With the annual development of the new energy vehicle industry, the production and sales of electric vehicles have exploded, and the demand for high-performance and high-efficiency motors is also increasing. The Ministry of Industry and Information Technology and the National Development and Reform Commission have proposed a plan to achieve a passenger car power density greater than 4kW/kg by 2025, further promoting the pursuit of higher power density motors. The high efficiency, lightweight, miniaturization, and low cost of electric drive systems are the future trends; The integration of electric drive systems and the flattening of motors are the main technical routes to achieve lightweight and miniaturization. Flat wire winding has gained rapid development due to its unique advantages compared to circular wire winding, becoming a hot research and development direction for new energy vehicle motors. Flat wire motors are also widely used both domestically and internationally, and the penetration rate of flat wire motors in new energy vehicle models is increasing year by year. This article will briefly introduce the flat wire winding technology, compare and analyze the advantages of flat wire motors, introduce the production and manufacturing process of Hair pin motors, summarize the new technologies and processes applied in flat wire motors in recent years, and future optimization research directions, providing reference for the research of flat wire motors.
1. Introduction to Flat Wire Motor Technology
Bar wound motor is commonly referred to as a flat wire motor in China. Flat wire motor technology refers to the use of flat copper wire windings in the stator of the motor to replace the original round copper wire windings, and is equipped with unique stator and rotor structure optimization, cooling scheme optimization, and control optimization technologies based on the special structure of the flat wire winding. A flat copper wire winding refers to a winding in which the slot structure of the motor stator is changed and replaced by fewer and thicker rectangular wires instead of more and thinner circular wires. Flat wire motors have been widely used in the field of new energy vehicles due to their advantages such as small size, high slot filling rate, high power density, good NVH performance, and better thermal conductivity and heat dissipation performance.
2. Comparison between Flat Wire Motors and Round Wire Motors
The energy loss of motors mainly includes motor copper loss, motor iron loss, wind friction loss, and stray loss, among which motor copper loss accounts for nearly 70%. Reducing motor copper loss can significantly reduce motor energy loss and improve motor power density. The formula for calculating DC copper loss is shown in equation (1). The rectangular copper wire used for the flat wire winding has a significant cross-sectional area change compared to the round and thin copper wire used for the round copper wire winding, which can effectively reduce the winding resistance and thus reduce copper loss. At the same time, rectangular wire windings have smaller gaps between flat wires compared to thin circular wire windings, and can accommodate more winding copper wires under the same stator slot volume, thus having a higher slot filling rate. The slot filling rate of the circular wire motor is about 40%, while the slot filling rate of the flat wire motor can reach up to 70%. Under high slot filling rate, the copper wire filling amount of the flat wire motor with the same motor power is less, and the size of the stator core and end is reduced, making the motor size smaller, saving materials and further improving the motor power density.
Compared to circular wire motors, flat wire motors have smaller slot sizes in the stator, which can effectively reduce cogging torque and electromagnetic noise. At the same time, rectangular wires have greater rigidity and also have a suppressive effect on armature noise. Combined with rotor magnetic poles and structural optimization, they have better NVH performance [1].
The winding ends of the flat wire motor are wound into special shapes, such as wavy, triangular, stepped, etc., as shown in Figure 2, which effectively reduces the size of the winding ends and is conducive to achieving miniaturization and lightweight [2]. At the same time, rectangular conductors reduce internal voids, increase the contact area between conductors and between conductors and iron core slots, and improve thermal conductivity and heat dissipation performance. There is a minimum air gap between the conductors at the end of the winding, which is more convenient for heat dissipation. At the same time, combined with end spray cooling technology, the heat dissipation performance of the flat wire motor is further improved; Under lower temperature rise conditions, the entire vehicle has better acceleration performance, effectively improving the vehicle's high-temperature power performance.
