Wheel drive critical bottleneck
Although the advantages of the hub drive are very obvious, there is still a lot of wait-and-see attitude in the industry, because there are a series of technical bottlenecks in the hub motor. In general, the core problem is four:
Among them, the quality of the unsprung quality that the automaker pays most attention to. The so-called unsprung mass is the mass under the damper spring.
The increase in the unsprung mass increases the rms value of the dynamic load of the tire and the vibration acceleration of the vehicle body. This will cause the vertical vibration amplitude of the electric vehicle to increase, affecting the grounding characteristics of the tire, which is not conducive to the dynamic control and ride comfort of the vehicle. (Imagine if the grip on the left side of the tire suddenly decreases, just as the grip on the right side increases, the car will be dangerous.)
In the automotive industry, the ratio of the sprung mass to the unsprung mass is generally evaluated. That is, the larger the value of m2/m1, the better the ride smoothness is. It is obvious that the unsprung mass of the concentrated motor is much smaller than that of the hub motor.
In addition, OEMs are also concerned about the purchase cost of hub motors. Dispersing one motor into two or four compared to a concentrated drive motor will increase the cost anyway. Although the operating cost of the whole life will decrease due to the increase of operating efficiency, the substantial increase in the cost of one equipment is always a threshold.
In contrast, due to the difficulty of heat dissipation caused by compact structure, due to the problem of demagnetization of the magnetic steel caused by overheating and vibration, the motor manufacturer can improve the design level and internal digestion, and the problem is relatively simple.
Current solution
As long as there is demand, the problem is not a problem and will definitely be resolved step by step. For these four types of problems, the industry has developed a variety of solutions, the following describes several.
Unsprung quality solution
There are two solutions to the problem of unsprung quality from the motor angle. The logic is very simple, one is to reduce the weight of the motor, using lightweight materials and high power density technology. Another route is to transfer the unsprung mass to the sprung. I focus on the latter technical route.
The most straightforward way to achieve weight transfer from unsprung to sprung is to provide an additional elastic support that supports the weight of the stator and rotor. Rubber bushings, spring damping, and universal joint structures have emerged. The working mechanism is the same. As shown in the figure below, another (k3, c3) elastic damping system is set up outside the original (k2c2) elastic damping system to form a transfer bypass.
