Method of twisting coil wire to make coil assembly for use in electric rotary machine

A method of producing a coil assembly to be wound in a stator core with a plurality of slots. The method prepares a first and a second coil wire each of which is made up of in-slot portions and turned portions, arranges the first and second coil wires in parallel with the turned portions being offset from each other in a lengthwise direction thereof, moves the first coil wire to establish engagement of the turned portions of the first and second coil wires, turns the first coil wire about a pivot where the turned portions engage, crosses the first coil wire over the second coil wire around the pivot, and turns the first coil wire around the pivot so as to twist the turned portions of the first and second coil wires together. This sequence of steps is repeated to twist or braid the first and second coil wires without undesirable deformation thereof.

CROSS REFERENCE TO RELATED DOCUMENT

The present application claims the benefit of Japanese Patent Application No. 2008-171586 filed on Jun. 30, 2008, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to a method of braiding or twisting coil wires to make a coil assembly which may be employed in an electric rotary machine such as a motor-generator for automotive vehicles.

2. Background Art

There have been proposed a variety of methods of producing a coil assembly for use in electric rotary machines such as motor-generators. For example, Japanese Patent First Publication No. 2002-176752 teaches winding a plurality of coil wires simultaneously using a pair of winding strip cores. Japanese Patent First Publication No. 2004-104841 teaches how to braid a first coil wire and a second coil wire. Specifically, the first coil wire is turned through 90° around the axis of the second coil wire, as having been shaped into triangular waves in a winding process, and moved so as to make an overlap therebetween by half a turn, and also turned through 90° around the axis of the second coil wire. This sequence of steps is repeated to braid the first coil wire over the second coil wire by half a turn.

The braiding of coil wires each of which has a plurality of turned portions may also be achieved by turning a first coil wire around a second coil wire, like in conventional wire stranding machines, and moving the first coil wire by a coil pitch per turn or holding two coil wires crossed a given angle and rotating them around each other.

The above methods are, however, needed to keep the angle which the first and second coil wires make with each other as great as possible in order to avoid physical interference between the turned portions during the rotation of the first and second coil wires. This may result in undesirable deformation of straight portions of the first and second coil wires which are to be disposed inside slots of a stator core, which leads to a difficulty in shaping the braided coil wires into a desired geometry.

SUMMARY OF THE INVENTION

It is therefore a principal object of the invention to provide a method of producing a coil assembly which may be employed in electric rotary machines and is made up of braided or twisted coil wires without undesirable deformation thereof.

According to one aspect of the invention, there is provided a method of producing a coil assembly for use in an electric rotary machine. The coil assembly is to be wound in a core in which a plurality of slots are formed. The coil assembly includes at least a first and a second coil wire each of which has a length made up of in-slot portions to be disposed in the slots of the core and turned portions. The in-slot portions extend substantially straight in parallel to each other. Each of the turned portions connects between adjacent two of the in-slot portions. The turned portions are located on a first side and a second side opposed across the width of a corresponding one of the first and second coil wires alternately. The method comprises the steps of: (a) an arrangement step of arranging the first and second coil wires substantially in parallel to each other, with the turned portions of the first coil wire being offset from those of the second coil wire in a lengthwise direction thereof; (b) an engagement step of moving the first coil wire substantially parallel to the second coil wire in a widthwise direction thereof to establish engagement of a first turned portion that is one of the turned portions of the first coil wire located on the first side with a first turned portion that is one of the turned portions of the second coil wire located on the second side; (c) a first turning step of turning a portion of the first coil wire, as located on a side of the engagement, relative to a portion of the second coil wire, as located on the side of the engagement, in the widthwise direction about a pivot where the first turned portions of the first and second coil wires engage; (d) a crossing step of crossing the portion of the first coil wire over the portion of the second coil wire around the pivot; (e) a second turning step of turning the portion of the first coil wire around the pivot relative to the portion of the second coil wire in a direction opposite that in the first turning step; and (f) a moving step of moving the first coil wire relative to the second coil wire to place the first and second coil wires so as to extend substantially parallel to each other in the lengthwise direction thereof to twist the first turned portions of the first and second coil wires together.

