Comparative Analysis of Key Technologies of Battery, Electric Drive and Electronic Control for Chinese and Foreign Electric Vehicles

1. Comparison of Chinese and foreign technologies in vehicle drive motors

The motor drive system is the main actuator in the driving of new energy vehicles, and the driving characteristics determine the main performance indicators of the vehicle. Benefiting from the huge domestic market capacity, the domestic drive motor industry has developed rapidly, and the drive motors matched by domestic electric vehicles are basically produced locally, which can basically meet the requirements of domestic new energy vehicles.

In terms of drive motor technology development, if we use patents as technical reserves, then in the past ten years, the total number of patent applications for China's vehicle drive motors is the world's first. However, the proportion of invention patents that most reflect the technical level is only 50%, while foreign invention patents account for more than 90% of the total number of applications. From this perspective, my country's patent applications pay more attention to "quantity", and abroad more heavy "quality".

In terms of the power density of the drive motor, the "13th Five-Year" national plan clearly states that the motor efficiency exceeds 4KW/KG, and the products of many domestic enterprises have been able to reach this target, but compared with the international advanced products, there are many differences in volume, manufacturing process, etc.

At present, the motors used in my country's new energy vehicles are mostly permanent magnet synchronous motors, AC asynchronous motors and other motors. From the perspective of motor type market structure, permanent magnet synchronous motors are mainly used. In 2017, the installed capacity of permanent magnet synchronous motors in the field of new energy vehicles reached 685100 units, accounting for more than 78%; the installed capacity of AC asynchronous motors was nearly 190000 units, accounting for 21.4%.

When comparing the power density with foreign motors, one thing to note is that Tesla model s uses induction motors. It is inappropriate to directly compare the power density of induction motors and synchronous motors. There are at least two points: First, AC motors do not have permanent magnets and use less materials, which reduces weight to a certain extent; On the other hand, AC motors can be self-excited and can establish higher magnetic field strength than synchronous motors. In addition, Tesla's patented copper core rotor technology also solves a big problem. These three points make Tesla's induction motor power density comparable to domestic permanent magnet synchronous motors.

Although the power density can represent the performance of the motor to a certain extent, it is not suitable for some occasions. For example, in some occasions requiring low speed, it is impossible to increase the power density by high speed, and the speed is too high for the motor. It will bring problems such as wind wear loss, bearing problems, NVH problems, etc. At this time, another indicator is used to characterize the performance of the motor: torque density. Because in many cases, the positive correlation between weight and volume is not necessarily power, but torque.

There are many ways to increase the torque density, such as Toyota prius, BMW, etc., which are obtained by increasing the reluctance torque ratio. Another method is to start from the control side and use the harmonic torque generated by the motor through harmonic injection. The final path is to increase the magnetic field energy per unit volume.

The reason why torque density has become a development direction is also driven by another factor, that is, the development of more and more prosperous independent drive technology, also known as distributed drive.

In this application, the motor is directly installed on the side of the wheel. It is called the wheel motor with a reducer, and it is called the hub motor without a reducer. The hub motor directly drives the wheel and rotates synchronously with the wheel. The speed of the wheel is actually very low, usually only a few hundred revolutions. In this case, the only way to pursue power density is torque density.

However, the torque density is also divided into applications. Passenger cars are mostly high-speed motors, so it is more difficult to make high torque density, while commercial vehicles that require low speed and high torque are relatively easy. At present, the domestic drive motor needs to focus on strengthening the design of rotor shape and reluctance torque utilization rate in the magnetic circuit design. How to greatly improve the heat conduction area without affecting the magnetic circuit performance is the focus of the design.

In the choice of induction motor and synchronous motor, Tesla's induction motor is not inferior to domestic synchronous motor, but why model 3 uses synchronous motor? This may be related to several factors: first, Tesla has entered China, and the use of synchronous motors has the meaning of pleasing China's rubidium-iron-boron material industry; second, it has signed agreements with relevant Chinese companies, and the supply chain is worry-free; third, induction motors usually have only one way to increase current to increase power density, but the side effects are quite obvious, while permanent magnet motors still have room for improvement and performance improvement.

In terms of core components, although China has the richest rare earth resources in the world, the cutting-edge technology for refining and processing rubidium, iron and boron is in the hands of Japan and Germany. In terms of silicon steel sheets, China can achieve 0.35mm, but Japan has reached 0.27mm. High-performance silicon steel sheets still rely on imports. The gap in high-speed bearings is even greater. Almost all high-speed motor bearings rely on imports and basically do not use domestic bearings.

2. Technical comparison of electric vehicle power battery at home and abroad

At present, the development of foreign power batteries from 18600 to 21700 is the general trend. According to the data released by Tesla, the energy density of 21700 batteries is 300wh/kg, which is 20% higher than the 18600 250wh/kg. However, in terms of cost, the 21700 battery is 170 USD/kwh, while the 18600 battery is 185 USD/kwh. It can be seen that compared with the 18600 battery, the energy density of the 21700 battery is increased by 20%, costs fell by 8%.

What does 21700 mean? 21 is the outer diameter of the battery and 70 is the height of the battery.

In theory, the capacity will increase simultaneously, but the side effects are also obvious: every 10% increase in energy density, the cycle life is reduced by 20%, the charge and discharge rate is reduced by 30-40%, and the battery has about 20% Temperature rise, so the 21700 is the result of comprehensive performance and economic balance.

In addition, from the manufacturing point of view, from the 18600 to 21700 production line is basically universal, will not invest too high equipment and technical reform costs.

Judging from the capacity of battery cells, the 21700 is greatly increased by 35% compared with the 18600, and the number of single batteries is reduced by 1/3, and the metal structural parts and conductive connectors required for battery PACK will also be simultaneously reduced, which is expected to reduce the weight by 10% and the cost by 24%.

The 21700 battery produced by Panasonic will be used on Model 3. Domestic passenger cars have not yet used 21700 models, but many domestic battery manufacturers have already arranged the production of 21700 batteries, and it is not too complicated to switch 18600.