Analysis of Key Technologies in the Motor Drive System of New Energy Vehicles

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In recent years, with the rapid development of China's transportation industry, the transportation sector has become the fastest growing area of energy consumption in China. The energy crisis and the intensification of environmental pollution have made the research and development of electric vehicles a strategic project for the sustainable development of the world's automobile industry. Countries around the world have also generally established the development of electric vehicles as an important way to ensure energy security and transform the low-carbon economy.


In recent years, with the rapid development of China's transportation industry, the transportation sector has become the fastest growing area of energy consumption in China. The energy crisis and the intensification of environmental pollution have made the research and development of electric vehicles a strategic project for the sustainable development of the world's automobile industry. Countries around the world have also generally established the development of electric vehicles as an important way to ensure energy security and transform the low-carbon economy.
In 1881, the first electric vehicle was manufactured by French engineer Gustave Trouve. It was a three wheel electric vehicle powered by a 0.1hp (British horsepower, 1hp=745.7W) DC motor and powered by lead-acid batteries. The weight of the entire vehicle and its driver was approximately 160kg. Two British professors created similar electric cars in 1883. Because the application technology was not yet mature enough to compete with the carriage at that time, these early constructions did not attract much public attention.
After the 1940s, semiconductor technology developed rapidly, followed by the emergence of thyristor and triode, especially the insulated gate bipolar junction transistor (IGBT), which came out in the 1980s, providing convenience for motor speed regulation and control. At the same time, with the rapid development of power electronics technology, it provided a technical basis for the replacement of oil powered internal combustion engines by electric powered motors.
1、 According to the national standard GB/T19596-2004 Electric Vehicle Terminology, electric vehicles can be divided into pure electric vehicles powered by dynamic power batteries, hybrid electric vehicles with coexisting motors and internal combustion engines, and fuel cell electric vehicles powered by fuel cells. These three types of electric vehicles all use one or more motor drive systems to convert electrical energy into mechanical energy, thereby driving the vehicle and recovering braking energy from the brakes, Thus achieving an improvement in energy utilization efficiency.
1. Pure electric vehicle
Pure electric vehicles are driven by electric motors, and their energy is completely provided by secondary batteries (such as lead-acid batteries, nickel pick batteries, nickel hydrogen batteries, or lithium-ion batteries). Due to the increasing scarcity of fossil fuels, pure electric vehicles are considered the future of the automotive industry. The typical power structure of a pure electric vehicle is shown in Figure 1. The electrical energy of the battery pack is replenished by the charging system after the vehicle has traveled a certain distance. The characteristic of pure electric vehicles is that they achieve zero emissions, do not rely on gasoline, and completely use electric energy to drive the vehicle. However, due to the energy density and power density of the battery being much lower than gasoline or diesel, the continuous driving range of pure electric vehicles is limited.
2. Hybrid electric vehicles
Hybrid electric vehicles can be divided into three hybrid modes: series, parallel, and hybrid based on their powertrain structure and energy flow transmission schemes. In series hybrid vehicles, engine power and electric motor power are transmitted through the electrical system; In parallel and hybrid electric vehicles, engine power and electric motor power are transmitted to the wheels through a specialized electromechanical coupling mechanism. Common electromechanical coupling mechanisms include planetary gear coupling, transmission coupling, and clutch coupling.
The powertrain of a series hybrid system, where the mechanical energy of the engine is converted into electrical energy through a generator, and the electric motor converts electrical energy into mechanical energy and transmits it to the drive axle. There is no direct mechanical connection between the drive axle and the engine. The advantage of this scheme is that the system control is simple, while the disadvantage is that it is difficult to cope with complex road conditions, the battery charging and discharging pressure is high, and the battery life requirement is high.
