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Home | Alpha Telephone | Domain Names | Web Hosting | Get Traffic | xrEvidence | xrSoccer United States Patent
Oil-filled electrical instrument An oil-filled electrical instrument is manufactured by connecting a coil-core assembly with a hollow member, forming a porous layer around the coil and hollow member, covering the porous layer with a substantially impervious layer of an uncured plastic material, curing the plastic material, and then introducing an insulation oil into the hollow member to impregnate the coil with the oil. The plastic layer minimizes the amount of insulation oil necessary to prevent corona discharge to thereby decrease the weight of the electrical instrument and minimize the possibility of accidental fire of the instrument.
Attorney, Agent or Firm: What we claim is: 1. A fly-back transformer, comprising a coil; a layer of porous material surrounding substantially all of the surfaces of said coil, said coil and said porous layer being impregnated with an insulation oil; a core assembled with said coil into a coil-core assembly; a casing of an electrically insulating material housing said coil-core assembly; a passage extending outwardly from said coil to the outside of said casing to provide a pouring port for the insulation oil; and a substantially impervious layer of a plastic material filling up the space defined between said core, said coil, said passage and said casing. 2. A fly-back transformer comprising a coil impregnated with an insulation oil, substantially all of the surfaces of said coil being covered with a layer of a plastic material of a characteristic such that the rate of variation of the viscosity of the plastic material with respect to the lapse of time when the plastic material is cured is one of zero and positive values; a core assembled with said coil to form a coil-core assembly; a casing of an electrically insulating material housing said coil-core assembly; a passage extending outwardly from said coil to the outside of said casing to provide a pouring port for the insulation oil; and a substantially impervious layer of a plastic material filling up the space defined between said core, said coil, said passage and said casing. 3. A fly-back transformer as claimed in claim 1 which further comprises means covering the opening of said insulation oil pouring port for enabling breathing of said transformer. 4. A fly-back transformer according to claim 3 in which said breathing enabling means comprises a bellows member. 5. A fly-back transformer as claimed in claim 2 which further comprises means covering the opening of said insulation oil pouring port for enabling breathing of said transformer. 6. A fly-back transformer as claimed in claim 5 in which said breathing means comprises a bellows member. FIElD OF THE INVENTION The present invention relates to an oil-filled electrical instrument, i.e., an electrical device disposed in a quantity of insulation oil, such as an oil-filled transformer, reactor or capacitor, and to a method of manufacturing this kind of electrical instrument. DESCRIPTION OF PRIOR ART Electrical instruments are classified into an oil-insulation type and a dry-insulation type. Oil-insulation is quite useful to prevent corona discharge and thus widely employed in the art. This type of insulation, however, requires a metal container for containing or receiving the insulation oil and electrical elements such as the coils and core. In some cases, the weight of the assembly is such that its transfer and installation are inconvenient. In addition, the prior art oil-filled electrical instrument is filled with such a large amount of insulation oil (usually mineral oil of a lower ignition point) that it is liable to produce a large fire when the oil is ignited by an accidental trouble such as a shortcircuit. The electrical elements and insulation oil are conveniently received in a closed container, the container preventing deterioration of the oil after a prolonged operative time of the electrical instrument. The closed container needs an arrangement to absorb the variation in the volume of the insulation oil caused either by the variation in the atmospheric temperature at which the electrical instrument is operated, or by temperature rise. Such an arrangement will be termed a "breathing mechanism" hereunder. Lack of a breathing mechanism will produce a partial vaccum within a closed container when insulation oil contracts, which in turn produces in the insulation oil voids about which corona discharges are produced to cause dielectric breakdown. On the other hand, if insulation oil is expanded, pressure will build up in the oil to deform the closed container until it is destroyed. The larger the amount of insulation oil in a closed container, the greater is the required breathing mechanism. In an attempt to reduce the amount of insulation oil necessary to fill up the space in a closed container, it has been proposed to dispose a spacer element in the container. The proposal, however, has failed to provide the desired result. Dry-insulation is useful to produce small-sized and light-weight electrical instruments but fails to avoid formation of voids in the instruments on electrical components such as conductors. The voids allow production of corona discharge which causes dielectric breakdown. Thus, dry-insulation is not employed in electrical instruments excepting those used with low voltages in order to assure prolonged operative lives of the instruments. SUMMARY OF THE INVENTION The present invention aims to provide novel and improved structures of oil-filled electrical instruments