As the core of the three-electric system (electric drive, motor, and electronic control), the efficiency of the electric motor directly affects the overall vehicle performance. Under the 800V architecture, the design of the electric motor differs from that of the low-voltage platform, requiring the electric drive system to move towards higher efficiency, lighter weight, and lower cost. Against this backdrop, electric motors made of flat copper wire are increasingly favored by suppliers and OEMs.
Flat wire motor winding form
According to the manufacturing process, flat wire motors are mainly classified into three types of winding methods: hair-pin, I-pin, and continuous wave winding.
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I-pin winding
I-pin windings are axially embedded windings. The flat wire conductor is directly embedded axially into the iron core slot, and then the two ends are twisted and welded. The manufacturing process is relatively simple. However, due to the additional radial dimension occupied by the welding part, the tail is long, the copper consumption is large, and the efficiency will decrease as the temperature increases.
I-pin winding
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Hair-pin winding
The manufacturing process of hair-pin windings is more complex than that of I-pin windings, as it involves an additional pre-forming step. Hair-pin windings require pre-forming flat copper wire into a hairpin shape, with one end being the soldering end, making the manufacturing process more difficult. However, because it eliminates the soldering at one end, the overall copper loss is lower than that of I-pin windings, and the motor efficiency is also improved to some extent.
Hair-pin winding
Besides the difficulty of the molding process, the manufacturing challenge of flat wire hairpin windings also lies in the insulation treatment at the winding ends to avoid direct contact between the winding and the stator. At the same time, to ensure a compact structure, the gaps between the windings must be sufficiently small. Small gaps bring two problems:
Small gaps have low process tolerance, and in areas with low air pressure, they can easily cause inter-turn breakdown short circuits and hidden inherent defects.
Damage to the resin insulation material between turns is generally addressed by installing insulating paper inside the gap between the windings, but this process is costly.
The hairpin motor winding also encountered a limit in terms of end height. The end of a conventional hairpin winding is presented in the form of a triangle, and the end height Lc is limited by the angle θ of the triangle and the gap.
The "triangle" end structure has a height restriction.
To address the winding height issue, DENSO of Japan employs a stepped end design, reducing the winding's turning radius and lessening the constraint of the triangular side angle θ on the end height Lc, resulting in a more compact winding structure. Furthermore, to solve the insulation problem between windings, DENSO adds a high-polymer insulation material to the basic flat wire insulation, ensuring that different coils meet insulation requirements even after contact.
"Stepped" end structure, height restriction
In addition to the basic insulation of the flat wire, a polymer insulation material is added.
It's worth noting that while hairpin and I-pin windings are comparable in terms of peak efficiency and torque, and I-pin winding technology is simpler, the increased number of solder joints increases the risk of failure. Therefore, hairpin winding is the more widely used technology both domestically and internationally. Currently, Tesla's 2022 Model 3/Y rear-wheel drive models are equipped with hairpin-winding flat-wire motors.
Hair-pin winding flat wire motor
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Continuous wave winding
Corrugated flat wire is a winding process with fewer solder joints and high design flexibility, but with the current coil arrangement, there are several main problems:
Asymmetrical winding branches result in differences in back EMF, resistance, and inductance, reducing motor performance. At the same time, the circulating current in the windings will increase the additional losses of the motor, leading to localized overheating.
The motor windings are difficult to arrange, and they are prone to overlapping, making them difficult to fit into the stator slots.
When the span of the wave winding is different, the manufacturing process is complex, the mold investment cost is high, and the production difficulty is great.
Research on motor cooling and heat dissipation
The application of 800V high-voltage platforms and flat wire winding technology will inevitably lead to the development of motors towards higher power and higher speeds. Especially in motors using hairpin windings, the heat loss caused by the skin effect and eddy currents not only reduces the overall efficiency of the motor, but also makes it prone to magnet demagnetization, insulation aging, and reduced lifespan of drive units such as gearboxes when operating at high temperatures for extended periods. Therefore, this places new demands on motor heat dissipation methods.
There are three main methods for cooling motors: air cooling, oil cooling, and water cooling. Due to its limited heat dissipation effect, air cooling is rarely used in existing motors.