A typical parallel hybrid system, where the motor and engine are dynamically coupled through a gear reduction mechanism. Parallel hybrid power has three driving modes: engine driven separately, electric motor driven separately, and hybrid engine and electric motor driven. The parallel hybrid powertrain has the following advantages: (1) there are two powertrain systems, the engine and the electric motor, each with a power equivalent to 50% to 100% of the vehicle's driving power, which is smaller in power, mass, and volume than the three powertrain systems of series hybrid vehicles.
(2) The engine can directly drive the vehicle, and the comprehensive energy conversion efficiency is higher than that of series hybrid vehicles. When the vehicle requires maximum output power, the electric motor can provide additional auxiliary power to the engine, so a low-power engine can be configured, which has better fuel economy than a series hybrid vehicle.
(3) The power battery pack matched with the electric motor has a smaller capacity and reduces the overall weight of the vehicle.
However, parallel hybrid electric vehicles require the assembly of structures such as transmissions, clutches, transmission shafts, and drive axles, as well as devices such as electric motors, power battery packs, and power combiners. Therefore, the power system structure is complex, and the structural layout and vehicle control are more difficult.
The famous hybrid car Pruis adopts a hybrid powertrain with a planetary gear structure as the coupling. The engine is connected to the planetary carrier of the planetary gear, the generator is connected to the sun gear, and the electric motor is connected to the gear ring. By controlling the working status of the clutch, two motors, and brake, multiple working modes can be achieved. Compared with the series hybrid system, the hybrid system has an additional transmission route for mechanical power, and an additional transmission route for electrical energy. The hybrid system has the following advantages: (1) the power, mass, and volume of the three powertrain systems are smaller than those of the series hybrid system.
(2) Electric motors can independently drive vehicles. Utilizing the low speed and high torque characteristics of electric motors to drive vehicles to start and achieve "zero pollution" driving in cities. When the vehicle requires maximum output power, the electric motor can provide auxiliary power for the engine, resulting in low engine power and good fuel economy.
However, the hybrid system requires two sets of drive systems; The engine transmission system needs to be equipped with transmission assemblies such as clutch, transmission, transmission shaft, and drive axle; In addition, there are electric motors, reducers, power battery packs, and specialized devices for combining or coordinating various energy sources (engine power and electric motor power).
3. Fuel cell electric vehicle Fuel cell is a device that converts the chemical energy of fuel and oxidants into electrical energy through electrochemical reactions, with high energy conversion efficiency and "zero emissions" characteristics, making it a candidate power source for electric vehicles. Fuel cell electric vehicles have the advantages of simple system structure, easy system layout, and are conducive to the lightweight of the entire vehicle. However, due to the short lifespan of fuel cells, low system power density, and difficulty in ensuring device reliability, the development of fuel cell electric vehicles has been slow in recent years.
Vehicle Motor Drive System
The automotive motor drive system is a key and common technology for electric vehicles. Due to the limitations of vehicle space and usage environment, vehicle motor drive systems are different from ordinary electric transmission systems. They require higher operating performance, specific power, and adaptability to harsher working environments. In order to meet these requirements, the technological development trend of automotive motor drive systems can be basically summarized as permanent magnetization of the motor, digital control, and system integration. The structure of the motor and its drive system. 1. High power density automotive motor controller. The main drive motor controller in electric vehicles generally adopts a typical three-phase bridge voltage source inverter circuit. Its main components include: power module, DC side support capacitor, and stacked busbar. According to the power level requirements of the vehicle for the controller, most power modules use insulated gate bipolar junction transistor (IGBT). The DC side support capacitor is the most important passive component in the controller, which is mainly used to absorb the DC side pulsating current caused by the power module switch, stabilize the DC side output voltage and current, and thus improve the service life of the battery, Its volume and weight have a significant impact on the power density of the controller. Therefore, the IGBT power module and DC side support capacitor are key to improving controller performance and cost control.
In order to improve the operation performance and reliability of IGBT power modules and reduce costs, the Institute of Electrical Engineering of the Chinese Academy of Sciences, together with domestic power module packaging enterprises, has carried out the research and development of domestic intelligent IGBT power modules with independent intellectual property rights. We have conducted extensive analysis, optimization, and process design work in the design of IGBT.
