JP7793932B2 - insulated wire - Google Patents

Description

This disclosure relates to insulated electrical wire.

2. Description of the Related Art Insulated wires are known in which an insulating coating is provided on the circumferential surface of a long conductor (see, for example, Patent Document 1).
The insulated wire is used, for example, in coils of industrial motors.

Industrial motors are driven at high voltages when high output is required. They are also inverter-driven, where the motor's speed is controlled by a variable voltage or variable frequency AC power supply using an inverter.

Japanese Patent Application Publication No. 9-106712

In inverter driving, switching causes reflection at points of impedance discontinuity, resulting in an inverter surge, a phenomenon in which a voltage about twice the output voltage is applied.
The high voltages applied to industrial motors for high-voltage operation and inverter surges generated by inverter drive can cause partial discharges in the insulating coating of the insulated wires used in the motor coils.Partial discharges can erode the insulating coating, causing insulation failure.

To solve this problem, it is possible to increase the partial discharge inception voltage and suppress the occurrence of partial discharges by using an insulating coating with a low relative dielectric constant. Here, one way to reduce the relative dielectric constant of the insulating coating is to create multiple voids within the insulating coating.

However, when multiple voids are provided in the insulating coating, there is a possibility that, for example, the multiple voids in the insulating coating may become connected and bonded (hereinafter also referred to as interconnected) along the thickness direction of the insulating coating. When an insulated electric wire having an insulating coating with such interconnected void portions (hereinafter also referred to as interconnected portions) is bent or stretched into a spiral shape in order to process it into a motor coil, the force applied in the tensile direction during bending or stretching may cause cracks in the insulating coating (hereinafter also referred to as coating cracks) to occur along the thickness direction, originating from the interconnected portions. These coating cracks may then reduce the insulating properties of the insulating coating.

The objective of this disclosure is to provide an insulated wire that suppresses the occurrence of partial discharges and coating cracks, even when used in the coils of motors that are driven at high voltages or by inverters.

One aspect of the present disclosure is an insulated wire having a long conductor and an insulating coating composed of one or more laminated insulating layers surrounding the conductor, wherein the insulating layer has a void region and a resin region. The void region is composed of resin and a plurality of voids. The resin region is composed of resin. The insulating layer has no interface between a first interface on the radial inner side and a second interface on the radial outer side, and the void region and resin region are arranged in this order from the first interface to the second interface.

With this configuration, the insulating layer included in the insulating coating has a void region with voids. This allows the relative dielectric constant to be lowered compared to an insulating coating that does not have an insulating layer with voids, making it easier to increase the partial discharge inception voltage.

Furthermore, within the insulating layer, the resin regions are positioned radially outward of the void regions. With this configuration, within the insulating layer, the resin regions are provided in areas far from the conductor, which is more likely to be subjected to tensile forces due to bending or stretching, and the void regions are provided in areas close to the conductor, which is less likely to be subjected to tensile forces due to bending or stretching. This makes it possible to prevent film cracks originating from the communication portions, even if tensile forces are applied due to bending or stretching.

1 is a cross-sectional view showing an outline of a cross section perpendicular to the longitudinal direction of an insulated wire in the present embodiment. FIG. 2 is a cross-sectional view schematically illustrating the inside of one insulating layer in the present embodiment. FIG. 2 is a cross-sectional view schematically illustrating a laminated structure of insulating layers in an insulating coating according to the present embodiment. FIG. 1 is a diagram showing an image of a cross section of an insulating coating taken by SEM. FIG. 2 is a diagram showing an enlarged image of a cross section of an insulating coating taken by SEM. FIG. 1 is a diagram showing measurement results in an example and a comparative example. FIG. 1 is a diagram schematically illustrating an example of a laminated structure of insulating layers inside an insulating coating according to a conventional technique. 1 is a diagram showing a schematic representation of a film crack and a communicating portion formed by the communication of pores. FIG.

[1. Configuration]
The insulated wire 1 of this embodiment will be described as being applied to an example in which it is an enameled wire used for a coil of a motor or the like.

FIG. 1 shows a schematic cross-sectional view of an insulated wire 1 taken along a plane perpendicular to the longitudinal direction thereof.
As shown in Fig. 1, an insulated wire 1 is composed of an elongated conductor 3 and an insulating coating 5 covering the circumferential surface of the conductor 3. In this embodiment, the conductor 3 has a circular cross section.

In this example, the conductor 3 is a commonly used metal conductor. Metals used for the conductor 3 may include, for example, copper, copper alloys, aluminum, or aluminum alloys. Conductor 3 may also be, for example, low-oxygen copper with an oxygen content of 30 ppm or less, or oxygen-free copper.

In this embodiment, the conductor 3 is a round copper wire having a diameter of 0.8 mm.
The insulating coating 5 covers the peripheral surface of the conductor 3 and prevents electrical conduction due to contact between an object outside the insulating coating 5 and the conductor 3 located inside the insulating coating 5 .

The "outside" here refers to the side where the insulating coating 5 is located relative to the conductor 3 along the radial direction of the conductor 3 in a cross section perpendicular to the longitudinal direction of the conductor 3, and the "inside" refers to the opposite side, where the conductor 3 is located relative to the insulating coating 5 along the radial direction of the conductor 3 in a cross section perpendicular to the longitudinal direction of the conductor 3.