In the arrangement step, the first and second coil wires are, as described above, offset from each other. Such an offset distance is greater than or equal to the diameter of the first and second coil wires and smaller than or equal to a length of the turned portions minus the diameter of the first and second coil wires. In other words, the offset distance is so selected that each of the in-slot portions of either of the first and second coil wires lies between adjacent two of the in-slot portions of the other of the first and second coil wires, that is, a portion of each of the turned portions of the first coil wire is laid to overlap a portion of one of the turned portions of the second coil wire, as viewed in the direction perpendicular to the length of the first and second coil wires. This arrangement permits the subsequent steps to be performed.

The above sequence of steps may be repeated to twist or brain the first and second coil wires together with the turned portions of the first coil wire being crossed over those of the second coil wire. In the first and second turning steps, the first coil wire is turned or rotated about the pivot where the turned portions of the first and second coil wires engages, thereby minimizing the angle which the first and second coil wires make with each other and which is required to enable the turned portions of the first and second coil wires to be crossed over each other. This enables the first and second coil wires to be braided without undesirable deformation thereof.

In the preferred mode of the invention, in the arrangement step, the turned portions of the first coil wire may be offset from those of the second coil wire by a pitch of the slots of the core, thereby facilitating ease of disposing the in-slot portion of the first and second coil wires in the slots of the core.

According to another aspect of the invention, there is provided a method of producing a coil assembly for use in an electric rotary machine. The coil assembly being to be wound in a core in which a plurality of slots are formed. The coil assembly includes a plurality of coil wires each of which has a length made up of in-slot portions to be disposed in the slots of the core and turned portions. The in-slot portions extend substantially straight in parallel to each other. Each of the turned portions connects between adjacent two of the in-slot portions. The turned portions are located on a first side and a second side opposed across the width of a corresponding one of the coil wires alternately. The method comprises the steps of: (a) an arrangement step of arranging a first and a second coil wire bundle, each of which is made up of a given number of the coil wires tied up together in a given condition, and placing the first and second coil wire bundles substantially in parallel to each other, with turned portions of the first coil wire bundle, each of which is a collection of the turned portions of the coil wires, being offset from those of the second coil wire bundle in a lengthwise direction thereof; (b) an engagement step of moving the first coil wire bundle substantially parallel to the second coil wire bundle in a widthwise direction thereof to establish engagement of a first turned portion that is one of the turned portions of the first coil wire bundle located on the first side with a first turned portion that is one of the turned portions of the second coil wire bundle located on the second side; (c) a first turning step of turning a portion of the first coil wire bundle, as located on a side of the engagement, relative to a portion of the second coil wire bundle, as located on the side of the engagement, in the widthwise direction about a pivot where the first turned portions of the first and second coil wire bundles engage; (d) a crossing step of crossing the portion of the first coil wire bundle over the portion of the second coil wire bundle around the pivot; (e) a second turning step of turning the portion of the first coil wire bundle around the pivot relative to the portion of the second coil wire bundle in a direction opposite that in the first turning step; and (f) a moving step of moving the first coil wire bundle relative to the second coil wire bundle to place the first and second coil wire bundles so as to extend substantially parallel to each other in the lengthwise direction thereof to twist the first turned portions of the first and second coil wire bundles together.

The above method is different from that in the first aspect only in that the first and second coil wire bundles, each of which is made up of the plurality of coil wires, are twisted or braided. Therefore, the arrangement step, the engagement step, the first turning step, the crossing step, the second turning step, and the moving step are substantially identical with those in the method of the first aspect.

For example, in the case where the coil assembly is fabricated by three-phase windings which are made by a total of twelve coil wires twisted, the in-slot portions of the first coil wire and the seventh coil wire, the second and eighth coil wires . . . , the fifth and eleventh coil wires, and the sixth and the twelfth coil wires are laid to overlap in the slots. Therefore, when the seven or more coil wires are used to fabricate the coil assembly, it increases the possibility that the coil wires physically interfere with each other or any of the coil wires is caught in the other coil wire during the twisting thereof. The use of the first and second coil wire bundles, each of which is made up of the coil wires twisted and tied up into a desired geometry, alleviates the above problem.

