1. Field of the Invention
The present invention relates to an insulated wire used for electrical equipments such as a motor or a transformer and, in particular, to an insulated wire with an insulating film formed by coating and baking an insulating varnish of a phenolic hydroxyl group-containing polyimide resin on a conductor.
2. Related Art
An enameled wire, which is a kind of an insulated wire, is formed by coating and baking an enamel on a conductor. A coil for electrical equipments obtained by coiling this enameled wire is broadly and practically used for large capacity or large size heavy electrical machinery. Furthermore, in recent years, the enameled wire is more likely to be used by high-voltage inverter control along with downsizing and high performance of the electrical equipments.
In general, an enameled wire is provided with an insulating film of insulating varnish on a conductor. When an electrical equipment such as a motor or a transformer (a voltage inverter) or the like is produced using this enameled wire, a major method is generally conducted such that the enameled wire is continuously wound into a coiled shape around a slot of a core (a magnetic core) of the motor or that the enameled wire wound in a coiled shape is engaged/inserted with/into the slot.
Meanwhile, in case of an enameled wire with a large cross-sectional area, namely, a large outer diameter, or an enameled wire having a rectangular conductor, not a method to form a long coil with many turns of wire by continuously coiling the enameled wire, but a method to form a long coil by forming a plurality of short small diameter coils with few turns of wire and connecting terminals of the enameled wire of these small diameter coils by welding (for example, electrical welding), has been proposed. The coil formed as above is used as a coil for small size electrical equipments requiring high-density magnetic flux, for example, a coil of an electric generator of a vehicle.
For a coil used for an electric generator of a vehicle, a double coated wire provided with a polyamide-imide insulating film formed by coating and baking insulating varnish composed of a polyamide-imide resin around a polyester imide insulating film which is formed by coating and baking insulating varnish composed of a polyester imide resin around a conductor or a single coated wire provided with an insulating film formed by coating and baking insulating varnish composed of a polyamide-imide resin around a conductor are mainly used.
Furthermore, in part, a double coated wire of which heat resistance and mechanical strength are improved by forming a polyimide insulating film by coating and baking insulating varnish composed of a polyimide resin around a conductor and providing an insulating film by coating and baking insulating varnish composed of a polyamide-imide resin around the polyimide insulating film, is used (for example, disclosed by JP-A-5-130759).
Furthermore, insulating varnish containing a saturated polyester resin and a resol-type phenolic resin is used for improving thermochromism (for example, disclosed by JP-A-62-127361).
In the meantime, although a polyamide-imide enameled wire and a polyimide enameled wire are heavily used as a heat-resistant enameled wire due to a high continuous heat resistance category such as 180-220° C. class, permittivity of the polyamide-imide insulating film or the polyimide insulating film is high due to its polar character while having high heat resistance. Furthermore, there was a problem in that the permittivity or a dielectric loss tangent is high due to high water absorption.
In case of the inverter control, high surge voltage generated from an inverter enters a motor and exerts a bad influence on a motor insulation system. When an insulation property between enameled wires wound in a coiled shape is not enough, the deterioration of the insulating layer is accelerated by corona discharge. In case of being over corona inception voltage of an insulating film against inverter surge voltage at the time of such high-frequency voltage application, the corona discharge is not generated and the operating life lasts long.
Thickening of the insulating film and lowering of permittivity are included as a method to raise the corona inception voltage.
Concretely, it is known that an enameled wire having low water absorption and excellent dielectric property is obtained by forming an insulating film by coating and baking insulating varnish formed by dissolving a phenol compound into an aromatic polyester resin around a conductor (for example, disclosed by JP-A-2003-16846).
Furthermore, in order to lower the permittivity of the insulating film, it has been suggested to coat and bake insulating varnish of fluorine system polyimide resin on the surface of the conductor.
However, the conventional insulating film has the problems that harmful fluorine system gas is generated during high-temperature baking despite low permittivity thereof, and low adhesiveness to the conductor of the insulating film may cause a loose part between the conductor and the insulating film, so that insulation breakdown may occur at low voltage during the voltage application.
Furthermore, the thickening of the insulating film is not preferable from the viewpoint of a lamination factor since it greatly affects to a motor design such that the motor grows in size.