Development Status and Application Introduction of Electric Vehicle Motors

This article reviews the current status of the development of electric vehicle motors, and provides a description and summary from different perspectives based on the typical motor design and application progress of more than 50 electric vehicle motors that have been mass-produced. From the perspective of motors, the main research and development status can be summarized as follows:

Motor Type Permanent Magnet

Permanent magnetization has become the industry standard, and the research and development and application of electric excitation and induction motors are gradually increasing. In addition, the research and development of some special motors is also ongoing. Such as switched reluctance motors, axial motors, rare earth-free permanent magnet motors, etc. Among the various pure electric and hybrid new energy vehicles on the market, permanent magnet synchronous motors account for the majority, and induction motors account for a small part. Compared with permanent magnet synchronous motors, AC induction motors are larger in size but moderately priced, but induction motors can be made very powerful and there is no demagnetization problem, so some large cars or electric vehicles that pursue performance, such as Tesla Model S and NIO ES8, all use induction motors. In general, permanent magnetization is a direction, but induction motors still have their own place in high-power and high-speed applications.

Other types of motors are mainly used in special vehicles;

Switched reluctance motors: Turntide Technologies has overcome many of these motor issues by placing sensors in the motor to track the rotor's motion. Engineers used machine learning algorithms to determine the right time to switch current on and off. Engineers designed a sensorless switched reluctance motor that minimizes vibration.

Axial flux motor: Products from Belgium-based Magnax reduce motor weight, size and cost. Their design aims to minimize the air gap between the rotor and stator teeth. The motor uses an SRS approach with two rotors, one on each side of the stator. In this configuration, the stator supports the electromagnetic gears but does not act as a support or yoke for the rotor. The yoke is a steel cylinder that accounts for two-thirds of the mass of the stator. Without the yoke, the motor is significantly lighter, and the company estimates that this approach can increase the range of electric vehicles by 7%.

Bidirectional Flux Motor: Linear Labs (Dallas-Fort Worth, Texas, USA) takes another approach, choosing to combine axial and radial flux designs in one motor. The company's "three-dimensional (3D) circumferential flux motor" consists of four rotors surrounding a stator. The central rotor rotates inside the stator, while the second rotor rotates outside the stator. The other two rotors are located on the left and right ends of the stator, for a total of four flux sources, each of which produces torque in the direction of motion.

Flat wire motor or round wire motor

The penetration rate of flat wire motors is increasing rapidly. In 2021, Tesla replaced its domestic flat wire motors, which led to a significant increase in penetration rate. The trend of flat wire motors has been determined. Many potential blockbuster models use flat wire motors, and the penetration rate is expected to increase rapidly to 95% in 2025. Many high-end models are equipped with flat wire motors. BYD's DMI models and e++ platform are all equipped with flat wire motors. Volkswagen MEB, NIO ET7, Zhiji L7, Zeekr 001 and other star models all use flat wire motors.

Flat wire motors can significantly improve conversion efficiency. Under WLTC conditions, flat wire motors have a conversion efficiency 1.12% higher than traditional round wire motors; under urban conditions (low speed and high torque), the efficiency difference between the two is 10%. According to a typical A-class car with a range of 500km (equipped with a 60kwh battery pack and a 150kw motor), under WLTC conditions, the battery cost of a flat wire motor is saved by 672 yuan, and under urban conditions, the battery cost is saved by 6,000 yuan.

The slot fill rate of flat wire is greater than that of round wire. When the slot fill rate is higher, the copper wire required for the same power motor is shorter, which reduces resistance and heat generation. Theoretically, the net slot fill rate of round wire is generally around 40%, while that of flat wire can be increased to 70%. Since the cross-section of round wire is circular, irregular gaps between the wires are inevitable, while the gaps between flat wires are smaller and the slot fill rate is higher. The high efficiency range of flat wire motors is much higher than that of round wire motors. The high efficiency range of round wire motors generally requires that the efficiency>85% interval accounts for no less than 85%, which is called "double 85". The efficiency>90% interval of flat wire motors accounts for no less than 90%, which is called "double 90". The efficiency of the motor is related to the speed and torque. The frequent start-stop conditions in urban conditions belong to low speed and high torque conditions, which is exactly the low efficiency range of round wire motors, and the conversion efficiency of flat wire motors is higher under this condition.

