I. Introduction: The “flat wire revolution” of new energy drive motor windings
With the explosive growth of the electric vehicle (EV) industry, the drive motor, as one of the “three major components” of electric vehicles (battery, motor, and electronic control), is undergoing a revolutionary upgrade from traditional round wire windings to flat wire (Hairpin/Flat Wire) windings. By 2025, global sales of new energy vehicles had exceeded 20 million units, with annual sales in China exceeding 12 million units, representing a penetration rate of over 50%. In mainstream new energy vehicle models, flat wire motors have become the de facto standard for high-power-density, high-efficiency drive motors.
Square/rectangular enameled copper wire ( flat wire ) is the core winding material for flat wire motors. Compared with the traditional circular enameled wire, the flat wire has significant advantages such as large cross-sectional area, high slot full rate, high conductivity, good heat dissipation performance, etc. It can increase the power density by 20-30% under the same motor volume, and is the preferred winding material for high-end electric vehicle drive motors such as 800V high-voltage platforms, high-speed motors, and oil-cooled/water-cooled motors.
As the world’s leading supplier of flat wires for new energy vehicles, LP Winding Wire specializes in the production of square/rectangular enameled copper wires with uniform film thickness, excellent insulation performance, high geometric accuracy, and high-speed winding . This article will systematically explain the technical characteristics and practical application of flat wires for new energy vehicles from multiple dimensions such as the technical principle of flat wires, paint film materials, key performance parameters, production processes, typical applications, selection guidelines, quality control, and market trends.
1.1 Market position of flat wire motors in new energy vehicles
Flat-wire motors dominate the new energy vehicle drive motor market. According to industry data, global new energy vehicle drive motor shipments will exceed 35 million units in 2025, of which flat-wire motor penetration will exceed 70% , and China’s new energy vehicle flat-wire motor penetration will reach more than 80%.
The reason why flat wire motors can quickly replace traditional circular wire motors is mainly due to its four core advantages :
Advantage 1: high slot full rate . The fill rate of the flat wire in the motor slot can reach 70-80%, which is much higher than the 40-50% of the circular wire. Under the same slot volume, the cross-sectional area of the copper wire is larger, the resistance is lower, and the efficiency is higher.
Advantage 2: High power density . The power density of the flat wire motor is 20-30% higher than that of the circular wire motor, which can output more power in a smaller volume, which is conducive to the miniaturization and lightweight of the motor.
Benefit 3: High efficiency . The copper loss of the flat wire motor is 15-25% lower than that of the round wire motor, the efficiency of the WLTP/CLTC working condition is improved by 2-3 percentage points, and the range is increased by 5-8%.
Advantage 4: Excellent heat dissipation . The contact area between the flat wire and the motor tank wall is large, the heat dissipation path is short, and the thermal resistance is low, which can support higher current density and peak power.
1.2 6 Core Advantages of Flat Wire
Flat wires have the following 6 core advantages over round wires:
Advantage 1: Large cross-sectional area . At the same wire height, the cross-sectional area of the flat wire is 1.2-1.4 times that of the circular wire, which can carry a larger current.
Advantage 2: high slot full rate . The full rate of the flat line groove is 70-80%, the full rate of the round line groove is 40-50%, and the copper filling amount is increased by 50-60% under the same groove volume.
Benefit 3: Short end size . The end height of the flat wire winding is 30-50% lower than the circular wire, and the axial dimension of the motor is more compact.
Advantage 4: Low DC resistance . Under the same number of winding turns and copper dosage, the DC resistance of the flat wire winding is 20-30% lower than that of the circular wire.
Advantage 5: Good heat dissipation . The contact area between the flat wire and the groove wall is large, and the thermal resistance is 30-50% lower, which can support higher current density.
Advantage 6: High mechanical strength . The flat wire has a large cross-sectional moment of inertia and strong resistance to deformation, and can withstand the centrifugal force when rotating at high speed.
1.3 5 main application scenarios for flat wires
Flat wires are widely used in the following 5 new energy vehicle scenarios:
Scenario 1: Bev drive motor . Flat wire is the mainstream choice for Bev drive motor windings, and flat wire motors are used in mainstream models.
Scenario 2: Plug-in hybrid electric vehicle (PHEV) drive motor . PHEV models have high power density and efficiency requirements for motors, and flat wire motors can meet the high power requirements of HEV/PHEV.
