The core of electric vehicle technology lies in the “three electric” system, that is, the electric drive system, the battery system and the electronic control system, which constitute the key technology of electric vehicles.The three-electric system accounts for more than 70% of the total cost of new energy vehicles, which is an important component affecting the vehicle’s sports performance and a standard to measure whether a car has a hard core strength.
(1) Electric drive system
Electric drive systems, also known as electric drive systems, are generally composed of motors, transmission mechanisms and converters.
① Motor
Electrical Machine, there are two types, one is to convert electrical energy into mechanical energy device, called Motor, the other is to convert mechanical energy into electrical energy device, called Generator.
Whether it is a motor or a generator, both are widely used in the car, in the electric drive system, the motor generally refers to the motor, also known as the drive motor, the main role is to convert the electrical energy stored in the power battery into mechanical energy, to provide forward power for electric vehicles.
The motor is mainly composed of two parts, stator and rotor, divided into DC motor and AC motor two large types.
For electric vehicles, the drive motor needs to meet the wide speed range, fast response, lightweight, high efficiency, energy recovery, high reliability, safety, sustainable cost reduction and other requirements, the current electric vehicles commonly used motors are AC motors, of which three-phase induction induction motor and permanent magnet synchronous motor are the most used two.
The three-phase asynchronous motor used in the Tesla Model S
② Transmission mechanism
Transmission mechanism refers to the motor output torque and speed transfer to the main shaft of the car, thereby driving the car to drive the mechanism, mainly including the two parts of the reducer and the differential.
The main role of the differential is to make the wheels on both sides of the car turn at different speeds, whether it is an electric car, or a fuel car component is the same, no more mention here.
For the reducer, it is generally composed of high-speed bearings and gears with different number of teeth, through the small gear on the input shaft with fewer teeth, meshing to the large gear with more teeth on the output shaft, to achieve the purpose of slowing down and increasing torque. The ratio of the number of teeth of the big and small gears is the transmission ratio. In fact, as a power transmission mechanism, the reducer has been widely used in a variety of mechanical traditional systems.
The reduction gear of electric vehicles can be regarded as the transmission of fuel vehicles, because the motor itself has a sufficient width of speed regulation performance, so the reduction gear is generally a fixed transmission ratio of single-stage reduction gear, that is, only one gear transmission. The following figure is the internal structure of an electric vehicle reducer, which can be seen as a relatively simple mechanism.
Internal structure of an electric vehicle reducer
③ Converter
Convertor/Converter refers to a device that changes one or more characteristics (voltage, current, waveform, phase number, frequency) of an electrical system. For electric vehicles, the Inverter and DC/DC converter are mainly included.
Among them, the main role of the inverter is to convert direct current into alternating current, convert the battery’s direct current into AC output to drive the motor, and convert the electrical energy into mechanical energy to drive the electric vehicle, and the inverter is directly related to the reliable and efficient operation of the drive motor.
The IGBT module is the core device of the inverter, which will be introduced in the following chapters.
DC/DC converter, the DC power supply voltage into any DC voltage, mainly used for DC high and low voltage conversion, such as the power battery high voltage (greater than 400V) to convert low voltage, for multimedia systems, air conditioning and other equipment (12V) power supply.
Internal structure of an electric vehicle inverter system
(2) Battery system
In order to distinguish 12V low-voltage lead-acid batteries, high-voltage batteries are generally called power batteries. The power battery is the core of the “three electricity”, and it is also the most expensive and complex system in the “three electricity”.
Compared with traditional 12V lead-acid batteries, lithium-ion batteries have high energy density, can achieve fast and deep charge and discharge, long life and other advantages, so the current power batteries are lithium-ion batteries. According to the different cathode materials, lithium-ion batteries for electric vehicles generally use ternary lithium batteries and lithium iron phosphate.
The power battery is generally composed of large and small cells, which have three forms of cylinder, soft bag and square due to different packaging forms;
The battery cell is formed into a module in series or parallel, and the module is then integrated into the thermal management system, battery management system and other components in series, and finally integrated into the battery pack, as shown in the figure.
(Composition of battery pack)
(3) Electronic control system
The electronic control system is the total console of electric vehicles, like the “brain of electric vehicles”, and its play determines the main performance indicators of electric vehicles such as energy consumption, emissions, power, handling, and comfort.
In general, the electric Control system of electric vehicles mainly includes three common subsystems, Vehicle Control Unit (VCU), Motor control Unit (Motor Control Unit), and motor control unit (VCU). MCU and Battery Management System (BMS), these controllers are through the CAN network to achieve mutual communication.
Vehicle Control Unit (VCU) is the control center of each electronic control subsystem of electric vehicles, which coordinates and manages the operating state of the entire electric vehicle. It is the main interface to interact with the driver, it receives various operating instructions from the driver, diagnoses and analyzes the status of the vehicle and components, controls the actions of the subsystem controller, and finally realizes the safe and efficient driving of the vehicle.
Motor Control Unit (MCU) is a unique core power electronic unit of electric vehicles, which controls the motor output specified torque and speed to drive the vehicle by receiving the travel control instructions of the vehicle controller.
Battery Management System (BMS) is the “brain” of the power battery system, which mainly collects and monitors the voltage, current, temperature and other data of the battery system, and realizes the functions of battery status monitoring and analysis, battery safety protection, energy control management and information management.