1. Field of the Invention
The present invention relates to an insulated wire having an insulation film formed by applying an insulation film paint on a surface of a conductor and baking it. Particularly, the invention relates to an insulated wire used as coil windings for electrical equipment such as motors.
2. Description of the Related Art
Generally, an insulated wire used for coils for electrical equipment, such as motors and transformers, is constructed such that a single layer or a plurality of layers of insulation film made by applying and baking an insulation film paint is formed on a conductor molded so as to have a cross-sectional shape (e.g. round or rectangle) to conform to the usage and shape of the coil. Due to the recent requirements for smaller size, higher performance, and energy conservation of electrical equipment, the application of inverter control for electrical equipment, such as motors and transformers, is becoming more and more popular. Also, to meet the demand, higher voltages and larger currents (greater electric power) are being used for the inverter control.
In the inverter control, a steep overvoltage (inverter surge voltage) can possibly occur, and there is concern that the increasing use of higher voltages as well as the inverter surge voltage may adversely affect the insulation system of the coil in electrical equipment. Specifically, electric fields concentrate in the small gaps among insulated wires constituting a coil, which may result in the occurrence of partial-discharge between adjacent insulated wires (between film and film) or between the insulated wire and the ground (between film and coil core). Partial discharge may cause corrosion deterioration (partial-discharge deterioration) of the insulation film, and with the progress of the partial-discharge deterioration, dielectric breakdown of the coil may occur.
To prevent deterioration due to partial discharge, it is desirable to suppress the generation of partial discharge between the insulation films, that is, it is desirable to make the partial-discharge start voltage in the insulation film high. To do so, for example, a method of increasing the thickness of the insulation film and a method of using a resin having a low specific dielectric constant for the insulation film can be exemplified. Generally, partial-discharge start voltage in an insulated wire is proportional to the thickness of the insulation film and is inversely proportional to the specific dielectric constant of the insulation film.
A method of making the insulation film thick, however, has a problem in that because the thickness of the film formed by one coating and baking process is usually thin, approximately several microns, a number of repetitions of the process needs to be increased. Consequently, production cost will increase. On the other hand, if an insulation film is formed by simply using a fluoric polyimide resin to decrease the specific dielectric constant, there is a problem in that weak adhesion between the insulation film and a conductor tends to cause peeling, resulting in the occurrence of dielectric breakdown.
Accordingly, there have been proposed various methods of increasing the adhesion between the insulation film and the conductor as well as simultaneously making the specific dielectric constant of the insulation film low. For example, JP-B 4177295 has disclosed an inverter-surge-resistant insulated wire composed of a resin material, having at least one enamel-baked layer provided on the outer periphery of the conductor and at least one extrusion-coated resin (excluding polyether ether ketone) layer on the outer periphery thereof, wherein the total thickness of the enamel-baked layer and the extrusion-coated resin layer is 60 μm or more, the thickness of the enamel-baked layer is 50 μm or less, and the tensile elastic modulus of the extrusion-coated resin layer is 1000 MPa or more at 25° C. and 10 MPa or more at 250° C. According to JP-B 4177295, it seems to be possible to provide an insulated wire having a high partial-discharge start voltage (approximately 900 V) without decreasing the adhesion strength between the conductor and the insulation film.
JP-A 2008-288106 has disclosed an insulated wire in which an enamel layer of 50 μm thick or less is formed on a conductor by applying and baking resin varnish, an extrusion-coated resin layer extrusion-coated with a thermoplastic resin having a specific dielectric constant of 4.5 or less is formed on the enamel layer, and protrusions are provided on the outermost layer of the extrusion-coated resin layer. In the insulated wire described in JP-A 2008-288106, it seems to be possible to increase corona characteristics between the insulated wire and a motor's slot into which the insulated wire is inserted and/or between adjacent insulated wires wound, thereby making it possible to make the insulation film thin. Additionally, when the insulated wire is inserted into the motor's slot, it seems that the surface of the insulation film is not damaged easily.
As stated above, with the further advancement in higher efficiency and higher output in electrical equipment, there is a need for insulated wires to further increase the partial-discharge start voltage (e.g., 1500V or more). Herein, in a conventional insulated wire having an enamel layer and an extrusion-coated resin layer such as those described in JP-B 4177295 and JP-A 2008-288106, it seems to be possible to make the partial-discharge start voltage high by increasing the thickness of the extrusion-coated resin layer so as to increase the thickness of the insulation film.
However, since characteristics of the resin composition of the conventional enamel layer greatly differ from those of the resin composition of the extrusion-coated resin layer, adhesion between the layers tends to become inadequate, causing inter-layer peeling or wrinkling to occur on the insulation film in severe processing conditions (e.g., where the wire is wound into a small radius), and consequently, resulting in the decrease in the partial-discharge start voltage. To address this problem, in the conventional insulated wires that are preferred embodiments described in JP-B 4177295 and JP-A 2008-288106, an adhesion layer is interposed between the enamel layer and the extrusion-coated resin layer to enhance the adhesion therebetween. However, when interposing the adhesion layer between those layers, a problem arises in that the production cost further increases.