In the preferred mode of the invention, in the arrangement step, the turned portions of the first coil wire bundle are offset from those of the second coil wire bundle by a distance associated with a pitch of the slots of the core.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly toFIGS. 1(a) and1(b), there is shown a stator10which is equipped with a coil assembly20according to the invention.FIG. 1(a) is a perspective view which shows the stator10.FIG. 1(b) is a side view ofFIG. 1(a).FIG. 2is a partially enlarged view which shows an essential part of the coil assembly20.

The stator10, as illustrated inFIGS. 1(a) and1(b), is designed for use in electric rotary machines such as motor-generators for automotive vehicles. The stator10has a rotor (not shown) retained inside an inner periphery thereof to be rotatable. The rotor has a plurality of permanent magnets arrayed on an outer circumference thereof facing an inner circumference of the stator10. The permanent magnets are so oriented as to have S-poles and N-poles arrayed alternately in the circumferential direction of the rotor. The stator10has a stator core12made of annular magnetic steel plates which have a given thickness and are staked in an axial direction of the stator10. The stator core12, as can be seen fromFIG. 2, has a plurality of pairs of slots14and15formed in an entire inner periphery thereof. Specifically, each of the slots14and15extends straight in a widthwise direction (i.e., a vertical direction, as viewed inFIGS. 1(a) and1(b)) of the stator core12and, as can be seen inFIG. 2, occupies only an inside portion of the thickness of the stator core12. The coil assembly20makes up three-phase stator windings. Each of the three-phase windings is disposed in each of the two adjacent slots14and15. The three-phase windings are disposed in the stator core12in units of circumferentially adjacent three of the pairs of the slots14and15.

FIG. 3is a perspective view which shows the coil assembly20.FIG. 4is a front view which shows a coil end of the coil assembly20.FIG. 5is a front view which shows one of coil wires30making up the coil assembly20.FIG. 6is a transverse sectional view which shows the coil wire30.FIG. 7is a perspective view which shows a turned portion42of the coil wire30.

Each of the coil wires30of the coil assembly20has, as illustrated inFIG. 5, the turned portions42arrayed at a given pitch or interval. The coil wire30is, as illustrated inFIG. 6, made of a copper conductor32and an insulating film wrapped about the conductor32to insulate the conductor32electrically. The insulating film includes an inner layer34and an outer layer36. The inner layer34covers the outer periphery of the conductor32fully. The outer layer36covers the outer periphery of the inner layer34fully. A total thickness of the insulating film (including thicknesses of the inner and outer layers34and36) is 100 μm to 200 μm. Such a great thickness of the insulating film eliminates the need for insulating the coil wires30electrically from each other using insulating paper.

The outer layer36is made of insulating material. The inner layer34is made of insulating material such as thermoplastics resin which is higher in glass-transition temperature than the outer layer36or polyamide having no glass-transition temperature. Therefore, when subjected to heat, as produced in the electric rotary machine, the outer layer36melts at an earlier time than the inner layer34, thereby causing portions of the coil wires30disposed in the same slot14to be bonded thermally at the outer layers36. The coil wires30in each of the slots14and15are, therefore, substantially changed into a one-piece winding, thus resulting in an increase in mechanical strength of the coil wires30in the slots14.

Each of the coil wires30includes, as illustrated inFIGS. 2 and 4, in-slot portions40each of which is to be disposed in either of the slots14and15of the stator core12and the turned portions42each of which connects between two of the in-slot portions40, as located away from each other in a circumferential direction of the stator core12, and extends outside either of opposed ends of the stator core12. Each of the coil wires30is so shaped that each of odd ones of the turned portions42are located 180° electric angle out of phase with an adjacent even one around the axial direction of the stator core12.

The middle of each of the turned portions42is, as illustrated inFIG. 7, substantially shaped in the form of a crank44without twisting.

The crank44is, as can be seen inFIG. 3(a), offset in the radial direction of the stator core12(i.e., a vertical direction, as viewed inFIG. 8(a)). The amount of offset of the turned portion42in the radial direction of the stator core12is within the width of the coil wire30, thereby enabling the turned portions42of the coil wires30to be wound tightly without physical interference between the turned portions42arrayed in the radial direction of the stator core12. This results in a decrease in radial width of the coil ends projecting from the ends of the stator core12, thus avoiding the overhanging of the coil assembly20in the radial direction of the stator core12.

