EP0318613A1

EP0318613A1 - High-voltage transformer and method for making same

Info

Publication number: EP0318613A1
Application number: EP87117994A
Authority: EP
Inventors: Tomokazu Himeji Seisakusho Umezaki
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1987-12-04
Filing date: 1987-12-04
Publication date: 1989-06-07
Anticipated expiration: 2007-12-04
Also published as: DE3781692D1; EP0318613B1; DE3781692T2

Abstract

A high-voltage transformer (101) such as an ignition coil for internal combustion engines and a method for making same are disclosed. A primary coil (3) and a secondary coil (4) are accommodated in a coil case (2) with the secondary coil disposed around the primary coil. An iron core having a plurality of iron core sections (610) is disposed in the coil case so as to surround the primary and secondary coils. Each of the iron core sections comprises a pair of first (612) and second (611) core members having a substantially similar configuration such as a channel form, an L-shaped form or the like. The first and second core membres are assembled in such a manner that the first members are in abutting engagement at one end thereof with the second members at one end thereof, but in a spaced face-to-face relation at the other end thereof with the second members at the other end thereof with a limited gap (613) formed therebetween. A resin (7) is filled into the coil case for electrically insulating the primary and secondary coils from the iron core as well as firmly securing them to the coil case.

Description

1. Field of the invention

This invention relates to a high-voltage transformer such as an ignition coil for internal combustion engines and a method for making the same.

2. Description of the Prior Art

Fig. 1 is a plan view showing a conventional high-voltage transformer for internal combustion engines, and Fig. 2 is a cross sectional view taken along line II-II of Fig.1. In these figures, the high-voltage transformer 1 in the form of an ignition coil illustrated comprises a generally cylindrical coil case 2 formed of a synthetic resin, a primary coil 3 in the coil case 2, a secondary coil 4 disposed in the coil case 2 so as to surround the primary coil 3, a cylindrical sleeve 5 disposed in and fixedly mounted on the coil case 2 substantially at the center thereof for receiving an illustrated rotary shaft of a distributor, and an iron core 6 disposed in the coil case 2 around the sleeve 5 so as to surround the primary coil 3 and the secondary coil 4.
The iron core 6 comprises an annular inner or central leg portion 601 disposed around the cylindrical sleeve 5 and radially inside the primary coil 3, four planar outer leg portions 602 disposed radially outside the secondary coil 4, a pair of first (or lower) and second (or upper) cross-shaped arm portions 603 and 604 interconnecting the inner and outer leg portions 601 and 602 for forming a substantially closed magnetic path which passes through the primary and secondary coils 3, 4 when these coils are energized. The annular inner leg portion 601 is in contact at its opposite ends with the inner surfaces of the lower and upper arm portions 603, 604. The outer leg portions 602 are slightly shorter than the inner leg portions 601 so that they are in contact at their lower end with the cross-shaped lower arm portions 603 but spaced from the upper arm portion 604 with a limited gap 605 formed therebetween.
A resin 7 is filled into the coil case 2 and impregnated into the spaces between the coils 3,4 and the iron core 6 for electrically insulating the coils 3, 4 and the iron core 6 from each other as well as for firmly securing or bonding them to the coil case 2. In this case, the lower arm portion 603 of the iron core 6 is moulded integrally with or otherwise firmly connected with the coil case 2, and it is exposed to the outside of the coil case 2 for dissipating heat which is generated during moulding of the coil case 2.
The above-described conventional high-voltage transformer 1 is produced in the following manner. First, the lower cross-shaped arm portion 603 of the iron core 6 is disposed in and integrally moulded or otherwise firmly connected with the coil case 2, and then the annular inner leg portion 601, the primary coil 3, the secondary coil 4 and the outer leg portions 602 are disposed in the coil case 2. Thereafter, the upper arm portion 604 of the iron core 6 is placed on the inner and outer leg portions 601, 602, and the resin 7 in a molten state is filled into the coil case 2 up to a predetermined level, impregnated into the spaces between the above members and solidified to firmly install these members in the coil case 2. In this manner, the resin 7 thus impregnated serves not only for securing the members to the coil case 2 but also for improving the electrical insulation therebetween.
The conventional high-voltage transformer 1 as constructed above operates as follows. As shown by an electric circuit in Fig. 3, current supplied from a battery 10 to the primary coil 3 is interrupted by an electronic circuit device 11 at an appropriate timing so that a high voltage is produced at the secondary coil 4 for sparking an ignition plug (not shown).
With the conventional high-voltage transformer 1 as described above, the iron core 6 has the cross-shaped lower and upper arm portions 603 and 604 each of which is composed of a stack of cross-shaped flat plates vertically piled one over another. Due to this construction, eddy-current losses are induced in the lower and upper arm portiosn 603 and 604 at their center. Further, such eddy-current losses are also induced in other portions of the iron core 6 such as the junctions between the inner and outer leg portions 601 and 602 and the cross-shaped lower and upper arm portions 603 and 604 where these core portions are joined with or abut against each other. As a result, there arises a problem in that the high voltage developed at the secondary coil 4 upon interruption of the current supplied to the primary coil 3 is reduced.
In addition, since the iron core 6 is composed of the annular inner leg portion 601, the planar outer leg portions 602 and the cross shaped lower and upper arm portions 603 and 604 all of which are of different configurations, the number of component parts of the iron core 6 is relatively large, and hence fabrication and assembly of the respective component parts are inefficient and costly.