Scenario 3: Extended Range Electric Vehicle (EREV) Generator . The generator of the extended range electric vehicle works under high speed and high power density conditions, and the flat wire is the ideal material for it.
Application scenario four: 800V high-voltage platform drive motor . The 800V platform drive motor has high working voltage and strict insulation requirements, and the thin paint film and high insulation strength of the flat wire meet the requirements of the 800V motor.
Application scenario 5: Electric drive assembly (electric drive axle) . The electric drive assembly integrates the motor, reducer, and controller, requiring the motor to be small and efficient. The flat wire motor is the core choice of the electric drive assembly.
II. Chemical Structure and Insulation Mechanism of Flat Wire
2.1 Geometry of rectangular sections
The flat line is designed with a rectangular cross-section (Rectangular Cross-Section). Typical flat line cross-section parameters include:
- Width (W): 1.5-10.0 mm, determines the number of winding turns and motor slot width
- Height (H): 0.8-5.0 mm, determines copper wire cross-sectional area and winding thickness
- Width-to-Height Ratio (W/H): 1.5-5.0, determines the geometric characteristics and winding difficulty of flat wire
- Corner Radius (R): 0.2-0.8 mm, reduces paint film stress concentration
The aspect ratio of the flat wire is the key geometric parameter . The larger the aspect ratio, the more “flat” the copper wire, and the higher the slot full rate, but the greater the difficulty of painting, the larger the bending radius when winding. The smaller the aspect ratio, the closer the copper wire is to the “square”, and the more uniform the paint film is applied, but the groove full rate is reduced.
2.2 Flat wire paint film material
Flat wires use paint film materials similar to round wires, mainly including:
Polyesterimide (PEI) paint film . Class F (155°C), good mechanical strength, good heat resistance. Suitable for general NEV drive motors.
Polyamide imide (Pai) paint film . Class H (180°C) with excellent mechanical strength and heat resistance. Suitable for high power density drive motors.
Polyimide (PI) paint film . Class N (200°C) with excellent heat resistance and excellent chemical resistance. Suitable for ultra-high temperature, high power density motors.
Polyesterimide + Polyamideimide (PEI + Pai) composite paint film . The inner PEI provides electrical insulation and the outer Pai provides mechanical strength and heat resistance. It has excellent comprehensive performance and is the mainstream paint film structure of new energy vehicle flat wires.
Polyimide + Polyamideimide (PI + Pai) composite paint film . The inner layer PI provides ultra-high temperature insulation and the outer layer Pai provides mechanical strength. Suitable for 800V high voltage platform, ultra-high power density motor.
2.3 Insulation mechanism of flat wire paint film
The insulation mechanism of the flat wire paint film is the same as that of the circular wire, mainly relying on the polar groups and multilayer composite structures of the paint film:
Molecular level : The imide ring structure of PEI, the amide bond + imide ring structure of Pai, and the imide ring structure of PI all form a tight molecular stack, which hinders the migration of carriers.
Aggregate layer : Multi-layer composite paint film forms gradient insulation properties , the inner layer PEI/PI provides high dielectric strength, and the outer layer Pai provides mechanical strength and wear resistance to avoid breakdown caused by single layer defects.
Geometric level : 4 corners of the cross section of the flat line (Corner) is the difficulty of painting the film. The paint film is prone to uneven thickness and stress concentration at the edges, which is a high incidence area for insulation failure. High-quality flat wires should keep the thickness of the paint film uniform at the edges and corners, with strong adhesion and no cracking.
2.4 Special Design Requirements for Flat Lines
Flat wires have the following special design requirements compared to round wires:
Special Requirement 1: Uniformity of angular paint film . The thickness of the paint film at the 4 edges of the flat line section should be consistent with that at the plane, with a deviation of ≤ 20%.
Special Requirement 2: Flexibility of the paint film . The flat wire needs to withstand deformation such as bending and twisting during winding and forming, and the paint film must have excellent flexibility to avoid cracking.
Special requirement 3: Paint film adhesion . The flat wire is subjected to large mechanical stresses in Hairpin forming, twisting, welding and other processes, and the adhesion of the paint film must be extremely strong.
Special Requirement IV: Geometric Precision . The width, height and chamfer size of the flat wire directly affect the motor slot full rate and winding consistency, and the geometric accuracy should be controlled within ± 0.02 mm.
Special request 5: Surface finish . The surface of the flat line should be smooth and flat, free of burrs, scratches, and oxidation spots, and Ra ≤ 0.8 μm.