which eliminate corona discharge and a method of manufacturing the improved electrical instruments. According to a feature of the present invention, there is provided an oil-filled electrical instrument, comprising an inner part, a layer of porous material surrounding said inner part, a substantially imprevious layer of a plastic material around said porous layer, said inner part and said porous layer being impregnated with an insulation oil, and a passage extending outwardly from said inner part through said layers to provide a pouring port for the insulation oil. According to another feature of the present invention, there is provided an oil-filled electrical instrument, comprising an inner part surrounded by a layer of a plastic material of a characteristic that the rate of variation of the viscosity of the plastic material with respect to the lapse of time when the plastic material is cured is zero or of a positive value, said inner part being impregnated with an insulation oil, and a passage extending outwardly from said inner part through said plastic layer to provide a pouring port for the insulation oil. According to a further feature of the present invention, there is provided a method of manufacturing an oil-filled electrical instrument having an inner part, said method comprising the steps of connecting said inner part with a hollow member, forming a porous layer around at least a part of said inner part and at least a part of said hollow member, covering said porous layer with a substantially impervious layer of an uncured plastic material, curing said plastic material, and introducing an insulation oil into said hollow member to impregnate said inner part with the oil. According to a still further feature of the present invention, there is provided a method of manufacturing an oil-filled electrical instrument having an inner part, said method comprising the steps of connecting said inner part with a hollow member, covering at least a part of said inner part and at least a part of said hollow member with a layer of uncured plastic material having a characteristic that the rate of variation of the viscosity of the plastic material with respect to the lapse of time when the plastic material is cured is zero or of a positive value, curing said plastic material, and introducing an insulation oil into said hollow member to impregnate said inner part with said oil. The present invention will be described by way of example with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a partial sectional side elevation of a transformer which is an embodiment of the present invention; and FIG. 2 is a similar view but illustrates a fly-back transformer which is another embodiment of the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS The transformer shown in FIG. 1 includes primary and secondary coils generally indicated by 3. An oil passage provided by a hollow member 4 interconnects a part of the coils 3 and the outside of the transformer. The coils and a part of the oil passage are surrounded by a layer 8 of an insulating porous material which may comprise a composite material formed therein with quite fine or small perforations communicated with each other to provide an oil permeable property. The porous layer 8 may be formed by the following process. A plastic material such as soluble phenolic or epoxy resin and a powdery inorganic material such as powdered calcium carbonate are mixed in a solvent to form a solution of the mixture. Coils 3 before assembled with other elements into a transformer are dipped into the solution and removed therefrom to allow the solvent on the coils to volatilize. Thereafter, the coils are heated to an elevated temperature to cure the plastic material contained in the mixture so that the coils are covered with a perforated layer. Thereafter, the coils are assembled with a core 2. The assembly is then placed in a casing 1. The coils 3 are then connected with lead wires 5, 10 and 11, respectively, which in turn are connected to terminals 6, 13 and 14, respectively. Thereafter, a liquid plastic material which forms a dense or a substantially impervious layer is poured into the casing 1 so as not to be introduced into the oil passage 4 so that the space in the casing 1 is filled with the plastic material 9. The terminal covers the top of the oil passage 4 and is provided with an opening 7 through which insulation oil 15 is poured into the oil passage 4 and thus into the coils 3 so that the coils are impregnated with the oil. Vacuum may be applied to the coils to facilitate the impregnation. The opening 7 is then sealed or closed by means of soldering to secure the lead wire 5 to the terminal 6. Reference numeral 12 denotes solder by which the lead wire 10 is secured to the terminal 13. The insulation oil thus poured into the transformer fills up the spaces not only in the coils 3 and the perforated plastic layer 8 but also any space which would be defined between the plastic layer 9 and the porous layer 8 to avoid corona discharge. Contrary to the invention, if the coils 3 are not connected with the hollow member 4 and thus have to be assembled into a transformer after impregnation with insulation oil, the insulation oil will flow out of the coils during assembly to produce non-impregnated parts in the coils with a resultant possibility of corona discharge. In addition, the insulation oil may disadvantageously be mixed with filling plastic material with resultant imcomplete curing of the plastic material and thus poor adhesion between the latter and the lead wires and terminals. The embodiment shown in FIG. 1 is equipped with the casing 1. The casing, however, is