The following description will be given on the assumption that the insulating coating 5 is made of a thermosetting resin, but the thermosetting resin may be polyimide or polyamideimide.
In this embodiment, the description will be given by taking an example in which polyimide is used as the thermosetting resin used for the insulating coating 5 .

The insulating coating 5 may consist of a single insulating layer 51 formed by applying a single coating layer containing a thermosetting resin around the conductor 3 and then baking (curing) the coating layer once. Alternatively, the insulating coating 5 may consist of multiple insulating layers 51 made of the same type of insulating coating, which are formed by repeatedly applying and baking the insulating coating described above. Each insulating layer 51 has a thickness of 1 μm or more but less than 10 μm (e.g., approximately 3 μm). The stacking of the multiple insulating layers 51 forms an interface between the inside and outside of the contact points between adjacent insulating layers 51, while the interior of the insulating layer 51 (i.e., the inner surface of the insulating layer 51 that contacts the outer surface of the conductor 3) does not have an interface. The interface of the insulating layer 51 here refers to, for example, the surface that forms the boundary between the insulating layer 51 and other layers. Specifically, it may be the layer between two radially adjacent insulating layers 51 or the boundary between the insulating layer 51 and a gas layer such as air. In the following, for each of the multiple insulating layers 51, the inner interface along the radial direction will also be referred to as the first interface, and the outer interface along the radial direction will also be referred to as the second interface.

Figure 2 is a cross-sectional view schematically showing the inside of one insulating layer 51. Figure 2 is also a cross-sectional view perpendicular to the longitudinal direction of the insulated wire 1. In Figure 2, the upper side of the page will be described as the outside of the insulated wire 1, and the lower side will be described as the inside of the insulated wire 1. Note that in Figures 3 to 5 and 7 to 8, the upper side of the page will be described as the outside of the insulated wire 1, and the lower side will be described as the inside of the insulated wire 1. For the sake of explanation, Figures 2, 3, 7, and 8 depict the upper and lower surfaces of the insulating layer 51 as flat, but the insulating layer 51 may have a curved shape that follows the shape of the circumferential surface of the conductor 3.

As shown in FIG. 2 , the insulating layer 51 has a plurality of voids Va therein. Hereinafter, a region of the insulating layer 51 in which no voids Va are formed and which is formed by resin will be referred to as a resin region 511, and a region containing resin and a plurality of voids Va will be referred to as a void region 513. That is, in this embodiment, the resin region 511 is a void-free region that does not have voids Va. Also, in this embodiment, the resin that constitutes the void region 513 is made of the same resin as the resin that constitutes the resin region 511.

In this embodiment, the size of the voids Va contained in the void region 513 is described as being between 0.1 μm and 2 μm. The shape of the voids Va is, for example, elliptical or circular.

In the insulating layer 51, the resin region 511 is located toward the outside of the insulating layer 51, and the void region 513 is located toward the inside of the insulating layer 51. In other words, the resin region 511 is located in an area far from the conductor 3 in the thickness direction (i.e., radial direction) of the insulating layer 51, and the void region 513 is located in an area close to the conductor 3 in the thickness direction of the insulating layer 51. The inner surface of the void region 513 is the first interface in the insulating layer 51, and the outer surface of the resin region 511 is the second interface in the insulating layer 51.

The thickness of the resin region 511 included in one insulating layer 51 is 5% to 70% of the thickness of the insulating layer 51 .
FIG. 3 is a cross-sectional view showing a schematic representation of the laminated structure of the insulating layer 51 in the insulating coating 5.

As shown in FIG. 3 , in each of the multiple insulating layers 51 that form the insulating coating 5, the region outside the insulating layer 51 is a resin region 511, and the region within the insulating layer 51 is a void region 513. That is, in the multiple stacked insulating layers 51, the resin regions 511 and the void regions 513 are alternately arranged adjacent to each other along the radial direction of the insulated wire 1. Specifically, as shown in FIG. 3 , this description applies to an example in which insulating layer 51a, insulating layer 51b, and insulating layer 51c are stacked in this order from the inside of the insulating coating 5. Note that, hereinafter, the resin regions 511 of insulating layers 51a, 51b, and 51c are also referred to as resin region 511a, resin region 511b, and resin region 511c, respectively, and the void regions 513 of insulating layers 51a, 51b, and 51c are also referred to as void region 513a, void region 513b, and void region 513c, respectively.

Between the inner insulating layer 51a and the adjacent insulating layer 51b, the resin region 511a of the insulating layer 51a is adjacent to the void region 513b of the insulating layer 51b. Similarly, between the adjacent insulating layer 51b and the adjacent insulating layer 51c of the insulating layer 51b, the resin region 511b is adjacent to the void region 513c.

The outer surface of the insulating layer 51 located at the outermost position in the insulating coating 5 is the surface of the resin region 511 (second interface).
Fig. 4 shows an image of a cross section of the insulating layer 51 taken by an SEM. Fig. 5 shows an enlarged view of the insulating layer 51. The enlarged view shown in Fig. 4 was taken with the SEM set at a magnification of 2000 times. SEM here is an abbreviation for scanning electron microscope.

As shown in FIGS. 4 and 5, in the insulating layer 51 photographed by SEM, the boundary between the resin region 511 and the void region 513 is not observed.
<Insulating film material>
The polyimide forming the insulating film 5 is produced by polymerizing a diamine and a tetracarboxylic dianhydride and imidizing the resulting polyamic acid.

