Introduction
Under the 800V high-voltage platform, vehicle battery, charging, and drive systems are being developed toward higher voltage architectures. The motor, as the core of the three-electric system, directly affects vehicle performance. In the 800V architecture, motor design differs from lower-voltage platforms and trends toward higher efficiency, reduced weight, and lower cost. Against this background, motors wound with flat copper wire have attracted increasing attention from suppliers and OEMs.
Flat-wire Winding Types
By manufacturing process, flat-wire motor windings are mainly classified into three types: Hair-pin, I-pin, and continuous wave windings.
I-pin winding
I-pin windings are axially inserted: the flat conductors are embedded axially into the stator slots and the ends are bent and welded. The manufacturing process is relatively simple, but the welded joints occupy additional radial space, resulting in longer tails, higher copper usage, increased temperature, and reduced efficiency.
Hair-pin winding
Hair-pin winding has a more complex manufacturing process than I-pin, including a preforming step. The flat copper wire is preformed into a U-shape, with the other end prepared for welding, making the process more challenging. However, because one end requires fewer welds, overall copper usage is lower than I-pin and motor efficiency can improve.
Beyond preforming difficulty, hairpin winding also faces challenges in insulating the winding ends to avoid direct contact with the stator. To maintain a compact structure, gaps between windings must be kept very small. Small gaps introduce two issues:
- Small process tolerances increase the risk of turn-to-turn breakdown and hidden defects in regions with minor air gaps.
- Damage to inter-turn resin insulation: this is typically addressed by inserting insulating paper into gaps, which increases process cost.
Hairpin motor end-winding height is also constrained. Conventional hairpin end-windings present a triangular shape; the end height Lc is limited by the triangle angle θ and the gap.
To address end-winding height, DENSO (Japan) adopted a stepped end design that reduces bending radius and relaxes the constraint imposed by the triangle angle θ, producing a more compact end-winding. To address inter-winding insulation, DENSO applied a polymer insulating layer on top of the base insulation of the flat wire so that touching coils still satisfy insulation requirements.
Although hairpin and I-pin windings perform similarly in peak efficiency and torque, I-pin has more welds and therefore a higher risk of failure at weld points. As a result, hairpin winding is widely adopted internationally. For example, some Tesla Model 3/Y rear-drive variants use hairpin flat-wire motors.
Continuous wave winding
Wave-wound flat wire uses fewer welds and offers high design flexibility. With current coil layouts, it faces several challenges:
- Asymmetric branches cause differences in back-EMF, resistance, and inductance, reducing motor performance. Circulating currents increase additional losses and can cause local overheating.
- Coil layout can be difficult, leading to overlap and difficulty inserting coils into stator slots.
- When winding spans differ, manufacturing becomes complex, tooling costs increase, and production difficulty rises.
Motor Cooling and Heat Dissipation
The 800V high-voltage platform and flat-wire winding processes drive motors toward higher power and higher speed. Especially in hairpin motors, losses from skin effect and eddy currents generate heat, reducing efficiency. Extended high-temperature operation can cause magnet demagnetization, insulation aging, and reduced service life of drive components such as gearboxes. These factors impose new requirements on motor cooling.
Motor cooling methods are mainly air cooling, oil cooling, and water cooling. Air cooling is limited in effectiveness and is rarely used in current high-performance motors.
Water-cooled motors borrow the engine cooling approach used in ICE vehicles, using a water-glycol mixture circulated through cooling passages in the housing. As motor power density increases, direct cooling of heat sources is needed for effective heat removal. Water's electrical conductivity, magnetic permeability, low boiling point, and potential expansion make current water-cooling solutions challenging for very high-power motors.
Oil cooling is currently the primary method. Cooling oil is non-conductive and non-magnetic, enabling direct cooling of shaft, stator, and other components inside the motor housing, achieving good cooling performance. By adjusting oil formulations, the coolant can also lubricate internal drive components. Well-formulated oil-cooling schemes are expected to be the mainstream approach for future motor cooling.
Some new OEM solutions consider integrated oil temperature sensing in multi-in-one motor designs. For example, Huawei's One-drive high-speed intelligent oil-cooled system plans to use spray-type oil channels to cool frictional heat sources such as shafts directly for improved cooling performance.
