability of the cores at the 
two inside coil assemblies 12, the distribution of the quantity of heat 
generated in the lengthwise direction of the fusing roller 10 is as shown 
in FIG. 17C. Assuming this type of distribution of the quantity of heat 
generated, even though the edge portion in the lengthwise direction of the 
fusing roller 10 is easily influenced by heat radiation, the temperature 
distribution in the lengthwise direction of the fusing roller 10 can be 
made almost uniform, thereby making it possible to achieve uniform fusing 
characteristics in the lengthwise direction of the fusing roller 10. 
The core 23 is preferably a ferrite core, which can include Zn ferrite, 
Mn-Zn ferrite, Ni-Zn ferrite or Mn-Mg ferrite. The intensity of the 
spontaneous magnetization of these materials is 400 to 500 G (0.5 to 0.6 
Wb/m.sup.2) and the cores can be chosen in the lengthwise direction of the 
fusing roller 10 depending on the properties of each ferrite, namely, the 
intensity of the spontaneous magnetization, i.e. difference in magnetic 
permeability. 
Moreover, although it is not shown in the figure, the distribution of the 
quantity of heat generated in the crosswise direction of the fusing belt 
30 can be changed by varying the magnetic permeability of the core 33. 
Further, the distribution of the quantity of heat generated in the 
lengthwise direction of the heated metal bodies 10, 30 can be changed by 
changing both the distance between the heated metal bodies (fusing roller 
10 or fusing belt 30) and the cores 23, 33 and the magnetic permeability 
of the cores 23, 33. 
A ninth embodiment of the invention is illustrated in FIGS. 22A-22D. This 
embodiment relates to a fusing device which utilizes a fusing belt 30. In 
this embodiment, the distance D1 between the cores 33 and the fusing belt 
30 is the same for all four of the coil assemblies 34. However, the cores 
33 for the two outside assemblies, illustrated in FIG. 22B, are larger 
than the cores for the two interior assemblies, as illustrated in FIG. 
22C. Since a larger core results in increased magnetic field strength, 
even though the number of windings remains the same, a greater amount of 
inductive heat will be generated at the outer portions of the fusing belt, 
relative to the center portion of the belt, as depicted in FIG. 22D. As a 
result, a uniform fusing action will occur across the width of the fusing 
belt. 
A further related embodiment of the invention is illustrated in FIGS. 
23A-23C. In this embodiment, the cores 33 of all of the coil assemblies 34 
are of the same size. To provide a greater amount of heat generation at 
the edges of the fusing belt 30, therefore, the coil assemblies 34 at the 
outside portions of the belt are positioned closer to the belt than the 
coil assemblies at the interior. Since the air gap between the cores and 
the belt is smaller for the outside assemblies, a stronger magnetic field 
is present at these locations, resulting in greater joule heating. 
Consequently, the amount of heat which is generated across the width of 
the belt varies, as shown in FIG. 23C, to compensate for heat loss due to 
radiation at the edges of the belt. 
Another embodiment of an inductive fusing device which employs a belt is 
illustrated in FIGS. 24A and 24B. In this embodiment, the inductive heat 
generation does not occur within the belt itself. Rather, a magnetic field 
is generated by coil assemblies 900 each comprised of a three-legged core 
100 whose center leg is surrounded by a coil 200. The coil assemblies are 
contained within a housing 400 made of a conductive material such as 
copper. The conductive housing forms a closed magnetic circuit with the 
coil assemblies 900, and becomes heated by the joule effect, in accordance 
with the principle described previously. 
A fusing belt 500 is located on the outside of the housing 400, and engages 
the lower surface of the housing. As a result, it becomes heated along the 
portion where the belt and the housing are in contact with one another. 
The belt 500 is driven in a path around the housing 400 by a driving 
roller 10000. As illustrated in FIG. 24A, the belt 500 moves in a 
counterclockwise direction. A pressing belt 600 is driven in a path around 
a pair of rollers 650 and 660, and contacts the fusing belt 500 along the 
portion of its length where it engages the housing 400. In the view of 
FIG. 24A, the pressing belt 600 moves in a clockwise direction. 
In operation, a sheet of paper 800, or similar record medium bearing a 
toner image, is fed into the nip between the fusing roller 500 and the 
pressing roller 600. As the paper passes from left to right, as viewed in 
FIG. 24A, it becomes heated by the inductive heating action of the coil 
assemblies 900 and the conductive housing 400, to thereby fuse the toner 
image on the paper. To prevent overheating of the paper, a temperature 
sensor 700, such as a thermistor, contacts the fusing belt 500 at a point 
where it engages the conductive housing, and regulates the power provided 
to the coils 200, as described previously. 
