As the new-energy vehicle industry continues to develop year by year, the production and sales of electric vehicles have surged, driving an ever-growing demand for high-performance, high-efficiency motors. The Ministry of Industry and Information Technology and the National Development and Reform Commission have proposed a plan to— By 2025, the power density of passenger vehicle electric motors is expected to exceed 4 kW/kg, further driving the pursuit of higher power density in electric motors. In the future, the trends in electric drive systems will be characterized by greater efficiency, lighter weight, smaller size, and lower costs. Among these, system integration and the use of flat-wire windings for electric motors represent the primary technological approaches for achieving lightweight and compact designs. Thanks to their unique advantages over round-wire windings, flat-wire windings have experienced rapid development and have become a hot research and development focus in the field of electric motors for new-energy vehicles. Flat-wire motors have been widely adopted both domestically and internationally, and the penetration rate of flat-wire motors in new-energy vehicle models has been steadily increasing year by year. This article will briefly introduce flat-wire winding technology and provide a comparative analysis to demonstrate the advantages of flat-wire motors. Introduction This document summarizes the manufacturing processes for hair-pin motors, outlines recent advances in new technologies and processes for flat-wire motor applications, and identifies future research directions for optimizing these motors, thereby providing a reference for flat-wire motor research.
1. Introduction to Flat Wire Motor Technology
Bar-wound motor ( The bar-wound motor—commonly referred to in China as the "flat-wire motor"—is a type of electric motor in which the stator winding is made using flat copper wires instead of the conventional round copper wires. The term "flat-wire motor technology" encompasses a set of integrated technologies that include specially optimized stator and rotor structures, enhanced cooling solutions, and advanced control strategies tailored specifically for the unique structure of the flat-wire windings. The flat copper wire winding involves modifications to the stator slot design, replacing the original numerous thin round conductors with fewer but thicker rectangular conductors. Thanks to its advantages—such as compact size, high slot fill factor, high power density, excellent NVH performance, and superior thermal conductivity and heat dissipation—flat-wire motors have found widespread application in the field of new-energy vehicles.
2. Comparison between flat-wire motors and round-wire motors

The energy losses in electric motors mainly include copper losses, iron losses, windage and friction losses, and stray losses. Among these, copper losses account for nearly 70%. Reducing copper losses can significantly decrease overall motor energy losses and improve the motor's power density. The formula for calculating DC copper losses is shown in Equation ( 1) As shown, compared to round copper wire windings, flat-wire windings made of rectangular copper wires exhibit a significant variation in cross-sectional area. This feature effectively reduces winding resistance and thereby lowers copper losses. Moreover, rectangular wire windings have smaller gaps between the flat wires than round-wire windings; consequently, under the same stator slot volume, more copper wire can be accommodated, resulting in a higher slot fill factor. While round-wire motors typically achieve a slot fill factor of around 40%, flat-wire motors can attain fill factors as high as 70%. With a higher slot fill factor, for the same motor power output, flat-wire motors require less copper wire, allowing for reductions in both the size of the stator core and end parts. This not only shrinks the overall motor dimensions but also further enhances the motor’s power density while saving materials.

Compared to round-wire motors, flat-wire motors feature smaller slot dimensions in the stator, which can effectively reduce cogging torque and thereby lower electromagnetic noise. Additionally, rectangular conductors have greater rigidity, which also helps suppress armature noise. Combined with optimized rotor pole design and structural enhancements, these motors achieve even better performance. NVH performance [1].

The ends of the flat-wire motor windings are wound into special shapes, such as wavy, triangular, or stepped forms, as shown in the figure. As shown in Figure 2, the effective reduction in the dimensions of the winding ends facilitates miniaturization and weight reduction [2]. Meanwhile, the rectangular conductor design minimizes internal air gaps and increases the contact area between conductors themselves as well as between conductors and the core slots, thereby enhancing thermal conductivity and heat dissipation performance. Minimal air gaps are maintained between the conductors at the winding ends, further promoting heat dissipation; combined with end-side oil-injection cooling technology, this approach significantly improves the thermal management of the flat-wire motor. Under conditions of lower temperature rise, the entire vehicle exhibits superior acceleration performance, effectively enhancing its high-temperature power output.

