Slant winding electromagnetic coil and ignition coil for internal combustion engine using same

Abstract: An electromagnetic coil which may be employed as an ignition coil for an internal combustion engine is disclosed. The electromagnetic coil includes a lower voltage winding portion and a higher voltage winding portion. The lower voltage winding portion is wound around a spool and includes a plurality of winding layers overlapped with each other and inclined at a given angle to the length of the spool. Each of the winding layers includes a collection of turns made up of a leading portion of wire. The higher voltage winding portion is wound around the spool adjacent the lower voltage winding portion and includes a plurality of winding layers overlapped with each other and inclined at a given angle to the length of the spool. Each of the winding layers includes a collection of turns made up of a trailing portion of the wire.

Description: Background of the invention The present invention relates to an electromagnetic coil suitable for use under application of high voltage, and more particularly to an ignition coil which develops high voltage to produce a spark used for ignition purposes in an internal combustion engine. Document JP-A-2151008, on which the preamble of present claim 1 is based, discloses an electromagnetic coil with a winding member having a given length, a first winding portion wound around the first length of the winding member, and a second winding portion wound around a second length of the winding member. A first winding portion includes a plurality of winding layers overlapped with each other and inclined at a given angle to the first length of the winding member, each of the winding layers of the first winding portion including a collection of turns made up of a leading portion of wire. Moreover, the second winding portion includes a plurality of winding layers overlapped with each other and inclined at a given angle to the second length of the winding member continuing from the first length, wherein each of the winding layers of the second winding portion include a collection of turns made up of a trailing portion of the wire. Document GB-A-0 501 830 discloses a similar conventional high tension field coil having first and second winding portions wound around a respective first and second length of a winding member. Again, the first winding portion includes a plurality of winding layers overlapped with each other and inclined at a given angle to the winding member. Documents JP-A-2-18572, JP-A-2-106910 and EP-A-0 142 175 teach further conventional electromagnetic coils. These electromagnetic coils are made up of a plurality of slant winding layers oriented at a given angle to the length of a spool so that each of the slant winding layers presents a circular cone. In the following discussion, this type of electromagnetic coil will be referred to as a slant winding electromagnetic coil. The slant winding electromagnetic coils may be distinguished in the shape of winding layers from typical electromagnetic coils made up of cylindrical winding layers each extending in a lengthwise direction of a bobbin. In such a slant winding electromagnetic coil, since each winding layer, as discussed above, extends radially so as to form a circular cone, the number of turns thereof is smaller than that of each of the cylindrical winding layers. This means that it is possible to decrease the number of turns of adjacent two of the winding layers to decrease a potential difference between the adjacent winding layers, thereby avoiding the dielectric breakdown for realizing an electromagnetic coil suitable for use under application of high voltage. Such an electromagnetic coil is, as discussed in the above publications, suitable for use in an ignition coil for internal combustion engines. Particularly, this type of electromagnetic coil may be employed as a secondary winding for developing high voltage in combination with a primary winding. The results of tests performed by the inventors of this application, however, showed that it was very difficult to arrange slant winding layers on a spool perfectly in an industrial manufacturing process, especially because an automatic winding machine which makes coils at high speeds is usually used in the industrial manufacturing process, and it is necessary to use thin wire for achieving the compact and lightweight structure of a coil. The slant winding requires the formation of a cone-shaped winding using a leading portion of wire to define a reference surface for arranging slant winding layers in a lengthwise direction of a spool. In order to form the cone-shaped winding easily, it is useful to make an irregular winding of a triangle shape in cross section using a leading portion of wire, but a drawback is encountered in that it is difficult to develop a potential difference across each turn of the irregular winding at a constant level. In the slant winding