IMPROVED HIGH VOLTAGE COIL SUBASSEMBLIES AND METHOD FOR THEIR PRODUCTION

Abstract: The present invention relates to a process for producing a high voltage coil subassembly by providing a primary coil on a molded bobbin, providing a bobbin for use as a secondary coil, winding bondable magnet wire in segments onto the secondary bobbin while simultaneously heating the wire whereby the wire bonds void-free to adjacent windings of the wire, connecting the primary coil and secondary coil to form a coil subassembly, and encapsulating the coil subassembly with an electrically insulating thermoplastic resin.

Description: TITLE IMPROVED HIGH VOLTAGE COIL SUBASSEMBLIES AND METHOD FOR THEIR PRODUCTION This application claims benefit of U.S. Provisional Application Serial No. 60/025,447, filed September 4, 1996. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to high voltage coils, and. more particularly, to improved high voltage coils having reduced opportunity for corona discharge for use in automotive ignition systems, television flyback transfoπners and neon sign transformers. 2. Description of the Related Art Ignition in automotive internal combustion engines, illumination of video screens, and neon signs each requires a source of high voltage. This is typically an induced voltage from a step-up transformer. In the automotive industry, a step-up transformer is referred to as an ignition coil, which can be seen in different views by reference to Fig. 1. In the television and video-screen industry a step-up transformer is referred to as a flyback transformer. High voltages, if not correctly insulated will ionize surrounding gases, e.g., air. resulting in corona, i.e.. a faint glow adjacent to the surface of an electrical conductor at high voltage. Corona, in turn, can produce a leakage pathway for the electric current, and the ensuing leak current can cause a runaway short and failure of the device. As used herein, the term "high voltage" means any voltage above 1 ,000 volts ( 1 KV). Only very small amounts of trapped gases or air bubbles are needed to cause corona, and, therefore, manufacturers of ignition coils impregnate their wound devices under vacuum with a thermosetting epoxy or a polyurethane resin because those polymers will generally fill voids and harden to form rigid or elastomeric barriers which insulate the device electrically and prevent corona. The manufacturing process for these induction coils involves winding two coils: a primary and a secondary coil. A primary coil has fewer windings and often of a larger gauge magnet wire than a secondary coil. The primary coil carries a line voltage, which in the case of an ignition coil is typically 12 volts, and which in the case of a flyback transformer is 1 10 or 220 volts. The primary coil is assembled inside the core of the secondary coil which, as shown in the sectional view seen in Fig. 2, has many more turns of a finer magnet wire. Surrounding the two coils and through the center of both is a steel lamination stack which provides a path for the magnetic field which is created as soon as the system is energized. The entire assembly, including the wound primary coil, is connected to line supply, inside the wound secondary with the steel lamination stack in a plane surrounding both coils and through their center. This assembly is then placed in a potting cup or housing which is then filled with potting, i.e., epoxy or polyurethane resin, insulation material, subjected to vacuum and cured with heat over a time period of typically two or more hours, or several hours. Typical potting cups loaded with a coil subassembly before being filled with potting material are shown in Fig. 2A. High voltage terminals are pre-attached to the secondary coil before potting, and cables are attached to those terminals to connect them to the spark plugs in an automotive coil or to the rear components on the TV tube where the high voltage is needed. The coil-winding industry has produced induction coils in this manner for many years with little change in concept but with some necessary improvements in design mostly to allow increases in productivity as demand has increased. One significant design change has occurred in the secondary coil. To reach output voltages required in the case of a typical ignition coil, for example, as high as 40,000 volts, the secondary coil needs to be segmented so as to multiply the increasing voltage. Previously segmentation was achieved by layering the coil and insulating between the layers with paper. Now the paper concept has been replaced with a molded bobbin having a plurality of flanges which act to partition the bobbin at predetermined intervals along its length into discreet winding bays. This type of molded bobbin is known as a segmented bobbin. It can be seen in Fig. 3, and its use is required for this invention. A step change in productivity occurred with the introduction of segmented bobbins as they allowed faster winding speeds on multiple-spindle winders, and such molded segmented bobbins could also include design features like pockets for insertion of high voltage terminals. Another change is now required to improve productivity still further because impregnation materials, which are used to prevent corona discharge, greatly extend manufacturing time because of their need to be oven cured. Cure time has become a bottleneck in the typical manufacturing process as demand for high voltage coils has