Examples of diamines that may be used include 1,4-bis(4-aminophenoxy)benzene (TPE-Q), 1,3-bis(4-aminophenoxy)benzene (TPE-R), 1,3-bis(3-aminophenoxy)benzene (APB), 4,4'-bis(4-aminophenoxy)biphenyl (BODA), and 4,4'-diaminodiphenyl ether (ODA).

Examples of tetracarboxylic dianhydrides that may be used include 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (DSDA), 4,4'-oxydiphthalic dianhydride (ODPA), 4,4'-(2,2-hexafluoroisopropylidene)diphthalic anhydride (6FDA), pyromellitic dianhydride (PMDA), and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA).

The polyimide, which is a polymer material used for the insulating film 5, may have its polymer ends capped.
The material used for capping may be a compound containing an acid anhydride or a compound containing an amino acid.

Examples of compounds containing an acid anhydride that may be used for capping include phthalic anhydride, 4-methylphthalic anhydride, 3-methylphthalic anhydride, 1,2-naphthalic anhydride, maleic anhydride, 2,3-naphthalenedicarboxylic anhydride, various fluorinated phthalic anhydrides, various brominated phthalic anhydrides, various chlorinated phthalic anhydrides, 2,3-anthracenedicarboxylic anhydride, 4-ethynylphthalic anhydride, and 4-phenylethynylphthalic anhydride.

The compound containing an amino group used for capping may be a compound containing one amino group.
The synthesis of the polyimide used for the insulating coating 5 will be described by taking an example in which the material is synthesized in a state of being dissolved in a solvent. Also, the synthesis of the polyimide will be described by taking an example in which the polyimide dissolved in a solvent is used as an insulating coating material.

Polar aprotic solvents such as N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and N,N-dimethylsulfoxide (DMF) may be used as solvents for synthesizing the polyimide used in the insulating coating 5 and for the coating material, as well as gamma-butyrolactone, dimethylimidazolidinone (DMI), cyclohexanone, methylcyclohexanone, and hydrocarbon solvents. Note that multiple of these solvents may be used in combination.

In this embodiment, the polyimide used in the insulating coating 5 is described as having a molar ratio of 100:100 between the acid anhydride component and the diamine component. The molar ratio between the acid anhydride component and the diamine component is not limited to 100:100, and may vary to the extent that the flexibility of the insulating coating 5 is not impaired. For example, the diamine component may be blended in excess of the acid anhydride component.

Polyimide blended at such a molar ratio can reduce the molecular weight and viscosity of the coating material. As a result, the workability of applying the insulating coating material to form the insulating coating 5 described below can be improved. For example, the blending molar ratio of the acid anhydride component to the diamine component may be in the range of 100:100.1 or more and 100:100.7 or less.

Conversely, the acid anhydride component may be blended in excess relative to the diamine component.
The insulating varnish that forms the insulating coating 5 is synthesized at a temperature that does not impair the properties of the polyamic acid. A specific temperature may be, for example, a temperature between 0°C and 100°C. After the insulating varnish that forms the insulating coating 5 is synthesized, the viscosity of the insulating varnish that forms the insulating coating 5 may be adjusted by stirring the varnish while heating it to, for example, a temperature between 50°C and 100°C.

In this embodiment, the pores Va of the pore region 513 of the insulating coating 5 are formed by using a foaming agent.
[2. Effect]
<Method for producing insulated wire>
The procedure for producing the insulated wire 1 will be described.

In this embodiment, an example will be described in which a round copper wire with a diameter of 0.8 mm is used as the conductor 3. An insulating varnish is applied to the conductor 3 using a die with a gap of 25 μm to 30 μm, and the insulating layer 51 is formed by baking in a furnace with a temperature gradient of 300°C to 400°C. Repeated application and baking of the insulating varnish results in multiple layers of insulating film 5, each with a thickness of 40 μm. This embodiment will be described as an example in which a single insulating layer 51 with a thickness of approximately 3 μm is formed by a single application and baking process.

<Method of synthesizing insulating paint>
The insulating coating material used for the insulating coating 5 of the insulated wire 1 was synthesized by the following procedure. Insulated wires 1 produced under different conditions will be described as Examples 1 to 3, respectively, and methods for producing insulated wires produced for comparison with the insulated wires 1 produced in Examples 1 to 3 will be described as Comparative Examples 1 to 6.

Example 1
Equal amounts of 4,4'-diaminodiphenyl ether and 1,3-bis(4-aminophenoxy)benzene, which serve as diamine raw materials (hereinafter also referred to as diamine raw materials), are dissolved in DMAc at a molar ratio of 1.00 mol of 4,4'-diaminodiphenyl ether to 1.00 mol of 1,3-bis(4-aminophenoxy)benzene.

Next, 1.03 mol of pyromellitic dianhydride and diphenyl-3,3',4,4'-tetracarboxylic dianhydride (s-BPDA), which serve as the acid dianhydride raw materials (hereinafter also referred to as acid dianhydride raw materials), are dissolved in equal amounts per 1.00 mol of the diamine raw material.

A polyimide coating material is obtained by stirring the mixture in nitrogen at room temperature for 12 hours. The polyimide coating material is an insulating coating material in which a polyimide precursor (polyamic acid) made from a diamine raw material and an acid dianhydride raw material is dissolved or dispersed in a solvent.
The polyimide paint obtained by this method will hereinafter also be referred to as insulating paint P1x (paint 1 shown in FIG. 6).