process, winding layers made of a trailing portion of wire may be shifted or crumbled. The turns of wire may be disordered at the end of winding due to a variation in length of a spool, a variation in tensile force acting on the wire during winding, or undesirable insertion of a portion of the wire into a groove formed in a flange provided at an end of the spool for withdrawing an end of the wire. When the above discussed irregular winding or irregularity of the winding caused by the disorder of the turns is included in the slant winding layers, it may cause some of the turns creating high voltages to be arranged adjacent to each other. It thus becomes difficult to estimate and manage the potential difference between the turns so that it is difficult to achieve high insulation expected in the slant winding electromagnetic coils. SUMMARY OF THE INVENTION It therefore is an object of the present invention to provide an electromagnetic coil which can be produced in an industrial manufacturing process and which has a low risk of dielectric breakthrough. According to the present invention this object is accomplished by the features indicated in present independent claim 1. Dependent claims 2 to 16 describe particular embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawings: Fig. 1 is a cross sectional view which shows a secondary winding of an electromagnetic coil according to the present invention; Fig. 2 is a cross sectional view which shows an ignition coil for an internal combustion engine using the electromagnetic coil in Fig. 1; Fig. 3 is a graph which shows a potential distribution of a secondary winding of an electromagnetic coil; Fig. 4 is a partially sectional view which shows a secondary winding according to the second embodiment of the invention; Fig. 5 is a partially sectional view which shows a secondary winding according to the third embodiment of the invention; Fig. 6 is a partially sectional view which shows a secondary winding according to the fourth embodiment of the invention; Fig. 7 is a partially sectional view which shows a secondary winding according to the fifth embodiment of the invention; Fig. 8 is a partially sectional view which shows a secondary winding according to the sixth embodiment of the invention; Fig. 9 is a partially sectional view which shows a secondary winding according to the seventh embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Figs. 1 and 2, there is shown an ignition coil for an internal combustion engine according to the present invention. Note that embodiments, as discussed below, will refer to obliquely overlapped winding layers each consisting of turns of wire arranged uniformly, but, in usual, a winding formed by an automatic winding machine has an inevitable yet allowable irregular turns. The ignition coil 2, as shown in Fig. 2, generally includes a cylindrical transformer 5, a control circuit 7, and a connection 6. The control circuit 7 is disposed on an end of the transformer 5 and selectively turns on and off a primary current flowing through the transformer 5. The connection 6 is disposed on the other end of the transformer 5 and supplies a secondary voltage produced by the transformer 5 to a spark plug (not shown) installed in the engine. The ignition coil 2 includes a cylindrical casing 100 made of a resin material. The cylindrical casing 100 defines a chamber 102 which has disposed therein the transformer 5 and is filled with an insulating oil 29 surrounding the transformer 5 and the control circuit 7. The cylindrical casing 100 also includes a control signal input connector 9 at an upper end of the chamber 102 and a bottom 104 at a lower end of the chamber 102. The bottom 104, as will be discussed later in detail, is closed by the bottom of a metallic cup 15. An outer peripheral wall of the cup 15 is surrounded by the connection 6 formed at the lower end of the casing 100. The connection 6 has formed therein a hollow cylinder 105 for insertion of the spark plug. A rubber-made plug cap 13 is disposed on an end portion of the cylinder 105. The cup 15 is disposed within the bottom 104 of the casing 100 by means of the so-called insert moulding to establish liquid-tight sealing between the chamber 102 and the connection 6. A compression coil spring 17 is retained by the bottom of the cup 15 for electric connection with an electrode of the spark plug inserted into the connection 6. The connector 9 includes a connector housing 18 and three connector pins 19 (only one is shown for the brevity of illustration). The connector housing 18 is integrally formed with the casing 100. The connector pins 19 partially project into the connector housing 18 from the inside of the casing 100. The casing 100 has formed in the upper end an opening 100a for mounting the transformer 5 and the control circuit 7 and injecting the insulating oil into the chamber 102 during assembly of the ignition coil 2. The opening 100a is closed by a metallic cover 33 which is tacked