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axial vs radial flux

Axial Flux vs Radial Flux: 4 Reasons Why Axial Flux Machines have a Higher Power Density

Daan Moreels
Posted by Daan Moreels on 31,January

Technology startups such as Magnax claim that electric motors and generators based on axial flux technology will be a step change in efficiency, size, reliability and cost-effectiveness versus the current generation of direct drive machines (based on Radial Flux).

But why is this?

There are 4 reasons why Axial Flux machines deliver a significantly higher power density:

1. Lever: For Axial Flux machines, the magnets are located further away from the central axis. This results in a larger "lever" on the central axis. Secondly, the Magnax Axial Flux Machines have two rotors, one at each side of the stator. Result: a large surface area in relation to the size of the machine.  

radial flux vs axial flux in electric machines 

2. Electromagnetic: Axial Flux machines have an inherently more efficient topology in the electromagnetic sense. For radial flux machines, 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 machines, the flux path is shorter: from the first magnet, through one core and straight on the other magnet. (only applicable for dual rotor topologies such as Magnax machines).

Additionally, in case of radial flux machines, the flux must make a “bend”: it must follow a 2-dimensional path. As a result, unlike axial flux machines, radial flux machines cannot use oriented electrical steel for the cores (stator). In case of axial flux machines, the flux path is one dimensional. As a result, Magnax can use grain oriented steel for its axial flux machines. This results in fewer iron losses when the flux passes the cores. Oriented steel makes it easier for the flux to pass which results in an additional efficiency gain of +/- 2%. Today, electrical steel is already in use for transformers (because they are stationary). And from now on; axial flux machines also allow the use of oriented steel.

3. Windings: in case of radial flux machines, a big part of the windings (as much as 50%) is not active (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 from additional electrical resistance (= heat dissipation) and has no function. It is what they call a “distributed winding” and results in much worse overall power/weight ratio compared with axial flux machines which have no coil overhang at all. In case of Magnax axial flux machines, “concentrated windings” are used where 100% of the winding is fully active (at least for Magnax axial flux machines).

axial vs radial with coil overhang.jpg

The coil overhang in a traditional radial flux motor is not beneficial for its functioning (left). At the right, the concentrated windings of the Magnax axial flux motor.

 

4. Cooling. In case of radial flux machines, the heat has to be transported through the stator to the outside of the machine. But steel is not a very good heat conductor. The “coil overhang” is also difficult to cool, because it is not directly in contact with the motor casing. In case of Magnax axial flux machines, the cooling is excellent because the windings are directly in contact with the exterior aluminium outside casing. And Aluminium conducts heat very well. As a result, 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 efficiency.

These advantages generally result in several additional percentages of efficiency, which makes a huge difference on a global scale.

For more information. Read our whitepaper:

download whitepaper 

 

Topics: Motor Technologies, axial flux

Magnax Axial Flux Direct Drive Generators and Motors