What Is Axial Flux Motor?
The world’s first motor is of axial flow type. Although the axial-flow motor is not new, it has only increased in the last two decades.
For many years, it was used in stable applications such as lift and agricultural machinery, but in the last decade, many developers have worked to improve the technology so that it uses electric motorcycles, airport pods, delivery truck applications such as To be done, electric cars, and even the aircraft too.
Let’s take a look at the axial flow motor’s unique design options:
- Using a rectangular copper wire is used to obtain the biggest viable copper filling factor (90 percent).
- Dual permanent magnet rotors are employed to get the largest torque-to-weight ratio probably.
- Yocelles stator, which loses the lowest flow route and total weight.
- The use of grain-oriented power steel reduces the original loss up to 85 percent.
- A unique technique to cool the windings to achieve the lowest viable stator temperature.
- Concentrated windings for lowest copper losses achievable no coil overhangs.
Axis of Power:
The new type of motors is gaining traction in the e-mobility sectors – the axial flow types.
For many years they have been used in stationary applications such as elevators & agricultural machinery, but over the past decade, many developers have been working on improving the technologies to make them suitable for applications such as electric motorcycles, airports pods, delivery trucks: the Electric cars, and even planes.
Conventional radial flux motors, which use permanent magnets or induction motors in an electric field, are undergoing extensive development with the aim of optimizing their weight and cost.
However, it can only go so far, so moving to an entirely different machine type such as axial flow may be a good option.
Axial flux permanent magnet machines generally provide more torque for a given amount of motor than a radial motor because the active magnetic surface area is the face of the motor’s rotor rather than the outer diameter.
This makes axial flux motors more compact; The axial length of the machine is much shorter than that of radial machines, a factor that is often important for an application such as an inwheel motor.
The thinner and lighter structure results in machines with higher power and torque densities than a comparable radial machine, without the need for very high-speed operation.
Axial flux motors can also be highly efficient, with efficiencies typically exceeding 96%. This comes from the short, one-dimensional flux path that is equal to or better than the best 2D radial flux motors on the market.
Motors are smaller, usually five to eight times, and can be two to five times lighter. Both these factors change the choices of electric vehicle EV platform designers.
Axial Flux Technology:
There are two major topologies of axial flux motors—the dual-rotor single stator, sometimes called a torus-style machine, and the single-rotor dual-stator. Most permanent magnet motors these days operate with a radial flux topology.
Here, the magnetic flux starts from a permanent magnet on the loop rotor, passes through the first tooths on the stators, then flows radially along the stator. It then passes through the second tooth to reach the second magnet on the rotor.
In dual-rotor axials flux topologies, flux loops start at the first magnets, pass axially through a stator tooth, & come immediately to the second magnet.
This means that the flux path is much shorter than in radial flux machines, allowing the motors to be smaller for the same power and higher power density and efficiency.
Conversely, in radial motors, the flux travels through the first tooth and then through the stator to the magnet on the next tooth. He, too, has to follow a two-dimensional path.
Since the flow path in axial flux machines is one-dimensional, grain-oriented electrical steel can be used. Steels make it easier for the flux to pass through, & as a result, efficiency is achieved.
Radial flux motor has traditionally used distributed windings, with half the windings not energized because they overhang the magnets.
Designs have improved winding methods, as coil overhangs result in additional weight, cost, electrical resistance, and more heat wasted. Axial flux machines have very short coil overhangs, and some designs use concentrated or segmented windings that are fully energized.
Fragmented stator radial machines do additional damage due to the breakdown of the flux path in the stator, but this is not a problem for axial machines. The design of coil windings is a major area where suppliers can differentiate among themselves.
Development of Axial Flux Motor:
However, axial flux motors face some serious design and production challenges, which have made them far more expensive than their radial counterparts, despite the technical advantages. Radial motors are well understood, and manufacturing methods and machinery are readily available.
A major challenge with axial machines is maintaining a uniform air gap between the rotor and stator, as the magnetic forces are much greater than with radial machines.
Dual rotor axial machines also have thermal issues, as the windings are located deep within the stator & between the two rotors discs, which makes it difficult to get out of the heat.
Axial flux motor has also been difficult to manufacture & for several reasons. Dual-rotors machines with a yokeless topology where the iron yoke of the stators is removed, but the iron teeth are kept, overcoming some of the problems while avoiding the need to scale the machine’s diameter and magnets.