increased. In addition to an extended manufacturing cycle for the coils, the existing process faces problems with reliably meeting EPA regulations governing emission of volatile organic compounds ( VOC) during the cure cycle, as well as being high cost. Low voltage coils, such as, for example, solenoid coils which actuate valves in fluid lines, use thermoplastics as an outside insulation that encapsulates the coil. Thermoplastic encapsulation alone, however, is not an acceptable replacement for impregnation materials and their corresponding cure times because the encapsulation material may not always penetrate the windings and, in such cases, will produce micro voids which lead to corona and eventual shorting out and early failure of the device.. SUMMARY OF THE INVENTION The present invention relates to an improved process for producing a high voltage coil subassembly which comprises providing a primary coil on a molded bobbin, providing a segmented bobbin for use as a secondary coil, winding bondable magnet wire onto the segmented bobbin while heating the wire to a temperature below the melting point of the bondable surface layer whereby the bondable surface layer flows and the wire bonds void-free to adjacent windings of the wire, connecting the primary coil and secondary coil to form a coil subassembly, and encapsulating the coil subassembly with a thermoplastic resin. The improvement comprises: (a) heating the bondable magnet wire by hot air or electrically to a temperature sufficiently high to cause the bondable surface layer to flow, but not melt, simultaneously as the wire is being wound onto the segmented bobbin whereby the wire will bond to adjacent windings of the wire without leaving any voids, i.e., very small pockets of entrained air; and (b) encapsulating the resulting coil subassembly with an electrically insulating thermoplastic resin. According to another aspect, the present invention is an improved high voltage coil which includes a coil subassembly produced by the process comprising: (a) providing a primary coil on a molded bobbin; (b) providing a bobbin for use as a secondary coil; (c) winding bondable magnet wire onto the bobbin in segments while simultaneously heating the bondable magnet wire to a temperature below the melting point of the bondable surface layer whereby the bondable surface layer flows during winding and the wire bonds void-free to adjacent windings of the wire; (d) connecting the primary coil and secondary coil to form a coil subassembly; and (e) encapsulating the coil subassembly with an electrically insulating thermoplastic resin. The coil subassembly is encapsulated by: (a) placing the subassembly in an encapsulation mold having a cavity designed therefore; (b) closing the mold; and (c) injecting a thermoplastic melt into the mold to surround and encapsulate the subassembly while simultaneously drawing a vacuum on the mold cavity. The process of the invention eliminates the need for epoxy or polyurethane resins and their oven curing and thereby substantially reduces the time of manufacture and overall cost of production. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of automotive ignition coils known in the art; Fig. 2 is a sectional view an automotive ignition coil of the type shown in Fig. 1 ; Fig. 2A is a view in perspective of typical potting cups, each loaded with a coil subassembly prior to impregnation with epoxy resin; Fig. 3 is a view in elevation of segmented bobbins; Fig. 4 is a perspective view of an encapsulated coil. Fig. 5 is a view in cross section of a holding pin mechanism of an encapsulation mold of the type suitable for practicing the invention where the holding pins are in a forward position; Fig. 6 is view in cross section of the holding pin mechanism of Fig. 5 where the holding pins are retracted; Fig. 7 is a view in elevation of an encapsulation mold that has hydraulic actuated holding pins; and Fig. 8 is a perspective view in of a sectioned flyback transformer and a corresponding empty potting cup. DETAILED DESCRIPTION The present invention is directed to the discovery that a high voltage coil may be made which is void free to avoid the problem of corona, and which may be encapsulated with a thermoplastic resin. Specifically, the present invention relates to a high voltage coil which includes an encapsulated coil subassembly and to a process for producing the high voltage coil subassembly by providing a primary coil on a molded bobbin, providing a bobbin for use as a secondary coil, winding bondable magnet wire in segments onto the secondary bobbin while simultaneously heating the wire so that the bondable adhesive surface coating flows and the wire bonds void-free to adjacent windings of the wire, connecting the primary coil and secondary coil to form a coil subassembly, and encapsulating the coil subassembly with an electrically insulating thermoplastic resin. To make the inventive coil subassembly, a primary coil is prepared using conventional methods such as by winding bondable magnet wire on a molded bobbin. Next, a secondary coil is prepared by winding a bondable magnet wire in segments onto a bobbin to make the secondary coil. Bondable magnet wire is a magnet wire of a gauge typically used for secondary coils, but having a melt adhesive outer surface coating applied over typically an insulation enamel inner layer. Pictures of these coils are shown in Figures 4 and 5. Bondable magnet wire is available