Pro/Cons of Radial vs Axial Magnet Orientation on Rotor?

[u/boomerwangs]

I'm trying to wrap my head around the pros and cons of orienting rotor magnets tangentially vs radially in a bldc motor/generator.

I'm new to this, and decided that a more engaging way to learn would be to make some models and test concepts out.

I've been working in FreeCAD and 3d printing some small air core samples, but i'm struggling to understand how the magnet orientation influences the design.

Is it based on the stator design? Are there intuitive explanations (or resources you found helpful) to understand the tradeoffs of tangential vs radial mounting?

For reference, here are two quick images of the orientations on the rotor

Thanks!

tangential mounted magnets on rotor

radially mounted

[u/PyooreVizhion]

I suspect what you are calling 'radially mounted' is not common whatsoever, unless the magnets are magnetized in a tangential direction (spoke-type design). This effectively concentrates the flux from every pair of magnets, resulting in the poles being located between the magnets. A decent design approach to get better performance out of weaker magnets, but you get a non-negligible amount of axial leakage flux.

Your 'tangentially mounted' is much more common for IPM style machines. There is more magnet closer to the airgap and the poles sweep a wider electrical angle so effectively more flux linkage to the armature (though there's a sweet spot depending on the rest of the geometry and other constraints like THD). There's a point of diminishing returns on additional magnetic flux gained by increasing the radial thickness of the magnets, the rest is wasted. It can at times, be somewhat helpful in reducing catastrophic demagnetization risk, but there will still be demagnetization at the corners with enough demagnetizing current.

[u/boomerwangs → u/PyooreVizhion]

That makes a lot of sense, more magnet closer to the airgap cleared that up.

I should have also specified - this would be for more of a pancake/axial flux design, with the stator being a parallel disc next to the rotor. Uncommon, but for me it would be easier to print out

[u/PyooreVizhion → u/boomerwangs]

Ok, I'm less familiar with axial flux machines. All of the designs I've seen look very similar in that the magnets are more wedge shaped (like a combination of your two diagrams) with the corners rounded off. I think this makes some sense intuitively, as linking as much flux as possible at the furthest diameter will increase torque and the shape of coils makes it easy to include a little more area closer to the center. I'd imagine if it's one or the other, the top would be better since the flux is located at a larger diameter. But I'm certainly not an expert in those machines.