Method for manufacturing coil

A method for manufacturing a coil is composed of the step of rolling a conductor of trapezoidal cross section or a conductor of rectangular cross section in the longitudinal direction thereof. An amount of deformation is gradually increased from a side of the conductor corresponding to the inner side of the coil toward a side of the conductor corresponding to the outer side of the coil. As a result, the conductor of trapezoidal cross section is formed into a coiled conductor of rectangular cross section, or the conductor of rectangular cross section is formed into a coiled conductor of trapezoidal cross section. In this way, rolling and coiling of the conductor are performed simultaneously to produce a coil.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for manufacturing a coil formed 
of a conductor of rectangular cross section and used in an electric 
apparatus such as a motor or a transformer. 
2. Description of the Related Art 
Conventionally, a conductor of rectangular cross section is wound as it is 
to obtain a coil. 
This method involves the formation of a projection, such as a thick-walled 
portion or a deformed portion, at the inner side of a coiled conductor. 
Since such a projection may cause dielectric breakdown or a like failure, 
the projection is made smooth by use of a coil surface leveler as 
disclosed in, for example, Japanese Patent Application Laid-Open (kokai) 
No. 57 (1982)-68222. 
However, the conventional method for manufacturing a coil involves the 
following drawbacks. 
First, since a separate surface leveler is required, the number of 
manufacturing apparatuses and the number of production steps increases, 
with the result that the cost of manufacture increases, and productivity 
decreases. 
Second, projections projecting in the axial direction of a coil can be 
eliminated, but other projections projecting toward the center of the coil 
may be newly generated, and deformed portions cannot be completely 
eliminated, thus resulting in failure to obtain a coil of high quality and 
excellent homogeneity. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method for manufacturing 
a coil which does not require employment of a separate surface leveler, 
thereby reducing production cost through decrease in the number of 
manufacturing apparatuses and the number of production steps, and which 
can improve productivity. 
Another object of the present invention is to provide a method for 
manufacturing a coil which does not involve the formation of a projection, 
such as a thick-walled portion or a deformed portion, on the coil, thereby 
obtaining a coil of high quality and excellent homogeneity. 
Still another object of the present invention is to provide a method for 
manufacturing a coil which can reduce the size of the coil and which can 
suppress bubble formation within ceramics to thereby establish good 
insulation. 
Yet another object of the present invention is to provide a method for 
manufacturing a coil which establishes reliable adhesion (a reliable bond) 
between ceramics and a conductor and which establishes an appropriate 
difference in thermal expansion between coating layers, thereby imparting 
significantly high mechanical rigidity to the coil. 
To achieve the above objects, the present invention provides a method for 
manufacturing a coil comprising the step of rolling a conductor of 
trapezoidal cross section or a conductor of rectangular cross section in 
the longitudinal direction thereof. At this time, an amount of deformation 
is gradually increased from a side of the conductor corresponding to the 
inner side of the coil toward a side of the conductor corresponding to the 
outer side of the coil. As a result, the conductor of trapezoidal cross 
section is formed into a coiled conductor of rectangular cross section, or 
the conductor of rectangular cross section is formed into a coiled 
conductor of trapezoidal cross section. In this way, rolling and coiling 
of the conductor are performed simultaneously to produce a coil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Embodiments of the present invention will next be described with reference 
to the drawings. The accompanying drawings are illustrative of the 
embodiments and are not meant to limit the scope of the invention. To 
clarify the invention, detailed description of known parts is omitted. 
A copper conductor Wt (W) of trapezoidal cross section as shown in FIG. 2 
is prepared. The conductor Wt of trapezoidal cross section is uncoated and 
has a cross-sectional area of about 10 mm.sup.2. 
In FIG. 1, reference numerals 11 and 12 denote a pair of rollers. The 
rollers 11 and 12 each have a cylindrical shape and are arranged in 
parallel to each other with a predetermined gap provided therebetween. 
In manufacture of a coil, as shown in FIGS. 1 and 3, the conductor Wt of 
trapezoidal cross section is caused to pass between the rollers 11 and 12. 