commercially from most magnet wire manufacturers. It is normally used in producing self-supporting structures from bundles of fine wires, such structures typically taking the form of television yoke coils, solenoid coils, and the like. The wire is also commonly used to wind motor parts, such as armatures and field coils. Such wire products are available, for example, from Phelps-Dodge Magnet Wire Corporation, of Fort Wayne, Indiana and Hopkinsville, Kentucky, under the trade designations SY-BONDEZE and AP-BONDEZE; similar wire products are also available from American Wire Corporation, of Sandy Hook, Connecticut (e.g., their PNB-1 wire product). Self-bondable wire is normally used by assembling the desired number of strands, forming them into the ultimate configuration, and then effecting integration in an appropriate manner, which will depend upon the nature of the bondable adhesive outer coating. Normally, integration can be achieved by either a heating method or a solvating method, the former being carried out by heating the free-standing coil or the wound part in an oven, or by passing a current through the wire to generate the necessary thermal energy. Solvent activation is achieved by a dipping, spraying or wiping technique. Commercial self-bondable magnet wire is available in a wide range of sizes, and with a variety of coatings thereon. Generally, the wire will have a duplex coating, consisting of a self-bondable adhesive outer surface layer, and an underlying base coat or layer which provides electrical, mechanical and/or chemical characteristics as desired for a given application. The underlying insulation will normally be provided by a natural or synthetic organic dielectric resinous material of the type that is conventionally used for wire coating purposes, exemplary of which are the polyesters, polyamides, the polyimides, the polvinyl formal resins, conventional varnishes, etc.; copolymers and interpolymers, as well as multilayer composite coatings are also commonly used. According to the present invention, the bondable wire for the secondary coil is heated simultaneously while it is being wound on the secondary bobbin. The temperature of the wire and the tension of the wire during the winding process are such that the bondable adhesive outer layer will flow, and the wire will bond to adjacent windings without leaving any voids, i.e., the secondary coil winding which results will be substantially void free. As used herein, the term "substantially void free" means that no voids are visible using a transmission electron microscope set to a magnification of 1000 mag. The wire may be heated by hot air or electrically. Then the wire is wound sufficiently tight to squeeze out air in each winding bay. The adhesive on the wire flows to occupy the same spaces as would be occupied by an impregnation material flowing under vacuum in prior art coils that used thermosetting resins. In a preferred embodiment the secondary coil bobbin is a segmented bobbin as is shown in Fig. 3. A segmented bobbin is preferred because the alternative layer winding system may result in a coil winding wich is difficult for a thermoplastic encapsulant to penetrate sufficiently to eliminate voids. The wound and bonded primary coil and secondary coil each includes conventional terminals or lead wires attached thereto. They are connected to form a coil subassembly by placing the primary coil in the core of the secondary coil and then placing the coil subassembly into an encapsulation mold. An example of an encapsulation mold which may be used is one which is similar to that used for molding the seamless cover on golf balls. This mold, which is shown in Figs. 5 and 6, has hydraulically actuated holding pins in the cavities of both the upper and lower mold halves. The encapsulation molding machine may be a rotary-table vertical-clamping injection-molding machine that is programmed to close the mold, inject thermoplastic melt in a controlled fashion around the coil subassembly. The holding pins are withdrawn at the appropriate and precise time, such that all holding-pin holes are filled with resin. It is preferred that while encapsulating the coil subassembly, a vacuum is applied to the mold cavity. The presence of a vacuum during encapsulation helps ensure there is no air entrapment at the bobbin crossovers, which are those regions of the segmented coil bobbin where the bondable magnet wire crosses from one winding bay to the next adjacent winding bay. The bondable adhesive coating on the outer surface of the magnet wire, while flowable, is not believed to be of sufficient thickness to entirely fill the crossover regions. The holding pins in the mold may be hydraulic actuated, as shown in Figure 7. The encapsulated coil subassembly is then ejected from the mold by the same holding pins, as they are reset in a forward position for the next shot of thermoplastic. Depending on the sophistication of the tooling, either the remaining components of a coil assembly, such as the lamination stack and exterior terminals, are loaded into a second encapsulation mold and encapsulated with the previously encapsulated coil subassembly. Alternatively, all components including the primary and secondary coils, lamination stack or iron cores to provide a continuous path for the magnetic field, plus a terminal to connect the input voltage and a high tension connector for the output voltage may be loaded into one mold. The outside