Axial Flux Motor for Electric Vehicles: A Comprehensive Overview
A new type of electric motor — the axial flux motor — is gaining increasing attention in the electric vehicle industry. For years, axial flux motors have been used in stationary applications such as elevators and agriculture machinery. However, over the past decade, many developers have been working to refine this technology, making it suitable for a wide range of applications, including electric motorcycles, airport shuttles, cargo trucks, electric vehicles, and even aircraft.
The Emergence of Axial Flux Motor: The World's First Generator
In 1831, British physicist Michael Faraday harnessed the phenomenon of electromagnetic induction to invent the world’s first true electric generator, known as the Faraday disk generator. This invention can be seen as the prototype of the axial flux motor. The Faraday disk generator consisted of a disk, brushes, and a horseshoe magnet. Faraday placed a rotatable metal disk within the magnetic field of the magnet and used a galvanometer to measure the current between the edge of the disk and its center. The experiment demonstrated that when the disk rotated, the galvanometer deflected, confirming the presence of current in the circuit—thus achieving the conversion of mechanical energy into electrical energy.
However, due to limitations in materials, structure, and manufacturing techniques, as well as the late emergence of demand for mobile electric equipment such as electric vehicles, axial flux motors remained largely undeveloped for nearly 200 years. The primary challenges included difficulties in electromagnetic calculations, material selection, complex manufacturing processes, and the intricate coupling between materials, structure, and techniques—all of which exponentially increased the difficulty of industrializing this technology.
Axial Flux Motor in the Era of Automotive Electrification
While radial flux motors have driven the automotive industry into the electrical age, axial flux motors outperform them in several key areas. Not only are they lighter and more compact, but they also deliver greater torque and power. This groundbreaking technology enables a smaller and lighter motor to generate more powerful force, opening up a world of new possibilities for automotive designers.
The most significant difference between axial flux motors and traditional radial flux motors lies in the direction of the magnetic flux. In axial flux motors, the magnetic flux is parallel to the motor’s axis, unlike in radial flux motors where it is perpendicular to the axis. This fundamental difference leads to a considerable change in the stator’s structure. Below are some structural diagrams to help illustrate this concept.
A New Benchmark in Performance
Traditional radial flux motors, which use permanent magnets or induction in the electric field, are currently being widely developed to optimize weight and cost. However, there is a limit to how much can be improved within this design. Therefore, switching to a completely different type of motor, like the axial flux motor, could be a promising alternative.
The torque in a radial flux motor is proportional to the square of the rotor radius, while in an axial flux motor, torque is proportional to the cube of the rotor radius. This means axial flux motors can produce significantly higher torque. For instance, with the same output power, an axial flux motor is 50% smaller and 50% lighter than a radial flux motor. A prime example is the axial flux motor in the Mercedes Vision One-Eleven all-electric concept car, which weighs only one-third of a conventional motor with the same power output.
Design and Efficiency Advantages
The size and weight advantages of axial flux motors not only greatly reduce the use of materials like copper, iron, and permanent magnets in production, but also allow for more flexible placement in tighter spaces, such as the recesses of car wheels. this reduces the need for components like transmissions and drive shafts, minimizing kinetic energy loss and improving drive efficiency. The direct torque compensation further enhances performance.
Streamlined Structure and Enhanced System Benefits
Due to their compact and lightweight design, axial flux motors require fewer structural elements for installation, saving even more weight. Additionally, their rotational speed does not reach the high levels seen in traditional radial flux motors, eliminating the need for reduction gears. This further reduces the overall system weight, benefiting other subsystems like braking and cooling. Overall, this can lead to a 5-10% improvement in both efficiency and range.
Technical Challenges for The Axial Flux Motor
Despite the technical advantages of axial flux motors and the significant progress made over the years, the industry has now grasped the working principles of these motors and has largely overcome key challenges in configuration and structural design, electromagnetic performance, thermal performance, and the development of calibration and control strategies. However, the large-scale application remains a challenge due to several design and production hurdles that must be addressed:
1. Immature Manufacturing Equipment
The industry’s understanding of axial flux motor manufacturing methods is still limited due to a lack of research and experience. Manufacturing equipment is not readily available, and the supply chain is underdeveloped.