A compound (high-boiling point solvent) that dissolves in the main solvent, DMAc, and has a boiling point of 210°C or higher is added to the insulating paint P1x as a foaming agent. The paint with the foaming agent added to the insulating paint P1x is also referred to as insulating paint P1.

Example 2
The diamine raw material, 4,4'-diaminodiphenyl ether, is dissolved in DMAc. Next, 1.05 mol of pyromellitic dianhydride, the acid dianhydride raw material, is dissolved in 1.00 mol of the diamine raw material. The solution is then stirred in nitrogen at room temperature for 12 hours to obtain a polyimide coating material.

The polyimide paint obtained by this method will hereinafter also be referred to as insulating paint P2x.
A compound (high boiling point solvent) that dissolves in DMAc, the main solvent, and has a boiling point of 210° C. or higher is added as a foaming agent to the insulating paint P2x. The paint obtained by adding the foaming agent to the insulating paint P2x (paint 2 shown in FIG. 6) is also referred to as insulating paint P2.

Example 3
Trimellitic anhydride and 4,4'-diphenylmethane diphenylmethane diisocyanate are dissolved in NMP. The molar ratio of 4,4'-diphenylmethane diphenylmethane diisocyanate dissolved in NMP is 1.00 mol of trimellitic anhydride to 1.02 mol of trimellitic anhydride. The solution is then stirred in nitrogen at 160°C for 4 hours to obtain a polyamideimide coating material. Polyamideimide coating material refers to an insulating coating material in which polyamideimide is dissolved or dispersed in a solvent.

The polyamideimide paint obtained by this method will hereinafter also be referred to as insulating paint P3x (paint 3 shown in FIG. 6).
A compound (high boiling point solvent) that dissolves in NMP, the main solvent, and has a boiling point of 210° C. or higher is added as a foaming agent to the insulating paint P3x. The paint obtained by adding the foaming agent to the insulating paint P3x is also referred to as insulating paint P3.

(Comparative Example 1)
In Comparative Example 1, the insulating paint P1x was used as the insulating paint without adding a foaming agent to the insulating paint P1x.

(Comparative Example 2)
In Comparative Example 2, the insulating paint P1 was prepared by adding degradable polymer particles having a particle size of about 1.0 μm as a foaming agent to the insulating paint P1x.

(Comparative Example 3)
In Comparative Example 3, the insulating paint P2x was used as the insulating paint without adding a foaming agent to the insulating paint P2x.

(Comparative Example 4)
In Comparative Example 4, the insulating paint P2 was prepared by adding degradable polymer particles having a particle size of about 1.0 μm as a foaming agent to the insulating paint P2x.

(Comparative Example 5)
In Comparative Example 5, the insulating paint P3x was used as the insulating paint without adding a foaming agent to the insulating paint P3x.

(Comparative Example 6)
In Comparative Example 6, the insulating paint P3 was prepared by adding degradable polymer particles having a particle size of about 1.0 μm as a foaming agent to the insulating paint P3x.

<Method for calculating the thickness ratio of the resin region>
The thickness of each of the resin region 511 and the void region 513, and the ratio of the thickness of each of the resin region 511 and the void region 513 to the thickness of the insulating layer 51, are calculated based on an image of the cross section of the insulated wire 1 taken with an SEM.

Specifically, first, the insulating coating 5 is cut in a direction perpendicular to the longitudinal direction of the enameled wire, and the cut surface is polished. Next, the polished cut surface is imaged using an SEM, and the SEM image is used for calculations.

The magnification when imaging using an SEM is adjusted appropriately, for example, within the range of 2000x to 5000x.

In this embodiment, the thickness ratio of the resin region 511 and the void region 513 is calculated as the ratio of the thickness of the resin region 511 and the void region 513 to the thickness of the insulating layer 51.

The thickness of the resin region 511 refers to the length along the thickness direction from the radially outer interface (second interface) of an insulating layer 51 to the outer boundary of the void Va located at the outermost radial position of the insulating layer 51. The thickness of the void region 513 refers to the length along the thickness direction from the outer boundary of the radially outermost void Va of an insulating layer 51 to the radially inner interface (first interface) of the insulating layer 51.

By measuring the thickness of the resin region 511 and the void region 513 using this method, it is easy to measure the thickness even in SEM images where the boundary between the resin region 511 and the void region 513 cannot be observed.

The thickness ratio of the resin region 511 and the void region 513 is not limited to being calculated separately from the resin region 511 and the void region 513 and the insulating layer 51. For example, if the thickness ratio of the resin region 511 to the insulating layer 51 is calculated, the remaining thickness ratio may be calculated as the thickness ratio of the void region 513.

The thickness of the resin region 511 and the void region 513 can be changed by adjusting, for example, the boiling point of the foaming agent or the amount of foaming agent added.
For example, when a foaming agent with a high boiling point is used, the thickness of the resin region 511 becomes smaller. Specifically, when the boiling point of the foaming agent is approximately 290°C, the thickness of the resin region 511 becomes approximately 10% or more and 20% or less. Conversely, when a foaming agent with a low boiling point is used, the thickness of the resin region 511 becomes larger. A foaming agent with a preferred boiling point is, for example, 210°C or more and 350°C or less. When the amount of foaming agent added is reduced, the thickness of the resin region 511 becomes larger, and when the amount of foaming agent added is increased, the thickness of the resin region 511 becomes smaller.