on the upper end of the casing 100. An O-ring 32 is disposed between the cover 33 and the end of the casing 100 for liquid-tight sealing. The transformer 5 includes a cylindrical iron core 502, magnets 504 and 506, a secondary spool 510, a secondary winding 512, a primary spool 514, and a primary winding 516. The iron core 502 is formed with thin silicon steel plates laminated in a circular form. The magnets 504 and 506 are attached to both ends of the iron core 502 using adhesive tape so as to have polarities producing magnetic flux in a direction opposite to that of magnetic flux produced under energization of the coil 2. The secondary spool 510 is made of a resin material and includes, as shown in Fig. 1, a hollow winding cylinder 530, flanges 510a and 510b formed at both ends of the cylinder 530, and a bottom 510c. A terminal plate 34 is disposed on the bottom 510c of the secondary spool 510 and electrically connected to a lead (not shown) extending from an end of the secondary winding 512. A spring 27 is mounted on the terminal plate 34 in engagement with the cup 15. The terminal plate 34 and the spring 27 work as a spool side conductor so that a high voltage developed across the secondary winding 512 is applied to the electrode of the spark plug through the terminal plate 34, the spring 27, the cup 15, and the spring 17. A cylinder 510g is formed on an end of the secondary spool 510 opposite to the bottom 510c in a coaxial relation with the secondary spool 510. The secondary spool 510 has therein a chamber within which the iron core 502 and the magnet 506 are disposed. The secondary winding 512 is wound around the periphery of the winding cylinder 530 of the secondary spool 510 in a manner, as will be described later in detail. The primary spool 514 is formed with a hollow cylinder which has flanges 514a and 514b formed at both ends thereof and is closed at an upper end by a cover 514c. Wound around the periphery of the primary spool 514 is the primary winding 516. The cover 514c of the primary spool 514 has formed thereon an annular portion 514f which extends downward as viewed in the drawing and is disposed within the cylinder 510g of the secondary spool 510 coaxially therewith. The cover 514c also has formed in the center thereof an opening 514d. Upon assembling of the primary spool 514 and the secondary spool 510, the iron core 502 having disposed on both ends thereof the magnets 504 and 506, is retained between the cover 514c of the primary spool 514 and the bottom 510c of the secondary spool 510. An auxiliary core 508 is disposed around the primary winding 516 wound around the primary spool 514. The auxiliary core 508 is made of a cylindrical silicon steel plate rolled so as to form a gap or slit between both side edges thereof which extends from the periphery of the magnet 504 to the periphery of the magnet 506. This reduces a short-circuit current flowing in a circumferential direction of the auxiliary core 508. The chamber 102 stores therein the insulating oil 29 with an air gap at the upper end portion thereof. The insulating oil 29 enters the lower opening of the primary spool 514, the opening 514d formed in the center of the cover 514c of the primary spool 514, the upper opening of the secondary spool 510, and given openings (not shown) to electrically insulate the iron core 502, the secondary winding 512, the primary winding 516, and the auxiliary core 508 from each other. The secondary winding 512, as shown in Fig. 1, consists of wire 520 covered with an insulating film made of amide imide. The material of the insulating film may alternatively be urethane or polyester imide. The wire 520 is wound 16,000 times coaxially around the winding cylinder 530 of the secondary spool 510 in a slant direction relative to the length of the secondary spool 510 so that a plurality of winding layers are obliquely overlapped with each other. In other words, the wire 520 is wound around the winding cylinder 530 so that each of the winding layers defines a conical surface decreased in diameter as reaching from the flange 510a to the flange 510b. The reason that a total number of turns of the secondary winding 512 is 16,000 is because the secondary voltage determined by the turns ratio of the primary winding 516 to the secondary winding 512 requires 30kV for producing an ignition arc at the spark plug. A maximum diameter of the wire 520 including the thickness of the insulating film is 0.07mm. The length of the winding cylinder 530 in an axial direction thereof is 61.5mm. The secondary winding 512 consists of three major portions: a first winding portion 531, a second winding portion 532, and a third winding portion 533. The first winding portion 531 consists of a collection of lower voltage winding layers overlapped in the form of a cone. Specifically, in a cross sectional view of Fig 1, the first winding portion 531 corresponds to a right triangle defined by a leftmost outer winding turn 531a close