III. Key Performance Parameters of New Energy Vehicle Flat Wire
3.1 Geometric parameters
Width (W) . Typical range 1.5-10.0 mm, accuracy ± 0.02 mm.
Height (H) . Typical range 0.8-5.0 mm, accuracy ± 0.02 mm.
Aspect ratio (W/H) . Typical range 1.5-5.0, selected according to motor design.
Chamfer radius (R) . The typical range is 0.2-0.8 mm, which affects the uniformity of the paint film.
Film thickness . The thickness of the new energy automobile flat wire paint film is 50-100 μm, and the thickness of the 800V high-pressure flat wire paint film is 80-120 μm.
Paint film thickness deviation . The film thickness deviation at the plane is ≤ ± 3 μm, and the film thickness deviation at the edges is ≤ ± 5 μm.
3.2 Electrical performance parameters
Dielectric breakdown voltage . The dielectric breakdown voltage of the flat wire paint film should be ≥ 3 kV (the thickness of the paint film is 50-80 μm), and the 800V high-pressure flat wire should be ≥ 5 kV.
Film continuity . The number of pinholes per 30-meter flat wire is ≤ 3 (IEC level 1), and the quality product can reach 0-0.5 per 100 meters.
Insulation resistance . ≥ 2,000 MΩ · m at room temperature, ≥ 20 MΩ · m at 150°C.
Dielectric loss tangent . tan δ ≤ 0.01 (1 kHz, 20°C), ≤ 0.05 (1 kHz, 180°C).
Corona resistance . 800V high voltage flat wire shall have good corona resistance and corona life ≥ 100 hours.
PDIV (Partial Discharge Start Voltage) . 800V High Voltage Flat Wire PDIV ≥ 1.5 kV (peak), ensure that no partial discharge occurs at 800V operating voltage.
3.3 Mechanical performance parameters
Tensile strength . Tensile strength of flat wire 200-280 MPa (soft state), 350-450 MPa (hard state).
Elongation . Elongation of flat wire ≥ 30% (soft state), ≥ 10% (hard state).
Flexibility . Flat wires should not crack under a 1 × W bending radius (round bar bending test).
Film adhesion . Flat wire paint film does not crack or peel off after bending 180° at 90°.
Paint film wear resistance . Single scratch force ≥ 8 N (PEI/Pai film), ≥ 10 N (PI film).
Hairpin formability . The flat wire should be able to withstand the stretching, bending, and twisting of the Hairpin molding process without cracking the paint film.
3.4 Thermal performance parameters
Temperature level . Class F (155°C) PEI + Pai film, Class H (180°C) Pai film, Class N (200°C) PI + Pai film.
Thermal shock . After 180°C × 30 min thermal shock, 1 × W is bent without cracking or peeling.
Thermal aging . Accelerated aging at 180°C, breakdown voltage retention ≥ 80% after 20,000 hours.
Softening breakdown temperature . ≥ 300°C (PEI + Pai film), ≥ 350°C (PI + Pai film).
Heat shock resistance . Flat wires do not crack or peel during temperature cycling tests from -40°C to 180°C.
ATF resistant . Dielectric strength retention ≥ 80% after 500 hours of immersion of the flat wire in ATF (Automatic Transmission Fluid).
3.5 Chemical performance parameters
ATF resistant . After the flat wire is soaked in ATF, the paint film does not soften, blister, or peel off significantly.
Coolant resistance . After the flat wire was immersed in the ethylene glycol coolant, there was no significant change in the paint film.
Hydrolysis resistance . The dielectric strength retention rate is ≥ 70% after the flat wire is exposed to 95°C and 95% humidity for 1000 hours.
Salt spray resistance . After the flat wire was exposed to 5% NaCl salt spray for 500 hours, there was no significant corrosion of the paint film.
IV. Production process of new energy vehicle flat wires
4.1 Preparation of rectangular copper rods
The production of flat wires begins with the preparation of rectangular copper rods . There are two ways to prepare rectangular copper rods:
Method 1: Round copper rod extrusion . The round copper rod is extruded through the extrusion die, extruded into a rectangular cross-section, and then stretched to the target size through multiple passes.
Method 2: Round copper rod rolling method . The round copper rod is passed through a rolling mill, rolled into a rectangular cross-section, and then stretched to the target size through multiple passes. The rolling method is the mainstream method because of its high production efficiency.