<Method for measuring porosity>
The porosity of the pore region 513 of the insulating coating 5 of the insulated wire 1 was measured by the underwater substitution method. Specifically, an insulated wire 1 having a predetermined length, such as 1 m, was immersed in water to replace the air inside the pores Va with water. The specific gravity of the insulated wire 1 with the pores Va replaced with water was measured before and after peeling of the insulating coating 5. The specific gravity of an insulated wire 1x having the same length and made of the same material as the insulated wire 1 but having an insulating layer 51x without pores Va was measured before and after peeling of the insulating layer 51x. The porosity was determined by comparing the specific gravity of the insulated wire 1 having the insulating coating 5 with the specific gravity of the insulated wire 1x without pores Va before and after peeling.

Porosity is calculated as follows: Porosity (%) = (ρ1 - ρ2) / ρ1 x 100. Here, ρ1 is the specific gravity of the entire insulating coating 5 when no pores Va are present, and ρ2 is the specific gravity of the entire insulating coating 5 including the pores Va.

In this embodiment, the porosity of the insulating coating 5 as a whole is described as being equal to or greater than 2% and less than 25%.
<PDIV measurement>
Using a twisted pair cable made from the insulated wire 1, a 50 Hz voltage was increased in the range of 10 V/s to 30 V/s in an atmosphere of 23°C and 50% humidity. The voltage at which a 50 pC discharge occurred 50 times was defined as the partial discharge inception voltage (PDIV).

The target PDIV value is 950 Vp when polyimide is used as the material for the insulating coating 5, and 830 Vp when polyamideimide is used as the material for the insulating coating 5.

<Flexibility test>
The flexibility test evaluates the flexibility of the insulated wire 1 by simulating the insulated wire 1 after being processed into a coil.

In Examples 1, 2, and Comparative Example 1, which used polyimide as the insulating coating 5, the insulated wire 1 was stretched by 30% and then self-diameter wrapped 50 times. The presence or absence of coating cracks was then checked. The target for coating cracks was no coating cracks.

In Examples 1, 2, and Comparative Example 1, in which polyamideimide was used as the insulating coating 5, the insulated wire 1 was stretched by 30% and then self-diameter wrapped 50 times. The presence or absence of coating cracks was then checked. The target for coating cracks was no coating cracks. In Figure 6, samples without coating cracks in the flexibility test were marked with a "◯" and samples with coating cracks were marked with an "X."

<Dielectric breakdown voltage (BDV) measurement>
Using a twisted pair cable produced from the insulated wire 1, a voltage was increased from 0.0 V to 20.0 kV at 50 Hz in the air, and the voltage at which dielectric breakdown occurred was taken as the dielectric breakdown voltage.

<Measurement results of Examples and Comparative Examples>
The measurement results of the insulated wires 1 of Examples 1 to 3 and the insulated wires of Comparative Examples 1 to 6 are shown in the table of FIG.

As shown in Examples 1 and 2 in the table in Figure 6, when the overall porosity is 20% or more, the target PDIV of 950 Vp is met when polyimide is used as the material for the insulating coating 5.

This is thought to be because the voids Va in the void region 513 can reduce the relative dielectric constant of the entire insulating coating 5, making it easier to increase the partial discharge inception voltage (PDIV) of the insulating coating 5.

While the PDIV meets the target PDIV, in Examples 1 and 2, the ratio of the thickness of the resin region 511 to the thickness of the insulating layer 51 is 50% and 20%, respectively, resulting in good flexibility. Self-diameter winding is good after 30% elongation, which is the target when polyimide is used for the insulating coating 5. Furthermore, insulated wires 1 in Examples 1 and 2 include insulating coatings 5 with void regions 513, and their breakdown voltages (BDV) are 16 kV and 15 kV, respectively. Furthermore, insulated wires 1 in Comparative Examples 1 and 3, which are formed with the same paint but have insulating coatings 5 without voids Va, have breakdown voltages (BDV) of 17 kV and 16 kV, respectively. In other words, the insulated wires 1 in Examples 1 and 2, which have insulating coatings 5 with void regions 513, can be evaluated as having breakdown voltages (BDV) comparable to those of insulated wires having insulating coatings 5 without voids Va.

On the other hand, in Comparative Examples 2 and 4, which do not have the resin region 511, film cracks occurred in the flexibility test. Furthermore, in Comparative Examples 2 and 4, which do not have the resin region 511, the breakdown voltages were 6 kV and 7 kV, respectively, which are lower than the breakdown voltages in the configurations with the resin region 511.

The insulated wire 1 of Example 3 uses polyamideimide for the insulating coating 5. In Example 3, the porosity is 20%. The PDIV of Example 3 is 850 Vp, which satisfies the target value of 830 Vp when polyamideimide is used for the insulating coating 5.

Furthermore, the insulated wire 1 of Example 3 can be evaluated as having good flexibility test results. That is, the self-diameter winding results after elongation of 20%, which is the target when using polyamideimide for the insulating coating 5, can be evaluated as good. Regarding breakdown voltage, the insulated wire of Comparative Example 5, which uses the same material but does not have voids Va, had a breakdown voltage of 17 kV, while the insulated wire 1 of Example 3 had a breakdown voltage of 15 kV. That is, the insulated wire 1 of Example 3, which has voids Va, can be evaluated as having a breakdown voltage similar to that of the insulated wire of Comparative Example 5, which does not have voids Va.

