A review of structural design for hairpin windings and ultra-high slot fill factor in electric motors for new energy vehicles

1 Fundamental principles and comparative advantages of flat-wire windings

The fundamental advantage of flat-wire or hairpin winding technology lies in the geometric efficiency of rectangular conductors packed into rectangular stator slots, eliminating the interstitial voids inherent to round-wire configurations (Zhu and Howe, 2007). While conventional random-wound round wire motors achieve slot fill factors of 0.35–0.45, and Litz wire configurations reach 0.35–0.40, hairpin and bar-wound motors routinely achieve slot fill factors of 0.60–0.80, with advanced designs approaching 0.75 in production EV traction motors (Shams Ghahfarokhi et al., 2022). In this review, the terms “flat-wire,” “rectangular conductor,” “bar winding,” “hairpin,” and “shaped conductor” are used interchangeably to denote windings formed from solid conductors of rectangular cross-section. The geometric slot fill factor is defined as the ratio of the total copper cross-section to the usable slot area, excluding insulation, slot liner, and wedge, whereas the effective copper fill factor additionally accounts for insulation build-up and manufacturable clearances; throughout this review, “ultra-high slot fill factor” refers to geometric fill factors above 70%, consistent with published hairpin machine data (Shams Ghahfarokhi et al., 2022). This increased copper packing directly translates to higher current capacity, reduced DC resistance, and improved power density; hairpin machines of a given power rating are reported to be markedly more compact than round-wire equivalents, with significantly higher torque density per unit axial length (Nuzzo et al., 2022). The thermal performance of flat-wire windings is substantially superior because the rectangular conductors sit flush against each other with minimal air gaps, creating efficient heat conduction pathways to the stator core and enabling lower winding hot-spot temperatures under continuous operation (Ha and Kim, 2021; Wang et al., 2022). Comparative driving-cycle studies report average efficiency advantages of roughly one percentage point for hairpin machines over equivalent round-wire machines, with the gap widening at higher speeds, where loss-minimizing control and careful conductor design become decisive (Islam et al., 2020; Lee et al., 2009). For round-wire alternatives, multi-strand Litz construction mitigates skin and proximity effects at the cost of reduced fill factor and complex, expensive terminations (Sullivan, 1999). The end-winding region of hairpin motors is significantly shorter than that of round-wire motors, which not only reduces copper usage and resistance but also decreases the axial length of the motor, enabling more compact packaging and potentially reducing the overall vehicle weight. However, these advantages come with inherent trade-offs: the larger cross-sectional area of solid rectangular conductors exacerbates skin and proximity effects at high frequencies, and the manufacturing process demands substantially higher precision than conventional round-wire winding. It should be emphasized that a high slot fill factor does not translate automatically into proportional gains in power density or efficiency: machine performance is determined simultaneously by current density, thermal limits, winding factor, AC losses, electromagnetic loading, cooling conditions, iron and magnet losses, and mechanical constraints, and a higher fill factor can itself increase AC losses and thermal-management demands (Bardalai et al., 2020). The profiled wire technology, including flat and rectangular shaped variants, represents a significant advancement for achieving high slot fill factors exceeding 0.70, and is adaptable to technologies such as stacked continuous hairpin winding, additive manufacturing, laser marking, and 3D printing (Zhu et al., 2025). Figure 1 distills the structural evolution from round-wire to flat-wire winding technologies and the associated manufacturing and design trade-offs, and Figure 2 maps the six principal trade-off dimensions that recur throughout this review. Section 2 describes the methodology used to select and analyze the literature.

2 Review methodology

This review was compiled following a structured literature search conducted between January and June 2025. The databases searched included Web of Science, Scopus, IEEE Xplore, and ScienceDirect, supplemented by Google Scholar for forward and backward citation tracing. The search combined the terms “hairpin winding,” “bar winding,” “flat wire,” “rectangular conductor,” “slot fill factor,” “AC losses,” “laser welding,” and “stator insulation” with “electric vehicle,” “traction motor,” and “permanent magnet synchronous motor.” Peer-reviewed journal articles published between 1992 and 2025 were considered, together with seminal works outside this window where historically required. Records were screened first by title and abstract and then by full text; studies were included when they reported quantitative results on winding architecture, slot fill factor, AC losses, thermal behavior, manufacturing processes, or insulation reliability of flat-wire machines, and were excluded when they concerned unrelated machine types or lacked quantitative content. An initial pool of 312 records was reduced to 168 after duplicate removal, 64 full texts were assessed, and 30 studies were finally included in this review. The comparative values reported in Table 1 are drawn from these sources; where no peer-reviewed data exist, for example, for the most recently introduced winding variants, values are explicitly labeled as manufacturer-reported estimates.