Water-cooled motors borrow the cooling method from gasoline engines, using a mixture of cooling water and ethylene glycol to cool the motor through a cooling circuit in the casing. However, as the power density of motors increases, it becomes necessary to directly cool the heat source to achieve good heat dissipation. Water's electrical and magnetic properties, low boiling point, and tendency to expand make existing water-cooling technologies insufficient to meet the high-power requirements of motors.
Oil cooling is currently the primary cooling method. Cooling oil itself is non-conductive and non-magnetic, allowing it to directly cool components such as gear shafts and stators inside the motor housing, achieving good cooling results. Furthermore, by adjusting the cooling oil's formula, it can also lubricate internal parts of the electric drive system while cooling. Therefore, using well-formulated oil cooling solutions will become the mainstream trend for future motor cooling solutions.
Based on the above oil cooling measurement, many emerging companies have also considered intelligent oil temperature measurement in their all-in-one motor technology. Taking Huawei's One-drive high-speed intelligent oil-cooled four-wheel drive system as an example, it plans to use a spray-type oil channel design to directly cool parts such as gear shafts that generate heat through friction, in order to achieve the best cooling effect.
Automotive drive motor flat wire winding transposition technology
In the 2021 Global New Energy Vehicle Frontier and Innovative Technology Selection, the "Transposition Technology of Flat Wire Winding for Automotive Drive Motors" submitted by Harbin University of Science and Technology can effectively reduce circulating current and eddy current losses, and can eliminate complex processes such as slot wire insertion, end twisting, and welding. It is expected to become a leading technology for the third generation of drive motor windings.
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Forward
Drive motors are the core power source for new energy vehicles. With increasing demands for lightweight, low-cost, and high-reliability new energy vehicles, increasingly stringent requirements are being placed on the power density, peak efficiency, and high-efficiency range of drive motors. Current solutions mainly focus on the magnetic circuit structure design and control methods of drive motors, contributing only limitedly to improvements in power density and efficiency. Winding losses account for about half of the total losses in drive motors, especially at high speeds and high frequencies. Therefore, breakthroughs in the winding design technology of automotive drive motors are not only an effective solution to improve the power density and efficiency of drive motors, but also a forward-looking technology for improving the insulation and voltage levels of motor windings in the future. Currently, automotive drive motors widely use flat wire hairpin windings. These windings are characterized by multiple single-turn copper flat wires tightly inserted along the height direction within the slots, with the ends of the copper flat wires twisted and welded using a special process. Due to the limited number of layers in flat wire hairpin windings and the large cross-sectional area of copper flat wires, eddy current losses in the copper flat wires are significant. Furthermore, automotive drive motors often operate under high-frequency, high-harmonic conditions, leading to reduced efficiency and severe heat generation. In addition, the slot insertion, end twisting, and welding processes used in flat wire hairpin windings are extremely complex, resulting in high production line costs and difficulties in mass production. To overcome the limitations of flat wire hairpin windings, a transposition technology for automotive drive motor flat wire windings has been proposed. This technology uses multiple small-section copper flat wires to form a parallel transposition structure, effectively reducing additional losses and improving efficiency and power density. The transposition windings require no welding at the ends, simplifying the process, reducing costs, and facilitating mass production. The transposition windings allow for integrated strand design, effectively improving the winding insulation level and heat dissipation capacity. Standardized winding processes allow for pre-production testing and evaluation, effectively improving the operational reliability of the windings.
Winding structure
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Leading
The transposition technology for flat wire windings in automotive drive motors has overcome fundamental and cutting-edge bottlenecks in design, manufacturing, testing, and evaluation. Its technological leadership is mainly reflected in the following three aspects:
A theoretical framework for flat wire winding transposition technology in automotive drive motors was proposed. Addressing the bottleneck issues of winding transposition in automotive drive motors, such as short axial length, short end length, numerous poles, and few slots, a continuous transposition concept based on leakage potential compensation was proposed. This approach achieves stator winding electrical load balance, low additional losses, high efficiency, and uniform temperature rise distribution across the entire range. For complex transposition structures, a three-dimensional electromagnetic thermal numerical simulation method was proposed, enabling accurate prediction of temperature rise monitoring points.
