Internally mounted and surface mounted magnets

Miles

Miles

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Comparison between rotors with internally mounted and surface mounted magnets.

The stator core geometry was adjusted for the flux available from the rotor with internally mounted magnets (IM). It was retained as a constant for the comparison with the rotor having surface mounted magnets (SM).

Even with a magnet volume one quarter that of the IM rotor, the SM rotor still created a greater flux density in the stator core.

Conclusion: IM is not really suitable for high pole counts or, there's something wrong with my methodology....
 

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Wouldn't the converse then also be true. ie The internal stator creates greater flux density in the gap using less copper and lamination steel ?
 
John in CR said:
Wouldn't the converse then also be true. ie The internal stator creates greater flux density in the gap using less copper and lamination steel ?
Click to expand...
There isn't a lot of difference in the flux density levels in the gap. The significant point is that such a high proportion of the flux, on the interior mounted example, follows the "short circuit", that a much bigger magnet is needed to achieve the same flux density in the airgap. The more poles there are, the greater proportion the "short circuit" assumes.
 
Internally mounted magnets are used only if your design takes advantage of reluctance torque
also are you setting the color scale limits manually or letting FEMM do auto? the default is auto
 
flathill said:
Internally mounted magnets are used only if your design takes advantage of reluctance torque
Click to expand...

It was the Headline motor that inspired me to investigate this:

l30.jpg
 
Re-plotted with manually equalised limits:
 

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I meant to ask when I saw it before. I take it on that Frontline rotor that those laminations holding the magnets as some kind of front and back iron serve the same purpose as in a stator, or do laminations somehow change the flux lines? If it's to reduce eddy currents, then why are our motor magnets nickle coated?
 
John in CR said:
I meant to ask when I saw it before. I take it on that Frontline rotor that those laminations holding the magnets as some kind of front and back iron serve the same purpose as in a stator, or do laminations somehow change the flux lines? If it's to reduce eddy currents, then why are our motor magnets nickle coated?
Click to expand...
It's just a constructional thing. The laminations aren't really needed on the rotor. If you can extract the rotor blank from inside the stator blank.....
 
You can't only go by magnet volume. Increasing the thickness of the magnet only works to a point and then you have rapidly increasing diminishing returns. Usually if you can spare the steel to channel/focus the flux its cheaper to make thin wide magnets. The surface mounted magnet appears to be wider so its not really apple to apples

also the internal version you're showing as a short circuit where the flux is folding back on itself before it jumps the gap, this could increase the reluctance torque but I would have to think about it more
 
flathill said:
The surface mounted magnet appears to be wider so its not really apple to apples
Click to expand...
It's only 0.4mm wider than the exposed width of the laminated rotor pole (4.5%). It was the area exposed to the gap that I was aiming to keep constant. It does make quite a difference to the comparison of magnet volumes, though: 744mm³ against 251mm³ is only 3:1... :oops:
 
So what's better for power density regular mounted magnets or magnets in a laminated rotor?
 
Ok. I'll start......

Internal magnets are more wasteful of flux, per pole. The number of poles you can use, for a given air-gap radius, is less because of this. So, you'd need to run a motor with internal magnets faster, for equivalent power. Assuming the max. speed was the same, the surface mounted motor should win. On the other hand, you could probably engineer an internal magnet rotor to run faster....
 
They usually doesn't have such an easy short circuit path.

It would be interesting to see how this design compares to surface magnets.
US07474029-20090106-D00000[1].jpg
http://www.strutpatent.com/patent/07474029/rotor-magnet-placement-in-interior-permanent-magnet-machines

It's like a mix between PMSM and reluctance motor, I think. Like flathill mentions.
 
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