As a result, the conductor Wt of trapezoidal cross section is rolled in 
the longitudinal direction thereof. The conductor Wt of trapezoidal cross 
section is rolled into the conductor Wt of rectangular cross section as 
shown in FIG. 1. The short side of the rectangular cross section has a 
length equal to the gap between the rollers 11 and 12. 
In this case, an amount P of deformation is gradually increased from a side 
Wi of the conductor Wt of trapezoidal cross section corresponding to the 
inner side of a coil toward a side Wo of the conductor Wt of trapezoidal 
cross section corresponding to the outer side of the coil. As a result of 
the difference in the amount P of deformation between the sides Wi and Wo, 
the conductor Wt of rectangular cross section ejected from between the 
rollers 11 and 12 is coiled into a circular coil C as shown in FIG. 3. In 
order to manufacture the coil C of a desired inner diameter D, the shape 
of the trapezoidal cross section of the conductor Wt and the amount P of 
deformation are determined accordingly. FIG. 4 shows the manufactured coil 
C. 
The thus-obtained coil C is subjected to insulation treatment to impart a 
desired dielectric strength thereto. The insulation treatment will next be 
specifically described with reference to FIGS. 5 to 7. 
The opposite ends of the coil C are pulled axially to thereby expand the 
coil C, i.e., expand the gap between the adjacent turns of the conductor 
Wt of rectangular cross section. The expanded coil C undergoes surface 
treatment, whereby a prime layer Lx is formed on the surface thereof. 
Nickel plating or chromium plating is an excellent surface treatment for 
the copper conductor Wt of rectangular cross section. In this case, 
plating is limited to a minimally required thickness so as to minimize 
magnetic effect. 
Notably, the prime layer Lx is employed for the following reason. Since the 
firing temperature for ceramics is usually 200.degree. C. or higher, the 
surface of the copper conductor Wt of rectangular cross section is 
oxidized, resulting in a weakened bond between the conductor surface and 
ceramics. Also, since the thermal expansion coefficient of copper is a 
bout three times that of ceramics, thermal shrinkage may cause ceramics to 
separate from the conductor. Surface treatment, such as plating treatment 
or oxidation treatment, of the conductor Wt of rectangular cross section 
facilitates intimate contact between the conductor surface and ceramics. 
Then, two ceramics layers La and Lb are sequentially formed on the prime 
layer Lx. 
Specifically, the first ceramics layer La is formed by the steps of: 
applying liquid ceramics onto the prime layer Lx; and firing the applied 
ceramics at high temperature. The liquid ceramics may be applied by 
dipping the coil C into the liquid ceramics or by spraying the liquid 
ceramics onto the coil C. In order to obtain a target thickness of coat, 
carrying out several repetitions of coating is effective. Particularly, 
forming thin layers of coat one on the other brings about a ceramics layer 
of good quality with no bubbles contained therein. 
Next, in order to form the second ceramics layer Lb, liquid ceramics is 
applied onto the first ceramics layer La. Since the second ceramics layer 
Lb is the last layer to be formed, after application of the liquid 
ceramics, the coil C is released from expansion and is allowed to shrink 
to its natural state. Then, the applied ceramics is fired to form the 
second ceramics layer Lb. Notably, through adjustment of the viscosity of 
the liquid ceramics, the ceramics layers La and Lb can be finished to 
their respective target thicknesses. 
Thus, the coil C insulated with ceramics is obtained. The coil C can serve 
as a final product as is. Alternatively, as shown in FIG. 5, the coil C 
may be attached to a coil bobbin 15. In this case, the coil bobbin 15 is 
formed through assembly of divided members. After the coil C is attached 
to the coil bobbin 15, ceramics or a like material may be filled into the 
gap between the coil C and the coil bobbin 15 as needed. 
Usually, ceramics is used as an electrical insulating material that can 
endure a working temperature greater than 250.degree. C. Generally, in 
manufacture of a coil insulated with ceramics, a ceramics-insulated 
conductor is coiled, or after an uninsulated conductor is coiled, ceramics 
is filled into gaps between turns of the coiled conductor. 
However, in the case of the method in which a ceramics-insulated conductor 
is coiled, since the ceramics-insulated conductor is difficult to elongate 
or bend, a coiling process encounters poor workability of the conductor. 
