The drive motor system is one of the three core systems of an electric vehicle. It is the main driving system for the vehicle's movement, and its characteristics determine the vehicle's main performance indicators, directly affecting the vehicle's power, economy, and user driving experience.
I. Introduction to the Drive Motor System
The drive motor system consists of a drive motor and a drive motor controller (MCU), and is connected to other systems of the vehicle through high and low voltage wiring harnesses and cooling pipes, as shown in Figure 1.
The vehicle control unit (VCU) sends commands to the motor controller (MCU) via the CAN network based on signals from the accelerator pedal, brake pedal, gear position, etc., to adjust the torque output of the drive motor in real time, so as to realize the vehicle's idling speed, acceleration, energy recovery and other functions.
The motor controller can monitor its own temperature, the motor's operating temperature, and the rotor position in real time, and transmit the relevant information to the vehicle controller (VCU), which in turn adjusts the operation of the water pump and cooling fan to keep the motor operating at the ideal temperature.
The technical specifications of the drive motor are shown in Table 1, and the technical specifications of the drive motor controller are shown in Table 2.
1. Drive motor
A permanent magnet synchronous motor (PMSM) is a typical drive motor (Figure 2), which has advantages such as high efficiency, small size, and high reliability. It is the actuator of a power system and the carrier of electrical energy converted into mechanical energy. It relies on a built-in rotary transformer and temperature sensor (Figure 3) to provide the motor's operating status information and sends the motor's operating status information to the MCU in real time.
The rotary transformer detects the rotor position of the motor. After being decoded by the rotary transformer decoder inside the motor controller, the motor controller can know the current rotor position of the motor, thereby controlling the corresponding IGBT power transistors to conduct, energizing the three coils of the stator in sequence, and driving the motor to rotate.
The function of the temperature sensor is to detect the temperature of the motor windings and provide the information to the MCU. The MCU then transmits the information to the VCU via the CAN line, thereby controlling the water pump, water circulation, cooling fan operation, and regulating the motor operating temperature.
The drive motor has a low-voltage interface and three high-voltage wire (V, U, W) interfaces, as shown in Figure 4.
The definitions of each terminal of the low-voltage interface are shown in Table 3. The motor controller obtains the motor temperature information and the current position information of the motor rotor through the low-voltage port.
2. Drive motor controller
The structure of the drive motor controller MCU is shown in Figure 5. It uses a three-phase two-level voltage source inverter and is the control core of the drive motor system. It is called the intelligent power module. It is based on IGBT (insulated gate bipolar transistor) and supplemented by drive integrated circuit and main control integrated circuit.
The MCU processes all input signals and sends the operating status information of the drive motor control system to the vehicle control unit (VCU) via the CAN 2.0 network . The drive motor controller contains a fault diagnosis circuit. When the motor malfunctions, it will activate an error code and send it to the VCU after certain conditions are met. At the same time, it will also store the fault code and related data.
The drive motor controller primarily relies on current sensors (Figure 6), voltage sensors, and temperature sensors to monitor the motor's operating status. Based on these parameters, it adjusts the voltage and current, and performs other control functions. The current sensor detects the actual operating current of the motor, including the bus current and three-phase AC current. The voltage sensor detects the actual voltage supplied to the motor controller, including the power battery voltage and the 12V battery voltage. The temperature sensor detects the operating temperature of the motor control system, including the temperature of the IGBT modules.
The drive motor controller is divided into a low-voltage interface and a high-voltage interface (Figure 7). The low-voltage interface terminal definitions are shown in Table 4.
II. Functions of the Drive Motor System
By observing the operating status of the drive motor, one can understand the basic functions of the drive system of a new energy vehicle. The operating status of the drive motor can be understood according to the driver's wishes: when accelerating in D gear, decelerating and braking, reversing in R gear, and driving in E gear.
1. Accelerate in D mode
When the driver engages the D gear and presses the accelerator pedal, the gear position information and acceleration information are transmitted to the vehicle control unit (VCU) via the signal line. The VCU then transmits the driver's intention to the drive motor control unit (MCU) via the CAN line. The drive motor control unit (MCU) then combines the information from the resolver sensor (rotor position) to supply three-phase AC power to the stator of the permanent magnet synchronous motor. The three-phase current generates a voltage drop across the resistance of the stator windings.
The rotating armature magnetomotive force and the established armature magnetic field generated by the three-phase alternating current cut the stator windings and induce an electromotive force in them; simultaneously, they drive the rotor to rotate forward at synchronous speed using electromagnetic force. As the accelerator pedal travel increases, the conduction frequency of the six IGBTs controlled by the motor controller rises, and the motor torque increases with the increase in current. Therefore, it essentially has maximum torque at startup. As the motor speed increases, the motor power also increases, and the voltage increases accordingly.
In electric vehicles, it is generally required that the output power of the motor remains constant, that is, the output power of the motor does not change with the increase of speed. This requires that the voltage remain constant when the motor speed increases. The output characteristic curve of the permanent magnet synchronous motor is shown in Figure 8.
At the same time, the motor controller will also sense the current power, current consumption, and voltage of the motor through current and voltage sensors, and transmit this information to the instrument and vehicle controller via the CAN network. Its specific working principle is shown in Figure 9.
2. When reversing in R gear
When the driver shifts into reverse (R), the driver sends a request signal to the VCU, which is then sent to the MCU via the CAN bus. At this time, the MCU combines the current rotor position (resolver sensor) information and changes the W/V/U power sequence by modifying the IGBT module, thereby controlling the motor to reverse.
3. Energy recovery during braking
When the driver releases the accelerator pedal, the motor continues to rotate due to inertia. Let the wheel speed be V_wheel and the motor speed be V_motor. Let the fixed transmission ratio between the wheel and the motor be K. When the vehicle decelerates, if V_wheel K < V_motor, the motor is still the power source. As the motor speed decreases, if V_wheel K > V_motor, the motor will rotate due to being dragged by the vehicle. At this time, the drive motor becomes a generator (Figure 10).
The BMS can calculate the corresponding maximum allowable charging current based on the battery charging characteristic curve (the relationship between charging current, voltage change curve and battery capacity) and parameters such as battery temperature. Based on the maximum allowable charging current, the MCU controls the IGBT module to maintain the angular velocity of the generator stator coil rotating magnetic field and the motor rotor angular velocity until the generator current does not exceed the maximum allowable charging current. This adjusts the current from the generator to the battery, and also controls the vehicle's deceleration. The specific process is shown in Figure 11.
