As one of the three core components of new energy vehicles, the development of in-wheel motors is something that new energy vehicle manufacturers are constantly monitoring. The 2020 target of 300kW/GB for power batteries is clear, and China's new energy vehicle development has created a number of world-class power battery companies. However, China does not yet have world-class drive motor companies. What's perplexing for new energy vehicle manufacturers is that while the pure electric vehicle route is clear, the drive motor is presented as a three-part system: "direct drive motor, wheel-side motor, and in-wheel motor." There's a lack of coordination between these components, unclear viewpoints among professionals, and even more confusion for users.
I. Development History of Vehicle Drive Motors
Trolleybuses have never been classified as automobiles, primarily because they lack onboard power. However, for vehicle management purposes, they are included in the public transport vehicle category. While trolleybuses are not automobiles, they are certainly road transport motor vehicles. Currently, pure electric vehicles and trolleybuses are fundamentally the same in principle; the main difference is that trolleybuses do not have onboard power batteries, while pure electric vehicles do.
The drive motors of trolleybuses are the same as those of pure electric vehicles. With the progress of technology, drive motors have also advanced. Trolleybuses have long used DC motors, while China's pure electric vehicles initially used asynchronous AC motors, and later permanent magnet synchronous motors.
Today, vehicle drive motors are basically all permanent magnet synchronous motors, and trolleybuses also use permanent magnet synchronous motors.
The direct drive motors, wheel-side motors, and hub motors discussed here are all permanent magnet synchronous motors. The terms "direct drive motors, wheel-side motors, and hub motors" refer to motors installed in different positions on the vehicle.
II. Similarities and Development Trends of Direct Drive Motors, Wheel-Side Motors, and Wheel Hub Motors
This discussion focuses on automotive drive motors. "Drive" refers to the motor that propels the tires, including direct-drive motors, wheel-side motors, and hub motors. The object being driven is always the wheel. Since this is a permanent magnet synchronous motor, its working principle is the same.
Its basic requirements:
Note: Different motors have different geometric dimensions, and naturally, their performance parameters will also differ.
Its development trend
(1) High power density: high speed, high torque density
(2) Wide speed range - adjust permanent magnet or back EMF
(3) Low cost - new materials and optimized design
III. Differences between direct drive motors, wheel-side motors, and hub motors
① Axles equipped with direct drive motors
A direct-drive motor is analogous to an engine. While an engine drives the tires, it requires intermediate components (gearbox, drive shaft, etc.). A direct-drive motor, however, connects directly to the traditional axle. The image below shows Dongfeng Dana axle products.
Figure 1. Dongfeng Dana axle with direct drive motor.
Its main feature is that the axle integrates the motor, gearbox, drive shaft, and differential, all of which are directly integrated and installed in the middle of the axle.
② Axles equipped with wheel-side motors
The ZF wheel-side electric drive axle is a typical example: two drive motors are arranged on both sides of the axle, and the wheels are driven by deceleration and torque increase through side reducers and wheel-side reducers. See Figure 2.
Figure 2 ZF wheel-side electric drive axle
③ Axles equipped with hub motors
The exploded view of the hub motor assembly is shown in Figure 3.
Figure 3 Hub Motor Assembly
The axle equipped with the hub motor is shown in Figure 4.
As explained above, the drive motor must be combined with the axle to function. The Dongfeng Dana axle with a direct-drive motor is developed based on the principle of traditional axles, retaining their advantages. However, the advantages of traditional axles are no longer visible in the ZF wheel-side electric drive axle and the Taitek Electromechanical wheel hub motor axle.
Direct drive motors, wheel-side motors, and hub motors are a trilogy, but they don't seem to have a progressive relationship. Theoretically, one might say that hub motors are the ultimate goal of pure electric vehicles, but from an engineering perspective, it doesn't seem to be the case yet.
IV. How to play the "three-part" approach of direct drive motor, wheel-side motor, and wheel hub motor?
① Currently, the market advantage lies in the combination of direct-drive motors and traditional axles. Traditional axles are engine-driven and meet the requirements of vehicle motion, namely, the requirements of varying vehicle speed, vehicle stability, and frequent vehicle bumps. Dongfeng Dana axles with direct-drive motors can answer these questions.
② Wheel-side motors equipped with axles. BYD and Yangtze Motors have experimented with promoting the application of wheel-side motors. However, from the perspective of automotive motion principles, equipping wheel-side motors with axles cannot answer how they can leverage the advantages of traditional axles.
③ How does a hub motor reduce speed and transmit power? It has a very large direct drive torque. Hub motors are subjected to significant impacts from the road surface; how can their reliability and durability be guaranteed? How does a hub motor overcome vehicle bumps? While the traditional axle is eliminated, its function cannot be omitted.
Hubei Tait Electromechanical Co., Ltd. states that its technology originates from the Dutch company (e-Traction). Founded in 1981, e-Traction is a pioneer in the development of hub motors and holds over 200 invention patents. Its technology has been tested on buses in 11 cities across 8 European countries, with the longest operating time exceeding 9 years.
Think about it carefully. The trial period has exceeded nine years, and it's still in trial. Since it's still a trial, with three buses in each of the 11 cities, the total number of buses wouldn't exceed 33. The idea that it will be mass-produced to 5,000 buses in China seems illogical. Meanwhile, the most conservative estimate for pure electric buses with a drive motor and traditional axles is over 200,000. Even the Dongfeng Dana axle with a direct-drive motor hasn't been widely adopted.
Core technologies are accumulated over time. How do we develop the three-part system of direct drive motors, wheel-side motors, and hub motors? We need to practice steadily and not rush to say things like who will replace whom or who will disrupt whom.
