The previous article, "A Comprehensive Guide to Domestic and International Hub Motor Research," summarized the hub motors that have emerged in recent years. However, most of them are conceptual, with relatively few actually used commercially. What exactly are the difficulties in hub motor drive? And in which field will breakthroughs be achieved first? This article will analyze the hub motor structure of GAC Trumpchi mentioned above. Please correct any errors or omissions.
In-wheel motor drive systems can be flexibly installed in the wheels of various electric vehicles, directly driving the wheel hub to rotate. Compared with traditional centralized drive methods such as internal combustion engines and single motors, its technological advantages and characteristics in terms of power configuration, transmission structure, handling performance, and energy utilization are extremely obvious, mainly manifested in:
The power control system has been changed from a rigid connection to a flexible connection, enabling stepless speed change between each wheel hub from zero to maximum speed and differential speed requirements between wheel hubs via an electronic controller. This eliminates traditional mechanical shifting, clutches, transmissions, drive shafts, and mechanical differentials, resulting in a simplified and unified drive system and vehicle structure, increased usable space, and improved transmission efficiency (theoretically 10%).
The freedom in vehicle layout and body styling design is greatly increased. Taking automobiles as an example, separating the load-bearing function of the chassis from the transmission function greatly simplifies the bridge structure, making it easier to diversify and serialize products with different body styles on the same chassis, shortening the new car development cycle and reducing development costs.
The torque of each wheel hub is independently controllable, with quick response and flexible forward and reverse rotation, resulting in superior instantaneous power performance and significantly improved driving ability to adapt to harsh road conditions.
It can easily achieve electric braking, electromechanical combined braking and energy feedback during the braking process of the wheel hub, and can also optimize the control and management of the efficient use of the vehicle's energy, effectively saving energy.
For electric vehicles driven by in-wheel motors, if four-wheel steering technology (4WS) is further introduced to reduce the turning radius, it may be possible to achieve zero-radius steering.
While wheel hub motors generally have a similar shape, mostly being flat, they differ significantly in motor type, structure, and drive method, and are classified as follows.
According to motor type, there are currently four main types of motors used in electric wheel hubs: permanent magnet motors (PM), asynchronous motors (IM), switched reluctance motors (SRM), and transverse flux motors (TFM). Among these, permanent magnet motors are the most widely used, while transverse flux motors are a highly competitive new type of low-speed, high-torque motor.
Classified by structural form: From the perspective of the main magnetic flux path, it encompasses all three basic forms: radial magnetic field, axial magnetic field, and transverse magnetic flux. From the perspective of motion, there are also inner rotor, outer rotor, and dual rotor structures. Among them, the dual rotor structure is the most innovative. The inner rotor is the driving rotor, and the outer rotor is the driven rotor. Power is transmitted between them through a set of planetary gears, achieving opposite rotations. This ensures that the speed at which the magnetic field cuts the conductor is the sum of the speeds of the inner and outer rotors. Clearly, this ingenious combination of speed superposition and mechanical linkage not only provides flexibility in motor design but also effectively mitigates load disturbances, smooths impact loads, and protects the battery.
According to the driving method, motors can be classified as follows: In direct drive, the motor usually adopts an external rotor structure, meaning the rotor directly drives the hub to rotate, resulting in a lower speed. In contrast, in indirect drive, the motor usually adopts an internal rotor structure, resulting in a higher speed. The speed reduction is achieved through a planetary gear and ring gear mechanism, which drives the hub to rotate; hence, it is also called a reduction drive.
Based on rotational speed: Hub motors are further divided into high-speed and low-speed types, but the corresponding speed ranges are not clearly defined and vary depending on the application. Generally, the definition of high and low speed ranges only has a relatively accurate meaning after the drive method is determined. That is, direct drive generally corresponds to low-speed motors (large size, high material consumption, low power density, low noise), while indirect drive mostly corresponds to high-speed motors (small size, low material consumption, high power density, high noise).
The hub motors used in the all-electric Trumpchi sedan employ direct external rotor drive, integrating the motor stator, rotor, and inverter into a single unit. Composed of eight logical sub-motors, they share a common rotor and utilize algorithms to achieve independent and coordinated control of each sub-motor. This "distributed" structure reduces the power requirements of each sub-motor, allowing for the use of small-sized, low-cost power electronics, resulting in a very compact motor design. Furthermore, through reasonable coordinated control of the eight sub-motors, the power and torque outputs of each sub-motor can be superimposed, achieving a powerful driving force for the entire motor. Simultaneously, if one sub-motor fails, the others can continue to operate normally, preventing the car from breaking down.
The structure of the hub motor is shown in the figure below. It consists of a rotor, bearings, stator, power and control electronic module, and sealing back plate.
So what are the key technical challenges hindering the commercial application of hub motors? They mainly include the following:
Electronic differential control technology
Because in-wheel motor-driven electric vehicles eliminate the mechanical transmission components of traditional automobiles, it is impossible to use a mechanical differential for differential control. Although electronic differentials have emerged, the vehicle exhibits significant directional instability when the speed exceeds a certain threshold. Currently, domestic and international researchers have begun to accumulate proprietary technology in this area.