SUMMARY OF THE INVENTION

The present invention is intended to obviate the above-described problems of the prior art.
An object of the present invention is to provide a high-voltage transformer and a method of making the same in which eddy-current losses in the iron core can be substantially avoided so that a voltage of a much higher magnitude is developed at a secondary coil.
Another object of the present invention is to provide a high-voltage transformer in which the respective component parts of an iron core have a simple and substantially uniform configuration thereby to improve productivity or assemblability of the entire iron core.
A further object of the present invention is to provide a high-voltage transformer and a method for making the same in which the number of junctions between the iron core component parts is greatly reduced to improve heat transmission therebetween, thereby expediting heat dissipation from the surfaces of the iron core which are exposed to the ambient air.
In order to achieve the above object, according to one aspect of the present invention, there is provided a high-voltage transformer which comprises:
a coil case;
a cylindrical sleeve in the coil case;
a primary coil disposed around the cylindrical sleeve in the coil case;
a secondary coil disposed in the coil case around the primary coil;
an iron core having a plurality of iron core sections disposed in the coil case so as to surround the primary and secondary coils, each of the iron core sections comprising a pair of first and second core members having a substantially similar configuration, the first and second core members being assembled in such a manner that the first members are in abutting engagement at one end thereof with the second members at one end thereof, but in a spaced face-to-face relation at the other end thereof with the second members at the other end thereof with a limited gap formed therebetween; and
a resin filled into the coil case for electrically insulating the primary and secondary coils from the iron core as well as firmly securing them to the coil case.
According to another aspect of the present invention, there is provided a method for making a high-voltage transformer which comprises the steps of:
moulding a plurality of first core members integrally with a coil case in such a manner that the first core members are vertically disposed in the coil case and integrally connected with the bottom portion of the coil case;
disposing a primary coil and a secondary coil on the first core members in the coil case with the secondary coil disposed around the primary coil;
placing a plurality of second core members on the first core members in such a manner that the second core members cooperate with the first core members to form iron core sections each of which surrounds the primary and secondary coils; and
filling a resin into the coil case so that the resin is impregnated inbetween the primary and secondary coils and the iron core members and solidified to electrically insulate the primary and secondary coils from the iron core as well as to secure them to the coil case.
According to a further aspect of the present invention, there is provided a method for making a high-voltage transformer which comprises the steps of:
fitting a secondary coil around a primary coil;
assembling a plurality of first and second core members onto the primary and secondary coils from the inside and the outside, respectively, to form iron core sections each of which surrounds the primary and secondary coils;
placing the primary and secondary coils and the iron core sections thus assembled into a coil case; and
filling a resin into the coil case so that the resin is impregnated inbetween the primary and secondary coils and the iron core and solidified to electrically insulate the primary and secondary coils from the iron core as well as to secure them to the coil case.
The above and other objects, features and advantages of the present invention will be more readily apparent from the following detailed description of a few preferred embodiments thereof when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a plan view of a conventional high-voltage transformer such as an ignition coil for internal combustion engines;
Fig. 2 is a cross sectional view taken on line II-II of Fig. 1;
Fig. 3 is a circuit diagram showing an electric circuit to which the transformer of Figs. 1 and 2 is electrically connected;
Fig. 4 is a plan view of an ignition coil for internal combustion engines in accordance with one embodiment of the present invention;
Fig. 5 is a cross sectional view taken on line V-V of Fig. 4;
