Introduction to Automobile Electric Drive System Technology and Design

The structure and design of pure electric vehicles are different from those of traditional internal combustion engine-driven vehicles. The author is sorting out the key technologies in the development of new energy powertrains to provide a theoretical basis and reference for the design and testing of powertrains. The plan is divided into three chapters to analyze the key technologies of electric drive in pure electric vehicle powertrains. Today, we will first introduce the technology and design of electric drive systems.

●Definition of motor drive system

According to the vehicle's power battery status and the vehicle's power requirements, the electrical energy output by the on-board energy storage or power generation device is converted into mechanical energy, and the energy is transmitted to the drive wheels through the transmission device. When the vehicle brakes, part of the vehicle's mechanical energy is converted into electrical energy and fed back to the energy storage device.

●Electric drive system technology and design

The task of the electric drive system technical parameter design is to reasonably match the technical parameters of the relevant components of the electric drive system to meet the power performance requirements of the vehicle and work efficiently. The electric drive system mainly includes components such as the motor, transmission mechanism and motor controller; the electric drive system technical parameter design mainly includes the power, torque, speed, voltage design of the motor, the transmission ratio of the reduction mechanism and the power supply capacity, output power, voltage, current and other designs.

New energy vehicle power design

●Dynamic parameters ◎ Maximum speed level The maximum speed that can be achieved on a good road surface. ◎ Acceleration time Starting from a standstill Acceleration time: The time required to reach a certain predetermined speed at maximum acceleration.

◎Maximum climbing grade

● Driving force calculation

●Driving force calculation

The driving resistance of a vehicle includes factors such as rolling resistance, air resistance, slope resistance and acceleration resistance.

Drive motor technology

● Basic requirements for motor design: ◎ The motor is the only power source for pure electric vehicles, and its performance is closely related to the performance of the entire electric vehicle. Therefore, the selection of the motor and parameter matching are one of the keys to designing the power system of pure electric vehicles. ◎ In order to drive the vehicle with high performance, the drive motor is required to have high dynamic performance of frequent starting/stopping, acceleration/deceleration, and torque control. ◎ At low speed or climbing, the torque should be high, and at high speed, the torque should be low. ◎ The motor speed regulation range should be wide, and it should work in both the constant torque area and the constant power area, while maintaining a high operating efficiency within the entire speed regulation range.

◎The motor of a pure electric vehicle should have a higher torque/inertia ratio, as wide a high efficiency zone as possible and good torque-speed characteristics; currently, the permanent magnet brushless motor system has the highest efficiency and torque-inertia ratio and has been widely used in electric vehicles.

●Permanent magnet brushless DC motor

The semiconductor switch device is used to realize electronic commutation, that is, the electronic switch device is used to replace the traditional contact commutation and brushes, and the interaction between the stator magnetic field and the rotor permanent magnetic field generates electromagnetic torque. It has high reliability, no commutation sparks, and low mechanical noise.

Figure. Permanent magnet brushless DC motor structure

●Permanent magnet synchronous AC motor

It is basically the same as permanent magnet brushless DC motor. The stator three-phase distributed winding and permanent magnet rotor have a sinusoidal induced electromotive force waveform in the magnetic circuit structure and winding distribution. The external stator voltage and current are also sinusoidal, provided by an AC transformer and frequency converter.

The difference between the two: ◎ Brushless DC motor is a system - brushless DC motor system, the motor and controller are designed in an integrated manner, with a high degree of interdependence. The motor and controller cannot exist and work independently, and the overall technical performance is assessed. Permanent magnet synchronous AC motor refers to a motor, the motor and controller can exist and work independently, and the performance of the motor itself is assessed.

◎The back EMF of brushless DC motors is mostly square wave, and basically no algorithm is needed for control. You only need to consider which winding to pass current based on the rotor position. The back EMF of permanent magnet synchronous AC motors is mostly sinusoidal wave, and variable frequency speed regulation requires certain algorithms.

●Design principles of motor technical parameters◎The peak parameters of the motor should meet the short-term limit working conditions of the vehicle.◎The common speed of the motor should be in the high-efficiency zone.◎For vehicles climbing long slopes, the motor overload working condition cannot be used to complete the task. The rated working condition should be selected to climb long slopes.◎The rated voltage and current of the motor should match the output voltage and current of the battery pack.

◎The motor voltage and current should match the motor controller technical parameters.

●Motor peak power and rated power design

During design, the power of the motor is usually determined based on the electric vehicle's maximum speed vmax (km/h), initial velocity v0, final velocity v, acceleration time and maximum climbing grade.

●Design of rated speed and maximum speed of the motor ◎For electric vehicles that are frequently started, braked and reversed, the main consideration when selecting the rated speed should be to shorten the starting and braking time to improve productivity.

◎The length of starting and braking time mainly depends on the flywheel torque and rated speed of the motor. Smaller flywheel torque and rated speed should be selected.

●Motor rated voltage design

The rated voltage of the motor is generally determined by the selected motor parameters and is proportional to the rated power of the motor. The higher the rated voltage of the motor, the greater the rated power of the motor; the rated voltage of the motor is generally in the range of 300 to 650V.

Summary: The motor design requires complex and high precision. Especially at low speed or climbing, the torque should be high, and at high speed, the torque should be low; the motor speed range should be wide, it should work in the constant torque area and the constant power area, and at the same time, it should maintain a high operating efficiency in the entire speed range; the motor of pure electric vehicles should have a high torque/inertia ratio, as wide a high efficiency area as possible and good torque-speed characteristics. At present, AC asynchronous motors and permanent magnet synchronous motors are widely used in pure electric vehicles, and major OEMs are constantly seeking breakthroughs in integration, cost savings and efficiency improvement.