I. Introduction
my country is a vast country with abundant resources, but its per capita resources are relatively scarce, it relies heavily on imported fossil fuels, and its energy consumption per unit of GDP is high. Therefore, developing high-efficiency new energy vehicles based on electric drive technology is of significant strategic importance to my country's energy security. At the same time, my country's internal combustion engine technology for automobiles still lags significantly behind leading manufacturers in Western developed countries, and it will be difficult to catch up within the next 10 years. Considering that the current gap between my country's electric drive technology and that of Western developed countries is not significant overall, vigorously developing new energy vehicles based on electric drive technology will be a crucial opportunity for Chinese automakers to catch up with leading Western automakers and achieve a leapfrog development.
For new energy vehicles, battery technology, motor technology, and motor controller technology are known as the three key electric technologies. Given the current lack of breakthroughs in battery technology, improving the efficiency, power density, safety, and reliability of motor drive systems has become the main research direction for new energy vehicle motor drive systems, and is also a key focus for policy-making and future development planning by the Chinese government and enterprises.
II. Key Technologies of Drive Controller
As the power conversion unit connecting the battery and motor in new energy vehicles, the motor drive controller is the core of the motor drive and control system. The application of high-performance power semiconductor devices, intelligent gate drive technology, and device-level integrated design methods will help to achieve high power density, low loss, and high efficiency motor controller design. At the same time, high-performance and high-reliability motor controller products also require high standards of electromagnetic compatibility (EMC), functional safety, and reliability design.
(I) Power Semiconductor Device Technology
The development of motor controllers is primarily driven by power semiconductor devices, evolving from silicon-based insulated-gate bipolar transistors (IGBTs) and traditional single-sided cooling packaging technology towards wide-bandgap semiconductors (such as SiC and GaN), customized module packaging, and double-sided cooling integration. Meanwhile, thanks to mature technological iterations and lower costs compared to wide-bandgap semiconductor devices, silicon-based IGBTs remain the primary choice for motor controller products for the foreseeable future.
In terms of silicon-based IGBT chip technology, Infineon Technologies has developed EDT2 chip technology to meet the high power density requirements of the new energy vehicle market, and has achieved mass production of 750V/270A IGBT chips. Japanese manufacturers such as Fuji Group have also successively developed high power density IGBT chip technology and have been applied to automotive IGBT module products in batches. In addition, compared with silicon-based devices (such as IGBT, MOSFET, etc.), SiC devices belong to the third generation of semiconductor material power devices, which have advantages such as high thermal conductivity, high temperature resistance, large band gap, high breakdown field strength, and large saturated electron drift rate. The junction temperature tolerance can reach 225 ℃ or even higher, which is far higher than the current silicon-based IGBT's highest application junction temperature of 175 ℃. SiC devices have faster switching speeds and can be applied to higher switching frequencies, making them more suitable for high-speed motor control. At the same time, compared with silicon-based IGBTs, SiC devices have significantly reduced switching losses and conduction losses, which helps to reduce the power consumption of the whole vehicle per hundred kilometers and improve the vehicle's range [1]. However, the cost of SiC devices is still much higher than that of silicon-based IGBTs, which is a major factor hindering the promotion of SiC devices.
Meanwhile, novel packaging technologies such as copper wire bonding, flip-chip bonding, silver sintering, and transient liquid phase welding can improve the current carrying capacity and lifespan of IGBT power modules, thus becoming current research hotspots. Currently, companies such as Denso, Delphi, Infineon, and CRRC Zhuzhou Times Electric Co., Ltd. have developed IGBT modules and motor controllers based on double-sided cooling, some of which have already been mass-produced and applied in vehicle manufacturing. Silicon-based IGBT-based motor controller designs will remain the mainstream choice for a considerable period, and silicon-based IGBT device chips and power module packaging technologies will continue to improve through continuous optimization and iteration.
(II) Intelligent Gate Drive Technology
Gate drive technology is the link between high-voltage power semiconductor devices and low-voltage control circuits in motor controllers and is the key to driving power semiconductor devices. In addition to basic isolation, driving and protection functions, IGBT gate drive also needs to combine the characteristics of IGBT itself to accurately control the turn-on and turn-off process so that IGBT can achieve the best trade-off between losses and electromagnetic interference (EMI) [2].
The two main features of intelligent gate drive are active gate control and monitoring and diagnostic functions. Active gate control is a method of actively and finely controlling the switching process of IGBT according to the working environment and operating conditions. Active gate control technology is a research hotspot in the current IGBT application field. Its basic idea is to divide the IGBT turn-on process and turn-off process into several different stages. For a certain problem, only the corresponding stage needs to be independently controlled, which has little (or even no) negative impact on other parameters [3].
In summary, the application of intelligent gate drive will help to fully utilize the performance of power semiconductor devices, such as reducing losses and improving voltage utilization, and enable online assessment of the health status of power semiconductor devices, thus meeting the goals of high safety and high reliability design for motor controllers.
