Reducing Eddy current losses

Question

Eddy currents are setup in any metallic block which is in the vicinity of changing magnetic fluxes. These primarily cause heat losses, and in certain cases causes damping of the relative motion between the metallic block (where the currents are induced) and the magnet producing the field.To reduce these effects, we use two strategies:-

1. Laminating the metallic core, that is to be in the vicinity of changing magnetic flux.
2. By drawing teeth along the piece of metal.

I fail to understand how these would reduce the eddy losses? The reason cited in the first case is the eddy current path will be blocked by the laminations. But from what I see, laminations just make the eddy currents go in smaller circles, and the path length traversed by the eddy currents are actually more than in the case without laminations, and therefore the resistance should increase, and the power losses should increase!
In the second case, the reason cited is the reduction in the area of the eddy current loops and hence a smaller magnetic moment for damping.($\vec \mu=I\vec A$)But again, although the area of each eddy current loop has decreased, but then individual loops in different "teeth" can produce individual moments, and the net area being same, the net moment will still be equal producing similar deacceleration!

Answer 1

Note that the voltage induced by the changing magnetic field is directional. To reduce the resulting currents, you only need to increase resistivity in the direction the current would flow.

That's what laminations do. Laminations are thin sheets of metal that conduct electricity (as a unintentional side effect of having desirable magnetic properties). These are stacked for form the bulk magnetic material, but with thin electrically insulating layers between the laminations. The net result is that current can flow in a loop in the plane of the laminations only, not perpendicular to the plains.

In a transformer or other magnetic device with the core made from laminated metal, the lamination planes are oriented so that the induced voltage will be perpendicular to them, thereby not allowing eddy currents to flow thru the bulk of the magnetic material.

Eddy currents can still flow within each lamination layer, since the layers aren't infinitely thin. However, they are thin relative to the whole device, so to a first approximation there are no eddy currents. How thin to make the layers is then a engineering tradeoff with the small amount of current that will still flow within each layer.

Another way to avoid this issue is to use material that is magnetically conductive but electrically not. Ferrite is such a material commonly used for this purpose. The downside is that the materials with good magnetic properies are also electrically conductive, which is why sometimes the lamination concept is used.

Answer 2

A third possibility is to use a very magnetically soft metal (minimal hysteresis) that has very high electrical resistance, thus amorphous (glassy) metals.

Metglas Fe/B/Si/P 25 µm foils have high magnetic susceptibility, very low coercivity, and high electrical resistance. Amorphous metals have rather lower saturation induction and larger magnetostriction than Fe-Si core steels,

Answer 3

This is where you are going wrong, according to farday's law of electromagnetic induction

$V_{induced} = {-d\phi}/dt$

So, actually potential is induced and not current, since circuit can be completed/current flows it flows! There are some examples in which current do not flow too, such as movement of conducting rod in magnetic field, giving rise to Motional EMF.

Anyway, since induced potential depends on net magnetic flux, it also depends on area where it is being developed, when slots are cut in a plate, or laminas are formed, the the area of individual slots is less ane hence lesser potential is induced in individual slots, but this potential when added all together may still be equal to original induced potential.

Now, as you noticed in totality the current has to travel a longer distance, it must be experiencing more resistance, this would directly imply that the current in this case would be lesser than the original case in accordance with $ I = V/R$ and therefore energy losses will also be lesser.

Since the current $I$ in totality has been reduced, the magnetic moment also reduces ! Hence the difference in deccleration