The 60kW high power density motor controller developed by the Institute of Electrical Engineering, Chinese Academy of Sciences, using intelligent IGBT power module and metal film capacitor technology, has a weight specific power of 4kW/kg and a volume specific power of 6kW/L. And successfully applied to the Lifan LF620 pure electric police vehicle, serving the 2010 Shanghai World Expo.
2. High power density vehicle motors
At present, electric vehicles mainly use asynchronous motors, permanent magnet motors, and switched reluctance motors. The driving motor of electric vehicles belongs to special motors and is a key component of electric vehicles. To ensure good performance of electric vehicles, the drive motor should have a wide speed range and high rotational speed, sufficient starting torque, small size, light weight, high efficiency, and strong dynamic braking and energy feedback performance. Currently, among the electric motors used in electric vehicles, DC motors have been basically replaced by asynchronous motors, permanent magnet synchronous motors, or switched reluctance motors.
Due to the advantages of compact structure, high efficiency, and high power density, permanent magnet synchronous motors have been widely used in electric vehicle applications in recent years. In order to further meet the special needs of vehicle applications, new types of special motors such as hybrid excitation motors and disc motors are also applied in the automotive field. The motors used in electric vehicles are developing towards high-power, high-speed, high-efficiency, and miniaturization.
(1) Permanent magnet synchronous motor
An electric motor is an electromagnetic device that uses a magnetic field as a medium to convert electrical and mechanical energy into each other. It plays a role in electric vehicles by converting the electrical energy in the battery into mechanical energy to drive the vehicle, or by converting excess mechanical energy when the car needs to brake into electrical energy and storing it in the battery. In order to establish the necessary air gap magnetic field for electrical energy conversion in the motor, the magnetic field can be generated by passing current through the motor winding, or by using a permanent magnet to generate the magnetic field. Due to the high remanence, high coercivity, and high magnetic energy accumulation of rare earth cobalt permanent magnets and neodymium iron boron permanent magnets, they can be used to manufacture permanent magnet motors to obtain strong air gap magnetic fields, reduce motor volume, have light weight, low losses, and high efficiency. The shape and size of the motor are flexible and diverse, suitable for the high power density requirements of automotive motors.
During the operation of the permanent magnet synchronous motor, the stator winding is connected with three-phase symmetrical current, the synchronous rotating magnetic field with the motor rotor is established in the air gap of the motor, and the phase and frequency of the current are adjusted by the control algorithm to achieve the stable operation of the motor in the full speed range.
(2) Hybrid excitation motor
The disadvantage of the permanent magnet flux linkage of permanent magnet motors being unable to be adjusted brings about weak magnetic field control problems under constant supply voltage: vehicle power performance requires the motor system to have a wide constant power speed range at high speeds to ensure the high-speed performance of the vehicle. Due to the limitation of battery voltage, most permanent magnet motor systems currently use the method of increasing the demagnetization current of the stator winding to offset the permanent magnet magnetic field, thereby achieving the goal of weak magnetic speed regulation under constant supply voltage. However, this method reduces system efficiency and power factor, increases controller cost, and also has poor stability during deep weak magnetic control and voltage safety issues during high-speed runaway. Hybrid excitation motor is a feasible technology to solve the above problems.
Hybrid excitation motor evolved from permanent magnet motor and electric excitation motor. By introducing electric excitation winding in permanent magnet motor, the motor achieves controllable excitation performance. The motor is more suitable for applications with wide speed range and high weak magnetic ratio, making up for the shortcomings of a single excitation method. The Electrical Research Institute of the Chinese Academy of Sciences takes the bypass hybrid excitation motor as the research object, and conducts in-depth research on the motor structure, motor parameter characteristics, motor mathematical model and excitation current planning. The bypass hybrid excitation motor inherits the advantages of high efficiency and high power density of permanent magnet motors to the greatest extent. The motor excitation is mainly provided by permanent magnet magnetic potential, and the electric excitation magnetic potential is mainly used to enhance or weaken the magnetic flux of the main magnetic circuit. By adjusting the size of the electric excitation current, the electric excitation magnetic assistance and weakening functions are achieved.