Method 3: Continuous casting and rolling method . The electrolytic copper solution is cast through a continuous casting machine into a rectangular copper rod, which is rolled and stretched to the target size. Suitable for mass production.
The key quality control points for rectangular copper rods include:
- Cross-sectional size accuracy: width, height deviation ≤±0.02 mm
- Corner radius: R 0.2-0.8 mm, corner symmetry ≤±0.05 mm
- Surface finish: Ra ≤0.8 μm, no burrs, scratches
- Internal structure: uniform grain size, no shrinkage, porosity, inclusions
- Copper purity: ≥99.97%, oxygen content ≤10 ppm
4.2 Annealing and Surface Treatment
Annealing . After drawing, the rectangular copper rod needs to be continuously annealed at an annealing temperature of 400-600°C to recrystallize the copper grains and restore softness.
Surface cleaning . Use pickling, electrolytic cleaning, ultrasonic cleaning and other methods to remove the oxide layer, oil stain and lubricant residue on the surface of the copper wire.
Surface roughening . Light mechanical or chemical treatment increases the surface roughness of the copper wire and improves the adhesion of the paint film. The surface roughness Ra 0.5-1.5 μm is appropriate.
4.3 Preparation of lacquer
The formulation of the flat line paint is similar to the round line, but requires special adjustments :
Viscosity adjustment . The viscosity of the flat wire paint is generally 10-20% lower than that of the round wire paint, which facilitates uniform application of the paint on the rectangular cross-section.
Leveling adjustment . Adding a leveling agent improves the leveling of the paint at the edges and avoids the accumulation or thinning of the paint film at the edges.
Thixotropy adjustment . Flat wire paint should have appropriate thixotropy, high viscosity at rest and low viscosity when flowing, to prevent the paint from settling in the paint tank.
Curing agent adjustment . The flat wire paint film requires good flexibility, and the amount of curing agent should be appropriately reduced to avoid brittle cracks caused by excessive cross-linking of the paint film.
4.4 Coating and baking process
Coating process . The flat wire coating adopts mold paint method , the mold hole shape is rectangular (with chamfer), and the coating amount is precisely controlled by parameters such as mold aperture, paint viscosity, and linear velocity.
The coating process typically includes:
- Base coat coating (Die 1) + baking
- Middle coat coating (Die 2) + baking (optional)
- Top coat coating (Die 3) + baking
Baking and curing . The coated film is cured in a high-temperature oven. The baking temperature is usually divided into multiple segments:
- Pre-baking section: 150-200°C, allowing solvent to evaporate smoothly
- Main baking section: 300-450°C, promote resin cross-linking curing
- Cooling section: gradually cool to room temperature to set the paint film
Coat multiple times . Flat wire paint film is thicker (50-100 μm) and requires multiple coatings and baking (usually 4-8), each application increases the thickness of the paint film.
4.5 Hairpin formability
Hairpin molding is a key application process for flat wires, which must be able to withstand the stretching, bending and twisting of Hairpin molding:
First step: linear unloading . Cut the flat wire to a specified length (usually 200-500 mm).
Step 2: U-bend . Bend the flat wire into a U shape (Hairpin shape) on a special bending device, usually with a bending radius of 1-2 × W.
Step 3: Annealing . The bent flat wire is subjected to stress relief annealing (200-300°C × 30 min) to eliminate bending stress.
Step 4: Removal of the oxide layer . The paint film and oxide layer at the bent end are removed by laser stripping or mechanical polishing, exposing the copper conductor.
Step 5: Turn your head . Twist and flare multiple Hairpin ends to form an electrical connection.
Step 6: Weld . Connect the ends of the Hairpin by TIG welding, ultrasonic welding or laser welding to form a three-phase winding.
Hairpin Forming Requirements for Flat Lines:
- Paint film does not crack or peel at the bend
- Maintain good dielectric strength after bending
- Copper wire cross-sectional area at the bend is not significantly reduced
- Strong paint film adhesion, does not fall off after bending
4.6 Online Inspection and Quality Control
On-line detection of geometric dimensions . Measure the width, height, and chamfer dimensions of the flat wire online with an laser profiler with an accuracy of ± 0.005 mm.
Online testing of film thickness . Use the Eddy Current Thickness Gauge or the X-Ray Thickness Gauge to measure the film thickness online, at the plane and at the edges, respectively.