Examples of poor workability include the following: back tension during 
coiling is limited; and the ceramics-insulated conductor cannot be bent at 
an acute angle. Accordingly, the final shape of a coil is limited. 
Meanwhile, the method in which ceramics is filled into gaps between turns 
of a coiled conductor is particularly applied to the case where a coil is 
formed of a thick conductor and assumes a small size. According to this 
method, after an uncoated conductor is coiled, ceramics is filled into 
gaps between turns of the coiled conductor. Thus, the gaps must be of a 
certain magnitude, resulting in an increase in coil size. If the gap is 
too small, ceramics may separate from the conductor, whose material is 
primarily copper, due to the difference in thermal expansion coefficient 
therebetween. Therefore, the gaps cannot be decreased. 
According to the present embodiment, not only can ceramics impart 
sufficient insulation property and heat resistance to the coil C, but also 
a very thin insulation layer can be formed. Thus, the size of the coil C 
can be reduced, and bubble formation within ceramics can be suppressed to 
thereby establish good insulation. Also, reliable adhesion (a reliable 
bond) can be established between ceramics and a conductor, and an 
appropriate difference in thermal expansion coefficient can be established 
between coating layers, thereby imparting significantly high mechanical 
rigidity to the coil C. 
FIG. 7 shows a change in thermal expansion coefficient between adjacent 
turns of the conductor Wt of rectangular cross section of the coil C 
manufactured by the method of the present embodiment. As shown in FIG. 7, 
the change in thermal expansion coefficient is gentle, so that the 
separation of the insulation layer from the conductor can be prevented. 
The effect of the difference in thermal expansion coefficient on the 
adhesion of the insulation layer can be checked by subjecting the coil C 
to heat shock. When the coil C is subjected to heat shock, the insulation 
layer may separate from the conductor due to the difference in thermal 
expansion coefficient therebetween unless appropriate measures are taken. 
In the coil C manufactured by the method of the present embodiment, the 
prime layer Lx and the ceramics layers La and Lb establish a gentle change 
in thermal expansion coefficient between adjacent turns of the coil C, 
thereby buffering heat shock. 
FIGS. 8 to 11 show modified embodiments of the method. According to the 
modified embodiment of FIGS. 8 and 9, a conductor W of rectangular cross 
section (a conductor Wr) is rolled into a coiled conductor of trapezoidal 
cross section. This method is similar to that of the basic embodiment 
described previously except that conical (tapered) rollers 21 and 22 are 
used, and has the advantage that general conductors of rectangular cross 
section can be used. 
According to the modified embodiment of FIGS. 10 and 11, a conductor W of 
rectangular cross section (a conductor Ws) is rolled into a coiled 
multilayer conductor Wm, which includes a plurality of conductor portions 
Wsp and Wsq of trapezoidal cross section integrated together by 
corresponding narrower connection portions Wsc. This method employs a pair 
of rollers 31 and 32 as shown in FIG. 10. The rolling roller 31 (32) 
includes a first roller portion 31p, an intermediate roller portion 31c 
for forming the connection portion Wsc, and a second roller portion 31q, 
which portions are sequentially formed in the axial direction thereof. 
This method is similar to that of the basic embodiment described 
previously. According to the present modified embodiment, the conductor 
portions Wsp and Wsq of trapezoidal cross section are formed substantially 
independent of each other. Thus, even when the conductor Ws of rectangular 
cross section has an elongated rectangular cross section, the respective 
rectangular conductor portions Wsp . . . can be rolled independently, so 
that the conductor Ws of rectangular cross section can be wound readily 
and smoothly, and the formed coil C is almost of a true circle. Further, 
since ceramics enters into grooves formed around the corresponding 
connection portions Wsc, the coil C assumes higher rigidity. For a certain 
shape of a coil to be manufactured, the connection portions Wsc may be 
previously formed on the conductor Ws of rectangular cross section. 
The present invention is not limited to the above-described embodiments. 
Regarding structural details, techniques, and the like, modifications and 
any omission or addition may be possible as needed without departing from 
the scope of the invention.