2. High Manufacturing Precision Requirements
Axial flux motors must maintain a uniform air gap between the rotor and stator, as the magnetic force is much stronger than in radial flux motors. Adjusting the air gap during the motor manufacturing process is challenging, making it crucial to precisely control the tolerance of key components.
3. Thermal management and Design Complexity
Axial flux motors feature a sandwich structure, making it difficult to dissipate heat from the central stator in dual-rotor designs or the central rotor in dual-designs. Additionally, due to their high power density, axial flux motors have a small heat capacity, leading to overheating issues. Innovative cooling methods, such as immersion oil cooling, oil-water hybrid cooling, liquid nitrogen cooling, and phase-change material cooling, are needed, along with the design of complex cooling and sealing structures.
4. High Costs Due to Complex Materials and Processes
For radial flux motors, the costs of magnets, silicon steel, copper, and structural components, as well as the manufacturing processes for mass production, are well-established within the industry. However, the industry’s understanding of the new materials and processes required for axial flux motors is still limited. While research and development costs can be reduced, the low maturity of industrialization, high costs, and significant cost uncertainties pose major obstacles to the mass production of these new axial flux motors.
Application of Axial Flux Motors in Electric Vehicles
Despite the technical challenges mentioned earlier, many companies have already developed relatively mature solutions, leading to practical applications of axial flux motors in the EV industry, including electric cars, buses, coaches, municipal vehicles, heavy trucks, vessels, and more.
Passenger Cars
Long before Mercedes-Benz adopted axial flux motors, they were already featured in several high-end vehicles. One of the most famous examples is the Koenigsegg Regera, unveiled at the 2015 Geneva International Motor Show. This 1500-hp hybrid beast is equipped with three axial electric motors – two mounted on the rear axle, providing 360 kW and 1600 N.m of torque, and a third motor delivering 160 kW and 350 N.m, responsible for engine connection, starting, power generation, and torque compensation. The unique pancake-like structure of the axial flux motor makes it ideal for placement at the engine’s flywheel end.
Other notable vehicles utilizing axial motors include the Drive eO PP03, the first electric car to win the Pikes Peak Hill Climb, as well as models like the Jaguar C-X75, Ferrari SF90 Stradale, and 296 GTB.
However, for most passenger cars, the primary challenge for EV development remains battery technology, particularly concerning range. While motors can be made to deliver extremely high power, if the battery technology cannot keep up, the vehicle’s usability is compromised. Even though radial motors are relatively compact compared to traditional internal combustion engines, they still offer sufficient space within conventional vehicle designs. Therefore, for most car manufacturers, adopting axial flux motors offers limited advantages. It’s often more cost-effective to use radial motors, which are less expensive and help optimize cost-performance ratios.
Because of this, axial flux motors are primarily used in small-batch, high-performance hypercars, where size and weight are critical factors.
Buses
Axial flux motors are particularly advantageous for buses. Their compact size and high power density align perfectly with the structural requirements of low-floor, wide-aisle, and fully flat-floor buses. Additionally, the unique characteristics of axial flux motors enable the development of distributed drive systems and independent suspension designs, which not only enhance safety redundancy but also improve vehicle range.
Our solution for buses with independent suspension integrates a gearbox, controller, four-airbag suspension, and electronic differential, resulting in shorter axial length, reduced weight, and a broader efficiency range. Compared to traditional drive systems, our solution reduces overall vehicle weight by approximately 700 – 800 kg and increases range by 20% with the same battery capacity. The drive motor is positioned directly adjacent to the wheels, allowing for a fully flat, continuous low floor throughout the bus. The widened rear aisle easily accommodates wheelchairs and strollers.
In one of our tests, buses equipped with our axial flux motor solution demonstrated a fuel economy improvement of over 10% and reduced electricity costs by $7698 over the vehicles’ lifecycle compared to traditional buses with similar capacity. The data also showed that our buses consume less electricity per kilometer, incur lower lifetime operating costs, and , due to the simplified structure and optimized design, experience less brake system wear, resulting in lower maintenance costs.