Fig. 6 is a graphic representation showing the relationship betwen the primary coil interruption current and the secondary coil voltage;
Fig. 7A is a schematic view of an iron core in accordance with another embodiment of the present invention;
Fig. 7B is a view similar to Fig. 7A, but showing a modified form of iron core;
Fig. 8 is a plan view of a high-voltage transformer for internal combustion engines in accordance with a further embodiment of the present invention and
Fig. 9 is a cross sectional view taken on line IX-IX of Fig. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference to a few presently preferred embodiments thereof as illustrated in the accompanying drawings. In the following description and the figures of the accompanying drawings, the same reference numerals as those employed in Figs. 1 and 2 designate the same or corresponding parts or members.
Referring first to Figs. 4 and 5, there is shown a high-voltage transformer 101 in the form of an ignition coil for internal combustion engines which is constructed in accordance with a first embodiment of the present invention. The transformer 101 of this embodiment is substantially similar in construction to the conventional transformer as illustrated in Figs. 1 and 2 except for the following. Namely, the transformer 101 of this embodiment has an iron core 6′ which comprises a plurality (four in the illustrated embodiment) of iron core sections 610 which are disposed around a cylindrical sleeve 5 so as to form a cross-shaped configuration. Each of the iron core sections 610 is composed of a pair of first (or upper) and second (or lower) channel-shaped core members 611 and 612 which are of one and the same construction and configuration, and each of which is formed of a plurality of channel-shaped flat iron plates which are stacked side by side. Each of the upper and lower core members 611 and 612 has a vertically extending planar inner leg portion 611a or 612a which is disposed around the cylindrical sleeve 5 and radially inside a primary coil 3, a vertically extending planar outer leg portion 611b or 612b which is disposed radially outside a secondary coil 4, and a laterally extending intermediate arm portion 611c or 612c interconnecting the inner and outer leg portions 611a and 611b or 612a and 612b. The outer leg portion 611b or 612b is slightly shorter than the inner leg portion 611a or 612a so that when the lower and upper core members 611 and 612 are assembled to form an iron core section 610, the lower end surface of the inner leg portion 611a of the upper core member 611 is placed in abutting engagement with the upper end surface of the inner leg portion 612a of the lower core member 612, whereas the lower end surface of the outer leg portion 611b of the upper core member 611 is placed in a spaced face-to-face relation with the upper end surface of the outer leg portion 612b of the lower core member 612 with a limited gap 613 formed therebetween. Thus, a substantially closed magnetic path is formed through these upper and lower core members 611 and 612 when the primary and secondary coils 3 and 4 are energized.
The above-described high-voltage transformer 101 is fabricated or assembled in the following manner. First, the channel-shaped lower core members 612 are integrally moulded with or otherwise fixedly connected with the coil case 2 in a cross form with the bottom or lower surfaces thereof exposed to the outside, and then the primary coil 3 and the secondary coil 4 are concentrically placed in the coil case 2 on the lower core members 612 such that the primary coil 3 is disposed around the inner leg portiosn 612a of the lower core member 612 with the secondary coil 4 arranged to surround the primary coil 3.
Thereafter, the channel-shaped upper core members 611 are placed on the lower core members 612 such that the lower surfaces of the inner leg portiosn 611a of the upper core member 611 are in abutting engagement with the upper surfaces of the inner leg portions 612a of the lower core member 612, whereas the lower surfaces of the outer leg portions 611b of the upper core member 611 are in a spaced face-toface relation with the upper surfaces of the outer leg portions 612b of the lower core member 612 with a limited gap 613 formed therebetween, as clearly seen from Fig. 5. Then, a molten resin 7 is filled into the coil case 2, impregnated inbetween the above members in the coil case 2 and solidified to firmly bond or secure them to the coil case 2.