Flat wire motors also have shortcomings, as they are greatly affected by skin effect. The skin effect guides the uneven distribution of current inside a conductor when there is an alternating current or magnetic field in the body. The closer to the surface of the conductor, the higher the current density; Reduced the effective energized copper wire area, increased the equivalent resistance of the winding, and increased high-frequency AC losses. The aspect ratio, placement direction, and winding distribution of conductors all have an impact on the skin effect, and the skin effect of windings in the same phase and slot is more severe than that of windings in different slots. Under the same groove depth and width, increasing the number of conductor layers is beneficial for reducing skin effect, reducing AC losses at high speeds, and improving motor performance. Another limitation on the development of flat wire motors is the high cost of their automated production lines, which is 2-3 times that of round wire stator automated production lines, and the huge initial investment of enterprises.
3. Classification of flat wire motors
Flat wire motors can be divided into concentrated winding flat wire motors, wave winding flat wire motors, and Hairpin (card issuing) flat wire motors according to product types, among which card issuing flat wire motor technology is widely adopted as the mainstream technology.
The concentrated winding is made of flat copper wire wound into a single tooth winding as shown in Figure 3 (a), with one tooth corresponding to a single tooth winding installed. Due to its short span coil end, the end size can be effectively reduced, and its process is simpler compared to the Hairpin flat wire motor. This structure has the disadvantages of large torque ripple and complex radial force due to its excessive fractional harmonics; In order to reduce cogging torque and torque pulse vibration, the structure needs to ensure roundness, coaxiality, and uniform distribution between teeth in the assembly process, with higher assembly requirements. Concentrated winding technology is widely used in the field of industrial motors, and some manufacturers in the field of new energy vehicle motors are also researching and applying it; Honda has adopted this technology in its Acura hybrid model, achieving good results with its unique fractional slot concentrated winding, segmented stator structure, and related optimization techniques. The Panasonic 270 series PHEV-P2 motor, which adopts centralized winding technology, is shown in Figure 3 (b) and is applied in hybrid solutions with its unique advantages.
The wave winding flat wire motor adopts continuous winding to form a whole and then insert it, or inserts it into the stator slot while winding, forming a wave shaped end. Compared with the Hairpin flat wire motor, it does not have solder joints, which can further shorten the height of the winding end and reduce the motor size. However, the slot size of this type of stator assembly is relatively wide, which leads to greater cogging torque, higher torque ripple, and poorer NVH performance. Therefore, it is necessary to cooperate with electromagnetic multi-objective optimization design and other measures to improve and optimize it; At the same time, its production cost is higher than that of the Hair pin motor.
Hair pin (U-pin) is also known as hairpin winding due to its winding shape resembling a "hairpin". It is made by pre forming one end of the enameled flat copper wire into a U-shaped shape, inserting it into the stator core slot, twisting the other end into a frog leg shape, and then welding it together to form a wavy winding. Another I-PIN winding process directly inserts straight copper wire into the stator core slot, and then simultaneously twists the two ends into a frog leg shape and welds them together to form a wave winding, eliminating the pre forming process in the U-PIN winding. U-PIN and I-PIN flat wire windings belong to the second generation of axial embedded windings, and they are on par in terms of maximum efficiency and peak torque. However, the latter has higher slot filling rate, sustained torque, and sustained power compared to the former; Due to the double number of solder joints in the latter, the size of the winding end is slightly increased, and the risk of solder joint failure is also higher. The Hair pin winding process is currently widely used both domestically and internationally.
4. Card issuing coil process
Card issuing technology is a large-scale, high-quality, and low cycle stator production technology. The process chain mainly consists of the following five steps: forming (straightening, stripping, cutting, and bending of enameled copper flat wires), inserting (stator slot lining and card issuing coil combination insertion), twisting, welding, and insulation [4]. The process steps are illustrated in Figure 4.
In addition to insulation paint film insulation, stator slot lining is also added between the copper wires of the card winding to separate the conductors from each other, in order to eliminate direct contact between turns or conductors and the stator core, improve insulation performance, and enhance short circuit protection. So it is necessary to insert insulation slot paper. Common slot paper shapes include O-type, C-type, B-type, S-type, etc., as shown in Figure 5. The B-type groove liner eliminates the gap at the corner of the S-type groove liner, enhancing the protection against short circuit faults [5].
