An electric motor, as the name suggests, is an electrical device that converts electrical energy into mechanical energy and vice versa. When electrical energy is converted into mechanical energy, the motor exhibits the operating characteristics of an electric motor; when electrical energy is converted into mechanical energy, the motor exhibits the operating characteristics of a generator. In most electric vehicles, during braking, mechanical energy is converted into electrical energy, which is then used by the generator to recharge the battery.
An electric motor mainly consists of a rotor, stator windings, a speed sensor, and components such as a housing and cooling system. In the field of new energy vehicles, permanent magnet synchronous motors are widely used. "Permanent magnet" refers to the addition of permanent magnets during the manufacturing of the motor rotor, which further enhances the motor's performance. "Synchronous" means that the rotor's speed and the current frequency of the stator windings are always kept consistent. Therefore, by controlling the input current frequency of the motor's stator windings, the speed of the electric vehicle can ultimately be controlled. How to adjust the current frequency is a problem that the electronic control unit must solve.
Compared to other types of motors, the biggest advantage of permanent magnet synchronous motors (PMSMs) is their high power density and torque density. Simply put, compared to other types of motors, PMSMs can provide the greatest power output and acceleration for new energy vehicles with the same mass and volume. This is the main reason why PMSMs are the preferred choice for many automakers in the new energy vehicle industry, where space and weight are extremely critical.
Besides permanent magnet synchronous motors, asynchronous motors have also gained widespread attention due to their use by Tesla. Compared to synchronous motors, the rotor speed of an asynchronous motor is always less than the speed of the rotating magnetic field (generated by the current in the stator windings). Therefore, the rotor appears to have a "mismatch" frequency with the current in the stator windings, which is why it is called an asynchronous motor.
Compared to permanent magnet synchronous motors, asynchronous motors have the advantages of lower cost and simpler manufacturing process; however, their disadvantages are that their power density and torque density are lower than those of permanent magnet synchronous motors.
Besides synchronous and asynchronous motors, in-wheel motors are another popular application in new energy vehicle motors. The biggest feature of in-wheel motors is that they integrate the vehicle's power unit, transmission, and braking system all within the wheel hub. Compared to traditional power units, the advantages of in-wheel motors are obvious: they eliminate many transmission components, resulting in a simpler vehicle structure. However, in-wheel motors still face many challenges in areas such as synchronous control and water sealing.
Analysis of electronic control technology
The electronic control unit (ECU) is equivalent to the ECU in a traditional car and is the main execution unit in an electric vehicle that controls high-voltage components. In addition to motor control, the ECU also controls related components such as the on-board charger and the DC-DC unit.
The core of the electronic control unit (ECU) is the control of the drive motor. The power unit's provider—the power battery—provides direct current (DC), while the drive motor requires three-phase alternating current (AC). Therefore, the ECU performs a process known in power electronics as inversion, converting the DC power from the power battery into AC power at the motor's input.
To achieve the inverter process, the electronic control unit (ECU) requires components such as a DC bus capacitor and IGBTs to work together. After the current is output from the power battery, it first passes through the DC bus capacitor to eliminate harmonic components. Then, through controlling the switching of the IGBTs and cooperating with other control units, the DC power is finally inverted into AC power, which ultimately serves as the input current for the motor. As mentioned earlier, by controlling the frequency of the three input currents of the motor and coordinating with the feedback values from the speed and temperature sensors on the motor, the ECU ultimately controls the motor.
Besides controlling the motor, the electronic control unit (ECU) is also the main control mechanism for components such as the on-board charger and the DC-DC converter. Charging is the opposite of motor control; it requires converting the alternating current (AC) supplied by the grid into direct current (DC) from the battery, a process known in power electronics as rectification.
The DC-DC unit is responsible for charging the 12V battery through the power battery. The electronic control unit needs to convert the high voltage at the power battery terminal into the low voltage at the 12V battery terminal to ultimately charge the new energy vehicle.
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