Technical interpretation of the principle of electric vehicle hub motor has great potential but is difficult to popularize

Posted 2020-09-16 00:00:00 +0000 UTC

In the new energy era, electric vehicles have become the darling of the market, and the driving form of electric vehicles is not single. Among them, there is a promising driving motor, hub motor, which is difficult to popularize at present. In fact, the hub motor is very common. The electric bicycle is the driving form. The wheel has an integrated motor, which sounds like a perfect driving scheme. The working principle of hub motor is permanent magnet synchronous motor, while wheel motor and hub motor refer to the motors installed in different positions of vehicles. Hub motor is the motor designed by integrating the "power system, transmission system and brake system" of the car. The idea of placing the motor in the hub became a reality more than 100 years ago. But now the application of this technology can hardly be seen in the cars running on the road. Simply put, because the speed of the car is too much faster than that of the electric bicycle, a higher power motor is needed to drive it. In the relatively small space in the hub, the heat dissipation of the motor is also a big problem. Poor heat dissipation will seriously affect the service life of the hub motor, mainly because the demagnetization phenomenon is too serious, there is no good solution at this stage. From the beginning, the technical principle of hub motor is that the motor assembly with integrated reducer is directly arranged in the hub, and four wheel motors directly drive four wheels. Compared with the traditional electric vehicle, the hub motor technology omits differential, half axle and even two-stage speed change device, which makes it can be flexibly placed in the wheels of all kinds of electric vehicles to directly drive the hub rotation. Compared with the traditional centralized driving mode such as internal combustion engine and single motor, it has obvious technical advantages and characteristics in power configuration, transmission structure, control performance, energy utilization, etc. Because the wheel hub motor has the characteristics of single wheel independent drive, it can be easily realized in the form of front drive, rear drive or four-wheel drive. It is very easy to realize full-time four-wheel drive on the vehicle driven by wheel hub motor. At the same time, the wheel hub motor can realize the differential steering of similar tracked vehicles through the different rotation speeds of left and right wheels or even reverse, greatly reducing the turning radius of vehicles. Under special circumstances, it can almost realize the in-situ steering (but at this time, the steering mechanism and wear of vehicles are large), which is very valuable for special vehicles. For the traditional vehicle, the clutch, transmission, transmission shaft, differential and even transfer case are indispensable. These parts not only make the structure of the vehicle more complex, but also need regular maintenance and failure rate. But the hub motor solves this problem well. In addition to the simpler structure, the vehicle driven by hub motor can obtain better space utilization, and the transmission efficiency is also much higher. Many new energy vehicles are driven by electricity, so the wheel motor drive is also in great use. Whether it is electric vehicle or extended range electric vehicle, hub motor can be used as the main driving force. Even for hybrid vehicles, wheel hub motors can be used as power for starting or accelerating, which is really a multi-purpose engine. At the same time, many technologies of new energy vehicles, such as brake energy recovery (i.e. regenerative braking), can also be easily realized on the wheel motor driven vehicles. Since hub motor has so many advantages, why can't it be popularized? What's more, it has been invented for more than 100 years, but has not been able to make a breakthrough in technology? Objectively speaking, the hub motor increases the unsprung mass and the moment of inertia of the hub, which has an impact on the vehicle control. For ordinary civil vehicles, some relatively light materials such as aluminum alloy are often used to make suspension parts, so as to reduce the mass under the spring and improve the response speed of the suspension. However, the hub motor just increased the unsprung mass by a large margin, which was contrary to our wishes, but also increased the moment of inertia of the hub, which was not good for the handling performance of the vehicle. In addition, the electric braking performance is limited, which requires a lot of electric energy to maintain the operation of the braking system. The main reasons why the hub motor can't be used in passenger cars are that the spring mass can't be reduced, the heat dissipation of the motor is difficult, the torque and reliability are insufficient due to the limited space in the wheel. Hub motor technology is to integrate motor, reduction gear, electric control system and cooling system in the narrow space of hub, which is just like adding frost to the ordinary passenger car with limited wheel space. The limited application of hub motor in the field of automobile is also in the field of electric bus, because the bus is huge, the layout of hub motor is relatively easy. Therefore, as far as the manufacturing and packaging level of hub motor is concerned, it may not bring great cost advantages to traditional electric vehicles in the field of general passenger vehicles.

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