In the context of environmental protection, electric vehicles have become a research hotspot in recent years. Electric vehicles can achieve zero or extremely low emissions in urban transportation, offering significant advantages in the environmental field, and countries around the world are striving to develop them. An electric vehicle mainly consists of three parts: a motor drive system, a battery system, and a vehicle control system. The motor drive system, which directly converts electrical energy into mechanical energy, determines the performance indicators of the electric vehicle. Therefore, the selection of the drive motor is particularly important.
1. Requirements of electric vehicles for drive motors
Currently, the evaluation of electric vehicle performance mainly considers the following three performance indicators:
(1) Maximum driving range (km): The maximum driving range of an electric vehicle after its battery is fully charged;
(2) Acceleration capability (s): The minimum time required for an electric vehicle to accelerate from a standstill to a certain speed;
(3) Maximum speed (km/h): The maximum speed that an electric vehicle can reach.
Motors designed specifically for the driving characteristics of electric vehicles have special performance requirements compared to industrial motors:
(1) Electric vehicle drive motors typically require high dynamic performance, including frequent start/stop, acceleration/deceleration, and torque control.
(2) In order to reduce the weight of the whole vehicle, the multi-stage transmission is usually eliminated. This requires the motor to provide higher torque at low speeds or when climbing hills, and it is usually able to withstand 4-5 times the overload.
(3) The speed regulation range should be as large as possible, and at the same time, the operating efficiency should be maintained within the entire speed regulation range.
(4) When designing the motor, try to design it with a high rated speed and use an aluminum alloy shell as much as possible. The high-speed motor is small in size, which helps to reduce the weight of the electric vehicle.
(5) Electric vehicles should have optimized energy utilization and regenerative braking energy recovery function. The energy recovered by regenerative braking should generally reach 10%-20% of the total energy.
(6) The working environment of electric motors used in electric vehicles is more complex and harsh, requiring motors to have good reliability and environmental adaptability, while ensuring that the cost of motor production is not too high.
2. Several commonly used drive motors
2.1 DC Motor
In the early stages of electric vehicle development, most electric vehicles used DC motors as their drive motors. These motors were technologically mature, easy to control, and offered excellent speed regulation, making them the most widely used type of speed-regulating motor. However, the complex mechanical structure of DC motors, such as brushes and mechanical commutators, limited their instantaneous overload capacity and the ability to further increase 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 operation heated the rotor, wasting energy, hindering heat dissipation, and causing high-frequency electromagnetic interference, affecting overall vehicle performance. Due to these drawbacks, DC motors have been largely phased out in current electric vehicles.
2.2 AC asynchronous motor
AC asynchronous motors are a widely used type of motor in industry. Their characteristics include a stator and rotor made of stacked silicon steel sheets, encapsulated at both ends with aluminum caps. There are no mechanically contacting parts between the stator and rotor, resulting in a simple structure, reliable and durable operation, and convenient maintenance. Compared to DC motors of the same power, AC asynchronous motors are more efficient and about half the weight. If vector control is used, controllability comparable to DC motors and a wider speed range can be achieved. Due to their high efficiency, high power density, and suitability for high-speed operation, AC asynchronous motors are currently the most widely used motors in high-power electric vehicles. Currently, AC asynchronous motors are mass-produced, with various mature products available. However, the rotor of the motor heats up significantly at high speeds, requiring adequate cooling. Furthermore, the drive and control systems of asynchronous motors are complex, and the cost of the motor itself is relatively high. 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.
2.3 Permanent Magnet Motor
Permanent magnet motors can be divided into two types based on the current waveform of their stator windings: brushless DC motors, which have rectangular pulse current, and permanent magnet synchronous motors, which have sinusoidal current. These two types of motors are largely similar in structure and working principle. Both use permanent magnets as rotors, reducing excitation losses. The stator has windings that generate torque via alternating current, making cooling relatively easy. Because these motors do not require brushes or mechanical commutation structures, they do not produce commutation sparks during operation, making them safe and reliable, easy to maintain, and with high energy efficiency.
The control system of a permanent magnet motor is simpler than that of an AC asynchronous motor. However, due to limitations in the manufacturing process of permanent magnet materials, the power range of permanent magnet motors is relatively small, typically with a maximum power of only a few tens of kilowatts, which is the biggest drawback of permanent magnet motors. Furthermore, the permanent magnet material on the rotor will experience magnetic degradation under conditions of high temperature, vibration, and overcurrent, making permanent magnet motors prone to damage under relatively complex operating conditions. Moreover, permanent magnet materials are expensive, resulting in a higher overall cost for the motor and its control system.
2.4 Switched Reluctance Motor
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.
Considering that DC motors and permanent magnet motors are poorly designed and adaptable to complex working environments when used as drive motors for electric vehicles, and are prone to mechanical and demagnetization failures, this article focuses on the following significant advantages of switched reluctance motors compared to AC asynchronous motors.
2.4.1 Motor body structure
Switched reluctance motors have a simpler structure than squirrel-cage induction motors. Their key advantage is that the rotor has no windings; it is simply composed of stacked ordinary silicon steel sheets. Most of the motor's losses are concentrated in the stator windings, which simplifies manufacturing, improves insulation, facilitates cooling, and provides excellent heat dissipation. This structure reduces the motor's size and weight, allowing for high output power in a small footprint. Due to the rotor's good mechanical elasticity, switched reluctance motors can be used for ultra-high-speed operation.
2.4.2 Motor drive circuit aspect
The phase current of a switched reluctance motor drive system is unidirectional and independent of the torque direction, allowing for four-quadrant operation of the motor to be achieved using only a single main switching device. The power converter circuit is directly connected in series with the motor's field winding, and each phase circuit is independently powered. Even if a phase winding or controller fails, only that phase needs to be stopped, without causing further disruption. Therefore, both the motor itself and the power converter are highly safe and reliable, making it more suitable for harsh environments than asynchronous motors.
2.4.3 Motor system performance
Switched reluctance motors have numerous control parameters, making it easy to meet the four-quadrant operation requirements of electric vehicles through appropriate control strategies and system design, while maintaining excellent braking performance even in high-speed operating ranges. Switched reluctance motors are not only highly efficient but also maintain high efficiency over a wide speed range, a feat unmatched by other types of motor drive systems. This performance is ideally suited for the operation of electric vehicles, significantly contributing to improving their driving range.
3. Conclusion
This paper focuses on comparing various commonly used drive motor speed control systems to highlight the advantages of switched reluctance motors as drive motors for electric vehicles, highlighting their current research focus in electric vehicle development. There is still significant room for development in the practical application of this type of special motor, requiring researchers to conduct more theoretical research while simultaneously addressing market demands to promote its practical application.