shape may be formed during the vacuum-assisted encapsulation. Thus, the invention makes use of bondable magnet wire, a segmented secondary bobbin and vacuum assisted injection molded encapsulation to replace epoxy and polyurethane resin impregnation materials. The inventive process described above reduces the time of manufacture of a coil assembly by at least 2 hours compared to a conventional method which uses thermoset resins because oven curing is eliminated, which is a big bottleneck in the manufacture of this style of induction coil. Televisions and video screens use fly-back transformers that are currently made by vacuum impregnation processes similar to the process for making a high voltage ignition coil using a thermoset resin, and those processes have similar problems. They refer to the secondary coil as the tertiary coil, and the tertiary coil also uses a segmented bobbin. An example of a finished sectioned part with it corresponding empty potting cup is shown in Fig. 8. The potting cup, which now becomes the outside housing, would be eliminated with the process the invention. Any electrically insulating, injection moldable thermoplastic resins which may be used in this invention, including 6,6-polyamide, 6-polyamide, 4,6-polyamide, 12,12-polyamide, 6,12-polyamide, and polyamides containing aromatic monomers, polybutylene terephthalate, polyethylene terephthalate, polyethylene napththalate, polybutylene napththalate, aromatic polyesters, liquid crystal polymers, polycyclohexane dimethylol terephthalate, copolyetheresters, polyphenylene sulfide, polyacylics. polypropylene, polyethylene, polyacetals, polymethylpentene, polyetherimides, polycarbonate, polysulfone, polyethersulfone, poyphenylene oxide, polystyrene, styrene copolymer, mixtures and graft copolymers of styrene and rubber, and glass reinforced or impact modified versions of such resins. Blends of these polymers such as polyphenylene oxide and polyamide blends, and polycarbonate and polybutylene terephthalate, may also be used. Optionally, such thermoplastic resins may include additives such as flame retardants, reinforcements, fillers, colored pigments, bulk fillers, plasticizers, and heat and light stabilizers. The amount of reinforcements or filler used can vary from about 1 to 70 weight percent based on the weight of the polymer and filler present. The preferred type reinforcement for use is fiberglass, and it is preferred that the fiberglass be present in the amount of about 15 to 55 weight percent based on the total weight of the polymer and filler present. The thermoplastic polymers can be prepared by known methods. When reinforcements or fillers are used, they can be added to the thermoplastic polymers during the preparation of the polymers or compounded in a separate step according to conventional compounding methods.

Claims: WHAT IS CLAIMED IS: 1. A process for producing a high voltage coil subassembly comprising: (a) providing a primary coil on a molded bobbin; (b) providing a bobbin for use as a secondary coil; (c) winding bondable magnet wire onto the bobbin in segments while simultaneously heating the bondable magnet wire to a temperature below the melting point of the bondable surface layer whereby the bondable surface layer flows during winding and the wire bonds void-free to adjacent windings of the wire; (d) connecting the primary coil and secondary coil to form a coil subassembly; and (e) encapsulating the coil subassembly with an electrically insulating thermoplastic resin. 2. The process of Claim 1 in which the coil subassembly is encapsulated by (a) placing the subassembly in an encapsulation mold having a cavity designed therefore; (b) closing the mold; and (c) injecting a thermoplastic melt into the mold to surround and encapsulate the subassembly while simultaneously drawing a vacuum on the mold cavity. 3. The process of Claim 1 , further comprising heating the bondable magnet wire with hot air. 4. The process of Claim 1 , further comprising heating the bondable magnet wire by passing an electrical current through the wire. 5. A high voltage ignition coil which includes a coil subassembly produced by the process comprising: (a) providing a primary coil on a molded bobbin; (b) providing a bobbin for use as a secondary coil; (c) winding bondable magnet wire onto the bobbin in segments while simultaneously heating the bondable magnet wire to a temperature below the melting point of the bondable surface layer whereby the bondable surface layer flows during winding and the wire bonds void-free to adjacent windings of the wire; (d) connecting the primary coil and secondary coil to form a coil subassembly; and (e) encapsulating the coil subassembly with an electrically insulating thermoplastic resin. 6. The high voltage ignition coil of Claim 5 in which the coil subassembly is encapsulated by: (a) placing the subassembly in an encapsulation mold having a cavity designed therefore; (b) closing the mold; and (c) injecting a thermoplastic melt into the mold to surround and encapsulate the subassembly while simultaneously drawing a vacuum on the mold cavity. 7. A transformer which includes a coil subassembly prepared by the process of Claim 1.

Inventors: BOYER THOMAS D, PATTERSON JAMES FRANCIS BOYS, SHEET M LANA, BOYER, THOMAS, D., PATTERSON, JAMES, FRANCIS, BOYS, SHEET, M., LANA
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1997
Main classification: H01F41
IPC codes: H01F27, H01F38, H01F41