to an inner wall of the flange 510a, an innermost winding turn 531b of the same winding layer as the winding turn 531a, and a leftmost inner winding turn 531c close to a corner between the winding cylinder 530 and the flange 510a. Similarly, the third winding portion 532 consists of a collection of higher voltage winding layers in the form of a cone. Specifically, in Fig. 1, the third winding portion 532 corresponds to a triangle defined by a winding turn 521b close to a corner between the flange 510b and the winding cylinder 530, an uppermost winding turn 521c of the same winding layer as the turn 521c, and the inner wall of the flange 510b. The second winding portion 532 consists of a collection of middle voltage winding layers arranged between the first winding portion 531 and the third winding portion 533. The potential difference developed across one turn of the secondary winding 512 assumes a potential distribution as shown in Fig. 3. As apparent from the drawing, the first winding portion 531 including a leading portion of the wire 520 creates a potential difference of about 2.5V every turn, and the potential difference every turn is increased as the number of turns is increased. The third winding portion 533 including a trailing portion of the wire 520 creates a potential difference of 15V to 16V. Specifically, a boundary portion between the second winding portion 532 and the third winding portion 533 and the third winding portion 533 develop the high voltage. The potential difference appearing across adjacent two of turns of the secondary winding 512, for example, the turn 521a and the turn 521b arranged in the lengthwise direction of the secondary spool 510 may be determined using the potential distribution in Fig. 3 and the number of turns of the wire 520 over adjacent winding layers 522 ranging from the turn 521a to the turn 521b. Specifically, the potential difference appearing across the turns 521a and 521b may be determined by multiplying the potential difference V developed across one turn, as derived from Fig. 3, by the number of turns n of the wire 520 over the adjacent winding layers 522 (i.e., V × n). An upper limit of the number of turns t H of adjacent two of the winding layers of the secondary winding 512 showing a maximum potential difference in the potential distribution of the secondary winding 512 may be expressed by the following equation. where n T is a total number of turns of the secondary winding 512 and V OUT is the voltage outputted by the secondary winding 512. t H ≤ n T / V OUT × 180 where n T is a total number of turns of the secondary winding 512 and V OUT is the voltage outputted by the secondary winding 512. From the equation (1), the number of turns t H of the adjacent winding layers 522 creating a maximum potential difference in the potential distribution of the secondary winding 512 will be less than or equal to about 96 since n T = 16,000 and V OUT = 30kV. Thus, a maximum potential difference Vmax developed across the adjacent winding layers 522 is 16(V) × 96 = 1,536(V). Specifically, the number of turns t H of the adjacent winding layers 522 is set to a value determined by the above equation (1) so that the potential difference appearing across the turns 521a and 521b shows about 1.5kV. The reasons for this may be summarized according to three points below. (1) Usually, the dielectric strength of amide imide used as the insulating film of the wire 520 is 3.0V to 4.0V in terms of a.c. voltage, while it is 6.5V to 8.0V in terms of d.c. voltage. For example, if the insulating film made of amide imide is subjected to intense heat of 150°C for 2000 hours, it will cause the dielectric strength thereof to be decreased to about 70%. Specifically, when the ignition coil 2 is used in an internal combustion engine, the dielectric strength of the insulating film is decreased to about 4.5kV to 5.5kV in terms of d.c. voltage. (2) The winding layers may be shifted or the arrangement of winding turns may be disordered during winding of the wire 520 around the secondary spool 514. For example, if a maximum diameter of the wire 520 is 0.05mm to 0.08mm, a winding pitch P 1 , as shown in Fig. 1, is two to four times the diameter of the wire 520, test results derived by the inventors of this invention showed that it was necessary to provide a safety factor of more than about three times the potential difference developed across adjacent two of the winding layers in view of the shifting of the winding layers and the disorder of the arrangement of the winding turns. (3) Having regard to the safety factor as discussed above, the dielectric strength of the wire 520, which would be decreased to about 4.5kV to 5.5kV when it is used under environmental conditions as mentioned above, needs to be considered as being decreased to about 1.5kV which is one-third of 4.5kV. it will thus be appreciated that the dielectric strength between the winding turns 521a and 521b of the adjacent winding