Axial-flux Designs Rev Up the Future of Electric Motors
While electric motors have primarily hinged on radial motor technology over the past 50 years, axial-flux designs are now surging ahead.
In recent years, electric motors (e-motors) have evolved to match the rising demand for lightweight, hyper, and electric vehicles. Though battery designs are getting more efficient and compact, motor designs are still large and robust.
EV manufacturers face a paradox: inexpensive (though less efficient) motors require additional batteries. Those batteries mean higher costs and more weight. Image used courtesy of Saietta Group
For over 50 years, electric motor designs depended on radial motor technology, which can be difficult to match to new cutting-edge battery designs. Despite the challenges in creating more efficient e-motor designs, one rising solution is axial-flux motors, which are a smaller and more power-efficient alternative.
Axial-flux vs. radial-flux motors. Image used courtesy of Magnax
This article will take a look at how three companies are innovating e-motors using axial-flux technology.
Mercedes-Benz Acquires YASA’s Axial-flux Designs
The first example of axial-flux technology comes from YASA. Mercedes-Benz recently acquired YASA’s axial-flux technology to be implemented into its AMG electric-only platform.
A key part of YASA's technology is its use of axial-flux motors. Axial flux stakes an advantage over radial motor technology because its spinning rotor has a larger diameter. This is because the rotor turns alongside the stator instead of within it.
Torque is inversely proportional to speed for electric motors, and torque equates to the force multiplied by the radius. Because YASA's axial-flux design involves a larger radius than radial motors, it generates more torque without altering the amount of force. This also yields a 30 percent increase in power density compared to radial motors.
YASA’s design approach uses less copper and iron materials, which may reduce BOM and save costs to manufacturers. Image used courtesy of YASA
YASA’s motor design removes the stator yoke and reduces iron mass by nearly 80 percent—a homage to the company name: Yokeless and Segmented Armature.
YASA has also developed a direct oil cooling solution that helps the motor reach higher power for a continuous amount of time. YASA's high-thermal-contact cooling oil is coupled with the close proximity of the rotor and stator.
Saietta Unveils Axial-flux Designs for Lightweight Vehicles
Similar to YASA is UK-based developer Saietta Group, which utilizes axial-flux motor technology to improve EVs. Saietta says its axial-flux motors can be easily integrated into lightweight zero-emission vehicles. Its technology can also be used as an alternative for small, internal combustion engine-powered motors.
The axial-flux motor, AFT140, can also be used for mid-power motorbikes operating at 96 V with a peak power of 45 kW. Image (modified) used courtesy of Saietta Group
Saietta’s AFT110 and AFT140 e-motors are under development; the AFT110 operates at 48 V and targets low-power motorbikes and 15hp marine motors while the AFT140 operates at 700 V and can be integrated with hypercars. These axial-flux designs have a small footprint, leaving more capacity for battery storage.
Magnax Introduces a Yokeless Axial-flux Motor
Magnax is a Belgium-based cleantech motor developer that aims to increase power density, efficiency, and reliability for automotive powertrains by integrating yokeless axial-flux technology. Through several years of research and technology development, Magnax was able to deliver new designs with higher power densities to replace traditional EV motors. Recently, the company announced it will be expanding its R&D efforts in yokeless axial-flux motors and power electronics.
Magnax’s AFX motors have higher power density and efficiency thanks to dual permanent magnet rotors. Image used courtesy of Magnax
The Magnax AXF290 is a yokeless axial-flux permanent magnet motor prototype that weighs 25 kg with an anticipated peak power density of 13 kW/kg. What allows this design to achieve high power densities is the compact nature of the yokeless motor.
In radial-flux designs, heat transfers through the stator to the outside of the machine, which makes it difficult to keep ambient temperatures from lowering. Magnax claims its patented design allows the windings to stay cool and the copper's resistance to stay low, providing higher power efficiency.
Axial-flux motor designs also impact electromagnetics characteristics because magnetic flux travels shorter distances.
Radial-flux Motors Hold Their Own
While YASA, Saietta, and Magnax all claim that axial-flux technology is the way to go, radial motor designs still hold their own in many applications.
One study investigating the differences in motor designs (when dealing with a 20-pole-pair radial-, axial-, and transverse-flux topology) found that radial-flux motors outperform their transverse flux and axial analogs when it came to torque density.
In another instance, a design calling for high motor efficiency and reduced I2R power losses may warrant a radial motor design as the most effective solution. This is because, for those specific design requirements, a motor with a high pole-pair of magnets is required.
What's your take on the different motor topologies for EVs? Share your thoughts in the comments below.