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Hair-PIN Motor Flat Wire, specifically enameled flat wire for hairpin motors, is a special type of insulated electromagnetic wire designed for the stator hairpin winding process. This wire features a standard rectangular cross-section, distinguishing it from traditional round enameled winding wire, and its structural characteristics are optimized for precision stacking processes. When a single strand of this flat wire is bent using specialized equipment, it forms a uniform U-shape resembling a hairpin, hence the name “Hair-PIN” wire. This wire is custom-designed exclusively for automotive drive motors and must not be interchanged with general-purpose transformer flat wire or standard industrial flat wire.
The overall structure consists of two core components: an inner layer of high-purity oxygen-free copper conductor and an outer layer of multi-layer composite insulating varnish. The length and width of the conductor are custom-manufactured based on the motor stator slot profile, number of winding layers, and power parameters, with extremely strict dimensional tolerances. Industry mass-production standards maintain tolerances within ±0.02 mm, ensuring tight stacking within the slots, orderly arrangement, and no gaps or misalignment. The insulation system employs a multi-layer gradient composite varnish structure, with each layer performing distinct functions such as adhesion, pulse resistance, and abrasion protection. Precise varnish-stripping areas are reserved at both ends of the wire, where the insulation layer is removed via a laser stripping process to meet the requirements for laser welding at the ends.
Compared to traditional round-wire motors, the bare copper slot fill rate in hairpin flat-wire motors can be increased to 70% to 78%, with the effective conductive area rising by more than 30%. This increase in effective copper area directly reduces the DC resistance of the windings, cutting copper loss during low-speed operation at its source. The prefabricated hairpin conductors feature uniform bend dimensions and neatly arranged ends, significantly shortening the length of ineffective end copper segments.
The flat-wire conductors in the card-issuing motors are uniformly made from TU1/TU2 ultra-high-purity oxygen-free copper base material, with a copper purity of ≥99.95%, far exceeding the standards for ordinary wires. The oxygen content of the base material is strictly controlled to within 20 ppm, eliminating issues such as conductive losses and resistance drift caused by impurities and oxides. High-purity copper offers excellent electrical conductivity uniformity, which minimizes DC losses in the windings and improves the motor’s electrical energy efficiency. All base materials undergo continuous precision rolling and high-temperature annealing in an inert gas atmosphere to completely eliminate rolling stresses.
The optimized copper material features uniform elongation and excellent toughness, preventing defects such as cracking, deformation, or wire breakage during bending and forming. The conductor’s tensile strength and elongation parameters are fully compatible with the forming process parameters of automated hairpin bending equipment. It can consistently bend to standard U-shaped hairpin angles without residual stress or microcracks, ensuring mass production stability.
The industry-standard wire specifications range from 1.0 to 6.0 mm in width and 0.3 to 2.5 mm in thickness, with customization available upon request. High-power-density motors use thin, wide flat wires to increase the number of stacking layers, while high-torque motors use thick, narrow flat wires to reduce resistance. The surface of the finished copper wire is free of burrs, scratches, and rolling dents, preventing localized electric field concentration that could cause micro-breakdowns in the insulation.
| Name | Hair-PIN Motor Flat Wire |
| Conductor | Copper and Aluminum |
| Dimension | Thickness(a) 0.5-5.6mm Width(b) 2.0-12mm |
| Thermal Class(℃） | 180(Class H), 200(Class C), 220 (Class C+), 240 (Class HC) |
| Standard | IEC, NEMA, GB, JIS, UL |
| Packing | 30Kg~150Kg Ply-wood spool(250*500/250*600) |
| Application | Hair-Pin Motor |
Rectangular-cross-section flat wires are tightly stacked without excess gaps; with standardized arrangement, the bare copper slot fill rate remains stable within the range of 70% to 78%. In contrast, traditional round-wire windings are limited by their circular structure, making it impossible to eliminate the inherent triangular gaps, resulting in a slot fill rate of only 42% to 48%. For the same stator slot volume, hairpin-style flat wire increases the effective copper cross-sectional area by more than 30%, significantly reducing the DC resistance of the winding.
In traditional random round-wire windings, the end wires are tangled and crisscrossed, with the axial protrusion from the stator core typically reaching 18 to 25 mm. A large amount of wasted end copper not only increases the motor’s weight but also occupies a significant amount of axial space in the chassis layout.
Hair-PIN prefabricated conductors have uniform bend dimensions, with an end protrusion height of only 8 to 12 mm, significantly reducing the axial dimensions. The proportion of wasted copper in the windings has been reduced from 25% to less than 12%, completely eliminating a large number of unnecessary heat sources and weight redundancy.
The flat-contact structure, exclusive to flat wire, completely resolves the pain points of traditional round-wire windings, such as air insulation and poor heat dissipation. Continuous, solid thermal conduction pathways between layers and along slot walls allow heat generated by the coil to be rapidly and evenly dissipated outward. The neat, open-ended structure is compatible with high-pressure oil-cooling sprays, providing a cooling oil contact area several times greater than that of round-wire windings.
In actual tests under equivalent continuous power output, the steady-state temperature rise of flat-wire windings is 18 to 22°C lower than that of round-wire windings. Lower long-term operating temperatures significantly slow the rate of thermal oxidation and cracking of the insulation varnish film, extending the service life of the windings. The motor has ample thermal safety margin; even under high summer temperatures, continuous hill climbing, or high-speed long-distance operation, it will not exceed temperature-limited power output.
Flat-wire motors achieve efficiency gains across all operating conditions, reducing DC losses at low speeds and AC losses at high speeds. Overall efficiency improves by an average of 1% to 3%, with efficiency gains of over 5% in the high-speed cruising range. For vehicle models equipped with flat-wire motors on an 800V high-voltage platform, CLTC combined range can be increased by 4% to 7%, resulting in significant energy consumption optimization.
The design goal of achieving a longer driving range can be met solely through motor technology upgrades, without the need to increase battery capacity. With the same battery capacity, the vehicle’s energy consumption per 100 kilometers can be reduced by 0.5 to 1.2 kWh, significantly lowering users’ operating costs. Industrial hairpin-wound variable-frequency motors can operate efficiently over the long term, and the energy savings generated can offset the additional cost of the wire upgrade. At the same time, the load on the cooling system is reduced, and energy consumption by auxiliary components such as oil pumps and fans decreases, creating a virtuous cycle of energy savings.
Hair-pin flat-wire windings utilize a prefabricated, standardized forming process, with bending, enamel stripping, and dimensional control all managed via CNC closed-loop control. The entire process—including stator wire insertion, twisting, laser welding, and varnish curing—is fully automated, requiring no manual intervention. The wire layout, electromagnetic parameters, and structural dimensions of each stator are highly consistent, resulting in extremely low production variability. Laser-welded joints are uniform and robust, ensuring stable contact resistance, with defect rates for cold solder joints and incomplete welds controlled below 0.1%.
The multi-layer composite insulation system for flat wires used in hairpin windings offers excellent resistance to corona discharge, steep pulses, and oil aging. It is perfectly suited for the ultra-steep voltage rise pulse conditions (50 to 100 V/ns) encountered in SiC (silicon carbide) controllers. The multi-layer insulation structure effectively disperses pulse electric field stress and suppresses partial discharge between strands and turns.