Each of the turned portions42also has cranks46which continue directly from the in-slot portions40and extend substantially along and on one of the axial ends13of the stator core12. This causes the interval between ends of a section of the turned portion42which is located away from the end13of the stator core12, in other words, the base of a triangle, as defined by the turned portion42and the end13, to be smaller than the interval between two of the slots14or15in which the in-slot portions40of the turned portion42are disposed, thus resulting in a decrease in height h of the coil ends.

If the length of each of the cranks46of the turned portions42which extend substantially parallel to the end13of the stator core12is defined as d1, and the interval between adjacent two of the slots14and15in the circumferential direction of the stator core12is defined as d2, a relation of d1≦d2is met. This avoids the physical interference of each of the cranks46of the coil wires30with one of the coil wires30(i.e., the turned portions42) extending from an adjacent one of the slots14or15without need for increasing the height of the coil ends in the axial direction of the stator core12or the width of the coil ends in the radial direction of the stator core12, thus avoiding the overhanging of the coil assembly20in the radial direction of the stator core12.

Each of the turned portions42of the coil wires30also has two consecutive cranks48formed between the central crank44and each of the outer cranks46. Specifically, each of the turned portions42extending outside one of the ends13of the stator core12has a total of the seven cranks44,46, and48. This results in a decrease in height h of the turned portions42as compared with when the turned portions42have no cranks. The cranks48are identical in configuration with the cranks44and46and extend substantially parallel to either of the ends13of the stator core12. In other words, each of the turned portions42is shaped stepwise outwardly from the central crack44.

The coil assembly20is, as described above, made of three-phase windings. Each phase has the coil wires30disposed in the two slots14and15per pole of the rotor. In other words, a total number of the slots14and15per pole of the rotor which are located adjacent each other in the circumferential direction of the coil assembly20(i.e., the stator core12) is 3×2=6. The in-slot portions40of each of the coil wires30are, as can be seen inFIG. 2, disposed in two of the slots14or15which are located six slots away from each other. Therefore, in order to avoid the physical interference between the turned portions42of the coil wires30extending outside adjacent two of the slots14and15of the stator core12, each of the turned portions42is preferably designed to have the seven (3×2+1) cranks44,46, and48. This permits the height and/or the width of the coil ends of the coil assembly20to be decreased.

FIGS. 8(a) to13(b) demonstrate a production method of the coil assembly20according to the first embodiment of the invention. The coil assembly20in this embodiment is made by twisting two bundles: a first coil wire bundle50and a second coil wire bundle60each of which is made up of six coil wires30which are twisted or entwined into a given shape basically in the same manner, as described below or in the second embodiment. The first and second coil wire bundles50and60are each tied tightly by a string or a holder from being lost in shape.

FIGS. 8(a),9(a),10(a),11(a),12(a), and13(a) are plan views.FIGS. 8(b),9(b),10(b),11(b),12(b), and13(b) are front views.FIGS. 8(a) and8(b) illustrate the first and second coil wire bundles50and60which have already been twisted partly from left ends of thereof as viewed in the drawing, in the same manner, as described below. The explanation of the production method will start from the illustrated condition for the sake of convenience.

The production method includes an arrangement step, as illustrated inFIGS. 8(a) and8(b), an engagement step, as illustrated inFIGS. 9(a) and (b), a first turning step, as illustrated inFIGS. 10(a) and10(b), a crossing step, as illustrated inFIGS. 11(a) and11(b), a second turning step, as illustrated inFIGS. 12(a) and12(b), and a moving step, as illustrated inFIGS. 13(a) and13(b).

First, in the arrangement step, the first and second coil wire bundles50and60are arranged substantially parallel to each other and shifted or offset, as illustrated inFIGS. 8(a) and8(b), by a given distance (e.g., a pitch of the slots14and15, that is, an interval between adjacent two of the slots14and15) in an axial direction or a lengthwise direction thereof (i.e., a lateral direction inFIGS. 8(a) and8(b)). A left portion of the first coil wire bundle50(which will be referred to as a second turned portion52below) is crossed over a left portion of the second coil wire bundle60(which will be referred to as a first turned portion61below). The first and second coil wire bundles50and60are placed, as viewed in the drawing, substantially parallel to each other. In the illustrated example, the second and first turned portions52and61are collections of the turned portions42, that is, ones of turned portions of the first and second coil wire bundles50and60which have already been twisted, as described above, in a previous one of twisting cycles.