Flat-wire Winding Transposition Technology for Automotive Drive Motors
In a 2021 evaluation of global new energy vehicle frontier and innovation technologies, a proposal from Harbin University of Science and Technology titled "Flat-wire winding transposition technology for automotive drive motors" aims to reduce circulating and eddy-current losses. The technique could eliminate complex processes such as slot insertion, end twisting, and welding, and is presented as a potential candidate for a third-generation drive-motor winding.
1. Practical motivation
Drive motors are the primary power source for electric vehicles. As requirements for vehicle lightweighting, cost reduction, and reliability increase, motor power density and peak efficiency become more demanding. Current improvements primarily focus on magnetic circuit design and control, offering limited gains. Winding losses account for roughly half of total motor losses and become more significant at high speed and frequency. Therefore, advances in winding design can be an effective route to improve power density and efficiency and to enable higher insulation and voltage classes.
Conventional flat-wire hairpin windings insert multiple single-turn flat conductors into slots along the height direction, with special processes for twisting and welding tails. Limitations on hairpin winding layers and the large cross-section of flat conductors result in significant eddy-current losses. In addition, motors often operate under high-frequency, high-harmonic conditions that increase losses and thermal stress. The slot insertion, end twisting, and welding processes for hairpin windings are complex and require costly production lines, complicating mass production.
To overcome these limitations, flat-wire winding transposition technology proposes using multiple smaller-section flat conductors arranged and transposed together. This approach can reduce additional losses, improve efficiency and power density; it can avoid welding at the winding end, simplifying the process and reducing cost; it supports integrated conductor design that can enhance insulation level and heat dissipation; and it enables standardized winding processes with pre-shipment testing and evaluation to improve operational reliability.
2. Technical developments
The proposed technology addresses multiple foundational and advanced bottlenecks in design, manufacturing, testing, and evaluation. Key aspects include:
- A theoretical framework for flat-wire winding transposition tailored to short axial length, short end-windings, many pole pairs, and few slots typical of automotive drive motors. A continuous transposition concept based on leakage flux compensation is proposed to achieve balanced electrical loading, low additional losses, high efficiency, and uniform temperature distribution. A 3D electro-thermal numeric simulation method was developed for accurate prediction of temperature rise monitoring points.
- A generalized transposition design methodology and software for series products, based on discrete integral methods, equivalent circuit network models, and multi-variable multi-objective optimization algorithms. These tools address design bottlenecks and support development of series and new products.
- Development of multi-path parallel conductor circulating current detection systems and evaluation techniques for transposed flat-wire windings. Traditional research has not resolved experimental testing challenges for closely packed parallel conductor arrangements. The proposed approach includes real-time testing methods, a measurement platform, and development of linear-array conductor current sensors, enabling online evaluation and diagnostic techniques for transposed windings.
3. Potential impact
Drive-motor winding technology has evolved from first-generation round-wire distributed windings to second-generation flat-wire hairpin windings, achieving higher slot fill, power density, and weight reduction. Flat-wire transposition technology could alter manufacturing complexity and reduce costs while offering potential improvements in power density, peak efficiency, and the extent of the high-efficiency operating region. It presents a combined-design approach addressing losses, insulation, and thermal management.
By transposing conductors and selecting appropriate transposition pitches, the technique aims to balance leakage flux linkage across individual conductors and minimize additional losses. Compared with round-wire distributed and flat-wire hairpin windings, this approach may improve efficiency, particularly under high-speed, high-frequency conditions, and widen the motor's high-efficiency operating range.
Transposed flat-wire windings offer flexible design options without the layer-count limitations of hairpin windings, enabling various winding arrangements tailored to technical requirements. Manufacturing-wise, the approach retains advantages of round-wire winding and end-of-line processes while removing slot insertion, end twisting, and welding steps typical of hairpin production. This can reduce manufacturing cost and allow integrated conductor design that facilitates higher insulation and voltage classes.
Overall, flat-wire winding transposition technology addresses multiple design, testing, and evaluation bottlenecks and can serve as an alternative to current hairpin windings. It has potential advantages in reducing full-operating-condition additional losses, lowering temperature rise, and improving operational reliability, while simplifying manufacturing and supporting higher insulation and voltage levels.