TABLE 1. Comparative overview of flat-wire winding architectures for electric vehicle traction motors. Slot fill factors are geometric values from the cited literature unless labeled manufacturer-reported; end-winding length and weld count refer to typical EV traction stators (60-slot/4-layer basis for conventional hairpin).

Figure 2 maps the principal trade-offs among slot fill factor, AC losses, thermal performance, manufacturing complexity, end-winding length, and insulation reliability that determine the selection of a flat-wire winding architecture for a given application.

3 Hairpin winding architecture and manufacturing process

The conventional hairpin winding process involves forming continuous copper wire into precise U-shaped conductors, inserting these hairpins axially into stator slots, twisting the open ends to create the desired winding pattern, and welding the connections to form continuous electrical circuits. Bar-wound stators entered high-volume automotive production with the GM Voltec transaxle (Rahman et al., 2011), and the production sequence typically includes: (1) wire forming and bending into hairpin shapes; (2) paper or film slot liner insertion; (3) hairpin insertion into stator slots; (4) end twisting to establish phase and pole connections; (5) enamel stripping from weld zones; (6) laser welding of twisted pairs; and (7) post-weld inspection and electrical testing (Loffredo et al., 2020). The number of winding layers is typically even, and design rules require that wires belonging to the same parallel path must cover all layers of the slot to ensure equal inductance, and all slots per pole of that phase to ensure equal back-EMF (Nuzzo et al., 2022). A rough maximum feasible solution for axial-insertion hairpin is approximately 72 slots with eight winding layers, beyond which the manufacturing complexity becomes prohibitive due to the exponential increase in weld joints. For a 60-slot, 4-layer stator, each slot contains 4 conductor ends, and conductors are welded in pairs, so a minimum of 60 × 4 ÷ 2 = 120 individual weld joints is required; a defect in any single joint creates an open circuit or high-resistance connection that can fail the motor, making welding the most failure-prone step in the entire manufacturing process (Kampker et al., 2023). The structural design of the stator lamination stack must accommodate hairpin insertion requirements, with sharp slot edges, liner wrinkling, slot mouth accessibility, and local springback during insertion all becoming critical design parameters that differ substantially from round-wire-optimized stacks (Guo et al., 2025). Hairpin winding changes the lamination stack from a purely magnetic part into a magnetic-and-process part, requiring early integration of winding and stack design to avoid yield issues. The slot opening must be wide enough to allow hairpin insertion but narrow enough to minimize magnetic air gap and cogging torque, creating a fundamental geometric trade-off that influences electromagnetic performance and manufacturing feasibility. The insulation system for hairpin windings must withstand higher mechanical stress during insertion compared to round wires, as the rectangular conductors with sharp edges can damage conventional enamel coatings. Advanced insulation materials such as polyimide (PI) films with enhanced abrasion resistance and thicker build are typically employed, along with careful slot liner design that protects the conductor enamel from the sharp edges of the laminated steel slots (Wang et al., 2014). Rectangular magnet wire is more expensive than round wire, but this cost is generally offset by efficiency and power-density gains that reduce overall system cost. The manufacturing process for hairpin motors is highly automatable via robotic insertion and laser welding, which lowers labor costs and improves production consistency compared to manual round-wire winding processes.

4 X-pin, continuous wave, and I-pin winding innovations

The evolution from conventional hairpin windings toward reduced end-winding length and simplified manufacturing has produced several innovative flat-wire configurations, each with distinct structural characteristics and process requirements. X-pin winding represents a significant advancement that eliminates the straight weld segments required in conventional hairpins, with end-winding heights reduced by several millimeters (manufacturer-reported) while maintaining weld quality and motor performance. The X-pin structure forms an X-shaped interlaced texture at the weld joint without flat-cutting, requiring only a short paint-removal length and employing low-temperature welding that avoids solder ball formation; manufacturers report weld strengths on the order of 600–800 N (manufacturer-reported). However, X-pin manufacturing demands extremely high wire feeding accuracy and twisting precision, with narrower pitches increasing creepage risk; for 800 V motors, the creepage distance must satisfy applicable cleanliness and insulation-coordination standards, which is difficult to meet with paint film alone. Continuous wave winding, also known as radially-inserted hairpin or S-winding, offers an alternative approach where conductors are pre-formed into continuous wave-like patterns that are inserted radially rather than axially, eliminating the need for welding between hairpins within the same phase (Kampker et al., 2023). This configuration enables higher slot numbers and winding layers (up to 12 layers) but requires open-slot stator structures, which may increase core and magnet losses due to the larger effective air gap. I-pin winding, adopted by companies such as United Automotive Electronic Systems (UAES), uses straight conductors inserted from both ends of the stator, requiring double-ended welding but offering design flexibility and compatibility with existing manufacturing infrastructure. The W-pin variation, used by BorgWarner and NIO, features the shortest end-winding among flat-wire types and does not require enamel stripping, cutting, or welding for certain connections, though it presents significant challenges in automatic wire insertion and magnetic leakage management. The inner-diameter insertion method for W-pin requires compressing the conductor to align concentrically with the stator core and then expanding using a fixture to enter the stator slot, which reduces slot fill factor by about 10%–15% (manufacturer-reported) compared to hairpin technology and introduces magnetic leakage risks. The optimum architecture is application-dependent: X-pin suits high-volume production, continuous wave winding high-reliability applications, and I-pin lower-volume or specialized platforms; no single architecture is universally superior, because the choice depends on motor topology, voltage, speed range, manufacturing strategy, cooling system, and application requirements. The N-pin and Mini-pin variations further reduce end-winding length by approximately 5–7 mm compared to conventional hairpins (manufacturer-reported), though they require even more precise manufacturing control and are currently limited to specific OEM platforms. Table 1 consolidates the comparative landscape of flat-wire winding architectures, distinguishing literature-reported values from manufacturer-reported estimates.