(III) Integrated Design of Power Components
To meet the requirements of high power density, long life and high reliability of new energy vehicles, most of the typical motor controller products in the world are directionally designed for power semiconductor modules[4]. The boundaries between power semiconductor devices and other electronic components are becoming increasingly integrated, and device-based integrated design has become a new trend in the development of motor controllers for new energy vehicles.
Device-level integrated design technology is mainly divided into physical integration and demand-level integrated design. Physical integration design is an integrated design method that studies the physical structure between various components of a motor to achieve a balance and optimization of parasitic parameters, heat dissipation, mechanical strength, etc., and to achieve the optimal design of mechanical, electrical, thermal, and magnetic aspects, ultimately achieving the design goal of high power density and high reliability of the motor controller. Demand-level integrated design technology refers to extending the requirements of the entire vehicle and electric drive system forward to the fields of IGBT chip design and power module packaging. Based on the design and performance requirements of the entire vehicle, it establishes a top-down optimization design method that is demand-oriented and moves from the system to the core components. The advantages it brings are an increase in the driving range of the entire vehicle or a reduction in battery capacity requirements.
(iv) Other key technologies
In addition to the three key technologies mentioned above, the following key technologies also need to be given attention in the future new energy vehicle industry.
(1) EMC and reliability design are also key technologies for the industrialization of motor controllers for new energy vehicles. EMC and reliability design are key indicators for evaluating power electronic products. More effective EMC design is a goal that the industry has been pursuing. Among them, establishing a high-frequency simulation model of EMC for "components-parts-controllers" based on the finite element analysis method, studying the failure mechanism, and combining it with experimental verification to finally achieve positive design for electromagnetic compatibility will gradually become the mainstream technical route.
(2) Automotive functional safety design can eliminate or significantly reduce various vehicle safety risks caused by functional abnormalities of electronic and electrical systems. Currently, the functional safety requirements of motor controllers are mostly ASIL C level, but in the future, the functional safety requirements of motor controllers may be upgraded to ASIL D level, which requires motor controller product design with higher complexity, stronger redundancy and higher reliability index [5].
(3) Reliability design of motor controller products. As the core drive unit of new energy vehicles, the reliability index of motor controller directly affects the driving experience and market reputation of the whole vehicle. German and American automotive electronics manufacturers jointly proposed the robustness verification (RV) method [6]. This method has been widely used by Infineon Technologies and Bosch Group in the reliability design analysis of semiconductor discrete devices. For complex systems such as motor controllers, its applicability and effectiveness are still being explored.
III. Key Technologies of Drive Motors
New energy vehicles are replacing traditional internal combustion engines with electric motors as their power output components. As new energy vehicles demand higher performance from drive motors, such as wider speed range, higher power density, and higher efficiency, rare-earth permanent magnet synchronous motors (PMSMs) are gradually replacing traditional DC motors and induction motors as the mainstream drive motor solution for new energy vehicles. However, with the continuous improvement in the power density and efficiency of drive motors, PMSMs manufactured using traditional structures and processes are increasingly unable to meet the competitive demands of the current market. Major traditional OEMs and emerging car manufacturers urgently need to find new technological solutions.
(I) Flat Copper Wire Technology
Hairpin (also known as flat copper wire) stator windings are shown in Figure 1. Using hairpin stator windings can increase the slot fill factor of the motor stator, thereby increasing the motor's power density. Furthermore, hairpin stator windings have shorter end dimensions, resulting in lower copper losses and better heat dissipation. Currently, the production technology, equipment, and patents for this type of motor are mainly led by traditional automotive powerhouses such as Japan, Italy, and Germany. Since 2018, domestic electric vehicle component suppliers such as Shenzhen Huichuan Technology Co., Ltd. and Songzheng Electric Vehicle Technology Co., Ltd. have also successively launched their own flat copper wire motor products.
However, compared to traditional round copper wire windings, flat copper wire windings exhibit a significant high-frequency skin effect. For high-power drive motors, the circulating current losses caused by hairpin stator windings are also more pronounced [7,8]. The manufacturing process of hairpin windings is complex, and the insulation layer is easily damaged after bending the flat copper wire, resulting in gaps or breaks. Reducing the skin effect and eddy current losses of hairpin stator windings is currently a hot research topic. Improving the material processing technology and manufacturing precision of hairpin stator windings will be beneficial to the localization and promotion of this technology.
(II) Multiphase permanent magnet motor technology
Multiphase motors have a lower bus voltage than traditional three-phase motors when outputting the same power, and have smaller torque ripple and stronger fault tolerance [9]. Therefore, they are suitable for electric drive systems of new energy vehicles with high requirements for noise, vibration, and acoustic roughness (NVH) [10]. Taking the dual three-phase permanent magnet synchronous motor as an example, the two sets of windings of the motor are 30° electrical angle apart in space, eliminating the 5th and 7th harmonic magnetomotive forces and greatly reducing the torque ripple of the motor [11,12]. At the same time, the dual three-phase permanent magnet synchronous motor adopts an isolated neutral line design for the two sets of windings. Compared with 4-phase and 5-phase motors, it reduces the order of the system, making it easier to analyze and control. When the motor and controller fail, the control algorithm does not need to be changed much to achieve fault-tolerant operation control of the motor system. Therefore, the dual three-phase permanent magnet synchronous motor has also become a hot topic in the research of electric drive systems for new energy vehicles.