layers 522 showing the maximum potential difference in the third winding portion 533 of the secondary winding 512 is about 1.5kV. Thus, it is advisable that the number of turns of the adjacent winding layers 522 be so determined that the potential difference Vmax appearing across the adjacent winding layers shows about 1.5kV. Therefore, in this embodiment, the wire 520 is wound in the third winding portion 533 so that a maximum number of turns, that is, the number of turns of the adjacent winding layers 522 is less than or equal to the number of turns t H determined by the equation (1), and the remaining winding layers are decreased in diameter as the flange 510b (i.e., the end of the secondary winding 512) is reached. The height of the adjacent winding layers 522 from the outer surface of the winding cylinder 530 in a radial direction of the third winding portion 533 is determined by the angle  at which the winding layers are oriented to the periphery of the winding cylinder 530 and the number of turns t H . The first winding portion 531 has a uniform height in a radial direction thereof which is established by setting the number of turns of adjacent two of the winding layers to a constant value. The second winding portion 532 between the first winding portion 531 and the third winding portion 533 has a tapered profile which is defined by winding the wire 520 so that outermost winding turns lie along a line extending from an outermost winding turn of the first winding portion 513 adjacent to the second winding portion 532 to an outermost winding turn of the third winding portion 533 adjacent to the second winding portion 532. In other words, the diameter of the second winding portion 532 is decreased at a given rate from the first winding portion 531 to the third winding portion 533. The number of turns of adjacent two of the winding layers in each of the second and third winding portions 532 and 533 will be greater than 96 when the number of turns of the adjacent winding layers 522 of the third winding portion 533 is set to a maximum number of turns (i.e., 96) determined by the equation (1), but all of the winding portions 531, 532, and 533 may alternatively be less than 96 in number of turns of adjacent two of the winding layers. The beneficial results in a winding process produced by locating the third winding portion 533 close to the flange 510b will be discussed below. In a turning point of the wire 520 on the periphery of the secondary spool 510, that is, a turning point from an innermost winding turn of the winding layer 520a, as indicated by black circles in Fig. 1, to an innermost winding turn of the winding layer 520b, as indicated by white circles, a tensile force produced inward in the radial direction of the third winding portion 533 and a sliding force produced when the wire 520 is being wound obliquely in an inward direction will act on the wire 520, thereby causing the wire 520 to be shifted in an advancing direction, but these forces are absorbed by the flange 510b, preventing the wire 520 from being disordered. The same is true for a turning point from an innermost winding turn of the winding layer 520a to an innermost winding turn of the winding layer 520b. According to the above first embodiment, a margin for degradation in dielectric strength of the insulating film of the wire 520 caused by use under high temperature environmental conditions is produced by setting the number of turns of the adjacent winding layers 522 developing the highest potential difference in the third winding portion 533 of the secondary winding 512 to a value less than or equal to a maximum value (i.e., 96) determined by the above equation (1). Specifically, this provides a safety factor of three times the degradation in dielectric strength of the insulating film of the wire 520 caused by the shifting of the wire 520 or disorder thereof, thereby establishing a sufficient dielectric strength of the wire 520 having a maximum diameter of 0.07mm in use of the ignition coil 2 in an internal combustion engine. Additionally, the number of turns is increased gradually from the third winding portion 533 to the first winding portion 531. The performance of the ignition coil 2 is thus enhanced greatly as compared with when the number of turns of each of the first and second winding portions 531 and 532 is equal to that of the third winding portion 533. While, in the above embodiment, the output voltage V out of the secondary winding 520 is 30kV, and the total number of turns t r of the secondary winding 520 is 16,000, only the output voltage V out may be changed to 35kV. In this case, the number of turns t H of the adjacent winding layers 522 developing the highest potential difference in the secondary winding 512 is given by an equation below. t H ≤ n T / V OUT × 155 In order to further improve dielectric withstanding ability of the ignition coil 2, the following equation may alternatively be used. t H ≤ n T / V OUT × 