An untwisted portion of the first coil wire bundle50is, as viewed inFIG. 8(b), placed in front of that of the second coil wire bundle60. Either of the first and second coil wire bundles50and60may be located outwardly in the direction in which the first and second coil wire bundles50and60overlap each other. In this example, the first coil wire bundle50will also be referred to as the outwardly located coil wire bundle.

In the engagement step, the first coil wire bundle50is moved, as illustrated inFIGS. 9(a) and9(b), upward parallel to the second coil wire bundle60to establish a mechanical engagement between the second turned portion52of the first coil wire bundle50located on the lower side in the widthwise direction thereof and the first turned portion61of the second coil wire bundle60located on the upper side in the widthwise direction thereof.

In the first turning step, as illustrated inFIGS. 10(a) and10(b), the first coil wire bundle50is turned, as indicated by an arrow a, in a counterclockwise direction, as viewed in the drawing, about the pivot P where the second turned portion52of the first coil wire bundle50engages the first turned portion61of the second coil wire bundle60until the fourth turned portion54of the first coil wire bundle50is spaced apart from the third turned portion63of the second coil wire bundle60by a distance L. The turning of the first coil wire bundle50around the engagement between the second turned portion52and the first turned portion61allows the angle required to braid the first and second coil wire bundles50and60to be decreased. The second coil wire bundle60may alternatively be turned in a clockwise direction relative to the first coil wire bundle50in the same manner, as described above.

In the crossing step, as illustrated inFIGS. 11(a) and11(b), a right portion of the first coil wire bundle50(i.e., the right side of the third turned portion53) is moved behind the second coil wire bundle60so as to cross the third turned portion53of the first coil wire bundle50over the second turned portion62of the second coil wire bundle60. In other words, the first and second coil wire bundles50and60are twisted one time at the pivot P.

In the second turning step, as illustrated inFIGS. 12(a) and12(b), the first coil wire bundle50is turned, as indicated by an arrow b, in the clockwise direction (i.e., the direction opposite to that in the first turning step) about the pivot P to place the first coil wire bundle50substantially parallel to the second coil wire bundle60in the lengthwise direction thereof. The second coil wire bundle60may alternatively be turned in the counterclockwise direction relative to the first coil wire bundle50in the same manner, as described above.

Finally, in the moving step, as illustrated inFIGS. 13(a) and13(b), the first coil wire bundle50is moved down until the first and second coil wire bundles50and60face in parallel to each other, that is, overlap each other fully, as viewed in the drawing. This places the first and second coil wire bundles50and60in a condition where the third turned portion53of the first coil wire bundle50is twisted or entwined with the second turned portion62of the second coil wire bundle60with a portion of the first coil wire bundle50on the right side of the third turned portion53(i.e., the fourth turned portion54or following turned portions) extending behind the second coil wire bundle60.

After the completion of the moving step ofFIGS. 13(a) and13(b), the locations of the untwisted portions of the first and second coil wire bundles50and60are opposite to those inFIGS. 8(a) and8(b). In other words, the untwisted portion of the second coil wire bundle60is located in front of that of the first coil wire bundle50(i.e., below the first coil wire bundle50, as viewed inFIG. 13(a)). Subsequently, the second coil wire bundle60is subjected to the same sequence of steps as inFIGS. 9(a) to13(a) to twist the fourth turned portion54of the first coil wire bundle50and the third turned portion63of the second coil wire bundle60together.

After completion of the above sequence of steps, the untwisted portion of the first coil wire bundle50is located in front of that of the second coil wire bundle60. The first coil wire bundle50is also subjected to the same sequence of steps as inFIGS. 9(a) to13(a) to twist subsequent turned portions of the first and second coil wire bundles50and60together.

The above sequence of steps makes up one twisting cycle. Ones of the turned portions of the first and second coil wire bundles50and60which have been twisted in a previous one of the twisting cycles will be handled as the second turned portion52of the first coil wire bundle50and the first turned portion61of the second coil wire bundle60in a following one of the twisting cycles.