The hybrid excitation motor has two working conditions: magnetic assistance and weak magnetic field: (1) magnetic assistance working condition: the magnetic circuit under the electric excitation magnetic assistance working condition. The electric excitation magnetic field lines on the N-pole side enter the motor rotor N-pole through the axial air gap from the electric excitation end cover, and are interconnected with the armature winding through the main air gap along with the permanent magnet magnetic field lines. Some of the magnetic field lines are closed through the end cover, while the other part enters the rotor S-pole through the motor yoke and main air gap, and enters the electric excitation bypass through the axial air gap on the S-pole side.
(2) Weak magnetic condition: The weak magnetic field of electric excitation is achieved by reversing the excitation current, and the reverse electric excitation magnetic potential and permanent magnet magnetic potential are established in the opposite direction of the magnetic field line in the electric excitation bypass under the auxiliary magnetic condition. Some permanent magnet magnetic field lines do not cross the main air gap and armature winding, achieving weak magnetic operation of the motor.
Overall, compared to traditional brushless permanent magnet motors, bypass hybrid excitation motors have significant advantages: increasing excitation at low speeds to improve output torque; Reduce or reverse excitation during high-speed operation to broaden the constant power weak magnetic zone of the motor; Reduce the iron loss of the motor during high-speed operation and improve efficiency; Dynamically adjust the excitation current to improve the dynamic performance of the generation voltage when the load changes; Reduce the weak magnetic potential of the armature reaction and reduce the risk of demagnetization during high-temperature operation of permanent magnets. Hybrid excitation is an important development trend for future automotive permanent magnet motors.
3. Vehicle Motor Control Technology
In response to the strong nonlinearity and parameter changes of the motor control system, as well as some requirements for high-speed and wide speed range of the motor system in automobiles, the Institute of Electrical Engineering of the Chinese Academy of Sciences focuses on the technical difficulties of safe, reliable, and efficient energy-saving operation control of high-performance motor drive systems suitable for vehicle operating conditions, and proposes dead zone compensation technology to solve the problem of low speed pulsation in low speed and light load operating conditions of pure electric vehicles; A deep magnetic weakening control method and decoupling control technology based on a single regulator have been proposed to solve the problems of high-speed motion control and high-speed power generation control in electric vehicle motor drive systems.
With the development of pure electric vehicles, there is an increasing demand for the constant power and weak magnetic characteristics of the motor. People hope that the output characteristics of the motor can fully cover the driving characteristics of the car, thereby eliminating the transmission mechanism, saving space, volume, and cost. Therefore, weak field control has become one of the important research directions for automotive motor control. According to research conducted by the Institute of Electrical Engineering of the Chinese Academy of Sciences, it has been found that in the commonly used dual current loop weak magnetic control, there is a possibility of irreversible loss of control, resulting in serious system failures and safety hazards. The Institute of Electrical Engineering has been committed to weak magnetic field control for many years and has proposed a reliable and globally controlled weak magnetic field control strategy. Through experimental verification, the constant power range can reach 1:6, fully meeting the needs of automotive applications.
4、 In summary, there is a certain gap between China and foreign countries in terms of traditional internal combustion engine vehicles. However, electric vehicles are still in the initial stage, and China has accumulated rich experience and solid theory in the field of motor drive. In recent years, it has also invested heavily in the research and development of key technologies for electric vehicles, cultivating a large number of research and development institutions and production enterprises. Although there are difficulties and twists in the current development, But it has not changed the determination of the government and industry to develop electric vehicles. Although traditional internal combustion engine vehicles still dominate the current vehicle market, the development of clean electric vehicles is a trend in terms of energy, environment, and technology. With the development of related technologies, electric vehicles will usher in a day of entry into the homes of ordinary people.