"Repmag" Part I — a Net Energy Output is Modelled

"Switching" permanent magnets

Although a permanent magnet cannot be switched on or off, its flux can be temporarily diverted as required, to the extent that it will no longer attract or repel another adjacent magnet.

In the next few posts I'll examine a particular case utilizing this principle.

"Repmag" permanent magnet motor

Here is an image of one of my own ideas for a permanent magnet motor, which does give some net energy output, according to the modelling I've done on it so far.

There are twelve identical 25mm IR × 62.5mm OR × 6mm × 20º arc shaped NdFeB35 permanent magnets, all magnetised in the same direction, through the z-axis (red). That means they will repel each other when placed close together, in the absence of any magnetic shielding. However, a steel magnetic shield is placed over half of the array as shown, with a small (0.25mm) airgap above and below the magnets.

The magnets rotate clockwise, when seen from above, about the z-axis. They close up against each other within the shield, and spread apart outside the shield. Although, as can be seen, the magnets' flux density is greatly increased when they have entered the shield, the flux lines will then be perpendicular to the magnet and shield surfaces. Very little flux should return around the straight sides of the magnets. (Recall that it is this flux that causes the mutual repulsion outside of the shield).

So, it would be reasonable to expect the magnets to separate outside the shield with more mutual repulsive force, and to close up within the shield with less mutual repulsive force. Also, because it is more separated from its neighbours, a magnet about to enter the shield should have a slightly higher overall flux density than one that has just left it, which would also help the desired movement.

Analysis in a silux model

I decided to analyse this idea by firstly finding all forces and torques on all the magnets at 2.5º intervals (of "spider" rotation — see below), and then assigning these quantities into the silux model shown below.

Repmag — silux model

This silux model has the magnets (black) and the steel shield (blue — only for illustration) as before. The magnets pivot around the left fixpoint. Each magnet has a roller attached to it, which slides in a spider (red), rotating around the right fixpoint, which is offset by 15mm. This ensures that the magnets remain correctly located at all times.

The model was started with a spider rotational speed of exactly 10 radians/second, with magnet forces and torques newly assigned from the magnetostatic modelling every 2.5º as discussed above.

Results from silux model for spider rotational speed and energy,
derived from the force and torque data from magnetostatic analysis

Results

The spider does at first accelerate, but then it decelerates, and accelerates again, finishing at 10.0705 radians/second after 30º, which is the end of one cycle of operation.

(Some further details: In the model the combined magnet and roller mass was an almost negligible 0.0482kg each, but the spider was made deliberately heavy, at m = 10kg and I = 0.0375kg-m², so that it would act as a flywheel. Therefore its energy (½Iω²) increased from 1.875 to 1.90153J in 0.05236 seconds, i.e. a power output of 0.507 watts.)

Conclusion

The analysis shows an increase in energy and power, which is more than the nominal 1% solution error used in the magnetostatic modelling.

If the magnets themselves, at 0.0258kg each, are considered to be the only "active" mass in the model, with everything else including the arbitrarily heavy spider excluded, this particular model indicates that 0.507/(12 × 0.0258) = 1.638 watts per kilogram of active mass could be achieved. This is better than the minimum figure of one watt per kilogram at which I'd consider building a physical prototype.

So, while no single model like this could be considered decisive, it does seem that further work on this idea would be worthwhile. 

Obviously further modelling is required. I have done some initial work on the mechanical design for a physical prototype for this idea, which I'll discuss next.