Removing the yoke, however, introduces new challenges, such as how to fix and place individual teeth without a mechanical yoke connection. It is also a challenge to beat the heat.
It can also be difficult to produce the rotor and maintain the required tolerances on the air gap, as the rotor discs exert an attractive force on the rotor.
However, the discs are directly connected to each other via a shaft ring, so the forces cancel each other out. This means that the internal bearing does not carry these forces, and its only function is to center the stator between the two rotor discs.
Although dual-stators single-rotors machines do not face the challenges of torus machines, the designs of the stator have been much more complex & difficult to automate.
Since it is unlike any conventional radial flux machine, manufacturing processes and machinery did not exist until recently. Adjusting airgap has also been difficult in machine manufacturing, as it requires precise control of the manufacturing tolerances of key components.
This has made stator design more complex, making automatic and volume production difficult, and costing motors more.
Electric Cars of Axial Flux Motor:
Reliability is of paramount importance in the automotive industry, and it has been a challenge to provide evidence of the reliability and robustness of various axial flux motors to convince manufacturers of their suitability for large-scale manufacturing.
This has prompted axial motor suppliers to carry out their own comprehensive verification programs, each capable of demonstrating that the reliability of their motors is indistinguishable from that of conventional radial flux types.
The only parts in axial flux motors that experience wears are the bearings. Due to the relatively short lengths of the axial flux design, they are located very close together and are also usually slightly ‘over-dimensioned.
In addition, lower mass rotors in axial flux machines are subject to fewer rotodynamic shaft loads, so the actual forces exerted on the bearings are much fewer than in radial flux motors.
One of the first applications for axial motors is the e-axle. The thinner width allows the motor and gearbox to be packed into the axle. In hybrid applications, the shorter axial length of the motor, in turn, keeps the overall length of the drivetrain smaller.
The next step is to mount the axial motor to the wheel of the vehicle. This allows power to go directly from the motor to the wheel, increasing the efficiency of the motor.
System complexity is also low, as the transmission, differential, and driveshaft are eliminated. However, no standard configuration appears to be emerging.
Each OEM is looking at specific configurations, as different sizes and form factors of axial motors change the design of an EV.
The higher power density of the motors means that a smaller axial motor can be used than a radial motor. This opens up new design options within the vehicle platform, such as where to place the battery pack.
Flux Efficiency:
Grain-oriented steels cannons are used with radial flux machines because the flux paths in these machines follow a non-linear 2D path.
In a yolkless axial flux topology, the grains of steel are oriented in the same direction as the flux path, so a more efficient grain-oriented electrical steel can be used for the core.
The low loss of grain-oriented material in the rolling (axial) direction and the high degree of permeability reduce stator core losses by up to 85% and give a slight increase in electromagnetic torque.
One way to boost the power of an axial motor is to combine multiple machines into a stack within a single, water-cooled housing.
This allows for much greater flexibility in design for a given application and allows for more standardized production techniques. Some e-mobility applications require direct-drive motor concepts.
A gearless design significantly reduces complexity and maintenance requirements. This maps well to axial flux motors, which already provide nominal torque at zero rpm.
For in-wheel designs, efficiency is optimized for a lower rpm range, with wheel speeds typically between 400 and 2000 rpm.
Only active parts (the stator and two rotors, for example) can be integrated inside OEM powertrain concepts. In e-axle systems, the small amount of motor can provide a compact powertrain design in the OEM market.
Segmented Armature:
The YASA Yokeless & Segmented Armatures motors topology is an example of a dual-rotor single-stator topology that has low manufacturing complexity for automated volumes productions. These provide powers densities up to 10 kW/kg at a speed of 2000-9000 rpm.
It uses a special controller which provides 200 kVA to the motor. The controller takes up a volume of 5 liters and weighs 5.8 kg, incorporates thermal management with dielectric oil cooling, and is suitable for use with axial flux motors as well as induction and radial flux motors.
It allows electric car OEMs and Tier 1 developers to choose the right motor for the application and available space. The smaller size and weight allow for a lighter vehicle and more battery, increasing the range.
Application of Axial Flux Motor:
The axial flux motor is ideal for high torque density and space-constrained applications. Most people assume that higher power density is only determined by faster speeds, and they ignore the effect of torque density.
The high-speed pursuit alone will result in limited service duration & noise. Indeed, the heart of high-speed motors is an increase in torque density, and axial flow motors are a common solution among them.