When the total of twelve coil wires30have been twisted in the above manner, the coil wires30are respectively turned or moved parallel to each other to make desired geometry of overlaps of the turned portions42. Ends of the coil wires30are welded to make a given number of connections thereof. Finally, the coil wires30are doughnut-shaped to complete the coil assembly20, as illustrated inFIG. 3.

As apparent from the above discussion, the production method of the coil assembly20twists the first and second coil wire bundles50and60together at the pivot P where one of the turned portions of the first coil wire bundle50intersects and engages a corresponding one of the turned portions of the second coil wire bundle60. This permits the angle which the first coil wire bundle50is turned and makes with the second coil wire bundle60in the first turning step to be decreased as compared with a conventional twisting manner, thus minimizing undesirable deformation of the first and second coil wire bundles50and60.

The use of the first and second coil wire bundle50and60each of which is made up of the six coil wires30tied up to make the coil wires30twisted to fabricate the coil assembly20minimizes the physical interference between the coil wires30or catching of one of the coil wires30in another during twisting thereof and ensures the ease of twisting the coil wires30.

Each of the first and second coil wire bundles50and60may alternatively be made of the number of the coil wires30other than six (6). The coil assembly20may alternatively be produced by using four or more coil wire bundles.

FIGS. 14 to 19demonstrate a production method of the coil assembly20according to the second embodiment of the invention. The production method, as referred to in this embodiment, braids or twists three coil wires70,80, and90to make the coil assembly20. Each of the coil wires70,80, and90is formed by pressing a piece of straight wire to have a plurality of in-slot portions and turned portions, like inFIG. 7.

FIG. 14illustrates the first to third coil wires70,80, and90which have already been twisted partly from left ends of thereof, as viewed in the drawing, in the same manner, as described below. The explanation of the production method of the coil assembly20will start from the illustrated condition for the sake of convenience.

The production method includes a sequence of an arrangement step, as illustrated inFIG. 14, an engagement step, as illustrated inFIG. 15, a first turning step, as illustrated inFIG. 16, a crossing step, as not shown, a second turning step, as illustrated inFIG. 17, and a moving step, as illustrated inFIG. 18.

First, in the arrangement step, the first, second, and third coil wires70,80, and90are arranged substantially parallel to each other and respectively offset, as illustrated inFIG. 14, by a given distance (e.g., approximately the pitch of the slots14and15) in an axial direction or a lengthwise direction thereof (i.e., a lateral direction inFIG. 14extending through the centers of the in-slot portions of the first to third coil wires70to90). In the illustrated example, the turned portions of the first, second, and third coil wires70,80, and90have already been crossed over each other from the left side, as viewed in the drawing, to the turned portions71,81, and91in a previous one of wire twisting cycles which will be described below. The first to third coil wires70to80are placed, as viewed in the drawing, parallel to each other.

Untwisted portions of the first, second, and third coil wires70,80, and90are, as can be seen inFIG. 14, laid to overlap each other in parallel. The untwisted portion of the first coil wire70is located behind those of the second and third coil wires80and90. The untwisted portion of the third coil wire90is located in front of those of the first and second coil wires70and80. The untwisted portion of the second coil wire80is located between those of the first and third coil wires70and90. In this example, the first coil wire70will also be referred to as the most-outwardly located coil in the direction in which the first to third coil wires70to90overlap each other. The first turned portion81of the second coil wire80has already been crossed over the first turned portion91of the third coil wire90frontward, as viewed in the drawing, in the previous wire twisting cycle. The following wire twisting cycle will start to twist or cross the first turned portion71of the first coil wire70over the first turned portions81and91of the second and third coil wires80and90.

In the engagement step, as illustrated inFIG. 15, the first coil wire70is moved upward parallel to the second and third coil wires80and90to establish a mechanical engagement of the turned portion78of the first coil wire70located on the lower side in the widthwise direction thereof, as indicated by P, with the first turned portions81and91of the second and third coil wires80and90located on the upper side in the widthwise direction thereof. Note that the arrangement step in the first wire twisting cycle places the first to third coil wires70to90in parallel to each other in the lengthwise direction thereof, and the engagement step of the first wire twisting cycle lifts the first coil wire70upward until the left end120of the first coil wire70hits or engages the turned portions181and191of the second and third coil wires80and90.