Comparative Example 6 had a lower porosity than Example 3, with the thickness of the resin region 511 being 2% of the thickness of the insulating coating 5, resulting in coating cracks during the flexibility test. Furthermore, the insulated wire 1 of Comparative Example 6 also had a lower breakdown voltage.

<Probable mechanism by which insulating film is formed on the periphery of a conductor>
A presumed mechanism by which the insulating coating 5 having the resin region 511 and the void region 513 is formed on the peripheral surface of the conductor 3 in the insulated wire 1 of the present disclosure will be described.

First, an insulating paint is applied to the conductor 3 to form the insulating coating 5. The film formed by applying the insulating paint is also called the paint film. Once the insulating paint is applied and baking begins on the conductor 3 with the paint film formed on it, the main solvent of the insulating paint evaporates. As the main solvent evaporates, the amount of main solvent in the paint that forms the paint film decreases. Furthermore, as the main solvent evaporates, phase separation occurs between the paint film and the blowing agent.

At this point, due to the reduction in the main solvent inside the paint film, phase separation occurs between the paint film and the foaming agent, and the foaming agent becomes dispersed throughout the paint film. The foaming agent dispersed within the paint film then volatilizes during further baking, forming voids Va. The portions of the paint film where these voids Va are formed become the void regions 513 of the insulating coating 5.

On the other hand, outside the paint film, the foaming agent is likely to be released from the paint film before phase separation occurs. Therefore, almost no foaming agent that phase separates from the paint film is present outside the paint film, and voids Va are not formed outside the paint film. The outer portion of the paint film that does not have voids Va becomes the resin region 511 of the insulating coating 5.

A blowing agent may be selected that has a boiling point that allows it to volatilize easily before phase separation outside the coating film. The phase separation state outside the coating film may also be adjusted to allow it to volatilize easily before phase separation.

As described above, each time the insulating paint is applied and baked, an insulating layer 51 is formed in which a resin region 511 is formed on the outside and a void region 513 is formed on the inside.
Furthermore, the method for applying the insulating varnish is not particularly limited, and any conventional application method may be used as long as it forms the resin region 511 and the void region 513 in the single insulating layer 51. Specifically, the insulating varnish is applied to the conductor 3, and then baked for about 1 to 2 minutes in an oven at a temperature of 350°C to 500°C, for example, to form the single insulating layer 51. By repeating this application and baking process, an insulating coating 5 having multiple insulating layers 51 is formed on the peripheral surface of the conductor 3. Furthermore, the thickness of the insulating coating 5 can be adjusted by adjusting the number of times the application and baking process are repeated.

The coating conditions may be adjusted depending on the type of foaming agent, the temperature of the heating oven, and the coating speed.
The thickness ratio of the resin region 511 in the insulating layer 51 may be adjusted by the type of foaming agent and the coating conditions.

By repeatedly applying and baking insulating paint, a plurality of insulating layers 51 are laminated.
According to this configuration, only one type of insulating paint is required to form the insulating coating 5 having the multiple insulating layers 51, which makes it easier to form the insulating coating 5 than when multiple insulating paints are prepared. Furthermore, since there is no need to change the painting equipment or painting conditions when multiple insulating paints are prepared, the painting process can be simplified.

Examples of foaming agents that can be used include ethyl glycols, propyl glycols, triglyme, and tetraglyme. Triglyme is also known as triethylene glycol dimethyl ether, and tetraglyme is also known as tetraethylene glycol dimethyl ether.

<Prevention of film cracking>
In the insulating coating 5 of the insulated wire 1 of this embodiment, the resin region 511 is a region outside the void region 513 in the multiple insulating layers 51 that are stacked.

For comparison, schematic diagrams of an insulating coating 9 having regions with voids throughout the insulating layer are shown in FIGS.
7 and 8 , as the number of voids Va inside the insulating layer increases, the voids Va become more interconnected. The interconnected voids Va will hereinafter also be referred to as interconnected portions S. Here, the interconnected portions of the voids Va are formed by connecting multiple voids Va together inside the insulating layer, forming a common internal space among the multiple voids Va.

In an insulating layer having a communicating portion S as shown in FIG. 8, when a force is applied to the insulated wire by bending or stretching, film cracks Cr are likely to occur starting from the communicating portion S.
On the other hand, in the insulating coating 5 of the insulated wire 1 of this embodiment, the outer region of the insulating layer 51, which is prone to tensile force due to bending or stretching, is the resin region 511 formed of resin. Therefore, in the resin region 511 formed of resin, communication portions where the voids Va communicate with each other are unlikely to occur. As a result, coating cracks originating from the communication portions are unlikely to occur.

<Measurement results for other examples>
Table 1 shows the measurement results for insulated wires 1 of Examples 4 to 6, which were produced using the same method as in Example 2 and in which the ratio of the thickness of resin region 511 to the thickness of insulating layer 51 was changed.

(Examples 4 to 6)
In Examples 4 to 6, an insulating paint was used in which a compound (high boiling point solvent) dissolved in the main solvent DMAc and having a boiling point of 210° C. or higher was added as a foaming agent to the insulating paint P2x.

