Axial-flux motors will be a major breakthrough for electric vehicles, offering a huge side benefit

Automakers are putting the main emphasis on batteries when looking to improve electric cars, focusing on increasing their energy density and reducing charging times. However, the importance of electric motors in this development should not be overlooked.

Manufacturers seem to favor the use of axial flux motors, as these have a beneficial impact on the overall design of the car and the battery.

Radial and axial flux motors are distinguished by the direction of movement of the electromagnetic flux. In axial flux motors, the flux moves along the axis of rotation of the motor, while in radial flux motors, the flux moves perpendicular to the axis of rotation.

In a simplified way, we can consider the axial motor as an inversion of the radial motor. In a radial motor, it is the rotor that sits inside the coils and rotates, while in an axial motor, it is the two outer discs that rotate around the central fixed disc (stator) along with the winding.

What advantages does the axial flux motor offer?

According to Dr. Tim Woolmer, Founder and CTO of YASA, a leading manufacturer of axial flux motors recently acquired by Mercedes-Benz, the main difference is the reduced size and weight of the axial flux motor compared to a equivalent power. In this explainer video, he points out that savings in weight and size have a cascading impact on the overall efficiency of electric cars.

Currently, electric cars are often considered overweight, to the point of being potentially dangerous, according to some sources in the United States. In Europe, the idea of ​​a tax based on weight is even considered. It is true that electric motors are not the heaviest part of an electric car, since it is the battery that generally takes this role. For example, an 80 kWh battery pack can weigh close to 500 kg. However, if the weight of the engine is reduced, this leads to a proportional reduction in the weight of other components, ranging from the battery to the chassis to the cooling system.

A 400 Nm axial flux motor occupies about the same diameter and is only one-sixth the size of a radial flux motor. Therefore, it has six times higher torque density than the radial flux motor, which means that a similar sized axial flux motor can deliver six times more torque. For example, a YASA axial motor produces 800 Nm of torque and almost 500 HP of power, while weighing only 25 kg.

This design has several advantages in terms of weight and cost reduction. A lighter motor requires fewer materials to manufacture (for example, a 50 kg radial flux motor contains about 25-30 kg of iron). Reducing the amount of materials used also reduces the energy consumption needed to produce it, which helps reduce the carbon footprint of the process.

Axial flux motor on the left and radial flux motor on the right.

In contrast, YASA claims its motors are between 5% and 10% more efficient than an equivalent radial flux motor. Thus, to maintain the same autonomy as an equivalent model, the size of the battery could be reduced proportionally.

At the same time, the weight of other systems could be reduced, such as the cooling system or the brakes, which would no longer need to be so bulky, because the motor is lighter, the battery is lighter, the chassis is lighter, etc.

Thus, according to YASA's calculations, up to 200 kg could be saved on an electric car, which would translate into lower energy consumption and greater range.

Why haven't axial flux motors been made until now?

Although the concept of the axial flux motor has been known since the 19th century, the difficulty of industrializing this type of motor, compared to the relative ease of manufacturing a radial flux motor, has so far prevented its arrival on the market.

Also, unlike radial flux motors, axial flux motors are usually much larger in diameter. However, YASA has succeeded in reducing the diameter of its axial flux motors using a unique process. Rather than having a stator (the "center disc" of the motor) as a single block, YASA split it into smaller sub-elements, reducing its weight by 80%. This approach resulted in a considerably lighter frame.

On the other hand, cooling is crucial for axial flux motors, but this has been a challenge so far. Internal coils tend to heat up, increasing resistance to rotation and ultimately reducing power. YASA solved this problem by using an oil-based liquid cooling system.

To avoid these disadvantages, YASA has chosen to focus on the production of very limited series. Initially, it was for the prototypes, which were used in particular during the Pikes Peak race. Then the production expanded to models with very limited distribution, such as the Koenigsegg Regera or the McLaren P1, and now YASA is picking up the pace to equip cars like the Ferrari SF90 and the McLaren Artura.

Although YASA is now owned by Mercedes-Benz, the German manufacturer allows YASA to manufacture engines for other brands. However, YASA's priority is to serve future Mercedes-AMGs.