Referring to FIG. 24B, the coil assemblies 900 all have the same size. 
However, they are positioned such that the assemblies near the outer edges 
of the housing 400 are positioned at a distance D1 which is closer to the 
lower surface of the housing, i.e., the surface which engages the belt 
500, than the distance D2 between the housing and the interior coil 
assemblies. For the reasons explained previously, a greater amount of heat 
will be generated at the edges of the housing, and hence the edges of the 
belt 500, to compensate for heat loss due to radiation. 
A similar arrangement can be employed in a fusing device which employs a 
fusing roller 10. Referring to FIG. 25, all of the coil assemblies within 
the fusing roller 10 have cores 33 which are of the same size, for ease of 
manufacture. The coil assemblies near the ends of the fusing roller 10 are 
positioned at a distance D1 from the portion of its surface which contacts 
the pressing roller 11. In contrast, the coil assemblies at the interior 
of the fusing roller 10 are located a greater distance D2 from this 
portion of the surface. As a result, in the area of the nip between the 
rollers 10 and 11 where fusing takes place, a greater amount of heat is 
generated at the ends of the roller, to provide uniform heating across the 
width of the fusing roller 10. 
In the preceding embodiments of the invention, the variation in the 
inductively generated heat is achieved by changing physical parameters of 
the magnetic circuit, e.g. the distance of the cores from the heated metal 
body, the permeability of the cores, and/or the size of the cores. In 
another approach, the electrical characteristics of the circuit connecting 
the cores to one another can be arranged to accomplish a similar result. 
For example, as shown in FIG. 26, coils 301, 302a, 302b and 303 are wound 
around a plurality of prismatic cores 23 inside a fusing roller 10. The 
coils 302a and 302b arranged at the center portion of the fusing roller 10 
are connected in parallel to each other, while the coils 301 and 303 
arranged at the end portions of the fusing roller 10 are connected in 
series with the coils 302a and 302b. 
Alternatively, as shown in FIG. 27, a plurality of coils 301, 302a, 302b 
and 303 are helically and uniformly wound around one cylindrical core 23, 
where the coils 302a and 302b arranged at the center portion of the fusing 
roller 10 are connected in parallel to each other, while the coils 301 and 
303 arranged at the end portions of the fusing roller 10 are connected in 
series with the coils 302a and 302b. In this case, each coil is wound with 
a uniform winding density. The above coil arrangement is shown in FIG. 28 
in the form of a schematic diagram. 
In various embodiments the number of coils is changed, which can be 
exemplified by a variety of schematic diagrams as follows. As shown in 
FIG. 29(a), there may be a coil arrangement in which three coils 302a, 
302b and 302c are connected in parallel to each other at the center 
portion, and coils 301a and 301b as well as coils 303a and 303b are 
connected in parallel to each other at the end portions to be connected in 
series with the coils at the center portion. As shown in FIG. 29(b), there 
may be a coil arrangement in which four coils 302a through 302d are 
connected in parallel to each other at the center portion, and three coils 
301a through 301c as well as coils 303a through 303c are connected in 
parallel to each other at the end portions. These end coils are further 
connected in series with the coils at the center portion. As shown in FIG. 
29(c), there may be a coil arrangement in which five coils 302a through 
302e are connected in parallel to each other at the center portion, and 
four coils 301a through 301d as well as coils 303a through 303d are 
connected in parallel to each other at the end portions. The end coils are 
further connected in series with the coils at the center portion. As shown 
in FIG. 29(d), there may be a coil arrangement in which six coils 302a 
through 302f are connected in parallel to each other at the center 
portion, and five coils 301a through 301e as well as coils 303a through 
303e are connected in parallel to each other at the end portions. The end 
coils are further connected in series with the coils at the center 
portion. 
When connecting the coils arranged at the center portion of the fusing 
roller 10 in parallel to each other and connecting the coils arranged at 
the end portions in series with the coils at the center portion, the 
number of coils connected in parallel to each other at the center portion 
is larger than the number of the coils in each end portion, as described 
in detail later. With this arrangement, the amount of current generated in 
the coils at the center portion is smaller than that in the coils arranged 
at the end portions. Therefore, the induction current at the center 
portion is smaller, so that the amount of heat generated at the center 
portion is smaller and the amount of heat generated at the end portions is 
larger. Consequently, the heated body has an almost uniform temperature 
distribution. At the center portion, the heat discharge amount is smaller 
and almost the entire electric field is used for the heat generation. At 
the end portions, the heat discharge amount is larger and a part of the 
electric field is formed outside the roller. 