(III) Permanent Magnet Synchronous Reluctance Motor Technology
The permanent magnet synchronous reluctance motor is a fusion of a permanent magnet synchronous motor and a reluctance motor. Compared with traditional permanent magnet synchronous motors, it has a smaller permanent magnet flux linkage and a larger reluctance torque, making it a low-rare-earth/rare-earth-free permanent magnet motor solution. Furthermore, it not only boasts a high torque-to-current ratio, high power density, and low magnetic saturation issues, but also a wider high-efficiency speed range. Therefore, this technology has been applied to BMW's i3 and i8 series models (see Figure 2).
Permanent magnet synchronous reluctance motors are a widely favored technology in the industry. However, they also face challenges such as complex rotor structure design, complex manufacturing processes, high manufacturing equipment costs, and large variations in the optimal current angle, making them a key focus and difficulty in current research. Therefore, the development of this technology will have a significant impact on companies that heavily rely on inexpensive rare-earth permanent magnets and have weak R&D and manufacturing capabilities.
(iv) In-wheel motor technology
Hub motors come in various forms, but domestic and international research has focused on external rotor hub motors [13-16]. The application of hub motors can bring a series of obvious advantages to new energy vehicles: eliminating mechanical transmission parts such as gearboxes, drive shafts, and differentials, enabling four-wheel distributed drive, and leaving more chassis space for battery packs. However, hub motors for drive motors are currently facing a series of new challenges, such as: significantly increasing unsprung mass and wheel rotational inertia, difficulty in handling motor waterproofing and dustproofing, heat dissipation, and more complex drive control algorithms [16]. Currently, foreign companies such as Protean and Elaphe have launched a series of product prototypes (see Figure 3) and have carried out localization cooperation with domestic companies such as Asia-Pacific Electromechanical Co., Ltd. and Wan'an Technology Co., Ltd. Domestic companies, led by Hubei Tait Electromechanical Co., Ltd., have also followed suit and launched a series of hub motor solutions for large commercial vehicles and special vehicles.
(V) Permanent Magnet Heat Dissipation Technology
The stability of permanent magnet performance is crucial to the output performance of vehicle drive motors. However, the increase in operating temperature often causes permanent magnet demagnetization, thereby reducing the torque output capability of the drive motor. Excessively high permanent magnet operating temperature can also lead to a reduction in the high-efficiency operating range and a decrease in the power factor of the drive motor [17]. In response to this problem, scholars at home and abroad have conducted a lot of theoretical research on permanent magnet temperature monitoring technology for permanent magnet motors [18]. However, in the drive motors of new energy vehicles, using a stable and low-cost temperature sensor to provide the necessary temperature monitoring function is still the only reliable option at present.
Current research on motor heat dissipation methods often focuses on the analysis of the stator and end windings. Studying the heat dissipation structure and methods from the perspective of the motor rotor would be significant for improving the dynamic stability of new energy vehicles. Furthermore, developing high-temperature resistant permanent magnets for high-power-density motors could fundamentally solve the problem of magnetic performance degradation of permanent magnets under high load and high-temperature conditions.
(vi) Other technologies
In the field of new energy vehicles, my country is still in the follower and initial stage. In the future, the specific areas that need to be focused on are: super copper wire technology, series and parallel winding switching motor technology, high voltage-resistant insulation material technology, and local demagnetization technology.
Furthermore, my country still lags significantly behind developed Western countries in several areas, including high-speed bearing technology, brushless electrically excited synchronous motor technology, and deep integration of motors and electronic controls. These areas require focused efforts in future industrial planning and research projects. If my country continues to rely solely on its advantage in rare earth permanent magnet resources, its new energy vehicle industry will inevitably face technological barriers related to environmental protection imposed by Western countries in future competition.
IV. Conclusion
The next 5 to 10 years will see a golden age of development for new energy vehicles, and my country, as the world's largest automobile market, will face a new round of industry reshuffling. Motor controllers based on traditional silicon-based IGBTs will remain the market mainstay for a considerable period. However, with the reduction in the production cost of SiC devices, highly reliable 800V high-voltage SiC drive systems will be the direction for the next generation of passenger vehicle drive controllers. my country needs to be wary of its reliance on the "rare earth permanent magnet dividend" and proactively develop cutting-edge motor design technologies, materials technologies, advanced manufacturing technologies, and high-precision processing equipment to counter the future technological barriers established by developed Western countries using their advanced low-rare-earth/rare-earth-free permanent magnet motor technologies.