Flat-wire motors also have certain drawbacks, as they are significantly affected by the skin effect. The skin (or surface) effect refers to the phenomenon whereby, when an alternating current or a time-varying magnetic field is present within a conductor, the current distribution inside the conductor becomes uneven: the closer one gets to the conductor's surface, the higher the current density becomes. This reduces the effective cross-sectional area of the conducting copper wire, thereby increasing the equivalent resistance of the winding and leading to greater AC losses at high frequencies. The aspect ratio of the conductor, its orientation, and the phase arrangement of the windings all influence the severity of the skin effect. For windings wound in the same slot, the skin effect is more pronounced than for windings wound in different slots. Under the same slot depth and width conditions, increasing the number of conductor layers can help mitigate the skin effect, reducing AC losses at high speeds and improving motor performance. Another limitation hindering the development of flat-wire motors is the high cost of their automated production lines—costs that far exceed those of automated production lines for round-wire stators. 2-3 times the amount, with substantial upfront investment by the enterprise.
3. Classification of Flat Wire Motors
Flat-wire motors, classified by product type, can be divided into concentrated-winding flat-wire motors and lap-winding flat-wire motors. Hairpin (hairclip) flat-wire motors—among which hairpin flat-wire motor technology is a widely adopted mainstream technology.

The concentrated winding is made by winding flat copper wire into single-tooth windings, as shown in the figure. As shown in Figure 3(a), each tooth is equipped with a single-phase winding. Due to the short pitch of the coil ends, this design effectively reduces the overall end-winding size, and its manufacturing process is simpler compared to that of hairpin-shaped flat-wire motors. However, this structure suffers from several drawbacks: it generates significant torque ripple and complex radial forces owing to the excessive presence of fractional-slot harmonics. To mitigate slot torque and torque ripple, the assembly process for this structure must ensure high precision in roundness, concentricity, and uniform distribution among the teeth, thus imposing higher assembly requirements. The concentrated winding technology is widely used in industrial motor applications and is also being explored by some manufacturers in the field of new-energy vehicle motors. For instance, Honda has adopted this technology in its Acura hybrid models, achieving impressive results through its unique fractional-slot concentrated windings, segmented stator structure, and related optimization techniques. The Songzheng 270-series PHEV-P2 motor, depicted in Figure 3(b), leverages the distinctive advantages of concentrated winding technology and is specifically tailored for hybrid powertrain applications.

The wave-wound flat-wire motor employs either a continuous winding process to form a monolithic structure that is then inserted, or a method in which the windings are wound and inserted into the stator slots simultaneously, creating wave-shaped end turns. Compared to this, Hairpin flat-wire motors feature no solder joints, which further reduces the height of the winding ends and thereby shrinks the overall motor size. However, this type of stator assembly has wider slot dimensions, resulting in greater tooth-slot torque, higher torque ripple, and poorer NVH performance. To address these issues, it requires coordinated electromagnetic multi-objective optimization design and other complementary measures for improvement and optimization. At the same time, the production cost of Hairpin motors is higher than that of hair-pin (U-pin) motors. The hair-pin (U-pin) winding, also known as the hairclip winding, gets its name from its winding shape resembling a hair clip: one end of the enameled flat copper wire is preformed into a U-shape and then inserted into the stator core slots; the other end is twisted and shaped like a frog's leg before being welded together to form a wavy winding [3]. Another winding process, the I-PIN winding, directly inserts straight copper wires into the stator core slots and then twists both ends simultaneously into frog-leg shapes before welding them together to create a wave winding, thus eliminating the need for the preforming step required in the U-PIN winding process. Both U-PIN and I-PIN flat-wire windings belong to the second-generation axial-slot embedded windings. Compared with each other, they are comparable in terms of maximum efficiency and peak torque; however, the I-PIN winding boasts a higher slot fill factor, sustained torque, and sustained power than the U-PIN winding. Because the I-PIN winding has twice as many solder joints as the U-PIN winding, its winding end dimensions are slightly larger, and the risk of solder joint failure is also higher. The hair-pin winding process is currently a widely adopted approach both domestically and internationally.
4. Card-issuing coil process

Hairpin winding technology is a large-scale, high-quality, and short-cycle stator manufacturing technique. The process chain primarily consists of the following five steps: forming (straightening enameled copper flat wires, stripping insulation, cutting, and bending), inserting (inserting stator slot liners and assembling hairpin windings), twisting the ends, welding, and insulation. [4]. The process step diagram is shown in Figure 4.

In addition to the insulation provided by the enamel coating between the copper wires of the winding, insulating shims are also inserted into the stator slots to separate the conductors from each other. This prevents direct contact between turns or between the conductors and the stator core, thereby improving insulation performance and enhancing short-circuit protection. Therefore, it is necessary to insert insulating slot liners; common shapes of slot liners include: Types O, C, B, and S are shown in Figure 5. The B-type groove lining eliminates the gaps at the corners found in the S-type groove lining, thereby enhancing protection against short-circuit faults [5].