As shown in Table 1, in the insulated wires 1 of Examples 4 to 6, the thickness ratio of the resin region 511 in the insulating layer 511 was 6% to 25%, and the target PDIV of 950 Vp was met when polyimide was used as the material for the insulating coating 5. This is thought to be because, as in Examples 1 and 2, the ratio of the void region 513, voids Va, was able to reduce the relative dielectric constant of the entire insulating coating 5, thereby increasing the partial discharge inception voltage (PDIV) of the insulating coating 5. Furthermore, in the insulated wires 1 of Examples 4 to 6, no coating cracks were found in the flexibility test.

<ATF resistance test>
The ATF resistance test was performed using the insulated wire 1 of Example 2 as Sample 1 and was evaluated by the following method. The measurement results are shown in Table 2.

The ATF resistance test begins by immersing Sample 1, consisting of a 25 cm long insulated wire 1 with a nearly circular cross section, in ATF (Automatic Transmission Fluid) with a water content of 0.2 wt%. The entire Sample 1 is immersed in ATF. Next, Sample 1 in this state is placed in a thermostatic chamber at 150°C for 1,000 hours. After 1,000 hours, Sample 1 is removed from the chamber, and any ATF adhering to Sample 1 is wiped off. Sample 1, from which the ATF has been wiped, is observed under a microscope with approximately 5x magnification to check for cracks on the surface of the insulating coating. Furthermore, to measure the relative permittivity of Sample 1, electrodes are formed on the surface of Sample 1 after it is removed from the chamber. To form the electrodes, a 100 mm long silver paste is applied to the insulating coating as the main electrode. Two 10 mm wide guard electrodes are then applied 10 mm away from the main electrode toward the edges of Sample 1. The silver paste used is Dotite D-550 manufactured by Fujikura Kasei, and it is recommended to use 10 mm wide tape for masking during application. The dielectric constant after immersion in ATF can be measured using the capacitance method between the silver paste and the conductor of Sample 1. To minimize the effect of absorbed moisture on the dielectric constant, the sample was heated in a constant temperature bath at 150°C for 1 hour to volatilize the moisture before measuring the dielectric constant. The frequency used when measuring the dielectric constant was 1 kHz. In Table 2, a "Good" is given if no cracks were found on the surface of the insulating coating after the ATF resistance test and the dielectric constant remained unchanged compared to before the ATF resistance test.

<Heat resistance test>
The heat resistance test was carried out by the following method using the insulated wires 1 of Examples 5 and 6 as Samples 2 and 3. The measurement results are shown in Table 2.

The heat resistance test begins by immersing samples 2 and 3, each consisting of an insulated wire 1 measuring 25 cm in length and having a nearly circular cross section, in ATF. The entire sample is immersed in ATF. Next, samples 2 and 3 are immediately removed from the ATF. After wiping off any ATF adhering to the samples, they are placed in a thermostatic chamber at 200°C for 1,000 hours. After 1,000 hours, samples 2 and 3 are removed from the chamber. After removing samples 2 and 3 from the chamber, they are observed under a microscope with a magnification of approximately 5x to check for cracks on the surface of the insulating coating. To measure the dielectric constant of samples 2 and 3, electrodes are formed on the surfaces of samples 2 and 3 after removal from the chamber. The electrode formation method and dielectric constant measurement method are the same as those used in the ATF resistance test described above. In Table 2, a "Good" is indicated if no cracks are observed on the surface of the insulating coating after the heat resistance test and the dielectric constant remains unchanged compared to before the heat resistance test.

As shown in Table 2, in Sample 1 using the insulated wire 1 of Example 2, after the ATF resistance test, no cracks or the like were found on the surface of the insulating coating, and the dielectric constant remained unchanged compared to before the ATF resistance test. Furthermore, as shown in Table 2, in Samples 2 and 3 using the insulated wires 1 of Examples 5 and 6, after the heat resistance test, no cracks or the like were found on the surface of the insulating coating, and the dielectric constant remained unchanged compared to before the heat resistance test. This is thought to be because, in the insulated wire 1 of this embodiment, the outermost surface of the insulating coating 5 is the resin region 511 that does not contain voids Va, and the absence of voids on the surface of the insulating coating 5 prevents ATF from penetrating into the interior of the insulating coating 5, thereby preventing an increase in the dielectric constant. In other words, the insulated wire 1 of this embodiment can be said to have good resistance to immersion in ATF.

3. Effects
(1) The insulated wire 1 of the above embodiment includes a conductor 3 formed in a long shape and an insulating coating 5 configured by laminating one or more insulating layers 51 that cover the conductor 3. The insulating layer 51 includes a void region 513 and a resin region 511. The void region 513 is configured of resin and a plurality of voids Va contained within the resin. The resin region 511 is configured of resin. The insulating layer 51 does not have an interface between a first interface on the radial inner side and a second interface on the radial outer side, and the void region 513 and the resin region 511 are arranged in this order from the first interface to the second interface.

With this configuration, the insulating layer 51 included in the insulating coating 5 has a void region 513 with voids Va. This makes it possible to lower the relative dielectric constant and increase the partial discharge inception voltage compared to an insulating coating 5 that does not have an insulating layer 51 with voids Va.

(2) In addition, within the insulating layer 51, the resin region 511 is disposed so as to be positioned radially outward of the void region 513.
According to this configuration, the resin region 511 is provided in the insulating layer 51 in a region far from the conductor 3 where a force in the tensile direction due to bending or stretching is likely to be applied, and the void region 513 is provided in a region close to the conductor 3 where a force in the tensile direction due to bending or stretching is unlikely to be applied. This makes it possible to suppress the occurrence of coating cracks Cr originating from the communicating portion S, even if a force in the tensile direction is applied due to bending or stretching.

