"Repmag" Part III

Further work required for a physical prototype

The modelling done so far, especially the magnetostatic/dynamic modelling described in Part I, is encouraging, and confirms the basic concept of significantly less repulsive force between adjacent magnets within the shield than outside of it. However, I would want to do further modelling before building a physical prototype. This would include larger devices, and ferrite magnets as well as neodymium-iron-boron ones. One obvious variation should also be modelled: the case where adjacent magnets all had alternate polarities, instead of the same polarity, so that they would attract instead of repelling.

Further work should also be done on the energy delivered by a magnet entering the shield separated from its neighbours, compared with the energy consumed when it exits the shield with closely adjacent neighbours. This is an important issue that needs to be fully resolved.

Various airgaps between the magnet and shield faces should also be modelled. It seems likely that very small airgaps will be desirable in a prototype, thus requiring very small flexibility/deformation of components.

Eddy currents

One reason for looking at ferrite, and also bonded, rather than sintered NdFeB magnets, would be to combat a potentially serious problem that can already be foreseen:— eddy current losses. In a device like this, magnetic flux density will vary greatly as the device operates, in the magnets themselves, and also in the shield. That means that eddy currents, and the resistive I²R losses they cause, will occur in all these components when running.

The traditional methods of reducing eddy currents to tolerable levels are to laminate conducting components, or else to substitute them with non-conducting components (such as ferrite).

Drawing 1

Repmag drawing 1

The "multi-layer" 2D drawing above is a start for a possible physical prototype. It was drawn to this stage mainly to show that, as required for this approach, the spider and its components can have a smaller radius than the distance from its pivot to the nearest magnet. Not all components are shown, such as bearings, framework etc.

The magnets (red) are attached to shafts, all pivoted to a fixed point at the black solid circle. The shafts carry linear motion bearings (purple) which are carried in equally-spaced pivots on the spider (green), which has an offset fixed pivot at the green solid circle.


Drawing 2

Repmag drawing 2

Here is another "multi-layer" 2D drawing of a slightly different approach which would give better rigidity (less deformation of components), although it would require the magnets themselves to have radial holes.

The magnets (red) have internal ball spline bearings so they can slide without rotation along the splined arms of a spider (blue) centered at O. At their outer radii the magnets carry rollers which bear against an offset circular track (green) centered at P. The shield (grey) is also centered at P.

Shield lamination options

Shield lamination

Various lamination options for the shield are shown above.

The best simple option for minimum reluctance of the magnetic circuits, i.e. Alt. A, with wedge-shaped laminations, is almost certainly impractical. Neither Alt. B or Alt. C are desirable because of their greater reluctance over the complete magnetic circuit between magnet faces (across the many inevitable small gaps between laminations, for these alternatives). Something like Alt. D could be the best option — although somewhat difficult, it should be achievable.