On-line detection of breakdown voltage . For each roll of flat wire sampling high voltage test, 800V flat wire should be ≥ 5 kV.
Pinhole detection . Use a pinhole tester to detect the continuity of the paint film, ≤ 3 pinholes per 30 meters.
Visual inspection . Check the surface defects of the flat lines with an industrial camera to identify burrs, scratches, uneven paint films, etc.
V. Comparison of flat wires and other motor winding materials
5.1 Flat vs. Round
Flat vs. round:
Cross-sectional area . At the same wire height, the cross-sectional area of the flat wire is 1.2-1.4 times that of the circular wire.
Groove full rate . Flat line 70-80%, round line 40-50%.
End height . The flat line is 30-50% lower than the round line.
DC resistance . Flat lines are 20-30% lower than round lines.
Efficiency . Flat wire motors are 2-3 percentage points more efficient than round wire motors.
Power density . The power density of the flat wire motor is 20-30% higher than that of the circular wire motor.
Production costs . Flat wire production equipment and processes are more complex than round wire and cost 30-50% more.
Difficulty of winding . Flat wire winding is difficult and requires special Hairpin forming equipment.
Flat lines have an overwhelming performance advantage over round lines, but are more costly.
5.2 Flat wire vs. copper-clad aluminum wire
Comparison of flat wire and copper-clad aluminum wire:
Material cost . Copper-clad aluminum wire costs 40-60% less than flat wire.
Weight . Copper clad aluminium wire is 30-50% lighter than flat wire.
Conductivity . Flat wire (pure copper) conductivity 100% IACS, copper-clad aluminum wire conductivity 60-70% IACS.
Weldability . Good flat wire weldability, poor copper-clad aluminum wire weldability.
Scenario . Flat wires for high power density, high efficiency drive motors; copper-clad aluminium wires for cost sensitive, lightweight demanding models.
Flat wires outperform copper-clad aluminium wires in performance, but at a higher cost.
5.3 Flat vs. Aluminum Flat
Flat wires vs. aluminum flat wires:
Material cost . Aluminum flat wire costs 50-70% less than flat wire.
Weight . Aluminum flat wire is 50-60% lighter than flat wire.
Conductivity . Flat wire (pure copper) conductivity 100% IACS, aluminum flat wire conductivity 60% IACS.
Mechanical strength . The mechanical strength of the flat wire is better than that of the aluminum flat wire.
Scenario . Flat wires for high power density, high efficiency drive motors; aluminum flat wires for lightweight, low cost models.
Flat wires are superior to aluminum flat wires in high power applications.
5.4 Comparison of different paint film flat lines
Comparison of different paint film flat lines:
PEI paint film flat wire . Class F (155°C) class, low cost, suitable for general NEV drive motors.
PEI + Pai composite film flat wire . Class H (180°C), with excellent comprehensive performance, is the mainstream choice for new energy vehicle flat wires.
PI + Pai composite film flat wire . Class N (200°C) class with excellent heat resistance for 800V high voltage, ultra high power density motors.
PI single film flat wire . Class N (200°C) with excellent heat resistance but poor mechanical strength.
PEI + Pai composite paint film is the best choice for current NEV flat wires.
VI. Typical Application of Flat Wire in New Energy Vehicles
6.1 Pure Electric Vehicle (Bev) Drive Motor
The pure electric vehicle drive motor is the largest application scenario of the flat wire. Flat-line motors are used in mainstream Bev models:
Model A . Flat wire motor, power density 5-6 kW/kg, CLTC efficiency 96-97%.
Model B . Flat wire motor, peak power 163-180 kW, maximum efficiency 97.5%.
Model C . Flat wire motor, peak power 180-300 kW, CLTC working efficiency 96%.
Model D . Flat line motor, peak power 230-405 kW, 800V high voltage platform.
Model E . Flat line motor, peak power 210-495 kW, 800V high voltage platform.
Model F . Flat wire motor, 800V high voltage platform, peak power 300-560 kW.
6.2 Plug-in hybrid vehicle (PHEV) drive motors
PHEV models have high power density and efficiency requirements for motors, and flat wire motors can meet the high power requirements of HEV/PHEV:
PHEV model A . Flat wire motor, peak power 132-160 kW, efficiency 96%.
PHEV model B . Adopt an extended range electric + flat line generator with a peak power of 100-150 kW.
PHEV model C . Adopt flat wire motor, HEV/PHEV dual mode.