With regard to the relation between the plurality of coils and the cores 
around which the coils are wound, it is acceptable to wind each coil 
around each core as shown in FIG. 26 while increasing the number of coils 
that are connected in parallel to each other at the center portion and 
decreasing the number of coils connected in parallel to each other at the 
end portions, or to wind a plurality of coils around one core while 
arranging a larger number of coils connected in parallel to each other at 
the center portion and arranging a smaller number of coils connected in 
parallel to each other at the end portions, as in the embodiment of FIG. 
27. 
The changes in the heat generation amount according to each coil 
arrangement achieved by combining the parallel connection with the series 
connection of the coils will now be described. FIG. 30 is an equivalent 
schematic circuit diagram showing the relation between the fusing roller 
and the coils. In this figure and the following equations, L1 is the 
inductance of the coil, R1 is the resistance of the coil, L2 is the 
inductance of the fusing roller, R2 is the resistance of the fusing 
roller, V is a voltage to be applied to the coil, I1 is a current flowing 
through the coil, 12 is a current flowing through the fusing roller 
(induction current), M is a mutual inductance and k is a coupling 
constant. In the illustrated equivalent circuit, the following equations 
(1) through (3) hold. 
EQU V.sub.1 =(R.sub.1 +j.omega.L.sub.1)I.sub.1 +j.omega.MI.sub.2(1) 
EQU O=j.omega.MI.sub.1 +(R.sub.2 +j.omega.L.sub.2)I.sub.2 (2) 
##EQU1## 
The current flowing through the fusing roller can be obtained from the 
above equations, yielding the following equation (4). 
##EQU2## 
Both the members of equation (4) are squared to obtain the following 
equation (5). 
EQU I.sub.2.sup.2 =.omega..sup.2 k.sup.2 L.sub.1 L.sub.2 I.sub.1.sup.2 
/(R.sub.2 +j.omega.L.sub.2).sup.2 (5) 
The heat generation amount of the fusing roller, W2, can be obtained from 
the following equation (6). 
EQU W.sub.2 I.sub.2.sup.2 R.sub.2 =.omega..sup.2 k.sup.2 L.sub.2 R.sub.2 
/(R.sub.2 +j.omega.L.sub.2).sup.2 L.sub.1 I.sub.1.sup.2 (6) 
where .omega., k, L.sub.2 and R.sub.2 are constants depending on the 
material and the shape of the fusing roller, and therefore, they can be 
set as in the following equation (7). 
EQU K=.omega..sup.2 k.sup.2 L.sub.2 R.sub.2 /(R.sub.2 +j.omega.L.sub.2).sup.2(7 
) 
Therefore, the heat generation amount W.sub.2 of the fusing roller can be 
obtained from the following equation (8), and it can be seen from equation 
(8) that the heat generation amount of the fusing roller is proportional 
to the inductance of the coil and the square of the coil current. 
EQU W.sub.2 =K.cndot.L1.cndot.I.sub.1.sup.2 (8) 
By means of equation (8), the heat generation amount of the fusing roller 
is obtained from the coils according to the aforementioned connection 
methods shown in FIG. 28 and FIGS. 29(a) through 29(d). It is assumed in 
the following equations that a current flowing through each coil arranged 
at the center portion is Ia, a current flowing through each coil arranged 
at the end portions is Ib, the heat generation amount obtained from each 
coil arranged at the center portion is Wa, and the heat generation amount 
obtained from each coil arranged at the end portions is Wb. 
First, in the case of the connection shown in FIG. 28, the currents flowing 
through the coils at the center portion and the coils at the end portions 
have the relation of 2.times.Ia=Ib according to the connection. Further, 
according to equation (8), the heat generation amount obtained from each 
coil arranged at the center portion is: 
Wa=K.times.L.times.Ia.times.Ia, and likewise, the heat generation amount 
obtained from each coil arranged at the end portions is: 
Wb=K.times.L.times.Ib.times.Ib. Therefore, the relation between Wa and Wb 
is: 
Wa=4.times.Wb, namely the heat generation amount per coil at the end 
portions is four times as great as the heat generation amount per coil at 
the center portion. 
Next, in the case of the connection shown in FIG. 29(a), the currents 
flowing through the coils at the center portion and the coils at the end 
portions have the relation of 3.times.Ia=2.times.Ib according to the 
connection. Further, according to equation (8), the heat generation amount 
obtained from each coil arranged at the center portion is: 
Wa=K.times.L.times.Ia.times.Ia, and the heat generation amount obtained 
from each coil arranged at the end portions is: 
Wb=K.times.L.times.Ib.times.Ib. Therefore, the relation between Wa and Wb 
is: 
Wa=9/4.times.Wb, namely the heat generation amount per coil at the end 
portions is 9/4 times as great as the heat generation amount per coil at 
the center portion. 