5 AC loss mitigation and electromagnetic design optimization

The primary electromagnetic challenge of flat-wire windings is the management of AC losses, which become increasingly significant at the high electrical frequencies characteristic of modern EV traction motors. Skin effect causes current to concentrate at the conductor surface, while proximity effect induces eddy currents from neighboring conductors, both phenomena leading to non-uniform current distribution and increased effective resistance (Ferreira, 1994). Four loss mechanisms must be distinguished: DC copper loss, proportional to conductor resistance; skin-effect loss arising from the conductor’s own alternating field; proximity-effect and circulating-current losses induced by neighboring conductors and parallel paths; and additional losses in the end-winding region and those associated with inverter-generated current harmonics (Popescu and Dorrell, 2013; Yamazaki and Abe, 2009). The AC-to-DC resistance ratio increases with conductor width and height, electrical frequency, and the number of layers per slot, and depends on the conductor position within the slot and the local magnetic field distribution. The severity of these effects depends on electromagnetic design parameters including magnetic flux density, slot height, and wire dimensions, with thicker rectangular conductors experiencing more severe AC loss penalties than thinner round wires at equivalent frequencies. Strategies to mitigate AC losses include: (1) optimizing conductor dimensions to balance high slot fill factor with acceptable skin-effect and proximity-effect losses; (2) increasing the number of parallel paths while ensuring strong electromagnetic symmetry to prevent circulating currents; (3) developing segmented conductors, transposed arrangements, and hybrid winding designs that reduce effective conductor dimensions without sacrificing slot fill (Bardalai et al., 2020; Islam et al., 2020; Zhu et al., 2025); and (4) employing variable cross-section flat wires that are narrower at the slot opening where magnetic field variation is most intense. The electromagnetic design of flat-wire motors must also account for the reduced flexibility in winding configurations compared to round wire. Hairpin motors have fewer turns per slot, meaning that even small design changes—such as adjusting the number of conductors—can significantly impact performance, requiring more upfront design optimization and reducing the ability to make late-stage design modifications. Finite element analysis is essential for predicting AC loss distributions within rectangular conductors, with specialized solvers that account for the exact conductor geometry and PWM harmonic content (Xu et al., 2025). The end-winding region presents particular challenges for AC loss prediction, as the three-dimensional geometry of twisted and welded connections creates complex current paths that are difficult to model accurately. Production EV platforms employing bar-wound stators, such as the Chevrolet Bolt, demonstrate the practical achievement of high copper fill factors in series production (Momen et al., 2019). The NVH advantages of hairpin windings are also significant, as the uniform magnetic field distribution and stiffer winding structure minimize electromagnetic noise and mechanical vibration, contributing to smoother vehicle operation. The AC loss management in high-speed hairpin motors requires particular attention to the inverter, as wide-bandgap devices such as SiC MOSFETs can operate at switching frequencies exceeding 20 kHz (Gerada et al., 2014); the relevant harmonic spectrum of the applied voltage, rather than the switching frequency alone, determines the additional winding losses (Yamazaki and Abe, 2009).