100 The equation (3) allows, for example, inexpensive urethane resin whose dielectric strength is smaller than that of polyamide imide to be used as the insulating film of the wire 520, thereby resulting in decreased manufacturing costs of the ignition coil 2. The dielectric withstanding ability of the secondary winding 512 may further be improved by decreasing a constant in the above equations, but the decrease in constant will cause the space factor of the secondary winding 512 to be decreased. Specifically, in order to obtain a given number of turns of the secondary winding 512 with a decreased space factor, it is necessary to prolong an axial length of the secondary spool 510. This increases the overall length of the ignition coil 2. It is therefore advisable that a lower limit of the constant in the above equations be determined in view of installation of the ignition coil 2 in a plug hole of an engine block. For instant, when the lower limit of the constant is 40, it provide an appropriate safety factor of the dielectric withstanding ability to the secondary winding 512, but it becomes difficult to install the ignition coil 2 in the engine for an increased size thereof. Fig. 4 shows the second embodiment of the secondary winding. The same reference numbers as employed in the above embodiments refer to the same parts, and explanation thereof in detail will be omitted here. In this embodiment, the number of turns of adjacent two of winding layers creating the highest potential difference in the secondary winding 630 is determined by the above equation (1). The wire 520 is wound obliquely around the secondary spool 510 in the same manner as in the first embodiment. The secondary winding 630 consists of first, second, and third winding portions 630a, 630b, and 630c. The first and the third winding portions 630a and 6530c have uniform diameters, respectively. The second winding portion 630b is decreased in number of turns at a constant rate from the first winding portion 630a to the third winding portion 630c. Specifically, the second winding portion 630b is of a tapered or conical shape. In the second embodiment, the length of the tapered second winding portion 630b is shorter than a total length of the tapered winding portions 532 and 533 of first embodiment, thereby allowing an operational control program of an automatic winding machine to be simplified. Fig. 5 shows the third embodiment of the secondary winding. The same reference numbers as employed in the above embodiments refer to the same parts, and explanation thereof in detail will be omitted here. The secondary winding 640, as can be seen from the drawing, includes six stepped windings 640a, 640c, 640e, 640g, 640i, and 640m and five tapered connection windings 640b, 640d, 640f, 640h, and 640j. Each of the stepped windings 640a to 640m has a constant diameter. The number of turns of adjacent two of winding layers creating the highest potential difference in the secondary winding 640 (i.e., adjacent winding layers extending from the periphery of the stepped winding 640m to a corner between the flange 510b and the outer surface of the winding cylinder 530) is determined by the above equation (1). The other stepped windings 640a to 640i are increased in diameter (i.e, the number of turns) in a stepwise fashion as reaching the flange 510a (i.e., the lower voltage side). The connection windings 640b to 640j connect adjacent two of the stepped windings 640a to 640m, respectively. The above structure of the secondary winding 640 increases the space factor thereof as compared with the third embodiment. This allows the number of turns of each of the primary winding 516 (see Fig. 2) and the secondary winding 640 to be increased for increasing the output voltage of the secondary winding 640. Fig. 6 shows the fourth embodiment of the secondary winding. The same reference numbers as employed in the above embodiments refer to the same parts, and explanation thereof in detail will be omitted here. The secondary winding 650 is decreased in diameter (i.e., the number of turns) at a varying rate from the flange 510a to the flange 510b so as to present a curved profile which is tapered at a rate increasing as the flange 510b is reached. Specifically, the number of turns of adjacent two of all winding layers is determined according to the equation (1) using the potential difference developed across one turn every number of turns, as shown in Fig. 3. This structure improves the space factor of the secondary winding 650 while optimizing the dielectric withstanding ability thereof. Fig. 7 shows the fifth embodiment of the secondary winding. The same reference numbers as employed in the above embodiments refer to the same parts, and explanation thereof in detail will be omitted here. The secondary winding 660 is decreased in diameter (i.e., the number of turns) at a constant rate from the flange 510a to the flange 510b to assume a frusto-conical profile. The number of