The paper insertion process involves inserting insulation slot paper into the stator slot in advance. As the number of flat wire layers increases, the difficulty of the process also increases significantly. The PIN forming process includes stamping, spring machine, and CNC equipment for automatic forming. The former has a fast forming speed and low cost, but causes significant damage to copper wires, while the latter has good versatility and less damage to copper wires but higher equipment costs. After PIN molding, it is pre inserted into the profiling fixture for molding. As the number of flat wire layers increases, the difficulty of automatic insertion across wires also increases. Next, insert all PINs in the profiling fixture into the corresponding design dimensions of the iron core, which requires high equipment accuracy. Then, the end of the winding is leveled and neat for welding through the processes of expanding, twisting, and cutting. At present, TIG welding and laser welding are the most popular welding methods for flat wire motors, and other companies are also testing welding methods such as CMT cold welding. After welding is completed, the winding needs to undergo electrical performance testing, phase resistance, inductance and balance testing, as well as voltage and resistance testing. After passing the testing, the coating can be applied. The coating process is divided into powder coating and liquid coating according to the different coating materials. The process sequence of the two is different. Powder coating is applied first in immersion paint, while liquid coating is applied first in immersion paint before application. The impregnation process includes traditional impregnation, vacuum impregnation, vacuum pressure impregnation, drip impregnation, EUV impregnation, etc. according to the different materials.
5. Research Development Trends
5.1. The number of conductor layers gradually increases
As shown in Figure 6, increasing the number of conductor layers can effectively reduce AC copper consumption, thereby reducing the total copper consumption of the motor and improving the overall performance of the motor. In order to further reduce the high-speed AC losses caused by skin effect, the card issuing motor adopts the method of increasing the number of conductor layers to optimize. From the already applied 4-layer, 6-layer, and 8-layer schemes to the currently being studied 12-layer and 16-layer schemes, there is a trend of gradually increasing the number of conductor layers. The main research difficulties lie in the limitations of process level and the control of manufacturing costs [6].
5.2. Optimize insulation
The more complex insulation groove paper makes the installation process more complex, and at the same time, it requires higher precision for CNC installation equipment. Research on better insulation processes has become an important direction. The Chevrolet Bolt motor simplifies the insulation slot lining and uses a simpler two-piece slot insulation to protect the winding from short circuiting to the stator core. The new design, as shown in Figure 7 (a), eliminates insulation between conductors compared to S-type B-type insulation slot lining, further improves slot filling rate, and simplifies stator manufacturing process. At the same time, in order to minimize the voltage potential between the wires in the slot, General Motors has made relevant optimizations to the winding layout. It is also possible to add a polymer insulation layer outside the insulation layer of the conductor base, as shown in Figure 7 (b), to eliminate the need for slot lining and solve the problem of inter turn insulation in windings, simplifying the production process [2].
5.3. Widely adopting oil cooling technology
The heat generated by the winding needs to pass through a longer path between the insulation layer in the slot, the stator core, and the casing before being carried away by water. The thermal resistance during this process can easily form local hotspots, making the efficiency of water-cooled heat dissipation inefficient. Oil cooling technology, which can directly contact the heat source and has no impact on the motor magnetic circuit, has become a research hotspot due to its higher heat dissipation efficiency. The main heat of the motor is concentrated at the end of the winding, and the unique end spray cooling of the flat wire motor can better achieve heat dissipation. Technologies such as oil circuit cooling, oil injection cooling, shaft oil swing cooling, and stator enclosed oil circulation cooling have been widely applied and studied in flat wire motors.
6. Conclusion
With the rapid development of new energy vehicles, flat wire motors have been widely used due to their unique advantages, and their penetration rate has been increasing year by year. Flat wire motors have important application research significance in the miniaturization, lightweight, and high-power density of new energy vehicle motors. This article briefly analyzes and introduces the advantages of flat wire motors compared to traditional circular wire motors, the production process of mainstream card winding, and the research and development trends of flat wire motor technology, providing reference for enhancing the understanding and research of flat wire motors and related technologies.
Source: Lan Pengyu, School of Mechanical, Electrical and Vehicle Engineering, Chongqing Jiaotong University