In the iron core 6′ as constructed above, since each of the upper and lower core members 611 and 612 is formed of a stack of channel-shaped iron plates which are disposed vertically in a parallel relation with the planes on which the magnetic fluxes generated upon energization of the primary and secondary coils 3 and 4 pass, there will be no eddy-current loss developed in the upper and lower arm portions 611c and 612c of the upper and lower core members 611 and 612. Moreover, each of the iron core sections 610 as assembled from the paired upper and lower core members 611 and 612 is only discontinued, except at the limited gap 613, at a location in which the lower end surface of the inner leg portios 611a of each upper core member 611 abuts against the upper end surface of the inner leg portion 612a of the corresponding lower core member 612. Thus, the number of junctions between the iron core component parts can be decreased to one as compared with three junctions in the conventional iron core 6 illustrated in Fig. 1 and 2. As a result, eddy-current losses occurring at the junctions or connecting poortions of the iron core 6′ can be considerably reduced. In addition, due to the decreased number of the iron core junctions, heat transmission of the iron core 6′ as a whole is substantially improved so that the heat generated in the iron core 6′ during operation of the transformer 101 can be efficiently dissipated into the ambient air from the surfaces of the iron core 6′ which are exposed to the ambient air.
The high-voltage transformer 101 as constructed above was tested in terms of the secondary coil voltage versus the primary coil interruption current. To this end, the primary and secondary coils 3 and 4 of the transformer 101 were connected in an electrical circuit in the same manner as in Fig. 3, and the voltage developed at the secondary coil 4 was measured by changing the interruption current at the primary coil 3. The results obtained are illustrated in Fig. 6 in which the primary coil interruption current is plotted as abscissa and the secondary coil voltage as ordinate. In Fig. 6, the solid line indicates the transformer 101 of the present invention, and the dashed line the conventional transformer 1 shown in Figs. 1 and 2. From this figure, it is clear that the transformer 101 of the present invention provides a higher voltage at the secondary coil 4 than the conventional transformer 1 does.
Further, since the iron core 6′ is constituted by a plurality of paired upper and lower core members 611 and 612 which are of one and the same structure, it is only necessary to fabricate a single kind of iron core component part, and thus the manufacturing cost of the iron core can be greatly reduced. In addition, the lower core members 612 can be integrally formed with the coil case 2 upon moulding thereof, the number of component parts of the iron core 6′ to be placed and assembled in the coil case 2 is decreased, thereby considerably enhancing the efficiency in assembly of the iron core 6′ and hence of the transformer 101 as a whole.
Fig. 7A shows an iron core section 610′ in accordance with another embodiment of the present invention. In this embodiment, the iron core section 610′ comprises a pair of L-shaped first and second core members 611′ and 612′ which are composed of a plurality of L-shaped flat iron plates 611A′ and 612A′, respectively. The first core member 611′ has a laterally extending upper arm portion 611a′ and an outer leg portion 611b′ which extends downwards from the outer end of the upper arm portion 611a′. The second core member 612′ has a laterally extending lower arm portion 612a′ and an outer leg portion 612b′ which extends upwards from the inner end of the lower arm portion 612a′. The outer leg portion 611b′ of the first core member 611′ is shorter than the inner leg portion 612a′ of the second core member 612′ so that when the upper and lower core members 611′ and 612′ are assembled, the upper end surface of the inner leg portion 612a′ abuts against the upper arm portion 61a′, whereas the lower end surface of the outer leg portion 611b′ is spaced from the lower arm portion 612b′ with a limited gap 613 formed therebetween. Although in this embodiment, the first and second core members 611′ and 612′ are slightly different in configuration from each other, an iron core formed of the iron core sections 610′ of this embodiment achieves substantially the same effects and advantages as those obtained by the iron core 6′ as illustrated in Figs. 4 and 5.
Fig. 7b shows a modification of the iron core section 610′. This modification is substantially similar to the iron core section 610′ of Fig. 7A except for the fact that the relationship between the first and second core members 611′ and 612′ of Fig. 7A is reversed or turned upside down.
