In the case of a 3 phase PMA radial flux type, used as a wind turbine, with magnets on the surface of the rotor, is it safe or even doable to stack another magnet with the same geometry on top of the existing ones, to reduce the air gap ? (supposing that the air gap is poorly designed in the first place by being way too large).

I suppose that the current magnets are glued to the rotor frame by epoxy. It would be cumbersome to knock them off easily.

The new magnets would be glued with epoxy too on top of the first layer.

The main problem I would see in this case is that a reduced air gap may cause the rotor to seize by striking the stator poles due to axial tilt in case of high vibrations / high RPM. or, the added magnets becoming loose at high RPM.

Any other mechanical issues to consider besides higher torque ?

In case of performance, I would expect higher starting speed and overall lower RPM at a given wind speed, lower destructive RPM tolerance, but higher power output and overall higher efficiency.

Anything I should take into account besides higher EMF at a given RPM for my rectifier circuit ?


2 Answers 2


If the magnets are epoxied as securely as they should be to resist centrifugal force and vibration, removing them may be quite difficult. However, if destroying them in removal is acceptable, it can probably be accomplished. Since grinding may be necessary, precautions should be taken to prevent dust from being inhaled by anyone.

Attaching another layer of magnets with epoxy might be possible. The rotor would need to be re-balanced.

If the air gap is excessive, reducing the air gap may be more effective than adding magnet material.

I would think the speed at which the generator produces a useable voltage (starting speed?) would be reduced not increased.


There is a simpler way : given (say) a 5 mm airgap you wish to reduce to 2mm, (having determined that gives enough clearance), cut 3mm sheet iron into segments that fit over the magnets. Slightly narrower than the magnets (80 or 90% of the width) to give a wider separation between adjacent (opposite) poles, to reduce magnetic field losses from fringing.

The iron provides a low reluctance path to the remaining airgap, which may improve performance.

It's cheap! and the magnets may hold the iron segments well enough for some initial performance measurements at some fraction of full speed. If worthwhile, then use decent epoxy (like West Systems marine epoxy) to hold them more permanently.

  • \$\begingroup\$ interesting. I suppose lamination steel would be best. \$\endgroup\$
    – rodv92
    Commented Nov 23, 2019 at 16:09
  • \$\begingroup\$ I think taminations serve no purpose since a permanent magnet is effectively DC. However you do want a high permeability iron which will not be saturated by the magnet behind it. \$\endgroup\$
    – user16324
    Commented Nov 23, 2019 at 16:14

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