Inverters have been employed in many types of electrical equipments, as an efficient variable-speed control unit. Inverters are switched at a frequency of several kHz to tens of kHz, to cause a surge voltage at every pulse thereof. Inverter surge is a phenomenon in which reflection occurs at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire in the propagation system, followed by applying a voltage twice as high as the inverter output voltage at the maximum. In particular, an output pulse occurred due to a high-speed switching device, such as an IGBT, is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and voltage decay due to the connection cable is also low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
As coils for electrical equipments, such as inverter-related equipments, for example, high-speed switching devices, inverter motors, and transformers, insulated wires made of enameled wires are mainly used as magnet wires in the coils. Further, as described above, since a voltage almost twice as high as the inverter output voltage is applied in inverter-related equipments, it has become required to minimize the inverter surge deterioration of the enameled wire, which is one of the materials constituting the coils of those electrical equipments.
In the meantime, partial discharge deterioration is a complicated phenomenon in which an electrical-insulation material undergoes, for example, molecular chain breakage deterioration caused by collision with charged particles that have been generated by partial discharge of the insulating material, sputtering deterioration, thermal fusion or thermal decomposition deterioration caused by local temperature rise, and chemical deterioration caused by ozone generated due to discharge. For this reason, reduction in thickness, for example, is observed in the electrical-insulation materials, which have been deteriorated as a result of actual partial discharge.
It has been believed that inverter surge deterioration of an insulated wire also proceeds by the same mechanism as in the case of general partial discharge deterioration. Namely, inverter surge deterioration of an enameled wire is a phenomenon in which partial discharge occurs in the insulated wire due to the surge voltage with a high peak value, which is occurred at the inverter, and the coating of the insulated wire causes partial discharge deterioration as a result of the partial discharge; in other words, the inverter surge deterioration of an insulated wire is high-frequency partial discharge deterioration.
Insulated wires that are able to withstand several hundred volts order of surge voltage have been required for the recent electrical equipment. That is, there is a demand for insulated wires that have a partial discharge inception voltage of several hundred volts order or more. Herein, the partial discharge inception voltage is a value that is measured by a commercially available apparatus called partial discharge tester. Measurement temperature, frequency of the alternating current voltage to be used, measurement sensitivity, and the like are values that may vary as necessary, but the above-mentioned value is an effective value of the voltage at which partial discharge occurs, which is measured at 25° C., 50 Hz, and 10 pC.
When the partial discharge inception voltage is measured, a method is used in which the most severe condition possible in the case where the insulated wire is used as a magnet wire is envisaged, and a specimen shape is formed which can be observed in between two closely contacting insulated wires. For example, in the case of an insulated wire having a circular cross-section, two insulated wires are brought into linear contact by spiral twisting the wires together, and a voltage is applied between the two insulated wires. Alternatively, in the case of an insulated wire having a rectangular cross-section, use is made of a method of bringing two insulated wires into planar contact through the planes, which are the long sides of the insulated wires, and applying a voltage between the two insulated wires.
In order to obtain an insulated wire that does not cause partial discharge, that is, having a high partial discharge inception voltage, so as to prevent the deterioration of the enamel layer of the insulated wire caused by the partial discharge, it is thought to utilize a method of using a resin having a low dielectric constant in the enamel layer or increasing the thickness of the enamel layer. However, the resins of commonly used resin varnishes generally have a dielectric constant between 3 and 5, and none of the resins have particular low dielectric constant. Further, upon considering other properties (heat resistance, solvent resistance, flexibility, and the like) required from the enamel layer, it is not necessarily possible to select actually a resin having a low dielectric constant. Therefore, in order to obtain a high partial discharge inception voltage, it is indispensable to increase the thickness of the enamel layer. When the resins having a dielectric constant of 3 to 5 are used in the enamel layer, if it is intended to obtain a targeted partial discharge inception voltage of 1 kVp or higher (a high peak value), it is necessary based on the experience to set the thickness of the enamel layer at 60 μm or more.
However, to thicken the in enameled layer, the number of times for passing through a baking furnace increases in a production process thereof, whereby making a film composed of copper oxide on a copper conductor surface thicker, this in turn, causing lowering in adhesion between the conductor and the backed enamel layer. For example, in the case of obtaining an enamel layer with thickness 60 μm or more, the number of passages through the baking furnace exceeds 12 times. It has been known that if this number of passages exceeds 12 times, the adhesive force between the conductor and the enamel layer is conspicuously lowered.
It is also thought to utilize a method of increasing the thickness that can be formed by a single baking step, in order not to increase the number of passages through the baking furnace. However, this method has a drawback that the solvent of the varnish is not completely vaporized and remains in the enamel layer as voids.
Further, it has become demanded to further improve various performances, such as heat resistance, mechanical properties, chemical properties, electrical properties, and reliability, in the electrical equipments developed in recent years, as compared to the conventional electrical equipments. Under the situations, excellent insulation property at high temperature and thermal aging resistance property as well as above-mentioned high partial discharge inception voltage have become required from insulated wires, such as enameled wires, that are used as magnet wires for electrical equipments for aerospace use, electrical equipments for aircraft, electrical equipments for nuclear power, electrical equipments for energy, and electrical equipments for automobiles.
To these problems, an attempt to provide a coated resin at the outer side of the enamel wire has been made (Patent Literatures 1 and 2). However, the insulated wires described in Patent Literatures 1 and 2 also still have room to improve a partial discharge inception voltage, insulation property at a high temperature, and a thermal aging resistance property. Further, Patent Literature 3 is cited as a technique for improving the partial discharge inception voltage.