Basic knowledge of wheel hub motor

The in-wheel motor drive used by electric vehicles belongs to the decentralized motor drive mode. There are usually two ways of decentralized motor drive: in-wheel motor and wheel-side motor. The so-called wheel-side motor drive mode means that each driving wheel is driven by a separate motor, but the motor is not integrated in the wheel, but is connected to the wheel through a transmission device (such as a rotating shaft). The drive motor of the wheel motor drive mode belongs to the range of sprung mass, and the suspension system has good vibration isolation performance. However, the motor installed on the body has a great influence on the overall layout of the vehicle, especially in the case of rear axle drive . Moreover, due to the sleeve-shaped movement between the body and the wheels, it also has certain restrictions on the universal transmission of the drive shaft. Therefore, the current development direction of the decentralized motor drive system is the hub motor.

(1) Structural form
According to the rotor form of the motor, the power system of the wheel teaching motor is divided into two structures: inner rotor type and outer rotor type, as shown in Figure 1. Usually, the outer rotor type uses a low-speed outer rotor motor, the motor speed is 1000~1500r/min, without any reduction device, the outer rotor of the motor is fixed or integrated with the wheel of the wheel, and the speed of the wheel is the same as the motor. The inner rotor type uses a high-speed inner rotor motor and is equipped with a reducer with a fixed transmission ratio. In order to obtain a higher power density, the speed of the motor is usually as high as 10000r/min. The reduction mechanism usually adopts a planetary gear reduction device with a transmission ratio of about 10:1.

Basic knowledge of wheel hub motor
Figure 1 Schematic diagram of the structure of the wheel valley motor power system

The advantages of the high-speed inner rotor hub motor are high specific power, light weight, small size, high efficiency, low noise and low cost; the disadvantage is that a reduction gear must be used, the efficiency is reduced, the unsprung mass is increased, and the maximum speed of the motor is affected by the coil It is limited by factors such as loss, friction loss and the bearing capacity of the transmission mechanism. The advantages of low-speed outer rotor hub motors are simple structure, small axial size, high specific power, torque control in a wide speed range, fast response speed, outer rotor directly connected to the wheel, no deceleration mechanism, and high efficiency; The disadvantage is that in order to obtain a larger torque, the volume and mass of the motor must be increased, so the cost is high, the efficiency is low during acceleration, and the noise is large. Both of these structures are used in current electric vehicles, but with the emergence of compact planetary gear transmission mechanisms, high-speed inner rotor drive systems are more competitive than low-speed outer rotor drive systems in terms of power density.
Due to the small electric braking capacity of the in-wheel motor power system, it cannot meet the requirements of the braking efficiency of the whole vehicle, and an additional mechanical braking system is usually required. The brakes in the in-wheel motor system can adopt drum or disc brakes according to the structure. The existence of the electric brake capacity of the motor can often reduce the design capacity of the brake appropriately. Most of the in-wheel motor systems adopt air-cooling methods for cooling, and some use water-cooling and oil-cooling methods to dissipate heat and reduce the temperature of heat-generating components such as motors and brakes, but the structure is more complicated.

(2) Advantages and disadvantages of the in-wheel motor drive mode
① Advantages.
a. The wheel hub motor drive mode can completely omit the transmission device, and the overall power utilization efficiency is greatly improved.
b. The in-wheel motor enables the overall layout of the vehicle to adopt a flat chassis structure, and the interior space and the degree of freedom of layout are greatly improved.
c. There are almost no high-power moving parts on the vehicle body, which reduces the vibration and noise of the vehicle and greatly improves the comfort.
d. This form is convenient to realize four-wheel drive, which is beneficial to improve the dynamic performance of the whole vehicle.
e. As an executive component, the in-wheel motor uses the advantages of fast response speed and accuracy to facilitate the realization of integrated vehicle dynamics control including drive-by-wire, drive-by-wire and vehicle-by-wire dynamics control to improve the active safety of the vehicle sex.
② Disadvantages.
a. The hub motor greatly increases the unsprung mass and at the same time increases the moment of inertia of the hub, which is detrimental to the handling performance of the vehicle.
b. The electric braking performance is limited, and a lot of electric energy is needed to maintain the operation of the braking system:
c. Since the electric braking capacity of the in-wheel motor system is small, it cannot meet the requirements of the braking performance of the whole vehicle, and an additional mechanical braking system is required; however, due to limited space, it is very difficult to design a mechanical braking system.
d. Without a vacuum pump driven by a traditional internal combustion engine, an electric vacuum pump is needed to provide braking assistance, but it also means greater energy consumption.
e. The working environment of the in-wheel motor is harsh, facing the influence of water, dust and other aspects, and it also has high requirements in terms of sealing. At the same time, the design of the in-wheel motor also needs to consider the heat dissipation problem separately.
f. The fluctuation of the running torque of Lunyi motor may cause vibration and noise of automobile tires, suspension and steering system, as well as other vehicle sound and vibration problems.

(3) Application of in-wheel motor drive mode
Typical electric vehicles driven by in-wheel motors are Japan’s Tokyo Electric Power IZA and Kokusai Electric’s PIVOT. The future direction of automobile development is informatization, intelligence and low-carbon. Electric vehicles with four-wheel independent drive (in-wheel motor drive) will be the best choice to achieve this goal.
The structure of the integrated in-wheel motor designed and manufactured by the French TM4 company is shown in Figure 2. It uses an external rotor type permanent magnet motor. The motor rotor shell is directly solidified with the wheel. It is an integral part of the wheel rim, and the motor rotor is integrated with the brake hub of the drum brake to realize the motor rotor, wheel and brake. The integration of a rotating moving object greatly reduces the quality of the integrated wheel hub motor system, and the degree of integration is quite high. The permanent magnet brushless DC motor of the integrated hub motor system has a rated power of 18.5kW, a peak power of 80kW, a peak torque of 670N·m, a rated speed of 50rmm, and a product height of 1385/min. The average efficiency under working conditions can reach 96.3%.
The EV96-1 electric vehicle developed by the Aiyingsi Electric Vehicle Research Institute of Harbin Institute of Technology in China also adopts the external rotor type wheel motor drive system. The dual characteristics of electric motor and asynchronous motor, its rated power is 6.8kw, the peak power is 15kw, integrated disc brake, air-cooled heat dissipation

Basic knowledge of wheel hub motor
Figure 2 TM4 integrated wheel valley motor system structure