Heavy Construction Equipment
In the construction machinery sector, axial flux motors are well-suited for powering vehicles like mining trucks and concrete mixers. In mining operations, where vehicles often undergo continuous, high-load work over extended periods, the motor’s performance and efficiency are critical to achieving optimal range and operational efficiency. Our range extender generators, powered by axial flux motors, reduce the axial size by more than 2/3, making installation more convenient and improving overall energy efficiency by over 20%.
The electric drive systems with axial flux motors feature a wide efficiency range, strong driving force, and high generation efficiency. These systems can effectively harness braking energy on slopes greater than 9%, significantly reducing energy consumption. Our drive assemblies, with peak motor power reaching 900 kW, are powerful enough for 105-150 ton mining trucks, offering broad applicability.
In 2021, a fleet at a metal mine purchased mining trucks equipped with our axial flux motors. According to the fleet’s operations manager, the mine has been in operation for nearly 50 years, with most ore now located at the pit bottom, requiring heavy loads and uphill climbs. The manager recounted a recent incident where a fully loaded fuel-powered mining truck struggled to climb out of a small rut, while an electric mining truck with our motor easily overcame the obstacle. Drivers also reported that these electric trucks are more comfortable to drive, with smoother operation due to the absence of frequent gear shifting and reduced cabin noise and vibration.
To support the operation of these fully-electric mining trucks, several charging stations have been established within the mining area. The charging speed is impressive, with about 1% charge gained per minute, meaning the battery can be fully charged in just over an hour. Additionally, the motor’s regenerative braking system helps recharge the battery during operation, ensuring sufficient power even for heavy uphill climbs with just on recharge during the day.
For concrete mixers, the transition from traditional fuel-powered to electric-powered vehicles represents a significant shift in power sources and transmission methods, especially for the mixer body. Traditional fuel mixers typically use hydraulic transmission systems that divert engine power to keep the mixing drum rotating. In the electrical era, however, a drive motor converts electrical energy into mechanical energy to ensure continuous operation of the mixing drum.
Considering the stability of the mixing drum and its adaptability to different working conditions, two key performance indicators are crucial for the electric motor: it must provide sufficient power and torque, and it must be as compact, lightweight, and efficient as possible. Compared to conventional fuel mixers, the axial flux motor replaces the traditional hydraulic transmission with an electric drive system. This system powers the mixing drum independently of the engine, allowing for continuous operation even when the vehicle is turned off, significantly reducing fuel consumption.
Engine idle time typically accounts for 30%-70% of a concrete mixer’s total operating time. For a 12-cubic-meter mixer operating eight hours a day, about 20 liters of fuel are consumed during idle time. Prolonged idling also adversely affects engine life and vehicle handling. By introducing the axial flux motor system, which decouples the engine power from the mixer drum power, fuel consumption is greatly reduced. The system also captures surplus engine power during operation to recharge the battery. This setup increases user value without adding load to the engine.
Marine Applications
Axial flux motors for marine applications reduce the installation height of motors by 40% compared to traditional motors, resulting in a lower center of gravity, increased stability, and easier handling. With weight reductions of up to 50%, the rotor’s moment of inertia is decreased by 30%, improving overall system stability. The use of special materials and surface-mounted magnets increases the rotor’s safety factor to 5.
Trucks
In the truck sector, axial flux motors achieve dual 90% efficiency, covering more operating conditions within the high-efficiency range and reducing overall vehicle energy consumption. The motors’ compact size allows for a more spacious chassis, enabling the use of higher-grade transmissions. The dual-mode shifting provides seamless power without interruption, delivering superior performance.
Motorcycles
Axial flux motors also offer significant advantages for motorcycles, increasing battery and storage space by 30% and extending range by 15%. Our motors boast an IP68 rating, providing better waterproofing than the IP67 rating of many competitors. These motors can also be used in tricycles or ATVs, with configurations supporting 1-4 motors depending on the vehicle’s needs.
Industrial Applications
Beyond vehicles, axial flux motors have industrial applications in sectors such as oilfields, mining, water conservancy, and ports. Their compact design and high efficiency make them well-suited for these demanding environments.
Our Axial Flux Motor Solutions
At Brogen EV Solution, we provide axial flux motor systems from 9 kW to more than 900 kW, suitable for different types of applications. We can also customize the system according to your specific requirements.
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