not essentially necessary and the layer of the filling plastic material 9 may form the outer surface of the transformer. In place of the mixture which forms the porous layer 8, another plastic material may be used which has such a characteristic that when it is cured, the viscosity of the plastic material is not once decreased but gradually increased, i.e., a characteristic that when the plastic material becomes solidified, the rate of variation in the viscosity of the plastic material with respect to the lapse of time is always zero or of a positive value. As a further alternative, coils 3 may be dipped into an uncured resin which is of a characteristic that upon curing forms a three-dimensional structure, such as, for example, silicone rubber, and to which silica is added to adjust the viscosity so that the coils are not impregnated with the resin by virtue of capillary effect. If the coils are removed from the uncured resin, the coating on the coils will form a dense layer when the coating is cured. If a plastic material is used which has a hysteresis characteristic that the viscosity of the plastic material when cured is once lowered (for example, the viscosity of a certain plastic material such as the usually used epoxy resin when cured is once decreased to 1/10 and then increased.), the plastic material will penetrate into the coils 3 due to the lowered viscosity of the material even if the plastic material was initially adjusted with respect to its viscosity, with a resultant disadvantage that the penetrated plastic material will be cured in the coils and block insulating oil during subsequent oil impregnation step. The method of the present invention, however, uses a plastic material such as silicone rubber having a characteristic that the viscosity of the material is not lowered during the curing of the material. Thus, the plastic material does not penetrate into the coils when covering layers are formed therearound. Preferably, the filling plastic material 9 is the same as the plastic material of the layer 8. This feature is advantageous in that the filling material 9 is completely adhered to the layer 8 to avoid any gap which would otherwise give rise to the production of corona discharge. Referring to FIG. 2, a second embodiment of the invention will be described hereunder. The embodiment comprises a fly-back transformer which is a kind of transformer disposed in the horizontal deflection circuit of a television image receiver to produce a high voltage by means of fly-back pulses. A fly-back transformer is known to have deficiencies such as increased loss and variation in its high voltage output due to large variations in its operation characteristics. The variations in operating characteristics are caused by a variation or increase in the stray capacitance, the stray capacitance changes being caused by deflection of the metal casing due to variation in the pressure of the insulating oil filling up the space. This pressure variation in turn takes place because the fly-back transformer is energized by fly-back pulses having a high frequency. It has been proposed to form the outer casing of a fly-back transformer from a plastic material in an attempt to eliminate the variation or increase in the stray capacitance. However, the proposed plastic casing is not completely satisfactory because the casing is not flexible and tends to be destroyed by the pressure of the oil within the casing. According to the present invention, a fly-back transformer is enabled to be equipped with an outer casing of a plastic material and to have a stable operating characteristic which is not changed by the variation in the oil pressure. The fly-back transformer shown in FIG. 2 can be manufactured by a method similar to the method described with reference to FIG. 1 and has an outer casing 1' of a plastic material. Coils 3 are disposed in the casing and have a part in communication with an oil passage 4' formed of a plastic material. The coils 3 and the oil passage 4' are surrounded by a layer 8 similar to the layer 8 in FIG. 1. The coils 3 are assembled with a core 2' of ferrite to form an assembly. The space between the assembly and the outer casing 1 is filled with a plastic material 9. Terminals 14' are connected to some of the windings of the coils 3. Rectifiers 16 are disposed within the oil passage 4'. The top of the passage 4' is covered with a bellows 17 of a metal having an opening 7 in the top surface thereof. Insulation oil 15 is poured through the opening 7 and flows into the oil passage 4' and thus into the coils 3 along paths indicated by dotted arrows 18. The opening 7 in the bellows 17 is closed after the pouring so that the bellows acts both as a terminal and as a breathing mechanism. A protector 19 for a terminal for high voltage output is mounted on the top surface of the plastic material 9. The above described and illustrated embodiments of the invention provide the following advantages: 1. Because an insulating layer of oil for preventing corona discharge is formed only in the coils and around the part through which high voltage is applied, a minimum amount of insulation oil is required with resultant decrease in the weight of the products and in the possibility of accidental fire. 2. As the coils are impregnated with oil by means of vaccum impregnation after the space between the coils and the outer casing is filled with a plastic material, the impregnation is complete and does not produce any void in transformer which would otherwise cause corona discharge. For U.S. patent law, rules, and procedures see MPEP. Disclaimer. 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