Magnet Material

[Yianie123.]

I have recently ran across information that Ceramic Magnets are better to use, much cheaper,  won't corrode but will have much less pull strength strength than neodymium magnets.  Has anyone used both materials and can share your experience?  Does pull strength matter?
Thank you.

[joestue]

Crane550 made a 2 or 3 to one chain drive speed increaser and used some relatively massive ceramic magnets.

In general, neglecting the efficiency and power density improvements that come with larger machines, the "performance" of a motor generator is approximately proportional to the energy density in the magnet. 10x ratio between ferrite and ceramic.

I just measured approximately 216 watts out of a ferrite magnet hvac fan motor at around 700 rpm, 50 volts at 2.5 amps 3 phase, about 3.5nm torque going in. 12 poles, 18 coils, frequency was 78hz out

The magnets are about 2.6 inches long, 3 inch outside diameter, .3 inches thick. That's like 6 cubic inches of ferrite, for 250 watts at 700rpm at 86% efficiency.
 Cogging torque is .25NM

If my rpm was actually 780 then its significantly less efficient than i thought.

[Yianie123.]

Thank you for your response.  I guess I'm trying to figure out if you use ceramic magnets, to build a Windturbine will you get good results.  There is a new book being written with Hugh Piggots and ceramic magnets are used.

[Adriaan Kragten]

Thank you for your response.  I guess I'm trying to figure out if you use ceramic magnets, to build a Windturbine will you get good results.  There is a new book being written with Hugh Piggots and ceramic magnets are used.

I have build radial flux PM-generators with ceramic magnets (ferroxdure) about 45 years ago when this was the only strong magnetic material available and later also with neodymium magnets. This is described in my public report KD 341. The remanence Br of neodymium magnets is about a factor four higher than for ceramic magnets but the low remanence of ceramic magnets can be compensated by concentration of the magnetic field. However, concentration is only possible for radial flux PM-generators. For a radial flux generator, the magnets are placed in radial grooves. Concentration is realised because the magnet area is much larger than the pole area if the armature has 12 poles. With concentration, it is possible to create a magnetic flux which is that strong that the stator is saturated and so the maximum torque level is rather high.

Concentration is very difficult for axial flux PM-generators because the magnetic pole is formed by the magnet itself. If ceramic magnets are used for an axial flux generator, you start already with a low remanence of the magnets and the flux density in the air gap is even much lower depending on the ratio in between the thickness of the air gap and the thickness of the magnets. So at a certain rotational speed, only a low voltage is generated in one turn of a winding and so a much lower power is generated for that rotational speed than for a PM-generator of the same size in which neodymium magnets are used. So for a certain maximum torque level, an axial flux generator with ceramic magnets must be much bigger than if neodymium magnets are used. Much bigger means more material for the magnets, more steel for the sheets and also more copper for the winding. So I doubt if using ceramic magnets makes an axial flux generator cheaper.

The pull strength isn't the criterium for judging magnets used in PM-generators. It is the remanence Br. The remanence is the strenght of the magnet flux in T in the magnet of the poles of the magnets are short-circuited with a mild steel iron bow which isn't saturated. The remanence depends on the magnet quality. The pull strength depends on the remanence but also on the magnet dimensions.

[Yianie123.]

Thank you for your reply.  Not quite sure why a new built it book is being written using ceramic magnets.it might be a fake web site.

[MattM]

When Hugh started I believe he used them.  You can get good results with them, too.

[joestue]

Judicious use of a tile saw could cut 2x6x1 inch magnets across the diagonal, to add 2 triangle sides to each 2" magnet, making a 2x4x6 trapezoid.

So you could put 14 magnets at about a 21 inch outer diameter disk, with 3/4 inch of space between the magnets at the od, and the magnets touching each other at the 9" diameter inner diameter

 42 magnets needed for 2 disks. On the order of 270$ for 42 magnets, plus shipping. This would buy you 27 cubic inches of neodymium, and might be about equivalent in output.


12 coils, AaBbCc repeating 4 times.