Do axial flux generators need iron cores?

[Njguy]

I have noticed that the majority of axial flux generator designs that people have built on the web don't have iron cores in the stator. Is this because the flux is already sufficiently dense due to the layout of the magnets that an Iron core is really not needed?

Here's two papers with in depth research
http://jfgieras.com/RJW_IEEE_MAG_2005.pdf
http://proceedings.ewea.org/ewec2009/allfiles2/276_EWEC2009presentation.pdf

 

[rumpfy]

The short answer is YES.
Where the magnets face each other with the rotor between them, the flux is perpendicular to the surface of the rotor. Lets call this surface the 'front' surface. At the 'back' surface, there needs to be a magnetically soft iron ring to conduct the flux between adjacent magnets.
These motors/generators generally have a winding which loops around each magnet, which is connected to the outside world and this winding is for magnetising the magnets.
I have a couple of these machines, often called 'pancake' motors, and they seem to suffer from demagnetising problems. I cant say why, but thats how I came by these machines.

 

[Njguy]

The short answer is YES.
Where the magnets face each other with the rotor between them, the flux is perpendicular to the surface of the rotor. Lets call this surface the 'front' surface. At the 'back' surface, there needs to be a magnetically soft iron ring to conduct the flux between adjacent magnets.
These motors/generators generally have a winding which loops around each magnet, which is connected to the outside world and this winding is for magnetising the magnets.
I have a couple of these machines, often called 'pancake' motors, and they seem to suffer from demagnetising problems. I cant say why, but thats how I came by these machines.

I appreciate the response although I must admit I'm having a had time interpreting what you meant. Also as far as I can tell in axial flux generators, the magnets are the rotor, the rotor is not between them as you say.

 

[rumpfy]

I didnt exactly answer your question. Apologies for that. I had my mind set on DC motors/generators of the pancake variety, with commutators..
The magnetic arrangement is still similar however. In both machine types, the flux is directed in parallel with the shaft axis. The air gap is necessary in both machine types to allow the rotor to rotate. There is also a need to guide the flux on the side away from the gap. Fig 6A in the IEEE paper shows the steel disc. In Fig this is part #2. Fig 4 shows the permanent magnets and the iron yoke/steel disc. Fig 4 also shows the air gap. In all machines the problem is how to maximise the flux density in the air gap and the way to do this is to eliminate all the air gaps in the magnetic circuit. The air gap where the rotor turns is an evil necessity.
Hope this is better.

 

[shortbus=]

The first paragraph of the second link tells why they don't use iron in the stators. It makes them easier to start rotating, because of no "cogging" due to the magnets and the iron. But the magnet rotors do need to be a 'magnetic pathway' to keep the magnetic force connected. By that I mean a continuous path through the rotor disc and shaft. Probablly not as important today with rare earth magnets as it was with Alnico magnets.

 

[dr pepper]

Firstly I dont mean disrespect to anyone involved in these.

I suspect that the reason why iron cores are not used is that you need a reasonable amount of magnetics knowledge to get it right and also the physical assembly (machining etc) makes iron cores difficult for a home constructor.

Cogging can be got round with design.

Axial flux generators are becoming more common, you see small ones springing up at the side of the road more and more, I suspect commercial ones do use iron cores at least to reduce the size of magnets and copper windings.