Figs. 8 and 9 shows a high-voltage transformer 101′ in accordance with a further embodiment of the present invention. The transformer 101′ of this embodiment is substantially similar to the transformer 101 of Figs. 4 and 5 except for the construction of an iron core 6˝. Specifically, the iron core 6˝ comprises a plurality (four in this embodiment) of iron core sections 610˝ which are disposed around a cylindrical sheeve 5 in a cross-shaped configuration. Each of the iron core sections 610˝ is composed of a pair of channel-shaped first (or inner) and second (or outer) core members 611˝ and 612˝ which are of one and the same construction and configuration, and each of which is formed of a plurality of channel-shaped flat iron plates which are stacked side by side. The inner core member 611˝ has a vertically extending planar inner leg portion 611a˝ which is disposed around the cylindrical sleeve 5 and radially inside a primary coil 3, an upper arm portion 611b˝ laterally extending radially outwards from the upper end of the inner leg portion 611a˝, and a lower arm portion 611c˝ laterally extending radially outwards from the lower end of the inner leg portion 611a˝. The outer core member 612˝ has a vertically extending planar outer leg portion 612a˝ which is disposed around the secondary coil 4, an upper arm portion 612b˝ laterally extending radially inwards from the upper end of the outer leg portion 612a˝, and a lower arm portion 612c˝ laterally extending radially inwards from the lower end of the outer leg portion 612a˝. The upper arm portions 611b˝ and 612b˝ are slightly shorter than the lower leg portions 611c˝ and 612c˝ so that when the upper and lower core members 611˝ and 612˝ are assembled to form the iron core section 610˝, the inner end surfaces of the lower arm portions 611c˝ and 612c˝ of the inner and outer core members 611˝ and 612˝ are placed in abutting engagement with each other, whereas the inner end surfaces of the upper arm portions 611b˝ and 612b˝ of the inner and outer core members 611˝ and 612˝ are placed in a spaced face-to-face relation with each other with a limited gap 613 formed therebetween. Thus, a substantially closed magnetic path is formed through these inner and outer core members 611˝ and 612˝ when the primary and secondary coils 3 and 4 are energized.
The above-described high-voltage transformer 101′ is fabricated or assembled in the following manner. First, the secondary coil 4 is concentrically fitted around the primary coil 3, and then, the channel-shaped inner core members 611˝ are mounted to the primary coil 3 from the inside, and the outer core members 612˝ are mounted to the secondary coil 4 from the outside in such a manner that the inner end surfaces of the lower arm portions 611c˝ and 612c˝ of the inner and outer core members 611˝ and 612˝ are in abutting engagement with each other, whereas the inner end surfaces of the upper arm portions 611b˝ of the inner core members 611˝ are in spaced face-to-face relation with the inner end surfaces of the upper arm portions 612b˝ of the corresponding outer core members 612˝ with the limited gap 613 formed therebetween, as clearly seen from Fig. 9. Subsequently, the primary and secondary coils 3 and 4 and the inner and outer core members 611˝ and 612˝ thus assembled are placed in the generally cylindrical bottomed coil case 2 with the inner core members 611˝ being fitted on the cylindrical sleeve 5 which is fixedly mounted on the bottom portion of the coil case 2. Thereafter, a resin 7 is filled into the coil case 2, impregnated inbetween the above members in the coil case 2 and solidified to firmly bond or secure them to the coil case 2.
As apparent from the foregoing, the transformer 101′ of this embodiment can achieve substantially the same effects and advantages as those obtained by the transformer 101 as illustrated in Figs. 4 and 5.
Although in the above-described embodiments, each of the first and second core members of the iron core is formed of a plurarity of stacked iron plates, it may be integrally formed from an iron blank as by forging. Also, in the above embodiments, four iron core sections are disposed around the cylindrical sleeve in a cross-shaped configuration, but the number of the iron core sections as required is not limited to this but any number of them may be employed. Thus, for example, two iron core sections may be disposed around the sleeve in diametrically opposite directions, or three iron core sections may be disposed around the sleeve and circumferentially spaced from each other at an angle of 120 degrees. Further, the bottom portion of the coil case may be open as shown in Fig. 5 or closed with appropriate apertures being formed therethrough for receiving iron core sections as well as permitting heat dissipation therefrom.

