Introduction
As economies and societies develop, living standards improve and environmental expectations rise. Vehicle exhaust emissions are a major source of environmental pollution, so energy and environmental issues have long constrained automotive development. Automakers and governments worldwide are actively promoting and researching new energy vehicles, which include battery electric vehicles, fuel cell vehicles, and plug-in hybrid vehicles. In the new energy vehicle sector, in-wheel motors are core components and play a decisive role. This article provides a brief introduction to in-wheel motor drive technology.
1. Overview of in-wheel motor drive technology
In regions such as Europe, the United States, and Japan, mature automotive supply chains for new energy vehicles have formed. The EU planned to produce over 5 million new energy vehicles by 2020 and allocated €1.43 billion for research and development. Japan planned to increase the share of new energy vehicles to 50% by 2020. China’s Ministry of Industry and Information Technology indicated in its "Energy-saving and New Energy Vehicle Industry Development Plan" that by 2020 the market share of new pure electric vehicles and PHEVs in China would reach 5 million units. Vehicle electrification is a major trend, and motors, as the primary drive source, play an important role in this development.
In-wheel motors are commonly used in the automotive industry. An in-wheel motor is installed within the relatively confined space of a wheel hub, making the motor system more sensitive to factors such as magnetic saturation, road excitation, and load, which allows precise control of vehicle performance. Compared with traditional centralized drive vehicles, vehicles using in-wheel motors have advantages including simplified chassis structure, flexible drive arrangement, improved driving stability, and higher transmission efficiency. Integrating the motor into the wheel hub simplifies the chassis, saves interior space, and increases layout flexibility and mass distribution options. Key technical requirements for in-wheel motors include high torque density; wide overload capacity to support fast starts and acceleration while maintaining high rotational speed within that range; and reliable operation with high precision in adverse weather conditions.
2. In-wheel motor drive types
2.1 In-rotor in-wheel motor
In-wheel motors can be classified as in-rotor or out-rotor types. The maximum speed for in-rotor in-wheel motors is about 1500 rpm. In-rotor designs generally have lower motor requirements because they incorporate a reduction mechanism, but this increases structural complexity and raises unsprung mass. In academic research, Japanese institutions have conducted extensive work on in-rotor in-wheel motors. For example, Keio University collaborated with several automakers to develop the ECO.
2.2 Out-rotor in-wheel motor
Out-rotor in-wheel motors differ from in-rotor types in that the rotor is the wheel hub itself, so they can drive the wheel directly without a reduction device. The maximum speed for out-rotor in-wheel motors is about 2000 rpm. Compared with in-rotor designs, out-rotor motors typically have a more compact structure, a smaller axial dimension, and higher transmission efficiency.
3. Application of in-wheel motor drive systems in electric vehicles
3.1 Drive schemes for electric vehicles
Electric vehicle drive schemes mainly include traditional centralized motor drive and in-wheel motor drive. The centralized motor drive approach is based on internal combustion engine vehicle architecture, replacing the engine and gearbox with a centralized electric drive system. This method swaps components without changing the vehicle structure or interior layout and is the fastest, simplest way to convert an internal combustion vehicle to electric. However, it is most suitable for low-volume production and cost-controlled updates. In-wheel motor drive systems place the motor inside the wheel hub, greatly simplifying vehicle layout and many traditional mechanical powertrain components, and increasing usable vehicle space.
3.2 Structure and technical parameters of in-wheel motors
In battery electric vehicles, in-wheel motors integrate propulsion, transmission, and control devices into the wheel hub, significantly simplifying the mechanical architecture. The main components of an in-wheel motor are the stator, a micro inverter, coils, the rotor, and wheel bearings. Figure 2 shows the overall structure of an in-wheel motor. The design may adopt a distributed motor arrangement; for example, a hub could be composed of eight motor units, each with an independent inverter, sharing a common rotor so that if one motor fails, the remaining motor units can continue to operate. Key technical parameters include rotor mass, rotational inertia, width, diameter, maximum winding temperature, continuous input current, reserve power, cable dimensions, total electrode mass, maximum output torque, and continuous output torque at an inlet cooling temperature of 50°C.
3.3 Characteristic analysis of in-wheel motor drives
In-wheel motor drives offer several benefits. Mechanical hard links in the powertrain are replaced by electronic control, and traditional components such as clutches and gearboxes can be omitted, simplifying the structure. Wheels driven by in-wheel motors have directly controllable torque, enabling more flexible vehicle dynamics. Because the drive wheels on each side are not connected by a rigid shaft, mechanical differentials and related components are unnecessary, reducing turning radius and, in some cases, enabling on-the-spot turning and tight maneuvering, which is valuable for special-purpose vehicles. Coordinated electrical braking and mechanical hybrid braking among individual driven wheels can reduce mechanical wear and transmission losses, improving overall efficiency. However, there are unresolved technical challenges: increased unsprung mass and rotor inertia raise suspension requirements, and early-stage designs must address sealing, waterproofing, and thermal management. In addition, core components for in-wheel motors in China still rely heavily on imports, which constrains development.
4. Conclusion
Research on in-wheel motor drive technology is an important direction for future electric vehicle drive systems and contributes to advancements in the new energy vehicle and environmental sectors. Due to rapid technological iteration, many technical issues remain to be solved. The development of in-wheel motor technology in China still has considerable room for improvement compared with abroad. Large-scale mass production will require solving multiple technical challenges; only with continued technical progress can in-wheel motors achieve broader application in China and international markets.
