What technical challenges do electric vehicle motor drive systems face?

In terms of automotive motors, a variety of motors including DC motors and AC asynchronous motors have been applied in the development of electric vehicles. Permanent magnet synchronous motors have been the first choice for electric vehicle motors since the 1990s because of their high power density, high efficiency in the entire working area, and good dynamic performance. Relevant scientific research work mainly focuses on improving power density, motor efficiency and satisfying driving performance. In recent years, due to the sharp rise in the price of permanent magnet materials, high-efficiency AC asynchronous motors and switched reluctance motors have been revisited by researchers and have achieved some results. However, the motor drive system used in electric vehicles is different from the motor drive system used in industrial applications in terms of manufacturing process.

Motor drive systems for industrial applications usually use AC asynchronous motors, and electric vehicles use permanent magnet motors. The motor can be mainly divided into several major components: the casing, the stator, the rotor and the rotating shaft. The industrial AC asynchronous motor equivalent to the power level of the vehicle motor adopts air cooling or natural cooling, and the casing is often made of aluminum profiles. The vehicle motor drive system is liquid-cooled, and the casing needs to be cast, machined and welded, and the aluminum welding process can withstand high pressure, severe ambient temperature changes and vibration. The manufacture of vehicle motor casings is difficult.

In terms of motor stator manufacturing, the stator part of the permanent magnet motor for vehicles adopts a segmented splicing structure to improve production efficiency. The stator structure adopts an automatic winding machine to realize stator winding embedding, which improves the efficiency and yield. In addition, the silicon steel sheet used in the permanent magnet motor for vehicles is also thinner and has less loss than the industrial motor, which puts forward new requirements for the core stacking process.

In terms of rotor manufacturing, the rotors of permanent magnet motors for vehicles are inlaid with high-performance NdFeB magnets. In order to improve productivity, a magnetizer needs to be used for overall magnetization. The overall magnetization technology is one of the key technologies of permanent magnet motors for electric vehicles.

Rare earth NdFeB permanent magnets for electric vehicles

Rare earth NdFeB permanent magnets for electric vehicles.jpg

In short, there are many differences between automotive permanent magnet motors and industrial motors in terms of raw materials, processes and production equipment.

Since the beginning of the 21st century, the technology of motor drive systems for vehicles has made remarkable progress. The global cumulative production of motor drive systems for vehicles represented by Toyota hybrid motors has exceeded 5 million sets, and large-scale production has been initially achieved. Compared with the international automotive motor drive system technology Yongping, China's automotive motor drive system performance indicators have also made significant progress. At the same time, the self-developed permanent magnet synchronous motor, AC asynchronous motor and switched reluctance motor products have been serialized, and the power range covers 0~200kW, which basically meets the needs of the whole vehicle and realizes the small batch matching for Chinese vehicle enterprises. However, the vehicle motor drive system technology still faces the following outstanding problems.

(1) Worldwide, the existing vehicle motor drive system technology cannot support the large-scale application of electric vehicles, and the material utilization of IGBTs and motors needs to be improved.

A 2010 U.S. Department of Energy report states: at present, the world's most advanced Toyota motor drive system has the smallest gap (only 10%) in terms of efficiency and performance with meeting the target of large-scale application, and the gap in cost is the most significant. The cost target value of the motor drive system for large-scale applications is 8~10 US dollars/kW (that is, equivalent to the unit power cost of the current internal combustion engine), and in fact, the unit power cost of the current international most advanced motor drive system is 30~35 US dollars/kW. That is to say, the cost of the current vehicle motor drive system is more than 35 times the target value of large-scale application, which is also one of the main reasons for the current high price of electric vehicles and the difficulty of large-scale application. Industrial manufacturing experience shows that although increased output will reduce unit power costs, technological breakthroughs are necessary to meet the large-scale application of electric vehicles. In other words, the existing vehicle motor drive system technology based on silicon-based IGBT, high magnetic permeability silicon steel and high temperature magnetic energy product permanent magnet, digital control and three-phase full-bridge power electronic converter technology cannot support the large-scale application of electric vehicles.

Motor drive system for vehicles

(2) The reliability and durability of China's automotive motor drive systems are low, and there is a lack of theoretical basis for reliability and durability evaluation, and the life of electric vehicle motor drive systems cannot be known through design and related tests.

The reliability of the vehicle motor drive system is an important issue in the current industrialization of electric vehicles in China. The formulation of the national standard GB/T29307-2012 "Reliability Test Method for Electric Vehicle Drive Motor System" is based on the idea of testing first and then revising, which is of practical significance, but the evaluation method of the design life and reliability of the brake release vehicle motor drive system cannot be theoretically, which reflects the current situation that the theory lags behind the demand and the technology.