Flat wire motors have good heat dissipation performance: the temperature rise is 10% lower than that of round wire motors. Because flat wires are in closer contact than round wires, heat dissipation is improved. Studies have found that the thermal conductivity between windings at high slot fill rate is 150% of that at low slot fill rate. The windings are anisotropic in thermal conductivity, and the axial thermal conductivity is 100 times that of the radial direction. Under lower temperature rise conditions, the vehicle can achieve better acceleration performance.

Low electromagnetic noise: The whole vehicle is quieter. The stress of the flat wire motor conductor is relatively large, the rigidity is relatively large, and the armature has better rigidity, which has a suppressive effect on the armature noise; a relatively small slot size can be used to effectively reduce the cogging torque and further reduce the electromagnetic noise of the motor.

Small size brings high integration efficiency, which is in line with the development trend of all-in-one electric drive: due to the higher slot fill rate of flat wire, the copper wire usage and corresponding stator of the same power motor are less, and the volume is expected to drop by 30%. In addition, the flat wire motor has a more advanced winding method, which makes the motor end easier to cut. Compared with the round wire motor, the end size is reduced by 15-20%, and the space is further reduced, realizing the miniaturization and lightweight of the motor.

The large-scale application of flat wire motors also needs to overcome some shortcomings, such as low yield rate, low speed, difficulty in standardization and patent barriers. In high-end models, in order to meet the pursuit of high performance, the number of flat wire motors has also begun to increase from the original single motor to dual motors, such as Porsche's first pure electric sports car Taycan, and some models will even be equipped with three motors.

Insulation The flat wire for automotive-grade new energy vehicles has high requirements for heat resistance. It is mainly enameled with three high-temperature resistant insulating materials, polyesterimide wire enamel, polyamideimide wire enamel, and polyimide wire enamel, which are resistant to temperatures ≥180°C. Polyesterimide varnish has good electrical properties and mechanical strength, and is resistant to heat shock and softening breakdown. It is used as the main material for primer coating in the manufacture of composite coated enameled wires of grade 180 and above, and is widely used in household appliances with high adhesion and Freon resistance. Polyamideimide varnish has high heat resistance. Not only is the paint film hard and non-softening, but it also has high adhesion to the conductor. It was the first to be industrialized and can be used for a long time at 210°C. It is used for coil windings of high-temperature resistant motor electrical and electronic components and is used as an insulating coating for electromagnetic wires. Polyimide varnish has excellent heat resistance, and is also resistant to aging and high-voltage electrical breakdown. It is mainly used for insulating varnish-coated electromagnetic wires, or as a high-temperature resistant coating in the electrical industry, aerospace, oil pipelines, etc.

High pressure

Research direction: 800V is considered to be the inevitable path for the next generation of electric vehicles. In 2019, Porsche released the world's first 800V model Taycan. Hyundai E-GMP5, Mercedes-Benz EVA, GM's third-generation pure electric platform and Volkswagen Trinity have all chosen the 800V voltage platform. Geely SEA Haohan platform, GAC, Chery, SAIC and other automakers are all planning 800V solutions, and 800V has become the commanding height of a new round of competition among automakers.

Advantages: 800V can significantly reduce the diameter of high-voltage wire harnesses, reduce heat generation, reduce quality, and save wire harness costs. The voltage level is increased from 400V to 800V. According to the simplest P=UI, under the condition of outputting the same power, the current transmitted by the 800V system is smaller, the cable diameter and weight can be reduced, saving the cost of the wire harness and installation space.

Disadvantages: The probability of corona corrosion increases under the 800V platform, and corona corrosion will cause serious damage to the motor insulation. Corona discharge refers to the local self-sustaining discharge of gas medium in an uneven electric field, which is the most common form of gas discharge. It usually occurs near the tip electrode with a very small radius of curvature, such as at the winding notch, inside the winding insulation layer, etc. Corona is air gap discharge, and part of the energy is converted into light, heat, sound, electromagnetic, etc., which will cause