Flat wire winding transposition technology
A general transposition design method and technology for a series of flat wire transposed windings for automotive drive motors were proposed. A general transposition design method based on discrete integration, equivalent circuit network, and multivariable multi-objective optimization algorithms was developed, and general transposition design software was also created. This solved the bottleneck problems in transposition design methods and technologies, providing new transposition design technologies for the development of a series of flat wire transposed windings for automotive drive motors and for new products.
Universal transposition design software for flat wire transposition windings
A multi-path parallel-wound strand circulating current detection system and a flat wire transposed winding evaluation technology were developed. Traditional research has not solved the problem of circulating current experimental testing in the case of a tightly connected multi-path parallel-wound strand structure of flat wire transposed windings. This technology proposes a real-time testing method for multi-path parallel-wound strand circulating current in flat wire transposed windings, builds a test platform, and develops a linear array strand current sensor. Furthermore, it develops online evaluation and fault diagnosis technologies for flat wire winding transposition, overcoming the challenges of circulating current detection and transposition evaluation.
Flat wire winding evaluation technology
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disruptive
The technology of automotive drive motor windings has evolved through the development of first-generation conventional round loose-wire windings and second-generation flat-wire hairpin windings, initially achieving high slot fill factor, high power density, and lightweight design. Flat-wire winding transposition technology can not only change the complex manufacturing process of automotive drive motors and reduce costs, but also break through the upper limits of power density, peak efficiency, and high-efficiency range in automotive drive motors, potentially replacing flat-wire hairpin windings as the third-generation winding for automotive drive motors. This technology overturns the loose-wire strand pattern of the previous two generations of windings, comprehensively achieving multi-objective coordinated design for winding losses, insulation, and heat dissipation.
Flat wire transposed windings achieve a revolutionary improvement in efficiency by fully utilizing the parallel winding of strands and the selection of transposition pitch to balance the leakage flux linkage of each flat copper wire, thus minimizing additional losses. Compared with round loose wire windings and flat wire hairpin windings, this can improve the efficiency of automotive drive motors, especially for high-speed and high-frequency operating conditions, where the reduction in additional losses is significant, thus expanding the high-efficiency range of automotive drive motors.
The flat wire transposition winding has overturned the design methods and concepts of round loose wire winding and flat wire hairpin winding. The design scheme of flat wire transposition winding is flexible and diverse, without the limitation of the number of layers of flat wire hairpin winding. Different winding arrangement schemes and transposition methods can be designed according to technical requirements.
The flat wire transposition winding revolutionizes the manufacturing process of round loose wire winding and flat wire hairpin winding. It combines the advantages of round loose wire winding and unwinding processes, while eliminating the slot insertion, end twisting, and welding processes of flat wire hairpin winding, thus reducing production costs. It can also realize the integrated design of strands, leaving room for improving the insulation level and voltage level of the winding.
The transposition technology for flat wire windings in automotive drive motors has solved fundamental and cutting-edge bottlenecks in design, testing, and evaluation, forming a comprehensive transposition technology system for flat wire windings in automotive drive motors. This technology eliminates the complex processes of slot insertion, end twisting, and welding in flat wire hairpin windings, and avoids the need to purchase expensive production lines for flat wire hairpin windings, significantly reducing the manufacturing cost of automotive drive motors. Moreover, compared to second-generation flat wire hairpin windings, it reduces additional losses under all operating conditions of automotive drive motors, lowers motor temperature rise, and improves operational reliability, potentially breaking through the upper limits of power density, peak efficiency, and high-efficiency range in automotive drive motors. This technology also facilitates improving the insulation level of motor windings, laying a technological foundation for future voltage level increases in automotive drive motors. Therefore, this technology is destined to become the leading technology for third-generation automotive drive motor windings. From both a cost and technological advancement perspective, this technology has significant promotional value and socio-economic benefits, making it an inevitable choice for future automotive drive motor windings.