In the paper-insertion process, insulating slot papers are pre-inserted into the stator slots. As the number of layers of flat conductors increases, the process difficulty also rises significantly. The PIN forming process includes stamping, spring machines, and automated forming using CNC equipment. The former offers fast forming speeds and low costs but causes greater damage to the copper wires; the latter boasts good versatility and minimal damage to copper wires, though its equipment costs are higher. After PIN forming, the pins are pre-inserted into conforming fixtures for shaping. As the number of flat wire layers increases, the difficulty of automatically inserting cross-wires also rises. Next, all the pins from the conforming fixture are inserted as a whole into the iron core according to their designed dimensions. This step places extremely high demands on equipment precision. Subsequently, the winding ends are leveled and smoothed through processes such as flaring, head twisting, and edge trimming to facilitate soldering. Currently, TIG welding and laser welding are the most popular methods for flat-wire motors; however, some other companies are experimenting with alternative welding techniques, such as CMT cold welding. Once welding is complete, the windings must first undergo electrical performance testing, including measurements of phase resistance, phase inductance, and their balance, as well as withstand voltage and insulation resistance tests. Only after passing these tests can the windings proceed to the coating process. Depending on the coating material used, the coating process is divided into powder coating and liquid coating, and the sequence of these two processes differs slightly. For powder coating, the coating is applied before impregnation with varnish; for liquid coating, the varnish impregnation is performed first, followed by the coating application. Impregnation processes vary based on the material used, including traditional impregnation, vacuum impregnation, vacuum-pressure impregnation, drip impregnation, and EUV impregnation.
5. Research Development Trends
5.1 The number of conductor layers gradually increases.

As shown in the figure As shown in Figure 6, increasing the number of conductor layers can effectively reduce AC copper losses, thereby lowering the overall copper loss in the motor and improving its overall performance. To further mitigate high-frequency AC losses caused by the skin effect, hairpin motors have adopted a strategy of increasing the number of conductor layers for optimization—from the already implemented 4-layer, 6-layer, and 8-layer configurations to the 12-layer and 16-layer schemes currently under investigation. There is a gradual trend toward increasing the number of conductor layers. The main research challenges lie in the limitations imposed by manufacturing process capabilities and the need to control production costs [6].

5.2、Optimize insulation

The more complex insulating slot paper makes the installation process more intricate and also places higher demands on the precision of CNC installation equipment. Consequently, research into even better insulation techniques has become an important area of focus. Chevrolet The Bolt motor simplifies the slot insulation by adopting a more straightforward two-piece slot insulation design, which protects the windings from short-circuiting to the stator core. The new design, as shown in Figure 7(a), eliminates the insulation between conductors compared to the S-type and B-type slot insulations, further enhancing the slot fill factor and simplifying the stator manufacturing process. Meanwhile, to minimize the voltage potential between conductors within the slot, General Motors has made relevant optimizations to the winding layout and other aspects. Additionally, a polymer-based insulation layer is applied over the basic conductor insulation, as illustrated in Figure 7(b), thereby eliminating the need for slot liners and addressing the issue of inter-turn insulation, thus simplifying the production process [2].

5.3 Widely adopt oil-cooling technology The winding heat must pass through the insulation layer within the slot. Only the longer path from the stator core to the housing is traversed by water, and the thermal resistance along this path easily leads to localized hot spots, thereby rendering the water-cooling approach inefficient. Oil cooling, which allows direct contact with the heat source and has no adverse effect on the motor’s magnetic circuit, has emerged as a research hotspot due to its superior cooling efficiency. The majority of the motor’s heat is concentrated at the winding ends; in particular, the unique end-region oil-injection cooling method used in hairpin-wire motors can more effectively dissipate heat. Technologies such as oil-path cooling, oil-injection cooling, centrifugal oil-extrusion cooling through the shaft, and sealed-stator oil-recirculation cooling have all been extensively studied and widely adopted in hairpin-wire motors.
6. Closing remarks
The rapid development of new-energy vehicles has led to the widespread adoption of flat-wire motors, which boast unique advantages and see their penetration rate increasing year by year. Flat-wire motors hold significant research and application value in the areas of miniaturization, weight reduction, and high power density for electric vehicle motors. This article briefly analyzes and introduces the advantages of flat-wire motors over traditional round-wire motors, the manufacturing processes for mainstream hairpin windings, and the current research and development trends in flat-wire motor technology, providing a reference for enhancing understanding and advancing research on flat-wire motors and related technologies. Source: School of Mechanical and Electrical Engineering & Vehicle Engineering, Chongqing Jiaotong University Lan Pengyu