With China's new energy vehicle industry clearly defining its pure electric drive technology roadmap, the selection and design of drive motors has become particularly important. The following discussion will explore the principles for selecting drive motors for electric vehicles.
I. Characteristics of Motors Driving Automobiles
(1) The dynamic performance requirements for the torque control of the drive motor are high. The better the dynamic performance, the higher the level of motor technology.
(2) The motor is required to provide high torque and is generally able to withstand 4-5 times the overload.
(3) The drive motor is required to maintain high operating efficiency and also to have a wide speed range.
(4) The motor is required to not only have good driving performance, but also to take into account the function of the generator, and the power generation efficiency is required to be high.
(5) The drive motor must meet automotive grade requirements; general industrial grade motors cannot be directly used in automobiles.
(6) Electric vehicles have relatively small interior space, requiring the drive motor to be lightweight and small in size.
II. Key Indicators of Constrained Drive Motors
(1) Acceleration capability (s): The minimum time required for an electric vehicle to accelerate from a standstill to a certain speed;
(2) Maximum speed (km/h): The maximum speed that an electric vehicle can reach.
(3) Maximum driving range (km): The maximum driving range of an electric vehicle after the battery is fully charged.
III. Comparison of Commonly Used Drive Motors in Electric Vehicles
1) DC motor
① Outline drawing and internal structure of DC motor
②Advantages: The motor technology is relatively mature, with easy control and excellent speed regulation.
③ Disadvantages: Mechanical structure, such as brushes and mechanical commutators; insufficient instantaneous overload capacity; limited increase in motor speed; wear and tear on the motor's mechanical structure during long-term operation, increasing maintenance costs; sparks from the brushes during motor operation cause the rotor to heat up, wasting energy, making heat dissipation difficult, and also causing high-frequency electromagnetic interference.
④ Basic Trend: DC motors have the widest application in the field of speed-regulating motors, and most early electric vehicles used DC motors as drive motors. However, their disadvantages are quite obvious and have not been overcome for a long time. Current electric vehicles have basically phased out DC motors.
2) AC asynchronous motor
①Outer view and internal structure of AC motor
② Advantages: The stator and rotor are made of stacked silicon steel sheets, and both ends are sealed with aluminum caps. There are no mechanical parts that come into contact with each other between the stator and rotor, resulting in a simple structure, reliable and durable operation, and convenient maintenance. AC asynchronous motors have advantages such as high efficiency, high power density, and suitability for high-speed operation.
③ Disadvantages: The rotor of the motor generates significant heat during high-speed operation, requiring adequate cooling. Furthermore, the drive and control systems of asynchronous motors are complex, and the motor itself is relatively expensive. Compared to permanent magnet motors and switched reluctance motors, asynchronous motors have lower efficiency and power density, which is detrimental to increasing the maximum driving range of electric vehicles.
④ Basic Trends: AC asynchronous motors are currently one of the most widely used types of motors in industry and are already in mass production. Many electric vehicle manufacturers currently choose AC asynchronous motors as starter motors, with basic control employing vector control. AC asynchronous motors also show strong viability in the application of large electric buses.
3) Permanent magnet motor
① Outline drawing and internal structure of permanent magnet motor
②Classification: Permanent magnet motors can be divided into two types according to the different current waveforms of the stator windings: one is a brushless DC motor, which has a rectangular pulse wave current; the other is a permanent magnet synchronous motor, which has a sinusoidal wave current.
③Advantages: Relatively easy to cool, does not produce commutation sparks during operation, safe and reliable operation, convenient maintenance, and high energy utilization rate.
④ Disadvantages: Permanent magnet motors have a relatively small power range, typically with a maximum power of only tens of kilowatts. Permanent magnet materials are prone to magnetic degradation and damage. Furthermore, permanent magnet materials are expensive, resulting in a higher overall cost for the motor and its control system.
⑤ Basic trend: Permanent magnet motors are now the mainstream motors in electric vehicles.
4) Switched reluctance motor
① Outline drawing and internal structure of switched reluctance motor
② Advantages: Switched reluctance motors have the simplest structure. Both the stator and rotor are double salient pole structures made of ordinary silicon steel sheets. The rotor has no windings, and the stator is equipped with simple concentrated windings. It has many advantages such as simple and robust structure, high reliability, light weight, low cost, high efficiency, low temperature rise, and easy maintenance.
③ Disadvantages: As a new type of motor, the performance of switched reluctance motors still needs to be improved and enhanced.
④ Basic trend: Compared with other types of drive motors, it has the excellent controllability of DC speed regulation system, and is suitable for harsh environments, making it very suitable as a drive motor for electric vehicles.
⑤ Basic Conclusion: When used as drive motors for electric vehicles, DC motors and permanent magnet motors are structurally and poorly adapted to complex working environments, making them prone to mechanical and demagnetization failures. The application of switched reluctance motors in electric vehicles is an inevitable trend.
IV. Selection Principles of Drive Motors for Electric Vehicles
1) Use technologically mature products, but not outdated ones;
2) Products that use the latest technology but have stable performance;
3) Currently, electric logistics vehicles generally use permanent magnet synchronous motors;
4) Electric buses should also use permanent magnet synchronous motors in principle, but using AC asynchronous motors with vector control is also a good option.
V. Basic Conclusions
1) Early DC motors have been largely phased out, and even trolleybuses are no longer in use.
2) Actively explore the use of switched reluctance motors, but large-scale adoption is not advisable at present;
3) Currently, it is not advisable to use vector control AC asynchronous motors in large quantities for electric logistics vehicles;
4) In principle, permanent magnet synchronous motors are recommended for electric buses with a length of 8 meters. For electric buses with a length of 10 meters or more, should AC asynchronous motors with vector control or permanent magnet synchronous motors be used? It is necessary to add more reference factors and conduct further research.