Intelligent Energy Management System
In layman's terms, this is a question of whether 1+1 equals 2. People's expected value is undoubtedly 2 (algebraic sum), but the actual result can only be less than, or at best close to, 2 (vector sum). Considering all aspects of a vehicle's power and energy demands, this constitutes the problem of optimal scheduling and management of limited onboard energy and power, or what might be called an intelligent energy management system. It is both a system engineering optimization technical solution and extremely difficult, starting with the rational allocation and management of energy from each wheel hub motor, and can also include considerations of energy feedback.
Reduction of unsprung mass in hub motors
Since in-wheel motor-driven electric vehicles require the drive motor, reduction gear, and brakes to be concentrated within the wheel, the unsprung mass of the vehicle will inevitably increase if effective measures are not taken. This increases the vertical vibration amplitude of the in-wheel motor-driven electric vehicle, affecting tire adhesion performance, hindering vehicle control, and reducing ride comfort. Furthermore, with the motor located within the wheel, it will bear significant impact loads from the road surface. Therefore, researching methods to reduce the unsprung mass of in-wheel motors is of great significance in guiding electric wheel design, structural improvements, and theoretical analysis.
Solutions for reducing unsprung mass typically include:
① Converting unsprung mass into sprung mass using special types of motors. For example, Johansen, Yang, and others proposed a method to convert the stator mass of a motor into sprung mass through a special planar motor design.
② Using the mass of the motor to construct a vibration absorber to control the negative effects of vertical vibration caused by unsprung mass. For example, Nagaya et al. used the mass of the motor to construct a vibration absorber to control the negative effects of vertical vibration caused by unsprung mass.
③ Change the ratio of sprung mass to unsprung mass. For example, B. Hredzak et al. designed a hub motor using a disc motor. As shown in the figure, this disc motor consists of two stators and one rotor. The two stators are fixed to the chassis, making them the sprung mass, while the rotor is connected to the wheel to drive the wheel to rotate. In this way, only the rotor part of the motor is on the wheel. This arrangement of the motor reduces the unsprung mass significantly compared to the method where the entire motor is placed inside the wheel. However, this drive form brings new problems: the impact of the ground on the wheel is directly transmitted to the rotor of the motor, which causes the air gap width of the motor to change continuously, affecting the output torque of the motor.
Dual-stator axial flux hub motor drive form
Braking integration technology
The hub motor is installed inside the hub of the drive wheel, occupying the space originally designated for the mechanical brake calipers and discs, making it impossible to use the existing mechanical brakes. Relying solely on the hub motor's regenerative braking suffers from insufficient braking force, inability to achieve regenerative braking when the battery is low, and low braking reliability. While integrated mechanical braking solutions exist for hub motors, these solutions are not yet mature. The ring-shaped brake discs used have a large braking arm and a small friction pad braking area, leading to issues such as easy deformation, significant vibration, and high heat generation. Their braking capability and reliability still need further verification.
Integrated solution for mechanical braking
Cooling technology for in-wheel motors
Wheel hub motors frequently operate under heavy loads and low speeds while climbing long slopes, and the motors are housed within the confined space of the wheel, making them prone to overheating and burnout due to insufficient cooling. However, hub motors are directly exposed to ground vibrations and impacts, as well as mud, sand, and gravel from the road surface, creating a very harsh working environment. From a protection and maintenance perspective, the better the motor's sealing, the better. This makes it even more difficult for the heat generated during operation to dissipate outside the motor, further complicating cooling. Therefore, the problem of heat dissipation and forced cooling of the motor urgently needs to be solved.
There are two main solutions for in-wheel motor cooling:
① In the structural design of electric wheels, the use of gas (wind) to cool the motor is considered, and an electric wheel structure that facilitates gas circulation is applied to cool the hub motor. For example, Ryunosuke Kawashima et al. designed a cooling fan specifically for cooling the brake disc and hub motor. This design installs a blade-shaped spoke inside the hub, using the rotation of the blade-shaped spoke to generate airflow to cool the brake disc and hub motor. Tests were conducted on actual vehicles using the designed cooling fan. The results showed that cars equipped with the cooling fan consume 2% to 4% more energy, but it enhances airflow and increases the gas vortex at the wheel inlet, resulting in better motor cooling.
② In the structural design of electric wheels, the use of liquid (water) to cool the motor is considered. This is achieved by setting up dedicated coolant (water) channels, which allow for heat exchange between the coolant and the motor to cool the hub motor. For example, Royji Mimtani et al. applied for a US patent in 2010 entitled "High-Efficiency Cooled Hub Motor". In this patent, an oil pump is installed at the end of the hub motor shaft. The oil pump forces oil from the tank into a specially designed cooling channel until it reaches the motor stator, where heat exchange between the oil and the stator cools the stator.
In summary, hub motors still face numerous challenges, including instability at high speeds, large unspring mass, difficulty in heat dissipation under highly sealed environments, brake integration issues, and the need for optimized energy management. However, many of these technical problems will be difficult to overcome without large-scale commercial applications. This paper argues that miniaturized low-speed electric vehicles are less sensitive to these issues, and the breakthrough for mass application should begin in this area. This will also accumulate technical expertise and resources for the application of hub motors in high-speed vehicles.