A vehicle motor drive system is a complex system that includes active switching devices (IGBT or MOSFET), passive devices, electronic components, sensors, mechanical components, insulating materials, electrical conductors and other energy flows and information flows coupled with each other. The complex system is under extremely harsh and frequently changing automotive road conditions, and the instantaneous or long-term failure of any component will directly affect the reliability of the system. Therefore, establishing the reliability model of the vehicle motor drive system and establishing the test system for the life and reliability evaluation of the vehicle motor drive system have become the key issues that need to be solved urgently in the electric vehicle motor drive industry, and it is also an interdisciplinary basic subject of materials science, electrical engineering, electronics, and mechanical engineering.

(3) There is a lack of accurate models and verification methods for the megahertz-level high chin of the vehicle motor drive system, and the solution to the electromagnetic compatibility (EMC) problem of the vehicle lacks theoretical support.

The vehicle motor drive system has high power, high density of internal electrical and electronic components, and the voltage change rate during the switching process of the inverter is as high as 109V/s. It is the main source of electromagnetic interference in the vehicle. Since the motor drive system is installed on the vehicle and is insulated from the ground, the chassis and body of the electric vehicle are complex in structure and diverse in materials, resulting in a very complicated propagation path for electromagnetic interference signals. On the one hand, the high voltage change rate of the main power loop of the motor drive system has a great impact on its own weak current control, vehicle control, battery management system, etc.; on the other hand, the motor shaft current and corona caused by the high voltage change rate shorten the motor life, which in turn poses a serious threat to the safety, reliability, and service life of electric vehicles. It is necessary to establish an accurate high-frequency model of the megahertz level of the vehicle motor drive system, which is used to study the internal relationship between the electromagnetic interference signal generated by the on-board motor drive system and its components, and the structure of the converter and the converter control, as well as the relationship between the propagation of the electromagnetic interference signal and the electric vehicle components and vehicle layout.

Motor drive system

Electric vehicles are divided into three types: pure electric vehicles, hybrid electric vehicles and fuel cell electric vehicles. Regardless of pure electric vehicles, hybrid electric vehicles or fuel cell electric vehicles, the vehicle drive motor system is both a key technology and a common technology. The performance requirements of the drive motor for electric vehicles are mainly reflected in the aspects of low speed and large torque, wide speed regulation range, large overload capacity, high power and small volume; at the same time, compared with the general industrial motor drive system, it also has the characteristics of harsh working environment and low cost, so the development technology and production technology of the vehicle drive motor system are more difficult. At present, various international automobile groups, multinational electric groups, and scientific research institutes have invested a lot of money and manpower to develop electric vehicle drive motor system technology. The development trend of electric vehicle drive system technology can basically be summarized as permanent magnetization, digitization and integration.

Permanent magnet motors have the advantages of high efficiency, large specific power, high power factor, high reliability and easy maintenance. The frequency conversion speed regulation system of vector control can make the permanent magnet motor have a wide speed regulation range. Therefore, the permanent magnetization of the motor has become one of the important development directions of the motor drive technology. The permanent magnet motor is the mainstream technology of electric vehicles, especially cars, and the permanent magnet reluctance type is more suitable for electric vehicle applications than the surface mount type. Digitalization is also an inevitable trend in the development of motor drive technology in the future. The digitization includes not only the digitization of the drive control, the digitization of the interface between the drive and the numerical control system, but also the digitization of the measuring unit. With the development of microelectronics and computer technology, the advent and commercialization of high-speed, high-integration, low-cost microcomputer-specific chips, and DSP, etc., make an all-digital control system possible. Using software to replace hardware to the greatest extent, in addition to completing the required control functions, it can also have other functions such as protection, fault monitoring, and self-diagnosis. Full digitization is one of the important development directions of electric vehicle control and even AC drive system.

The integration of the motor drive system mainly includes two aspects. One refers to the integration of the motor and the engine assembly or the motor and the gearbox; the development of the motor drive technology towards the direction of integration is conducive to reducing the weight and volume of the entire system, and can effectively reduce the manufacturing cost of the system; the other is power electronics integration, including functional integration (multi-inverter + DCDC + battery management + vehicle control), physical integration (power modules, drive circuits, passive devices, control circuits, sensors, power supplies, etc.), application of new devices such as Trench+ FS IGBT, and a high degree of system integration based on monolithic integration, hybrid integration and system integration technologies.