turns of adjacent two of winding layers creating the highest potential difference in the secondary winding 660 is determined by the above equation (1). Fig. 8 shows the sixth embodiment of the secondary winding. The same reference numbers as employed in the above embodiments refer to the same parts, and explanation thereof in detail will be omitted here. The sixth embodiment is designed for applying the high voltage to two spark plugs through both ends of the secondary windings 670. Specifically, the secondary winding 670 consists of two higher voltage winding portions 670a and 670c and one lower voltage winding portion 670b. The lower voltage winding portion 670b is located at substantially the center of the secondary spool 510 in a lengthwise direction and has a constant diameter. The higher voltage winding portions 670a and 670c are decreased in diameter from the lower voltage winding portion 670b in opposite directions. The number of turns of adjacent two of winding layers creating the highest potential difference in the secondary winding 670 is determined according to the above equation (1). Fig. 9 shows the seventh embodiment of the secondary winding which presents substantially the same profile as that in the first embodiment, but is different therefrom in shape of the secondary spool 510 and in that a winding arrangement of turns of a trailing portion of the wire 520 is more regular than that of a leading portion of the wire 520 in a coaxial direction. The same reference numbers as employed in the above embodiments refer to the same parts, and explanation thereof in detail will be omitted here. The winding cylinder 530 of the secondary spool 510 extends straight along the longitudinal center line of the secondary spool 510 without any partitions. The secondary spool 510 has the flanges 510a and 580a at both ends thereof. The flange 580a is located on the winding end side and has a flared or conical inner surface 580b oriented at a given obtuse angle of 6 to the periphery of the winding cylinder 530 (i.e., the longitudinal center line of the secondary spool 510). The conical shape of the flange 580a serves to prevent winding turns made of the trailing portion of the wire 520 from being disordered. Usually, a gap may be foamed in a winding end portion due to variations in length of a spool and in tensile force acting on a wire during a winding process. The conical surface 580b of the flange 580a alleviates this problem. Specifically, the conical surface of the flange 580a serves to hold an arrangement of turns of a high voltage winding portion adjacent to the flange 580a, thereby assuring high insulation thereof. The flange 580a has formed therein a groove 580c for withdrawing the trailing portion of the wire 50 outside the secondary spool 510. The groove 580c extends from an edge of the flange 580a to a location above an outermost turn of the wire 520 close to the conical surface 580b for preventing turns of the wire 520 close to the flange 580a from being pushed out of the secondary spool 510. This avoids shifting of the winding layers of the secondary winding 512. An inclined surface 580e is defined as a reference surface for slant winding of the wire 50 by an irregular winding portion 580d which is formed by an automatic winding machine. The irregular winding portion 580d is of a triangular shape in cross section defined by an outer surface of the winding cylinder 530 and an inner surface of the flange 510a and consists of a collection of turns wound irregularly. The inclined surface 580e thus facilitates easy winding of the wire 520 in the slant direction throughout the length of the secondary spool 510. The left end portion, as viewed in the drawing, of the secondary winding 512 is designed so as to create lower voltage through the ignition coil 2 similar to the above embodiments. Specifically, a leading edge of the irregular winding portion 580d is connected to a power source (i.e., 12V) for the ignition coil 2. Thus, a potential difference developed across the irregular winding portion 580d is relatively low, thereby preventing dielectric withstanding and insulating abilities of the secondary winding 512 from being degraded greatly. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate a better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modification to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims. For example, the winding direction of each winding layer of the secondary winding in the above embodiments is reversed between adjacent two of the winding layers, however, it may be oriented in the same direction (i.e., one of inward and outward directions). Additionally, the wire is wound from the periphery of the secondary winding to the outer surface of the secondary spool and vice versa in the above embodiments, however, it may be returned from the middle of an adjacent winding layer. In other words, the number of turns of one winding layer may be decreased alternately.