Claims

1. A high voltage transformer comprising:
a coil case;
a cylindrical sleeve in the coil case;
a primary coil disposed around said cylindrical sleeve in said coil case;
a secondary coil disposed in said coil case around said primary coil;
an iron core having a plurality of iron core sections disposed in said coil case so as to surround said primary and secondary coils, each of said iron core sections comprising a pair of first and second core members having a substantially similar configuration, said first and second core members being assembled in such a manner that said first members are in abutting engagement at one end thereof with said second members at one end thereof, but in a spaced face-to-face relation at the other end thereof with said second members at the other end thereof with a limited gap formed therebetween; and
a resin filled into said coil case for electrically insulating said primary and secondary coils from said iron core as well as firmly securing them to said coil case.

2. A high-voltage transformer as claimed in claim 1, wherein each of said first and second core members is of a channel-shaped configuration.

3. A high-voltage transformer as claimed in claim 2, wherein said channel-shaped first and second core members are vertically assembled in such a manner that the end surfaces of said first core member face the end surfaces of said second core member in a direction parallel to the axis of said cylindrical sleeve.

4. A high-voltage transformer as claimed in claim 2, wherein said channel-shaped first and second core members are laterally assembled in such a manner that the end surfaces of said first core members face the end surfaces of said second core members in a direction perpendicular to the axis of said cylindrical sleeve.

5. A high-voltage transformer as claimed in claim 1, wherein each of said first and second core members is of an L-shaped configuration.

6. A high-voltage transformer as claimed in claim 5, wherein each of said L-shaped first and second core members has a lateral arm portion and a vertical leg portion, the vertical leg portion of each first core member being shorter than that of each second core member.

7. A high-voltage transformer as claimed in claim 1, wherein each of said first and second core members is formed of a plurality of flat iron plates which are stacked side by side.

8. A high-voltage transformer as claimed in claim 1, wherein each of said first and second core members is integrally formed from a single iron blank by forging.

9. A high-voltage transformer as claimed in claim 1, wherein said first and second core members are disposed around said cylindrical sleeve in a cross-shaped configuration.

10. A high-voltage transformer as claimed in claim 1, wherein said first and second core members are disposed around said cylindrical sleeve in a diametrically opposite directions.

11. A high-voltage transformer as claimed in claim 1, wherein said first and second core members are disposed around said cylindrical sleeve and circumferentially spaced from each other at an angle of 120 degrees.

12. A method for making a high-voltage transformer comprising the steps of:
moulding a plurality of first core members integrally with a coil case in such a manner that said first core members are vertically disposed in said coil case and integrally connected with the bottom portion of said coil case;
disposing a primary coil and a secondary coil on said first core members in said coil case with said secondary coil disposed around said primary coil;
placing a plurality of second core members on said first core members in such a manner that said second core members cooperate with said first core members to form iron core sections each of which surrounds said primary and secondary coils; and
filling a resin into said coil case so that said resin is impregnated inbetween said primary and secondary coils and said iron core members and solidified to electrically insulate said primary and secondary coils from said iron core as well as to secure them to said coil case.

13. A method for making a high-voltage transformer as claimed in claim 12, wherein each of said first and second core members is of a channel-shaped configuration.

14. A method for making a high-voltage transformer as claimed in claim 13, wherein said channel-shaped second core members are vertically assembled onto said first core members.

15. A method for making a high-voltage transformer as claimed in claim 12, wherein each of said first and second core members is of an L-shaped configuration.

16. A method for making a high-voltage transformer comprising the steps of:
fitting a secondary coil around a primary coil;
assembling a plurality of first and second core members onto said primary and secondary coils from the inside and the outside, respectively, to form iron core sections each of which surrounds said primary and secondary coils;
placing said primary and secondary coils and said iron core sections thus assembled into a coil case; and
filling a resin into said coil case so that said resin is impregnated inbetween said primary and secondary coils and said iron core and solidified to electrically insulate said primary and secondary coils from said iron core as well as to secure them to said coil case.

17. A method for making a high-voltage transformer as claimed in claim 16, wherein each of said first and second core members is of a channel-shaped configuration.

18. A method for making a high-voltage transformer as claimed in claim 16, wherein each of said first and second core members is of an L-shaped configuration.