6.3 Extended Range Electric Vehicle (EREV) Generator
The generator of the extended range electric vehicle works under high speed and high power density conditions, and the flat wire is the ideal material for it.
Mainstream extended-range electric vehicles use flat-line generators, 1.5T range extender + flat-line generators, and the peak power of the generator is 100-150 kW.
6.4 800V High Voltage Platform Drive Motor
The 800V high-voltage platform drive motor requires higher insulation performance for flat wires:
800V model A . 800V platform, flat wire motor, peak power 230-405 kW, CLTC efficiency 96-97%.
800V model B . 800V platform, flat wire motor, peak power 930 kW (4 motors).
800V model C . 800V platform, flat wire motor, peak power 495 kW.
800V model D . 800V platform, flat wire motor, peak power 560 kW.
6.5 Electric Drive Assembly (Electric Drive Axle)
The electric drive assembly integrates the motor, reducer, and controller, requiring the motor to be small and efficient. The flat wire motor is the core choice of the electric drive assembly:
Electric drive assembly A . Eight-in-one electric powertrain, integrated flat wire motor, 89% efficiency.
Electric drive assembly B . Flat-wire motors + single-speed transmissions + inverters with industry-leading integration.
Electric drive assembly C . ASS ‘Y electric drive, integrated flat wire motor, peak power 150-300 kW.
VII. Guidelines for the Selection of New Energy Vehicle Flats
7.1 Recommended specifications for different applications
Application 1: Standard Bev drive motor (400V platform) . PEI + Pai paint film flat wire is recommended, the width is 2.0-4.0 mm, the height is 1.0-2.5 mm, the thickness of the paint film is 50-80 μm, and the temperature class is 180°C.
Application 2: 800V high voltage Bev drive motor . Recommended PI + Pai film flat wire, width 2.0-5.0 mm, height 1.0-3.0 mm, film thickness 80-120 μm, temperature class 200°C, PDIV ≥ 1.5 kV.
Application 3: High speed Bev drive motor (> 18,000 rpm) . PI + Pai paint film flat wire is recommended, with a width of 1.5-3.0 mm and a height of 0.8-2.0 mm, requiring ultra-high geometric accuracy and high mechanical strength.
Application 4: PHEV drive motors . PEI + Pai film flat wire is recommended, with a width of 1.5-3.0 mm, a height of 0.8-2.0 mm, and a film thickness of 50-80 μm, requiring high power density.
Application Five: Extended Range Generator . PEI + Pai paint film flat wire is recommended, with a width of 1.5-3.0 mm and a height of 0.8-2.0 mm, which requires high efficiency and high reliability.
Application 6: Fuel Cell Vehicle (FCV) drive motors . PI + Pai paint film flat wire is recommended, which is required to be resistant to hydrogen permeation and low dielectric loss.
7.2 Common Misunderstandings in Model Selection
Myth 1: The thicker the paint film, the better . Overly thick paint film will reduce the groove full rate and affect the motor power density. The appropriate film thickness should be selected according to the operating voltage.
Myth 2: The larger the aspect ratio, the better . The aspect ratio is too large, the angular paint film is difficult to apply, and the winding bending radius is large. The appropriate aspect ratio should be selected according to the motor design.
Myth 3: Ignore the quality of the angular paint film . The corner is a high incidence area of flat line insulation failure, and special attention should be paid to the uniformity and adhesion of the corner paint film.
Myth 4: Ignore Hairpin formability . Flat wire is subjected to stretching, bending, twisting in Hairpin molding, and the paint film must have excellent moldability.
Myth 5: Ignore certification requirements . Tier 1 suppliers of the main engine plant need to comply with IATF 16949 and other certifications, and suppliers with corresponding certifications should be selected.
Myth 6: Ignore cooling medium tolerance . The flat wire of the oil-cooled/water-cooled motor should be resistant to ATF oil, ethylene glycol coolant and other media to avoid failure of the paint film.
VIII. Production quality control of flat wires
8.1 Raw material quality control
Quality control of copper rods . Copper purity ≥ 99.97%, oxygen content ≤ 10 ppm, resistivity ≤ 0.01724 Ω · mm ²/m (20°C), rectangular cross-section geometric accuracy meets the standard.
PEI resin quality control . Solid content 25-30%, viscosity 1,000-3,000 cP, molecular weight 30,000-80,000, residual solvent ≤ 1%.