(3) In this embodiment, polyimide is used as the thermosetting resin used as the material of the insulating coating 5 .
With this configuration, the insulating coating 5 has the mechanical properties, low relative dielectric constant, and heat resistance of polyimide.

(4) In this embodiment, the outermost layer of the insulating coating 5 is made up of a plurality of resin regions 511 that do not contain pores, and the surface of the insulating coating 5 does not have pores.
With this configuration, even if the insulating coating 5 having pores comes into contact with ATF (Automatic Transmission Fluid), the ATF can be prevented from penetrating into the insulating coating 5. Therefore, in the insulated wire 1 according to this embodiment, the relative dielectric constant of the insulating coating 5 is unlikely to increase even if the insulating coating 5 comes into contact with ATF. Furthermore, cracks (coating cracks) caused by ATF can be made less likely to occur in the insulating coating 5.

4. Other Embodiments
(1) In the insulated wire 1 of the above embodiment, the thickness of the resin region 511 is 5% to 70% of the total thickness of the insulating coating 5 .

Here, the thickness of the resin region 511 may be 20% or more of the total thickness of the insulating coating 5 .
Such a thickness ratio is suitable for suppressing film cracking because the ratio of the resin region 511 is greater than the ratio of the void region 513 having the voids Va.

(2) The thickness of the resin region 511 may be 50% or less of the total thickness of the insulating coating 5 .
Such a thickness ratio makes it easy to increase the proportion of the voids Va in the void region 513 in the entire insulating coating 5, and to reduce the relative dielectric constant of the insulating coating 5. This makes it easy to improve the partial discharge inception voltage (PDIV) of the insulating coating 5. This makes it easy to suppress the occurrence of partial discharges in the insulated wire 1.

(3) In the above embodiment, the thickness ratio of the resin region 511 to the insulating layer 51 is 5% or more and 70% or less. Furthermore, the thickness ratio of the void region 513 to the insulating layer 51 is 30% or more and 95% or less.

However, the thickness ratio of the resin region 511 and the thickness ratio of the void region 513 are not limited to those based on the insulating layer 51. For example, they may be calculated based on the thickness of the entire insulating coating 5. Specifically, based on the insulating coating 5, the total thickness of the resin region 511 included in each of the multiple insulating layers 51 included in the insulating coating 5 may be 5% or more and 70% or less, and the total thickness of the void region 513 included in each of the multiple insulating layers 51 included in the insulating coating 5 may be 30% or more and 95% or less.

(4) Note that the insulating coating 5 is not limited to being formed by stacking multiple insulating layers 51 made from the same material. For example, the insulating coating 5 may include insulating coatings 5 formed from different insulating paints. In this case, the insulating coatings 5 may be formed using different painting equipment and painting conditions for each insulating paint.

(5) An adhesive layer may also be provided between the conductor 3 and the insulating coating 5. The adhesive layer may be made of a material that can improve the adhesion between the conductor 3 and the insulating coating 5. There are no particular limitations on the thickness of the adhesive layer, but it is preferable that it does not impair the flexibility of the insulated wire 1. It is also preferable that the thickness of the adhesive layer does not reduce the partial discharge inception voltage. For example, the thickness of the adhesive layer is preferably 1 to 10 μm.

(6) The insulating coating 5 and the insulating paint used to form the insulating coating 5 may further contain additives. The type of additive is not particularly limited, but may be added, for example, to improve the strength of the insulating coating 5, improve the surface slipperiness of the insulating coating 5, improve the abrasion resistance of the insulating coating 5, improve the elongation characteristics, reduce the dielectric constant, or make the insulating coating semi-conductive. Furthermore, antioxidants may be used as additives.

(7) In the above embodiment, the cross-sectional outer shape of the insulated wire 1 including the conductor 3 and the insulating coating 5 is circular. However, the outer shape of each is not limited to a circular shape and may be rectangular or polygonal.

1, 1x...insulated wire, 3...conductor, 5, 9...insulating coating, 51, 51a, 51b, 51c, 51x...insulating layer, 511, 511a, 511b, 511c...resin region, 513, 513a, 513b, 513c...void region, P1, P2, P3, P1x, P2x, P3x...insulating paint, S...communicating portion, Va...void.

Claims (5)

a conductor formed in an elongated shape;
an insulating coating formed by laminating one or more insulating layers covering the conductor;
An insulated wire having
The insulating layer is
a pore region formed of a resin and a plurality of pores;
a resin region made of the resin;
and
an insulating layer having no interface between a first interface on the radially inner side and a second interface on the radially outer side, and the void region and the resin region are arranged in this order from the first interface to the second interface. The insulated wire according to claim 1,
the insulating layer has a surface of the pore region disposed at the first interface and a surface of the resin region disposed at the second interface. The insulated wire according to claim 1 or 2,
An insulated wire, wherein a ratio of a thickness of the resin region contained in the insulating layer to a thickness of the insulating layer is 5% or more and 70% or less. The insulated wire according to claim 3,
An insulated wire, wherein a ratio of a thickness of the resin region contained in the insulating layer to a thickness of the insulating layer is 20% or more. The insulated wire according to any one of claims 1 to 4,
an insulated wire, wherein a ratio of a thickness of the resin region included in the insulating layer to a thickness of the insulating coating is 50% or less;