Claims: An electromagnetic coil comprising: a winding member (530) having a given length; a first winding portion (531) wound around a first length of said winding member (530); and a second winding portion (532) wound around a second length of said winding member (530), wherein said first winding portion (531) includes a plurality of winding layers overlapped with each other and inclined at a given angle () to the first length of said winding member, each of the winding layers of said first winding portion (531) including a collection of turns made up of a leading portion of wire; and said second winding portion (532) includes a plurality of winding layers overlapped with each other and inclined at said given angle () to the second length of said winding member (530) continuing from the first length, each of the winding layers of the second winding portion (532) including a collection of turns made up of a trailing portion of the wire, the potential difference across one turn increases from said first winding portion (531) to said second winding portion (532), characterized in that the winding layers of said first winding portion (531) and said second winding portion (532) are arranged along the length of said winding member (530) so as to define a conical surface tapered decreased in diameter as reaching from said first winding portion (531) to said second winding portion (532). An electromagnetic coil as set forth in claim 1, characterized in that an irregular winding portion is provided in said first winding portion, said irregular winding portion being formed with turns of the wire due to a variation in length of a spool, a variation in tensile force acting on the wire during winding, or undesirable insertion of a portion of the wire into a groove formed in a flange provided in the spool. An electromagnetic coil as set forth in claim 1, characterized in that the electromagnetic coil (512) is a high voltage developing coil which develops a high voltage through electromagnetic induction, and wherein said second winding portion (532) includes adjacent two of the winding layers which have the number of turns t H given by the following equation: t H ≤ n T / V OUT x 180 where n T is a total number of turns of said first and second winding portions, and V OUT is an output voltage outputted by the electromagnetic coil. An electromagnetic coil as set forth in claim 1, characterized in that said second winding portion (532) is smaller in diameter than said first winding portion (531). An electromagnetic coil as set forth in claim 1, characterized in that said second winding portion (532) is decreased in diameter with respect to said first winding portion (531) at a given rate. An electromagnetic coil as set forth in claim 1, characterized in that said winding member (530) is formed with a spool having formed at an end thereof a flange (580a) which has a tapered surface engaging said second winding portion (532). An electromagnetic coil as set forth in claim 6, characterized in that said tapered surface of said flange (580a) is oriented at an obtuse angle () to a longitudinal center line of said spool. An electromagnetic coil as set forth in claim 1, characterized in that said winding member (530) is formed with a spool having formed at an end thereof a flange (510b) engaging said second winding portion (532), the flange (510b) having formed therein an opening through which the trailing portion of the wire passes, the opening being located in a radial direction of the spool above an outer peripheral portion of an end of said second winding portion (532) engaging the flange (510b). An electromagnetic coil as set forth in claim 8, characterized in that the opening is formed with a groove (580c) extending inward from an outer peripheral portion of the flange (580a). An electromagnetic coil as set forth in claim 3, characterized in that the adjacent two of the winding layers of said second winding portion (532) has the number of turns t H given by the following equation: t H ≤ n T / V OUT x 100 An electromagnetic coil as set forth in claim 3, characterized in that a diameter of said first winding portion (531) is greater than that of said second winding portion (532). An electromagnetic coil as set forth in claim 3, characterized in that the number of turns of each of the winding layers of said second winding portion (532) is smaller than that of said first winding portion (531). An electromagnetic coil as set forth in claim 11, characterized in that a diameter of each of the winding layers of said first winding portion (531) and said second winding portion (532) is decreased at a given rate from the first winding portion to the second winding portion. An electromagnetic coil as set forth in claim 13, characterized in that the winding layers of said first winding portion (531) and said second winding portion (532) are arranged so as to define a tapered profile. An electromagnetic coil as set forth in claim 13, characterized in that a profile defined by the winding layers of said first winding portion (531) and said second winding portion (532) is changed in a stepwise fashion. An electromagnetic coil as set forth in any of the preceding claims, characterized in that the electromagnetic coil (512) is a secondary winding of an ignition coil for an internal combustion engine.

Inventors: KAWANO KEISUKE, KOJIMA MASAMI, NAKAZAWA KATSUMI, OOSUKA KAZUTOYO, SATOU YOSHITAKA, SUGIURA AKIMITSU, KAWANO, KEISUKE, KOJIMA, MASAMI, NAKAZAWA, KATSUMI, OOSUKA, KAZUTOYO, SATOU, YOSHITAKA, SUGIURA, AKIMITSU
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1996
Main classification: H01F27
IPC codes: H01F41, H01F27, H01F38