Next, in the case of the connection shown in FIG. 29(b), the currents 
flowing through the coils at the center portion and the coils at the end 
portions have the relation of 4.times.Ia=3.times.Ib according to the 
connection. Further, according to equation (8), the heat generation amount 
obtained from each coil arranged at the center portion is: 
Wa=K.times.L.times.Ia.times.Ia, and likewise, the heat generation amount 
obtained from each coil arranged at the end portions is: 
Wb=K.times.L.times.Ib.times.Ib. Therefore, the relation between Wa and Wb 
is: 
Wa=16/9.times.Wb, namely the heat generation amount per coil at the end 
portions is 16/9 times as great as the heat generation amount per coil at 
the center portion. 
Next, in the case of the connection shown in FIG. 29(c), the currents 
flowing through the coils at the center portion and the coils at the end 
portions have the relation of 5.times.Ia=4.times.Ib according to the 
connection. Further, according to equation (8), the heat generation amount 
obtained from each coil arranged at the center portion is: 
Wa=K.times.L.times.Ia.times.Ia, and likewise, the heat generation amount 
obtained from each coil arranged at the end portions is: 
Wb=K.times.L.times.Ib.times.Ib. Therefore, the relation between Wa and Wb 
is: 
Wa=25/16.times.Wb, i.e. the heat generation amount per coil at the end 
portions is 25/16 times as great as the heat generation amount per coil at 
the center portion. 
Next, in the case of the connection shown in FIG. 29(d), the currents 
flowing through the coils at the center portion and the coils at the end 
portions have the relation of 6.times.Ia=5.times.Ib according to the 
connection. Further, according to equation (8), the heat generation amount 
obtained from each coil arranged at the center portion is: 
Wa=K.times.L.times.Ia.times.Ia, and likewise, the heat generation amount 
obtained from each coil arranged at the end portions is: 
Wb=K.times.L.times.Ib.times.Ib. Therefore, the relation between Wa and Wb 
is: 
Wa=36/25.times.Wb, such that the heat generation amount per coil at the end 
portions is 36/25 times as great as the heat generation amount per coil at 
the center portion. 
The heat generation amount of the coils at the center portion and the heat 
generation amount of the coils at the end portions can be thus controlled 
in various ways according to each connection method. 
In an induction heat fusing device according to one embodiment of the 
invention as described above, the heat generating efficiency is increased 
and the distribution of the quantity of heat generated in the lengthwise 
direction of the heated metal bodies can be freely set by utilizing a 
simple construction in which a magnetic flux is generated in a direction 
at a right angle to the heated metal bodies and the distance between the 
heated metal bodies and the cores is changed. Because the coils are 
manufactured with a uniform winding density, the ability to mass-produce 
coils is enhanced. 
Furthermore, the temperature distribution in the lengthwise direction of 
the heated metal bodies can be made almost uniform, considering the 
effects of heat radiation, thereby making it possible to achieve uniform 
fusing characteristics in the lengthwise direction of the heated metal 
bodies. 
Furthermore, according to an alternate embodiment of the invention, the 
heat generating efficiency is increased and the distribution of the 
quantity of heat generated in the lengthwise direction of the heated metal 
bodies can be freely set by utilizing a simple construction in which a 
magnetic flux is generated in a direction at a right angle to the heated 
metal bodies and the magnetic permeability of the cores is changed. Again, 
because the coils are manufactured with a uniform winding density the 
ability to mass-produce coils is improved. 
In yet another embodiment of the invention, the effects of heat loss due to 
radiation can be compensated by varying the size of the cores which are 
used to generate the magnetic field that causes inductive heating. Again, 
uniform winding density is utilized, to facilitate mass production of the 
coils. 
According to another embodiment of the present invention, the heating 
temperature of the heated body can be made uniform merely by changing the 
connection method of the plurality of coils. As such, there is no 
requirement to wind the coil in a special winding manner, and consequently 
a high productivity is assured. Furthermore, it is possible to greatly 
vary the heat generation amount since there is no restriction in terms of 
shape, and this allows the heat generation amount to be easily corrected 
even when there is a great temperature decrease due to the heat discharge 
at the end portions of the heated body. 
It will be appreciated, of course, that although the various embodiments of 
the invention have been described individually, they can be combined in 
various manners to achieve a desired variation in heat generation along 
the length of the heated metal body.