EP0318613B1

EP0318613B1 - High-voltage transformer and method for making same

Info

Publication number: EP0318613B1
Application number: EP19870117994
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: 1992-09-09
Anticipated expiration: 2007-12-04
Also published as: EP0318613A1; DE3781692T2; DE3781692D1

Description

This invention relates to a high-voltage transformer according to the preamble of claim 1 and a method for making the same.
Such a transformer is known from the Abstract of JP-A-61 231 708, and will be explained in more detail with reference to the Figs. 1 to 3.
Fig. 1 is a plan view showing the said 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 unillustrated 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 consisting of a 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 portion 601 so that they are in contact at their lower end with the cross-shaped lower arm portion 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 pedetermined 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 portions 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 corrent 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.
DE-A-3 423 524 discloses another type of ignition coil comprising an iron core section formed by a pair of first and second core members having the same U-channel shaped configuration, which improves the productivity of the entire iron core.
The present invention is intended to obviate the above-described problems of the prior art transformer according to the Abstract of JP-A-61 231 708.
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 according to claim 1.
According to another aspect of the present invention, there is provided a method for making the said high-voltage transformer according to claim 7 or claim 8.
The above and other objects, features and advantages of the present invention will be more readily apparent from the following detailed description of two preferred embodiments thereof when taken in conjunction with the accompanying drawings as follows:

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. 4A: is a schematic view of an iron core in accordance with a first embodiment of the present invention;
Fig. 4B: is a view similar to Fig. 4A, but showing a modified form of iron core; and
Fig. 5: is a graphic representation showing the relationship between the primary coil interruption current and the secondary coil voltage.

The present invention will now be described just with reference to the Figs. 4A and 4B which each show a schematic view of an iron core as to be used in connection with a high-voltage transformer as illustrated in the Figs. 1 and 2, wherein the iron core according to Fig. 4A or Fig. 4B is to replace the prior known iron core 6.
Fig. 4A shows an iron core section 610' in accordance with a first 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 621a'. 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 611a', 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. 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 all the effects and advantages as referred to above.
Fig. 4B shows a modification of the iron core section 610'. This modification is substantially similar to the iron core section 610' of Fig. 4A except for the fact that the relationship between the first and second core members 611' and 612' of Fig. 4A is reversed or turned upside down.
A high-voltage transformer constructed in connection with iron core sections as illustrated in Figs. 4A or 4B 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 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. 5 in which the primary coil interruption current is plotted as abscissa and the secondary coil voltage as ordinate. In Fig. 5, the solid line indicates the transformer 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 of the present invention provides a higher voltage at the secondary coil 4 than the conventional transformer 1 does.
A high-voltage transformer in connection with an iron core as described with reference to the Figs. 4A and 4B is fabricated or assembled in the manner as disclosed in claim 7 or claim 8.
Although in the above-described embodiments, each of the first and second core members of the iron core is formed of a plurality 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 or closed with appropriate apertures being formed therethrough for receiving iron core sections as well as permitting heat dissipation therefrom.

Claims

A high-voltage transformer comprising:
a coil case (2);
a cylindrical sleeve (5) in the coil case (2);
a primary coil (3) disposed around said cylindrical sleeve (5) in said coil case (2);
a secondary coil (4) disposed in said coil case (2) around said primary coil (3);
an iron core (6) having a plurality of iron core sections (610') disposed in said coil case (2) so as to surround said primary and secondary coils, each of said iron core sections (610') comprising core members (611'; 612') assembled in abutting engagement end to end but leaving at one point an air-gap (613); and
a resin (7) filled into said coil case (2) for electrically insulating said primary (3) and secondary (4) coils from said iron core as well as firmly securing them to said coil case (2);
characterized in
that the said iron core sections (610') each comprise a pair of first (611') and second (612') core members having almost the same L-shaped configuration, wherein the first and second core members (611'; 612') each have a shorter leg portion (611a', 612a') of the same length and a longer leg portion (611b'; 612b') of a slightly different length so that the air-gap results when assembled together.
A high-voltage transformer as claimed in claim 1, wherein each of said first and second core members (611'; 612') is formed of a plurality of flat iron plates which are stacked side by side.
A high-voltage transformer as claimed in claim 1, wherein each of said first and second core members (611'; 612') is integrally formed from a single iron blank by forging.
A high-voltage transformer as claimed in claim 1, wherein core sections (610') are disposed around said cylindrical sleeve (5) so as to form a core having a cross-shaped configuration when seen in cross-section taken perpendicular to the longitudinal axis of the cylindrical sleeve.
A high-voltage transformer as claimed in claim 1, wherein core sections (610') are disposed around said cylindrical sleeve (5) in diametrically opposite directions about the longitudinal axis of the cylindrical sleeve.
A high-voltage transformer as claimed in claim 1, wherein core sections (610') are disposed around said cylindrical sleeve (5) to form cores which are circumferentially spaced from each other at an angle of 120 degrees.
A method for making a high-voltage transformer according to claim 1 comprising the steps of:
moulding a plurality of first L-shaped core members integrally with a coil case in such a manner that it is vertically disposed in that coil case and one of its legs is 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 L-shaped core members on said first L-shaped 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.
A method for making a high-voltage transformer according to claim 1 comprising the steps of:
fitting a secondary coil around a primary coil;
assembling a plurality of first and second L-shaped core members onto said primary and secondary coils from the inside and the outside of the coils, 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.