Pai resin quality control . Solid content 20-30%, viscosity 2,000-8,000 cP, glass transition temperature ≥ 270°C, residual solvent ≤ 1%.
PI resin quality control . Solid content 15-25%, viscosity 1,000-5,000 cP, molecular weight 50,000-150,000, residual solvent ≤ 1%.
8.2 Process Quality Control
Preparation process of rectangular copper rod :
- Cross-sectional size accuracy ±0.02 mm
- Corner radius R 0.2-0.8 mm
- Surface roughness Ra 0.5-1.5 μm
- Internal structure without shrinkage, porosity, inclusions
Annealing process :
- Annealing temperature 400-600°C ±5°C
- Elongation after annealing 30-40%
- Resistivity after annealing meets standards
Coating process :
- Paint temperature 20-30°C ±1°C
- Paint viscosity 1,000-3,000 cP ±5%
- Coating speed 5-30 m/min ±0.5%
- Paint film thickness online measurement accuracy ±3 μm
Baking process :
- Baking temperature gradient setting meets process specifications
- Each stage temperature control accuracy ±5°C
- Tension control 50-200 g ±5 g
- Baking atmosphere flow rate stable
8.3 Quality control of finished products
Geometric dimension test : each batch sampling width, height, chamfer size, accuracy ± 0.02 mm.
Paint film thickness test : The thickness of the paint film at the sampling plane and at the edges of each batch, the deviation at the plane is ≤ ± 3 μm, and the deviation at the edges is ≤ ± 5 μm.
Electrical performance testing :
- Dielectric breakdown voltage: sampling inspection per batch
- Paint film continuity: 100% online detection
- Insulation resistance: sampling inspection per batch
- Corona resistance test: 800V high-voltage flat wire sampling
- PDIV test: 800V high-voltage flat wire sampling
Mechanical performance testing :
- Tensile strength: sampling inspection per batch
- Elongation: sampling inspection per batch
- Mandrel bending: sampling inspection per batch
- Hairpin forming test: sampling inspection per batch
Thermal performance testing :
- Heat shock: sampling inspection per batch
- Softening breakdown: sampling inspection per batch
- Thermal aging: periodic type test
- ATF oil resistance: periodic type test
- Coolant resistance: periodic type test
Appearance inspection
- Paint film color uniform and consistent
- No bubbles, impurities, particles
- No scratches, indentations
- No paint film cracking at corners
IX. Summary
Square/rectangular enameled copper wire (flat wire) As the core material of new energy vehicle drive motor windings, it has become the de facto standard for high-end electric vehicle drive motors such as 800V high-voltage platforms, high-speed motors, and oil-cooled/water-cooled motors with its high slot full rate, high power density, high efficiency, excellent heat dissipation, and excellent mechanical strength .
PEI + Pai composite paint film structure is the mainstream choice for new energy vehicle flat wires, which can meet the heat resistance requirements of Class H (180°C); PI + Pai composite paint film structure is the preferred choice for 800V high-voltage platforms and ultra-high power density motors, which can meet the heat resistance requirements of Class N (200°C) and high-voltage insulation requirements of PDIV ≥ 1.5 kV.
Correct selection is key to the success of flat wire applications. The appropriate paint film structure (PEI/PEI + Pai/Pai + Pai), film thickness (50-150 μm), geometric dimension (width 1.5-10.0 mm × height 0.8-5.0 mm), certification level (IATF 16949, UL, TÜV) should be selected taking into account the motor power density, operating voltage, operating temperature, heat dissipation, mechanical stress, industry certification and other factors.
As the world’s leading supplier of flat wires for new energy vehicles, LP Winding Wire can provide a full range of PEI, PEI + Pai, PI + Pai paint films, with a width of 1.5-10.0 mm and a height of 0.8-5.0 mm. With an annual flat wire production capacity of 10,000 tons, UL, TÜV, IATF 16949 and other international certifications, LP Winding Wire is the preferred partner for new energy vehicle drive motor manufacturers.
With the explosive growth of the new energy vehicle industry, the popularity of 800V-1000V high-voltage platforms, the breakthrough of high-speed motors, and the penetration of oil-cooled motors, the application of flat wires in new energy vehicles will continue to grow at a high speed, and continue to develop in the direction of nano-modification, high PDIV, high thermal conductivity, high voltage resistance, and automated Hairpin molding, with a broad market prospect.