What characteristics must the motor in an electric vehicle meet?
Due to the characteristics of electric vehicles, the motors used have high requirements. To increase the top speed, the motor should have high instantaneous power and power density (W/kg); to increase the driving range on a single charge, the motor should have high efficiency; and since electric vehicles operate with variable speeds, the motor should have high overall efficiency at both high and low speeds; in addition, it should have strong overload capacity, large starting torque, and fast torque response. Electric vehicles start at low speeds and climb hills, but require high torque; during normal operation, less torque is needed, while the speed is very high. The motor exhibits constant torque characteristics at low speeds and constant power characteristics at high speeds, and its operating speed range should be wide. Furthermore, the motor should be robust, reliable, have some dust and water resistance, and its cost should not be too high.
Currently, among mature motor technologies, switched reluctance motors seem to better meet the needs of electric vehicles in terms of various technical characteristics, but they have not yet become widespread. Permanent magnet synchronous motors are more widely used, such as in the Kia K5 Hybrid, Roewe E50, Denza, and BAIC EU260. Tesla's Model X and Model S both use asynchronous motors. Furthermore, motors can be classified into DC motors and AC motors based on current type. The table below provides a general overview of the characteristics of four typical electric motor types.
In the early stages of electric vehicle development, most electric vehicles used DC motors as their drive motors. This type of motor technology was relatively mature, easy to control, and offered excellent speed regulation, making it the most widely used type of speed-regulating motor. However, the complex mechanical structure of DC motors limited their instantaneous overload capacity and the potential for further increases in motor speed. Furthermore, prolonged operation caused wear and tear on the motor's mechanical structure, increasing maintenance costs. In addition, the sparks from the brushes during motor operation heated the rotor, generating high-frequency electromagnetic interference that affected the performance of other electrical components in the vehicle. Due to these drawbacks, DC motors have been largely phased out in modern electric vehicles.
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, further enhancing the motor's performance. "Synchronous" means that the rotor's speed and the current frequency of the stator windings remain consistent. Therefore, by controlling the input current frequency of the motor's stator windings, the vehicle's speed can ultimately be controlled. Compared to other types of motors, the biggest advantage of permanent magnet synchronous motors is their high power density and torque density. Simply put, compared to other types of motors, permanent magnet synchronous motors can provide the greatest power output and acceleration for new energy vehicles with the same mass and volume. This is the main reason why permanent magnet synchronous motors are the preferred choice for many automakers in the new energy vehicle industry, where space and weight are extremely critical. However, permanent magnet synchronous motors also have their own disadvantages. Under conditions of high temperature, vibration, and overcurrent, the permanent magnet material on the rotor will experience magnetic degradation, making the motor prone to damage under relatively complex operating conditions. Furthermore, permanent magnet materials are expensive, resulting in a higher overall cost for the motor and its control system.
Compared to permanent magnet synchronous motors (PMSMs), asynchronous motors offer advantages such as lower cost, simpler manufacturing process, reliable and durable operation, easier maintenance, and the ability to withstand significant temperature variations. Conversely, large temperature fluctuations can damage PMSMs. While asynchronous motors are less advantageous in terms of weight and size, their wide speed range and peak speed of around 20,000 rpm allow them to meet the high-speed cruising requirements of vehicles in this class, even without a secondary differential. Regarding the impact of weight on driving range, the high-energy-density 18650 battery can compensate for the motor's weight disadvantage. Furthermore, the excellent stability of asynchronous motors is also a key reason for Tesla's choice.
As a new type of motor, the switched reluctance motor has the simplest structure compared to other types of drive motors. Both the stator and rotor are double-salient pole structures made of ordinary silicon steel sheets stacked together. The rotor has no windings, while the stator has simple concentrated windings. It boasts numerous advantages, including simple and robust structure, high reliability, light weight, low cost, high efficiency, low temperature rise, and ease of maintenance. Furthermore, it possesses the excellent controllability of DC speed control systems and is suitable for harsh environments, making it ideal for use as a drive motor in electric vehicles. Experts once predicted it to be a dark horse in the electric vehicle industry.