In the first turning step, as illustrated inFIG. 16, the first coil wire70is turned, as indicated by an arrow c, in a counterclockwise direction, as viewed in the drawing, that is, a direction in which a plane extends between the first and second coil wires80and90about the pivot P where the turned portion78of the first coil wire70engages the first turned portions81and91of the second and third coil wires80and91until the turned portion72the first coil wire70is spaced apart from the turned portions of the second and third coil wires80and90by a distance L2. Note that the distance L2is a minimum interval between the turned portion72of the first coil wire70next to the turned portion78defining the pivot P and the second and third coil wires80and90. The turning of the first coil wire70around the engagement (i.e., the pivot P) of the turned portion78of the first coil wire70with the first turned portions81and91of the second and third coil wires80and91allows the angle required to braid or twist the first to third coil wires70to90to be decreased as compared with the conventional manner. The second and third coil wires80and90may alternatively be turned in a clockwise direction relative to the first coil wire70in the same manner, as described above. In the first turning step of the first wire twisting cycle, the first coil wire70is turned in the counterclockwise direction about the engagement of the left end120of the first coil wire70with the turned portions181and191of the second and third coil wires80and90.

In the crossing step, a right portion of the first coil wire70(i.e., the right side of the first turned portion71) is moved ahead of the second and third coil wires80and90around the pivot P to cross the second turned portion72over the second turned portions82and92of the second and third coil wires80and90. In other words, the first to third coil wires70to90are twisted one time at the pivot P.

In the second turning step, as illustrated inFIG. 17, the first coil wire70is turned, as indicated by an arrow d, in the clockwise direction (i.e., the direction opposite to that in the first turning step) about the pivot P to place the first coil wire70substantially parallel to the second and third coil wires80and90in the lengthwise direction thereof. The second and third coil wires80and90may alternatively be turned in the counterclockwise direction relative to the first coil wire70in the same manner, as described above.

In the moving step, the first coil wire70is moved down, as illustrated inFIG. 18, until the first coil wire70face in parallel to the second and third coil wires80and90, that is, they overlap each other fully. This places the first to third coil wires70to90in a condition where the second turned portion72of the first coil wire70is twisted or crossed over the second turned portions82and92of the second and third coil wires80and90. By the above sequence of steps, a portion of the first coil wire70on the right side of the first turned portion71, as viewed inFIG. 18, has been moved from the outside to the inside of those of the second and third coil wires80and90(i.e., from behind the second coil wire80to ahead the third coil wire90).

After the completion of the moving step ofFIG. 18, that is, the wire twisting cycle, the location of the untwisted portion of the first coil wire70relative to those of the second and third coil wires80and90is reverse to that inFIG. 14. Specifically, the untwisted portion of the second coil wire80immediately adjacent the second turned portion82is located most rearward. Subsequently, the second coil wire80is subjected to the same sequence of steps as inFIGS. 14 to 18to tie the third turned portion83of the second coil wire80together with the third turned portions73and93of the first and third coil wires70and90.

After completion of the above sequence of steps, the untwisted portion of one of the first to third coil wires70to90which is located most backward or outward is also subjected to the same sequence of steps as inFIGS. 14 to 18to tie subsequent turned portions of the first to third turned portions70to90together. The above described twisting cycle is repeated several times to braid or twist, as illustrated inFIG. 19, the first to third coil wires70to90completely.

For example, four coil wire bundles each of which is formed by the first to third coil wires70to90twisted in the above manner are prepared. The four coil wire bundles are then twisted in the same manner, as described inFIGS. 14 to 18, to make a set of a total of twelve coil wires70to90. This wire set is processed in the manner, as described in the first embodiment, and doughnut-shaped to complete the coil assembly20, as illustrated inFIG. 3.

The wire set may alternatively be made by preparing and twisting twelve coil wires each of which is identical in shape with the first to third coil wires70to90in the same manner, as described inFIGS. 14 to 18, or by preparing the coil wire bundle(s) and twisting a required number of coil wires around the coil wire bundle(s), in sequence, in the same manner, as described inFIGS. 14 to 18.