6 Manufacturing quality control, insulation integrity, and future perspectives

The manufacturing quality control of flat-wire windings represents a critical determinant of motor reliability, with welding defects, insulation damage, and conductor misalignment being the primary failure modes. Laser welding has become the dominant joining technology for hairpin connections, offering precise energy control and minimal heat-affected zones, though the process requires strict control of weld penetration, pull strength, and insulation integrity (Raffin et al., 2023). The principal welding defects include insufficient penetration, excessive penetration, porosity, cracking, spatter, conductor misalignment, incomplete joining, excessive heat input, and damage to adjacent enamel insulation; each defect class produces distinct electrical and optical signatures, and quantitative quality monitoring based on process data and machine learning has been demonstrated for hairpin laser welding (Raffin et al., 2023). Laser welding should not be treated as a single homogeneous process: the choice among infrared, green-wavelength, and ring-mode laser sources, together with beam parameters, shielding gas, and focal offset, determines weld pool geometry, spatter formation, and thermal damage to adjacent insulation. The transition from conventional hairpin to X-pin welding has been enabled by advancements in laser welding technology that minimize damage to adjacent enameled wires, allowing the elimination of protective straight segments while maintaining weld quality. Insulation integrity is particularly critical for flat-wire motors because the sharp edges of rectangular conductors and the high insertion forces during assembly create mechanical stress on the enamel coating that can lead to microscopic cracks and eventual electrical failure (Grubic et al., 2008). Slot liners must protect against sharp-edged lamination stacks during assembly and operation, control conductor separation, and survive higher insertion precision demands with less room for wrinkles, cracks, or overlap drift. In inverter-fed machines, the turn insulation is subjected to repetitive steep-fronted voltage pulses whose rise time and repetition rate govern electrical stress and partial discharge (PD) behavior (Kaufhold et al., 2000; Wang et al., 2014); reflected-wave overvoltages originating from the inverter-cable-motor interface further aggravate this stress (Persson, 1992). For high-voltage traction systems, the insulation system must therefore satisfy turn-to-turn, conductor-to-ground, and phase-to-phase requirements with adequate creepage and clearance distances, while electrical treeing, thermal aging, insertion abrasion, and vibration-induced degradation constitute the principal long-term failure mechanisms (Grubic et al., 2008; Giangrande et al., 2020). Thermal performance must be analyzed by separating three contributions: improved conduction paths within the slot due to the flat conductor geometry, reduced thermal resistance at the conductor-to-stator interface, and enhanced convective heat removal when direct oil cooling is used—while recognizing that AC-loss heat generation increases with fill factor and frequency (Boglietti et al., 2009; Gai et al., 2019; Ha and Kim, 2021). Experimentally validated thermal network models for oil-spray-cooled motors and porous-medium flow models of oil-cooled hairpin machines provide quantitative design support (Park and Kim, 2023; Yang et al., 2024). The development of low-surface-energy coatings and optimized slot insertion processes aims to reduce insulation abrasion, while model-based closed-loop process control and machine learning algorithms enable real-time anomaly detection and defect prediction during manufacturing (Loffredo et al., 2020). Ten research gaps emerge from this review: (1) electromagnetic design—multi-objective frameworks that jointly optimize fill factor, saliency, and harmonic content; (2) AC-loss prediction—fast analytical models for arbitrary rectangular conductor arrangements; (3) thermal management—experimentally validated conjugate models of oil-impinged hairpin end-windings; (4) manufacturing and welding—in-line defect detection and closed-loop laser welding control; (5) insulation reliability—partial discharge behavior and lifetime models under fast PWM transients; (6) automation and process monitoring—data-driven process control across forming, insertion, and twisting; (7) design optimization—topology-level co-optimization of winding and lamination stack; (8) high-speed operation—AC loss scaling and rotor cooling beyond 20,000 rpm; (9) high-voltage operation—creepage, clearance, and PD margins at 800 V and above; and (10) cost and industrial scalability—process windows and yield models for high-volume production. The future trajectory of flat-wire winding technology points toward increasingly automated production systems that integrate forming, insertion, welding, and inspection within single continuous processes. The convergence of hairpin technology with additive manufacturing, die-compressed stator windings, and 3D-printed conductor geometries offers potential pathways to further increase slot fill factors beyond current limits while simplifying manufacturing complexity. The standardization of flat-wire design rules and manufacturing process windows will be essential for scaling this technology across the diverse landscape of NEV platforms, from compact urban vehicles to heavy-duty commercial trucks. As the industry continues to push toward higher power densities and efficiency targets, flat-wire winding technology—evolving from conventional hairpin through X-pin to continuous wave configurations—will remain a central pillar of advanced electric motor structural design. The integration of hairpin windings with advanced cooling technologies such as direct oil spray cooling and the development of higher-temperature insulation materials will further extend the performance envelope of flat-wire motors, enabling continuous power densities that approach or exceed those of conventional internal combustion engines. The ongoing research into variable cross-section conductors, transposed flat-wire arrangements, and hybrid round-flat winding configurations promises to address the persistent challenge of AC loss management while maintaining the manufacturing and thermal advantages of rectangular conductors.

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Footnotes

Edited by: Adel Razek, Laboratoire Génie électrique et électronique de Paris (GeePs), France

Reviewed by: Manoel Kolling, Federal University of Santa Catarina, Brazil