Axial Flux – The future technology in motor for the electric vehicles
Traditional radial flux motor technologies have been dominating the motor industry for the last 150 years, but suffer from problems with machine cost and size. To address this problem, this particular type of electrical machine exhibits superior characteristics: the Axial Flux Permanent Magnet Machine. It not only has greater engineering benefits but also reduces environmental damage.
Typically, the electric motors are designed and constructed in order to use the radial flux distribution, where rotor and stator have a small radial air gap between them whereas, Axial flux motors are versatile and the winding can be varied by geometric arrangement according to the design specific diameter, making it possible to considerably reduce the total volume occupied by the machine.
Axial flux designs have posed some serious design and production challenges as well: –
Mechanical challenges: the high magnetic forces acting between the rotor and the stator produces an engineering and material difficulty in maintaining a high-tolerance uniform air gap within these two components.
Thermal challenges: Windings in an axial flux motor are located deep within the stator and between the two rotor discs – which presents a greater challenge in terms of cooling than for a radial flux direct drive design.
Manufacturing challenges: AF devices have thus far been very difficult to
manufacture as the design of the stator iron has continued to be based on that of RF motors, using a stator yoke to close the flux loop within the motor.
The axial flux electric motors have specific positioning of their magnets, which are in planes parallel to the coils, which allows generating magnetic flux over a smaller rotary volume resulting in a decrease in the moment of inertia and the overall mass of the rotor.
Let’s have a look at their unique design choices: –
- For the highest possible copper fill factor (90%), a rectangular copper wire is used.
- For the highest possible torque-to-weight ratio, Dual permanent magnet rotors are used.
- Yokeless stator, for the shortest possible flux paths and a lower overall weight
- Lowering of core losses by as much as 85% is achieved by using grain-oriented electrical steel
- A patented system for cooling the windings, for lowest possible stator temperatures
- Concentrated windings, for the lowest possible copper losses (no coil overhangs).
Axial flux permanent magnet motors operate excellently at a much wide range of rotational speeds, which makes them perfectly suitable for high-speed-low-torque as well as low-speed-high-torque applications. There are 4 reasons why Axial Flux motors deliver a significantly higher power output with reduced weight density:
1.Lever – The magnets are located further away from the central axis for Axial Flux machines. This results in a higher “leverage” on the central axis.
2.Windings – In case of radial flux machines, a large part of the windings is inactive (the part located at the exterior of the stator teeth which is only used to make loops (so-called “coil overhang”). The coil overhang results in additional electrical heat dissipation. Thus, called as “distributed winding” and results in much worse overall power/weight ratio whereas, in case of axial flux machines, they have no coil overhang at all. In the case of Magnax axial flux machines, “concentrated winding” are used where 100% of the winding is fully active (at least for Magnax axial flux machines).
3.Electromagnetic – In terms of electromagnetism, axial flux machines have an inherently more efficient topology. For radial flux motors, the magnetic flux moves through the first tooth and then via the stator back to the next tooth to the magnets. In contrast, Axial Flux motors, the flux path is shorter i.e. from the first magnet, through one core and direct on the other magnet. (only fit for dual rotor topologies such as Magnax machines).
Also, in the case of radial flux machines, the flux must follow a 2-dimensional path. In the case of axial flux machines, the flux path is one dimensional. As a result, Magnax can use grain-oriented steel for its axial flux motors. This outcome in fewer iron losses when the flux passes the cores. Additionally, the use of Oriented steel makes it easier for the flux to pass which results in an additional efficiency gain.
4. Cooling – In the case of radial flux machines, the heat has to be transported through the stator to the outside of the machine. But as we know, steel is not a very good heat conductor. The “coil overhang” is also challenging to cool, because it is not directly in contact with the motor casing. In the case of Magnax axial flux machines, the cooling is excellent because the windings are in direct contact with the exterior aluminium outside casing. As we know, aluminium conducts heat very well, the windings of Magnax axial flux machines stay cool while the resistance of the copper remains low. And this results again in a much higher performance. These advantages generally improves the efficiency that do make a huge difference on the global scale.
Currently, Autobot India is researching on the application of Axial Flux Motors in the Electric Vehicle and it’s various iterations of drivetrain configurations. We think Axial Flux motor technology is the next big technology in the EV sector which will help us increase the range and performance of the electric vehicle substantially.