Axial Flux VS Radial Flux Motor:
The architecture of the motors are variants of what is known as an axial flow, or axial lag, machine. Because of their very low axial length-to-diameters ratios, axial flux motors are often referred to as ‘flats’ or ‘pancake’ motors.
They produce lots of torques, but their speed is limited due to the design of the rotor. These can be mentioned as one of the axial fluxes motor’s disadvantages.
Distinctions, as their name suggests, are that in axial flux machines, the magnetic flux direction is parallel to the motor rotation axis, whereas, in radial flux motors, the magnetic flux direction is radial.
Radial flux motors have the classic cylindrical design, while axial flux motors are built like a biscuit tin, and they only lend themselves to optional installations depending on size.
Because they are longers but smaller in diameter, conventional radial flux is a natural match between two wheels on the same axle.
The magnets on the rotors of the axial flux motor, like all forms of AC synchronous electric motor, are attracted to the spinning field generated by the surrounding ring of independent electromagnets in the stator.
Since the switching of the magnets that causes the field to rotate is not perfectly smooth, the rotor suffers from a small ‘torque ripple’ known as ‘cogging’ as it turns.
Most hybrid system motors are of the radial flux type, in which a rotor wound with permanent magnets rotates within the stator.
Since there is a small gap between the outer surface of the rotors and the inner diameter of the stators, these motors are known as radial gap or radial flux machines, and they can generate a lot of power as they run at high speeds. Can.
An axial flux motor is considered better than radial flux motors. There are four reasons why axial flux motor provides significantly more power while having a lower weight density:
#1. Lever
For an axial flux motor, the magnets are placed further away from the central axis. As a result, there are more “leverages” on the central axis.
#2. Windings
In terms of winding, the axial flux motor has a significant advantage over the radial motor. It has more active winding copper and less overhang, which means it can more easily increase the number of turns and generates less heat due to the final impact.
In addition, the winding can come into contact with metal, which is an excellent heat conductor. This translates into a simple cooling system.
In a radial flux motor, a large portion of the winding is idle (the portion located outside the stator teeth that is only used to form the loop (coil overhang). The coil overhang adds to the electrical heats dissipations.
Thus, it is ” distributed winding,” and this results in a significantly lower total power/weight ratio, whereas in axial flow machines, there is no coil overhang. In a radial machine, the heat must be removed through the stator core, which Must be made of low thermal conductivity steel.
#3. Electromagnetic
Because axial flux motors contain greater amounts of electromagnetically active materials, the percentage of winding mass that produces no torque but causes loss is smaller in axial flux machines.
In radial flux motors, the magnetic flux first travels through the teeth and then through the stator to the magnets.
In contrast, axial flux motors have a shorter flux path, i.e., from the first magnet, through one core, and directly onto the second magnet. Furthermore, in the cases of radial flux machines, the flux must travel in two dimensions. The flow path in axial flow machines is one-dimensional.
As a result, iron losses are less as the current travels through the core. In addition, the use of oriented steel allows flux to pass through more easily, resulting in additional efficiency gains.
#4. Cooling
In the cases of radial flux machines, the heat must be transferred to the exterior of the machine through the stator. However, steel is a poor heat conductor. It is particularly difficult to cool the “coil overhang” because it is not in direct contact with the motor casing.
Axial flux motors provide good cooling because the windings are in close contact with the outer aluminum shell. Because aluminum conducts heat easily, the windings of axial flux motors remain cool while copper has a low resistance.
And it leads to much better performance. These benefits generally increase efficiency, which has a significant impact on a worldwide scale.
Disadvantages of Axial Flux Motor:
Despite its technical advantages, the axial flux motor offers significant design and manufacturing problems, making it much more expensive than its radial counterpart.
The axial flux motor is versatile, and the winding can be varied by geometric arrangement according to the design-specific diameter, leading to a significant reduction in the total volume occupied by the machine.
Axial flow design also poses some serious design and production challenges: Mechanical Challenges: Due to the strong magnetic forces acting between the rotor and the stator, maintaining high-tolerance uniform air gaps between these two components is a challenging engineering & materials problem.
Thermal Issues: The windings of an axial flux motor are located deep within stators & between the two rotors discs, posing bigger cooling problems than a radial flux direct drive system.
Axial flux electric motors have magnets that are located in planes parallel to the coil, allowing them to generate a small rotational amount of magnetic flux, resulting in a reduction in the moment of inertia and the overall mass of the rotor.
